Mangrove forests are the dominant intertidal ecosystem of tropical coastlines. In New Caledonia, mangroves act as a buffer zone between Ni open-cast mines and a lagoon registered as a UNESCO World Heritage site. Across the intertidal zone, mangroves are composed of three main stands; with from the seaward side to the landward side: Rhizophora spp., Avicennia marina, salt-flat. This botanical gradient relies on the duration of tidal immersion, which imposes sedimentary gradients of water content, salinity, oxygenation, and organic content. The objective of this PhD thesis was to improve our undestanding of the biogeochemistry of iron and nickel in mangrove sediments and to charaterize the role of edaphic parameters on trace metals dynamic. Particular emphasis was focused on the mineralogical (bio)transformation of Fe and Ni bearing phases and on crystal chemistry. Both XANES and EXAFS data showed that Fe and Ni speciation strongly followed the redox boundaries marking the intertidal and depth zonations. Fe(III)-bearing goethite and phyllosilicates, herited from lateritic outcrops, were the major Fe and Ni hosts in the upper mangrove sediments. These mineral species were fully preserved at depth in the dry and oxic salt flat area. By contrast, beneath the vegetated Rhizophora and Avicennia stands, goethite rapidly disappeared with depth. In these anoxic horizons, sulfate reduction occurred, and pyrite and organic complexes became the dominant Ni-species. At the limit between oxic and anoxic layers, intense re-oxidation of aqueous Fe(II) and Fe-sulfides led to the formation of ferrihydrite, lepidocrocite and likely goethite. The relative proportion of the newly formed poorly ordered iron-oxyhydroxides was found to be higher in the Rhizophora mangrove stand. Moreover Ni incorporation in pyrite was less developed beneath Rhizophora stand. Such lateral evolution of Ni speciation may be related to reoxidation of Ni-bearing pyrites in the Rhizophora stand, which is subjected to periodic alternation of reducing and oxidizing events due to daily tidal fluctuations. The latter may be a major cause for continuous Fe reductionoxidation cycles in the vegetated mangrove stands, and could significantly affect iron and nickel mass balances in mangroves. Indeed, Ni was found to be immobile in the salt flat, to accumulate beneath Avicennia and to be partially leached beneath Rhizophora. Eventually, Fe and Ni behavior in mangrove sediments currently oxidizing in response to isolation from the sea, was studied. In the layer of the oxidation front, really high concentrations of dissolved Ni were measured, while Ni concentrations in the solid phase were 4 times lower than in the pyritized horizon, and 2.5 times lower than in the upper sediment. These results suggest that mangrove sediment oxidation was a cause of Ni loss. This PhD thesis allows a better assesment of the mineralogical processes that lead to the fixation or the release of trace metals by mangroves, and is thus useful for the management of mangroves that are situated downstram lateritic watershed.

Spéciation et dynamique du fer et du nickel dans les sédiments de mangrove situés en aval de massifs ultrabasiques (Nouvelle-Calédonie)

Photo de la mangrove en aval de creek Mine (Baie de vavouto)

Vincent NOËL Résumé

La mangrove est un écosystème majeur des zones intertidales des côtes tropicales. En Nouvelle-Calédonie, les mangroves sont des zones tampon entre les massifs ultrabasiques, plus ou moins exploités, et un lagon inscrit au patrimoine mondial de l'UNESCO. Le long de l'estran, cet écosystème se décompose principalement en trois zones de végétation qui se développent parallèlement au trait de côte, avec du front de mer vers les terres : Rhizophora spp., Avicennia marina, tannes. Ce gradient botanique dépend de la durée d'immersion des marées, qui impose des gradients de teneur en eau, de salinité de l'eau interstitielle, d'oxygénation, et de teneur en matière organique dans les sédiments.

L'objectif de cette thèse était d'améliorer notre compréhension du cycle biogéochimique du fer et du nickel dans les sédiments de mangrove, et de déterminer le rôle des paramètres édaphiques sur la mobilité de ces éléments métalliques. Ce travail est plus spécifiquement focalisé sur l'évolution de la cristallochimie de Fe et Ni en liant avec les (bio)transformations minéralogiques. Les analyses de spectroscopie d'absorption à rayons X montrent clairement que les spéciations de Fe et Ni évoluent en fonction des gradients redox qui marquent d'une part, la zone intertidale, et d'autre part, la profondeur. La goethite et les phyllosilicates, hérités des bassins versants latériques, sont les principaux hôtes du Fe et Ni dans la partie supérieure des sédiments de mangrove. Ces espèces minérales sont intégralement préservés en profondeur des sédiments des tannes, qui sont pauvres en matière organique et bien oxygénés.

En revanche, sous les Rhizophora et les Avicennia, la goethite disparait rapidement avec la profondeur . Dans ces horizons inférieures anoxiques riches en matière organique, la sulphato-réduction se développe, et la pyrite et les complexes organiques sont les principales phases porteuses du Ni. A la limite entre les couches oxiques et anoxiques, une intense réoxydation du Fe (II) aqueux et des sulfures de fer conduit à la formation de ferrihydrite, lépidocrocite, et probablement de goethite. La proportion relative des oxyhydroxydes de fer néoformés et mal cristallisés, est plus élevé dans la mangrove à Rhizophora. En outre l'incorporation du Ni dans la pyrite est également moins prononcée dans cette zone. Une telle évolution latérale de la spéciation du Ni peut être liée à la réoxydation fréquente des pyrites porteuses de Ni en lien avec le balayage quotidien de cette zone par les marées. Ce cycle tidal, qui touche plus particulièrement la zone à Rhizophora du fait de sa position basse dans l'estran, peut être une cause majeure des cycles de réduction et d'oxydation des phases porteuses de Fe, et pourrait affecter de manière significative les bilans de masse du fer et du nickel dans les mangroves. En effet, le Ni tend à être immobile dans les tannes, à s'accumuler sous Avicennia, et à être partiellement libéré sous Rhizophora. Finalement, le comportement du Fe et du Ni dans des sédiments de mangrove subissant une oxydation intense, en réponse à un isolement de la mer, a été étudié. Au niveau du front d'oxydation, des concentrations très élevées de Ni en solution ont été mesurées , tandis que les concentrations en Ni dans la phase solide étaient quatre fois plus faible que dans l'horizon pyritisé , et 2,5 fois plus faibles que dans le sédiment supérieur. Ces résultats suggèrent que l'oxydation des sédiments de mangrove est une cause de perte en Ni pour l'écosystème. Cette thèse a permis une meilleure connaissance des processus minéralogiques qui conduisent à la fixation ou la libération des élémentes traces métalliques par les mangroves, et est donc utile pour la gestion des mangroves qui sont situés en aval de bassins versants latéritiques.

Table des matières

Problématique et cadre de la recherche A. Contexte du projet de recherche

La mangrove est un écosystème spécifique de la zone intertidale, ayant développé des capacités d'adaptation à des conditions hypersalines et dont la distribution des différentes espèces végétales est fonction de la salinité [START_REF] Walsh | Mangroves, a review[END_REF], la durée d'inondation [START_REF] Mckee | Soil physicochemical and mangrove patterns species -reciprocal distribution effects ?[END_REF], le taux de sédimentation [START_REF] Ellison | Impacts of sediment burial on mangroves[END_REF], ou bien encore la teneur en sulfures des sédiments [START_REF] Nickerson | Association between pore water sulphide and distribution of mangroves[END_REF]. La mangrove revêt une importance capitale, aussi bien au niveau écologique qu'économique, et représente, avec la forêt tropicale humide, un des écosystèmes les plus productifs en domaine terrestre, i.e. 30.0 Tmol y-1 C [START_REF] Twilley | Carbon sinks in mangrove forests and their implications to the carbon budget of tropical coastal ecosystems[END_REF][START_REF] Jennerjahn | Relevance of mangroves for the production and deposition of organic matter along tropical continental margins[END_REF][START_REF] Alongi | Nutrient-use efficiency in arid-zone forests of the mangroves Rhizophora stylosa and Avicennia marina[END_REF][START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF]. Actuellement, elle occupe environ 75 % des littoraux tropicaux sur près de 200 000 km2, et sa végétation est constituée par ~19 familles de palétuviers, se décomposant en ~27 genres et ~70 espèces [START_REF] Ellison | Mangrove communities[END_REF]. Cependant, du fait de la croissance démographique, d'une urbanisation accrue, de l'expansion des activités industrielles, de la prospection et de l'exploitation des ressources naturelles, la mangrove disparaît actuellement à un taux de 1 à 2% par an. Ce taux est équivalent voire supérieur à celui des écosystèmes menacés, tels que les récifs coralliens ou la forêt tropicale primaire (UNEP- WCMC, 2006 ;[START_REF] Valiela | Mangrove forests: one of the world's threatened major tropical environments[END_REF]Wilkie et Fortuna, 2003). La destruction de cet écosystème se déroule partout à travers le monde, et particulièrement dans les pays émergents, où se trouvent 90 % des mangroves. Une des menaces évoquée pour cet écosystème est l'accumulation potentielle d'éléments traces métalliques (ETM) au sein de son substratum, en relation avec sa richesse en matières organiques (MO) [START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF]. Cependant, il n'y a encore que très peu de connaissances sur le devenir et l'impact des ETM dans ces écosystèmes, de même que sur la capacité de ces derniers à stocker les ETM ou bien à les relarguer vers l'océan et les zones récifales frangeantes.

En Nouvelle-Calédonie, la superficie des mangroves est particulièrement étendue (35 000 ha sur près de 80 % du littoral occidental et 14 % du littoral oriental) et leur richesse floristique est très significative (24 espèces, dont 1 endémique). Du fait d'un contexte géomorphologique particulier, la mangrove calédonienne constitue une zone tampon entre un lagon de plus de 20.000 km2, délimité par la plus longue barrière récifale continue au monde, et les massifs ultrabasiques (péridotites) et leurs couvertures d'altération, couvrant 1/3 du territoire calédonien. Ces massifs sont caractérisés par leur grande richesse en Fe et dans une moindre mesure en Mn, Ni, Co, Cr qui font de la Nouvelle-Calédonie, le 3 ème producteur mondial de nickel (9,3 % des réserves mondiales de nickel recensées par l'USGS en 2009).

En période de crue cyclonique, les processus naturels d'érosion et de sédimentation représentent des facteurs importants de transfert des matériaux latéritiques riches en ETM vers le littoral, la mangrove, les récifs frangeants et le lagon. Par conséquent, les mangroves de Nouvelle-Calédonie subissent des pressions naturelles importantes et représentent un formidable laboratoire naturel pour mener à bien un projet de recherche concernant la dynamique des ETM au sein de cet écosystème. En outre, ces phénomènes de transferts érosifs peuvent être localement accentués par l'exploitation minière qui, depuis la fin du XIXe siècle, a remanié environ 300 millions de m 3 de stériles latéritiques riches en ETM, dont une partie a été transportée par érosion vers les zones littorales à la faveur d'épisodes climatiques violents.

Dans ce contexte, une volonté s'est exprimée de limiter les dégradations environnementales potentielles. Elle se traduit par la formulation d'un code minier (loi de pays 2010), et de codes de l'environnement des provinces Nord (2008) et Sud (2009). Désormais l'activité minière est encadrée depuis l'ouverture de la mine (études préliminaires, état initial) jusqu'à sa fermeture (plan de restauration). Les motivations premières de KNS sont :

-établir un état des lieux du comportement des métaux au sein des sédiments de mangroves situés en aval de la future zone d'exploitation du massif du Koniambo

B. Objectifs

Le projet a pour objectifs d'identifier les mécanismes contrôlant la dynamique de certains Fe et Ni au sein des sédiments d'une mangrove située en aval d'un massif ultrabasique minéralisé. Une attention particulière est portée aux relations entre l'évolution de la spéciation des éléments métalliques au sein des environnements littoraux de dépôt et la composition chimique des eaux porales, tout en tenant compte de la zonation de la diversité floristique intertidale. Cette étude s'inscrit dans une optique de meilleure connaissance globale des processus régissant l'écosystème mangrove.

Les questions spécifiques auxquelles nous nous proposons de répondre sont les suivantes :

-Quelle sont les relations entre les paramètres externes (saison, marée, type de végétation) et les conditions redox du milieu ?

-Quelle est la spéciation des éléments métalliques dans les fractions solides (modes d'association avec les phases minérales et la matière organique) et les eaux porales de cet écosystème ?

-Quelles sont les voies de transformation microbienne et/ou abiotique des phases porteuses des éléments métalliques engendrées par la variabilité des conditions redox et comment influent-elles sur la spéciation des éléments métalliques dans les solides et les eaux porales ?

-Quelles sont les effets de l'activité minière sur le cycle naturel des éléments métalliques ? -Quel est le devenir des éléments métalliques suite aux modifications importantes des paramètres physico-chimiques engendrées par l'aménagement du territoire ? Par exemple, quel est le devenir de ces éléments au cours de l'oxydation des sédiments isolés des marées ?

Nous souhaitions, en premier lieu, obtenir une meilleure compréhension du fonctionnement biogéochimique des sédiments de mangroves en fonction de la végétation et des conditions environnementales. L'étude a contribué à montrer l'influence des paramètres physicochimiques sur les processus de transformation/néoformation des phases minérales, et le rôle de ces dernières sur la dynamique de Fe et Ni. Nous confirmons également le rôle de la matière organique (MO) sur la dynamique de ces éléments dans cet écosystème spécifique.

Finalement, cette étude permet de déterminer les conditions dans lesquelles la mangrove agit comme piège vis-à-vis du nickel, ainsi que les conditions qui influencent la stabilité de ce piège, à longue échelle.

Au-delà de ces résultats qui sont transposables à d'autres mangroves se développant dans des conditions physiographiques similaires, les résultats attendus concernent la gestion environnementale des mangroves de Nouvelle-Calédonie vis-à-vis de l'activité minière qui est en plein essor.

C. Plan du manuscrit

-La première partie de ce mémoire de thèse est consacrée à une étude bibliographie d'une part, sur les mangroves et leur intérêt dans le cycle des éléments métalliques, et d'autre part sur le contexte particulier de la Nouvelle-Calédonie, véritable laboratoire naturel pour l'étude de l'impact anthropique sur le cycle des éléments trace métalliques.

-La seconde partie de ce mémoire de thèse est consacrée à la présentation des sites d'études, de la stratégie d'échantillonnage et la démarche analytique entreprise au cours de la thèse.

-Le troisième chapitre est consacré à la détermination de la spéciation quantitative du fer dans les sédiments de mangrove. Les relations entre les transformations minérales et les paramètres externes (végétation, bioturbation, etc.), sont recherchées. Elle a donné lieu à une publication soumise dans Geochimica et Cosmichimica Acta.

-Le quatrième chapitre s'intéresse au comportement du nickel au cours de ces transformations minérales au sein des sédiments de mangrove. L'objectif principal était de comprendre l'influence à long terme du cycle d'oxydoréduction sur la mobilité réelle de cet élément à l'interface continent-océan. Les résultats sont présentés sous la forme d'une publication en préparation pour Environmental Science and Technology.

-Le cinquième chapitre est consacré à la cinétique de synthèse des pyrites en présence d'ETM dissous, ainsi qu'à l'étude du comportement de ces ETM vis-à-vis des phases minérales précipitées au cours de la pyritisation. Elle fera l'objet d'une publication.

-Dans le sixième chapitre, l'attention est portée sur les effets de l'activité anthropique sur le cycle biogéochimique naturel du fer et du nickel. Les résultats sont présentés sous la forme d'une publication en préparation pour Science of the Total Environment.

-Enfin, la septième et dernière partie décrit les conclusions et perspectives du travail. Elle qui se développe dans la zone de balancement des marées des régions littorales tropicales et subtropicales [START_REF] Trochain | Ecologie végétale de la zone tropicale non désertique[END_REF][START_REF] Blasco | Les mangroves[END_REF]. Une acceptation plus large considère la mangrove comme l'ensemble de l'écosystème colonisé par cette végétation. Il se développe le long de plus de 75% des régions intertropicales entre 32°N (îles des Bermudes) et environ 39°S (Victoria, Australie) [START_REF] Day | The productivity and composition of mangrove forests, Laguna de Terminos, Mexico[END_REF]Food and Agriculture Organisation FAO, 1994 ;IFRECOR, 2006). Selon les estimations, elle couvre de par le monde une surface de 100 000 km 2 à 181 000 km 2 [START_REF] Blasco | Les mangroves[END_REF][START_REF] Spalding | World Mangrove Atlas[END_REF]Giri et al., 2011, Figure I-1). Les mangroves représentent ainsi un des écosystèmes majeurs de la biosphère tropicale.

Figure I-1 :

Distribution des forêts de mangrove dans le monde (Giri et al., 2011).

Cette forêt littorale de faible diversité spécifique est principalement composée de ligneux appelés palétuviers [START_REF] Tomlinson | The Botany of Mangroves[END_REF]. Ces derniers se développent préférentiellement sur des cordons littoraux, sableux ou argileux, à l'abri des courants marins qui érodent le littoral et empêchent les propagules (organe asexué de propagation apte à se développer en nouvelle plante) de s'implanter. Ainsi, cet écosystème se développe essentiellement dans des zones calmes et peu profondes, tels que dans les estuaires et les systèmes lagunaires. Les palétuviers sont régulièrement inondés d'eau saline, depuis des zones quotidiennement inondées par les marées jusqu'à des zones inondées uniquement durant les marées de vives eaux. Ces eaux sont également diluées par les ruissellements d'eau douce en provenance du milieu continental.

Ces dernières sont porteuses de sédiments, chargés en minéraux, qui viennent se déposer dans le marécage. Ainsi, les mangroves doivent leur forme et leur nature à des influences aussi bien marines que terrestres.

Les rôles écologiques de la mangrove sont multiples et agissent à différents niveaux, qu'ils soient physiques, chimiques ou biologiques :

-Tout d'abord, elle joue un rôle physique, en stabilisant le trait de côte, et protégeant les côtes contre les vagues et l'érosion due au vent [START_REF] Wolanski | Hydrodynamics of mangrove swamps and their coastal waters[END_REF][START_REF] Furukawa | Currents and sediment transport in mangrove forests[END_REF]UNEP-WCMC, 2006). De plus, il s'est avéré que les mangroves modèrent les effets des tempêtes et des cyclones côtiers [START_REF] Dahdouh-Guebas | Transitions in ancient inland freshwater resource management in Sri Lanka affect biota and human populations in and around coastal lagoons[END_REF].

-D'autre part, elle joue un rôle important de piégeage physique et chimique. Elle piège notamment les sédiments telluriques issus de l'érosion des bassins versants, assurant la protection des récifs côtiers et réduisant la turbidité de l'eau sortant vers le lagon, dont la limpidité est nécessaire pour la survie des herbiers et des coraux situés en aval de la mangrove.

-Elle joue également le rôle de régulateur des échanges de nutriments entre terre et mer, et contrôle notamment le transfert des éléments métalliques majeurs et traces le long des côtes tropicales [START_REF] Ponnamperuma | The chemistry of submerged soils[END_REF][START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Lacerda | Biogeochemistry of Mangrove Soil Organic Matter : a Comparison Between Rhizophora and Avicennia Soils in South-eastern Brazil[END_REF][START_REF] Tam | Spatial variation of heavy metals in surface sediments of Hong Kong mangrove swamps[END_REF][START_REF] Burton | Sulfur biogeochemical cycling and novel Fe-S mineralization pathways in a tidally re-flooded wetland[END_REF]. La mangrove est notamment reconnue aujourd'hui comme un filtre contre les pollutions anthropiques biologiques.

-Par ailleurs, les processus de stockage et de transfert du carbone entre les différents réservoirs de son cycle biogéochimique jouent un rôle essentiel sur la pression de CO 2 dans l'atmosphère [START_REF] Berner | Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over phanerozoic time[END_REF][START_REF] Hedges | Global biogeochemical cycles: progress and problems[END_REF][START_REF] Donato | Whole-island carbon stocks in the tropical Pacific: Implications for mangrove conservation and upland restoration[END_REF].

-L'importance des mangroves comme zones de nourriture, de reproduction et d'alevinage pour de nombreux poissons et mollusques commerciaux est reconnue (MacNae, 1974 ;[START_REF] Martosubroto | Relationship between tidal forests and commercial shrimp production in Indonesia[END_REF].

-Enfin, dans de nombreux pays, la mangrove est un lieu de récolte et d'utilisation traditionnelle de produits utilisés par la population locale, elle produit du bois pour Chapitre 1 : Etat de l'art sur les éléments métalliques dans les mangroves le charbon, mais aussi pour les constructions d'habitations. Elle fournit aussi de nombreuses plantes qui alimentent l'artisanat et la pharmacopée locale. (Walters et al., 2008) La mangrove est ainsi un écosystème essentiel des zones tropicales, et motive les études pour une meilleure compréhension du fonctionnement géochimique de son écosystème.

Malheureusement, cet écosystème est grandement menacé du fait de la croissance de population en zone côtière et de sa surexploitation, de la déforestation et du "traitement" des rejets anthropiques. A l'échelle de la planète, sa superficie totale diminue de 1 à 2 % par an [START_REF] Duke | A world without mangroves?[END_REF].

I-A.2. La mangrove, un écosystème zoné

A l'échelle d'un estuaire, les différents genres et familles s'organisent en bandes de végétation, chacune dominées par une espèce végétale différente. Ces faciès successifs sont grossièrement parallèles au trait de côte et offrent une zonation répondant aux exigences écologiques de chaque type de végétation. Cette zonation est communément observée dans les mangroves de par le monde, elle varie en fonction de la durée d'inondation des marées -liées à l'élévation topographique - [START_REF] Mckee | Soil physicochemical and mangrove patterns species -reciprocal distribution effects ?[END_REF][START_REF] Baltzer | Les formations végétales associées au delta de la Dumbéa (Nouvelle Calédonie) et leurs indications écologiques, géomorphologiques et sédimentologiques mises en évidence par la cartographie[END_REF], la salinité [START_REF] Walsh | Mangroves, a review[END_REF], les propriétés des microorganismes présents [START_REF] Quénée | Transformations minéralogiques et texturales de matériaux rocheux, mortiers et bétons d'ouvrages variés -Approche de la cinétique des mécanismes et identification des facteurs responsables[END_REF], les conditions d'oxydoréduction et des teneurs en sulfures des sédiments [START_REF] Nickerson | Association between pore water sulphide and distribution of mangroves[END_REF][START_REF] Mckee | Soil physicochemical and mangrove patterns species -reciprocal distribution effects ?[END_REF][START_REF] Youssef | Mangrove zonation in Mobbs Bay-Australia[END_REF], du taux de sédimentation [START_REF] Ellison | Impacts of sediment burial on mangroves[END_REF] et de la dynamique de développement végétal. La majorité de ces facteurs étant fortement corrélée à la durée et la fréquence d'inondation des marées, ainsi une immersion prolongée entraine une anoxie du milieu, l'oxygène ne diffusant que quelques millimètres à travers un sédiment saturé en eau [START_REF] Pastor | Modeling biogeochemical processes in sediments from the Rhône River prodelta area (NW Mediterranean Sea)[END_REF], tandis qu'un faible temps d'inondation entraine une plus forte évaporation, donc l'augmentation de la salinité, ainsi qu'une meilleure oxygénation du sédiment. La zonation peut donc être plus spécifiquement représentée en fonction de Du front de mer vers l'intérieur des terres:

-La zone à Rhizophora : Rhizophora, reconnaissable à ses racines échasses, est le genre le mieux adapté à l'immersion et aux sols gorgés d'eau. Ce genre peut résister sans souffrir à l'anoxie. Il se développe sur des sols d'une salinité proche du milieu marin ou d'un milieu saumâtre (entre 19 et 49 g.L -1 , de moyenne 40g.L -1 ) et dans des conditions anoxiques (Eh<100 mV ; sédiment continuellement sous la colonne d'eau) ou suboxiques (Eh entre 100 et 400 mV) au dessus de -6 cm, dût à la bioturbation de la macrofaune benthique ou de la porosité élevée du sédiment [START_REF] Zoneco | Typologies et biodiversité des mangroves de Nouvelle-Calédonie[END_REF]. Ainsi, il se développe opposées [START_REF] Mckee | Soil physicochemical and mangrove patterns species -reciprocal distribution effects ?[END_REF][START_REF] Marchand | Pore water chemistry in mangrove sediments: relationship with species composition and developmental stages. (French Guiana)[END_REF]. Le système racinaire plus développé de Rhizophora [START_REF] Hesse | Some differences between the soils of Rhizophora and Avicennia mangrove swamp in Sierra Leone[END_REF], enrichit le sédiment en carbone organique de façon plus significative que le système racinaire d'Avicennia (de [START_REF] De Granville | Aperçu sur la structure des pneumatophores de deux espèces des sols hydromorphes en Guyane[END_REF], ce qui influence les conditions redox dans les sédiments suite à la consommation d'oxygène lors de la dégradation de la matière organique produite [START_REF] Alongi | Below-ground decomposition of organic matter in forests of the mangroves Rhizophora stylosa and Avicennia marina along the arid coast of Western Australia[END_REF]. Le substrat sous Rhizophora est principalement anoxique, sauf dans des zones à forte bioturbation, générée par les crabes, ou bien dans des zones de plus forte élévation topographique, position qui entraîne un dessèchement périodique du sédiment. Ceci étant la dynamique des marées est susceptible de modifier temporairement les conditions d'oxygénation, en particulier lors de l'apport d'eau de mer saturée en oxygène à la marée montante. A contrario, les lenticelles, qui sont des organes de respiration présents sur les pneumatophores des Avicennia, ont la capacité de diffuser de l'oxygène dans le sédiment [START_REF] Scholander | Gas exchange in the roots of mangrove[END_REF] et de provoquer des conditions suboxiques à oxiques. Il existe une corrélation négative entre la densité de pneumatophores et le degré d'anoxie du sédiment [START_REF] Mckee | Reexamination of pore water sulphide concentrations and redox potentials near the aerial roots of Rhizophora mangle and Avicennia germinans[END_REF], ces racines peuvent créer des rhizosphères oxydantes plus larges que les autres espèces, le dégazage d'O 2 étant estimé entre 28 et 179mmol.m -2 .d -1 [START_REF] Nedwell | Dynamic nature of the turnover of organic carbon, nitrogen and sulfur in the sediments of a Jamaican mangrove forest[END_REF].

La nature réductrice ou oxydante du sédiment apparaît être une conséquence de l'élévation topographique, entrainant une saturation en eau plus ou mois importante, mais aussi de la nature du palétuvier et du développement de la mangrove. En conclusion, la distance au front de mer et le type de végétation influence largement les conditions physicochimiques du milieu, et par conséquence, peuvent modifier plus spécifiquement la spéciation, la dynamique et la biodisponibilité des métaux vis-à-vis des organismes vivants, le long de la zone intertidale [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]Marchand et al., 2011[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF].

I-A.4. Etat de l'art sur les processus biogéochimiques dans les sédiments de mangrove

La géochimie des sédiments de mangrove diffère profondément de la géochimie des sédiments continentaux et marins avoisinant du fait de la forte variabilité des conditions physico-chimiques (redox, pH, salinité) qui s'y développent. Cette spécificité induit, d'une part, une grande diversité des processus de décomposition des MO (Marchand et al., 2006[START_REF] Marchand | O r g a n i c c a r b o n s o u r c e s a n d transformations in mangrove sediments: a Rock-Eval pyrolysis approach[END_REF], et d'autre part, de nombreuses dissolutions/précipitations des phases minérales [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF].

Cette particularité du point de vue des cycles d'oxydation et de réduction qui interviennent au niveau des interfaces liquide-solide favorisent le développement d'une intense activité bactérienne basée sur les réactions d'oxydoréduction de Fe 3+ /Fe 2+ et de SO 4 2-/S 2-qui interviennent dans plusieurs cycles métaboliques vitaux (en particulier la respiration hétérotrophe).

Dans ces environnements sédimentaires hydromorphes, les sulfates constituent la principale source de soufre disponible à l'interface eau -sédiment. La réduction des sulfates en sulfures d'hydrogène (H 2 S) s'opère sous l'action de bactéries sulfato-réductrices, en présence de MO réactive [START_REF] Morse | Pyrite formation under conditions approximating those in anoxic sediments: II. Influence of precursor iron minerals and organic matter[END_REF][START_REF] Wilkin | Formation processes of framboidal pyrite[END_REF]. En milieu marin, la forme qui prédomine est HS -. Par la suite, le sulfure d'hydrogène peut évoluer suivant plusieurs voies :

soit former des composés organo-soufrés avec la MO, soit réagir avec du fer ferreux (1) ou d'autres cations bivalents (2) [START_REF] Billon | Géochimie des métaux et du soufre dans les sédiments des estuaires de la Seine et de l'Authie[END_REF][START_REF] Hmaidi | Évolution des environnements de dépôt au Quaternaire terminal dans le bassinméridional d'Alboran : apport de l'étude de la carotte 93TG-21[END_REF][START_REF] Lesven | Devenir des éléments traces métalliques au sein du sédiment, un compartiment clé de l'environnement aquatique[END_REF], pour précipiter des sulfures métalliques, soit réduire directement les oxyhydroxydes de Fe(III) en sulfures de fer, y compris la pyrite [START_REF] Neal | Iron sulfides and sulfur species produced at hematite surfaces in the presence of sulfate-reducing bacteria[END_REF][START_REF] Hellige | Pathways of ferrous iron mineral formation upon sulfidation of lepidocrocite surfaces[END_REF]. Dans un environnement riche en Fe, les sulfures produits par la réduction bactérienne des sulfates sont précipités essentiellement sous forme de mackinawite (FeS), de greigite (Fe 3 S 4 ) et de pyrite (FeS 2 ) [START_REF] Marchand | The composition of sedimentary organic matter in relation to the dynamic features of a mangrove-fringed coast in French Guiana[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF][START_REF] Burton | Sulfur biogeochemical cycling and novel Fe-S mineralization pathways in a tidally re-flooded wetland[END_REF]. A l'inverse, dans les vasières intertidales qui occupent la même situation physiographique en zone tempérée, les sulfures libres, dans un contexte faible en fer, correspondent à la forme dominante du soufre sédimentaire. Les sulfures peuvent notamment s'associer au soufre élémentaire pour former des polysulfures (S 4 2-, S 5 2-, S 6 2-, …) [START_REF] Billon | Géochimie des métaux et du soufre dans les sédiments des estuaires de la Seine et de l'Authie[END_REF][START_REF] Kribi | Décomposition des matières organiques et stabilisation des métaux lourds dans les sédiments de dragage[END_REF][START_REF] Gabelle | Étude de la contamination des sédiments par les métaux dans les canaux et rivières du nord de la France[END_REF][START_REF] Lesven | Devenir des éléments traces métalliques au sein du sédiment, un compartiment clé de l'environnement aquatique[END_REF][START_REF] Zhou | Évaluation de la biodisponibilité des métaux dans les sédiments[END_REF] qui sont des précurseurs connus comme nécessaires pour la formation de la pyrite [START_REF] Rickard | Chemistry of iron[END_REF].

La sulfato-réduction est généralement considérée comme le processus de respiration dominant [START_REF] Lallier-Vergès | The relationship between environmental conditions and the diagenetic evolution of organic matter derived from higher plant in a present mangrove swamp system (Guadeloupe, French West Indies)[END_REF][START_REF] Alongi | Below-ground decomposition of organic matter in forests of the mangroves Rhizophora stylosa and Avicennia marina along the arid coast of Western Australia[END_REF]. Cependant, des études récentes dans les sédiments de mangrove riche en Fe, ont suggérées que le rôle de la respiration du Fe dans l'oxydation du carbone peut être comparable ou supérieur à la réduction du sulfate [START_REF] Nielsen | Phosphorus dynamics during decomposition of mangrove (Rhizophora apiculata) leaves in sediments[END_REF][START_REF] Canfield | Aquatic Geomicrobiology[END_REF]. En condition anoxique, la réduction microbienne des oxyhydroxydes de Fe(III) peut conduire à la transformation de ces phases minérales en oxyhydroxydes de Fe(II)-Fe(III), tels que magnétite [START_REF] Lovley | Anaerobic production of magnetite by a dissimilatory iron-reducing microrganism[END_REF] et rouille verte (fougérite) [START_REF] Génin | Thermodynamic Equilibria in Aqueous Suspensions of Synthetic and Natural Fe(II)-Fe(III) Green Rusts: Occurrences of the Mineral in Hydromorphic Soils[END_REF] et des sulfures de fer [START_REF] Johnson | Biogeochemical cycling of iron and sulphur in leaching environments[END_REF].

Inversement, les variations de pH, de teneur en oxygène, de taux de saturation en eau, de température, de surface spécifique des phases minérales réductrices, et de la capacité métabolique des bactéries, peuvent entrainer de nombreux phénomènes d'oxydation dans les sédiments marins [START_REF] Billon | Géochimie des métaux et du soufre dans les sédiments des estuaires de la Seine et de l'Authie[END_REF]. Par exemple, les sulfures (FeS, greigite, FeS 2 amorphe) peuvent s'oxyder par simple contact avec l'oxygène dissous [START_REF] Quénée | Transformations minéralogiques et texturales de matériaux rocheux, mortiers et bétons d'ouvrages variés -Approche de la cinétique des mécanismes et identification des facteurs responsables[END_REF][START_REF] Billon | Géochimie des métaux et du soufre dans les sédiments des estuaires de la Seine et de l'Authie[END_REF], provenant de la diffusion d'oxygène par les systèmes racinaires [START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF], de la bioturbation, par exemple due aux crabes [START_REF] Ferreira | Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate[END_REF], ou bien au cours des premières heures de la marée haute [START_REF] Caetano | Tidal flushing of ammonium, iron and manganese from inter-tidal sediment pore waters[END_REF]. L'oxydation des sulfures de fer peut conduire à la transformation de ces phases minérales en oxyhydroxydes de Fe(III), tels que la ferrihydrite, la schwertmannite [START_REF] Majzlan | Thermodynamics of iron oxides: Part III. Enthalpies of formation and stability of ferrihydrite (!Fe(OH)3), schwertmannite (!FeO(OH)3/4(SO4)1/8), and e-Fe2O3[END_REF], la goethite, ou bien la jarosite [START_REF] Parkhurst | User's Guide to PHREEQC (Version 2) -A Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations[END_REF].

En conséquence, l'évolution des dépôts sédimentaires de mangroves est dominée et caractérisée par les cycles biogéochimiques du fer et du soufre.

I-B. Capacités potentielles des sédiments de mangrove à la rétention des éléments métalliques

I-B.1. Recensement des concentrations des ETM dans les sédiments de mangroves

Par contraste avec les polluants organiques, les éléments traces métalliques (ETM) ne peuvent pas être dégradés biologiquement ou chimiquement, et donc peuvent s'accumuler localement.

Par conséquent, et en raison de leur toxicité potentielle pour les organismes vivants [START_REF] Toro | Acid volatile sulfide predicts the acute toxicity of cadmium and nickel in sediments[END_REF], notamment humain, leur dynamique dans l'écosystème de mangrove a reçu beaucoup d'attention dans les dernières décennies [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF][START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF].

Les concentrations en ETM, de nombreux écosystèmes de mangroves ont été répertoriées dans les articles de revue de [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF]) et de Bayen (2012). Le Tableau I-1, modifié de [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF], répertorie les concentrations de nombreux ETM (Cd, Cu, Cr, Hg, Mn, Ni, Pb, Zn, et Fe) pour les sédiments des écosystèmes de mangroves situées à Hong Kong [START_REF] Chan | Heavy metal concentrations in coastal sea water and sediments from Tolo Harbour, Hong Kong[END_REF][START_REF] Fung | s e s s m e n t o f h e a v y m e t a l p o l l u t i o n i n H o n g K o n g 's marine environment[END_REF]Tam et Wong, 1995b ;[START_REF] Ong Che | Heavy metals in Metapenaeus ensis, Wriocheie sinensis and sediment from the Mai Po Marshes, Hong Kong[END_REF], en République populaire de Chine [START_REF] Jingchun | Distribution and speciation of some metals in mangrove sediments from Jiulong River Estuary, Peoples Republic of China[END_REF], en Malaisie [START_REF] Kamaruzzaman | Geochemistry of sediment in the major estuarine mangrove forest of Terengganu region, Malaysia[END_REF], en

Australie [START_REF] Mcconchie | Heavy metals in marine biota, sediments, and waters from the Shark Bay area, Western Australia[END_REF]Mackey et al., 1992), en Inde [START_REF] Ray | Assessment of Godavari estuarine Mangrove ecosystem through trace metal studies[END_REF], aux

Philippines [START_REF] Prudente | Heavy metal concentrations in sediments from Manila Bay, Philippines and inflowing rivers[END_REF], au Mexique (Soto-Jimenez et Paez-Osuna , 2001), en

Tanzanie [START_REF] Dewolf | Heavy metal levels in sediments of four Dar es Salaam mangroves. Accumulation in, and effect on the morphology of the periwinkle, Littoria scabra (Mollusca:Gastropoda)[END_REF], au Brésil [START_REF] Lacerda | H e a v y m e t a l a c c u m u l a t i o n b y m a n g r o v e a n d s a l t -m a r s h sediments[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF], et en Nouvelle-Calédonie (Marchand et al., 2011a(Marchand et al., , 2011b[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF].

Pour certains sites d'étude, l'accumulation des ETM peut être très importante. Par exemple, l'étude de Marchand et al. (2012), montre notamment que les concentrations de Ni dans les sédiments de mangrove de Nouvelle-Calédonie, en aval de massifs ultrabasiques (21 à 65 µmol.g -1 ) sont 11 à 36 fois supérieures à la concentration maximale répertoriée dans les mangroves du monde (1,8 µmol.g -1 ; [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF]. Il paraît alors évident, de renforcer les études sur la biodisponibilité et la mobilité des ETM accumulés dans ces sédiments [START_REF] Caçador | Accumulation of Zn, Pb, Cu and Ni in sediments between roots of the Tagus estuary salt marshes, Portugal[END_REF][START_REF] Lacerda | A worldwide evaluation of the status of mangroves with emphasis on ecosystems functioning, environmental impacts and sustainable use[END_REF]Machado et al., 2002 ;[START_REF] Adeyeye | Trace metals in soils and plants from Fadama farms in Ekiti State, Nigeria[END_REF][START_REF] Cox | Trace métal distributuion within marine and estuarine sédiments of western moreton bay, Queensland, Australia : relation to land use and setting[END_REF][START_REF] Gonzalez-Mendoza | C o o r d i n a t e d r e s p o n s e s of phytochelatin synthase and metallothionein genes in black mangrove, Avicennia g e r m i n a n s , exposed to cadmium and copper[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF][START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF].

Tableau I-1 : Exemple de la concentration des éléments traces métalliques mesurée dans les sédiments de mangrove dans le monde (modifié de [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF].

I-B.2. Mécanismes de piégeage des éléments métalliques dans les mangroves

La mangrove, grâce sa position géomorphologique et son réseau racinaire, piège physiquement les sédiments. En effet, les racines des arbres créent petit à petit leur substrat en retenant les alluvions, et augmentent ainsi la sédimentation, permettant alors à d'autres plantules de prendre racine. Ainsi, cet écosystème se comporte comme un bassin de rétention des particules provenant des systèmes fluviaux continentaux. D'autre part, les matières en suspension de la colonne d'eau sont efficacement piégées [START_REF] Furukawa | Currents and sediment transport in mangrove forests[END_REF].

La mangrove possède également toutes les conditions requises pour une forte capacité de piégeage chimique de nombreux éléments, tels que les ETM [START_REF] Tam | Spatial variation of heavy metals in surface sediments of Hong Kong mangrove swamps[END_REF][START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. Les observations de Martin et Kaplan (1998), montrent que la fraction de métaux biodisponibles, apportée par les eaux continentales, diminue généralement au cours du temps, suggérant que ces derniers interagissent soit avec les divers composants dissous, soit avec la fraction solide.

Un grand nombre d'interactions avec les divers composants des eaux interstitielles peut affecter la biodisponibilité des ETM. En outre, l'augmentation de la salinité entraîne la formation de complexes métal -chlorure stables, diminuant ainsi la biodisponibilité des ETM [START_REF] Lerner | Plant responses to enviromental stresses: from phytohormones to génome reorganization[END_REF][START_REF] Mader | Heavy metals: problems and solutions[END_REF]. D'autre part, la matière organique dissoute peut également former de nombreux complexes avec les ETM, comme le Ni [START_REF] Doig | Nickel partitioning in formulated and natural freshwater sediments[END_REF].

Par ailleurs, les ETM peuvent interagir avec la fraction solide du sédiment, selon différents processus chimiques :

(1) par adsorption des ETM à la surface des matériaux composants le substrat, notamment les particules à grains fins qui s'accumulent physiquement dans les sédiments de mangroves, tels que les argiles et les oxydes métalliques [START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Silva | Metals reservoir in a red mangrove forest[END_REF][START_REF] Tam | Retention and distribution of heavy metals in mangrove soils receiving wastewater[END_REF], 1997 ;[START_REF] Defew | An assessment of metal contamination in mangrove sediments and leaves from Punta Mala Bay, Panama[END_REF][START_REF] Cuong | Heavy metal contamination in mangrove habitats of Singapore[END_REF][START_REF] Janaki-Raman | Trace metal enrichments in core sediments in Muthupet mangroves, SE coast of India: application of acid leachable technique[END_REF][START_REF] Macfarlane | Accumulation and partitioning of heavy metals in mangroves: a synthesis of field studies[END_REF][START_REF] Chatterjee | An assessment of trace element contamination in intertidal sediment cores of Sunderban mangrove wetland, India, for evaluation of sediment quality guidelines[END_REF]. Cette capacité dépend de la nature des matériaux, du redox et de la concentration relative des ions présents dans la solution aqueuse du sédiment, et des conditions de pH (Hutchings et [START_REF] Saenger | Ecology of mangroves: St. Lucia[END_REF][START_REF] Lerner | Plant responses to enviromental stresses: from phytohormones to génome reorganization[END_REF], les cations métalliques étant plus mobiles à pH acide.

(2) par complexation des ETM avec la matière organique. En effet, la matière organique présente de fortes affinités pour les cations métalliques [START_REF] Nissenbaum | Organic-metal interactions in recent sediments: the Rôle of humic substances[END_REF].

D'autre part, le système racinaire des palétuviers génère de grandes quantités de matière organique dans le sédiment [START_REF] Alongi | Below-ground decomposition of organic matter in forests of the mangroves Rhizophora stylosa and Avicennia marina along the arid coast of Western Australia[END_REF]. En effet, le carbone organique total (COT) dans les sédiments de mangrove est en moyenne de 2,2% [START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF], cependant il existe de fortes variations entre les sites et la concentration en COT peut atteindre des valeurs supérieure à 10 % (Marchand et al., 2011b). Dans un environnement enrichi en matière organique, cette dernière peut exercer un fort contrôle sur le cycle des ETM par complexation, tel que pendant l'oxydation de phases réductrices porteuses d'ETM [START_REF] Fritzsche | Fast microbial reduction of ferrihydrite colloids from a soil eluent[END_REF].

(3) par incorporation dans la structure des minéraux en cours des transformations minéralogiques. En effet, les conditions réductrices régnant en profondeur dans les sédiments de mangroves peuvent directement affecter la mobilité et la biodisponibilité des ETM présents dans le sédiment et l'eau. Des études ont montré que les sulfures minéraux de type MeS (Acid Volatile Sulfides) peuvent se former avec Cu, Pb, et Zn, car ils sont moins solubles que FeS (Morse et Luther III, 1999 ;[START_REF] Wen | Mobilization of heavy metals from Lean River sediment[END_REF][START_REF] Billon | Géochimie des métaux et du soufre dans les sédiments des estuaires de la Seine et de l'Authie[END_REF]. Ceci aurait pour conséquence une diminution de la mobilité de ces éléments en raison de la forte insolubilité d de leurs sulfures métalliques [START_REF] Tack | Chemical Speciation and Fractionation in Soil and Sediment Heavy Metal Analysis : A Review[END_REF][START_REF] Wen | Mobilization of heavy metals from Lean River sediment[END_REF]. Les cations métalliques peuvent également être substitués dans la structure des sulfures de fer (Huerta-Diaz et [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF]Burton et al., 2008a, b;Johnston et al., 2010a, b). La variation saisonnière de la pluviométrie influence le niveau piézométrique sous la mangrove, particulièrement dans les mangroves éloignées du littoral, pour lesquelles l'influence des marées est faible (Marchand et al., 2006). Durant les périodes d'intenses précipitations, le niveau élevé de la nappe devrait entraîner des conditions anoxiques sur la quasi-totalité du profil sédimentaire. Par opposition, durant la saison sèche, la chute du niveau de la nappe favoriserait le développement de conditions suboxiques dans la partie supérieure du sédiment. Il pourrait en résulter une alternance entre précipitation et dissolution de certaines phases minérales porteuses d'ETM [START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Lacerda | The fate of heavy metals in suspended matter in a mangrove creek during a tidal cycle[END_REF][START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF][START_REF] Tam | Spatial variation of heavy metals in surface sediments of Hong Kong mangrove swamps[END_REF], Marchand et al , 2006a;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF], et engendrer des variations dans la salinité, la température et le pH, qui ont un impact sur les propriétés de rétention et/ou libération des métaux par les argiles, les matières organiques, les oxydes et les sulfures (Lacerda , 1998;Rai , 2008) , 2007).

Si la mangrove

Figure I-4 :

Localisation géographique de la Nouvelle-Calédonie et distribution des mangroves le long des côtes calédoniennes (Thollot, Cahier de l'ORSTOM ; Giri et al., 2011).

Dans les mangroves calédoniennes jusqu'à 24 espèces végétales de palétuviers ont été recensées [START_REF] Duke | Mangrove biodiversity in New Caledonia. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF]. Malgré cette grande diversité, la flore de ces mangroves est cependant dominée par deux espèces : Rhizophora spp., qui couvre 50 % des zones de mangrove, et Avicennia marina, sur plus de 15 %. Ces zones boisées, sont bordées par une bande de terre hyper-salée peu végétalisée, les tannes, qui représente près de 26 % de la zone intertidale, sur l'ensemble de la Nouvelle-Calédonie.

La morphologie du trait de côte, à l'Ouest, permet le développement d'une forte zonation de la végétation, qui peut être observée sur tout le long de l'île. En effet, du front de mer vers le haut de l'estran, nous pouvons observer la présence de forêts à Rhizophora, puis de forêts à Avicennia, et enfin des tannes.

Ces mangroves, bien développées, sont de très bons laboratoires naturels, pour de mener des études comparatives de l'influence des palétuviers et de la topographie de l'estran sur le cycle des ETM.

Nouvelle-Calédonie

I-C.2. Les Massifs ultrabasiques de Nouvelle-Calédonie : une source importante d'ETM en amont des mangroves

L'île principale « Grande Terre », est recouverte pour un tiers de sa superficie par des massifs ultrabasiques [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Latham | Altération et pédogenèse sur roches ultra-basiques en Nouvelle-Calédoniegenèse et évolution des accumulations de fer et de silice en relation avec la formation du modelé[END_REF][START_REF] Perrier | Bio-géodiversité fonctionnelle des sols latéritiques miniers : Application à la restauration écologique (Massif du Koniambo[END_REF][START_REF] Becquer | Sources of trace metals in Ferralsols in New Caledonia[END_REF] : un grand massif dans le sud et un chapelet de klippes ultramafiques le long de la façade Ouest [START_REF] Latham | Altération et pédogenèse sur roches ultra-basiques en Nouvelle-Calédoniegenèse et évolution des accumulations de fer et de silice en relation avec la formation du modelé[END_REF]Figure I-5). Ces roches mantelliques affleurent à la surface terrestre, suite aux processus d'obduction datant du Miocène/Pliocène [START_REF] Brand | Nickel laterites: classification and features[END_REF][START_REF] Dalvi | The past and the future of nickel laterites[END_REF]. La composition chimique des formations latéritiques dépend de la nature de la roche-mère. [START_REF] Brand | Nickel laterites: classification and features[END_REF][START_REF] Gleeson | Nickel laterites : A review[END_REF] horizons latéritiques sont souvent de l'ordre de 1 %pds Ni, mais les minerais dits latéritiques peuvent atteindre plus de 1,6 % pds Ni [START_REF] Brand | Nickel laterites: classification and features[END_REF]. minerai ont été extraits, ce qui représente environ 300 millions de m 3 de stériles latéritiques qui ont été remaniés. L'impact de l'activité minière sur les mangroves autour de l'île n'a pas été évalué. On estime néanmoins que quarante cours d'eau et, indirectement, les marais à mangroves autour des estuaires, en aval, sont touchés par l'activité [START_REF] Bird | The impacts of opencast mining on the rivers and coasts of New-Caledonia[END_REF]. [START_REF] Holthus | Les oceans : Amenuisement desressources, dégradation de l'environnement[END_REF][START_REF] Lebigre | Mangroves. Annexe V, publication Ensemble Pour La Planète[END_REF]. Cette pression démographique au travers des rejets urbains peut aussi exercer un déséquilibre du fonctionnement biogéochimique des sédiments de mangrove, en venant perturber les paramètres physico-chimiques du sédiment. Il existe deux saisons principales et deux saisons de transitions [START_REF] Trescases | L'évolution géochimique supergène des roches ultrabasiques en zone tropicale: formation des gisements nickélifères de Nouvelle-Calédonie[END_REF][START_REF] Latham | Altération et pédogenèse sur roches ultra-basiques en Nouvelle-Calédoniegenèse et évolution des accumulations de fer et de silice en relation avec la formation du modelé[END_REF][START_REF] Traoré | Serpentinisation hydrothermale et altération latéritique des roches ultrabasiques en milieu Tropical: évolution géochimique et minéralogique de la minéralisation en platine de la Rivière des Pirogues[END_REF][START_REF] Perrier | Bio-géodiversité fonctionnelle des sols latéritiques miniers : Application à la restauration écologique (Massif du Koniambo[END_REF] :

Les

Aménagement du territoire.

La

-« L'été austral » (de 19 à 29 °C), de la mi-novembre à la mi-avril, correspondant à une saison chaude et humide avec de la pluie et des cyclones.

-Une première « saison de transition », de la mi-avril à la mi-mai, avec une décrue de la pluviosité.

-« L'hiver austral », de la mi-mai à la mi-septembre, avec des précipitations modérées et des températures minimales -Et enfin une deuxième « saison de transition » (de 15 à 24°C), de la mi-septembre à la mi-novembre, saison sèche avec une hausse des températuree. une autre [START_REF] Maitrepierre | Bilan des mesures météorologiques effectuées pour le projet Koniambo[END_REF][START_REF] Perrier | Bio-géodiversité fonctionnelle des sols latéritiques miniers : Application à la restauration écologique (Massif du Koniambo[END_REF].

Le niveau de précipitation annuel moyen en Nouvelle- [START_REF] Roche | Projet Koniambo, Etude environnementale de base, Volume I, Description du milieu physique continental[END_REF].

Les cyclones sont suffisamment fréquents en

Climat de la zone d'étude

A l'échelle de notre zone d'étude (le massif du Koniambo), la température et la pluviométrie varient beaucoup. Sur la partie sommitale (930m), on mesure une moyenne de 270mm de pluie avec une température de 27°C en février (mois le plus humide) et de 45mm de pluie avec 21°C en septembre (mois le plus sec).

La répartition saisonnière est caractérisée par un maximum pendant les 3 premiers mois de

Contexte de la zone d'étude

Le projet d'exploitation des latérites du massif du Koniambo prévoit plus de 450 millions de tonnes de minerais exploitables (titrant entre 1,5 et 3,5 %) sur une durée initiale de 25 ans, avec un prolongement possible au-delà de 50 ans. Les carrières prévues dans le plan

II-A.2. Mangrove de la baie de Dumbea

Localisation et description du site d'étude

Un troisième site de mangrove a été étudié. Il est située à l'embouchure de la rivière Dumbéa, située approximativement à 15 km au nord de Nouméa. La Dumbéa est un des nombreux petits fleuves (25km de long) qui s'écoulent sur la côte ouest de la Nouvelle Calédonie, perpendiculairement aux axes de plissement et à l'allongement général de l'île. Son bassin versant couvre 200km 2 , 80% de sa superficie totale s'étend sur des roches ultrabasiques et 20%, en aval, sont constitués par un ensemble plissé de roches sédimentaires (pélites calcaires et grès), volcaniques acides (rhyolites et tufs rhyolitiques) et volcaniques basiques (basalte) [START_REF] Baltzer | Les formations végétales associées au delta de la Dumbéa (Nouvelle Calédonie) et leurs indications écologiques, géomorphologiques et sédimentologiques mises en évidence par la cartographie[END_REF].

Climat de la zone d'étude

Dumbea est caractérisé par une précipitation moyenne de 1109mm par an. Cependant, de par sa grande diversité géographique, cette région a une pluviosité très contrastée, pouvant varier du simple au double (de 900 mm en baie de Gadji à 2 000 mm à la Couvelée). Le mois de Septembre est le mois le plus sec (précipitations moyennes de 46 mm), tandis que les le mois de Mars est recensé comme le mois le plus humide (~148 mm). La température moyenne annuelle est de 23°C, avec un maximum au mois de février et un minimum en juillet-août.

Contexte du site d'étude

Au cours d'une de ses prospections sur les rives de la Haute-Dumbéa, Jules Garnier découvre, le 24 septembre 1864, des roches enrichies en nickel, les garniérites. La fièvre du nickel s'empare alors des pionniers et de nombreuses mines exploitent le filon. En 1877, il en existe 17 à Dumbéa, et ces dernières sont exploitées jusqu'au milieu du 20 ème siècle. Suite aux nombreuses exploitations minières, et aux intenses processus d'érosion causés par les fortes pluies caractéristiques du climat calédonien, un grand nombre de particules chargées en éléments métalliques se sont déposés le long du rivage, et plus précisément dans la mangrove, enrichissant les sédiments en fer et en nickel [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. D'autre part, durant les dernières décennies, l'aménagement du territoire au sein de la baie de Dumbea, contribue à la diminution de la surface de ces mangroves impactées. La construction L'ensemble des missions de terrain a été réalisé à la fin de la saison des pluies, pendant les périodes d'avril à mai 2011 pour les prélèvements effectués à la baie de Vavouto, et en mai 2009 pour les prélèvements effectués à Dumbea.

Concernant la mangrove de la baie de Vavouto

Pour les deux sites d'étude, l'ensemble du travail réalisé a été identiquement répété le long de trois différentes stations de prélèvements : une station dans les Tannes, une autre dans les sédiments à Avicennia et une dernière dans les sédiments à Rhizophora.

Pour chaque station, les mesures et les échantillonnages sont réalisés le long de carottes de 50 cm de profondeur. Deux séries de prélèvements de carottes ont été réalisées. Une première série a été prélevée en aérobie, tandis que la seconde a été prélevée sous flux d'azote. Chacun des prélèvements de cette seconde série a été stocké dans des flacons en verre scellés sous flux d'azote. Les échantillons ont ensuite été temporairement préservés à la température de 4°C. Au laboratoire, les échantillons solides ont été immédiatement introduits en boîte à gants sous atmosphère azotée, afin d'être séchés sous vide. Ils ont ensuite été broyés, homogénéisés et enfin stockés en conditions anoxiques. Pour chaque série, les prélèvements ont été réalisés tous les 5 cm, le long des cinquante premiers centimètres de profondeur. Ce procédé est répété pour chacun des 6 points géographiques étudiés, à l'origine de 60 prélèvements en aérobie et 60 prélèvements en anoxie.

Parallèlement aux prélèvement de la phase solide et aux mesures des paramètres physicochimique, les eaux porales ont également été extraites des carottes de sédiments à l'aide de seringues couplées à des bougies poreuses en polypropylène de type Rhizon ® [START_REF] Song | Novel use of soil moisture samplers for studies on anaerobic ammonium fluxes across lake sediment -water interfaces[END_REF]. Après avoir été collectées dans un tube PVC, chaque carotte de sédiments a été introduite dans un sac à gants sous atmosphère d'azote. Les rhizons sont insérés latéralement à travers le carottier, via des trous prépercés. Après 3 h, environ 15 ml d'eau interstitielle ont été recueillis d'une section de sédiment de 150 cm 3 . Le volume ainsi extrait, est décomposé en deux fractions, la première fraction est alors acidifiée à pH 2 avec HNO 3 Suprapur ® (Merck), pour la mesure de la concentration en cations dissous, alors que l'autre fraction reste brute afin de mesurer l'alcalinité, la spéciation du soufre, du fer et de l'azote, ainsi que le phosphore, les fluorures, la silice et le carbone organique dissous. Cette seconde fraction est introduite dans des flacons en verre scellés en conditions anoxiques. L'ensemble des échantillons est préservé à 4°C, puis stockés jusqu'à analyse.

Concernant la mangrove de la baie de Dumbea

Pour ce site d'étude, un seul point géographique a été sondé. Les différents prélèvements et mesures ont étés effectués le long d'une carotte de 70 cm de profondeur, et ont été réalisés selon le même procédé et les mêmes conditions que celles décrites précédemment pour le prélèvement dans la baie de Vavouto.

II-B. Méthode analytique

II-B.1. Démarche analytique générale

Les paramètres physico-chimiques du sédiment ont été directement mesurés sur le terrain, durant les missions de prélèvements, tandis que les prélèvements de la fraction solide ont été analysés à l'IMPMC et à l'Université de Nouvelle Calédonie (UNC), et les prélèvements d'eaux porales ont étés analysés à dans les laboratoires de l'Institut de Recherche et Développement de Nouméa ainsi qu'au Laboratoire de Géochimie des Eaux (LGE) de l'IPGP.

Mesures des paramètres physico-chimiques in-situ

Pour l'ensemble des points géographiques étudiés, la mesure des paramètres physicochimiques a été directement réalisée sur le terrain le long d'un premier carottage. L'ensemble des mesures est répété tous les 5 cm de 2,5 cm à 47,5 cm de profondeur. La salinité a été déterminée en utilisant un réfractomètre Atago après extraction d'une goutte d'eau interstitielle, le pH en utilisant une électrode de verre connectée à un pHmètre WTW, le potentiel redox en utilisant une électrode Pt et Ag/AgCl connectée à un pH/mV mètre WTW, puis la teneur en eau a été déduite après mesure de la masse volumique saturée et de la masse volumique sèche.

Analyses de la fraction liquide

Les analyses d'alcalinité ont été réalisées par colorimétrie, quelques heures après le prélèvement, tandis que le dosage du fer et du soufre dissous ont été réalisés 24h après les prélèvements. Les mesures de Fe 2+ et Fe total dissous, et de HS -ont été réalisées par colorimétrie, en sac à gants. Les concentrations en SO [START_REF] Belzile | Testing readsorption of trace elements during partial chemical extractions of bottom sediments[END_REF][START_REF] Bermond | Estimation des cations métalliques lies à la matière organique à l'aide de réactifs chimiques : approche cinétique[END_REF]Bermond et al., , 1992 ; ;[START_REF] Khebioan | Accuracy of sélective extraction procédure for métal spéciation in model aquatic sédiments[END_REF][START_REF] Nirel | Pitfalls of sequential extractions[END_REF][START_REF] Rendell | Adsorption as a control of metal concentrations in sediment extracts[END_REF], ces phénomènes risquent d'être particulièrement accentués dans les sédiments de mangrove riches en petites particules de fortes réactivité de surface, tels que les phyllosilicates. D'autre part, la non sélectivité des différents réactifs chimiques [START_REF] Rendell | Adsorption as a control of metal concentrations in sediment extracts[END_REF][START_REF] Auliitia | Sediment analysis-lability of selectivrly extracted fractions[END_REF]Whalley et al., 1983) Koniambo [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF][START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]. Cette étude apporte une connaissance approfondie de la cristallochimie du Ni dans les minéraux hérités de l'altération des massifs ultrabasiques, qui viennent enrichir le sédiment des mangroves via le transport fluvial. Ces auteurs démontrent que, la spectroscopie EXAFS permet de distinguer le nickel présent dans les oxydes de manganèse, du nickel associé aux oxydes de fer, ainsi que de celui incorporé dans les phyllosilicates [START_REF] Manceau | Crystal chemistry of trace elements in natural and synthetic goethite[END_REF]Dublet et al., 2011). D'autre part, la spectroscopie EXAFS permet notamment de souligner les modes de substitution atomique et les effets direct de cette substitution sur la structure du minéral. Par exemple, elle a permis de montrer que l'incorporation du nickel dans la goethite était possible par substitution de Fe 3+ par Ni 2+ (Singh et al., 2000[START_REF] Singh | Incorporation of Cr, Mn and Ni into goethite (alpha-FeOOH): mechanism from extended Xray absorption fine structure spectroscopy[END_REF][START_REF] Carvalho-E-Silva | Incorporation of Ni into natural goethite: An investigation by Xray absorption spectroscopy[END_REF]. Elle a est également permis de suspecter des différences de complexes pour le Ni adsorbés à la surface de différents types d'oxyhydroxides de fer (tel que la goethite, la ferrihydrite et l'hématite) [START_REF] Arai | Spectroscopic Evidence for Ni(II) Surface Speciation at the Iron Oxyhydroxides at Water Interface[END_REF].

Actuellement, peu de données existent sur la complexation du Ni par des complexes organiques (Musgrave et al. 2001;[START_REF] Pattanaik | XAFS Studies of Nickel and Sulfur Speciation in Residual Oil Fly-Ash Particulate Matters (ROFA PM)[END_REF] (Ostergren al., 1999 ;[START_REF] Morin | XAFS détermination of the chemical form of lead in smelter---contaminated soils and mine tailings: Importance of adsorption processes[END_REF] ; Référence de Ni-Goethite [START_REF] Manceau | Crystal chemistry of trace elements in natural and synthetic goethite[END_REF][START_REF] Manceau | Deciphering Ni sequestration in soil ferromanganese nodules by combining X-ray fluorescence, absorption, and diffraction at micrometer scales of resolution[END_REF][START_REF] Beauchemin | Principal component analysis approach for modeling sulfur K-XANES spectra of humic acids[END_REF][START_REF] Isaure | Quantitative Zn speciation in a contaminated dredged sediment by µ-PIXE, µ-SXRF, EXAFS spectroscopy and principal component analysis[END_REF]Scheinost et al., 2002 ;[START_REF] Levard | Role of natural nanoparticles on the speciation of Ni in andosols of la Reunion[END_REF]. [START_REF] Malinowski | Factor Analysis in Chemistry[END_REF][START_REF] Wasserman | EXAFS and principal component analysis: a new shell game[END_REF][START_REF] Ressler | Quantitative speciation of Mn-bearing particulates emitted from autos burning (methylcyclopentadienyl) manganese tricarbonyl-added gasolines using XANES spectroscopy[END_REF]. Le principe de l'ACP est de ramener un nombre important de spectres EXAFS expérimentaux à un nombre réduit de spectres EXAFS synthétiques constituant des vecteurs propres qui constituent une base orthonormée du système linéaire. Manceau et al., 2003 ;2007 ;[START_REF] Isaure | Quantitative Zn speciation in a contaminated dredged sediment by µ-PIXE, µ-SXRF, EXAFS spectroscopy and principal component analysis[END_REF]Scheinost et al., 2002 ;Sarret et al., 2004 ;[START_REF] Voegelin | Changes in Zinc Speciation in Field Soil after Contamination with Zinc Oxide[END_REF][START_REF] Jacquat | Formation of Zn-rich phyllosilicates, Zn-layered double hydroxide and hydrozincite in contaminated calcareous soils[END_REF][START_REF] Levard | Role of natural nanoparticles on the speciation of Ni in andosols of la Reunion[END_REF]Dublet et al., 2011 ;Johnson et al., 2013).

II
Ensuite

EXAFS analysis of iron cycling in mangrove sediments downstream a lateritized ultramafic watershed (Vavouto Bay, New Caledonia)

Vincent Noël 1,2 , Cyril Marchand 3 , Farid Juillot Corresponding author: Guillaume Morin; e-mail: guillaume.morin@impmc.upmc.fr

August 2013

Submitted to Geochimica Cosmochimica Acta

ABSTRACT

Mangrove forests are the dominant intertidal ecosystem of tropical coastlines. In New Caledonia, mangroves act as a buffer zone between massive Fe lateritic deposits and a lagoon registered by UNESCO as a World Heritage site. Across the intertidal zone, mangrove is composed of three main vegetal stands (i.e. Rhizophora spp., Avicennia marina and salt flat).

This botanical gradient relies on the duration of tidal immersion, which imposes gradients of pore-water salinity, oxygenation, and organic content in the sediment. In the present study, we have determined the distribution and speciation of Fe in mangrove sediments along this botanical gradient by using X-ray Absorption Spectroscopy (XAS) data at the Fe K-edge.

Both XANES and EXAFS data show that iron speciation strongly follows the redox boundaries marking the intertidal and depth zonations. Fe-bearing minerals eroded from lateritic outcrops, mainly Fe(III)-bearing goethite and phyllosilicates, are the major Fe hosts in the upward horizons. These mineral species progressively disappear with increasing depth where pyrite forms, in the hydromorphic Rhizophora and Avicennia zones. Sulfate reduction is not observed in the drier salt flat zone. In addition to these reduction processes, intense reoxidation of aqueous Fe(II) and Fe-sulfides leads to the formation of ferrihydrite, lepidocrocite and likely goethite in the upper Avicennia and Rhizophora sediments. The relative proportion of the newly formed poorly ordered iron-oxyhydroxides is found to be higher in the Rhizophora mangrove stand, which is the closest to the shore. Tidal fluctuations may thus be a major cause for continuous Fe reduction-oxidation cycles in the vegetated mangrove stands, which could significantly affect the iron mass balance in mangrove systems.

III-1. INTRODUCTION

Mangrove forests are the dominant intertidal ecosystem of tropical coastlines, covering between 150,000 and 200,000 km 2 [START_REF] Tomlinson | The botany of mangroves. Cambridge tropical biology series[END_REF][START_REF] Valiela | Mangrove forests: one of the world's threatened major tropical environments[END_REF]. These intertidal forests play a fundamental role in the ecological balance of these areas, having a significant impact on global carbon cycling [START_REF] Bouillon | Mangrove production and carbon sinks : A revision of global budget estimates[END_REF]. They also control the transfer of major and trace metals between land and sea along the tropical coastlines [START_REF] Ponnamperuma | The chemistry of submerged soils[END_REF][START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Lacerda | Biogeochemistry of Mangrove Soil Organic Matter : a Comparison Between Rhizophora and Avicennia Soils in South-eastern Brazil[END_REF][START_REF] Tam | Spatial variation of heavy metals in surface sediments of Hong Kong mangrove swamps[END_REF][START_REF] Burton | Sulfur biogeochemical cycling and novel Fe-S mineralization pathways in a tidally re-flooded wetland[END_REF].

Mangrove sediments are redox-stratified, with a succession of oxic, suboxic and anoxic zones, across the intertidal zone and with increasing depth along the sediments [START_REF] Alongi | The role of soft-bottom benthic communities in tropical mangrove and coral reef ecosystems[END_REF][START_REF] Kristensen | Benthic metabolism and sulfate reduction in a southeast Asian mangrove swamp[END_REF]Kristensen et al., , 1992;;[START_REF] Mckee | Soil physicochemical and mangrove patterns species -reciprocal distribution effects ?[END_REF]. These strong redox gradients in mangrove sediments play a dominant role on the speciation and solubility of redox sensitive elements, notably iron [START_REF] Torres | Porewater dynamics and the formation of iron sulfide in a mangrove ecosystem, Sepetiba Bay, Brazil[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF], as well as trace metals [START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF][START_REF] Ferreira | Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate[END_REF]Marchand et al., 2006aMarchand et al., , 2011aMarchand et al., , 2011b[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. In these environments composed of organic-rich and water saturated sediments, anaerobic sulfate reduction is usually considered as the dominant respiration process [START_REF] Alongi | Coastal Ecosystem Processes[END_REF]. However, recent studies have suggested that the role of Fe respiration in carbon oxidation may be comparable to, or higher than, sulfate reduction in Fe-rich mangrove environments [START_REF] Nielsen | Phosphorus dynamics during decomposition of mangrove (Rhizophora apiculata) leaves in sediments[END_REF][START_REF] Canfield | Aquatic Geomicrobiology[END_REF]. These reduction reactions yield high levels of ferrous iron and sulfides into anoxic mangrove sediments, primarily under the form of pyrite (FeS 2 ), Fe monosulfides (FeS) being negligible, as determined by sequential extraction procedures [START_REF] Holmer | Biogeochemical cycling of sulfur and iron in sediments of a south-east Asian mangrove, Phuket Island, Thailand[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. These reduced Fe phases can re-oxidize upon oxygenation of the surface sediment, leading to the formation of ferric oxyhydroxides [START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF]. For instance, based on sequential extraction procedure and Mössbauer spectroscopy analyses, lepidocrocite, and to a lesser extent goethite and ferrihydrite, were identified as the dominant ferric oxyhydroxides forming in Brazilian mangrove sediments [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. In addition, iron-rich smectites, such as nontronites have also been recognized as major Fe hosting minerals in mangrove sediments and could then also be part of the redox cycling of iron [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF][START_REF] Souza-Júnior | Smectite in mangrove soils of the State of São Paulo, Brazil[END_REF]. Apart from these important findings, few studies have analyzed iron speciation and mineralogy in mangrove sediments as a function of the geological context, the vegetal cover and the distance to the sea, which are key parameters to better depict the biogeochemical cycling of this element at the continent/ocean interface.

For this purpose, the present study aimed at determining iron speciation in iron-rich mangrove sediments of New Caledonia, where mangroves act as a buffer zone between massive Fe lateritic deposits developed over ultramafic rocks [START_REF] Becquer | Mineralogical, chemical and charge properties of Geric Ferralsols from New Caledonia[END_REF]Quantin et al., 2002) and a lagoon of over 20 000 km 2 (recently registered by UNESCO as a World Heritage site).

This ecosystem is composed of different zones, each dominated by a specific botanical species, which is commonly observed in mangrove swamps throughout the world. A recent study suggest that the main factor controlling the distribution of vegetal species in mangroves in New Caledonia is the soil salinity (Marchand et al., 2011a), which in turn is controlled by the duration of tidal immersion and thus by sediments elevation [START_REF] Baltzer | Géodynamique de la sédimentation et diagenèse précoce en domaine ultrabasique -Nouvelle Calédonie[END_REF]. This distribution in the tidal area may be responsible for differences in sediment geochemistry, especially metal speciation and bioavailability, between the different parts of mangrove forests.

In the present study, X-ray absorption spectroscopy at the Fe K-edge was chosen to directly determine iron speciation in mangrove sediments because this method has demonstrated its ability at distinguishing among the wide variety of possible Fe-bearing phases, including poorly crystalline iron oxyhydroxides minerals [START_REF] Maillot | New insight into the structure of nanocrystalline ferrihydrite : EXAFS evidence for tetrahedrally coordinated iron (III)[END_REF], occurring in complex heterogeneous media like sediments (O'day et al. 2004). Complemented by classical mineralogical analyses at both the macro and micro scales, the quantitative distribution of iron species is interpreted in relation with pore-water chemistry, as a function of physicochemical gradients with depth and across the intertidal zonation in order to improve our understanding of the biogeochemical cycling of this element in this peculiar intertidal ecosystem.

III-2. MATERIALS AND METHODS

III-2.1. Natural samples studied

III-2.1.1. Natural setting of the studied site

New Caledonia is located in the South-West Pacific Ocean, and extends over ~500 km long for ~50 km wide . The climate of the main island (20°S-23°S) is tropical and it is semi-arid along the west coast, where the mangrove studied here develops [START_REF] Maitrepierre | Bilan des mesures météorologiques effectuées pour le projet Koniambo[END_REF]. The sampling campaign was performed at the end of the rainy season in May 2011. The studied mangrove is located in the Vavouto bay, at the mouth of the ultramafic Koniambo outcrop (20°59' S; 164°49'E). This outcrop consists of deeply weathered ultramafic rocks, forming several tens of meters thick lateritic regoliths hosting high concentrations of iron and other transition metallic elements like nickel, cobalt and chromium (e.g. Fandeur et al., 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]. III-2.1.2. Solid sediment sampling Despite a great diversity of the mangrove flora in New Caledonia, including more than 20 different species [START_REF] Duke | Mangrove biodiversity in New Caledonia. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF], the same main species distribution can be observed over the whole island. On the one hand, the Rhizophora spp., representing more than 50% of mangrove areas, develop mainly in the zones swept by tides daily. On the other hand, the Avicennia marina, representing more than 15% of mangrove areas, develop at higher elevations, at the borders of the salt flat, which occupies the higher area in the intertidal zone [START_REF] Virly | Atlas des mangroves de Nouvelle-Calédonie. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF]. Three sediments cores were collected at low tide with an Eijkelkamp gouge auger within the intertidal zone c), one in each mangrove stand, from the inland to the sea front: i) the salt flat zone, not colonized by vascular plants; ii) the zone dominated by Avicennia marina forest; and iii) the zone dominated by Rhizophora forest. For each core, the sediment samples were collected, under nitrogen flow, every 5 cm from 0 to 60 cm depth, and conserved stored under anoxic conditions, in order to preserve them from oxidation. These solid samples were then kept frozen until they were introduced in a glove box N 2 atmosphere in order for being vacuum dried, ground with an agate pestle and mortar, homogenized and stored in sealed containers in an anoxic glove box.

III-2.1.3. Pore-water sampling

Upon the solid sediment sampling campaign, pore-waters were extracted from sediment cores using soil moisture samplers Rhizon® [START_REF] Song | Novel use of soil moisture samplers for studies on anaerobic ammonium fluxes across lake sediment -water interfaces[END_REF]. After being collected in PVC tube, each sediment core was introduced in a glovebag under nitrogen. The soil moisture samplers were then directly inserted into the core through holes into the PVC tube and they were connected to a syringe using luer-lock fittings. Around 15 ml of pore-water was collected after 3 h for a section of core of 150 cm 3 . All samples were acidified to pH 2 with Suprapur® HNO 3 , and stored in acid cleaned 14 ml polypropylene tubes, in a cold room (T = 4°C) until analysis (Marchand et al., 2006b). Pore-water salinity was determined using an Atago refractometer after extracting a drop of pore-water from the core. The pH and redox potential (Eh) were measured in situ along the sediment cores; pH with a glass electrode (model SenTix® 81), previously calibrated with pH 4.0 and 7.0 standards (NIST scale), and Eh with a combined Pt and Ag/AgCl (AgCl reference electrode, 25°C) electrode connected to both types of electrodes were carefully inserted into the sediment core until stable measurements were reached, and thoroughly washed with seawater between two measurements. Calibration of the redox electrode was periodically checked using a 430 mV standard solution and dionised water, and data are reported relative to a standard hydrogen electrode (i.e., after adding 202 mV to the values measured at 25 °C.

III-2.2. Chemical analyses

Bulk concentrations of major (Si, Mg, Fe, Al, K, Ca) and trace (Ni, Cr, Mn) elements in the dried solid samples were measured at the Institut de Mineralogie et de Physique des Milieux Condenses (IMPMC) using a XEPOS (Spectro X Lab) X-ray fluorescence spectrometer equipped with a Pd cathode and operating at 50 kV and 40 mA. Pore-water sub-samples were used to determine aqueous Fe(II), hydrogen sulfide (H 2 S), sulfate (SO 4 2-) and chloride (Cl -) concentrations. Aqueous Fe(II) and H 2 S were measured by colorimetric methods using a SpectroDirect (AQUALYTIC) spectrophotometer, immediately after pore-water extraction. Aqueous Fe(II) was analyzed by the ferrozine method [START_REF] Viollier | Van Cappellen The Ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters[END_REF] at a wavelength of 562 nm (limit of detection = LOD: 0.4 mmol/L), and H 2 S by the methylene blue method of [START_REF] Cline | Spectrophotometric determination of hydrogen sulfide in natural waters[END_REF] at a wavelength of 665 nm (LOD: 0.2 µmol/L).

Sulfate and chloride were determined by capillary electrophoresis at the Laboratoire des Moyens Analytiques (LAMA) of IRD -Noumea, and by ion chromatography at University Paris Diderot.

Total organic carbon (TOC) in solid sediments was determined using an SSM-5000A combined with TOC-L Series analyzer on powdered samples at IRD -Noumea. Samples were first lyophilized and then pyrolysed at 900°C during 50h in the presence of platinum and cobalt, which act as oxidation catalysts. TOC is determined as the sum of pyrolysed and residual organic. All of the carbon in the sample is pyrolysed in CO 2 ; this gas is also analyzed by infrared spectrophotometer. TOC is determined as TOC = 95% TC [START_REF] Marchand | Influence of mangrove zonation on CO2 fluxes at the sediment -air interface (New Caledonia)[END_REF].

III-2.3. Mineralogical analysis

Mineralogical composition of the solid samples was determined by X-ray powder diffraction (XRD) using CoKα radiation in order to minimize the X-ray absorption of Fe. Samples were mounted within the glove box and transferred in an anoxic sample chamber for XRD data collection in order to avoid any changes in the mineralogy related to possible redox reaction with air. Data were collected in Bragg-Brentano geometry using a Panalytical® X'Pert Pro diffractometer equipped with an X'celerator® detector. A continuous collection mode was applied over the 3-120° 2θ range with a 0.033° 2θ step and counting 4h for each sample.

In order to determine the mineralogical composition of the clay fraction of the sediments, the < 2 µm size fraction of the sediment samples was isolated in deionised water in the presence of air, using sedimentation procedures based on the Stokes' law. Oriented specimens of the clay fractions were prepared by suspending few tens of milligrams of each clay fraction sample in deionised water and dropping a few hundreds of microliters of this suspension on a silicon sample holder. After air drying, physico-chemical treatments were then applied to these oriented specimens before XRD analyses in order to characterize their mineralogical properties [START_REF] Brindley | Crystal structures of clay minerals and their X-ray identification[END_REF][START_REF] Eslinger | Clay minerals for petroleum geologists and engineers. Economic paleontologists and mineralogists[END_REF]. A first solvation with ethylene glycol (EG), which is designed to reveal the swelling capacity of phyllosilicates, was performed by leaving oriented specimens for 12 h at 60°C above an EG solution in a sealed desiccator. A second solvation with CsCl-Hydrazine-DMSO was performed according to the protocol of Jackson and Abdel-Kader (1978). Thermal treatments, which are designed to reveal the collapsing capacity of hydrated phyllosilicates, were performed by heating oriented specimens at 350 and 500°C during one hour.

Scanning Electron Microscopy (SEM) observations and analyses were performed at IMPMC with a Field Emission Gun Scanning Electron Microscope (GEMINI ZEISS Ultra55)

operating at 15 kV. Samples to be analyzed were deposited as ground powder on carbon tape and coated with a thin carbon film.

Pellets for infrared transmission measurements were obtained by pressing a mixture of about were recorded between 500 and 4,000 cm -1 with a resolution of 1 cm -1 using a Nicolet 7600

Fourier Transform Infrared (FTIR) spectrometer.

III-2.4. X-ray Absorption Spectroscopy (XAS)

III-2.4.1. XAS data collection Fe K-edge X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near-edge Structure (XANES) spectra of 7 mangrove sediment samples (Salt flat 2.5 and 47.5cm;

Avicennia 27.5 and 47.5 cm; and Rhizophora 2.5 and 12.5 cm) were collected in transmission detection mode at 80K on the XAFS beamline (ELETTRA, Trieste, Italy) using a Si(111) double crystal monochromator. XANES and EXAFS data of 3 mangrove sediment samples (Rhizophora 7.5, 27.5 and 47.5 cm) and of the Fe sulfide model compounds were collected in transmission detection mode at 20K on the SAMBA beamline (SOLEIL, Saint-Aubin, France) using a Si(220) double crystal monochromator, equipped with dynamic sagittal focusing of the second crystal [START_REF] Briois | SAMBA : The 4 -40 keV X-ray absorption spectroscopy beamline at SOLEIL[END_REF]. In order to strictly preserve the oxidation state of iron during the analyses, all mangrove samples and model compounds were mounted on the cryostat sample rod within a glove bag or a glove box next to the beamline and carried to the beamline in a liquid nitrogen bath before being rapidly transferred into the cryostat.

Energy was calibrated by setting to 7112 eV the energy position of the first inflection point in the K-edge of a Fe foil recorded in double transmission setup. For each mangrove sample and model compound, 2 to 6 scans were recorded, depending on the Fe concentration that varied between 1 and 30 wt% Fe. Data were averaged and normalized using the ATHENA software [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]. EXAFS spectra were then background subtracted from these normalized data using the XAFS code [START_REF] Winterer | XAFS -a data analysis program for materials science[END_REF]. Radial distribution functions around the Fe absorber were obtained by Fast-Fourier-transformation of the k 3 -weighted experimental χ(k) function using a Kaiser-Bessel apodization window with the Bessel weight fixed at 2.5. III-2.4.2. Principal Component Analysis (PCA) and Linear Combination Least-Squares Fitting (LC-LSF) of the EXAFS spectra EXAFS spectra of the 10 samples from the three mangrove sediment profiles were analyzed by principal component analysis (PCA) [START_REF] Wasserman | EXAFS and principal component analysis: a new shell game[END_REF][START_REF] Ressler | Quantitative speciation of Mn-bearing particulates emitted from autos burning (methylcyclopentadienyl) manganese tricarbonyl-added gasolines using XANES spectroscopy[END_REF] using the SIXPACK code [START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF]. The minimum number of principal components necessary to fit the whole set of experimental EXAFS spectra was chosen on the basis of the minimum value of the factor indicator function [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF][START_REF] Malinowski | Factor Analysis in Chemistry[END_REF][START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF] and the quality of the reconstruction of the whole set of experimental EXAFS spectra with the first, the two first, the three first and the four first components extracted from the PCA analysis (Figure III-SI-6). Target transformation (TT) was then used to select the EXAFS spectra of model compounds that most likely represent the set of experimental EXAFS spectra [START_REF] Malinowski | Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry[END_REF][START_REF] Ndiba | Heavy metal immobilization through phosphate and thermal treatment of dredged sediments[END_REF]. This selection was based on the quality of the TT fits assessed by the normalized sum of squared residual (NSSR TT :

∑(k 3 χ Target -k 3 χ Transform ) 2 / ∑(k 3 χ Target ) 2 ).
The Fe K-edge spectra of the mangrove sediments were analyzed by linear combination leastsquares fitting (LC-LSF) using the model compounds spectra selected from the TT analysis.

LC-LSF of the XANES and EXAFS spectra were performed using the ATHENA software [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF] and using a home made software based on Levenberg-Marquardt minimization algorithm [START_REF] Hohmann | Molecular-level modes of As binding to Fe (III) (oxyhydr)oxides precipitated by the anaerobic nitrate-reducing Fe(II)-oxidizing Acidovorax sp. strain BoFeN1[END_REF][START_REF] Pantke | Green rust formation during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1[END_REF][START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF], respectively. Linear coefficients were only constrained to be positive. The accuracy of this fitting procedure ranges between ± 25% and ± 5% of the stated values of each individual contribution, and components below 10% are considered as not significant [START_REF] Ostergren | Quantitative Speciation of Lead in Selected Mine Tailings from Leadville , CO[END_REF][START_REF] Cancès | XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant[END_REF]Juillot et al., 2011). As for the TT procedure, the quality of the fit was assessed on the basis of a normalized sum of squared residual of the following form

(NSSR: ∑(k 3 χ exp -k 3 χ calc ) 2 / ∑(k 3 χ exp ) 2 ).

III-2.4.3. Model compounds for XAS analyses

A large set of experimental Fe K-edge XAS spectra from natural and synthetic model compounds was used for LC-LSF analysis of the XANES and EXAFS spectra of the mangrove sediments. Our Fe K-edge model compounds data set includes experimental data already collected for a large variety of synthetic Fe compounds: Fe(III) oxyhydroxides as

goethite (α-FeOOH), hematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ), akaganeite (β-FeOOH), 2-
Line and 6-Line ferrihydrite [START_REF] Maillot | New insight into the structure of nanocrystalline ferrihydrite : EXAFS evidence for tetrahedrally coordinated iron (III)[END_REF][START_REF] Hohmann | Molecular-level modes of As binding to Fe (III) (oxyhydr)oxides precipitated by the anaerobic nitrate-reducing Fe(II)-oxidizing Acidovorax sp. strain BoFeN1[END_REF]) and lepidocrocite (γ- phosphate (FePO 4 ) (Cosmidis et al., accepted) and siderite (FeCO 3 ) (Dublet et al., accepted).

In addition, our data set includes Fe K-edge XAS data for natural phyllosilicate model compound (Cosmidis et al., accepted; Othmane et al., accepted), such as illite from Le Puy en Velais, France (Ildefonse et al., 1998), Mg-rich chlorite (kindly provided by D. Beaufort, Universite de Poitiers, France), biotite (Mineralogy Collection of IMPMC, Paris, France), serpentine from a highly crystalline serpentine sample from the New Idria Mining District (California, USA) containing 3.45 wt% Fe, and Garfield nontronite (Washington, USA) [START_REF] Bonnin | Sites Occupancy of Fe 3 + in Garfield Nontronite: A Spectroscopic Study[END_REF]. A Fe-humate model compounds consisting of purified colloidal material from the Rio Negro River (Brazil) [START_REF] Allard | Tracing source and evolution of suspended particles in the Rio Negro Basin (Brazil) using chemical species of iron[END_REF] was also used as model compounds for

Fe-natural organic matter complexes (Cosmidis et al., accepted).

To complete this data set, several Fe sulfide model compounds were also synthesized and analyzed at the Fe K-edge for the present study. Pyrite (FeS 2 ) was synthesized according to a protocol derived from that reported by [START_REF] Wei | Aqueous synthesis of finely divided pyrite particles[END_REF]. Mackinawite (FeS) was synthesized following the procedure by [START_REF] Donald | Low temperature anaerobic bacterial diagenesis of ferrous monosulfide to pyrite[END_REF], and greigite (Fe 3 S 4 ) was synthesized following the procedure by [START_REF] Klekowski | An association of mangrove mutation, scarlet ibis, and mercury contamination in Trinidad, West Indies[END_REF]. Finally, the sulfate (SO 4 2-) to chloride (Cl -) molar concentration ratio in the salt flat is equal to that measured for the sea-water (0.05). In contrast, this ratio is slightly lower than that of seawater beneath Avicennia and it is significantly lower than that of sea-water beneath Rhizophora, at the exception of the topmost horizons (i.e. above 10 cm depth) where it reaches a value that is twice that of sea-water at 7.5 cm depth (Figure III-3

III-3. RESULTS

III-3.1. Physico-chemical analyses of the mangrove sediments

III-3.4. X-Ray absorption spectroscopy analysis of iron speciation

The pre-edge region and the full normalized Fe K-edge XANES spectra obtained on the mangrove sediment for each studied zone (salt flat, Avicennia and Rhizophora) are shown in Figs. A4 and A5 (Appendix). It is well known that the position of the pre-edge features depends on the redox state of Fe in Fe-bearing minerals [START_REF] Wilke | Oxidation state and coordination of Fe in minerals : An Fe K-XANES spectroscopic study[END_REF][START_REF] Galoisy | High-resolution XANES spectra of iron in minerals and glasses: structural information from the pre-edge region[END_REF][START_REF] Farges | The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study[END_REF]. Although the actual number and positions of absorption features in the pre-edge region of Fe K-edge XANES spectra depends on the symmetry and distortion of the site, it is considered that Fe(III) is characterized by absorption features centered around 7114 eV, whereas Fe(II) is characterized by absorption features centered around 7112 eV [START_REF] Wilke | Oxidation state and coordination of Fe in minerals : An Fe K-XANES spectroscopic study[END_REF][START_REF] Galoisy | High-resolution XANES spectra of iron in minerals and glasses: structural information from the pre-edge region[END_REF][START_REF] Farges | The effect of redox state on the local structural environment of iron in silicate glasses: a combined XAFS spectroscopy, molecular dynamics, and bond valence study[END_REF]. Examination of our data indicates that the absorption features in the pre-edge region of the XANES data collected in the surface sediment beneath the three studied zones are centered around 7114 eV, which suggests a major occurrence of Fe(III). The position of these absorption features does not change with depth in the sediment beneath the salt flat, whereas it clearly moves towards 7112 eV in those beneath the two vegetated zones (Avicennia and Rhizophora). Such an evolution suggests that the redox state of Fe changes from major Fe(III) near the surface to PCA indicates that 88% of the total variance of the system of our 10 EXAFS spectra from the mangrove sediment samples can be explained by the four first principal components (Table

III-SI-2, Figure III-SI-6
), with an minimum IND parameter value of 0.17 [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF].

Adding the fifth principal component explains 92% of the system with an IND value of 0.184.

Among our large set of model compound spectra, the TT analysis helped in selecting the most appropriate ones for the LC-LSF of the mangrove samples EXAFS spectra (Table III-SI-3).

References with SPOIL values below 6 were considered as possible components, and could be classified as good, fair and acceptable components according to the classification of [START_REF] Malinowski | Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry[END_REF]. Pyrite and schwertmannite were found as good; 2-Line and 6-Line ferrihydrite, nontronite and illite (as proxi of Fe-poor smectite) akaganeite, and Fe-bearing serpentine were found as fair; and phlogopite, green rust, lepidocrocite, goethite, magnetite, amorphous Fe(III) phosphate, chlorite, were found as acceptable. However, the high NSSR TT values for phlogopite, green rust and chlorite (>40%) suggested that these Fe species were not dominant in the samples. The high SPOIL (>6) and NSSR TT of vermiculite, oxidized or not vivianite, greigite, hematite, maghemite, Fe-humate, biotite, mackinawite, siderite indicated that these Fe species were not relevant in the studied soils. These latter model compounds were thus not considered for LC-LSF.

III-3.5. Quantitative Fe speciation in the mangrove sediments

LC-LSF fits of the Fe-K edge EXAFS data from the mangrove sediments were first performed with four of the model compounds identified as good of fair components by the TT analysis, i.e, SPOIL < 6 in Table III-SI-3. Among these candidates, four model compounds corresponding to mineral species identified by XRD and SEM analysis of the mangrove sediments were selected as components for the LC-LSF analysis. They consisted of goethite, serpentine, pyrite and an illite-nontronite 65/35 combination (Figure III-SI-2). The choice of this latter component relies on the fact that for all mangrove sediments studied, the best fits were obtained by including both an illite (proxi of Fe-poor smectite) and a nontronite component in the LC-LSF. However, standard deviations on the relative proportions of these two fitting components were as large as 50%, which may be explained by the similarities in the Fe-K edge EXAFS spectra of these Fe-bearing 2:1 phyllosilicates (Figure

III-SI-2).
Consequently, a linear combination of illite and nontronite spectra in proportions 65/35 was used as a fitting component for all mangrove sediment samples in order to account for the Fe- (1) Calculated as Σ(k

3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2
Based on these four components fits, LC-LSF fits were then performed with addition of one or more iron oxyhydroxides that were identified as fair components by the TT analysis (Table III-SI-3). Among these iron oxyhydroxides, only ferrihydrite and lepidocrocite improved significantly (more than 25%) the goodness of fit (NSSR in In the sediments collected in the Rhizophora stand, surface sediments (down to 12.5 cm) are characterized by high proportions of Fe oxyhydroxides (42 -59 %) consisting mostly of goethite and 2line ferrihydrite. However this Fe oxyhydroxide pool disappears at depth, in favor of pyrite. This latter component appears at 12.5 cm depth (10 %) and its proportion increases largely with depth to reach 70 % at 27.5 cm. As in the sediments collected beneath Avicennia stand, the remaining pool of Fe is hosted by phyllosilicate minerals, whose contribution decrease with depth.

III-4. DISCUSSION

III-4.1. Physico-chemistry of the studied mangrove sediments in relation with physiography and vegetation gradient

Within the present transect across the intertidal zone, from the salt flat to the Rhizophora stand, water content and salinity gradients are known to be controlled by the duration of tidal immersion [START_REF] Baltzer | Géodynamique de la sédimentation et diagenèse précoce en domaine ultrabasique -Nouvelle Calédonie[END_REF]Marchand et al., 2011a). This zonation is considered as the main factor determining the specific distribution of mangrove vegetation across the intertidal zone in the semi-arid area. In turn, the vegetal cover influences the organic matter content of the sediment. Forested mangrove sediments are highly enriched in organic carbon compared to the salt flat sediments. Even if the mixing of riverine and marine material contributes to sediments enrichment in organic carbon [START_REF] Bouillon | Resource utilization patterns of epifauna from mangrove forests with contrasting inputs of local versus imported organic matter[END_REF][START_REF] Marchand | Early diagenesis of carbohydrates and lignin in mangrove sediments subject to variable redox conditions (French Guiana)[END_REF], as observed in the upper sediment of salt flat , the largest source of organic carbon to mangrove sediments can be attributed to mangrove litter and root system.

Observations in Australia [START_REF] Alongi | Below-ground decomposition of organic matter in forests of the mangroves Rhizophora stylosa and Avicennia marina along the arid coast of Western Australia[END_REF] and New-Caledonia [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF] showed that differences of root system between Avicennia and Rhizophora, influence considerably total organic carbon (TOC). The more developed root system of Rhizophora tends to increase the TOC of the sediment compared to the radial cable root system of Avicennia [START_REF] Hesse | Some differences between the soils of Rhizophora and Avicennia mangrove swamp in Sierra Leone[END_REF] that develops predominantly in the subsurface zone [START_REF] De Granville | Aperçu sur la structure des pneumatophores de deux espèces des sols hydromorphes en Guyane[END_REF]).

In the present study, visual inspection of the sediments core collected under the Avicennia pneumatophores could explain this difference since this cable root system has the capacity to diffuse oxygen into the sediment [START_REF] Scholander | Gas exchange in the roots of mangrove[END_REF][START_REF] Thibodeau | Differential oxidation of mangrove substrate by Avicennia germinans and Rhizophora mangle[END_REF][START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF]. In addition, bioturbation by crab burrows [START_REF] Ferreira | Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate[END_REF] may also favor oxygen diffusion in the sediment. However at depth, the organic content is higher It also means that the daily submersions by oceanic water does not supply enough oxygen to pore waters to reset the system to "oxic" conditions.

(

III-4.2. Fe-bearing mineral species inherited from eroded materials

The Fe concentrations measured in the studied mangrove sediments (4 -14 wt%) are substantially higher than world average [START_REF] Ferreira | Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]Marchand et al., 2011aMarchand et al., , 2011b)). The highest Fe concentrations are similar to those already measured in other mangrove sediments of New Caledonia (i.e. 15 wt% Fe at Dumbea; [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF].

The Fe enrichment in the mangrove sediments from the Vavouto mangroves studied here and in those from Dumbea results from the geological setting of their watersheds. These latter consist of lateritic regoliths developed over ultramafic rocks. For instance, the studied mangrove sediments are located in the Vavouto bay that is connected to the weathered Koniambo outcrop. This regolith is composed of an upper laterite unit principally made of goethite, with minor amounts of hematite and Mn-oxides, and a deeper saprolite unit essentially consisting of serpentine and talc, with minor amounts of olivine and pyroxene (Fandeur et al., 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]. In such a geological context, erosion events related to tropical storms are responsible for the large export of Fe-rich particulate material towards mangrove sediments. Moreover, in New Caledonia, this export is enhanced by open-cast mining of the Ni ores associated with lateritic regoliths over ultramafic rocks, as indicated by the higher Fe concentrations found in the Dumbea mangrove that is related to former mining sites [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF].

XRD data indicate that the surface horizons of the sediment in the Vavouto mangrove contain significant amounts of goethite and serpentine (Figure III-4), which is in agreement with the major occurrence of these mineral species in the weathered Koniambo outcrop. However, other ferric oxyhydroxides components, like lepidocrocite and ferrihydrite, are also suggested in significant proportions by EXAFS data . In addition, phyllosilicate components other than serpentine, such as smectites, can also be found . Since these latter ferric oxyhydroxides and possibly iron-bearing smectites were not detected in the regolith of the Koniambo outcrop (Fandeur et al. 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]), these mineral species could have formed within the mangrove sediments and then contributed to the iron cycling. This is also the case of pyrite that is found predominantly in the sub-oxic to anoxic horizons of the studied sediments (Figure III-4).

III-4.3. Pyritization with depth and across the intertidal zone

The major change in iron speciation is observed in the sediments beneath Avicennia and Rhizophora, where ferric oxyhydroxides disappear gradually with depth whereas the fraction of pyrite progressively increases (Figure III-4d and4f). With concentrations reaching 60 -70% of the total Fe pool below 30 cm depth (Table III-1), the major fraction of Fe occurs as pyrite in the deepest horizons of the sediment beneath Avicennia and Rhizophora. Conversely, the ferric oxyhydroxides to pyrite transition is not observed in the sediments under salt flat.

This latter observation is likely due to the oxic character of these sediments that is not favorable to sulfate reduction activity, as indicated by their lower microbial biomass compared to sediments beneath Rhizophora and Avicennia (Otero et al., 2006 a,b).

The pyrite fraction increased with depth, with concentrations reaching 60 -70% of the total Fe pool beneath Avicennia and Rhizophora below 30 cm depth (Table III-1 The concentration of pyrite in the Vavouto mangrove sediments is significantly higher than that already reported for other intertidal marshes [START_REF] King | Short-term endproducts of sulfate reduction in a salt marsh: Formation of acid volatile sulfides, elemental sulfur, and pyrite[END_REF][START_REF] Oenema | Pyrite accumulation in salt marshes in the Eastern Scheldt, southwest Netherlands[END_REF][START_REF] Krairapanond | Distribution of organic and reduced sulfur forms in marsh soils of coastal Louisiana[END_REF][START_REF] Otero | Spatial variation in pyritization of trace metals in salt-marsh soils[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. However, acid volatile sulfides (AVS), which are commonly reported for salt marshes and marine sediments [START_REF]Acid volatile sulfide ( AVS )[END_REF], were not detected in the studied sediments. Although the presence of these AVS in the studied mangrove sediments cannot be totally excluded, their lack of detection by XANES and EXAFS suggests that their proportion is below the detection limit of these methods (estimated at 10% of the total Fe content). If present, such compounds could act as precursors for the formation of pyrite in the deep anoxic horizons of the mangrove sediments.

Conversely these reactive compounds can rapidly oxidize in the oxygenated upper horizons of the sediment [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. Fe-organic complexes and Fe(II) carbonate are not detected as significant Fe species, although they were proposed by [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF] based on selective extraction procedures.

Several biogeochemical processes may be responsible for the mineral transformations observed with depth in the studied organic-rich sediments. Mangrove sediments are characterized by high rates of sulfate reduction [START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF]. Accordingly, below sulfide build-up. According to laboratory synthesis of pyrite at ambient temperatures, sulfide may reduce ferric oxyhydroxide to Fe(II), which may end in the precipitation of pyrite [START_REF] Rickard | Kinetics and mechanism of pyrite formation at low temperatures[END_REF][START_REF] Pyzik | Sedimentary iron monosulfides: kinetics and mechanism of formation[END_REF]Burton et al., 2006b). Furthermore, bacterial respiration of ferric oxyhydroxides can be as important as sulfate reduction in Fe-rich mangrove environments [START_REF] Nielsen | Phosphorus dynamics during decomposition of mangrove (Rhizophora apiculata) leaves in sediments[END_REF][START_REF] Canfield | Aquatic Geomicrobiology[END_REF]. Finally, combination of iron and sulfate bioreduction reactions could favor the precipitation of Fe(II) sulfides as mackinawite (FeS), that may also act as transient phases in pyrite formation processes [START_REF] Butler | Framboidal pyrite formation via the oxidation of iron (II) monosulfide by hydrogen sulphide[END_REF]. SEM investigations in the studied sediments indicate abundant framboids consisting of submicron size cuboctahedral crystals that can be assigned to pyrite, based on bulk XRD data . Formation of framboidal pyrite has been attributed to both microbial activity and abiotic processes. Early hypotheses relied on pyritic fossilization of bacterial colonies [START_REF] Love | Microorganisms and the presence of syngenetic pyrite[END_REF]. Others models suggest that pyrite framboids would result from the replacement of an organic globule [START_REF] Kalliokoski | Morphology, mode of formation, and diagenetic changes in framboids[END_REF][START_REF] Rickard | The origin of framboids[END_REF]. [START_REF] Wilkin | Formation processes of framboidal pyrite[END_REF] suggested that pyrite framboids could result from magnetic aggregation of uniformly sized greigite (Fe 3 S 4 ) microcrystals, resulting from transformation of initial Fe mono-sulfide (FeS) microcrystals, which finally react to pyrite (FeS 2 ). In the mangrove sediments studied here, SEM-EDS observations and analyses reveal that pyrite framboids are often embedded with a thin carbon rich material from an unknown origin (Figure III-7a). In addition, alignments of pyrite framboids are observed in Rhizophora sediments and could be related to formation of pyrite along root debris . Interestingly, the presence of roots have been proposed as favoring AVS and pyrite formation by more than 20 times those measured in nearby unvegetated zones [START_REF] Wind | Localization of sulfate reduction in planted and unplanted rice field soil[END_REF]. Such effect could be explained by the high availability of fresh organic matter for bacterial respiration in root debris. Roots are also known as preferential oxygen diffusion pathways, inducing the formation of iron oxyhydroxides coatings [START_REF] Skinner | Biomineralization processes of iron and manganese[END_REF] that could be further reduced to iron sulfide under strict anoxic conditions.

III-4.4. Iron cycling in the studied mangrove sediments

A major fraction of goethite observed in the studied mangrove sediments likely originates from the weathered Koniambo outcrop that overhangs the Vavouto Bay. However, Fe K-edge EXAFS results indicate that these sediments also contain significant proportions of lepidocrocite and ferrihydrite with a fraction that can be as high as that of goethite in the surface horizons. These Fe oxyhydroxides are indicators of Fe(II) oxidation processes in soils and sediments [START_REF] Cornell | The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses[END_REF]. Especially, in saline environments, lepidocrocite is the main product of the oxidation of Fe(II) [START_REF] Sung | Kinetics and Product[END_REF].

Moreover, The low pH value recorded at the top of Avicennia and Rhizophora cores suggest an ongoing Fe(II) oxidation process. Consequently, in the studied mangrove sediments, ferrihydrite and lepidocrocite and part of goethite may derive from the oxidation of Fe sulfides minerals or of aqueous Fe(II) species, as proposed by [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF] in Brazilian mangroves. Indeed, tidal emersion, bioturbation by the root system or by crabs, and oxygen transport by Avicennia roots, favor oxygen diffusion towards the sediment, and can then cause partial oxidation of the reduced phases. In our study, the nature and proportion of the ferric oxyhydroxides is found to be related with the position along the intertidal zone and with the type of vegetation cover.

In the salt flat, ferric oxyhydroxides represents more than 60% of total Fe concentration along the whole core profile (Table III-1). Goethite (30 -43 % of total Fe) is the most abundant ferric oxyhydroxide, with ferrihydrite (15 -20% of total Fe) and minor amount of lepidocrocite (≤ 10% of total Fe). The occurence of ferrihydrite and lepidocrocite suggests that redox cycling of iron also occurs in the salt flat sediments. This cycling could be due to the activity of Fe-reducing bacteria (IRB) [START_REF] Pollock | Alkaline iron (III) reduction by a novel alkaliphilic , halotolerant , Bacillus sp . isolated from salt flat sediments of Soap Lake[END_REF].

In the upper sediments beneath Avicennia, the ferric oxyhydroxide component represents 35% of total Fe and consists of goethite and lepidocrocite in similar proportions (Table III-1). Accordingly, oxygen diffusion via the Avicennia root system may favor the formation and/or preservation of ferric oxyhydroxides in the vicinity of the roots occurring down to ~20 cm depth. Below this depth, measured Eh values indicate suboxic to anoxic conditions. Therefore, sulfide oxidation appears to be limited to the uppermost horizon of the sediment beneath Avicennia, which is in agreement with the fact that the SO 4 2-/Cl -molar ratio is higher than that of seawater only above 2.5 cm depth . This limited oxidation process may explain the slight pH decrease to 6 in the uppermost horizon , where iron sulfide minerals as well as dissolved sulfide species may also oxidize, and explain the formation of ferric oxyhydroxides.

In the sediment beneath Rhizophora, the proportion of the ferric oxyhydroxide components in the upper horizons (50 -60 % of total Fe) is significantly higher than beneath Avicennia.

Goethite is mostly preserved at about 7.5 cm depth, where lepidocrocite and ferrihydrite are virtually absent. In contrast these latter phases are abundant and goethite is strongly depleted above and below this 7.5 cm horizon (Table III-1). This specific horizon is characterized by a high sulfate production as indicated by SO 4 2-/Cl -molar ratio that is more than two times higher than that of seawater . Such a high sulfate production can be explained by high sulfide oxidation rate that could affect both the dissolved and the solid sulfide pool, which could include pyrite and possibly reactive AVS that, if present, were however below the detection limit of EXAFS (10% of total Fe). Such oxidation of iron sulfides could explain the release of dissolved Fe(II) into the pore-water at 7.5 cm depth. Further diffusion and oxidation of this dissolved Fe pool could explain the abundance of ferrihydrite and lepidocrocite above and beneath this horizon. Conversely, at this particular depth only a small fraction of iron released into the pore-waters oxidizes and precipitates. Altogether, these observations indicate that the rate of sulfide oxidation in the upper horizons of the sediments of the Rhizophora zone is significantly higher than for the Avicennia zone. This difference could be caused by larger intrusion of oxygen-rich seawater during the onset of high tide [START_REF] Caetano | Tidal flushing of ammonium, iron and manganese from inter-tidal sediment pore waters[END_REF]. Ferrihydrite and lepidocrocite formed upon these sulfides oxidation events, and which are more soluble than goethite, may be rapidly reduced and converted back to iron sulfides once the anoxic conditions are restored in the submerged sediments (severalminutes to few hours) as shown in the laboratory by [START_REF] Canfield | The reactivity of sedimentary iron minerals toward sulfide[END_REF].

III-4.5. Origin and evolution of the Fe-bearing phyllosilicate minerals in the mangrove sediments and contribution to the iron cycling

Beyond iron oxyhydroxide and sulfide minerals, 21 to 57% of total Fe is found incorporated in the structure of phyllosilicate minerals, including serpentine and Fe-rich smectite (Table III For all sediment cores collected along intertidal zone of studied mangrove (salt flat, Avicennia, Rhizophora), the fraction of Fe hosted by phyllosilicate minerals decreased with depth (Table III-1). This result suggests either that the sedimentary regimes of the Vavouto mangrove evolved over time, or that Fe-bearing phyllosilicates dissolved. Recent studies indicated that Fe(III) associated with clay minerals, like nontronite, can be reduced to Fe(II)

by Fe-or sulfate-reducing bacteria [START_REF] Kostka | Respiration and Dissolution of Iron (III) -Containing Clay Minerals by Bacteria[END_REF][START_REF] Li | Iron reduction and alteration of nontronite by a sulfate-reducing bacterium[END_REF]. However, even if it has been suggested for Brazilian mangrove [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF], the importance of this process, so far observed only in laboratory experiments [START_REF] Li | Iron reduction and alteration of nontronite by a sulfate-reducing bacterium[END_REF][START_REF] Kostka | Respiration and Dissolution of Iron (III) -Containing Clay Minerals by Bacteria[END_REF], remains to be demonstrated in field conditions. Moreover, [START_REF] Canfield | The reactivity of sedimentary iron minerals toward sulfide[END_REF] calculated that the half-life of Fe(III) associated with silicates before pyritization was approximately 100,000 years. All these remarks suggest that the decrease in the fraction of Fe hosted by phyllosilicate minerals with depth is due to an evolution of the sedimentary regime of the Vavouto mangrove rather than to Fe-bearing phyllosilicates dissolution. This latter observation indicates that the contribution of Fe-bearing phyllosilicates to the iron cycling in the studied mangrove sediments is low compared to those of Fe oxyhydroxides and sulfides.

III-5. CONCLUSION

This study demonstrates that Fe speciation in the mangroves of New Caledonia is mainly driven by strong redox cycling mainly influenced by the distance to the shore. Goethite and serpentine inherited from lateritic regoliths are major phases of the upper layers of the mangrove sediments. Goethite converts to pyrite with depth in the organic rich sediments beneath the vegetated mangroves. However, in addition to this reducing process, the presence of significant amounts of newly formed poorly ordered ferric oxyhydroxide phases indicates that pyrite oxidizes, likely during low tide periods and upon the onset of the high tide. During the high tide periods the sediments may become anoxic again leading to reduction of these ferric phases. The intensity of this iron redox cycling significantly increases from the salt flat to the Rhizophora, i.e., from the land to the shore.

Finally, the average Fe concentration in the sediments decreases across the intertidal zone, from the salt flat (13 wt%) to, the Avicennia and the Rhizophora stands (7 -8 wt%),

indicating a loss of Fe from the land to the shore 4c,and 4e). Such a trend suggests that, over long term cycling of Fe in the mangrove sediments, the amount of Fe released by the reductive dissolution of ferric oxyhydroxide minerals and by the oxidation of iron sulfide minerals may not be fully balanced by the amount of Fe immobilized via the formation of iron sulfides and by the precipitation of ferric oxyhydroxides.

Several trace metal elements as Ni, Co, Cr, have high affinities for ferric oxyhydroxides [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF] and/or iron sulfides (Huerta-Diaz and [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF][START_REF] Otero | Spatial variation in pyritization of trace metals in salt-marsh soils[END_REF]. Although further studies are needed to evaluate the role of iron minerals in the cycling of trace metals in mangroves, the better understanding of the intense iron redox cycling occurring in mangrove sediments provided by the present study, may help in better predicting the fate of these trace metals in these fragile intertidal ecosystems, and that this ecosystem may act as a trace metal sink. Wilke et al. American Mineralogist, 2001, 86, 714-730, corrected for Fe(0) first inflection point taken at 7112 eV in the present study. [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF]. a SPOIL classification (Malinowski, 1978): 0-1.5, excellent;1.5-3, good;3-4.5, fair;4.5-6, acceptable;and >6, unacceptable reference.

Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2
.5 3.03E-02 - - - - - - - - - - - - 47.5 3.20E-02 - - - - - - - - - - - - (1) Calculated as Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2 (2) Improvement

IV-1. INTRODUCTION

Covering between 160,000 and 200,000 km 2 along more than 70% of tropical and subtropical coastlines [START_REF] Tomlinson | The botany of mangroves. Cambridge tropical biology series[END_REF][START_REF] Valiela | Mangrove forests: one of the world's threatened major tropical environments[END_REF], mangroves play a key role in the ecological balance of these areas [START_REF] N Nagelkerken | The habitat function of mangroves for terrestrial and marina fauna: a review[END_REF]. These intertidal forests receive inputs from natural or anthropized watersheds, and may act as a filter between land and sea for various contaminants as trace metals [START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Lacerda | The biogeochemistry and trace metals distribution of mangrove rhizospheres[END_REF]Wang, 1995;Tam and Wong, 1995;2000). As a result, and due to their potential toxicity to living organisms (Di [START_REF] Toro | Acid volatile sulfide predicts the acute toxicity of cadmium and nickel in sediments[END_REF], their dynamic in mangrove ecosystem has received significant attention during the last decades (Yim and tam, 1999;[START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF][START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF]Pi et al., 2011;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF][START_REF] Nath | Trace metal biogeochemistry in mangrove ecosystems: A comparative assessment of acidified (by acid sulfate soils) and non-acidified sites[END_REF]. This is especially the case in New Caledonia where mangroves act as a buffer zone between massive lateritic deposits enriched in heavy metals as Cr, Ni, Co and Mn [START_REF] Becquer | Mineralogical, chemical and charge properties of Geric Ferralsols from New Caledonia[END_REF][START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF][START_REF] Perrier | Biogeochemistry of a regolith: The New Caledonian Koniambo ultramafic massif[END_REF]Fandeur et al., 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF], and a lagoon of over 20,000 km 2 , recently registered as an UNESCO World Heritage site.

Mangrove sediments are known to have a large capacity to accumulate transition metal cations (i.e., Cr, Mn, Fe, Co, Ni, Cu, Zn; [START_REF] Tam | Retention and distribution of heavy metals in mangrove soils receiving wastewater[END_REF][START_REF] Tam | Spatial variation of heavy metals in surface sediments of Hong Kong mangrove swamps[END_REF][START_REF] Qiu | Accumulation and partitioning of seven trace metals in mangroves and sediment cores from three estuarine wetlands of Hainan Island, China[END_REF][START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF], which is partly attributed to their large contents in organic matter that can act as complexing agent [START_REF] Nissenbaum | Organic-metal interactions in recent sediments: the Rôle of humic substances[END_REF]. In addition, organicrich mangrove sediments are subjected to sulfate reduction processes [START_REF] Howarth | Pyrite formation and the measurement of sulfate reduction in salt marsh sediments[END_REF][START_REF] King | Patterns of sulphate reduction and the sulphur cycle in a South Carolina salt marsh[END_REF], which lead to precipitation of sulfide minerals (Huerta-Diaz and [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF]Burton et al., 2008a;2008b;Johnston et al., 2010a;2010b) able to trap chalcophile transition metal cations [START_REF] Morse | Chemical influences on trace métal-sulfide interactions in anoxic sediments[END_REF]Burton et al., 2006). However, successive oxic and anoxic diagenetic reactions depends on the biogeochemical conditions that operate in the sediments as a function of time, and which are driven by the distance to the seafront, the magnitude of the tide and the nature of the mangrove tree species [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]Marchand et al, 2011[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. Complex redox cycling in mangrove sediments may thus significantly affect the speciation of heavy metals along the intertidal zone [START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Lacerda | The fate of heavy metals in suspended matter in a mangrove creek during a tidal cycle[END_REF][START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF]Tam andWong, 2000, Marchand et al., 2006a;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF].

Such biogeochemical cycles have been already studied in the mangrove sediments of the Island of Pai Matos (Cananeia -SP, Brazil), in which Fe speciation determined from sequential extactions has been shown to significantly change as a function of the redox boundaries marking the intertidal and depth zonations [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. In the same way, Noël et al. (submitted) have fully determined Fe speciation in mangrove sediments from New Caledonia using X-ray Absorption spectroscopy, and this latter study confirms that the crystal-chemistry of this element is driven by the redox gradient generated by both the physiographic position along the intertidal zone and the depth along the sediments.

The objective of the present study is to improve our undestanding of the biogeochemistry of nickel in mangrove sediments, especially accross the intertidal zone where redox driven mineralogical (bio)transformations are susceptible of influencing Ni crystal-chemistry and bioavailability. Complemented by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, X-ray absorption spectroscopy (XAS) was used for this purpose because this method has demonstrated its ability at quantifying the wide variety of Ni-species in lateritic materials that are known to be the source of nickel in mangrove sediments in New caledonia [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]2013). The quantitative spatial distribution of nickel species is determined along three cores drilled beneath typical mangrove tree covers (Rhizophoras spp.,

Avicennias marina and salt flat), which help evaluating the long-term influence of the redox gradient in the mangrove sediments on the actual mobility of Ni in these fragile ecosystems.

IV-2. MATERIALS AND METHODS

IV-2.1. Field site and sampling location

Located in the South Pacific Ocean, New Caledonia is orientated along a northwest-southeast axis. The west coast of the main island (20°S -23°S), where the studied mangrove develops, is characterized by a semi-arid climate. The sampling campaign was performed in May 2011, at the end of the rainy season, in the Vavouto bay (20°59'S; 164°49'E), at the mouth of the Talea-Coco river that drains the ultramafic Koniambo regolith. As in many mangrove ecosystems, the intertidal zone in New Caledonia can be decomposed in 3 areas defined in terms of vegetation composition, from the sea front to the inland [START_REF] Duke | Mangrove biodiversity in New Caledonia. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF][START_REF] Virly | Atlas des mangroves de Nouvelle-Calédonie. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF].

The first zone is swept daily by tides and is dominated by Rhizophora spp. The second zone is located at the border of the salt flat and is dominated by Avicennia marina. The third zone is the salt flat which is not colonized by vascular plants. One sediment core was collected in each of these three mangrove stands, at low tide, with an Eijkelkamp gouge auger. For each core, the sediment samples were collected under nitrogen flow, every 5 cm from 0 to 60 cm depth, and conserved under anoxic conditions, in order for preventing them from oxidation.

These sediment samples were then kept frozen until being introduced in a glove box under N 2 atmosphere where they were vacuum dried, hand-ground in an agate mortar, homogenized and stored in anoxic conditions in sealed bottles.

IV-2.2. Mineralogy and chemistry of the mangrove sediments studied

Located upstream the studied mangrove, the Koniambo regolith consists of deeply weathered ultramafic rocks and thus hosts high concentrations of iron, and to a lesser extent, nickel, cobalt, chromium and manganese [START_REF] Perrier | Biogeochemistry of a regolith: The New Caledonian Koniambo ultramafic massif[END_REF]Fandeur et al., 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]. It is composed of an upper laterite unit consisting of goethite with minor amounts of hematite and Mn-oxides, and of a deeper saprolite unit consisting of serpentine and talc, with minor amounts of olivine and pyroxene (Fandeur et al., 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]). Erosion of these materials down to the mangrove areas has influenced the mineralogy of the mangrove sediments (Noël et al., submitted). In the top sediment horizons, serpentine (Mg 3 Si 2 O 5 (OH) 4 ), quartz (SiO 2 ) and halite (NaCl) are associated with talc (Mg 3 Si 4 O 10 (OH) 2 ) and goethite (α-FeOOH). In addition, due to the strong redox gradient observed with depth in the mangrove sediment and from the inland to the sea front (Noël et al., submitted) goethite disappears in favor of pyrite (FeS 2 ) in the deeper horizons beneath the Avicennia and Rhizophora stands.

IV-2.3. Model compounds for X-ray Absorption Spectroscopy (XAS) analyses

A Synthesis procedures for these latter model compounds are detailed in Supporting Information.

IV-2.4. Mineralogical and chemical analyses

The chemical composition of the mangrove sediments and of the natural and synthetic model compounds was determined by X-Ray Fluorescence (XRF) spectrometry, at IMPMC with a XEPOS (Spectro X Lab) X-ray fluorescence spectrometer equipped with a Pd cathode and For each sample, between 2 and 18 XAS scans were averaged and normalized using the ATHENA code [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]. EXAFS spectra were then extracted from these normalized data using the XAFS code [START_REF] Winterer | XAFS -a data analysis program for materials science[END_REF]. Fourier transforms (FT) of the EXAFS spectra were obtained by Fast-Fourier-transformation of the k 3 -weighted

IV-3. RESULTS

IV-3.1. Macro-and micro-scale distribution of Ni in the mangrove sediments

From the salt flat to the Rhizophora stand, Ni concentration in the studied sediment samples ranges from 1000 to 5300 mg.kg -1 , with an average of 2680 mg.kg -1 (Table IV-1). This average value is higher than world average [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF], but the highest Ni concentration measured in the studied samples is similar to that already measured in other mangrove sediments from New Caledonia [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. Such large Ni concentration likely results from erosion of the Koniambo lateritic regolith later the geological setting of the corresponding watershed, which consists of a (i.e. the outcrop) developed over ultramafic rocks.

In order to evaluate the Ni mass balance in the mangrove sediments, the Ni concentration was compared to that of Ti taken as geochemical invariant 3c,3e). For each mangrove stand, the Ni/Ti ratio was normalized to that of the upper sediment level (0 -2.5 cm) that could be regarded as representative of detrital sedimentation. In the sediments Principal Component Analysis (PCA) indicates that the 12 EXAFS spectra of the bulk sediment samples can be described by four principal components. Indeed the IND parameter reached a minimum value (i.e. 0.10) when considering the four first principal components of the PCA [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF], which explained 83% of the total variance of the system (Table IV-SI-1). In addition, the set of 12 EXAFS spectra was very well reconstructed when considering these four first components (Figure IV-SI-1). Target Transformation analysis helped in selecting the most appropriate model compounds for the Linear Combination Least-Square Fitting (LC-LSF) procedure. All the model compounds with a SPOIL value below 6

Figure IV-1: SEM images and SEM-EDS analyses of pyrite grains from the sample collected at 47.5 cm depth beneath the Rhizophora stand. The elemental maps in (a) show the homogeneous distribution of Ni in framboidal pyrite. SEM image in (b) show clusterized pyrite grains with sizes ranging from few micrometers (blue) to a few hundred of nanometers (purple) and then to a few tens of nanometers (red). SEM images in (c) show two types of framboidal pyrite, a first type constituted of large cuboctahedral pyrites (blue) and a second type constituted of small spherulitic pyrites (red). Corresponding SEM-EDS spectra are displayed in (d).

were considered as possible components [START_REF] Malinowski | Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry[END_REF]. Among all tested model compounds, the Ni-phyllosilicates (kerolite, saponite, and talc) yielded the lowest SPOIL values and can be classified as excellent references (SPOIL ≤ 1.5) (Table IV-

Figure IV-4: Experimental (black lines) and calculated (red lines) Ni K-edge k 3 -weighted EXAFS spectra EXAFS spectra (a) and their corresponding Fourier-Transforms (b) of the synthetic Ni:pyrite (top) and natural bravoite (bottom). Feff-calculated EXAFS spectra of pyrite and vaesite were least -squares adsjusted to the experimental data by varying σ(Å), S 0 2 and ΔE 0 parameters values (see Table IV-SI-4 for quantitative results).

IV-4. DISCUSSION

IV-4.1. Evolution of the Ni-speciation along the major redox gradients

The present study show that in the salt flat sediments as well as in the upper horizon beneath the Avicennia and the Rhizophora stands Ni is essentially hosted by phyllosilicate minerals and goethite, which have be shown by [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF] to be major Ni species in the Koniambo regolith, located upstream the studied mangrove. This result confirms that a major Ni input to the studied mangrove is related to erosion of the lateritized ultramafic outcrops.

Beyond these erosion -sedimentation processes, the present study also reveals major changes in Ni speciation along the redox gradients that develop across the intertidal zone, from land to the sea and with depth in the hydromorphic mangrove sediments beneath the Avicennia and Rhizophora stands. In these suboxic to anoxic sediments, Ni-bearing goethite disappears gradually with depth and Ni, for one part incorporated in pyrite and, for the other part, IV-SI-3). These processes can be related to the development of reductive dissolution of ferric oxyhydroxydes and, at depth, sulfate reduction and pyrite formation, both processes having been recently reported for the studied mangroves (Noël et al., submitted). However, in the present study, the partition of Ni between sulfides minerals and Ni-organic complexes contrasts with the results reported for iron speciation (Noël et al., submitted), for which Fe-organic complexes were not found to be significant.

IV-4.2. Crystal-chemistry of the Ni-pyrites across the intertidal zone

The present study shows that pyrite acts as an efficient sink for Ni in the anoxic horizons beneath the Avicennia and Rhizophora stands. This process is consistent with the ability of several transition metals (Ni, Co, As, Cu …) to be incorporated within the structure of sulfide minerals (Huerta-Diaz and [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF][START_REF] Aragon | Microscopic analysis of pyrite in the sediments of two Brazilian mangrove ecosystems[END_REF]. Moreover, Fe-sulfide mineral phases have been reported to dominate the partitioning of Ni in the solid fraction of recent sediments [START_REF] Nissenbaum | Organic-metal interactions in recent sediments: the Rôle of humic substances[END_REF][START_REF] Baltzer | Géodynamique de la sédimentation et diagenèse précoce en domaine ultrabasique -Nouvelle Calédonie[END_REF]Burton et al., 2006;[START_REF] Doig | Nickel partitioning in formulated and natural freshwater sediments[END_REF]. In the present study, Ni K-edge EXAFS results show that the Ni sulfide pool in the mangrove sediments studied essentially consists of Ni 2+ incorporated in pyrite and no evidence for pure Ni-sulfide phases could be found by SEM-EDXS analyses. Furthermore, in the mangrove sediments studied here, no Acid Volatile Sulfide (AVS), as mackinawite or greigite, were detected by either Ni-K edge EXAFS (this study) or Fe K-edge EXAFS (Noël et al., submitted), thus indicating that, if present, such phases represent less than 10% of the Ni and Fe pools. The absence of these intermediate phases that are known to evaluate with time to form pyrite [START_REF] Morse | The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters[END_REF] suggests that the pyritization process is particularly intense in the studied mangroves [START_REF]Acid volatile sulfide ( AVS )[END_REF]. EXAFS results also revealed changes in the crystal-chemistry of the Ni-bearing pyrites across the intertidal zone. Indeed, beneath Rhizophora a unique type of pyrite is found that can be accounted for by our In agreement with EXAFS results, distinct generations of pyrites, differing in their morphologies and Ni contents were recognized by SEM-EDXS analyses. Moreover, the proportions of these different pyrite phases vary across the intertidal zone. Small-sized pyrites with spherulite shape and with higher Ni content are more abundant in the sediments beneath Avicennia, while large cuboctahedral Ni-poor pyrites are more abudant beneath Rhizophora. Since the size and cristallinity of pyrite can directly influence its resistance to oxydation [START_REF] Silverman | Bacterial oxidation materials in coal[END_REF], small Ni-rich pyrites would be more sensitive to oxidative dissolution. This latter process could explain the lower abundance of small Ni-rich pyrites in the Rhizophora zone that is the closest to the shore and is thus subject to intense reduction-oxidation events upon tidal fluctuations (Noël et al. submitted).

IV-4.3. Ni complexation by organic matter

EXAFS results indicated that a significant fraction of Ni is bound to organic ligands, likely carboxyl moieties, in the anoxic horizons beneath Rizophora, and to a lesser extent, beneath

Avicennia. These results can be related to the strong accumulation of organic matter (OM)

(between 3 and 16%; Table IV-1) in the mangrove sediments beneath vegetated stands. Such

Ni complexation by organic ligands is consistent with previous studies of mangrove sediments based on selective chemical extractions [START_REF] Ray | Assessment of Godavari estuarine Mangrove ecosystem through trace metal studies[END_REF][START_REF] Chen | Distribution and accumulation of Heavy metals in the sédiments of Kaohsiung Harbor, Taiwan[END_REF][START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. However the nature, distribution and persistence of metal-organic complexes in mangrove sediments may be highly variable [START_REF] Alongi | Below-ground decomposition of organic matter in forests of the mangroves Rhizophora stylosa and Avicennia marina along the arid coast of Western Australia[END_REF][START_REF] Marchand | Pore water chemistry in mangrove sediments: relationship with species composition and developmental stages. (French Guiana)[END_REF][START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF]. For instance humic substances are major components of degraded OM and they can bind a significant fraction of metal ions (Nissenbaum and Kaplan, 1972). In soil humic substances, carboxyl and other oxygenated carbon moeities are known as major ligands for metal cations. By contrast, marine humic substances are less oxygenated and contain more sulfur groups (Nissenbaum and Kaplan, 1972), either because of the actual nature of the original OM or because of sulfidation of this OM during early diagenesis (e.g., [START_REF] Raiswell | The influence of bottom water oxygenation and reactive iron content on sulfur incorporation into bitumens from Jurassic marine shales[END_REF]Andersen and Pratt, 1995;[START_REF] Kohnen | Origin and diagenetic transformations of C25 and C30 highly branched isoprenoid sulphur compounds: Further evidence for the formation of organically bound sulphur during diagenesis[END_REF][START_REF] Kohnen | Identification and geochemical significance of cyclic di-and trisulphides with linear and acyclic isoprenoid carbon skeletons in immature sediments[END_REF][START_REF] Wakeham | Organic sulfur compounds formed during early diagenesis in Black Sea sediments[END_REF]. Our EXAFS results indicates that Ni is mainly complexed to oxidized groups in the studied mangrove sediments. However, further studies are required to fully describe the actual nature of theses Ni-organic complexes found here. In addition, interaction with Dissolved Organic Matter (DOM) in the pore water, where concentrations can be really high in mangrove sediments (Marchand et al., 2006), should be investigated. Nevertheless, our results suggest that Ni complexation by organic ligands and Ni incorporation in pyrite are competing upon redox cycles in the mangrove sediments studied. Indeed, in the sediments beneath the Avicennia stand where anoxic conditions are more stable and persistent with time (Noël et al. submitted), EXAFS results that Ni is preferentially trapped upon pyritization. On the opposite, in the sediments beneath the Rhizophora stand where the redox cycle is intensified due to tidal fluctuations (Noël et al., submitted), Ni-organic complexation dominates over Ni sequestration in pyrite. This difference could be explained by oxidative dissolution of Nipyrites during the partial re-oxidation processes that take place at the beginning of the high tide period in these sediments [START_REF] Canfield | The reactivity of sedimentary iron minerals toward sulfide[END_REF], which could promote Ni 2+ release to pore waters and further bonding to organic ligands.

IV-4.4. Potential influence of redox cycling on Ni mobility across the intertidal zone

Our results show major changes in Ni speciation, both vertically and laterally across the intertidal zone, in the studied mangrove sediments. These changes are accompanied by significant variations in the Ni concentration across the intertidal zone and with depth. Beneath the salt flat, in which Ni species inherited from the erosion of lateritic material are preserved, the Ni concentration normalized to that of Ti suggests that Ni is neither stored nor leached. This result is in agreement with those from previous studies that showed that salt flat is characterized by the quasi absence of redox cycling [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]Noël et al., submitted). Beneath the Avicennia stand, the average Ni concentration normalized to that of Ti suggests a storage of Ni with depth, which could be related to the dominant incorporation of Ni in pyrite observed in these sediments . On the opposite, beneath the Rhizophora stand, the evolution of this ratio with depth indicates that Ni does not accumulate but is rather slightly leached at depth. Such behavior could be interpreted as resulting from periodic reoxydation of Ni-bearing pyrites, which may favor Ni leaching and complexation to organic matter. This hypothesis is supported by previous study of Fe speciation in these mangrove sediments, which showed that Fe-sulfides are subject to periodic reoxidation in the Rhizophora stand likely because of tidal fluctuations (Noël et al., submitted).

Model compounds for X-ray Absorption Spectroscopy analyses

Ni-Pyrite (Fe 0.99 Ni 0.01 S 2 ) was synthesized according to a protocol derived from that reported by [START_REF] Wei | Aqueous synthesis of finely divided pyrite particles[END_REF]. 0.67 g FeCl 3 .6H 2 O and 6 mg Cl 2 Ni.6H 2 O were dissolved separately in 10 mL of double-distilled and oxygen-free water, in an anaerobic chamber.

Meanwhile, 0.6 g Na 2 S.9H 2 O was dissolved in 30 mL of double-distilled and oxygen-free water, and subsequently added to the metal solution while stirring. The suspension obtained was sealed in anoxic conditions and was gently stirred in the anaerobic chamber during 14 days. The solid fraction was collected after centrifugation at 8,500 G for 10 min, and it was then rinsed three times with double-distilled and oxygen-free water, and finally dried in Ni-Greigite (Fe 2.97 Ni 0.03 S 4 ) was synthesized following the procedure by [START_REF] Chang | Journal of Colloid and Interface Science Fabrication , characterization , and application of greigite nanoparticles for cancer hyperthermia[END_REF].

1.1 g Fe(SO 4 ).9H2O and 9.6 mg Cl 2 Ni.6H 2 O were dissolved separately in 10 mL of doubledistilled and oxygen-free water, in an anaerobic chamber. Meanwhile, 0.96 g Na 2 S.9H 2 O was dissolved in 20 mL of double-distilled and oxygen-free water, and subsequently added to the metal solution while stirring. pH was adjusted to 3.8 with a 1 M HCl solution. The suspension obtained was sealed in anoxic conditions and was gently stirred in the anaerobic chamber during 1 month. The solid fraction was collected after centrifugation at 8,500 G for 20 min, and it was then rinsed three times with distilled water, and finally dried in dessicator. XRD analyses confirmed the purity of the synthetic greigite (Figure IV-SI-7) and XRF analyses indicated that it contains 1mol% Ni.

Amorphous NiS (am-NiS) was prepared by neutralizing a Ni(II) salt solution with a basic Na 2 S solution, following a procedure modified from (Helz et al. 1999).

Ni-adsorbed iron sulfides (mackinawite and pyrite) Mackinawite and pyrite were synthetised according to the protocols described above without adding Ni. For sorption experiments, 100 mg of synthetic mackinawite was suspended in 100 mL of double-distilled water and dispersed with 15 min ultrasonic waves. In order to yield a constant ionic strength during the Ni sorption experiments, pH was adjusted to 5 with a 0.1 M HNO 3 solution. Then 500 mL double-distilled and oxygen-free water was added to the mackinawite suspension.

The pH of these suspensions was again adjusted to 5 with a 0.1 M HNO 3 solution. Finally, 800 mg KNO 3 were dissolved in 300 mL double-distilled and oxygen-free, and the pH was adjusted to 5 with a 0.1 M HNO 3 solution. This solution water 300 mL was added to the mackinawite suspension, and the pH was adjusted to 5 with a 0.1 M HNO 3 solution.

Meanwhile, 2.4 mg of Ni(NO 3 ) 2 .6H 2 O were dissolved in 100 mL double-distilled water. pH of solutions was then adjusted to the required pH 5 with a 0.1 M HNO 3 solution and added to the mackinawite suspension while stirring. After 24 h reaction, the final pH was controlled and the solid fraction was separated by centrifugation at 8,500 G for 10 min, washed three times and dried in dessicator. The same protocol was repeated for pyrite. XRD analysis indicated that the products obtained after this treatment were still pure mackinawite and pyrite (Figure IV-SI-7). XRF analyses indicated that pyrite and mackinawite contains 1 and 1mol%

Ni, respectively.

Ni-citrate was synthesized by first dissolving 2.3 mg of Cl 2 Ni.6H 2 O in 0.5 mL of a solution of 1 M citric acid and 1 mL of a solution of 1 M NaOH. The solution was then stirred and heated while at 50 ° C until total evaporation and precipitation of the complex. XRF analyses indicated that it contains 1mol% Ni. experiments, energy was calibrated by setting the first inflection point of the absorption edge of a Ni metal foils at 8333 eV, with the foil placed after the second gas-filled ion chamber detector.

Ni-gluthation and

Samples were prepared as pressed pellets of finely hand-ground and homogenized powders of either low Ni concentration samples (no dilution) or high Ni concentration samples (diluted in cellulose). In order to strictly preserve the oxidation state of these samples during the analyses, pellets were mounted on the cryostat sample rod within a glove box and carried to the beamline in a liquid nitrogen bath before being rapidly transferred into the He cryostat.

For each sample, between 2 and 18 XAS scans were averaged and normalized using the ATHENA code [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]. EXAFS spectra were then extracted from these normalized data using the XAFS code [START_REF] Winterer | XAFS -a data analysis program for materials science[END_REF]. Fourier transforms (FT) of the EXAFS spectra were obtained by Fast-Fourier-transformation of the k 3 -weighted experimental χ(k) function using a Kaiser-Bessel apodization window with the Bessel weight fixed at 2.5.

In addition to this LSC-LSF analysis of EXAFS data of the natural mangrove sediment samples, the local structure around Ni in the Ni-pyrite and Ni-rich-pyrite model compounds was determined by fitting the Ni K-edge EXAFS spectra with theoretical spectra calculated using the Feff 8.10 code [START_REF] Ankudinov | Real-space multiple-scattering calculation and interpretation of X-ray-absorption near-edge structure[END_REF]. Calculations were performed on clusters of 8 Å in diameter around the central Ni absorbing atom, using all single and multiple scattering paths (nleg = 8), and without using the self consistent potential option for faster calculations.

Calculated spectra were least-squares fit to experimental ones using a Levenberg-Marquard minimization algorithm including an overall energy threshold shift (∆E 0 ) and an overall Debye-Waller σ as the two only fitting variables. As detailed in the result section, the structures of the atomic clusters were modified from the crystallographic structures of pyrite [START_REF] Bayliss | Crystal structure refinement of a weakly anisotropic pyrite[END_REF] and vaesite [START_REF] Nowack | Charge Densities in CoS2 and NiS2 (Pyrite Structure)[END_REF] for the Ni-pyrite and Ni-rich-pyrite samples, respectively.

EXAFS data analysis by Principal Component Analysis (PCA) and Linear Combination

Least Square Fitting (LC-LSF)

The Ni K-edge EXAFS spectra of 12 samples from the three studied sediment cores was first analyzed by Principal Component Analysis (PCA) using the SIXPACK code [START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF], in order for estimating the minimum number of components necessary to fit this set of EXAFS spectra [START_REF] Wasserman | EXAFS and principal component analysis: a new shell game[END_REF][START_REF] Ressler | Quantitative speciation of Mn-bearing particulates emitted from autos burning (methylcyclopentadienyl) manganese tricarbonyl-added gasolines using XANES spectroscopy[END_REF]. Appropriate model compounds spectra were then selected from our large set of natural synthetic model compounds spectra (see Table IV-SI-3) by Target Transform (TT) analysis. The details of this PCA-TT procedure are reported in the Supporting Information. The 12 Ni K-edge EXAFS spectra were then fit by a Linear Combination Least Squares Fitting (LCF-LSF) procedure using a home made software based on Levenberg-Marquardt minimization algorithm. The quality of the LCF-LSF fits was estimated by a R-factor parameter (Rf) of the following form:

Rf = ∑ [y exp - y calc ] 2 / ∑ [y exp ] 2 .
The accuracy of this fitting procedure ranges between ± 25% and ± 5% of the stated values for each individual contribution and contributions below 10% are considered as not significant [START_REF] Ostergren | Quantitative Speciation of Lead in Selected Mine Tailings from Leadville , CO[END_REF][START_REF] Cancès | XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant[END_REF]Juillot et al., 2011).

Principal Component Analysis (PCA)

The minimum number of principal components necessary to fit the whole set of experimental EXAFS spectra was chosen on the basis of the minimum value of the Factor Indicator Function [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF][START_REF] Malinowski | Factor Analysis in Chemistry[END_REF][START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF] and the quality of the reconstruction of the whole set of experimental EXAFS spectra with the first, the two first, and the three first components extracted from the PCA analysis (Figure IV-SI-1). Target Transformation (TT)

was then used to select the EXAFS spectra of model compounds that most likely represent the set of experimental EXAFS spectra [START_REF] Malinowski | Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry[END_REF][START_REF] Ndiba | Heavy metal immobilization through phosphate and thermal treatment of dredged sediments[END_REF]. This selection was based on the quality of the TT fits assessed by the Normalized Sum of Squared Residual (NSSR:

∑(k 3 χ Target -k 3 χ Transform ) 2 / ∑(k 3 χ Target ) 2 ).
Chapitre IV : Le cycle du nickel dans les sédiments de mangrove 123 [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF]. (Malinowski, 1978): 0-1.5, excellent;1.5-3, good;3-4.5, fair;4.5-6, acceptable;and >6, unacceptable reference. Calculated as

Table IV-SI-2: Results of the Target Transformation performed on the EXAFS spectra from various model compounds using the firstfour obtained by PCA (

Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2

Chapitre V : Mécanisme de formation des pyrites et étude du comportement des éléments traces au cours de la pyritisation

Chapitre V : Mécanisme de formation des pyrites et étude du comportement des éléments traces au cours de la pyritisation

Intérêt et résumé de l'étude

L'étude des transformations minérales dans les sédiments de mangroves (cf. Chapitre III), montre que la pyrite est la phase cristalline majeure qui précipite dans les conditions anoxiques des sédiments hydromorphes des zones à Avicennia et à Rhizophora. Ce sulfures de fer a une forte affinité pour les ETM, et peut ainsi contribuer à les immobiliser comme nous l'avons montré au chapitre précédent. Nous avons en effet montré au Chapitre IV qu'une large fraction de Ni est incorporée dans la structure des pyrites au cours de la précipitation de cette dernière et est ainsi immobilisée tant que les conditions restent réductrices. Cependant le mécanisme de formation de la pyrite est très complexe, et encore mal compris. Les études sur les sédiments marins, montrent que d'autres types de sulfures de fer, tel que la mackinawite (Fe) 1+x S x , la pyrrhotite Fe x S (1-x) et la greigite Fe 3 S 4 , peuvent précipiter comme phases intermédiaires au cours de la pyritisation. De nombreux auteurs considèrent ces monosulfures de fer comme les précurseurs de la pyrite. Cependant, les études récentes démontrent que la pyrite précipiterait suite à la réaction d'oligomères [FeS] dissous avec les polysulfures (Rickard et al., 2007), et non par transformation à l'état solide de précurseurs solides de type FeS. Ces derniers précipitent cependant plus facilement que la pyrite, et sont connus pour évoluer vers un pôle pyrite, probablement par dissolution-reprécipitation. L'ensemble de ces processus est important à prendre en compte afin de comprendre le comportement des éléments métalliques en traces (ETM) au cours de la pyritisation.

Dans ce chapitre, plusieurs questions portant sur le comportement des éléments métalliques au cours de la pyritisation sont soulevées :

-Quels sont les éléments métalliques qui sont efficacement piégés par la pyrite ? -Est-ce que les éléments métalliques sont piégés par les monosulfures de fer qui précipitent en premier, et dans ce cas, sont-ils en partie relargués ou piégés au cours de(s) transformation(s) minéralogique(s) en faveur de la précipitation de pyrite ? -L'interaction de certains éléments métalliques avec le Fe et le S dissous peut-elle influencer le processus de formation de la pyrite ? Afin de répondre à ces questions, ce chapitre décrit les études de suivi cinétique de synthèses de pyrites au laboratoire, avec et sans l'ajout d'ETM. Le protocole de synthèse de pyrite utilisé pour cette étude a été mis au point au cours de la thèse et s'inspire de celui de Wei et al. (1996). Il utilise une source de Fe(III) et de S(II). Quatre suivis cinétiques ont étés réalisés, une expérience témoin, mettant en jeu uniquement du Fe(III), et 3 synthèses où 1% molaire du Fe(III) est remplacé par un élément trace métallique divalent dissous (Mn 2+ , ou Co 2+ , ou Ni 2+ ), dans la solution de sel métallique initiale. Dans un premier temps, l'évolution de la composition minéralogique et de la cristallochimie du Fe au cours du temps a été suivie dans l'expérience témoin, sans ETM. Ceci a permis de caractériser les phases minérales et leurs transformations, jusqu'à l'obtention de la pyrite. Ensuite, les successions minéralogiques et les cinétiques de formation de la pyrite ont été comparées avec les expériences réalisées en présence de métaux dissous. Ceci a permis de caractériser l'influence de chacun de ces ETM sur les transformations minérales et leurs cinétiques. D'autre part, un regard particulier a été porté sur le suivi cinétique de la concentration des métaux dissous dans le surnageant, afin d'estimer l'affinité de ces ETM pour les différentes phases minérales. Finalement, l'étude de la spéciation du Ni, associée à la fraction solide, a été menée par spectroscopie EXAFS, afin d'identifier la nature de ces interactions entre cet élément et les différentes phases minérales formées lors de la pyritisation.

En premier lieu, la réduction du Fe(III) par le sulfure d'hydrogène produit du Fe(II) qui précipite immédiatement avec les sulfures sous la forme de Fe 1+x S. La concentrations en Fe(II) en équilibre avec ce précurseur est suffisante pour assurer la sursaturation vi-à-vis de la pyrite, à condition que les ions HS -dissous réagissent avec S(0) pour former des polysulfures, initiant ainsi la nucléation de la pyrite. En présence de Co 2+ , la précipitation de la pyrite est 2 fois plus rapide, tandis qu'en présence de Ni 2+ , la pyrite précipite jusqu'à 5 fois plus rapidement. L'analyse de la chimie de la phase aqueuse, couplée aux études de microscopie électronique à balayage montrent que seul le Ni 2+ et le Co 2+ sont piégés dans la fraction solide dès le début de la réaction. Ces derniers sont aussi efficacement piégés au cours de l'évolution de Fe 1+x S en pyrite, suggérant que cette étape n'augmente pas la mobilité de ces ETM. Plus spécifiquement, les analyses par TEM-EDXS montrent que le Ni est préférentiellement localisé au coeur des grains de pyrites, montrant ainsi que cet élément participe à la nucléation. Nous suggérons que les ions Ni 2+ , accélèrent la nucléation de la pyrite en formant une solution solide entre les pôle purs pyrite (FeS 2 ) et vaésite (NiS 2 ), dont le produit de solubilité est plus faible que celui de la pyrite. A partir de cette nucléation, la croissance Chapitre V : Mécanisme de formation des pyrites et étude du comportement des éléménts métalliques traces au cours de la pyritisation 133 cristalline de pyrite s'opère progressivement. Nous proposons un mécanisme similaire pour le Co 2+ , tandis que le Mn 2+ n'interagit ni avec la mackinawite, ni avec la pyrite.

V-1. INTRODUCTION

Transition metals are known to incorporate the structure of precipitating sulfide minerals in low-temperature environments, being then rapidly removed from the dissolved phase (Huerta-Diaz and [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF][START_REF] Aragon | Microscopic analysis of pyrite in the sediments of two Brazilian mangrove ecosystems[END_REF]Noël et al., in prep., a). Pyrite and other reactive iron-sulfide minerals, as Acid Volatile Sulfides (AVS; [START_REF] Morse | The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters[END_REF], provide sinks for potentially toxic trace elements, decreasing their potential mobility and bioavailability in sulfidic sediments (Chapman et al., 1998;[START_REF] Morse | Chemical influences on trace métal-sulfide interactions in anoxic sediments[END_REF]Simpson et al., 2002). However, the replacement of AVS, as mackinawite, by pyrite may enhance or decrease the reactivity of metals during diagenesis (Burton et al., 2006). Pyrite synthesis experiments showed that pyrite formation kinetic depend on the nature of the substrate (organic substrate, FeS 0 ; Rickard et al., 2007aRickard et al., , 2007b)), and on the interaction of dissolved iron and sulfide with the various components of the medium (as aldehydic carbonyl; Rickard et al., 2001). The possible effects of trace contaminants, as transition metal cations, in lowtemperature experimental sulfide systems have not been investigated (Rickard et al., 2007). It is however expected that such impurities may have an effect on the distribution of pyrite in natural systems.

We present the chemical, mineralogical and crystal-chemistry study of pyrite precipitation from Fe(III) source and sulfide ions, with addition of dissolved Mn 2+ or Co 2+ or Ni 2+ . The particles precipitating during chemical synthesis were analyzed, using X-ray diffraction (XRD), Wide-angle X-rays scattering (WAXS), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). Fe K-edge and Ni K-edge X-ray absorption spectroscopy (XAS) was chosen to directly determine iron and nickel speciation. These results were interpreted in relation with dissolved metals concentration, and provided evidence that Ni 2+ and Co 2+ speed up the pyrite formation involved of mackinawite, for an effective trapping of these metals in the structure of the pyrite.

V-2. MATERIALS AND METHODS

V-2.1. Pyrite synthesis and kinetic sampling

Pyrite (FeS 2 ) was synthesized according to a protocol derived from [START_REF] Wei | Aqueous synthesis of finely divided pyrite particles[END_REF], with or without dissolved Mn 2+ , or Co 2+ , or Ni 2+ . All solutions were prepared with double-distilled and oxygen-free water, in an anaerobic chamber. For the control experiment absorption spectroscopy. Technical details and analytical procedures for these analyses are reported in Supporting Information.

V-3. RESULTS

In the control experiment, the particles sampled in the beginning (0.5h) and at the end (216h) of the reaction were analyzed by wide-angle X-ray scattering. The calculated PDF of mackinawite based on the crystal structure reported by [START_REF] Redfern | Synthesis and Rietveld crystal structure refinement of mackinawite[END_REF] is in good agreement with the PDF calculated from the experimental WAXS pattern obtained for the solid phase after 0.5h. In contrast the PDF for the particles at 216h (end of the control experiment) is in good agreement with the calculated PDF of pyrite, based on the crystal structure by [START_REF] Bayliss | Crystal structure refinement of a weakly anisotropic pyrite[END_REF] (Figure V-SI-1).

In between these two initial and final stages, mineralogical transformations were followed by XRD and EXAFS at the Fe and Ni K-edges. In a first stage, the interaction of dissolved Fe 3+ with HS -leads to the precipitation of mackinawite and elemental sulfur .

No other solid-phases were observed by XRD, during the first 180h. Then after 216h, the Bragg peaks of elemental sulfur and mackinawite disappear, while those of the pyrite appear LC-LSF analysis of the Ni K-edge spectra of solid samples collected at 29 and 30h indicated that they consist of a mixture of the Ni-bearing precursor (principally of amorphous NiS and a little of Ni-mackinawite) and Ni-pyrite. The proportion of Ni-pyrite increased from 43% at 29h, to 58% at 30h 3d;.

Figure V-1: X-ray powder diffraction patterns of the solids collected over the course of the control experiment without trace metal cation impurity (orange; a), the experiment performed with dissolved Mn 2+ (blue; b), with dissolved Co 2+ (purple; c), and with dissolved Ni 2+ (black; d). Bragg peak indexation indicates mackinawite (M), elemental sulfur (S), and pyrite (P). The low angle region of the XRD patterns is not shown because of a broad diffuse band caused by the Kapton window of the anoxic sample chamber (2θ ~ 7°). The wide-angle regions of the XRD patterns have been

Table V-1 : (a, b) Quantitative results of the best LC-LSF fits performed on the k 3 -weighted Ni K-edge EXAFS data of the solid fraction collected in experiment performed with dissolved Ni 2+ , expressed as the relative proportion of the selected model compounds. (c) Values of the parameters used to fit the Ni K-edge EXAFS data collected at 48h, with Feff-calculated EXAFS data. (see Figure V-3). (a) Initial Ni speciation (LS-LCF)

Amorphous NiS

Ni-Mackinawite NSSR (1) 0.5h 90 23 0.0736

(b) Transition Ni speciation (LS-LCF)

Initial Final d,f)

σ (Å) ΔE0 (eV) 48h 0.067 -3.425 (1) Calculated as Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2

. Experimental data and LC-LSF fits for (a) and (c) are displayed in black and red lines, respectively (see Table V-1 for quantitative results). For the pyrite sample obtained after 48h, red lines indicated Feff-calculated EXAFS spectra after least-squares fit of the σ(Å), and ΔE 0 parameters (see Table V-1 for quantitative results).

V-4. DISCUSSION

In our experiments, Fe(III) initially reacts with dissolved sulfide ions to form ferrous iron and elemental sulfur (S(0)) (Eq. ( 1)). Then poorly ordered iron monosulfide, we show it has the mackinawite local structure, immediately precipitates via the reaction between aqueous sulfide and aqueous ferrous iron species (Eq. ( 2)) [START_REF] Berner | Sedimentary pyrites formation[END_REF]Shoonen et al. 1991).

2Fe 3+ + HS -= 2Fe 2+ + 1/8 S 8 + H + (1)

Fe 2+ + x Fe 3+ + HS -= Fe 1+x S + H + (2)
Finally, in the presence of elemental sulfur, Fe 1+x S is converted to pyrite. It has been proposed

that this conversion involves the formation of a dissolved [FeS] transition intermediate, which

reacts with the nucleophilic polysulfides to form FeS 2 [START_REF] Rickard | Kinetics and mechanism of pyrite formation at low temperatures[END_REF][START_REF] Luther | Pyrite synthesis via polysulfide compounds[END_REF][START_REF] Rickard | Kinetics of pyrite formation by the H,S oxidation of iron(H) monosulfide in aqueous solutions be-tween 25°C and 125°C: The rate equation[END_REF]. These polysulfide species form after the reaction between S(0) and S(-II) [START_REF] Hartler | Rate of sulfure dissolution in aqueous sodium sulfide[END_REF]Boulegue et al., 1977), which explain the dissolution of elemental sulfur, as also

V-5. CONCLUSION

Ni 2+ and, to a lesser extent Co 2+ , could have directly influenced the mineralogical transformations of iron sulfides in present and past environments, promoting pyrite precipitation.

For solid fraction collected at 216 h from the control experience, the best fit was obtained from the refinement in real space of the Pyrite structure. Starting unit cell and atomic positions were those of a preliminary solved structure from single crystal. The agreement between the calculated and the experimental PDF is characterized by the following reliability factor:

where G obs is the PDF extracted from the diffraction data, G calc is the PDF calculated from the model and p is the list of parameters refined in the model.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

Scanning Electron Microscopy (SEM) observations and analyses were performed at IMPMC with a Field Emission Gun Scanning Electron Microscope (GEMINI ZEISS Ultra55) operating at 15 kV. Samples to be analyzed were deposited as ground powder on carbon tape and coated with a thin carbon film. TEM analysis were carried out on a JEOL 2100F

VI.A. Effets de l'exploitation minière sur le cycle biogéochimique de Fe et Ni

La méthodologie employée pour les analyses chimiques, minéralogiques et spectroscopiques réalisées sur la mangrove de Creek Mine (baie de Vavouto) est identique à celle mise en oeuvre sur la mangrove de référence (Creek Coco, baie de Vavouto). Elle est décrite dans les chapitre III et IV.

VI.A.1 Contraste chimique et minérale le long des profils sédimentaires de la mangrove de Creek Mine

La mangrove de Creek Mine, en aval d'une zone anciennement exploitée du massif ultrabasique du Koniambo, se distingue visuellement de la mangrove de référence, n'ayant subi jusqu'alors aucune influence minière (cf. Chapitre II, -Dans les tannes, les concentrations en Fe varient entre 5 et 28 %pds dans les sédiments de l'horizon supérieur (2,5 à 12,5 cm), tandis qu'elles sont proche de 4,9% pds en dessous de 15 cm de profondeur. Concernant les concentrations en Ni, elles varient entre 0,12 et 1,23 % pds dans l'horizon supérieur, et se stabilisent à 0,057% pds de 15 à 50 cm de profondeur.

-La concentration moyenne en Fe, dans l'horizon supérieur de la carotte de sédiment collectée sous Avicennia, est de 20 % pds, tandis qu'elle est approximativement de 5,32 % pds en dessous de 25 cm de profondeur. De même la concentration moyenne en Ni est de 0,68% pds pour l'horizon de surface et de 0,11% pds en dessous.

-Finalement, pour les sédiments sous Rhizophora, les concentrations en Fe varient entre 14,7 et 23,2 % pds, et celles du Ni entre 0,55% pds et 0,84% pds, dans l'horizon supérieur

(2,5 à 12,5 cm). Ces deux concentrations se stabilisent approximativement sous 15 cm de profondeur, à 4.3% pds pour le Fe et 0,047% pds pour le Ni.

Figure VI-1 : Compositions chimiques et minéralogiques des sédiments de mangrove de

Creek Mine, le long des carottes prélevées dans les tannes, les zones à Avicennia, et celles à Rhizophora. Le panneau de gauche montre les carottes prélevées (a, e, i), et les évolutions verticales des concentrations (wt %) de Fe (rouge), de S (noire) (b, f, j), et de Ni (vert) (c, g, k) enregistrées dans la fraction solide le long de ces carottes. Le panneau de droite (d, h, l) montre les diffractions des rayons X collectés pour les échantillons représentatifs des variations minérales le long du profil sédimentaire. L'indexation des pics de Bragg indique la présence de goethite (Gt), serpentine (Srp), talc (Tlc), quartz (Qz), halite (H) et pyrite (Py). La région des faibles angles n'est pas montrée à cause de la large bande de diffusion de la fenêtre en capton du dispositif anoxique (2θ ~ 7°). C'est pourquoi cette région a été coupée pour plus de clarté.

Globalement, l'horizon supérieur se caractérise par un enrichissement conséquent en Fe et en La fraction de Fe dans la pyrite reste cependant en dessous du seuil de détection de l'EXAFS, soit environ 10% du fer total. D'autre part, des oxyhydroxides de fer potentiellement néoformés suite à l'oxydation du Fe(II) ne sont pas détectés. En effet, la figure VI-2c ne montre pas la présence de ferrihydrite, ni de lepidocrocite, contrairement aux sédiments de mangrove de la zone de référence qui montre la présence de ces deux oxyhydroxides de fer dès les premiers centimètres de profondeur (cf. Chapitre III, Figure 6f et 6i).

En conséquence, les transformations minérales en condition anoxique, au sein de l'horizon de goethite sont très faibles, traduisant une faible réactivité des goethites, à l'échelle de notre étude (30 ans d'exploitation).

VI.A.4 Conclusion

L'exploitation minière modifie la sédimentation détritique. La proportion de goethite des minéraux hérités augmente, enrichissant ainsi le sédiment en Fe et en Ni. Les études sur les transformations minérales au sein des sédiments de mangrove (cf. chapitre III), montrent que cet oxyhydroxyde de fer est peu sensible aux conditions anoxiques de ces sédiments. En effet, sur une échelle de 30 ans, cette couche de goethite est faiblement réactive, conservant les ETM piégé dans leur structure.

Il paraît donc plus judicieux de travailler sur une échelle de temps plus longue en se posant deux questions essentielles :

-Si l'échelle de temps joue sur la réactivité des goethites, et donc sur la mobilité du Fe et du Ni, quel est le devenir de ces ETM ? Plus spécifiquement, est-ce que Fe et Ni sont piégés, en partie, par pyritisation et/ou par complexation organique dans les sédiments de mangrove comme le montre notre étude sur la mangrove de référence (cf. chapitre III) et du Ni (cf. chapitre IV et V) ?

-Dans le cas où ces ETM s'accumuleraient dans les sédiments par pyritisation, que deviennent ces métaux si le système est perturbé par une phase intense d'oxydation ?

ABSTRACT

The stability of Fe(II)-sulfides in the anoxic horizons of mangrove sediments make this ecosystem a potential long-term sink for heavy metals that are delivered at the land-sea interface in the inter-tropical region. However, in a context of increasing anthropogenic pressure, the urbanization of coastal areas leads to artificial isolation of mangrove sediments from the sea, which modifies their bio-physico-chemical functioning. In the present study, we have combined Fe and Ni K-edges XAS data with mineralogical analyses of the solid fraction (XRD and SEM-EDS) and chemical analyses of the pore-waters to characterize the changes in Fe and Ni cycling in the sediments of a former mangrove that is currently re-oxidizing in response to isolation from the sea in New Caledonia. In this country, a large fraction of the mangroves extend downstream massive Ni-rich lateritic deposits, which explains the elevated Fe and Ni concentrations measured in the solid fraction of the studied mangrove sediments compared to those classically reported worldwide. In addition, the results of the physicochemical analyses indicate unusual suboxic/oxic conditions at intermediate depths in the studied mangrove, compared to other mangroves worldwide. These peculiar conditions for mangrove sediments that were originally under anoxic/suboxic conditions directly result from their isolation from the sea. They favor the oxidative dissolution of Ni-bearing Fe(II)-sulfides (mainly pyrite in the studied sediments) and the subsequent precipitation of Fe(III)hydroxides, which strongly acidifies the pore-waters. Although normally limited in the nearneutral conditions that prevail in the surface horizons of mangrove sediments, the partial dissolution of Ni-bearing goethite and/or phyllosilicates is also enhanced in such an acidic context. The very high concentrations in Fe and Ni measured in the pore-waters at this reoxidation front directly result from these reactions. The high levels of dissolved Ni are of particular concern with respect to the biodiversity of the mangrove ecosystem. However, Ni K-edge EXAFS data indicate that both incorporation within the structure of pyrite and sorption at the surface of this mineral species strongly immobilize the fraction of this dissolved Ni that percolate in the underlying anoxic horizons. This mechanism explains the 2 times larger total Ni concentration measured in the solid fraction of the bottom anoxic horizons compared to that in the surface oxic/suboxic horizons.

These results emphasize the importance of Ni interaction with Fe(II)-sulfides (either incorporation within the structure or sorption at the surface) for mitigating the impact of the re-oxidation of former Ni-rich mangrove sediments after isolation from the sea through the storage of this element in the anoxic/suboxic bottom horizons. Considering the known affinity of sulfides towards Mn, Co, Cu and Zn, this conclusion can be extended to this set of transition elements.

VI-B-1. INTRODUCTION

Mangroves cover more than 70% of tropical and subtropical coastlines [START_REF] Tomlinson | The botany of mangroves. Cambridge tropical biology series[END_REF][START_REF] Valiela | Mangrove forests: one of the world's threatened major tropical environments[END_REF] and act as an ecologic filter between land and sea for various contaminants, especially trace metals [START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF]Mackey et al., 1992;[START_REF] Lacerda | The biogeochemistry and trace metals distribution of mangrove rhizospheres[END_REF][START_REF] Tam | Retention and distribution of heavy metals in mangrove soils receiving wastewater[END_REF]Tam and Wong, 1995;2000;[START_REF] Qiu | Accumulation and partitioning of seven trace metals in mangroves and sediment cores from three estuarine wetlands of Hainan Island, China[END_REF][START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF].

Concentrations of heavy metals recorded in mangrove ecosystems are highly variable, probably reflecting the different natural and anthropogenic sources, as well as their short-and long-term occurrences [START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF]. Due to their persistence, potential toxicity, and bioavailability, these elements represent a major potential threat for mangrove biodiversity [START_REF] Toro | Acid volatile sulfide predicts the acute toxicity of cadmium and nickel in sediments[END_REF][START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF]. As a consequence, their dynamic in this ecosystem has received significant attention during the last decades [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF][START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF][START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF][START_REF] Nath | Trace metal biogeochemistry in mangrove ecosystems: A comparative assessment of acidified (by acid sulfate soils) and non-acidified sites[END_REF].

The accumulation of metals in mangrove sediments is partly attributed to their ability to efficiently trap suspended material from the water column [START_REF] Furukawa | Currents and sediment transport in mangrove forests[END_REF], their large contents in dissolved or solid organic matter and high accumulations of fine particles with a large specific surface area (like clay minerals), both known for their strong affinity towards these elements [START_REF] Nissenbaum | Organic-metal interactions in recent sediments: the Rôle of humic substances[END_REF][START_REF] Harbison | Mangrove Muds-A Sink and a Source for Trace Metals[END_REF][START_REF] Tam | Retention and distribution of heavy metals in mangrove soils receiving wastewater[END_REF].

Furthermore, the organic-rich mangrove sediments are redox-stratified, with more reductive conditions at depth in response to organic matter decomposition and sulfate reduction processes [START_REF] Howarth | Pyrite formation and the measurement of sulfate reduction in salt marsh sediments[END_REF][START_REF] King | Patterns of sulphate reduction and the sulphur cycle in a South Carolina salt marsh[END_REF]. Whereas precipitation of ferric oxyhydroxides such as lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and ferrihydrite (5Fe 2 O 3 .9H 2 O) is the main process in relation with oxic to suboxic conditions in the surface horizons due to air-input through bioturbation, roots functioning and tidal mixing (Marchand et al., 2011;Noël et al., submitted), this redox stratification leads to an increasing precipitation of Fe(II)-sulfide minerals downward mangrove sediments (Huerta-Diaz and [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF][START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF]Burton et al., 2008a;2008b;Johnston et al., 2010a;2010b).

This strong mineral gradient play a direct role on the speciation and solubility of redox sensitive elements, notably iron that is the main constituent of the Fe(II)-sulfide minerals that form at depth [START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF][START_REF] Torres | Porewater dynamics and the formation of iron sulfide in a mangrove ecosystem, Sepetiba Bay, Brazil[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]Noël et al., submitted). However, it also plays an indirect role on the transition metal cations because these elements have been demonstrated to trap more or less efficiently at trace levels within Fe(II)-sulfides in sulfidic environments [START_REF] Clark | Redox stratification and heavy metal partitioning in Avicennia-dominated mangrove sediments: a geochemical model[END_REF][START_REF] Morse | Chemical influences on trace métal-sulfide interactions in anoxic sediments[END_REF]Burton et al., 2006a;2006b;2007;2008a;2008b;[START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]Johnston et al., 2010a;2010b;[START_REF] Nath | Trace metal biogeochemistry in mangrove ecosystems: A comparative assessment of acidified (by acid sulfate soils) and non-acidified sites[END_REF].

The relative stability of Fe(II)-sulfides in anoxic horizons of mangrove sediments can then lead this ecosystem to act as a long-term sink for heavy metals, provided the reducing conditions are preserved. Indeed, cumulative effects of aerobic and anaerobic oxidation processes have already been evidenced to affect the capacity of pyritization in mangrove sediments to act as a sink for trace metals [START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF][START_REF] Machado | Trace metal pyritization variability in response to mangrove soil aerobic and anaerobic oxidation processes[END_REF]. Therefore, the production of pyrite and its preservation can be considered as the key parameter for estimating the retention capacity of mangrove sediments towards trace metals. However, in a context of increasing demographic pressure and urbanization of coastal areas, artificial isolation of mangroves from the sea modifies the hydrological conditions at the land-sea interface (Alongi, 2002;[START_REF] Nath | Trace metal biogeochemistry in mangrove ecosystems: A comparative assessment of acidified (by acid sulfate soils) and non-acidified sites[END_REF]. These modifications can yield significant drying and re-oxidation of the surface horizons of mangrove sediments through perturbation of their inundation-dessication cycles and, in turn, alter their trapping capacity towards trace metals through destabilization of Fe(II)-sulfides. This has already been demonstrated for the reoxidation of sulfidic soils that lead to the formation of Coastal Lowland Acid Sulfate Soils (CLASS) (Andriesse and van Mensvoort, 2006;[START_REF] Burton | Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal Lowlands[END_REF]2008a;Claff et al., 2011).

An accurate description of the perturbation of the (bio)geochemical cycles of potentially toxic trace metals caused by such re-oxidation process in relation with the perturbation of the hydrological cycle is then of prime importance to better evaluate the effects of the demographic pressure on metal-rich mangrove sediments.

The objective of the present study was to contribute to this topic by estimating the changes in Fe and Ni speciation in the sediments of a former mangrove that is currently re-oxidizing in response to isolation from the sea after road construction in New Caledonia. In this country, a large fraction of mangroves extend downstream massive lateritic deposits enriched in heavy metals (i.e. essentially Fe and Ni; [START_REF] Becquer | Mineralogical, chemical and charge properties of Geric Ferralsols from New Caledonia[END_REF][START_REF] Quantin | Oxide weathering and trace metal release by bacterial reduction in a New Caledonia Ferralsol[END_REF][START_REF] Perrier | Biogeochemistry of a regolith: The New Caledonian Koniambo ultramafic massif[END_REF][START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF] and precedent investigations showed that this geomorphological context leads to strong accumulation of metals in mangrove sediments (Marchand et al., 2001;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF]. In the present study, Fe and Ni speciation was assessed by X-ray absorption spectroscopy (XAS) because this technique has successfully demonstrated its ability at distinguishing among the wide variety of Fe-bearing phases in mangrove sediments (Noël et al., submitted) and Ni-bearing phases in lateritic materials that are known to be the source of nickel in the studied mangrove sediments [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]2013). XAS data were complemented by more classical mineralogical (XRD) and crystal-chemistry (SEM) analyses in order for depicting the vertical evolution of the quantitative distribution of Fe and Ni species and for discussing the changes in the crystal-chemistry of these elements as a function of the environmental conditions along the studied mangrove sediments. The results obtained were then combined with aqueous chemistry of pore-waters, in order for evaluating the potential consequences of the re-oxidation of metal-rich mangrove sediments after perturbation of the hydrological cycle in response to isolation from the sea. Despite a great diversity of the mangrove flora in New Caledonia, including more than 20 different species [START_REF] Duke | Mangrove biodiversity in New Caledonia. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF], two major species can be found across the whole island. On the one hand, the Rhizophora spp. develops at the lowest position where salinity is equal to that of sea water and it represents more than 50% of the mangrove areas. On the other hand,

VI-B-2. MATERIALS AND METHODS

VI

Avicennia marina develops at higher elevation, at the borders of the salt flat where salinity is the highest, and it represents more than 15% of the mangrove areas, [START_REF] Virly | Atlas des mangroves de Nouvelle-Calédonie. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF]. The studied former mangrove initially developed beneath Avicennia marina forest, but it is now covered with continental herbaceous species since it has been artificially isolated from the sea after road construction there are over 10 years.

VI-B-2.1.2. Solid sediment and pore-waters sampling After extraction of a 70 cm long sediment core with an Eijkelkamp gouge auger, nine solid samples were collected every 5 cm from 0 to 30 cm depth and every 10 cm from 30 to 70 cm depth. These samples were collected under nitrogen flow and conserved under anoxic conditions in order to preserve them from oxidation. Once at the lab, they were introduced in a glove box filled with N 2 atmosphere in order for being vacuum dried, ground with an agate pestle and mortar, homogenized and stored in sealed containers.

In addition to these solid sediments samples, pore-waters were extracted from a second sediment core with soil moisture samplers Rhizon® [START_REF] Song | Novel use of soil moisture samplers for studies on anaerobic ammonium fluxes across lake sediment -water interfaces[END_REF]. After collection in a PVC tube, this second sediment core was introduced in a glove bag filled with nitrogen.

Forteen soil moisture samplers were then directly inserted into the core through holes into the PVC tube and connected to a syringe using luer-lock fittings. The volume of sediment sampled with this setup is estimated at 150 cm 3 for each syringe and around 15 mL of porewater was collected after 3 h of depression in each syringe. These 15 mL were divided in three aliquots of 5 mL each. The first 5 mL were immediately used to analyze the dissolved iron and sulfur speciation. The second 5 mL were stored in sealed glass containers in order for analysis of sulfates, chlorides, nitrates and dissolved organic carbon (DOC). Finally, the third 5 mL were acidified to pH 2 with Suprapur® HNO 3 and stored in acid cleaned 14 mL polypropylene tubes in order for analysis of dissolved metals. All these operations were performed in a glove bag filled with nitrogen.

Pore-water salinity was determined on the field with an Atago refractometer after extracting a drop of pore-water from a third sediment core. The pH and redox potential (Eh) were also measured in situ along this third sediment cores. The pH measurements were performed with a glass electrode (model SenTix® 81) previously calibrated with pH 4.0 and 7.0 standards (NIST scale) and the Eh measurements were performed with a combined Pt and Ag/AgCl (AgCl reference electrode, 25°C) electrode. During these measurements, the pH and Eh electrodes were carefully inserted into the sediment core and the value of the measured parameter was considered only after a stable reading has been reached. Each electrode was thoroughly washed with seawater between two measurements. Calibration of the redox electrode was periodically checked using a 430 mV standard solution and double-distilled water, and data are reported relative to a standard hydrogen electrode (i.e. after adding 202 mV to the values measured at 25 °C).

VI-B-2.2. Chemical analyses

Bulk concentrations of major (Si, Mg, Fe, Al, K, Ca) and trace (Ni, Cr, Mn) elements in the dried solid samples were measured at the Institut de Mineralogie, de Physique des Materiaux et de Cosmochimie (IMPMC) of University Pierre et Marie Curie (UPMC) with a XEPOS™ (Spectro X Lab) X-ray fluorescence spectrometer equipped with a Pd cathode and operating at 50 kV and 40 mA. Total organic carbon (TOC) in solid sediments was determined at the Laboratoire de Geochimie des Eaux (LGE) of University Paris Diderot /IPGP with a SSM-5000A combined with TOC-L Series analyzer on powdered samples.

All analyses for dissolved species were preformed at LGE. Dissolved Fe concentrations in pore-waters were measured by ICP-AES with an ICAP™ (ThermoFisher Scientific) spectrometer after 100 times dilution and dissolved Ni concentration were measured by HR-ICP-MS with a Element2™ (Thermofisher Scientific) equipped with an integrated FAST (ESI) injection system after 300 times dilution. Certified reference materials (SLRS4) were intercalated during the analytical series. Aqueous Fe(II) and H 2 S were measured by colorimetric methods using a SpectroDirect™ (AQUALYTIC) spectrophotometer. Aqueous Fe(II) was analyzed by the ferrozine method of [START_REF] Viollier | Van Cappellen The Ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters[END_REF] at a wavelength of 562 nm (limit of detection = LOD: 0.4 µmol/L) and H 2 S was analyzed by the methylene blue method of [START_REF] Cline | Spectrophotometric determination of hydrogen sulfide in natural waters[END_REF] at a wavelength of 665 nm (LOD: 0.2 µmol/L). Finally, sulfate, chlorides and nitrates concentrations were determined by ion chromatography with Dionex IonPac AS12.

VI-B-2.3. Mineralogical analysis

Mineralogical composition of the solid samples was determined by X-ray powder diffraction (XRD) using CoKα radiation in order to minimize the X-ray absorption of Fe. In order to avoid any changes in the mineralogy related to possible redox reaction with air, samples were mounted within a glove box filled with nitrogen and transferred in an anoxic sample chamber to the X-Ray diffractometer that was equipped with a capton-sealed sample holder. Data were collected in Bragg-Brentano geometry using an X'Pert Pro™ (Panalytical) diffractometer equipped with an X'celerator™ detector. A continuous collection mode was applied over the 3-100° 2θ range with a 0.016° 2θ step and counting 4h for each sample. The morphology, organization and chemical composition of the particles in the mangrove sediments was studied by Scanning Electron Microscopy (SEM) at IMPMC, with a GEMINI Ultra55™ (ZEISS) Field Emission Gun Scanning Electron Microscope. Elemental composition of the particles was performed using EDXS with a 15 kV electron beam collimated with a 60 µm aperture. Samples to be analyzed were deposited as ground powder on carbon tape and coated with a thin carbon film. were needed for Fe K-edge data, whereas 6 to 17 scans were needed to achieve an acceptable signal/noise ratio for Ni K-edge. The actual number of scans depended on the Ni concentration that varied between 720 and 6050 ppm Ni. XAS data were averaged and normalized using the ATHENA software [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]. EXAFS spectra were then background subtracted from these normalized data using the XAFS code [START_REF] Winterer | XAFS -a data analysis program for materials science[END_REF]. Fourier transforms (FT) of the EXAFS spectra were obtained by Fast-Fourier- (Noël et al., in prep., a). The data set also includes several synthetic organometallic Ni complexes like Ni-citrate, Ni-cysteine, and Ni-gluthation (Noël et al., in prep., a). To complete this data set, a Ni-bearing ferrihydrite sample was synthesized according to a protocol derived from that reported by Adra et al. (2013) and analyzed at the Ni K-edge for the present study.

VI

VI-B-2.4.4. Principal Component Analysis (PCA) and Linear Combination Least-Squares Fitting (LC-LSF) of Fe and Ni K-edge EXAFS spectra Fe K-edge EXAFS spectra of all samples from the sediment profile were analyzed by principal component analysis (PCA) [START_REF] Wasserman | EXAFS and principal component analysis: a new shell game[END_REF][START_REF] Ressler | Quantitative speciation of Mn-bearing particulates emitted from autos burning (methylcyclopentadienyl) manganese tricarbonyl-added gasolines using XANES spectroscopy[END_REF] using the SIXPACK code [START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF]. The minimum number of principal components necessary to fit the whole set of experimental EXAFS spectra was chosen on the basis of the minimum value of the factor indicator function [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF][START_REF] Malinowski | Factor Analysis in Chemistry[END_REF][START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF]. Target transformation (TT) was then used to select the EXAFS spectra of model compounds that most likely represent the set of experimental EXAFS spectra [START_REF] Malinowski | Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry[END_REF][START_REF] Ndiba | Heavy metal immobilization through phosphate and thermal treatment of dredged sediments[END_REF]. This selection was based on the quality of the TT fits assessed by the normalized sum of squared residual (NSSR TT :

∑(k 3 χ Target -k 3 χ Transform ) 2 / ∑(k 3 χ Target ) 2 ).
The Fe K-edge spectra of the mangrove sediments were analyzed by linear combination leastsquares fitting (LC-LSF) using the model compounds spectra selected from the TT analysis.

LC-LSF of the EXAFS spectra were performed using a home made software based on Levenberg-Marquardt minimization algorithm [START_REF] Hohmann | Molecular-level modes of As binding to Fe (III) (oxyhydr)oxides precipitated by the anaerobic nitrate-reducing Fe(II)-oxidizing Acidovorax sp. strain BoFeN1[END_REF][START_REF] Pantke | Green rust formation during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1[END_REF][START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF], respectively. Linear coefficients were only constrained to be positive. The accuracy of this fitting procedure ranges between ± 25% and ± 5% of the stated values of each individual contribution, and components below 10% are considered as not significant [START_REF] Ostergren | Quantitative Speciation of Lead in Selected Mine Tailings from Leadville , CO[END_REF][START_REF] Cancès | XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant[END_REF]Juillot et al., 2011). As for the TT procedure, the quality of the fit was assessed on the basis of a normalized sum of squared residual of the following form (NSSR:

∑(k 3 χ exp -k 3 χ calc ) 2 / ∑(k 3 χ exp ) 2 ).
The model compounds used for the analysis of Ni K-edge XANES and EXAFS spectra were essentially chosen on the basis of the results of the analysis of the Fe K-edge XANES and EXAFS spectra. Then, the same LC-LSF procedure than that described for Fe K-edge EXAFS spectra was used to distinguish and quantify the different Ni species in the studied sediments. . These Fe and Ni concentrations are elevated compared to the Fe concentrations ranging from 0.5 to 6 wt% recently measured in Brazilian mangroves [START_REF] Ferreira | Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF] and the Ni concentrations ranging from 0 to 100 mg/kg reported by [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF] ). This observation suggests that this latter Fe-sulfide is an important Febearing mineral species in the deep part of the studied sediments. This hypothesis is in agreement with the XRD results . It also agrees with the results of a precedent investigation, which showed that goethite and pyrite were the major phases involved in the cycling of iron in the mangrove sediments located downstream a lateritized ultramafic watershed in New-Caledonia (Noël et al., submitted).

The results of the Principal Component Analysis (PCA) indicate that the set of the EXAFS spectra from the 10 samples of bulk sediments studied at the Fe K-edge can be explained with three principal components. Indeed the IND parameter reached a minimum value (i.e. 0.064) when considering the three first principal components of the PCA [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF], which explained 90% of the total variance of the system (Table VI-SI-1). Among the model compounds considered as possible candidates for Fe speciation in this study (see Materials and Methods), Target Transformation analysis helped in selecting the most appropriate ones for the Linear Combination Least-Square Fitting procedure. The model compound yielding a Normalized Sum of Squared Residue (NSSR TT = Σ (k 3 χ exp -k 3 χ model ) 2 / Σ (k 3 χ model ) 2 ) lower than 0.15 after this TT procedure were considered as possible components [START_REF] Malinowski | Theory of error for target factor analysis with applications to mass spectrometry and nuclear magnetic resonance spectrometry[END_REF][START_REF] Ndiba | Heavy metal immobilization through phosphate and thermal treatment of dredged sediments[END_REF]. Among all tested model compounds, goethite, pyrite, several Febearing phyllosilicates (Fe-bearing serpentine, illite and nontronite) yielded the lowest NSSR TT values (Table VI-SI-2) and were best reconstructed (Figure VI-SI-1). This suggests studied, the best fits were obtained by including several Fe-bearing phyllosilicates such as illite, nontronite and serpentine as model compounds, in addition with goethite or pyrite (depending on the sample analyzed). Moreover, the standard deviations on the relative proportions of these three Fe-bearing phyllosilicates as fitting components were as large as 50%. This may be explained by the similarities in their Fe K-edge EXAFS spectra (Noël et al., submitted). Consequently, a linear combination of illite, nontronite and serpentine spectra with 30/27/43 respective proportions was used to account for the Fe-clays component for all studied sediment samples. The composition of the EXAFS spectrum for this model compound is the statistical average of all the compositions that provided the best LC-LSF fits of the studied mangrove sediments.

The LC-LSF fits of the EXAFS data from all samples studied at the Fe K-edge were then performed with goethite, pyrite and a Fe-clays model compound that corresponds to a composite of illite, nontronite and serpentine spectra with 30/27/43 respective proportions (Figure VI-SI-1). In addition to these three components, ferrihydrite was also tested during the LC-LSF fits because this mineral has already been identified in mangrove sediments in Brazil [START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF] and in New Caledonia (Noël et al., submitted) and because the TT procedure classified it as a fair component for the studied samples (Table VI-SI-2). The results indicate that such an addition of ferrihydrite improved significantly (more than 25%) the goodness of the LC-LSF fits. This improvement is especially important (i.e. 70% compared to LC-LSF fits performed without ferrihydrite) for the samples collected in the oxic/suboxic part of the studied sediments (Table VI . Such an addition of a ferrihydrite component improved the NSSR of the fits up to 45% (15 cm depth, Table VI-SI-3). On the opposite, addition of other Fe-oxyhydroxides that have also been retained as fair components during the TT procedure (i.e. lepidocrocite, maghemite) did not improve significantly the quality of the LC-LSF fits (Table VI-SI-2). In the same way, addition of schwertmannite (Fe 8 O 8 (OH) 6 SO 4 ) or jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) that have been identified as a major Fe-bearing species in acidic sulfate soils [START_REF] Burton | Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal Lowlands[END_REF]2008;2011) did not improve significantly the quality of the LC-LSF fits (Table VI-SI-2). Finally, despite the large T.O.C of the studied mangrove sediments, addition of Fe(III)-humate as a possible model compound during the LC-LSF fits of Fe K-edge EXAFS data did not improve the quality of the fits.

The results of this LC-LSF fitting procedure performed on all samples studied at the Fe Kedge show that the sediments collected between 05 and 40 cm depth are characterized by a large fraction of Fe hosted by Fe-oxyhydroxides (mean value around 61% of total Fe), with goethite representing between 45 and 76 % of total Fe and ferrihydrite representing between 10 and 30 % of total Fe . With values ranging from 14 to 39% of total Fe, incorporation in phyllosilicates also represents a significant fraction of Fe speciation in these samples . On the opposite, the results of the LC-LSF fitting procedure performed on all samples studied at the Fe K-edge indicate that the sediments collected between 50 and 70 cm depth are characterized by pyrite as the main Fe pool, with proportions ranging from 55% to 87% of total Fe (Figure . The remaining fraction of Fe in these bottom samples is incorporated in Fe-bearing phyllosilicates, with values ranging from 10 to 34% of total Fe . [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF], whereas the occurrence of first-, second-and thirdneighbor peaks at ~1.83 Å, ~2.98 Å and ~3.56 Å (uncorrected for phase shift) is characteristic of the distances found for Ni-bearing iron sulfides (Noël et al., in prep., a).

VI

VI-B-3.3.2 Quantitative analysis of Ni K-edge EXAFS data

The visual inspection of XANES and EXAFS spectra suggests that the LC-LSF fits of the Ni K-edge EXAFS data has to be performed with three-components: a Ni-bearing phyllosilicate, a Ni-goethite, and a Ni-pyrite. However, for each sample studied, several Ni-bearing phyllosilicates (i.e. montmorillonite, serpentine and talc) yielded similar fitting results. Moreover, addition of two or three of these model compounds in a same LC-LSF fit yielded again better fitting results (data not shown). As for Fe, these results are likely due to the similarity between the Ni K-edge EXAFS spectra of these Ni-bearing phyllosilicates, which render them difficult to differentiate [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF] . However, the results of these different LC-LSF fitting solutions show that Ni-sorbed pyrite is the best model compound for the samples collected at 30 and 40 cm depth . The results of this LC-LSF fitting procedure applied on the 7 samples studied at the Ni Kedge show that the sediments collected above 25 cm depth are characterized by a major association of Ni with phyllosilicates (around 70% of total Ni; Figure and a significant association with goethite (around 30% of total Ni; Figure . These two Ni associations also appear in the samples collected between 30 and 40 cm depth, with Ni-clays and Ni-goethite representing about 35% and 25% of total Ni, respectively. In addition, these samples also hold a significant fraction of . In New Caledonia, Avicennia trees are the best adapted to high situation in the intertidal zone, and develop in the borders of saltflat. They are, thus, less frequently immerged by tides, which induce pore-water salinities always higher than the one of seawater, with mean values that can reach up to 50 in the upper sediment, and 90 at depth (Marchand et al, 2011, 2012, andunpublished data). In the present study, the salinity recorded within the first 30 cm depth of the studied core is intermediate between those of seawater (35 g/L) and freshwater (< 0.5 g/L; Figure . This low salinity compared to a conventional mangrove suggests that the studied mangrove is no longer fed by seawater but only by rainwater, which interpreted as the result of its isolation from the sea after road construction. In such a context, rainwater dissolves the salts precipitated during dry periods, which lead to a progressive evolution of the sediments from high-salinity to low-salinity conditions. Although such a decrease of the salinity might have adverse effects on the mangrove trees, [START_REF] Lambs | Mangrove trees growing in a very saline condition but not using seawater. Rapid Communications in Mass Spectrometry Lambs L[END_REF] showed that mangroves subjected to freshwater inflows could continue to exist. At the studied site, it is considered that the sediments within the first 25 cm depth are regularly leached by rainwater, but without being permanently saturated. The salinity decrease then probably results from the long-term alternation of the saturated/unsaturated conditions that lead to alternative salts dissolution/precipitation events and, in turn, to a decrease of the salinity. It is not observed at depth either because the salts dissolved near the surface are transported downward with percolating rainwater or because the bottom horizon of the studied mangrove are still under the tide influence through groundwater flow.

The decrease of the salinity at the surface of the studied mangrove sediments likely explains their colonization by Achrostichum aureum, although they are located at a place that was initially covered with Avicennia marina species (Marchand et al., 2011). Since [START_REF] Scholander | Gas exchange in the roots of mangrove[END_REF], it is known that Avicennia pneumatophores constitute a pathway for the exchange of oxygen between atmosphere and sediments. In addition, the Avicennia cable root system has the capacity to diffuse oxygen into the sediment, creating oxidized rhizosphere [START_REF] Thibodeau | Differential oxidation of mangrove substrate by Avicennia germinans and Rhizophora mangle[END_REF]. Consequently, sediments beneath Avicennia trees are usually characterized by oxic to suboxic conditions in the upper root layer. In addition, the fact that these trees develop at an intermediate position in the tidal zone increases the exchange with the atmosphere, not being always saturated (Marchand et al., 2011). However, redox values measured within Avicennia stands in New Caledonia usually ranged between 200 and 300 mV in the upper 20 cm, and between 0 and 200 mV below (Marchand et al., 2011a[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. Consequently, the oxic to suboxic conditions measured, in this study, along the first 50 cm depth (300 -490 mV; Figure . This high-oxygen level is likely explained by the above-mentioned inputs of oxic rainwater in the surface horizons of the studied sediments, together with the lack of replenishment by seawater after evaporation upon dry periods.

The pH of the studied sediments, especially along the 50 first cm depth, is considerably more acidic (i.e. between 2.9 and 5.2; Figure than that usually measured beneath Avicennia stand in New Caledonia (close to 6 ; Marchand et al, 2011;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF]. This observation, together with the high Eh values, suggests that the oxygen inputs in relation with rainwater favor oxidation reactions that release H + ions in the pore-waters. The large concentrations in nitrates (Table VI-SI-1), suggest that the intense (bio)degradation of organic material is one of these oxidation reactions, at least at the surface of the studied sediments (Marchand et al., 2006b). Indeed, both constituents result from the degradation of organic material by the activity of denitrifying bacteria [START_REF] Herbert | Nitrogen cycling in coastal marine ecosystems FEMS[END_REF] and this reaction is known to release H + ions in the porewaters [START_REF] Duke | Mangrove biodiversity in New Caledonia. Typologies et Biodiversité des mangroves de Nouvelle-Calédonie[END_REF][START_REF] Machado | Trace metal pyritization variability in response to mangrove soil aerobic and anaerobic oxidation processes[END_REF]. However, in such a context of increased oxygen levels in former reduced sediments where Fe(II)-sulfides would have formed, the chain reactions that probably contributes at the largest extent to the low pH is oxidativedissolution of these Fe(II)-sulfides followed by oxidation of aqueous Fe(II) to Fe(III) and precipitation of Fe(III)-oxyhydroxides [START_REF] Sullivan | Iron sulfide oxidation and the chemistry of acid generation[END_REF]Burton et al., 2006;Johnson et al., 2013). This assumption is supported by the SO 4 2-/Cl -molar ratio measured in the porewaters that is higher than that of seawater all along the studied core. This ratio is around 0.16 down to 25 cm depth, where it starts to increase to reach a value up to 4 times greater than that of seawater around 45 cm and 60 cm depth . Moreover, precedent studies performed on non-perturbed mangroves in New Caledonia indicated that, beneath Avicennia marina, the sediments above 20-25 cm depth can be considered as non-pyritized, whereas those below 20-25 cm depth can be considered as pyritized (Marchand et al., 2011;Noël et al., submitted). Finally, the vertical evolution of the SO 4 2-/Cl -molar ratio measured in the studied core is correlated to that of the total Fe concentration in the pore-waters . All these observations fully support the hypothesis of an oxidation of the Fe(II)-sulfides pool in the studied sediments, as already reported for other mangroves [START_REF] Giblin | Pyrite formation in marshes during early diagenesis[END_REF][START_REF] Johnston | The acid flux dynamics of two artificial drains in acid sulfate soil backswamps on the Clarence River floodplain, Australia[END_REF][START_REF] Macdonald | Impacts of runoff from sulfuric soils on sediment chemistry in an estuarine lake[END_REF].

All the above observations indicate then that the evolution from near-neutral anoxic/suboxic conditions towards acidic suboxic/oxic conditions resulting from isolation from the sea modified the vegetative cover that evolved from Avicennia marina to herbaceous continental species and had an influence on the physico-chemical parameters along the studied mangrove sediments. Since the studied mangrove is located downstream Ni-rich lateritic regoliths that developed in ultramafic rocks, it is particularly enriched in Fe and Ni and these modifications of the physico-chemical parameters likely influenced, in turn, the speciation of these elements. However, the downward evolution of the physico-chemical parameters suggests a vertical gradient for this influence, which means that the potential consequences of this oxidation/acidification on the biogeochemical cycles of Fe and Ni has to be addressed has a function of depth.

VI-B-4.2. Consequences of the re-oxidation of the studied mangrove sediments on the Fe cycling

Above 25 cm depth, Fe K-edge XAS data indicate that goethite is the most abundant Fe(III)oxyhydroxides, which is in agreement with the fact that this mineral species is the most abundant Fe(III)-oxyhydroxides in the lateritic regoliths developed in the ultramafic rocks of New Caledonia [START_REF] Perrier | Biogeochemistry of a regolith: The New Caledonian Koniambo ultramafic massif[END_REF]Fandeur et al., 2009;[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]. Indeed, the studied mangrove is located downstream some of these lateritic regoliths and erosion events related to tropical storms are then responsible for the large inputs of lateritic material [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. In addition to goethite, Fe K-edge XAS data also indicate the occurrence of Fe as ferrihydrite and Fe-bearing phyllosilicates .

Fe-bearing phyllosilicates have also been identified in the lateritic regoliths developed in the ultramafic rocks of New Caledonia [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF]. Together with goethite, this mineral species is then considered as inherited from erosion. On the opposite, ferrihydrite is considered to entirely result from the oxidation of former Fe(II)-sulfides and/or aqueous Fe(II) species [START_REF] Cornell | The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. Indeed, despite the suboxic/oxic conditions that prevail within the 25 first cm depth of mangrove sediments, it is known that the concomitant release of dissolved sulfide upon (bio)reduction of sulfates from seawater and ferrous iron upon ferric (bio)reduction of Fe(III)-oxyhydroxides can lead to the pyritization of Fe(III)-oxyhydroxides [START_REF] Queens | Pyrite formation under conditions approximating those in anoxic sediments I. Pathway and morphology[END_REF][START_REF] Burton | Sulfur biogeochemical cycling and novel Fe-S mineralization pathways in a tidally re-flooded wetland[END_REF][START_REF] Hellige | Pathways of ferrous iron mineral formation upon sulfidation of lepidocrocite surfaces[END_REF]Noël et al., submitted). The past colonization of the studied mangrove sediments by Avicennia marina whose root system is known to diffuse oxygen down to 20-25 cm depth is then in agreement with the hypothesis of an oxidation of former Fe(II)-sulfides and/or aqueous Fe(II) species, followed by precipitation of Fe(III)-oxyhydroxides, to explain the occurrence of ferrihydrite in the surface horizons. This hypothesis has already been proposed

to explain the occurrence of both ferrihydrite and lepidocrocite in the mangrove sediments of the Vavouto Bay in New Caledonia (Noël et al. in press). In mangroves connected with the sea, the tidal cycles induce a balance between pyritization of Fe(III)-hydroxides and oxidative dissolution of Fe(II)-sulfides followed by precipitation of Fe(III)-oxyhydroxides, which explains the near-neutral pH in the upper horizons of the sediments [START_REF] Marchand | Relations entre les caractéristiques physico-chimiques des sediments de mangrove et le type des palétuviers -Implications sur la repartition spatiale des espèces[END_REF]2011;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF][START_REF] Nath | Trace metal biogeochemistry in mangrove ecosystems: A comparative assessment of acidified (by acid sulfate soils) and non-acidified sites[END_REF]. In the studied mangrove, the oxidation of the surface sediments induced by the isolation from the sea likely generated a stronger oxidation of the former Fe(II)-sulfides and/or aqueous Fe(II) species, which in turn induced a larger precipitation of Fe(III)-oxyhydroxides. This perturbation of the oxidation/reduction cycling between Fe(II)-sulfides and Fe(III)-oxyhydroxides likely explains the more acidic pH measured in the surface horizons of the studied mangrove sediments (Figure VI-5a), compared to the pH values usually reported for the upper horizons of mangrove sediments [START_REF] Marchand | Relations entre les caractéristiques physico-chimiques des sediments de mangrove et le type des palétuviers -Implications sur la repartition spatiale des espèces[END_REF]2011;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF][START_REF] Nath | Trace metal biogeochemistry in mangrove ecosystems: A comparative assessment of acidified (by acid sulfate soils) and non-acidified sites[END_REF].

Between 25 cm and 50 cm depth (i.e. between 25 cm and 45 cm depth) correspond to the re-oxidation front within the studied mangrove sediments after their isolation form the sea.

Considering the high sulfates concentration in the pore-waters of these horizons, precipitation of secondary Fe(III)-hydroxy-sulfates might have also occurred after oxidation of pyrite.

Indeed, in such a context of sulfates-rich pore-waters, the nature of precipitating Fe(III)oxyhydroxides or Fe(III)-hydroxy-sulfates depends on pH, aqueous Fe and ligands speciation and kinetic constraints (Schwertmann and Murad, 1983;[START_REF] Parkhurst | User's Guide to PHREEQC (Version 2) -A Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations[END_REF][START_REF] Majzlan | Thermodynamics of iron oxides: Part III. Enthalpies of formation and stability of ferrihydrite (!Fe(OH)3), schwertmannite (!FeO(OH)3/4(SO4)1/8), and e-Fe2O3[END_REF]. For instance, in acidic to near-neutral sulfate-rich waters precipitation of schwertmannite will be favored, whereas ferrihydrite, lepidocrocite and goethite will precipitate in low-sulfates equivalent waters with increasing pH [START_REF] Regenspurg | Formation and stability of schwertmannite in acidic mining lakes[END_REF][START_REF] Majzlan | Speciation of iron and sulfate in acid waters: aqueous clusters to mineral precipitates[END_REF]. In addition, although precipitation of schwertmannite will be favored in acidic sulfates-rich waters, this mineral species will progressively transform towards goethite upon ageing [START_REF] Bigham | Schwertmannite and the chemical modeling of iron in acid sulfate waters[END_REF][START_REF] Jonsson | Schwertmannite precipitated from acid mine drainage: phase transformation, sulphate release and surface properties[END_REF]Burton et al., 2008).

These various Fe(III)-oxyhydroxides and/or Fe(III)-hydroxy-sulfates could then account for the Fe(III)-species precipitated after oxidation of aqueous Fe(II) to Fe(III) in these re-oxidized horizons (i.e. between 25 cm and 45 cm depth). This hypothesis is in agreement with the visual inspection of the studied sediments that show a yellow brown color for these horizons, whereas the other horizons show a more dark brown color . It is also in agreement with the significant fraction of ferrihydrite found in these horizons during the LC-LSF analysis of the Fe K-edge EXAFS data . This LC-LSF analysis also suggests the occurrence of schwertmannite, as indicated by the improved quality of the fits performed on some of the samples collected between 25 cm and 45 cm depth (Table VI-SI-5). However, the Fe K-edge EXAFS spectrum of this Fe(III)-hydroxy-sulfate is very similar to that of goethite [START_REF] Maillot | New insight into the structure of nanocrystalline ferrihydrite : EXAFS evidence for tetrahedrally coordinated iron (III)[END_REF][START_REF] Maillot | Structure and reactivity of As(III)-and As(V)-rich schwertmannites and amorphous ferric arsenate sulfate from the Carnoulès acid mine drainage, France: Comparison with biotic and abiotic model compounds and implications for As remediation[END_REF], which is the major Fe(III)-species identified in these horizons . As a consequence, although the occurrence of small amounts of schwertmannite can not be dismissed and despite the very high SO 4 2-/Cl -molar ratio measured in the pore-waters of these horizons, only ferrihydrite and goethite were retained as the major Fe-oxyhydroxides in the samples collected between 25 cm and 45 cm depth. Among these two Fe(III)-oxyhydroxides, goethite is considered to be mainly inherited from erosion, although a small fraction could result from the ageing of ferrihydite and/or schwertmannite (Schwertmann and Murad, 1983;Cornell and Giovanelli, 1985;1986;[START_REF] Bigham | Schwertmannite and the chemical modeling of iron in acid sulfate waters[END_REF][START_REF] Jonsson | Schwertmannite precipitated from acid mine drainage: phase transformation, sulphate release and surface properties[END_REF]Burton et al., 2008). On the opposite, as already mentioned above, ferrihydrite is considered to entirely result from the oxidation of Fe(II)-sulfides and/or aqueous Fe(II) species [START_REF] Cornell | The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses[END_REF][START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF]. In addition to pyrite, intermediate Acid Volatile Sulfides (AVS) like mackinawite or greigite can form during pyritization of Fe(III)-oxyhydroxydes [START_REF] Morse | The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters[END_REF]. However, these AVS were not detected in the bottom horizons of the studied sediments. Such an absence could be related to the intensity of the pyritization since past studies have demonstrated that AVS are not stable and tend to evolve towards pyrite, when pyritization is very intense [START_REF]Acid volatile sulfide ( AVS )[END_REF] 6c). Another possible explanation for such an absence of dissolved Fe(III) in the pore-waters of the bottom horizons is that a fraction of dissolved Fe is indeed oxidized to Fe(III), like at the overlying re-oxidation front, but this fraction of dissolved Fe(III) is rapidly reduced back to Fe(II) upon reaction with the surface of the large amounts of pyrite that remain in the bottom horizons because the anoxic/suboxic conditions are preserved [START_REF] Morse | The chemistry of the hydrogen sulfide and iron sulfide systems in natural waters[END_REF]Rimstidt and Vaughan, 2003). In this context, the precipitation of Fe(III)-oxyhydroxides is not favored, which explains the lack of detection of these mineral species by Fe K-edge XAS data . The quite high pH values measured in these bottom horizons (i.e. between 5 and 6; Figure , and the fact that these pH values are close to those usually recorded in mangrove sediments (Marchand et al, 2006;2011;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF], are in agreement with this hypothesis of an absence of precipitation of Fe(III)-oxyhydroxides due to a lack of oxidation of dissolved Fe(II) to Fe(III).

Below 50 cm depth,

VI-B-4.3. Consequences of the re-oxidation of the studied mangrove sediments on the Ni cycling

Above 25 cm depth, Ni K-edge data indicate that Ni is associated with goethite and phyllosilicates . Since these mineral species were identified as the major Ni-bearing species in the lateritic regoliths [START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF], they are considered to result from erosion. The oxic/suboxic conditions that prevail in the surface horizons of the studied mangrove sediments (Figure VI-5a) are favorable to the stability of these Ni-bearing goethite and phyllosilicates. However, the Ni concentration measured in the pore-water of these horizons is significantly larger than that already measured in other mangrove sediments from New Caledonia (Figure VI-6d and Table VI-SI-2; [START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. This difference can be explained by the significantly low pH measured in these horizons compared to that reported for the same surface horizons in other mangrove from New Caledonia (i.e. pH 3-5 vs. pH 6-7; Marchand et al., 2011;[START_REF] Bayen | Occurrence, bioavailability and toxic effects of trace metals and organic contaminants in mangrove ecosystems : A review[END_REF]. In such acidic conditions, the stability of Nibearing goethite and phyllosilicates is significantly decreased and a fraction of these mineral species likely dissolves and releases Ni in the pore-waters. Moreover, in such acidic conditions, the sorption capacity of the remaining fraction of goethite and phyllosilicates towards dissolved Ni is also significantly decreased [START_REF] Elzinga | Reaction condition effects on nickel sorption mechanisms in illite-water suspensions[END_REF][START_REF] Trivedi | Ni and Zn Sorption to Amorphous versus Crystalline Iron Oxides: Macroscopic Studies[END_REF][START_REF] Xu | Ni(II) complexation to amorphous hydrous ferric oxide: An X-ray absorption spectroscopy study[END_REF][START_REF] Arai | Spectroscopic Evidence for Ni(II) Surface Speciation at the Iron Oxyhydroxides at Water Interface[END_REF] -11). The difference between these two levels of Ni incorporation within the structure of pyrite is interpreted as resulting from the direct influence of this element on the crystal growth and stability of this mineral species (Noël et al., submitted).

In addition to incorporation within the structure of pyrite, Ni K-edge EXAFS data indicate that half of total Ni occurs as Ni sorbed at the surface of pyrite . This second mode of Ni association with pyrite is unusual for Ni-rich mangrove sediments of New Caledonia where connection with the sea is conserved since, in that case, the bottom horizons are rather characterized by a total incorporation of Ni within the structure of Fe(II)sulfides (Noël et al., in prep., a). As already mentioned for Fe, the anoxic/suboxic conditions measured and the pH values similar to those usually reported for bottom horizons of the mangrove sediments suggest that isolation from the sea did not directly modified the physicochemical conditions in the bottom horizons of the studied mangrove sediments. These observations then strongly suggest that the significant fraction of Ni sorbed at the surface of pyrite does not result from dissolution/precipitation cycles of Ni-bearing pyrite that could have been generated upon partial re-oxidation of these bottom horizons. This fraction of Ni sorbed at the surface of pyrite is then considered to result from the downward percolation of dissolved Ni from the overlying re-oxidation front. This hypothesis is in agreement with the already reported affinity of Ni and other divalent cations for the surface of Fe(II)-sulfides [START_REF] Kornicker | The interactions of divalent cations with the surface of pyrite[END_REF][START_REF] Parkman | Reactions of copper and cadmium ions in aqueous solutions with goethite, lepidocrocite, mackinawite, and pyrite[END_REF]. It is also in agreement with the significant lower Ni concentration measured in the solid fraction of the intermediate horizons (i.e. at the re-oxidation front), compared to that measured in the solid fraction of the bottom horizons . This latter observation suggest that, although it reduces the storage capacity of the sediments at the reactive front, the re-oxidation of the studied mangrove after isolation from the sea strongly amplifies the storage capacity of the bottom anoxic/suboxic horizons towards Ni through a combination of co-precipitation with Fe(II)-sulfides and sorption at the surface of these mineral species.

VI-B-5. CONCLUDING REMARKS

The results of this study indicate that the re-oxidation of former reduced mangrove sediments in response to their isolation from the sea can lead to a large release of metals in the porewaters. This release is especially important at the re-oxidation front where former Fe(II)sulfides are strongly oxidized and precipitation of secondary Fe(III)-oxyhydroxides generates very acidic conditions. In this context, the dissolved metals move downward in the underlying anoxic horizons where they are trapped upon interaction with Fe(II)-sulfides (either incorporation within the structure or sorption at the surface). This study then emphasizes the importance of Fe(II)-sulfides for mitigating the dispersion of metals in former mangrove sediments that oxidized after their isolation from the sea. However, the efficiency of this mitigation is strongly dependant on the stability of the anoxic/suboxic conditions in the bottom horizons, which means that it might be significantly decreased for long-term isolation from the sea. Moreover, reverse situations with re-connection of the mangrove sediments with the sea might also yield an increased mobility of metals. This increased mobility of metals might be due to the restoration of reducing conditions in oxidized surface horizons, as already demonstrated for tidally re-inundated degraded acid sulfate soils wetlands (Claff et al., 2011).

It could also simply results from an increased in the ionic strength of the pore-waters that would yield a competitive desorption of metals from Fe(II)-sulfides in the bottom nonoxidized horizons, as already described by Wong et al. (2010 ;2013). All these observations emphasize the need for further long-term monitoring of mangrove sediments after isolation from the sea in order to evaluate the actual consequences of this anthropogenic forcing on the metals mobility at the land-sea interface. [START_REF] Malinowski | Determination of the number of factors and the experimental error in a data matrix[END_REF]. Cependant, la dynamique des marées est susceptible de modifier temporairement les conditions d'oxygénation, en particulier lors de l'apport d'eau de mer saturée en oxygène à la marée montante [START_REF] Caetano | Tidal flushing of ammonium, iron and manganese from inter-tidal sediment pore waters[END_REF]. Les conditions anoxiques peuvent alors de nouveau se rétablir en quelques heures [START_REF] Canfield | The reactivity of sedimentary iron minerals toward sulfide[END_REF]

Figure VII-2 : Présentation générale du cycle du Fe à l'interface oxique-anoxique du sédiment

En conditions anoxiques, une partie des oxyhydroxydes ferriques sont susceptibles d'être réduits, soit par l'activité des bactéries ferri-réductrices, soit par la réaction chimique de Fe(III) avec H 2 S résultant de l'activité des bactéries sulfato-réductrices (Wei et al., 1996 ;Rickard et al., 2007). Le Fe(II) ainsi produit précipite avec les polysulfures sous forme de Une oxygénation partielle du sédiment peut se produire par introduction d'oxygène dans le sédiment, soit i) par bioturbation [START_REF] Ferreira | Effects of bioturbation by root and crab activity on iron and sulfur biogeochemistry in mangrove substrate[END_REF], soit ii) par diffusion de l'oxygène par le système racinaire des palétuviers [START_REF] Kristensen | Organic carbon dynamics in mangrove ecosystems: A review[END_REF], ou alors iii) par les va-et-vient des marées [START_REF] Caetano | Tidal flushing of ammonium, iron and manganese from inter-tidal sediment pore waters[END_REF]. L'oxygénation partielle du sédiment, a pour conséquence d'oxyder les sulfures de fer tels que les pyrites, libérant ainsi le Fe(II) qui peut s'oxyder en et al., 1969).

VII-B. La pyritisation des éléments traces métalliques

La pyrite apparait comme la phase cristalline majeure dans les conditions anoxiques des sédiments de mangrove (cf. Chapitre III). D'un point de vue plus général, cette phase domine une grande part des sédiments marins anoxiques, caractérisés par un enrichissement en Fe. La latérisation intense, en conditions tropicales, des roches ultrabasiques de Nouvelle-Calédonie, est à l'origine de profils latéritiques enrichis en Cr (jusqu'à 10 % en poids), ainsi qu'en Co (jusqu'à 2 % en poids). Tout d'abord, le chrome est un élément potentiellement toxique, ainsi cette grande quantité de chrome dans les sols latéritiques est d'une grande préoccupation pour l'environnement. En effet, les ions Cr 3+ qui pourraient s'accumuler dans les sédiments de mangrove suite à la sédimentation détritique sont susceptibles de s'oxyder par interaction avec les oxydes de manganèse (Fendorf et Zasoski, 1992;Silvester et al., 1995;Weaver et Hochella, 2003) en Cr 6+ , classés comme cancérigène par l'Agence Internationale pour la Recherche sur le Cancer (AIRC). D'autre part, le cobalt est également toxique lorsque les quantités concernées deviennent importantes (la limite de toxicité pour les plantes est de 13 mg/kg, selon l'EPA, Environmental Protection Agence) ; la forme réduite étant généralement la plus toxique (Borch et al., 2010). Ainsi, l'étude du comportement de ces ETM dans les sédiments de mangrove semblent en effet capitale afin d'évaluer leur mobilité au sein de ces sédiments.

Actuellement, nos études ont montré que le cobalt a une forte affinité pour la pyrite, et peut accélérer la précipitation de la pyrite en présence de mackinawite (cf. Chapitre IV). Ces particularités nous laissent suggérer que son comportement dans les sédiments de mangrove, contrôlé par la pyritisation, est singulièrement proche de celui du nickel. Cependant, le manganèse, qui ne présente pas d'affinité avec les sulfures de fer (mackinawite et pyrite, cf. [START_REF] Macfarlane | Zinc distribution and excretion in the leaves of the grey mangrove Avicennia marina (Forsk.) Vierh[END_REF]. En outre, des échantillons de tissus des palétuviers ont montrés une augmentation des concentrations de métaux par rapport aux sédiments, soutenant l'idée que les palétuviers peuvent bio-accumuler des éléments métalliques [START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF]. D'autre part, les racines des palétuviers peuvent servir de barrières au transfert des ETM. En effet, des concentrations élevées en ETM ont été mesurées dans les systèmes racinaires par rapport aux parties hautes des palétuviers [START_REF] Tam | Accumulation and distribution of heavy metals in a simulated mangrove system treated with sewage[END_REF]Mac-Farlane et Burchett, 2002). À l'heure actuelle, ce processus est mal compris et nécessite un investissement considérable en travail pour mieux comprendre les processus impliqués (Ebrahimpour et Mushrifah, 2008;[START_REF] Gonzalez-Mendoza | C o o r d i n a t e d r e s p o n s e s of phytochelatin synthase and metallothionein genes in black mangrove, Avicennia g e r m i n a n s , exposed to cadmium and copper[END_REF]Machado et al., 2002).

Une première étude d'analyse des concentrations des ETM vers les différents organes (racines, tiges, feuilles) des palétuviers pourrait être menée afin d'estimer le transfert des ETM vers les différents organes (racines, tiges, feuilles) des palétuviers, et de comprendre le rôle de la végétation sur le devenir des ETM. (projet en cours porté par le CEREGE) Finalement, cette étude pourrait également être étendue à l'ensemble de l'écosystème des mangroves afin d'estimer la capacité de bioaccumulation des métaux lourds dans les différents organismes de la faune de cet écosystème, tels que les poissons et les crabes, qui représentent notamment un ressource alimentaire essentielle pour les populations locales.

  Un regard particulier est porté sur les systèmes côtiers et récifaux. En effet, le transport terrigène peut affecter les milieux en aval, engorgeant les rivières, dégradant la qualité des eaux et étouffant les écosystèmes littoraux sensibles à la turbidité. C'est pourquoi, l'étude d'impact suite à la réalisation de projets susceptible d'affecter le domaine public maritime est obligatoire et indispensable (Arrêté n° 2002-1567 du 30 mai 2002 relatif aux études d'impact préalables à la réalisation d'aménagements ou d'ouvrages prévues dans la loi de pays n° 2001-017 du 11 janvier 2002 sur le domaine public maritime de la Nouvelle-Calédonie et des provinces). C'est dans ce contexte général que s'inscrit cette thèse, financé par Koniambo Nickel SAS, société d'exploitation minière de Ni du massif du Koniambo (province Nord, Nouvelle-Calédonie).

-

  et prédire les effets potentiels de l'exploitation minière et de l'aménagement des installations relatives à l'entreprise, sur cet écosystème Ce projet de recherche est développé au sein de l'Institut de Minéralogie et de Physique des Milieux Condensés (IMPMC) de Paris VI, en partenariat avec l'Institut de Recherche pour le développement (IRD) de Nouméa, l'Université de la Nouvelle Calédonie (UNC), et le Laboratoire de Géochimie des Eaux (LGE) de l'Institut de Physique du Globe de Paris (IPGP, Paris VII).

  présente une synthèse et une discussion conduites en reprenant les résultats majeurs et elle propose des perspectives de recherche.La mangrove, écosystème singulier à l'interface entre le milieu terrestre et le milieu marin, est souvent définie comme étant l'ensemble des arbres, arbustes et herbiers à caractère halophile,

  l'élévation topographique (Figure I-2).

Figure I- 2 :

 2 Figure I-2 : Courbe de fréquence sellons laquelle chaque espèce végétale occupe un niveau topographique, et rapport de ces courbes de fréquence avec les hauteurs caractéristiques des marées dans une mangrove de Nouvelle Calédonie (d'après[START_REF] Baltzer | Les formations végétales associées au delta de la Dumbéa (Nouvelle Calédonie) et leurs indications écologiques, géomorphologiques et sédimentologiques mises en évidence par la cartographie[END_REF].

-

  Figure I-2), balayées moins régulièrement par les marées (quelques heures par semaine -mois). -Les Tannes présentent des salinités très élevés (>100g.L -1 ; ZONECO, 2007). La salinité de cette zone est bien trop importante pour que les palétuviers puissent s'y développer, ce qui implique que ces zones soient uniquement couvert de quelques salicornes, voir dépourvues de végétation. Le milieu est généralement suboxique sur les 50 premiers centimètres. Ils se développent dans les zones à plus forte topographie, et ne sont immergés qu'en période de forte marée (1,6 -1,8 m au-dessus du 0 des cartes marines ; Figure I-2).

Figure I- 3 :

 3 Figure I-3 : Coupe topographique schématisée de la zone intertidale d'une mangrove de Nouvelle-Calédonie, et répartition des gradients de salinité, des conditions redox, et du carbone organique total.

  est reconnue comme un écosystème ayant développé de fortes capacités à la rétention des métaux sur le court terme, il semble plus complexe d'évaluer la biodisponibilité et la mobilité de ces contaminants sur le long terme du fait de l'alternance des conditions redox.

Figure I- 5 :

 5 Figure I-5 : Localisation des massifs de péridotites ou massifs ultrabasiques de Nouvelle-Calédonie. D'après les travaux de l'ORSTOM dans « Bulletin APBG n°2. 1990 »

  et localement jusqu'à 40 %pds Ni dans des veines. Ces assemblages sont alors appelés garniérite qui constitue le minerai classique de Ni en Nouvelle-Calédonie. -L'unité supérieure latéritique est généralement dominée par la goethite, α-FeOOH. Elle contient aussi des proportions plus ou moins grandes d'oxydes de manganèse (asbolane, lithiophorite, birnessite, cryptomélane, ramsdellite, todrokite), d'hématite (α-Fe 2 O 3 ), de maghémite (γ-Fe 2 O 3 ), de magnétite (Fe 3 O 4 ), de chromite (FeCr 2 O 4 ), de gibbsite (Al(OH) 3 ), et de phyllosilicates résiduels (serpentines et talcs). Les teneurs en Ni dans ces

  processus d'altération et d'érosion actuels de ces massifs ultrabasiques altérés, sont susceptibles de charger les eaux de pluies des bassins versants en aval des mangroves, de particules enrichies en ETM (Figure I-6). A cet égard, Marchand et al. (2012) montrent que les concentrations de Ni dans les sédiments de mangrove de Nouvelle-Calédonie, sont 10 à 100 fois supérieure en aval de massifs ultrabasiques, à celle en aval de massifs qui ne sont pas constitués de roches ultrabasiques.

Figure I- 6 :

 6 Figure I-6 : Schéma simplifié du couple érosion -sédimentation en Nouvelle-Calédonie (modifié d'aprèsBaltzer et Trecases, 1971).

  Nouvelle-Calédonie offre les conditions requises pour l'étude des effets anthropiques sur les paramètres physico-chimiques et le cycle des métaux au sein des sédiments de mangrove. I-C.3.2. Pression climatique La Nouvelle Calédonie subit plusieurs influences météorologiques et géographiques qui créent un climat très contrasté et irrégulier de type océanique (chaud et humide).

Figure I- 7 :

 7 Figure I-7 : Carte des moyennes annuelles de précipitations en mm, reconstituée par la méthode Aurelhy sur la période 1971-2000 au pas de 4 km (isohyètes tous les 250 mm). D'après Météo France (2004).

  Nouvelle-Calédonie pour avoir un impact sur la distribution et le fonctionnement des mangroves, en effet, durant ces 25 dernières années, il y a eut 6 passages cycloniques dans le sud du territoire et 11 passages dans le nord. Par exemple, le cyclone Erica, en 2004, a largement affecté la mangrove de Ouémo à Nouméa. Outre l'impact direct de destruction dû à la force des houles cycloniques, les cyclones induisent une forte sédimentation en zone lagunaire au débouché des rivières, particulièrement dans les bassins versants touchés par l'exploitation minière. Du fait de sa position à l'interface entre Terre et Mer, la mangrove de Nouvelle-Calédonie subit de fortes pressions environnementales naturelles à la faveur dde la baie de Vavouto Localisation et description de la zone d'étude La mangrove étudiée se situe au Nord-Ouest de la Nouvelle Calédonie, à 270 km au nord de Nouméa, au nord de Koné et au sud de Voh. Elle se développe en aval du massif ultrabasique du Koniambo, qui s'étend sur 20 km de long (orienté NW-SE) et sur 6 à 10 km de large avec une altitude maximale de 930 m. Le socle du massif est constitué pour l'essentiel de harzburgite, avec des intrusions de dunite et de gabbro, et repose sur un substrat d'origine basaltique (Fandeur, 2009). Le massif est latéritisé sur environ 21km 2 . Il forme deux bassins versants à l'Ouest (rivières Pandanus et Taléa) et un bassin versant à l'Est, Voh (215km 2 ), Le bassin de la Taléa est constitué des 2 sous-bassins de la Rivière Coco (Creek Coco) (41,5 km 2 ) et de la Rivière Mine (Creek Mine) (9,5 km 2 ) (Etude d'impact environnemental et social, Projet Koniambo, 2005).

  l'année. La pluviométrie annuelle moyenne sur le massif du Koniambo est de 2200 mm avec une variabilité interannuelle pouvant être très importante (à Koné : 1970 mm en 1999 et 770 mm en 2001). Les quantités de précipitation et les températures varient en fonction de l'altitude et des saisons. La comparaison des taux d'évapotranspiration et de la pluviométrie mensuelle fait ressortir un déficit en eau entre septembre et décembre. Les précipitations sont très variables et quelques événements pluvieux majeurs peuvent être responsables de la précipitation totale enregistrée dans un mois donné.

Figure II- 1 :

 1 Figure II-1 : (a) Localisation de la baie de Vavouto située en aval du massif ultrabasique du Koniambo. (b) Localisation des bassins versants du massif du Koniambo. (c) Les deux sites étudiés sont également localisés, avec d'une part la mangrove de dite « de référence » (en bleu ; en aval de creek Coco), et d'autre part la mangrove potentiellement sous la pression d'une ancienne intense exploitation minière (en rouge ; en aval de creek Mine).

  d'exploitation à 25 ans ont une emprise sur l'ensemble des basins versants, mais les données recueillies dans le cadre de l'étude d'impact environnementale de 2005, soulignent que la position géographique par rapport au site prochainement exploité et le faible débit (faible dilution), font de Creek Coco et de Creek Mine, les sources potentielles de pollution les plus préoccupantes pour la mangrove. Une première exploitation minière du massif du Koniambo a déjà été réalisée par Falconbrige dans les années 1980 à 1990. Cette dernière exploitait la partie du massif située sur le bassin versant de Creek Mine. L'exploitation minière peut donc avoir affecté la mangrove alimentée par ce bassin versant. Au contraire, Creek Coco n'a subi aucun apport en métaux qui serait d'origine anthropique car aucune carrière n'a été exploitée à cet endroit durant tout le XXe siècle. La présence éventuelle d'ETM le long de ce bassin versant et dans la mangrove alimentée par celui-ci, est principalement contrôlée par l'érosion naturelle, accentuée par les fortes dépressions et les cyclones. Ainsi, la zone d'étude est naturellement divisée en deux sites (Figure II-1) : -un premier site correspondant à une zone de la mangrove alimentée par un système fluvial encore non affecté par l'exploitation minière (rivière Coco / Creek Coco), appelé mangrove de référence -et un second site correspondant à une zone de la mangrove alimentée par un réseau fluvial situé au niveau d'une ancienne zone d'exploitation des minerais nickélifères (rivière CF / Creek Mine), appelé mangrove de Creek Mine Sur le terrain, ces deux sites se distinguent visuellement. En effet, en période de crue, la rivière CF de Creek Mine est particulièrement enrichie de matières en suspensions, tandis que la rivière Coco de Creek Coco est moins turbide (Figure II-1). Plus spécifiquement, les sédiments en surface de la mangrove sous influence minière se distinguent de la mangrove de référence par des sédiments de couleur rouge (Figure II-2).

Figure II- 2 :

 2 Figure II-2 : Localisation et description photographique des différentes zonation constitutives (Tannes, Avicennia et Rhizophora) de la mangrove de référence (a) et de la mangrove en aval de l'ancienne exploitation minière (b). (Image Bluecham)

Figure II- 3 :

 3 Figure II-3 : Localisation de la baie de Dumbea, et de la zone étudiée (a). Présentation photographique de la zone étudiée (b), de la carotte de sédiment analysée (c), et profil de pH mesuré le long du profil sédimentaire (d).

  de routes et de parcs d'activités a notamment pour conséquence d'isoler une partie de ces mangroves, de l'influence des marées. Certaines d'entre elles sont à présent colonisées par de la végétation terrestre, et abritent des mares d'eaux saumâtres (Figure II-3), traduisant une évolution des conditions physico-chimique du sédiment. Cette évolution peut directement affecter le cycle des ETM potentiellement accumulés au sein de ces sédiments. Le site choisi pour cette étude est une ancienne mangrove à Avicennia, isolée des marées par la construction d'une route (Figure II-3). Cette mangrove se distingue des mangroves de la baie de Dumbea par une forte acidité des eaux porales (pH 3-4) le long des 50 premiers centimètres de profondeur. Ces valeurs de pH sont plus classiquement mesurées lorsque les sédiments de mangrove sont soumis à des périodes d'exondation et/ou d'oxygénation plus ou moins longue, où les conditions réductrices ne sont plus permanentes, ce qui provoque l'oxydation des pyrites. Intérêt du site d'étude Cette mangrove, précédemment située en aval d'anciennes exploitations minières, est à présent isolée des marées. Ce site d'étude est spécifiquement choisi, afin d'étudier le devenir des métaux potentiellement piégés dans le sédiment de mangrove, au cours de l'oxydation importante de ce dernier. II-A.3. Echantillonnage Le sédiment est un système biphasé constitué d'une part, d'une phase solide regroupant une fraction minérale, une fraction organique, ainsi que des organismes vivants (plantes, bactéries …), et d'autre part, d'une phase liquide appelée eau interstitielle. Chacune de ces phases sont en perpétuelle interaction, et peuvent naturellement évoluer dans le temps suite à la modification d'un ou plusieurs facteurs physico-chimiques et biologiques comme l'oxygénation, la température, le pH, la compaction, la pression, la biomasse. Lorsque l'on effectue un prélèvement au sein d'un sédiment, il est particulièrement primordial préserver des conditions anoxiques, afin d'éviter l'oxydation des sulfures et du Fe(II).

  Seuil K de Ni. Les gammes de références au seuil du Ni sont constituées de Ni adsorbé ou incorporé dans la structure des oxyhydroxides de Fe et les oxydes de Mn (Figure II-7 a et b ; Dublet et al., 2011), et de Ni incorporé dans la structure des phyllosilicates (Figure II-7 c et d; Dublet et al., 2011). Dans le cadre de cette étude, l'accent a été particulièrement mis sur l'enrichissement de la gamme de référence par des complexes organométalliques (Figure II-7 e et f), ainsi que des espèces de Ni adsorbé ou incorporé dans la structure des sulfures de fer et d'autre type de sulfures métalliques (Figure II-7 g et h).

Figure III- 1 :

 1 Figure III-1: Map of the studied area (Vavouto Bay, New Caledonia) showing: (a) the location of New Caledonia in the Pacific Ocean and (b) the location of mangrove in the bay and the position of the cores. The bottom panel (c) shows a lateral schematic representation of the three sampling zones.

  Dissolved metals concentrations in pore-waters were measured by ICP-AES (Icap ThermoFisher Scientific at University Paris Diderot) after 10 times dilution and HR-ICP-MS with an integrated FAST (ESI) injection system (Element 2 Thermofisher scientific at University Paris Diderot) after 100 times dilution. Certified reference materials (SLRS4) were intercalated during the analytical series.

FeOOH) ;

 ; Fe(II,III) (hydr)oxides as green-rust (Pankte et al., 2012) and magnetite (Fe 3 O 4 ); Fe(II) hydroxide (Ona-Nguema et al., 2009), vivianite ((Fe 3 (PO 4 ) 2 •8H 2 O) and Fe(III)

For

  each mangrove stand, i.e. salt flat, Avicennia marina forest, and Rhizophora spp. forest, the measured physico-chemical parameters at low tide, including water content, salinity, total organic carbon, redox potential and pH, are reported as a function of depth in Figure III-2. In the salt flat, the water content is ~25 wt% throughout the length of the core, while it ranges between 30 and 65 wt % in the sediment beneath the Avicennia and Rhizophora stands, the lower values being observed near the surface. The salinity increases progressively from Rhizophora to Avicennia stands and was the highest in the salt flat. Maximum salinity values, for each stand, are found at 20 -30 cm depth (Figure III-SI-1). Total organic carbon (TOC) values are significantly higher beneath the Avicennia and Rhizophora stands (4-16 wt% range) than in the salt flat sediments (0.4 -1.5 wt% range) (Figure III-2). A particular TOC-rich horizon occurs around 20 -30 cm depth in the sediments beneath Avicennia while it is found around 0 -10 cm depth in those beneath Rhizophora (Figure III-2).

Figure III- 2 :

 2 Figure III-2: Physicochemical parameters measured as a function of depth in the mangrove sediments cores collected beneath salt flat, Avicennia, and Rhizophora. The left panels (a, c, e) show pH (pink line) and Eh potential (gray line). The right panels (b, d, f) show Total Organic Carbon (black line) and water content in weight % (blue line).

Figure III- 3 :Figure III- 4 :

 34 Figure III-3: Concentration and speciation of dissolved Fe and S in the pore-waters as a function of depth under salt flat, Avicennia, and Rhizophora. The red line on the left panels (a, c, e) corresponds to total dissolved Fe concentration, and the green and blue bars represent the proportion of dissolved Fe 2+ and Fe 3+ , respectively. The purple line on the right panels (b, d, f) corresponds to dissolved SH2S concentration and the black bars represent the dissolved SO 4 2-/Cl -molar ratio (the vertical dotted line indicates the sea water dissolved SO 4 2-

  Figure III-5. These data indicated that the clay fraction of the studied sediments was composed of serpentine and talc, as mentioned previously, and also contains a large amount of smectite associated to minor amount of illite (Figure III-5).

Figure

  Figure III-5: X-ray powder diffraction pattern of clay fraction from the top (2.5 cm) and bottom (47.5 cm) of a sediment core collected beneath Rhizophora: Smect (smectite), Tlc (talc), Ill (illite), and Srp (serpentine). The diffractograms are comprised of oriented aggregates of the clay fraction heated at 350 °C and 550°C, solvated with ethylene glycol (EG) and air-dried (AD). Patterns were recorded in air on glass sample holders.

  II) at depth in the sediments beneath Avicennia and Rhizophora. These observations are confirmed by the results of the LC-LSF fitting of the Fe K-edge XANES spectra of sediment samples performed with the Fe K-edge XANES spectra of the goethite, serpentine and 65/35 illite-nontronite model compounds. The results of these LC-LSF fits show a quite constant composition with roughly equal fractions of Fe(III)-bearing goethite and phyllosilicates in the sediment beneath the salt flat, whereas they show the disappearance of Fe(III)-bearing goethite and the apparition of Fe(II)-bearing pyrite downward the sediments beneath Avicennia and Rhizophora (TableIII-SI-1). These results emphasize the evolution of Fe(III) towards Fe(II) at depth in the sediment beneath the two vegetated zones, whereas Fe remains as Fe(III) along the sediment beneath the salt flat.However, assessing the actual fractions of the various Fe(III) and Fe(II)-bearing mineral species on the basis of XANES data is not achievable because these data are very similar for the various model compounds used. On the opposite, with much more contrasted signatures, EXAFS looks best suited to decipher between these various model compounds and to quantify these relative proportions in the studied sediment samples.The Fe K-edge EXAFS spectra and their corresponding Fourier-Transformed of selected bulk samples collected beneath the salt flat, the Avicennia stand, and the Rhizophora stand, are presented in Figure 6b, 6h, respectively. The EXAFS spectra of salt flat samples do not show major variations with depth, while EXAFS spectra beneath Avicennia and Rhizophora show distinct features from the top to the bottom of the sediments profiles. These changes are also observed in the corresponding Fourier-Transforms, which show variations in the position and shape of both the first and secondneighbor peaks 6h). EXAFS data of the sample collected at the top of the cores from the three zones studied, have similar shapes, with a first and second neighbor peaks at 1.55 Å and ~2.75 Å, respectively, in the Fourier Transform, uncorrected for phase shift. In contrast, EXAFS spectra of the deepest samples beneath Avicennia and Rhizophora exhibit a different shape and compare well with those of pyrite 6g ;.

  bearing 2:1 phyllosilicate component. The composition of the EXAFS spectrum for this model compound is the statistical average of all the compositions that provided the best EXAFS fit of mangrove sediments Fe K-edge EXAFS spectra. The illite model compound used in the LC-LSF analysis of EXAFS data is interpreted here as a proxy for dioctahedral 2:1 phyllosilicate, including especially smectite that was identified in every samples of the studied mangrove sediments. Illite component was combined with nontronite, in order to obtain the best fits for our EXAFS spectra. As previously observed by[START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF] in Brazil mangrove sediments, the infrared spectra of the New Caledonia mangrove sediment samples studied here show the characteristic peaks of nontronite (peaks at 3560 cm -1 and close to 798 and 820 cm -1 ). As shown in Figure III-SI-3, the peak at 3560 cm -1 , attributed to vibrations generated by the stretching of AlFeOH bonds[START_REF] Frost | Vibrational spectroscopy of ferruginous smectite and nontronita[END_REF], is also observed in our Fe-containing illite model compound (Le Puy, France). The intensity of this peak for the Vavouto mangrove sediment samples is intermediate between that of illite and nontronite. This observation strongly supports our modeling of the 2:1 phyllosilicate component in EXAFS spectra of the mangrove sediments using a linear combination of the illite and nontronite EXAFS spectra. Altogether, these results indicate that the 2:1 phyllosilicate component in the studied mangrove sediments likely consists in a Fe-rich smectite.

  stand indicates that the highest levels of TOC observed below 20 cm depth (Figure III-2d) are related with higher proportion of red tissues, characteristic of Rhizophora wood. Hence, one may infer that Avicennia could have developed on sediments that were previously colonized by Rhizophora forest[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. Conversely, low TOC content in the deeper part of the sediments core under Rhizophora (below to 15 cm depth) could be inherited from previous Avicennia forest cover.The changes in the amounts of water and TOC across the intertidal zone and with depth likely influence redox conditions and pH. Low duration of tidal immersion and low organic content, associated with permeable coarse deposit in the salt flat, are expected to favor higher oxygen transfer downward into the sediment, inducing oxic to suboxic conditions. Conversely, the sediments beneath Avicennia and Rhizophora are suboxic in the upper horizons, and most likely anoxic below (Figure III-2c and e). More precisely, Eh value gradually decreases within the first 20 cm depth beneath Avicennia, while the Eh value readily decreases within the 5 cm depth beneath Rhizophora (Figure III-2e). Avicennia

  Figure III-2d) (likely inherited from ancient Rhizophora forest) and the conditions are anoxic. In the upper sediment, increasing oxygen concentration is expected to favor the biodegradation of organic material and/or the oxidation of Fe sulfides, causing the observed pH decrease (Figure III-2c, and 2e). In summary, oxygenation of the sediments decreases from the salt flat to the Rhizophora stand, and with depth, which likely influences iron biogeochemical cycling, as discussed below.

  ). The mineralogical transformation of ferric oxyhydroxides towards pyrite with depth is not correlated with a major change in the total Fe concentration in the sediments (FigureIII-6f and 6i), which suggests a gradual process. Although likely driven by sulfate and ferric iron (bio)reduction processes, the mechanisms of such a transformation are not yet documented in complex natural systems such as mangrove sediments. Recent laboratory investigations of lepidocrocite to pyrite transformation via abiotic pathway revealed possible solid-state transformation[START_REF] Hellige | Pathways of ferrous iron mineral formation upon sulfidation of lepidocrocite surfaces[END_REF]).

  15 cm depth, high dissolved sulfide concentrations are measured beneath Avicennia (200 -800 µM) (Figure III-2d) and at a lesser extent beneath Rhizophora (100 -200 µM) (Figure III-2f), where higher dissolved Fe(II) concentration and FeS formation may inhibit dissolved

Figure III- 7 :

 7 Figure III-7: Scanning electron microscope (SEM) images showing (a) individual framboidal pyrite, (b) root debris upholstered of framboidal pyrites and (c) mixture of organic materials and small mineral particles (less than 2 µm) showing an Fe-S association (EDS spectrum in the right bottom corner).

  Figure III-SI-1:Measured salinity in the mangrove sediments cores sampled in each mangrove stand across intertidal zone (salt flat, Avicennia, Rhizophora).

Figure III-SI- 2 :

 2 Figure III-SI-2: Experimental Fe K-edge EXAFS spectra of the model compounds used for LC-LSF analysis of the mangrove sediment EXAFS spectra.The Fe-rich smectite spectrum is obtained as a linear combination of the illite and nontronite in a 65/35 ratio.

Figure III-SI- 4 :

 4 Figure III-SI-4: Detail of the pre-edge region for the Fe K-edge XANES data collected on the mangrove sediment samples beneath (a) salt flat, (b) Avicennia and (c) Rhizophora. Preedge data of sediment samples are compared to those of the model compounds used for the LC-LSF fits of XANES and EXAFS data. The dark-gray and light-gray vertical shaded zones indicate expected positions of the centroïd pre-edge signal for Fe(III) at 7114.5 eV and Fe(II) at 7113 eV according to values reported byWilke et al. American Mineralogist, 2001, 86, 714-730, corrected for Fe(0) first inflection point taken at 7112 eV in the present study.

Figure

  Figure III-SI-5: LC-LSF analysis of the Fe K-edge XANES spectra of the sediment samples beneath (a) salt flat, (b) Avicennia and (c) Rhizophora. Experimental and calculated curves are displayed as black and red lines respectively. Experimental XANES spectra of the model compounds used for this LC-LSF analysis are displayed above and below the mangrove sediment data.

  Figure III-SI-6: Reconstruction of the whole set of experimental EXAFS data with (a) the first, (b) the two first (c) the three first and (d) the four first components extracted by the PCA procedure. Black and red lines correspond respectively to experimental and reconstructed data. From top to bottom : sample collected in salt flat, beneath Avicennia marina, and beneath Rhizophora.

  Figure III-SI-7 :Two Linear Combination Fitting solutions for the Fe K-edge EXAFS spectra of mangrove sediments samples collected beneath Salt flat. The LC-LSF fits include Fe-rich smectite, Fe-bearing serpentine and goethite, as model compounds spectra with (a) or without (b) additional ferrihydrite(Fh). Experimental k3-weighted EXAFS functions (black lines) and LC-LSF fits (red lines) of Fe K-edge EXAFS spectra of the bulk sediment samples.

Figure

  Figure III-SI-8:Two Linear Combination Fitting solutions for the Fe K-edge EXAFS spectra of mangrove sediments samples collected beneath Rhizophora. The LC-LSF fits include Ferich smectite, Fe-bearing serpentine and goethite, as model compounds spectra with (a) or without (b) additional ferrihydrite(Fh). Experimental k3-weighted EXAFS functions (black lines) and LC-LSF fits (red lines) of Fe K-edge EXAFS spectra of the bulk sediment samples.

Figure III-SI- 9 :Figure

 9 Figure III-SI-9:Two Linear Combination Fitting solutions for the Fe K-edge EXAFS spectra of sample at 2.5 cm of core collected beneath Avicennia. These LC-LSF fits include Febearing smectite, Fe-rich serpentine and goethite, as model compounds spectra with or without additional lepidocrocite(a). Corresponding Fourier Transform are shown (b). Experimental spectra and LC-LSF fit curves are displayed as black and red solid lines, respectively.

  large set of experimental Ni K-edge XAS spectra from natural and synthetic model compounds was used for LC-LSF analysis of the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of the mangrove sediments. Our Ni K-edge model compounds data set includes already described and collected data for Ni-goethites, Ni-bearing Mn oxides, as well as Ni-silicates, Niphyllosilicates[START_REF] Dublet | Ni speciation in a New Caledonian lateritic regolith: A quantitative X-ray absorption spectroscopy investigation[END_REF], and Ni-bearing siderite[START_REF] Dublet | XAS Evidence for Ni Sequestration by Siderite in a lateritic Ni-deposit from New Caledonia[END_REF]. In addition, a natural Ni-rich pyrite (Fe 0.88 Ni 0.12 S 2 ) as well as synthetic amorphous NiS were also analyzed by XAS in the present study. To complete this data set, Fe sulfides containing 1 mol% Ni were synthesized and consisted of Ni-subsituted sulfides (Ni-pyrite, Ni-mackinawite, Nigreigite), and Ni-sorbed sulfides (Ni-sorbed pyrite, Ni-sorbed mackinawite). Organometallic complexes were also prepared, including Ni-citrate, Ni-cysteine and Ni-tri-glutathion.

  operating at 50 kV and 40 mA. The morphology, organization and chemical composition of the particles in the mangrove sediments was studied by Scanning Electron Microscopy (SEM), at IMPMC, with a Field Emission Gun Scanning Electron Microscope (GEMINI ZEISS Ultra55). Elemental composition of the particles and chemical distribution maps were performed using Energy Dispersive X-ray Spectroscopy with a 15 kV electron beam collimated with a 60 µm aperture. Ni/Fe ratio were estimated by the intensity ratio of the Ni Kα and Fe Kβ lines. Samples to be analyzed were deposited as ground powder on carbon tape and coated with a thin carbon film. The mineralogical purity of model compounds was assessed by powder XRD with a Panalytical® X'Pert Pro diffractometer. Technical details and analytical procedures for these analyses are reported in the Supporting Information.IV-2.5. X-ray Absorption Spectroscopy (XAS) data collection and analysisX-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) fluorescence yield spectra at the Ni K-edge were recorded at 10K on BL 11-2 at the Stanford Synchrotron Radiation Lightsource (SSRL, CA, USA) for 12 selected samples from the three studied sediment cores. XAS data for model compounds were recorded at 20 K in fluorecence detection mode on the SAMBA beamline (SOLEIL, Saint-Aubin, France) and at 40K in transmission detection mode on the BM23 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). For all experiments, energy was calibrated by setting the first inflection point of the absorption edge of a Ni metal foils at 8333 eV. In order to strictly preserve the oxidation state of the samples during XAS data recording, sample pellets were mounted on the cryostat sample rod within a glove box and carried to the beamline in a liquid nitrogen bath before being rapidly transferred into the He cryostat.

  Figure IV-SI-6). The spatial distribution of the Fe, S and Ni concentrations measured by SEM-EDXS in framboidal pyrites shows that Ni is homogeneously distributed when associated with this type of pyrite (Figure IV-1a). Based on these observations, systematic investigation of Ni/Fe ratios revealed that the pyrite formed in the sediments beneath the

  of the Ni K-edge XAS data for the mangrove sediments The Ni K-edge EXAFS spectra of bulk sediments samples collected in the salt flat and beneath the Avicennia and the Rhizophora stands are displayed in Figure IV-2, together with their corresponding Fourier-Transforms. The EXAFS data of the samples from the top of the three studied cores compare well with those of Ni-bearing phyllosilicates and Ni-bearing goethite (Figure IV-SI-3). Accordingly, XANES spectra of these top horizons sediment samples show characteristic features of Ni-bearing phyllosilicates XANES spectra (i.e. the shape of the white line at 8352 eV and the characteristic oscillation at 8352 eV; Figure IV-SI-2). On the opposite, the EXAFS data of the samples at the bottom of the cores beneath the Rhizophora and Avicennia stands (Figure IV-2c, and 2e) compare well with that of Ni-pyrite (Figure IV-SI-3). However, the discrepancy between the XANES spectrum of Ni-pyrite and those of the deep sediment samples indicates that, one or more other major Ni species are present. Owing to the similarity in XANES spectra for various compounds as forinstance, Nigoethite and Ni-citrate, or various Ni-sulfide bound compounds, detailed speciation of Ni in the complex natural samples studied here required a full analysis of the EXAFS data set.

  SI-2), which suggests that such Ni species are likely to occur in the studied samples. In addition, the SPOIL value obtained for am-NiS, Ni-citrate, Ni-gluthation, Ni-rich pyrite, Ni-talc, Nimontmorillonite, Ni-mackinawite, Ni-bearing and Ni-sorbed goethite, Ni(OH) 2 , Ni-cysteine and Ni-pyrite indicated that these references are good to fair (SPOIL ≤ 2.24 and ≤ 4.75, respectively) according to Malinowski (1978) (Table IV-SI-2).

Figure IV- 2 :

 2 Figure IV-2:Experimental (black lines) and LC-LSF fits (red lines) of the Ni K-edge k 3weighted EXAFS spectra of the mangrove sediment samples collected beneath the salt flat (a), Avicennia (c) and Rhizophora (d) stands, and their correspondingFourier- Transforms (b, d, e). See Table IV-SI-3 for quantitative results.

  model compounds spectra, Ni-montmorillonite, Ni-sorbed goethite, Nicitrate, and Ni-rich pyrite had the lowest NSSR values (i.e. the deviation between the experimental and the reconstructed spectra; Table IV-SI-2), which suggest that these Ni species are also likely to occur in the studied samples. IV-3.3. Quantitative speciation of Ni in the mangrove sediments based on LC-LSF EXAFS Analysis According to the results of the PCA and Target Transformation analysis, the best LC-LSF fits were obtained using the following four model compounds spectra: a proxy for Ni-clays (Nibearing phyllosilicates combination), Ni-goethite, Ni-pyrite and Ni-citrate (Figure IV-2; Table IV-SI-3). Concerning the Ni-bearing phyllosilicates, attempts to fit mangrove sediment data by including Ni-bearing montmorillonite, serpentine, talc and kerolite as independent components yielded standard deviations on the relative proportions of these four components as large as 50%, which may be explained by the similarities in the Ni K-edge EXAFS spectra of these Ni-bearing phyllosilicates (Figure IV-SI-3; Dublet et al., 2012). Consequently, an artificial model compound consisting of a linear combination of Ni-bearing montmorillonite, serpentine, talc, and kerolite spectra in the 25/33/27/15 respective proportions was used as a fitting component for all studied sediment samples in order to account for the Ni-clays component. These proportions correspond to the statistical average of all the compositions that provided the best LC-LSF fits when performed with the four clay model compounds included as independent components. The Ni-citrate model compound was considered as a proxy for Ni-carboxylate complexes associated to organic matter. Including this component improved significantly the quality of the fits (i.e. R-factor improved by 46%) for all samples where it represents more that 10% of total Ni (Figure IV-SI-4). Although the Ni-pyrite (1 mol% Ni) best matches the Ni-sulfide component in the mangrove sediments studied, addition of the Ni-rich pyrite model compound as a fifth component improved the quality of the fits (i.e. R-factor improved up to 34%; Figure IV-SI-5) for two sediments samples beneath Avicennia (TableIV. Including a fifth component did not improve the fit quality for any of the other samples, which confirmed the PCA results that indicated the whole set of EXAFS spectra could be well reconstructed using four components.

Figure IV- 3 :

 3 Figure IV-3: Chemical composition and Ni speciation of the studied mangrove sediments collected in salt flat, and beneath the Avicennia and Rhizophora stands. (a), (c), and (e) show the vertical evolution of the Ni/Ti (red lines) and Fe/Ti (black lines) ratios and (b), (c) et (d) show the results of the LC-LSF procedure expressed as the relative proportion of each of the model compounds quantified normalized to 100%.

  ligands Table

synthetic

  Ni-pyrite (1 mol% Ni) model compound (Figure IV-3f; Table IV-SI-3). In contrast, beneath Avicennia, an additional Ni-rich pyrite component occurs and it is well modeled by our natural Ni-rich-pyrite (12 mol% Ni) model compound (Figure IV-3d; Table IV-SI-3). Such variations in the amount of Ni incorporated within the pyrite structure can be distinguished by EXAFS because the extent of the structural relaxation around the Ni 2+ ions in pyrite depends on the Ni substitution rate, as shown by our theoretical simulation of the model compounds EXAFS data (Figure IV-4).

  dessicator. XRD analyses confirmed the purity of the synthetic pyrite (Figure IV-SI-7) and XRF analyses indicated that it contains 1mol% Ni.Ni-Mackinawite (Fe 0.99 Ni 0.01 S) was synthesized following the procedure by[START_REF] Donald | Low temperature anaerobic bacterial diagenesis of ferrous monosulfide to pyrite[END_REF]. 0.93 g Fe(SO 4 ).7H 2 O and 6 mg Cl 2 Ni.6H 2 O were dissolved separately in 10 mL of double-distilled and oxygen-free water, in an anaerobic chamber. Meanwhile, 0.6 g Na 2 S.9H 2 O was dissolved in 10 mL of double-distilled and oxygen-free water, and subsequently added to the metal solution while stirring. The suspension obtained was sealed in anoxic conditions and was gently stirred in the anaerobic chamber during 24h. The solid fraction was collected after centrifugation at 8,500 G for 20 min, and it was then rinsed three times with double-distilled and oxygen-free water, and finally dried in dessicator. XRD analyses confirmed the purity of the synthetic mackinawite (Figure IV-SI-7) and XRF analyses indicated that it contains 1mol% Ni.

  Ni-cysteine were obtained by dissolving 10 mg of Cl 2 Ni.6H 2 O in 10 mL of double-distilled and oxygen-free water containing 200 mg of gluthation or cysteine. The pH was then raised and stabilized to 8 with NaOH. The suspensions obtained were gently stirred during 2 h. The solid fraction were collected after evaporation in a dessicator. XRF analyses indicated that it also contains 1 mol% Ni.

Figure IV-SI- 1 :Figure

 1 Figure IV-SI-1: Reconstruction of the whole set of experimental EXAFS data with (a) the first, (b) the two first (c) the three first and (d) the four first components extracted by the PCA procedure. Black and red lines correspond respectively to experimental and reconstructed data. From top to bottom : sample collected beneath the salt flat, beneath the Avicennia stand, and beneath the Rhizophora stand.

Figure IV-SI- 5 :

 5 Figure IV-SI-5: Two different results of LC-LSF fits performed on the k 3 -weighted Ni K-edge EXAFS data of mangrove sediment samples collected beneath the Avicennia stand. The LC-LSF fits displayed in the A panel were performed with Ni-clays, Ni-bearing goethite, Ni-poor pyrite, Ni-citrate and Ni-rich pyrite model compounds, whereas those displayed in the B panel were performed only with Ni-clays, Ni-bearing goethite, Ni-poor pyrite and Ni-citrate model compounds. Experimental data and and LC-LSF fits are displayed in black and red lines, respectively (see Table IV-SI-4 for quantitative results).

Figure IV-SI- 6 :

 6 Figure IV-SI-6: Evolution of pyrite particles diameter as a function of the Ni/Fe ratio as measured by SEM-EDS analyses on framboidal pyrites from deep samples collected beneath the Avicennia and Rhizophora stands. The Ni/Fe ratio corresponds to the ratio of the intensities of the Ni Kα and the Fe Kβ lines.

(

  Figure V-1a). A similar evolution was observed in the experiments performed with the addition of dissolved transition metal cations, but with major changes in the kinetic of pyrite formation. Indeed, pyrite precipitated within 120h in the experiments performed with addition of dissolved Co 2+ (Figure V-1c), and within 48h in experiment performed with addition of dissolved Ni 2+ (Figure V-1d). In contrast dissolved Mn 2+ did not influence the rate of pyrite formation (Figure V-1b).

  Figure V-1: X-ray powder diffraction patterns of the solids collected over the course of the control experiment without trace metal cation impurity (orange; a), the experiment performed with dissolved Mn 2+ (blue; b), with dissolved Co 2+ (purple; c), and with dissolved Ni 2+ (black; d). Bragg peak indexation indicates mackinawite (M), elemental sulfur (S), and pyrite (P). The low angle region of the XRD patterns is not shown because of a broad diffuse band caused by the Kapton window of the anoxic sample chamber (2θ ~ 7°). The wide-angle regions of the XRD patterns have been cut for clarity.

Figure V- 2 :

 2 Figure V-2: Dissolved concentrations of Mn 2+ , Co 2+ and Ni 2+ (µmol/L) during pyrite synthesis experiments.

Figure V- 3 :

 3 Figure V-3:Ni K-edge EXAFS spectra of the solid samples collected over the course of pyrite synthesis in the presence of dissolved Ni 2+ in the starting solution. Ni K-edge k 3 -weighted EXAFS at 0.5h (a),29 and 30h (c) and 48h (d). Corresponding Fourier-Transforms are given in(b, d, f). Experimental data and LC-LSF fits for (a) and (c) are displayed in black and red lines, respectively (see TableV-1 for quantitative results). For the pyrite sample obtained after 48h, red lines indicated Feff-calculated EXAFS spectra after least-squares fit of the σ(Å), and ΔE 0 parameters (see TableV-1 for quantitative results).

  observed in our experiments (Figure V-2).We show that the presence of dissolved Co 2+ , and to a larger extent of dissolved Ni 2+ in the starting solution drastically accelerate the conversion of a poorly ordered mackinawite-like precursor into pyrite, which is obtained after only 48 hours in the presence of Ni 2+ instead of 9 days with only iron. Both Co 2+ and Ni 2+ , exhibited a high affinity for the solid phase during the whole synthesis reaction but the addition of dissolved Ni 2+ , did not influence the nature of the Fe 1+x S precursor minerals, which was found to exhibit a poorly ordered and/or mackinawite local structure (Figure V-SI-2). Moreover, Ni was principally precipitated as amorphous NiS, while a minor fraction of Ni (< 20%) was incorporated in the mackinawite precursor 3b,. During the dissolution/precipitation process affecting the poorly ordered mackinawite-like precursor, Ni 2+ and Co 2+ are incorporated into pyrite without loss. Moreover the core of the pyrite crystals is enriched in Ni as evidenced from TEM-EDXS analyses, which suggests that Ni 2+ cations are directly involved into the pyrite nucleation process.Rickard et al. (2007a) suggested that spontaneous pyrite nucleation required relatively high supersaturations, catalytic effects of active surfaces, or both. The hypothesis that we retain to explain the increasing rate of pyrite formation in the presence of dissolved Ni 2+ or Co 2+ is that these cations increase the supersaturation with respect to Ni-rich or Co-rich pyrite because the vaesite (NiS 2 ) or cattierite (CoS 2 ) end-members are less soluble than pyrite (FeS 2 ).

Figure V- 4 :

 4 Figure V-4: STEM-HAADF images (a) and SEM-EDXS analyses (b, c, d) of particles collected at 48h, from the experiment performed with addition of dissolved Ni 2+ . Element map of distribution of Fe, S and Ni show that Ni is preferentially distributed to center of pyrites particles.

Figure V -

 V Figure V-SI-1: Wide-angle X-rays scattering (WAXS) of the control experiment samples collected at 0.5h and 216h. Calculated curves correspond to mackinawite and pyrite structure, respectively.

Figure V-SI- 2 :

 2 Figure V-SI-2: Fourier-Transforms of Fe K-edge k 3 -weighted EXAFS spectra of the solid fraction sample collected in the control experiment (a) and in the experiment performed with dissolved Ni 2+ (b).

Figure V-SI- 5 :

 5 Figure V-SI-5: TEM picture in the bright field mode of particles collected at 48h from experiment performed wit dissolved Ni 2+ (a, c, e), and their SAED (b, d, f). The particles had the characteristic of particles assemblies, while Bragg peaks distribution were compatible with the structure of a crystal. Contrails spread and numerous additional diffraction peaks highlight the poor crystallinity of the particles.

  Figure II-1 et 2). En effet, celle-ci est caractérisée par la présence d'un substrat de couleur ocre, qui s'étend sur 15 -25 cm de profondeur (Figure VI-1). Cet horizon, appelé « horizon supérieur » dans la suite du texte, se distingue chimiquement des sédiments des horizons plus profonds. Au travers de la zone intertidale, de l'intérieur des terres vers le front de mer, les concentrations de Fe et de Ni sont contrastés entre le haut et le bas de chaque profil sédimentaire :

  Ni. En effet, les concentrations en Fe et en Ni sont respectivement 3 à 4 fois et 6 à 15 fois supérieure à celle des sédiments plus profonds (Figure VI-1).L'horizon supérieur, enrichi en ETM, se distingue également par sa composition minéralogique. La composition minéralogique de l'ensemble des échantillons, le long des trois profils sédimentaires, est composée de phyllosilicates, de types serpentine et talc, de quartz et de halite. Cependant, les diffractogrammes des échantillons de l'horizon enrichi en Fe et en Ni (horizon supérieur) se distinguent par la présence des pics de Bragg caractéristiques de la goethite, tandis que les diffractogrammes des échantillons des sédiments plus profonds ne montrent pas la présence d'oxyhydroxide de fer, mais indiquent la présence de pyrite.VI.A.2 Influence de l'exploitation minière sur la minéralogie et la cristallochimie du Fe et du Ni des matériaux hérités de l'érosionDans la mangrove de référence de la baie de Vavouto, les sédiments de surface sont principalement constitués de goethites et de phyllosilicates, hérités de l'érosion du massif (cf.Chapitre III, FigureIII-4). Cependant, nous n'observons pas de contraste chimique le long des carottes de sédiments collectés dans chacune des zones de cette mangrove. Plus spécifiquement, le sédiment des tannes de la mangrove de référence est chimiquement et minéralogiquement homogène le long des 50 premiers centimètres de profondeur (cf.Chapitre III, Figure. Etant donné que le sédiment des tannes, peu réactif, ne présente pas de transformations minéralogiques marquées[START_REF] Otero | Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia -SP , Brazil )[END_REF] Noël et al. submitted), ce sédiment est essentiellement influencé par la sédimentation détritique. L'homogeneité de composition avec la profondeur dans le tanne de la mangrove de référence suggère ainsi que la sédimentation détritique est plus ou moins homogène au cours du temps. Au contraire, comme nous l'avons vu précèdemment, la variation chimique et minéralogique marquée vers 15 cm de profondeur dans les tannes de la mangrove de Creek Mine, indique que le régime sédimentaire de la mangrove sous influence minière, a radicalement changé au cours du temps (Figure VI-1). D'autre part, la cristallochimie du Fe dans les matériaux hérités du massif, est également contrastée entre la mangrove de référence et la mangrove sous influence de l'exploitation minière. Seulement 20 à 53 % du Fe est apporté dans le sédiment de la mangrove de référence sous la forme de goethite, contre 57 à 95 % du Fe pour le sédiment de Creek Mine (Figure VI-2).

Figure VI- 2 :

 2 Figure VI-2: Spectroscopie d'absorption des rayons X au seuil K du Fe des échantillons de sédiments prélevés à 2,5 cm de profondeur dans les tannes (haut), les sédiments à Avicennia (milieu), et les sédiments à Rhizophora (Bas). (a) Spectres EXAFS des échantillons de la mangrove de référence (noire), de la mangrove de Creek Mine (bleu) et fit LCF (rouge); (b) transformées de fourrier correspondantes ; et (c) résultats de la procédure LCF exprimée en concentration massique (wt % Fe) de chaque phase porteuse de Fe dans les échantillons de sédiments de mangrove. Gt: goethite; Fh : ferrihydrite ; Lp : lepidocrocite ; Smec: smectite ; Srrp : serpentine.

Figure VI- 3 :

 3 Figure VI-3: Spectroscopie d'absorption des rayons X au seuil K du Ni des échantillons de sédiments prélevés à 2,5 cm de profondeur dans les tannes (haut), les sédiments à Avicennia (milieu), et les sédiments à Rhizophora (Bas). (a) Spectres EXAFS des échantillons de la mangrove de référence (noire), de la mangroves de Creek Mine (bleu) et fit LCF (rouge); (b) transformées de fourrier correspondantes ; et (c) résultats de la procédure LCF exprimée en concentration massique (wt ppm Ni) de chaque phase porteuse de Ni dans les échantillons de sédiments de mangrove. Gt: goethite; Py: pyrite.

  -B-2.1. Natural samples studied VI-B-2.1.1. Natural setting of the studied site New Caledonia is located in the South-West Pacific Ocean. The studied natural setting is located on the west coast of the main island (20°S-23°S), which is characterized by a semiarid tropical climate. Sampling was performed at the beginning of the dry season in June 2009 in a former mangrove that developed at the mouth of the Dumbea River. The catchment of this river is composed of ultramafic rocks and lateritic soils (BRGM, 1965) that have been exploited from the end of the 19th century to the middle of the 20th century for their large Fe and Ni contents. Although these old open mines are no longer exploited, their presence likely favors the erosion of the lateritic regolith and the transport of metal-bearing sediments downstream the Dumbea catchment (i.e. towards the shoreline, and more specifically the mangrove) during strong climatic events.

  K-edge X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Nearedge Structure (XANES) spectra of the sediment samples were collected at 10K on the 11.2 beamline (SSRL, Menlo Park, California, USA). In addition to these natural samples, Fe and Ni K-edge EXAFS and XANES spectra of several synthetic sulfides (see details below) were collected at 20K on the SAMBA beamline (SOLEIL, Saint Aubin, France) using a double crystal monochromator equipped with Si(111) crystals. For all the studied samples, Fe K-edge XANES and EXAFS data were collected in transmission detection mode, while Ni K-edge XANES and EXAFS data were collected in fluorescence detection mode using a Canberra ® high-throughput Ge 30-element solid-state array detector.Natural samples and synthetic reference compounds were prepared as pressed pellets of finely hand-ground and homogenized powders of either low Ni concentration samples (no dilution) or high Fe concentration samples (diluted in cellulose). In order to strictly preserve the oxidation state of Fe during the analyses, all mangrove samples and model compounds were mounted on the cryostat sample rod within a glove bag or a glove box next to the beamline and carried to the beamline in a liquid nitrogen bath before being rapidly transferred into the cryostat. For Fe K-edge data, the energy of the X-Ray beam was calibrated by setting to 7112 eV the first inflection point on the Fe K-edge spectrum recorded on a Fe foil in double transmission setup. The same procedure was used for Ni K-edge data, with the Ni K-edge spectrum recorded on a Ni foil set to 8333 eV. For each bulk sediment sample, only 2 scans

Figure VI- 5 :

 5 Figure VI-5: Vertical evolution of (a) the pH (pink line) and Eh (gray line), (b) the Total Organic Carbon (purple line), (c) the total Fe concentration in the solid fraction (red line), and (d) the total Ni concentration in the solid fraction (green line) along the studied mangrove sediments. The horizontal bars in the panels c and d correspond to the samples whose XRD patterns are presented in Figure 3.

Figure VI- 6 :

 6 Figure VI-6: Vertical evolution of (a) salinity (purple line) and (b) the dissolved SO4 2-/Cl - molar ratio (blue line), (c) the total Ni concentration in the pore-waters (green line), and (d) the total dissolved Fe concentration (red line).The green and blue bars represent the proportion of dissolved Fe2+ and Fe3+, respectively. All dissolved SO4 2-/Cl -molar ratio measured in the pore-water were greater than that of the sea water(0.05; Gibblin, 1988).

  It then decreases to reach a mean value around 1100 ppm Ni from 20 cm to 40 cm depth and increases back towards its maximum value around 6000 ppm Ni at 50 cm depth to stabilize around 4500 ppm Ni from 50 to 70 cm depth

  for mangroves worldwide. These large amounts of Fe and Ni are related to the position of the studied mangrove that is located downward Ni-rich lateritic regoliths developed in ultramafic rocks. The mineralogical composition of the sediments was assessed by XRD analysis of three samples (i.e. 10 cm, 40 cm and 70 cm depth) representative of the studied core (Figure VI-7). Although the relative proportions of these mineral species change along the studied core, XRD data indicate the occurrence of serpentine (Mg 3 Si 2 O 5 (OH) 4 ), quartz (SiO 2 ), talc (Mg 3 Si 4 O 10 (OH) 2 ) and halite (NaCl) all along the sediments (Figure VI-7). In contrast, goethite (α-FeOOH) is identified as one of the major mineral species above 40 cm depth but disappears below 50 cm depth and pyrite (FeS 2 ) is found as one of the major mineral species at depth whereas it is not detected by XRD above 40 cm (Figure VI-7). VI-B-3.2 Fe speciation in the solid fraction of the sediments along the studied core VI-B-3.2.1 Qualitative analysis of Fe K-edge XAS data The Fe K-edge XANES spectra of the samples collected at 5 cm, 10 cm, 25 cm, 30 cm, 40 cm, 50 cm and 70 cm depth are presented in Figure VI-SI-2. These Fe K-edge XANES spectra show a vertical evolution downward the studied core, with a dampening of the intensity of the white-line and the apparition of a shoulder on the low-energy side of this white-line (Figure VI-8a). These changes suggest the occurrence of a reduced form of Fe in the studied sediments below 50 cm depth, which is confirmed by the similarity between the XANES spectrum of the sample collected at 50 cm depth and that of pyrite (Figure VI-SI-2a). Considering the similarity of the XANES spectra of the samples collected above with those of goethite and Fe-bearing phyllosilicates (i.e. the characteristic shape of the white line at 7130 eV and the characteristic feature around 7150 eV; Figure VI-SI-2a), these observations suggest a vertical change in Fe speciation form ''oxides'' towards ''sulfides'' downward the studied core.

Figure

  Figure VI-7: X-ray powder diffraction patterns of N 2 -dried bulk samples considered as representative of the various units identified along the studied mangrove sediments. Bragg peak indexation indicates goethite (Gt), serpentine (Srp), talc (Tlc), quartz (Qz), halite (H) and pyrite (Py). The low angle region of the XRD patterns is not shown because of a broad diffuse band caused by the Kapton window of the anoxic sample chamber (2θ ~ 7°). The wideangle regions of the XRD patterns have been cut for clarity.

Figure VI- 8 :

 8 Figure VI-8: Vertical evolution of (a) Fe K-edge and (b) Ni K-edge XANES spectra along the studied mangrove sediments. In order to facilitate comparison, only the samples analyzed at both the Fe and Ni K-edges are presented here.

  confirmed by the visual inspection of the Fe K-edge EXAFS spectra and their corresponding Fourier-Transforms of the 10 samples of bulk sediment studied at the Fe K-edge (Figure VI-9). The EXAFS spectra of the samples collected between 0 and 40 cm depth do not show major variations with depth (Figure VI-9a), and their Fourier-Transforms show first and second neighbor peaks at the same positions (i.e. ~1.52 Å and ~2.62 Å, respectively, uncorrected for phase shift; Figures VI-9b). In contrast, the EXAFS spectra of the samples collected in the lower part of the sediments (from 50 to 70 cm depth) show distinct features compared to those of the sediments above (Figures VI-9a). These changes are also observed in the corresponding Fourier-Transforms, which show variations in the position and shape of both the first and second-neighbor peaks (Figures VI-9b). The XAS data of the samples collected at the top of the studied core compare well with those of Fe-bearing phyllosilicates and Fe-bearing goethite (Figure VI-SI-1), whereas the XAS data of the samples collected at the bottom of the studied core compare well with those of Ni-pyrite (Figure VI-SI-1

  -B-3.3 Ni speciation in the solid fraction of the sediments along the studied core VI-B-3.3.1 Qualitative analysis of Ni K-edge XAS data The Ni K-edge XANES spectra of the samples collected at 5 cm, 10 cm, 25 cm, 30 cm, 40 cm, 50 cm and 70 cm depth are presented in Figure IV-10. As for the data at the Fe K-edge, these Ni K-edge XANES spectra show a vertical evolution downward the studied core, with a dampening of the intensity of the white-line and the apparition of a shoulder on the lowenergy side of this white-line (Figure VI-10b). These changes suggest the occurrence of a reduced form of Ni in the studied sediment below 50 cm depth, which is confirmed by the similarity between the XANES spectrum of the sample collected at 50 cm depth and that of Ni-bearing pyrite (Figure VI-SI-2b). As for the data at the Fe K-edge, the Ni K-edge XANES spectra of the samples collected above are more similar to those of Ni-bearing goethite and Ni-bearing phyllosilicates (i.e. the characteristic shape of the white line at 8350 eV and the characteristic feature around 8365 eV; Figure VI-8b), which indicates a vertical change in Ni speciation from ''oxides'' towards ''sulfides'' downward the studied core. This assumption is confirmed by the visual inspection of the Ni K-edge EXAFS spectra and their corresponding Fourier-Transforms of 7 of the 10 bulk samples analyzed at the Fe K-edge (Figure VI-10). These Ni K-edge EXAFS spectra show distinct features from the top to the bottom of the studied sediments, with a first group of samples within the first 25 cm depth, a second group between 25 and 40 cm depth and a third one below 50 cm depth (Figure VI-10a). This vertical evolution of the XAS data is also visible on the corresponding Fourier-Transforms. Indeed, the FT of the samples above 25 cm depth are characterized by first-and second-neighbor peaks around ~1.62 Å and ~2.78 Å (uncorrected for phase shift), whereas those of the samples below 50 cm depth are characterized by first-, second-and thirdneighbor peaks at ~1.83 Å, ~2.98 Å and ~3.56 Å (uncorrected for phase shift) (Figure VI-10b). The occurrence of first-and second-neighbor peaks around ~1.62 Å and ~2.78 Å (uncorrected for phase shift) is characteristic of the distances found for Ni-bearing iron oxides and Ni-bearing clays

  . Consequently, as previously performed for the LC-LSF fitting of the Fe K-edge EXAFS data, a linear combination of Nibearing montmorillonite, serpentine and talc EXAFS spectra (with 62/22/16 respective proportions) was used as the Ni-clays component for the LC-LSF fits of the EXAFS data from the samples studied at the Ni K-edge. Considering the visual inspection of the Ni K-edge XANES spectra, the LC-LSF fits of the EXAFS data from the samples collected below 30 cm depth were performed with addition of a nickel sulfide or a Ni-bearing or Ni-sorbed iron sulfide, compared to the EXAFS data from the samples collected between 25 and 40 cm depth. The comparison of the quality of the LC-LSF fits performed with additional Ni sulfide (i.e. NiS), Ni-bearing iron sulfides (i.e. Nipyrite, Ni-rich pyrite, Ni-greigite and Ni-mackinawite), Ni-sorbed iron sulfides (i.e. Ni-sorbed mackinawite and Ni-sorbed pyrite) and Ni bound to sulfur in organic complexes (i.e. Niglutathion and Ni-cysteine) indicates a good agreement for all Ni-bearing and Ni-sorbed iron sulfides (Figures VI-

Finally

  , addition of the Ni-sorbed pyrite model compound to the fitting procedure for the samples collected in the lowest part of the studied core (from 50 to 70 cm depth) also improved significantly the quality of the LC-LSF fits (i.e. 46% average for all samples where Ni-sorbed pyrite represent more 10% of total Ni (Figure VI-SI-6 and Table VI-SI-9).

Figure VI- 10 :

 10 Figure VI-10: Vertical evolution of the Ni K-edge (a) k 3 -weighted EXAFS spectra along the studied mangrove sediments and (b) their corresponding Fourier-Transforms. The black and red lines correspond to the experimental data and the LC-LSF fits, respectively. See Table VI-SI-4 for quantitative results.

  Finally, the sediments collected between 50 and 70 cm depth are characterized by a major fraction of Ni hosted by pyrite (both as sorbed and incorporated Ni). This association of Ni with pyrite is total in the sample collected at 50 cm depth, whereas it represents only 70% of total Ni in the sample collected at 50 cm depth where Ni-clays represent around 30% of total Ni (Figure VI-12;Table. of the isolation from the sea on the physico-chemical properties of the studied mangrove sediments

  Fe K-edge XAS data indicate a major contribution of pyrite to Fe speciation (i.e. 55 to 87 % of total Fe; Figure. This result is in agreement with the anoxic/suboxic conditions that prevail in these bottom horizons. In these conditions, the pyritization of the Fe(III)-oxyhydroxydes inherited from erosion of the lateritic regoliths is much stronger than in the overlying horizons. These conditions very favorable to pyritization likely explains the occurrence of cuboctahedral pyrites with a diameter larger than 10 micrometers in these bottom horizons.

Figure VI- 11 :

 11 Figure VI-11: SEM images and SEM-EDS analyses of pyrite grains from the sample collected at 70 cm depth. The SEM images in (a) show two types of pyrite: a first one constituted of cuboctahedral pyrites of approximately ten micrometers (red) and a second one constituted of spherulitic pyrites of a few hundred of nanometers (green). The SEM-EDS spectra corresponding to these two types of pyrite are displayed in (b) and (c).

  . Indeed, since the Eh values measured in the bottom horizons of the studied mangrove are very low (i.e. below 250 mV; Figure VI-5a), pyritization is likely very intense. As a consequence, although a minor fraction of AVS like mackinawite or greigite cannot be excluded, their contribution to Fe speciation, if any, is considered as very low in the bottom horizons (i.e. below 50 cm depth) of the studied mangrove sediments. The very low Eh values measured in the bottom horizons of the studied sediments suggest that these horizons below 50 cm depth are not concerned by the re-oxidation of the studied mangrove sediments as a consequence of their isolation from the sea. On the opposite, the very high SO 4 2-and total Fe concentrations measured in the pore-waters of these horizons (Figure VI-6b) suggest that an oxidation of pyrite took place (or is taking place), like at the overlying re-oxidation front. This apparent discrepancy is likely explained by the fact that a fraction of the dissolved Fe(II) released upon oxidation of pyrite at the overlying re-oxidation front percolated downward and accumulated in the bottom horizons. This fraction of dissolved Fe released in the pore-waters of the bottom horizons is likely not oxidized to Fe(III) since Fe in the pore-waters of these horizons occurs exclusively as Fe(II) (Figure VI-

  , which means that the fraction of Ni released in the pore-waters upon the partial dissolution of the Ni-bearing goethite and phyllosilicates remains in the dissolved form.Between 25 cm and 45 cm depth, Ni K-edge XAS data indicate that Ni is associated with pyrite, as well as with goethite and phyllosilicates. As already mentioned for the surface horizons (i.e. above 25 cm depth), Ni association with goethite and phyllosilicate is considered as inherited from erosion. On the opposite Ni association with pyrite is not, since this association has never been identified in the lateritic regoliths of New Caledonia. Moreover, Ni K-edge XAS data suggest that the fraction of Ni associated with pyrite occurs in the form of Ni sorbed at the surface of this mineral species, rather than in the form of Ni incorporated within pyrite structure. As already mentioned for Fe, Eh and pH values as well as SO 4 2-and total Fe concentrations in the pore-waters of the intermediate horizons suggest that they are former reduced horizons where pyrite precipitated before being strongly oxidized when conditions became oxic/suboxic after isolation from the sea.

Figure VI- 12 :

 12 Figure VI-12: Schematic representation showing (a) both the vertical evolution of the color of the sediments and the redox conditions along the studied core, together with (b) the quantitative Fe speciation derived by the analysis of the Fe K-edge EXAFS data and (c) the quantitative Ni speciation derived by the analysis of the Ni K-edge EXAFS data. For both elements, the quantitative speciation has been normalized to the concentration in the solid fraction. Gt: goethite; Fh: ferrihydrite; Fe-clays: Fe-bearing illite-nontronite-serpentine combination with 30/27/43 respective proportions; Ni-clays: Ni-bearing montmorilloniteserpentine-talc combination with 62/22/16 respective proportions; Py: pyrite. The Ni-pyrite contains 1% mol Ni and the Ni-rich pyrite contains 10% mol Ni.

( 1 )

 1 Calculated as Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2Table VI-SI-6:Quantitative results of the best LC-LSF fits performed on the k 3 -weighted Fe K-edge EXAFS data of the studied mangrove sediment samples, expressed as the relative proportion of the selected model compounds. Gt: goethite; Fh: ferrihydrite; Py: pyrite. The Fe-clays model compound corresponds to a Fe-bearing illite-nontronite-serpentine combination with 30/27/43 respective proportions.

( 1 )

 1 Figure VI-SI-1: Fe K-edge EXAFS spectra of selected Fe model compounds (black) and their corresponding target transforms (red, Table VI-SI-2) calculated with the first 3 components obtained by the PCA procedure performed on the experimental EXAFS spectra of the 10 studied sediment samples (Table VI-SI-1).

  . Ce phénomène contribue fortement à l'intensification du cycle de précipitation/dissolution et/ou d'adsorption/désorption ; plus la zone est susceptible d'être balayée par les marées (comme les sédiments à Rhizophora, Figure I-2 ; Baltzer et al. 1969), plus ce phénomène s'intensifie. C'est à ce processus que nous attribuons l'ensemble des observations suggérant l'existence d'un gradient de la fréquence du cycle d'oxydoréduction, diminuant le long de l'estran des zones à Rhizophora aux tannes, en passant par les zones à Avicennia (Figure VII-1).

Figure VII- 1 :

 1 Figure VII-1 : Schématisation d'un profil latéral de mangrove le long de l'estran, représentant les conditions redox en fonction de la profondeur et l'intensité des cycles d'oxydoréduction associés, pour chacune des zones constituant la mangrove.

  pyrite (Figure VII-3) ; ce processus de pyritisation peut également conduire à la précipitation d'autres sulfures de fer (mackinawite, pyrrhotite, greigite ; Morse et Wang 1997 ; Donald et Southam 1999). Ces derniers ne sont pas représentés ici puisqu'ils sont considérés comme des phases minérales mineures en raison du fait qu'ils sont sous le seuil de détection de la spéciation EXAFS (cf. Chapitre III ; Figure III-6).

  Fe(III) en présence de bactéries ou non. Le Fe(III) peut ensuite précipiter pour former des oxyhydroxydes de fer tels que la ferrihydrite, la lepidocrocite et la goethite. Les oxyhydroxydes de fer mal cristallisés, telle que la ferrihydrite peuvent se transformer en phase cristalline plus stable, telle que de la goethite (FigureVII-4).La répétition des cycles de dissolution/précipitation génère successivement des sulfures de fer et des oxyhydroxydes de fer. Seules des conditions stables permettent la croissance cristalline des pyrites, ou la transformation des oxyhydroxydes de fer mal cristallisés en phases plus stables, donc plus cristallines. Plus le cycle d'oxydoréduction est répété, moins les phases peuvent se développer. La plus forte proportion des oxyhydroxydes de fer amorphes observés par spéciation quantitative des spectres EXAFS au seuil du Fe (cf. Chapitre III, Figure, sous Rhizophora peut être due à une plus forte instabilité des conditions redox causée par une plus forte répétition des cycles de précipitation/dissolution. L'hypothèse retenue est la fréquence du cycle des marées. En effet, les sédiments à Rhizophora sont régulièrement brassés par les marées (quelques heures par jours), tandis que les sédiments qui se développent sous Avicennia ne sont immergés que quelques heures par semaine/mois(Baltzer

Figure VII- 3 :

 3 Figure VII-3 : Processus biogéochimique de réduction des oxydes de fer en conditions anoxiques.

Figure VII- 4 :

 4 Figure VII-4 : Processus biogéochimique d'oxydation partielle des phases ferreuses.

Figure VII- 5 :

 5 Figure VII-5 : Présentation générale du cycle du Ni à l'interface oxique-anoxique du sédiment Recyclage du Ni durant la réduction des oxydes de fer porteurs de Ni.

Figure VII- 6 :

 6 Figure VII-6 : Recyclage du Ni suite à l'oxydation partielle des phases réductrices porteuses de Ni.

Figure VII- 7 :

 7 Figure VII-7 : Schématisation conceptuelle du profil sédimentaire d'un sédiment hydromorphe sous Avicennia, au cours de l'exploitation minière (sur une échelle de 100 ans).

Figure VII- 8 :

 8 Figure VII-8 : Recyclage du Ni à l'interface entre l'horizon pyritisé et le front d'oxydation durant l'oxydation d'un profil sédimentaire sous Avicennia (cf. Chapitre VI-B).

  Figure VII-A-1), la particularité de la sensibilité de ces pyrites à l'oxydation, pourrait engendrer une forte remobilisation du Ni, favorisant ainsi la complexation organométallique dans le piégeage à long terme du Ni (Figure VII-9), comme le suggère les résultats de la cristallochimie du Ni (cf. Chapitre IV, Figure IV-3). En conséquence, les pyrites fortement enrichies en Ni pourraient être utilisées comme un traceurs de la fréquence des cycles d'oxydoréduction d'un front géochimique.

Figure VII- 9 :

 9 Figure VII-9 : Schématisation du comportement du Ni au cours de l'oxydation de deux types de génération de pyrites, plus ou moins enrichie en Ni, dans un environment riche en matière organique (exemple des sédiments de mangroves sous Rhizophora).

  Chapitre IV), se comporte probablement différemment que Co et Ni. Ainsi, l'étude de la spéciation du Mn dans les sédiments de mangrove permettrait également de cibler des mécanismes différents au sein de ces sédiments. L'étude du cycle de ces ETM pourrait être de nouveau menée par spectroscopie d'absorption des rayons X au seuil K. En effet, l'analyse de la spéciation EXAFS du Mn et du Co au seuil K ont déjà été réalisées dans les profils latéritiques des massifs ultrabasiques de Nouvelle-Calédonie(Dublet et al., en préparation), et fournissent ainsi, une bonne connaissance de la spéciation du Mn et du Co des matériaux hérités par altération/érosion de ces massifs. (6) Bilan des pertes et gains des ETM La comparaison des concentrations en Fe et en Ni pour chaque zone caractéristique, ainsi que l'étude de l'évolution de ces concentrations, comparées aux invariants géochimiques, permettent de suspecter la présence de processus de stockage et/ou de relarguage de ces ETM, en relation avec les conditions physico-chimiques du sédiment et des facteurs forçant extérieurs (cf. Chpaitre IV, et VI-B.). Les concentrations en amont et en aval des sédiments de mangrove doivent être sondés en détails afin d'estimer le taux de perte et de gain des ETM pour les sédiments de mangrove. Une attention particulière doit être portée sur le transfert des ETM et leurs spéciations dans les eaux de ruissèlement des bassins versant impactés afin d'estimer la quantité d'ETM entrant et sortant du système. (7) Transfert des éléments métalliques vers les organismes vivants En ce qui concerne le transfert d'ETM vers les palétuviers, certains systèmes d'adaptation des métaux jouent un rôle important. En effet, pour lutter contre l'hypersalinité du substrat, les palétuviers de type Avicennia peuvent exclure les sels des feuilles par des glandes excrétrices ; ce système permet également d'éliminer les ETM en excès dans leurs tissus

La mangrove calédonienne : milieu privilégié pour l'étude du cycle des ETM

  . Sur une échelle de temps plus courte, l'influence des flux et reflux successifs de la marée pourrait être comparable à celle des variations saisonnières, et influencer également la spéciation et la mobilité des ETM.

	Appartenant à l'ensemble mélanésien, l'archipel de la Nouvelle-Calédonie se localise dans le
	Pacifique sud occidental (19-23°S ; 158-172°E), à l'Est Nord-Est de l'Australie (Figure I-4).

I-C.

L'archipel est constitué d'une île principale appelée « Grande Terre », d'îles au Nord (les îles Bélep), d'une île au Sud (l'île des Pins), et des quatre îles Loyauté à l'Est

(Ouvéa, Lifou, Tiga, et Maré)

. La superficie totale émergée de la « grande terre » est de 16 372 km 2 , elle s'étend selon un axe NW-SE sur 400 km de long, et 50 à 70 km de large.

I-C.1. Distribution et zonation de la mangrove en Nouvelle-Calédonie

Les mangroves de Nouvelle-Calédonie couvrent une superficie de 35 100 ha (dont 10 000 ha de tannes) répartie sur plus de 80 % du littoral occidental et près de 20 % de la côte est, de la Grande-Terre

(Zonéco, 2007)

. Entre terre et mer, elles constituent une zone tampon entre, d'une part, un lagon de plus de 20 000 km 2 délimité par la plus longue barrière récifale continue au monde et dont de nombreux sites sont classés patrimoine mondial par l'UNESCO, et d'autre part les massifs ultrabasiques et leurs couvertures d'altération. La topographie actuelle de l'île est marquée par une chaîne centrale montagneuse aux vallées encaissées. La côte Est, dans le prolongement de la chaîne centrale, plonge dans le lagon en falaises escarpées. À l'inverse, la côte Ouest est relativement plane sauf au niveau des klippes ophiolitiques. La morphologie de la côte Ouest est donc plus propice au bon développement de la mangrove (ZONECO

  Ces aménagements conduisent à l'isolement ou la disparition de la mangrove. Par exemple, l'aménagement de la bande littorale dans la zone urbaine de Nouméa et la construction de certaines routes littorales sur la mer, entre 1955 et 1993, ont conduit à la destruction de 380 ha de mangroves et autres milieux biologiques

Une des principales sources de dégradation des mangroves provient d'abord d'une pression démographique grandissante sur la zone côtière. En outre, les constructions de grandes infrastructures liées au transport (routes, aéroports et ports) ou au commerce (zones industrielles et commerciales) sont très souvent des projets publics qui mobilisent des moyens importants.

  Le régime hydrologique des rivières calédoniennes est caractérisé par des crues très violentes de courte durée et des étiages très importants de longue durée. Il en résulte des crues soudaines, tout au long de l'année, pendant lesquelles la majorité du transport sédimentaire a

	D'autre part, les massifs ophiolitiques sont également caractérisés par une variation
	importante des régimes de précipitations : le grand massif péridotitique du Sud est le plus
	arrosé, avec 2000 mm/an et des maxima à 4000 mm/an au-dessus de 1000 m d'altitude
	(Moniod et al., 1968), tandis que les klippes de la côte ouest reçoivent environ 1700mm d'eau
	par an en moyenne.

Calédonie est d'environ 1700 mm avec une température moyenne annuelle de 22°C. Il existe cependant un écart important d'une région à une autre, avec moins de 1000 mm/an pour certains secteurs de la côte Ouest, à plus de 4000 mm/an au voisinage des sommets les plus élevés (Mont-Panié, Mont Humboldt, Mont Kouakoué ; Figure I-7). Plus spécifiquement, la grande variabilité spatiale intimement liée au relief très contrasté entre la façade Ouest et la façade Est (Roche, 2001), conduit à une précipitation plus importante sur la façade Est de la Nouvelle-Calédonie que celle de la façade Ouest. Le chapelet de Klippes ultrabasiques situé le long de la façade Ouest joue un rôle de barrière naturelle. Les pluies venant du Sud-Est précipitent fortement sur la façade Est. Au cours du franchissement des massifs ultrabasiques vers l'ouest, les nuages libèrent leur eau, produisant d'importantes précipitations depuis la base jusque sur les parties hautes des massifs pour enfin redescendre moins chargés en pluie sur la façade Ouest (Latham, 1986). Ainsi, la côte ouest de l'île principale est caractérisée par un climat tropical semi-aride (Figure I-7). lieu. Le potentiel élevé d'érosion des sols conjugué à d'importants événements pluviométriques, amènent les cours d'eau à transporter des charges élevées en sédiment. Enfin, le phénomène « El Niño Southern Oscillation » et plus particulièrement la zone de convergence intertropicale (ZCIT) influence le niveau d'activité cyclonique

Analyses de la fraction organique et minérale

	sont d'abord dosés en ICP-OES, puis en ICP-MS selon les concentrations. Enfin, en vue Les techniques analytiques conventionnelles utilisées dans le cadre de cette étude sont bien
	d'une meilleure connaissance de l'ensemble des espèces qui peuvent interagir avec les cations connues et leurs principes de fonctionnement bien documentés dans la littérature. Les aspects
	métalliques dans la phase liquide, les nitrites, les nitrates, les phosphates, et la silice dissoute, instrumentaux ne sont donc pas détaillés ici, cependant l'ensemble des protocoles et
	ont été analysés par auto-analyseur Axflow et le carbone organique dissous par analyseur conditions expérimentales de ces différentes techniques analytiques sont détaillés dans les
	Shimadzu. articles et annexes correspondants des chapitres III à VI.
	En revanche, l'analyse des données de spectroscopie d'absorptions des rayons X est
	relativement complexe. Cette technique représente une grande part de cette étude. Il semble
	En premier lieu, les teneurs totales en carbone organique ont été déterminées dans la fraction donc justifié de détailler quelques aspects de la méthodologie adaptée.
	solide à l'aide d'un analyseur Shimadzu. Concernant la fraction minérale, une étude chimique
	et minéralogique systématique a été réalisée sur l'ensemble des échantillons collectés, pour II-B
	chacun des profils de sédiments. Les concentrations chimiques totales en Na, Ca, K, Mg,Fe,
	Mn, Al, Cr, Ni, Co ont étés déterminées par Fluorescence X, tandis que la diffraction des
	rayons X a permis de déterminer la composition minéralogique des échantillons naturels. La
	conservation des conditions réductrices au cours de la diffraction des rayons X, a été réalisée
	grâce à un système développé par la cellule projet de l'IMPMC (non publié). Les diffractions
	ont étés réalisées d'une part en utilisant une anode au cobalt (λ=0,17889 nm) pour minimiser
	l'absorption du Fe, et d'autre part avec une anode au cuivre (λ=0,15406 nm), pour minimiser
	l'absorption du manganèse.
	La composition chimique et minéralogique des échantillons a été déterminée en fonction de la
	profondeur, ce qui a permis de mettre en évidence l'influence de certains paramètres physico-
	chimiques (Eh, pH, salinité) sur la dynamique de transformation des phases minérales, en
	localisant les fronts actifs de ces transformation. Les échantillons destinés aux analyses plus
	détaillées du solide ont ainsi pû être sélectionnés en fonction de la localisation des fronts
	redox.
	Afin de déterminer les différentes familles de phyllosilicates constituant le solide, la
	minéralogie de la fraction inférieure à 2 µm a été étudiée, d'une part, par DRX à partir de
	préparations orientées et de divers traitements thermiques ou physico-chimiques adaptés à
	l'étude des argiles, et d'autre part, par spectroscopie infrarouge. La distribution des phases
	porteuses des ETM dans les échantillons naturels étudiés au cours de cette thèse, a été
	observée en microscopie électronique à balayage (MEB). Enfin, une attention particulière a
	4 été portée à l'étude de la spéciation des ETM (plus spécifiquement le Fe et le Ni) dans la 2-et Cl -ont été mesurées par
	électrophorèse capillaire, et par chromatographie ionique. Les éléments métalliques dissous fraction solide par spectroscopie d'absorption des rayons X (XAS, X-ray absorption
	dans les différents compartiments : eaux des rivières, eaux de surface et eaux interstitielles), spectroscopy).

.2. Spéciation par spectroscopie d'absorption à rayons X

  La Figure II-4 représente un spectre d'absorption au seuil K du Ni (éjection d'un électron de la couche 1s) d'un échantillon de saponite synthétique. Deux régions spectrales se distinguent, en fonction de la gamme d'énergie des rayons X par rapport à l'énergie de liaison des électrons des couches électroniques proches du noyau. Le spectre XANES (X-Ray Absorption Near Edge Structures) et le spectre EXAFS (Extended X-Ray Absorption Fine Structures)

	La spectroscopie d'absorption des rayons X (XAS, X-ray absorption spectroscopy) est une
	technique exploitant le rayonnement synchrotron et pouvant donner des informations sur
	l'état d'oxydation ainsi que la structure locale (nature et nombre de voisins, distances
	interatomiques) d'un élément spécifique. Le principe de cette technique, ainsi que la
	méthodologie utilisée pour identifier les différentes spéciations d'un élément chimique dans
	un échantillon naturel et quantifier les proportions de chacune de ces spéciations, est abordé
	dans la partie ci-dessous.
	II-B.2.1. Principe de la méthode
	La spectroscopie d'absorption des rayons X repose sur la propriété des atomes à absorber les
	rayons X lorsque ces derniers ont une énergie proche de l'énergie de liaison des électrons des
	couches électroniques proches du noyau (Teo, 1986). Cette technique consiste donc à suivre
	l'évolution du coefficient d'absorption d'un échantillon en fonction de l'énergie du faisceau
	de rayons X incident. Ce coefficient d'absorption est lui-même déduit de la mesure de
	l'intensité du signal en sortie de l'échantillon, c.a.d. soit l'intensité des rayons X transmis, soit
	celle de la fluorescence émise en conséquence de l'absorption.
	Dans une expérience d'absorption des rayons X, l'échantillon est irradié par un faisceau de
	rayons X dont l'énergie varie au voisinage du seuil d'absorption de l'atome que l'on souhaite
	étudier. Il existe plusieurs seuils d'absorption, le seuil K correspond à l'énergie qui induit une
	transition d'un niveau de coeur 1s vers un état vide situé juste au-dessus du niveau de Fermi.
	L'énergie de ce seuil K est spécifique de chaque élément, par exemple celui du fer est à 7112
	eV alors que celui du nickel est à 8333 eV.

Figure II-4 : Schématisation

  des différentes régions spectrales composant un spectre d'absorption des rayons X au seuil K du Ni, du composé de référence de saponite synthétique.

	La région du pré-seuil se situe où l'énergie des photons est inférieure à celle du seuil étudié éléments. Par exemple, si l'environnement local du nickel est différent d'une phase minérale à
	(E 0 ). Dans cette gamme énergétique, les électrons de coeur peuvent être excités vers les une autre, les signaux EXAFS correspondants sont différents.
	premiers états électroniques vides ou partiellement remplis. Lorsque l'énergie des rayons X
	émis atteint l'énergie d'ionisation (énergie du seuil d'absorption), l'énergie d'un photon II-B.2.2. Intérêt de la spectroscopie XANES et EXAFS pour l'étude de la spéciation des ETM
	incident est suffisamment grande pour éjecter un électron de coeur vers le continuum, se dans la fraction solide des sédiments de mangrove
	traduisant par une brusque augmentation du coefficient d'absorption. L'électron éjecté, appelé Les travaux portant sur l'étude de la spéciation des ETM dans les sédiments de mangrove
	photoélectron, possède une énergie cinétique Ec = E -E 0 (pour un photon incident d'énergie jusqu'à maintenant été basés sur des attaques chimiques sélectives. Cependant, les protocoles
	E et une énergie de seuil E 0 ) qui reste faible dans la région du post-seuil, et est principalement d'extractions séquentielles comportent des limites expérimentales quant à leurs aptitudes à
	soumis à de la diffusion multiple par les atomes voisins. permettre d'atteindre la localisation des ETM dans les différents compartiments du sédiment.
	La position du seuil dans le spectre XANES est particulièrement sensible au degré Ces limites dépendent d'une part des protocoles, mais également de la composition du
	d'oxydation de l'élément analysé, ainsi qu'à la nature et la géométrie du site autour de cet sédiment étudié. Par exemple, au cours d'une attaque chimique, certaines études ont montré
	atome (site tétraédrique ou octaédrique par exemple). Ainsi, pour certains éléments, les l'existence de phénomènes de refixation des ETM extraits
	spectres XANES sont suffisamment caractéristiques pour permettre une bonne approche de la
	spéciation, en termes de redox (Morin et al. 2003) et de coordinance (Miot et al. 2009 ;
	Fandeur et al. 2009 ; Reiche et al. 2001).
	Le spectre EXAFS (Extended X-Ray Absorption Fine Structure) correspond à la partie du
	spectre qui s'étend de 50 eV jusqu'à plus de 1000 eV au-delà du seuil. Dans cette gamme
	d'énergie, le photoélectron est doté d'une énergie cinétique plus élevée que dans la région du
	seuil. Ces photoélectrons peuvent être modélisés par une onde électronique sphérique, qui se
	propage à partir de l'atome absorbeur. En l'absence de voisins (atomes isolés), le coefficient
	d'absorption décroît de manière monotone après le seuil, tandis que dans un solide, l'onde
	électronique sortante de l'atome central peut interférer avec celles rétrodiffusées par les
	atomes voisins. Ces interférences se traduisent sur le coefficient d'absorption par la présence
	d'oscillations appelées oscillations EXAFS. Ce spectre est donc particulièrement sensible à
	l'environnement atomique de l'atome absorbeur, principalement aux distance interatomiques
	entre l'atome absorbeur et ses voisins, au nombre d'atomes voisins, et dans une moindre
	mesure, à la nature des voisins (∆Z > 10).
	Les énergies de liaison des électrons étant caractéristiques du numéro atomique, la
	spectroscopie d'absorption des rayons X est une technique chimiquement sélective. Elle
	Le spectre XANES (X-Ray Absorption Near Edge Structure) correspond à la partie du spectre permet ainsi de sonder spécifiquement l'environnement atomique de l'élément chimique
	qui s'étend de quelques eV en dessous du seuil jusqu'à environ 50 eV au-dessus. Le spectre d'intérêt, et ceci même pour de très faibles concentrations (> 1-100 ppm) et quelle que soit la
	XANES est composé d'une part du pré-seuil, et d'autre part du seuil et du post-seuil. cristallinité de la phase qui porte cet élément, et indépendamment de la présence d'autres

Figure II-5 : schématisation du principe de la décomposition linéaire par moindres

	II-B.2.3. Analyse quantitative des spectres EXAFS par combinaison linéaire
	II-B.2.3.1 Méthodologie
	Les échantillons naturels de sédiments de mangrove sont susceptibles de contenir plusieurs
	formes chimiques (espèces) d'un même élément métallique choisi comme élément absorbeur
	pour l'analyse EXAFS.. Chacune de ces espèces contribue au signal EXAFS de l'échantillon
	total. La contribution de chacune de ces espèces au spectre EXAFS est proportionnelle à sa
	concentration dans l'échantillon. En d'autres termes, chaque spectre EXAFS d'un échantillon
	naturel doit pouvoir être décomposé en une combinaison linéaire de spectres EXAFS de
	composés de références correspondant à espèce de l'élément étudié (Figure II-5).
	. Peu de données sont également
	carrés disponibles en ce qui concerne le Ni adsorbé ou incorporé dans la structure des sulfures de (c) d'un spectre expérimental (a ; échantillon de sédiment de la mangrove de référence de la
	fer. Cependant, compte tenu des différences structurales entre les sulfures de fer de type FeS baie de Vavouto, prélevée à 5 cm de profondeur sous Avicennia) avec trois composés modèles
	de références (b ; Ni incorporé dans la structure d'une goethite, d'une serpentine et d'un et les pyrites (FeS 2 ) (Rickard et al., 2007), la spectroscopie d'absorption des rayons X devrait talc). Le résultat de la décomposition linéaire est en rouge.
	permettre de distinguer la proportion de Ni structural dans chacun des types de sulfures de fer.
	Cette étude a donc également pour objectif d'apporter des informations supplémentaires sur la Ainsi, la décomposition linéaire des spectres EXAFS par les spectres de référence adaptés
	cristallochimie du Ni dans les sulfures. permet de quantifier leurs contributions relatives, et ainsi d'estimer la proportion d'élément
	absorbeur, comme le Fe ou le Ni, total sous ses différentes formes. La qualité de la
	décomposition linéaire est estimée par une méthode de moindres carrés, via le paramètre
	NSSR (Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp ) 2 ). Cette approche a déjà été largement utilisée avec succès
	pour quantifier la spéciation des ETM dans les sols

Seuil K de Fe. Les gammes de références au seuil du Fe sont constituées d'oxyhydroxides de

	Chapitre II : Sites d'études, échantillonnages et démarches analytiques
	particulier est notamment porté sur la position du seuil qui sur l'état d'oxydation, afin de
	repérer la présence ou non, de composés plus ou moins réduits.
	II-B.2.3.2. Base de données des composés modèles de références
	Les composés de références sont choisis en fonction des observations chimiques et
	minéralogiques effectuées sur les échantillons analysés. Une bonne connaissance de la
	, des analyses par décomposition linéaire sont systématiquement testées. Le choix des minéralogie des particules en provenance des massifs, ainsi que des phases minérales
	composés de références structuralement les plus pertinentes, ainsi que le nombre de néoformées dans les sédiments et sols typiques de ces environnements, est également
	composantes, s'appuie sur les résultats obtenus en ACP. Cependant, dans le cas où le nombre primordiale pour compléter la gamme de composés de références.
	de composantes principales est faible, et où une composante domine largement les autres, des Les composés de références sont issus de synthèses de laboratoire. Divers protocoles de
	analyses par décomposition linéaire ont été systématiquement testées, en utilisant un des synthèses ont été adaptés de la littérature en fonction du composé souhaité. Les protocoles
	composés de référence sélectionné par l'analyse ACP en association avec d'autres. La qualité utilisés pour synthétiser les composés modèles sélectionnés sont soit décrits dans les articles
	des décompositions linéaires des différents couples de composés de références a ensuite été référents, répertoriés en Figure II-6 et II-7, soit décrits en annexes des articles figurant aux Figure II-6 : Spectres EXAFS enregistrés au seuil K du Fe des composés de références systématiquement comparée. Après cette étape systématique, la différence de qualité de Chapitres IV à VI. utilisés dans cette étude, pour l'analyse quantitative des spectres EXAFS des échantillons décomposition linéaire a été testée avec l'ajout d'un composé de référence. Pour cette naturels par combinaison linéaire par moindres carrés. Les figures de gauches (a, c, e)
	Les composés de références sont systématiquement analysés par DRX, et au MEB ou au montrent les spectres EXAFS, tandis que celles de droite (b, d, f) présentent les transformés dernière étape, seuls les composés de références, expliquant plus de 10% de la spéciation de l'élément chimique, et améliorant la qualité de la décomposition linéaire de plus de 25%, ont MET, afin de contrôler leur pureté. D'autre part, la fluorescence X et l'ICP-AES ont permis de fourrier correspondantes.
	été retenus (Ostergren et al., 1998). D'autre part, le choix des composés de références pour la de déterminer la concentration de l'élément chimique (Fe ou Ni) incorporé, adsorbé ou
	résolution du spectre EXAFS d'un échantillon, est également comparé aux résultats obtenus, complexé. Enfin, ces composés de références sont ensuite analysés par spectroscopie
	sur l'ensemble des échantillons présentant des caractéristiques chimiques et minéralogiques d'absorption à rayons X.
	similaires.
	En parallèle de l'étude des spectres EXAFS, un regard particulier est porté sur la
	reconstitution des transformés de fourrier. Les transformés de fourrier des spectres EXAFS
	traduisent le nombre et la nature et la distance interatomique des atomes voisins de l'atome Fe (Figure II-6 a et b), de Fe incorporé dans la structure des phyllosilicates (Figure II-6 c et
	absorbeur, et sont très sensibles aux interférences entre les contributions provenant de d), de Fe complexé par la matière organique, ainsi que de phosphates, carbonates et sulfures
	différents types de voisins. Ainsi, les décompositions linéaires ont été réalisées sur le spectres de Fe(II) crsitallisés (Figure II-6 e et f).
	EXAFS mais le choix des combinaisons de références pour la reconstitution d'un spectre
	EXAFS a été validé, si la solution obtenue reproduit correctement la transformée de Fourier.
	Finalement, la procédure d'analyse des spectres EXAFS par décomposition linéaire est
	répétée pour les spectres XANES. Le choix des références sélectionnées pour la résolution
	des spectres EXAFS est uniquement validé, si il explique également la structure et l'évolution
	des spectres XANES de l'ensemble des échantillons sélectionnés. Inversement, l'analyse des
	spectres XANES peut également orienter le choix des composés de références. Un regard

Chapitre III : Le cycle du fer dans les sédiments de mangrove, caractérisation des transformations minérales et des processus biogéochimiques Chapitre III : Le c y c l e d u f e r d a n s l e s s é d i m e n t s d e mangrove, caractérisation des transformations minérales et des processus biogéochimiques Intérêt et résumé de l'étude

  Le long d'un transect recoupant la zone intertidale depuis la mer vers la terre, la mangrove est composée de trois principaux couverts végétaux (Rhizophora spp., Avicennia marina et les tannes), distribués en fonction de la durée d'immersion de la marée, qui impose des gradients de teneur en eau et de salinité. Cette distribution des couverts végétaux modifie en retour la teneur en matière organique des sédiments. La combinaison de ces facteurs génère de forts gradients d'oxygénation dans le sédiment, la déplétion en oxygène se développant de la surface vers la profondeur et de l'intérieur des terres vers le front de mer. Ces gradients redox, combinés avec la présence d'une forte teneur en matière organique donneuse d'électrons, permettent la dissolution réductive des oxyhydroxydes de fer et la sulfato-réduction dans les zones anoxiques et influencent ainsi fortement le devenir des phases porteuses du fer. Sous

	La mangrove est un écosystème complexe caractérisé par de forts gradients spatio-temporels
	des paramètres physico-chimiques (teneur en eau, salinité, potentiel d'oxydoréduction, pH).
	Cette spécificité induit d'une part, une grande diversité des processus de décomposition des
	matières organiques, et d'autre part, la succession de réactions de dissolutions/précipitations
	des phases minérales ( Cf. Chapitre I.A4). L'ensemble de ces processus est fortement
	susceptible d'influencer la mobilité des éléments traces métalliques dans les mangroves. Une
	meilleure compréhension des processus biogéochimiques se produisant dans les sédiments de
	mangrove, est primordial afin de mieux prédire le devenir des éléments traces métalliques
	dans ces écosystèmes intertidaux fragiles. Dans ce chapitre, nous nous sommes plus
	spécifiquement intéressés aux processus de transformations minéralogiques et à la
	Figure II-7 : Spectres EXAFS enregistrés au seuil K du Ni des composés de références utilisés dans cette étude, pour l'analyse quantitative des spectres EXAFS des échantillons compréhension des facteurs qui contrôlent leur distribution le long de l'estran.
	naturels par combinaison linéaire. Les figures de gauches (a, c, e, g) montrent les spectres EXAFS, tandis que celles de droite (b, d, f, h) présentent les transformés de fourrier Les processus biogéochimiques des sédiments de mangrove sont, pour une large part,
	correspondantes. représentés par les cycles d'oxydoréduction du fer et du soufre (cf. Chapitre I.A4). La
	sédimentation détritique des mangroves en aval de massifs ultrabasiques est dominée par des
	dépôts d' oxyhydroxydes de fer (Fandeur, 2009; Dublet et al., 2012). Le fer est donc
	l'élément chimique le plus adapté pour l'étude de la caractérisation des processus
	biogéochimique au sein des sédiments de mangrove. Ce chapitre est focalisé sur l'étude de la
	distribution et de la spéciation de Fe dans les sédiments de mangrove le long du gradient
	botanique et de la profondeur. La cristallochimie de Fe à été étudiée à l'aide de la
	Noël et al., in prep spectroscopie d'absorption des rayons X au seuil K de Fe, sur les échantillons de la mangrove
	de la rivière Coco (mangrove de référence, cf. Chapitre II A.1) dont l'embouchure se trouve
	dans la baie de Vavouto.
	L'analyse des spectres XANES et EXAFS montre que les minéraux ferrifères, principalement
	la goethite et les phyllosilicates, hérités de l'érosion des massifs latéritiques, sont les

principaux porteurs de Fe dans les horizons superficiels des sédiments de mangrove. Ensuite, la cristallochimie de Fe évolue le long de l'estran et avec la profondeur, en suivant les gradients redox, dépendant des teneurs en eau de la teneur en carbone organique du sédiment. les tannes, oxique à suboxique, les phases minérales héritées de la sédimentation détritique sont peu modifiées. En revanche, dans les sédiments localisés dans les zones hydromorphes à Avicennia et à Rhizophora, où les conditions sont très rapidement anoxiques (5 à 20 cm de profondeur), les oxyhydroxydes de fer disparaissent progressivement avec la profondeur tandis que le Fe(II) et les sulfures forment essentiellement de la pyrite.

Inversement, dans la partie supérieure des sédiments sous Avicennia et Rhizophora, la présence de quantités significatives de minéraux de type ferrihydrite et de lepidocrocite ainsi que de sulfates dissous, montrent que le Fe(II) dissous et/ou les sulfures de fer s'oxydent, conduisant à la précipitation d'oxyhydroxydes de fer néoformés. Cette oxydation peut être la conséquence de l'intrusion de l'eau de mer oxygénée au début de la marée haute. Ensuite, pendant les périodes de marée haute, le sédiment peut redevenir anoxique en quelques heures, ce qui conduit à une réduction de ces phases ferriques. L'absence de ferrihydrite et la plus faible proportion de lépidocrocite sous Avicennia par rapport à Rhizophora, suggère que les conditions d'oxydoréduction sont plus stables sous Avicennia, moins régulièrement réapprovisionné par l'eau de mer. Cette plus grande stabilité des conditions redox limiter la réoxydation des sulfures et favoriser l'évolution des oxyhydroxydes de fer nanocristallins vers la goethite. Cette différence suggère que l'intensité des cycles redox semble augmenter de manière significative le long de l'estran, des tannes vers rhizophora, en passant par Avicennia.

1,3 , Georges Ona-Nguema 1 , Eric Viollier 4 , Gregory Marakovic 2 , Luca Olivi 5 , Ludovic Delbes 1 , Frédéric Gelebart 1 , Guillaume Morin 1

1 Institut de Minéralogie et de Physique des Milieux Condensés (IMPMC), UMR 7590, CNRS, UPMC, 4 place Jussieu, 75005 Paris, France 2 Koniambo Nickel SAS (KNS), B.P.679, 98860 Koné, Nouvelle Calédonie 3 Institut de Recherche pour le développement (IRD), UR 206/UMR 7590 IMPMC, 98848 Nouméa, New Caledonia 4 Laboratoire de Géochimie des Eaux, UMR 7154, Université Paris Diderot, Paris 7 and Institut de Physique du Globe (IPGP), Bâtiment Lamark, Case Postale 7052, 75205 Paris cedex 13, France 5 Sincrotrone Trieste (ELETTRA), Area Science Park, Strada Statale, 34012 Basovizza, Trieste, Italy

Table III -

 III

	Sample (cm)	% Fe-Gt*	% Fe-Fh*	% Fe-Lp*	% Fe-rich Smect*	% Fe-Srp* % Fe-Py*	Sum of components (%)	NSSR
	Salt Flat
	2.5	28	22	-	34	13	-	97	8.88E-003
	47.5	41	17	-	14	24	-	96	1.27E-002
	Avicennia forest
	2.5	14	-	12	48	31	-	105	2.64E-002
	17.5	10	-	-	36	14	42	102	2.00E-002
	42.5	-	-	-	18	10	67	95	1.39E-002
	Rhizophora forest
	2.5	15	34	10	25	22	-	106	1.26E-002
	7.5	41	-	-	43	14	-	98	3.38E-002
	12.5	-	33	13	29	16	10	101	1.08E-002
	27.5	-	-	-	16	10	69	95	3.03E-002
	47.5	-	-	-	13	14	55	82	3.20E-002

1: Results of the best LCF fits of the k3-weighted Fe K-edge EXAFS spectra of the mangrove sediments, expressed as the relative proportion of the model compounds components. Gt: goethite; Fh: ferrihydrite; Lp: lepidocrocite; Fe-rich Smect: Fe-rich smectite component modelled by an illite-nontronite combination 65/35; Srp: serpentine; Py: pyrite.

Table III -

 III . Addition of a ferrihydrite component improved the NSSR of 40% for the salt flat samples (Figure III-SI-7), and of 60% for the 2.5 cm and 12.5 cm Rhizophora samples. Addition of a lepidocrocite component improved the NSSR of 31% for the 2.5 cm Avicennia sample (TableIII-SI-4), with a better reproduction of the FT peak at ~2.73 Å (uncorrected for phase shift). Both ferrihydrite and lepidocrocite components were then systematically included in the fits in order to compare the whole set of EXAFS spectra on a same basis.However, fitted components below 10% are considered as not significant in TableIII-1 andFigure III-6. For the 2.5 cm and 12.5 cm Rhizophora samples, adding both lepidocrocite and ferrihydrite improves NSSR of 11% and 30%, respectively, compared to a fit solution without lepidocrocite. For all mangrove samples studied, including Fe-humic complex, siderite, vivianite, mackinawite and greigite, did not improve the fit quality, in line with the fact that these components were found to be unacceptable by the TT analysis.

	Figure III-6: Fe K-edge X-ray absorption spectroscopy data of the mangrove sediment
	samples beneath salt flat, Avicennia and Rhizophora. (a, d, g) Experimental k 3 -weighted

Altogether, the results of the LC-LSF analysis of the mangrove sediments EXAFS data indicate that in the sediments collected in the salt flat, Fe is mainly present as Feoxyhydroxides (~60 %), including goethite and, at a lesser extent ferrihydrite and lepidocrocite. The remaining fraction of Fe (~40%) is incorporated into the structure of phyllosilicate minerals, (i.e. illite-nontronite 65/35 combination and serpentine).

In the sediments collected in the Avicennia stand, the proportion of Fe hosted by Feoxyhydroxide minerals (20 -35 %) is much lower than in the salt flat, with about equal amounts of goethite and lepidocrocite, ferrihydrite is absent in these samples. In addition, the proportion of ferric oxyhydroxides regularly decreases with depth, from 25% in the upper horizon to undetectable amounts at 42.5 cm depth. Concomitantly, pyrite appears as an essential Fe pool, with proportions of that increase from 42% at 27.5 cm, to 67% at 42.5 cm depth

. The remaining fraction of Fe is incorporated in phyllosilicate minerals (27 -57 %), this latter fraction decreasing with depth. EXAFS functions (black lines) and LCF curves (red lines);

(b, e, h) corresponding Fourier Transforms; (c, f, i)

results of the LCF procedure expressed as mass concentration (wt % Fe) of each Fe-bearing species in the bulk sediment samples. Gt: goethite; Fh: ferrihydrite; Lp: lepidocrocite; Fe-rich smect: Fe-rich smectite component modelled by an illite-nontronite combination 65/35; Srp: serpentine; Py: pyrite.

  -1). Although the serpentine component is a common constituent of the lateritic source materials of the studied sediments, dioctahedral 2:1 phyllosilicates are not. EXAFS and IRTF analyses indicated that this latter component corresponds to a Fe-rich smectite in the New Caledonia mangrove sediments studied. Such mineral has been previously reported as a major Fe bearing phase in mangrove sediments, and it was proposed to be either newly formed or inherited from past marine transgressions[START_REF] Souza-Júnior | Smectite in mangrove soils of the State of São Paulo, Brazil[END_REF].

Table III -

 III SI-1: Results of the LC-LSF analysis of the Fe K-edge XANES spectra of the mangrove sediments, expressed as the relative proportion of the various model compounds used. Gt: goethite; Fe-rich Smect: Fe-rich smectite (modelled by an illite-nontronite combination 65/35); Srp: serpentine; Py: pyrite.

	Sample (cm)	% Fe-Gt*	%Fe-rich Smct*	%Fe-Srp*	%Fe-Py*	Sum of components (%)	NSSR (1)
	Salt flat
	2.5	31	49	19	-	99	8.77E-003
	47.5	38	30	31	-	99	3.49E-002
	Avicennia forest
	2.5	10	60	32	-	102	2.72E-002
	17.5	10	41	10	43	104	5.96E-003
	42.5	-	22	13	65	100	7.70E-003
	Rhizophora forest
	2.5	37	42	22	-	101	1.16E-002
	7.5	49	22	28	-	99	2.40E-002
	12.5	23	42	21	14	100	6.90E-003
	27.5	-	17	16	65	98	4.75E-003
	47.5	-	12	29	55	96	4.70E-003
	(1) Calculated as

Table III -SI-2:

 III Results of the Principal Component Analysis (PCA) of 10 bulk EXAFS spectra.

a Cumulative variance. b Indicator function

  Component Eingenvalue Variance Cum. Var.

				a	IND	b
	1	127.687	0.479	0.479	0.30664
	2	67.004	0.251	0.731	0.18008
	3	23.739	0.089	0.820	0.17231
	4	16.540	0.062	0.882	0.17027
	5	10.907	0.04	0.923	0.18460
	6	7.912	0.027	0.950	0.23127
	7	6.186	0.023	0.973	0.26056
	8	2.652	0.009	0.983	0.54372
	9	2.479	0.009	0.993	1.81966
	10	1.819	0.006	1.0	NA

Table III -

 III SI-3: Target testing of reference spectra using the first four components obtained by PCA

	References	SPOIL a	NSSR TT (%)
	Pyrite	2.224	2.566
	Schwertmannite	2.8196	8.855
	Ferrihydrite 2L	3.7397	12.141
	Nontronite	3.7736	11.789
	Ferrihydrite 6L	3.7784	12.928
	Illite	3.8651	7.7
	Akaganeite	4.0977	17.233
	Fe-bearing serpentine	4.4989	38.486
	Fe-bearing phlogopite	4.6722	65.9
	Green rust Cl -	4.996	75.133
	Lepidocrocite	5.2011	25.333
	Goethite	5.3112	19.099
	Magnetite	5.4031	45.611
	am-Fe 3 PO 4	5.5184	22.311
	Chlorite	5.6002	66.523
	Fe-bearing vermiculite	6.3149	40.096
	Oxidized vivianite	6.8787	23.213
	Vivianite	7.1584	50.101
	Greigite	7.1851	58.528
	Hematite	7.6694	73.258
	Maghemite	8.2819	45.112
	Fe 3+
	Humate	9.2074	28.474
	Biotite	9.476	46.9
	Mackinawite	17.0911	51.848
	Siderite	24.8274	96.208

Table III -

 III SI-4: NSSR improvement of LC-LSF performed with the four first components (illite-nontronite 65/35 combination, serpentine, goethite and pyrite), with the addition of iron oxyhydroxide. These iron oxyhydroxides are selected by target transform of PCA (TableIII-SI-3). Fh: ferrihydrite; Lp: lepidocrocite; Aka: akaganeite; GR: green rust; Schw: schwertmannite; Mt: magnetite.

		With 4 first	Addition of	Addition of	Addition of	Addition of	Addition of	Addition of
		compounds	Schw		Fh2L		Aka		GR	Lp		Mt
	Sample (cm)	NSSR	NSSR	% NSSR % NSSR % NSSR % NSSR % NSSR %
	Salt Flat
	2.5	1.63E-02 1.22E-02 25 8.88E-03 46 1.46E-02 10 1.63E-02 0 1.54E-02 6 1.56E-02 4
	47.5 1.93E-02 1.55E-02 20 1.27E-02 34 1.70E-02 12 1.91E-02 1 1.93E-02 0 1.91E-02 1
	Avicennia forest
	2.5	3.85E-02 3.81E-02 1 3.74E-02 3 3.82E-02 1 3.84E-02 0 2.64E-02 31 3.56E-	8
	002 17.5 2.00E-02 2.00E-02 0 1.86E-02 7 1.99E-02 1 1.99E-02 1 2.00E-02 0 1.91E-02 5
	42.5 1.39E-02	-	-	-	-	-	-	-	-	-	-	-
	Rhizophora forest
	2.5	4.41E-02 4.41E-02 0 1.41E-02 68 3.90E-02 12 4.17E-02 6 3.99E-02 10 4.23E-02 4
	7.5	3.38E-02 3.38E-02 0 3.38E-02 0 3.38E-02 0 3.28E-02 3 3.38E-02 0 2.98E-02 12
	12.5 3.45E-02 3.45E-02 0 1.65E-02 52 3.06E-02 11 3.34E-02 3 3.29E-02 5 2.56E-02 24
	27

Table IV - 1 :

 IV1 Ni concentration, Eh, pH, Total Organic Carbon (TOC), and mineralogical composition in the studied mangrove sediments collected in the salt flat, and beneath the Avicennia, and the Rhizophora stands.

	Sample (cm) Ni (ppm) Eh (mV)	pH	TOC (%)	OxFe	Srp	Smec	Py
	Salt Flat
	2.5		347	7.7	1.5	x	x	x	-
	7.5		299	7.5	0.9	x	x	x	-
	12.5		267	7.2	0.4	x	x	x	-
	17.5		275	7	0.4	x	x	x	-
	22.5		289	7	0.3	x	x	x	-
	27.5		304	6.9	0.4	x	x	x	-
	32.5		268	6.9	0.4	x	x	x	-
	37.5		274	6.8	0.2	x	x	x	-
	42.5		87	6.9	0.3	x	x	x	-
	47.5				0.4	x	x	x	-
	Avicennia
	2.5		147	5.9	6.5	x	x	x	-
	7.5		66	6.6	7.3
	12.5		36	6.8	9.4
	17.5		-179	6.8	7.7	x	x	x	x
	22.5		-70	6.7	14.7
	27.5		-93	6.6	15.3
	32.5		-83	6.7	11.6
	37.5	996	-160	6.6	7.3
	42.5		-142	6.5	10.3	-	x	x	x
	47.5		-131	6.5	8.7
	Rhizophora
	2.5		241	5	13,7	x	x	x	-
	7.5		-127	6.5	15.5	x	x	x	-
	12.5		-72	6.6	9.2	x	x	x	x
	17.5	817	-65	6.6	6
	22.5		-116	6.6	9.2
	27.5		-121	6.5	5.4	-	x	x	x
	32.5	516	-118	6.5	8.9
	37.5		-167	6.5	4
	42.5		-117	6.3	5.5
	47.5		-52	6.3	10.2	-	x	x	x

Table IV -

 IV SI-1: Results of the Principal Component Analysis (PCA) performed on the EXAFS spectra from the 12 bulk samples analyzed.

	Component	Eingenvalue	Variance	Cum. Var. a	IND b
	1	166.056	0.548	0.548	0.14434
	2	47.228	0.155	0.704	0.1065
	3	22.127	0.073	0.777	0.10448
	4	17.267	0.057	0.834	0.10283
	5	10.069	0.033	0.867	0.12076
	6	8.410	0.027	0.895	0.14975
	7	7.074	0.023	0.918	0.19946
	8	6.02	0.019	0.938	0.29328
	9	5.731	0.018	0.957	0.47673
	10	4.923	0.016	0.974	0.98397
	11	4.124	0.013	0.987	3.73837
	12	3.738	0.012	1	NA

a Cumulative variance. b Indicator function

Table IV -

 IV

	References	SPOIL a	NSSR (%)
	Ni-kerolite	0.7539	4.798
	Ni-saponite	0.9197	6.803
	Ni-talc	1.3586	4.527
	Ni-rich srp	1.8793	9.019
	NiS	2.2356	41.763
	Ni-citrate	2.4789	21.327
	Ni_gluthation	2.9387	46.833
	Ni-rich pyrite	3.3643	30.691
	Ni-montmorillonite	3.4482	8.892
	Ni-mackinawite	3.5247	60.106
	Ni-goethite	3.8706	24.046
	Ni-adsorbed gt	3.9857	10.821
	Ni(OH) 2	4.1606	35.313
	Ni-cysteine	4.4826	44.684
	Ni-pyrite	4.7448	45.295

a SPOIL classification

Table IV -

 IV SI-3: Quantitative results of the best LC-LSF fits performed on the k 3 -weighted Ni K-edge EXAFS data of the studied mangrove sediment samples, expressed as the relative proportion of the selected model compounds. The Ni-clays model compound corresponds to a Ni-bearing montmorillonite-serpentine-talc-kerolite combination with 25/33/27/15 respective proportions. The Ni-poor pyrite contains 1% mol Ni and the Ni-rich pyrite contains 10% mol Ni.

	Sample (cm)	% Ni-goethite	% Ni-clays	% Ni-citrate	% Ni-pyrite	% Ni-rich pyrite	Sum (%)	NSSR (1)
	Salt Flat
	2.5	25	74	-	-	-	99	4.02E-002
	47.5	28	66	-	-	-	94	7.40E-002
	Avicennia forest
	2.5	18	92	-	-	-	110	5.03 E-002
	17.5	19	42	12	32	-	105	1.30 E-001
	22.5	-	28	33	34	11	106	3.94 E-002
	32.5	-	23	26	50	12	111	4.48 E-002
	47.5	-	56	18	44	-	118	6.82 E-002
	Rhizophora forest
	2.5	21	85	-	-	-	106	3.46 E-002
	7.5	10	94	-	-	-	104	4.75 E-002
	12.5	19	35	27	21	-	101	4.33.E-002
	27.5	-	19	49	34	-	102	5.02 E-002
	47.5	-	38	23	49	-	110	3.06E-002
	(1)

Table IV -

 IV

	Samples	σ (Å)	ΔE 0 (eV)
	Ni:-Py (1 mol% Ni)	0.088	1.912
	Ni-rich-pyrite (12 mol% Ni) 0.066	0.406

SI-4 : Values of the parameters used to fit the Ni K-edge EXAFS data of the synthetic Ni-poor and Ni-rich pyrites with Feff-calculated EXAFS data. (see Figure IV-4).

Final Ni speciation (FEFF calculated)

				NSSR (1)
	29h	43	58	0,0655
	30h	28	69	0,0392
	(c)

Table VI -

 VI . This concentration is stable around 13 wt% Fe from 5 to 15 cm depth. It then

	reaches its maximum value (i.e. 29 wt% Fe) at 20 cm depth. Finally, it ranges from 11 to 17
	% wt% Fe, from 25 to 40 cm depth and reaches its lower value (~9 wt% Fe ) below 50 cm
	depth (Figure VI-5c; Table VI-SI-2). Ni concentration is stable around 2900 ppm Ni from 5 to
	20 cm depth (Figure VI-5d; Table VI-SI-2).

,

  Fe K-edge XAS data indicate a Fe speciation similar to that in the overlying horizons. On the opposite to a recent study performed in the mangrove sediments of the Vavouto Bay, these data do not indicate In addition, the traces of dissolved Fe(III) revealed at 40 and 30 cm depth suggest that a partial oxidation of aqueous Fe(II) to Fe(III) occurred (or is still occurring). This observation, together with the very low pH values measured in these horizons (i.e. between 3 and 4;Figure, strongly suggest that precipitation of secondary Fe(III)-oxyhydroxides took place (or is still taking place). All these observations suggest that these intermediate horizons

	the occurrence of Fe(II)-sulfides (Figures VI-12b; Table VI-SI-6). However, Ni K-edge XAS
	data emphasize the occurrence of reduced forms of Ni, and more precisely of Ni associated
	with pyrite (Figures VI-8, and 12a; Table VI-SI-7). These intermediate horizons then likely
	contain remnants of pyrites, probably below the detection limit of Fe K-edge XAS data (<
	10% of total Fe), which suggests that they are former reduced horizons where pyrite firstly

precipitated and where it was secondly strongly oxidized. This hypothesis is in agreement with the quite high Eh values measured in these horizons, as well as with the very high SO 4 2and total Fe concentrations measured in their pore-waters

.

Table VI -

 VI . The second pool is a Ni-rich pyrite represented by the model compound that contains 10 % mol Ni and it accounts for 11% to 36% of total Ni (FigureVI-12c; Table VI-SI-7). These results are in agreement with SEM observations and SEM-EDS analyses, which show the coexistence of two types of pyrite, more or less enriched in Ni in these bottom horizons. The first type corresponds to small framboidal pyrite grains that are enriched in Ni, whereas the second type corresponds to large cuboctahedral pyrite grains that are less enriched in Ni (FigureVI

Table VI -

 VI SI-1: pH, Eh(mV), salinity (g/L), and total Organic Carbon (TOC) in the N 2 -dried mangrove sediments, and nitrates in pore-waters.

	Sample (cm)	pH	Eh (mV) Salinity (g/L) TOC (%) Nitrates (mg/L)
	2	5.1700	298.00	24.500
	5	4.2800	376.00	24.500	5.635	3561
	10	3.5100	457.00	20.500	7.356	5185.7
	15	4.0700	435.00	21.500	6.164	2682.2
	20	3.4200	474.00	15.000	2.367	1866.9
	25	3.0900	486.00	30.000	5.1615	1188
	30	2.9300	449.00	35.000	5.572	1823.7
	40	2.9800	380.00	32.500	4.6625	1704.8
	45	3.2300	354.00	37.000		1722.9
	50	3.9100	322.00	44.000	9.025	1444.2
	55	5.2700	267.00	40.500		2463.2
	60	5.5300	179.00	40.000	8.116	1237.1
	65	5.6000	127.00	40.000
	70	5.8600	110.00	38.000	6.503	1080.3
	Table VI-SI-2: Dissolved Ni, and dissolved Fe(II), Fe(III), and Fe Total , and the dissolved SO4 2-/Cl -molar ratio (blue line) in pore-waters, and concentration of dissolved Ni and Fe in
	the N 2 -dried mangrove sediments.
	Sample
	(cm)

Ni (µmol/L) Fe(II) (mmol/L) Fe(III) (mmol/L) Fe (mmol/L) SO 4 2-/Cl - Ni (ppm) Fe (%) 5

		68.474	3.3088	42.094	0.045402	0.16473	2765.5	16.167
	10	175.17	75.508	28.755	0.10426	0.16074	3148.0	14.167
	15	460.34	48.194	5.7739	0.053968	0.16063	2624.3	17.500
	20	593.17	270.95	0.0000	0.27095	0.16151	910.83	29.167
	25	719.04	336.12	0.0000	0.33612	0.15807	721.50	11.000
	30	531.29	9122.3	1135.3	10.258	0.17863	1429.3	14.000
	40	274.72	14448	9023.2	23.471	0.20347	1467.2	17.000
	45	133.91	29926	0.0000	29.926	0.20311
	50	76.475	14115	0.0000	14.115	0.16650	6044.3	8.3833
	55	167.16	14927	0.0000	14.927	0.17621
	60	51.126	33147	0.0000	33.147	0.19800	4214.2	9.6333
	70	129	8980.2	0.0000	8.9802	0.15336	3431.0	9.0167

Table VI -SI-3:

 VI Results of the Principal Component Analysis (PCA) performed on the Fe Kedge EXAFS spectra from the 10 bulk samples analyzed

a Cumulative variance. b Indicator function

Table VI -

 VI

SI-4: Results of the Target Transformation performed on the Fe K-edge EXAFS spectra from various model compounds using the first three components obtained by the PCA procedure (Table VI-SI-3). The Fe-clays model compound corresponds to a Ni-bearing illite-

nontronite-serpentine combination with 30/27/43 respective proportions.

Table VI -SI-5:

 VI

NSSR improvement of the LC-LSF fits performed on the Fe K-edge EXAFS spectra with the first two components (Fe-bearing serpentine-illite-nontronite combination with 43/30/27 respective proportions and goethite), with the addition of iron oxyhydroxides. Schw: schwertmannite; Aka: akaganeite; Fh: ferrihydrite; Lp: lepidocrocite; Mh: maghemite; Hem: hematite.

Références bibliographiques .....................................................................................................227 Problématique et cadre de la recherche

Le nickel est l'élément trace métallique le plus répandu dans le paysage calédonien. Présent en très forte concentration dans les dépôts latéritiques enrichis des massifs ultrabasiques, il est par conséquent susceptible de s'accumuler dans les sédiments de mangrove de Nouvelle-Calédonie[START_REF] Marchand | The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)[END_REF]. En effet, les concentrations de Ni enregistrées dans les sédiments en aval de massifs ultrabasiques, sont 10 à 100 fois supérieures aux concentrations en Ni répertoriées dans les sédiments de mangroves dans le monde[START_REF] Lewis | Fate and effects of anthropogenic chemicals in mangrove ecosystems: a review[END_REF]. A titre d'exemple, les analyses chimiques des sédiments de mangrove de la rivière Coco (secteur de référence, cf. Chapitre II A.1) de la baie de Vavouto, montrent que les concentrations en Ni varient entre 1000 et 5300 mg.kg -1 , avec une moyenne de 2680 mg.kg -1 . L'objectif de ce chapitre est de comprendre le comportement du nickel au cours des processus biogéochimiques précédemment caractérisés (cf. Chapitre III). Cette étude tente plus particulièrement de déterminer l'influence de ces processus sur la cristallochimie et la mobilité du Ni afin de mieux évaluer l'éventuel rôle de protection des sédiments de mangroves envers le lagon Néo-Calédonien.La cristallochimie de Ni a été déterminée par spectroscopie XANES et EXAFS, au travers de la zone intertidale et en fonction de la profondeur, de la mangrove de la rivière Coco (mangrove de référence, Baie de Vavouto ; cf. Chapitre II-A.1). Afin de comparer la cristallochimie du Ni avec celle du Fe, les échantillons analysés sont en grande partie ceux précédemment étudiés en spectroscopie d'absorption des rayons X au seuil du Fe (baie de Vavouto, rivière Coco ; Cf. Chapitre III). Cette étude met particulièrement l'accent sur le rôle des pyrites, principale phase minérale précipitée dans les zones anoxiques du sédiment, dans le piégeage du Ni à court et à long terme. Cette étude spectroscopique a également été couplée avec une étude par microscopie électronique à balayage, afin d'évaluer la distribution du Ni en fonction de la morphologie des différentes générations de cristaux de pyrites. L'analyse spectroscopique indique que la spéciation de Ni dans les horizons de surface des sédiments de mangrove est dominée par les minéraux apportés par la sédimentation détritique.En effet, dans les horizons de surface de la mangrove, tout au long du transect au travers de l'estran, Ni est essentiellement incorporé dans la structure des phyllosilicates et de la goethite, héritées des matériaux latéritiques érodés. Dans les horizons plus profonds, les

(1) Calculated as Σ(k 3 χ exp -k 3 χ fit ) 2 /Σ(k 3 χ exp )2

ACKNOWLEDGMENTS

This study was funded by Koniambo Nickel SAS (KNS). The implementations of this study and access to studied mangrove have been greatly facilitated by Sylvain Capo (KNS-XTRATA) and Jacques Loquet, president of environmental committee Koniambo. Laure Cordier and Mickael Tharaud (LGE, Paris Diderot) are acknowledged for measurements by ICP-AES and ICP-MS respectively. The authors thank Imène Esteve for her help with Scanning Electron Microscope analyses, Jessica Brest for her help in sample handling and model compound syntheses under anoxic conditions, and Jean-Louis Robert for him help with Infra-Red spectrometer. The technical staffs at ELETTRA (Trieste, Italy) are greatly acknowledged for technical support during XAS measurements. The authors thank Thierry Allard (IMPMC) for providing the iron-humate complex reference compound. ACKNOWLEDGMENTS This study was performed within the framework of a UPMC Paris 6/CNRS (IMPMC)/IRD -Koniambo Nickel SAS (KNS) joint project. Field sampling campains have been greatly facilitated by Sylvain Capo (KNS-GLENCORE), Andy Wright (KNS), Gregory Marakovic (KNS) and Jacques Loquet (Koniambo Environmental Committee). The authors thank Imene Esteve (IMPMC) for her help during Scanning Electron Microscope observations and analyses and Ludovic Delbes (IMPMC) for handling the X-ray diffraction experiments under anoxic conditions. The technical staffs at SSRL (Stanford, USA) and ESRF (Grenoble, France) are also greatly acknowledged for their technical support during XAS measurements.

ACKNOWLEDGEMENTS

This study was funded by Koniambo Nickel SAS (KNS), within the framework of a project aimed at understanding the biogeochemical behavior of metals in the sediments of mangroves from New Caledonia. The access to the studied site was greatly facilitated by Sylvain Capo (KNS-XTRATA), Andy Wright (KNS) and Jacques Loquet (President of The Koniambo Environmental Committee). Laure Cordier and Mickael Tharaud (LGE, Paris Diderot/IPGP) are acknowledged for ICP-AES and ICP-MS measurements, respectively. The authors wish to thank Imene Esteve for help during SEM observations and SEM-EDS analyses and Ludovic Delbes for handling the X-ray diffraction equipment under anoxic conditions. The technical staffs at SSRL (Stanford, USA), ESRF (Grenoble, France), SOLEIL (Saclay, Paris) and ELETTRA (Trieste, Italy) are greatly acknowledged for technical support during XAS measurements.

SUPPORTING INFORMATION

EXAFS analysis of iron cycling in mangrove sediments downstream a lateritized ultramafic watershed (Vavouto Bay, New Caledonia) Vincent Noël 1,2 , Cyril Marchand 3 , Farid Juillot 1,3 , Georges Ona-Nguema 1 , Eric Viollier 4 , Gregory Marakovic 2 , Luca Olivi 5 , Ludovic Delbes 1 , Frédéric Gelebart 1 , Guillaume Morin 1 . Corresponding author: Guillaume Morin; e-mail: guillaume.morin@impmc.upmc.fr

August 2013

Submitted to Geochimica Cosmochimica Acta

This files contains results of pH, infrared spectra, and Fe K-edge pre-edge rgion and XANES spectra, Fe K-edge EXAFS Principal Component Analysis (PCA) and additional LCF fits analysis of Fe K-edge EXAFS.

Chapitre IV : Le cycle du nickel dans les sédiments de mangrove

Intérêt et résumé de l'étude

Ni cycling in mangrove sediments from New Caledonia

Vincent Noël 1,2 , Guillaume Morin 1 , Farid Juillot 1,3 , Cyril Marchand 3 , Jessica Brest 1 , John Bargar 4 , Manuel Munoz 5 , Grégory Marakovic 2 , Sandy Ardo 1 , and Gordon E. Brown Jr.

Environmental Science and Technology

Corresponding author: Vincent Noël; e-mail: vincent.noël@impmc.upmc.fr experimental χ(k) function using a Kaiser-Bessel apodization window with the Bessel weight fixed at 2.5.

The Ni K-edge EXAFS spectra of 12 samples from the three studied sediment cores was analyzed by Principal Component Analysis (PCA) using the SIXPACK code [START_REF] Webb | SIXpack: a graphical user interface for XAS analysis using IFEFFIT[END_REF], in order for estimating the minimum number of components necessary to fit this set of EXAFS spectra [START_REF] Wasserman | EXAFS and principal component analysis: a new shell game[END_REF][START_REF] Ressler | Quantitative speciation of Mn-bearing particulates emitted from autos burning (methylcyclopentadienyl) manganese tricarbonyl-added gasolines using XANES spectroscopy[END_REF]. Appropriate model compounds spectra were then selected by Target Transform (TT) analysis of our large set of natural synthetic model compounds spectra. The details of this PCA-TT procedure are reported in Supporting Information. The 12 Ni K-edge EXAFS spectra were then fit by a Linear Combination Least Squares Fitting (LCF-LSF) procedure using a home made software based on Levenberg-Marquardt minimization algorithm. The quality of the LCF-LSF fits was estimated by a R-factor parameter (Rf) of the following form:

The accuracy of this fitting procedure ranges between ± 25% and ± 5% of the stated values for each individual contribution and the detection limit for minor species is estimated to be ~10%. Contributions below this limit are considered as not significant [START_REF] Ostergren | Quantitative Speciation of Lead in Selected Mine Tailings from Leadville , CO[END_REF][START_REF] Cancès | XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant[END_REF]Juillot et al., 2011).

In addition to this LSC-LSF analysis of EXAFS data of the natural mangrove sediment samples, the local structure around Ni in the Ni-pyrite and Ni-rich-pyrite model compounds was determined by fitting the Ni K-edge EXAFS spectra with theoretical spectra calculated using the Feff 8.10 code [START_REF] Ankudinov | Real-space multiple-scattering calculation and interpretation of X-ray-absorption near-edge structure[END_REF]. Calculations were performed on clusters of 8 Å in diameter around the central Ni absorbing atom, using all single and multiple scattering paths (nleg = 8), and without using the self consistent potential option for faster calculations.

Calculated spectra were least-squares fit to experimental ones using a Levenberg-Marquard minimization algorithm including an overall energy threshold shift (∆E 0 ) and an overall Debye-Waller σ as the two only fitting variables. As detailed in the result section, the structures of the atomic clusters were modified from the crystallographic structures of pyrite [START_REF] Bayliss | Crystal structure refinement of a weakly anisotropic pyrite[END_REF] and vaesite [START_REF] Nowack | Charge Densities in CoS2 and NiS2 (Pyrite Structure)[END_REF] for the Ni-pyrite and Ni-rich-pyrite samples, respectively.

SUPPORTING INFORMATION

Ni cycling in mangrove sediments (New Caledonia)

Environmental Science and Technology

Corresponding author: Vincent Noël; e-mail: vincent.noël@impmc.upmc.fr This files contains results of Ni K-edge EXAFS Principal Component Analysis (PCA), additional LCF fits analysis of Ni K-edge EXAFS spectra, and Scanning Electron Microscopy (SEM). Additional materials and methods, and model compounds for X-ray Absorption Spectroscopy are given at the beginning of this document.

Additional materials and methods

Extraction of clay fraction

In order to determine the Ni-bearing speciation by X-ray absorption spectroscopy in the clay fraction of the sediments, 1g of bulk sample and 0.25g sodium hexametaphosphate was stired in 50mL of distilled water, and the < 2 µm size fraction of the sediment samples was isolated in deionised water in the presence of air, using sedimentation procedures based on the Stokes' law. Finally, fraction was purified with a 1 M H 2 O 2 solution, to dissolve the organic matter, et a 0.1 M HCl solution, to dissolve the iron oxyhydroxides. The solid fraction was collected after centrifugation at 30,000 G for 10 min, then it was then rinsed three times with distilled water and finally dried in dessicator.

X ray diffraction

Mineralogical characterization of model compounds by XRD was performed with a Panalytical® X'Pert Pro diffractometer using Co K-alpha radiation in order to minimize incident X-ray absorption by Fe. XRD data were recorded in continuous scan mode between 5 and 80° 2θ with 0.017° steps using an X'celerator® detector, and counting 4h per scan.

Samples were mounted within the glove box and transferred in an anoxic sample chamber for XRD data collection in order to avoid any changes in the mineralogy related to possible redox reaction with air.

XAS data collection

XANES and EXAFS Ni K-edge spectra were collected on 12 samples from the three studied sediment cores. The data were recorded in fluorescence-yield at 10 K on the high-flux wiggler beamline BL 11-2 at the Stanford Synchrotron Radiation Lightsource (SSRL, CA, USA).

XAS data for synthetic samples were recorded in both fluorescence-yield at 20 K on the SAMBA beamline (SOLEIL, Saint-Aubin, France) and transmission detection mode at 40K on the BM23 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Fluorescence-yield spectra were collected with a Canberra ® high-throughput Ge 30element solid-state array detector. The energy of the incoming x-ray beam was monitored over the 8200-9100 eV energy range, with a Si(220) double-crystal monochromator on beamlines BL 11-2 (SSRL) and BM23 (ESRF), and using a double crystal monochromator equipped with Si(111) crystals on beamline SAMBA (SOLEIL). Dynamic sagittal focusing of the second crystal was used at this latter beamline [START_REF] Briois | SAMBA : The 4 -40 keV X-ray absorption spectroscopy beamline at SOLEIL[END_REF]

France

To be submitted Chapitre V : Mécanisme de formation des pyrites et étude du comportement des éléménts métalliques traces au cours de la pyritisation without transition metal cation impurities, 4 mL a 0.625 M FeCl 3 .6H 2 O solution was added in 42 mL of double-distilled and oxygen-free water, while stirring. Then, 4mL a 0.1 M Na 2 S.9H 2 O solution was subsequently added to the ferric solution while stirring. For experiment performed with dissolved Mn 2+ , or Co 2+ , or Ni 2+ , 3.9 mL a 0.625 M FeCl 3 .6H 2 O solution and 0.1mL a 0.1 M metal (Mn 2+ , or Co 2+ , or Ni 2+ ) chloride solution was added in 42 mL of double-distilled and oxygen-free water, while stirring. Then, 4mL a 0.1 M Na 2 S.9H 2 O solution was subsequently added to the metal solution while stirring. The four suspensions were sealed in anoxic conditions and gently stirred in the anaerobic chamber. Over the course of pyrite formation, aliquots of 3 mL were regularly collected from the reacting suspensions.

The solid phase was harvested after centrifugation at 8,500 G for 10 min, and it was then rinsed three times with double-distilled and oxygen-free water, and finally dried under vacuum within the anaerobic glove box. The aqueous phase, was collected after centrifugation, filtered through 0.22 µm cellulose filters, and stored under anoxic conditions at 4°C, until analyzes.

V-2.2. Aqueous phase analyses

Dissolved metals concentrations in the aqueous phase, were measured by ICP-AES (Icap ThermoFisher Scientific at University Paris Diderot) after 100 times dilution and HR-ICP-MS with an integrated FAST (ESI) injection system (Element 2 Thermofisher scientific at University Paris Diderot) after 2000 times dilution. Certified reference materials (SLRS4) were intercalated during the analytical series.

V-2.3. Mineral and crystallography analyses of solid phase

Mineralogical composition of the solid samples was determined by X-ray diffraction (XRD).

Poorly ordered phases were analyzed by Wide-angle X-ray scattering (WAXS) and pair distribution function (PDF) analysis. In order to avoid any possible redox reaction of the solid samples with air, XRD and WAXS analyses were performed using an anoxic sample chamber developed at IMPMC and in sealed glass capillaries, respectively. Micro and nano-scale distribution of the transition metal cations were mapped by Scanning Electron Microscopy (SEM), and transmission electron microscopy (TEM) coupled with energy dispersive X-ray analysis (EDXS). The molecular environment of Ni and Fe was probed using X -ray

AKNOWLEDGMENTS

The authors thank Imène Esteve (IMPMC) for her help with Scanning Electron Microscope analyses, also that Ludovic Delbes and Frédéric Gélébart (IMPMC) for handling the X-ray diffraction experiments under anoxic conditions. Laure Cordier and Mickael Tharaud (LGE, Paris Diderot) are acknowledged for measurements by ICP-AES and ICP-MS respectively.

The technical staffs at SOLEIL (Paris, France) and at ESRF (Grenoble, France) are greatly acknowledged for technical support during XAS measurement.

SUPPORTING INFORMATION

France

To be submitted This files contains WAXS spectra, Fe K-edge EXAFS results and additional LCF fits analysis of Ni K-edge EXAFS, TEM images and their SAED, SEM images. Additional materials and methods are given at the beginning of this document.

Additional materials ands method X ray diffraction

Mineralogical composition of the solid samples was determined by X-ray powder diffraction (XRD) using CoKα radiation in order to minimize the X-ray absorption of Fe. Data were collected in Bragg-Brentano geometry using a Panalytical® X'Pert Pro diffractometer equipped with an X'celerator® detector. A continuous collection mode was applied over the 3-80° 2θ range with a 0.016° 2θ step and counting 3h for each sample.

Wide-angle X-ray scattering

Total X-ray scattering data were collected at the IMPMC, using a Panalytical Empyrean diffractometer equipped with an Ag anode and a point detector, in a θθ transmission geometry, on samples in borosilicate capillaries. Measurements up to 2θ=148° with Ag K α radiation (0.56 Å or 22.16keV) allowed us to obtain data up to Q max = 21.8 A -1 , where Q is the magnitude of the scattering vector, Q= (4πsinθ)/λ).

To minimize the statistical noise level at high Q values (due to low form factor at high angle), the exposure time used for 2°<2θ<50° (10s per 0.2° step) was doubled for 50°<2θ<90° and quadrupled for 90°<2θ<148°.

The pair distribution function G(r) gives the probability of finding a pair of atoms separated by a distance r. It is experimentally obtained from the sine Fourier transform of the scattering function S(Q) [START_REF] Egami | Underneath the Bragg Peaks: Structural Analysis of Complex Materials[END_REF][START_REF] Farrow | Relationship between the atomic pair distribution function and small-angle scat-tering: Implications for modeling of nanoparticles[END_REF] as shown in the following equation :

PDFgetX3 [START_REF] Juhas | PDFgetX3: a rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions[END_REF] was used to process the experimental powder diffraction intensity to generate G(r). The observed PDF was fitted with the calculated one from structure models using PDFgui [START_REF] Farrow | PDFfit2 and PDFgui: computer programs for studying nanostructure in crystals[END_REF]. Regarding the solid fraction collected at 0.5 h from the control experience, the best solution was found by using the Mackinawite structure described by [START_REF] Redfern | Synthesis and Rietveld crystal structure refinement of mackinawite[END_REF]. microscope (IMPMC, Paris) operating at 200 kV, equipped with a field emission gun, a highresolution (UHR) pole piece and a Gatan energy filter GIF 2001. Scanning transmission electron microscopy (STEM) in high angle annular dark field (HAADF) allowed Z-contrast imaging. Qualitative elemental compositions were determined by EDXS performed using a JEOL detector with an ultrathin window allowing detection of light elements.

X ray absorption spectroscopy

Fe and Ni K-edge X-ray Absorption Fine Structure (EXAFS) spectra of the Fe sulfide model compounds were collected in transmission detection mode at 20K on the SAMBA beamline (SOLEIL, Saint-Aubin, France) and at 40K on the BM23 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). using a Si(220) double crystal monochromator, equipped with dynamic sagittal focusing of the second crystal [START_REF] Briois | SAMBA : The 4 -40 keV X-ray absorption spectroscopy beamline at SOLEIL[END_REF]. In order to strictly preserve the oxidation state of iron during the analyses, all mangrove samples and model compounds were mounted on the cryostat sample rod within a glove bag or a glove box next to the beamline and carried to the beamline in a liquid nitrogen bath before being rapidly transferred into the cryostat. Energy was calibrated by setting to 7112 eV the energy position of the first inflection point in the K-edge of a Fe foil recorded in double transmission setup. For each mangrove sample and model compound, 2 to 6 scans were recorded, depending on the Fe concentration that varied between 1 and 30 wt% Fe. Data were averaged and normalized using the ATHENA software [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF].

EXAFS spectra were then background subtracted from these normalized data using the XAFS code [START_REF] Winterer | XAFS -a data analysis program for materials science[END_REF]. Radial distribution functions around the Fe absorber were obtained by Fast-Fourier-transformation of the k 3 -weighted experimental χ(k) function using a Kaiser-Bessel apodization window with the Bessel weight fixed at 2.5.

Model compounds for Ni K-edge XAS analyses

A large set of experimental Ni K-edge XAS spectra from natural and synthetic model compounds was used for LC-LSF analysis of the extended X-ray absorption fine structure (EXAFS) spectra of the particles collected in experiments. Our Ni K-edge model compounds data set includes already described and collected data for natural Ni-rich pyrite (Fe 0.88 Ni 0.12 S 2 ), synthetic amorphous NiS, and Fe sulfides containing 1 mol% Ni, consisted of Ni-subsituted sulfides (Ni-pyrite, Ni-mackinawite, Ni-greigite), and of Ni-sorbed sulfides (Nisorbed pyrite, Ni-sorbed mackinawite). Synthesis procedures also that the description of EXAFS spectra of these model compounds are detailed in Noël et al. (in prep., a).

VI.B. Evolution de la spéciation du Fe et du Ni au cours de l'oxydation intense d'une ancienne mangrove sous influence minière

Fe and Ni behavior during re-oxidation of former mangrove sediments in response to isolation from the sea (Dumbea Bay, New Caledonia) [START_REF] Cosmidis | Biomineralization of mixed valence iron-phosphates in the anoxic water column of Lake Pavin (Massif Central, France)[END_REF]; Fe(II) carbonate as siderite (FeCO 3 ) [START_REF] Dublet | XAS Evidence for Ni Sequestration by Siderite in a lateritic Ni-deposit from New Caledonia[END_REF]; and Fe(II) sulfides as pyrite (FeS 2 ), mackinawite (FeS) (Noël et al., submitted). In addition, our data set includes Fe K-edge XAS data for natural phyllosilicates model compound [START_REF] Cosmidis | Biomineralization of mixed valence iron-phosphates in the anoxic water column of Lake Pavin (Massif Central, France)[END_REF]Othmane et al., in press), such as illite from Le Puy en Velais, France (Ildefonse et al., 1998), Mg-rich chlorite (kindly provided by D. Beaufort, University of Poitiers, France), biotite (Mineralogy Collection of IMPMC, Paris, France), serpentine from a highly crystalline serpentine sample from the New Idria Mining District (California, USA) containing 3.45 wt% Fe, and Garfield nontronite (Washington, USA) [START_REF] Bonnin | Sites Occupancy of Fe 3 + in Garfield Nontronite: A Spectroscopic Study[END_REF]Noël et al., submitted). A Fe-humate model compounds consisting of purified colloidal material from the Rio Negro River (Brazil) [START_REF] Cosmidis | Biomineralization of mixed valence iron-phosphates in the anoxic water column of Lake Pavin (Massif Central, France)[END_REF]. To complete this data set, a greigite (Fe 3 S 4 ) sample was also synthesized following the procedure by Noël et al. (in prep., a) that these Fe species are likely to represent Fe speciation in the 10 samples of bulk sediments studied at the Fe K-edge. species. This process is in agreement with the already known ability of sulfide minerals to incorporate a large set of transition metals (Huerta-Diaz and [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF][START_REF] Aragon | Microscopic analysis of pyrite in the sediments of two Brazilian mangrove ecosystems[END_REF]. These mineral species can then act as a very efficient sink for these elements under the anoxic conditions of mangrove sediments. Several studies already indicated that sulfides dominate the partitioning of Ni in the solid fraction of recent sediments, this element being incorporated into Fe(II)-sulfides or forming a pure Ni-S phase (Huerta-Diaz et [START_REF] Huerta-Diaz | Pyritization of trace metals in anoxic marine sediments[END_REF]Burton et al., 2006;[START_REF] Doig | Nickel partitioning in formulated and natural freshwater sediments[END_REF]. Such an incorporation of Ni within the structure of pyrite at the bottom of mangrove sediments is also suspected in a parallel study performed in the Vavouto Bay that is located downstream the Koniambo ultramafic regolith (Northern Province, New Caledonia; Noël et al., in prep., a). In the present study, the results of the LC-LSF fits performed on the Ni K-edge EXAFS data from the samples collected in (1) la modélisation des équilibres solide/liquide et la prédiction de la spéciation des métaux dissous ;

(2) la compréhension des mécanismes de pyritisation en présence de métaux ;

(3) la réduction des oxyhydroxydes de fer par les bactéries ferri-et/ou sulfato-réductrices en présence ou en absence de métaux afin de déterminer les paramètres qui contrôlent la formation des pyrites par voie microbienne ;

(