Emmanuelle Sisavath
email: sisavath_emmanuelle@hotmail.fr

Aude Mazuel

Stephan J Jorry

Nathalie Babonneau

Patrick Bachèlery

Béatrice De Voogd

Marie Salpin

Laurent Emmanuel

Luc Beaufort

Samuel Toucanne

Processes controlling a volcaniclastic turbiditic system during the last climatic cycle: Example of the Cilaos deep-sea fan, offshore La Réunion Island

Keywords: Turbidites, La Réunion Island, Indian Ocean, Late Quaternary, Volcaniclastic system

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

 , 2012, 281, pp.180-193. 10.1016/j.sedgeo.2012.09.010. insu-00771704

Introduction

Recent studies based on high-resolution stratigraphy show that past climate and sea-level changes have had a significant influence on deep ocean turbidite sedimentation at both orbital and millennial timescales.

These relationship has been demonstrated over the last few years along both siliciclastic ( [START_REF] Covault | Highstand fans in the California borderland: The overlooked deep-water depositional systems[END_REF]], [START_REF] Jorry | Bundled turbidite deposition in the central Pandora Trough (Gulf of Papua) since Last Glacial Maximum: Linking sediment nature and accumulation to sea level fluctuations at millennial timescale[END_REF], [START_REF] Toucanne | Activity of the turbidite levees of the Celtic-Armorican margin (Bay of Biscay) during the last 30,000 years: Imprints of the last European deglaciation and Heinrich events[END_REF], [START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: influence of climate change on fan sedimentation[END_REF], [START_REF] Jorry | Turbiditic levee deposition in response to climate changes: The Var Sedimentary Ridge (Ligurian Sea)[END_REF] and [START_REF] Toucanne | External controls on turbidite sedimentation on the glacially-influenced Armorican margin (Bay of Biscay, western European margin)[END_REF] among others) and carbonate margins (e.g., [START_REF] Droxler | Glacial versus interglacial sedimentation rates and turbidite frequency in the Bahamas[END_REF], [START_REF] Schlager | Highstand shedding of carbonate platforms[END_REF], [START_REF] Andresen | Timing and distribution of calciturbidites around a deeply submerged carbonate platform in a seismically active setting (Pedro Bank, Northern Nicaragua Rise, Caribbean Sea)[END_REF] and [START_REF] Jorry | Deepwater carbonate deposition in response to re-flooding of carbonate bank and atoll-tops at glacial terminations[END_REF]). Surprisingly, few studies have yet addressed the timing of gravity deposits around volcanic islands, and the forcing factors controlling the sediment delivery in such situations remain unclear ( [START_REF] Alibés | Determination of sediment volumes, accumulation rates and turbidite emplacement frequencies on the Madeira Abyssal Plain (NE Atlantic): a correlation between seismic and borehole data[END_REF] and [START_REF] Frenz | Provenance and pathways of late Quaternary turbidites in the deepwater Agadir Basin, northwest African margin[END_REF]).

The conventional sequence stratigraphy model for clastic systems states that deep marine systems preferably grow during falls in sea-level and at lowstand. However, several studies have demonstrated that some turbidite systems do not follow the classic sequence stratigraphy concepts. [START_REF] Covault | Submarine fans at all sea-level stands: Tectonomorphologic and climatic controls on terrigenous sediment delivery to the deep sea[END_REF] showed that deep-sea deposition occurs at all sea-level states. Terrigenous sediment delivery to the deep-sea depends on many factors, such as the tectono-morphologic character of the margin, climatic forcing and terrestrial sediment source.

The influence of climate and sea-level changes on sediment delivery to volcaniclastic basins is poorly defined and remains a matter of debate. [START_REF] Quidelleur | Causal link between Quaternary paleoclimatic changes and volcanic islands evolution[END_REF] and [START_REF] Mcmurtry | Giant landslides, mega-tsunamis, and paleo-sea level in the Hawaiian Islands[END_REF] suggested that most large volume landslides affecting volcanic islands occur at glacialinterglacial transitions (Terminations) and concluded there was a causal relationship between flank collapses of volcanic islands and global climate change. However, recent data contradict these results, as these showed no link between climate-driven changes and volcanic flank collapses [START_REF] Harris | Large eruption-triggered ocean-island landslide at Tenerife: Onshore record and long-term effects on hazardous pyroclastic dispersal[END_REF][START_REF] Longpré | Age of the El Golfo debris avalanche, El Hierro (Canary Islands): New constraints from laser and furnace 40Ar/39Ar dating[END_REF][START_REF] Rodriguez-Gonzales | The Holocene volcanic history of Gran Canaria island: implications for volcanic hazards[END_REF]. In contrast, the influence of volcanic activity has been widely examined, especially off the Canary Islands. In this area, the main turbidite activity has coincided with phases of high volcanic activity [START_REF] Schmincke | Volcanic evolution of Gran Canria reconstructed from apron sediments: synthesis of VICAP Project Drilling[END_REF][START_REF] Schneider | Epiclastic sedimentation during the upper Miocene-lower Pliocene volcanic hiatus of Gran Canaria: evidence from sites 953 and 954[END_REF].

Since 2006, several oceanographic cruises have been conducted on the submarine flanks of La Réunion Island (Indian Ocean). These cruises led to the discovery of five volcaniclastic deepsea fans linked to major erosional structures visible on land [START_REF] Saint-Ange | A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): gradualism versus catastrophism[END_REF][START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. La Réunion Island offers the opportunity to study a deep depositional system related to an isolated oceanic island, situated far from continental influences. The aim of this paper is to establish the first stratigraphy of the Cilaos turbidite system based on a set of Küllenberg piston cores. We discuss how volcanic activity, climate and sea-level variations have interacted and controlled the input of sediment offshore of La Réunion island over the last 140 ka, leading to the building of a deep-sea fan spreading over hundreds of kilometers on the sea floor.

Physical setting

General setting of La Réunion Island

La Réunion Island is an isolated volcanic system located 750 km from Madagascar in the western part of the Indian Ocean (Fig. 1). It belongs to the Mascarene Archipelago and resulted from the activity of the hotspot that formed the Deccan Trapps (65 Ma ago) and subsequently the Mascarene Plateau and Mauritius Island [START_REF] Bonneville | Evidence from geoid data of a hotspot origin for the southern Mascarene Plateau and Mascarene Islands (Indian Ocean)[END_REF][START_REF] Duncan | Reunion hotspot activity through tertiary time: initial results from the Ocean Drilling Program, leg 115[END_REF][START_REF] Morgan | Hot spot tracks and the opening of the Atlantic and Indian Oceans[END_REF]. It is the youngest and largest island in this group and the only one that has active volcanism today. The island is located on an isolated compartment of the oceanic crust (~67 Ma) bordered by two fracture zones (FZ) separated by 350 km: the Mahanoro FZ to the west and Mauritius FZ to the east (Fig. 1).

La Réunion Island s.s. is the emerged part of a volcanic edifice rising from approximately -4200 m (the oceanic floor) to more than 3000 m above sea level. The subaerial island represents only three percent of the whole edifice [START_REF] De Voogd | Vertical movements and material transport during hotspot activity: seismic reflection profiling offshore La Reunion[END_REF]. The submarine flanks of La Réunion Island are mostly built by accumulation of debris avalanche deposits (Lénat and Labazuy, 1990;[START_REF] Oehler | Recurrence of major flank landslides during the last 2-Ma-history of Reunion Island[END_REF]. In the southwestern part of the island, no major failures or flank collapses have produced deposits for 1 Ma [START_REF] Oehler | Growth and collapse of the Reunion Island volcanoes[END_REF].

The island is elliptical in shape (50 × 70 km) and composed of two basaltic shield-volcanoes:

Piton des Neiges and Piton de la Fournaise (Fig. 1). Activity of Piton de la Fournaise (2632 m high) started less than 0.6 Ma ago and this volcano is still highly active [START_REF] Gillot | Eruptive history of the Piton de la Fournaise volcano, Réunion Island, Indian Ocean[END_REF]. Its morphology is marked by a succession of calderas open to the sea on their eastern sides (Fig. 1). The historic volcanic activity of Piton de la Fournaise has been described by [START_REF] Bachelery | Structural characteristics of historical eruptions of the Piton de la Fournaise (Reunion Island)[END_REF], [START_REF] Lenat | Age and nature of deposits on the submarine flanks of Piton de la Fournaise (Reunion Island)[END_REF], [START_REF] Michon | Morphology of Piton de la Fournaise basaltic shield volcano (La Reunion Island): Characterization and implication in the volcano evolution[END_REF], [START_REF] Peltier | Magma transport and storage at Piton de La Fournaise (La Reunion) between 1972 and 2007: A review of geophysical and geochemical data[END_REF]2008) and [START_REF] Stieltjes | A statistical and probabilitic study of the historic activity of Piton de la Fournaise, Reunion Island, Indian Ocean[END_REF].

The Piton des Neiges volcano (3070 m high) occupies the northwestern two thirds of the island. The principal and most original feature of this volcano is the existence of three major erosional depressions, called "cirques", opened in the heart of the volcano (Fig. 1). The cirques were partly filled by unconsolidated detrital rocks such as volcanic debris, avalanche deposits, debris flow deposits and other breccia [START_REF] Arnaud | Les processus de demantelement des volcans, le cas d'un volcan bouclier en milieu oceanique : le Piton des Neiges (Ile de La Réunion)[END_REF][START_REF] Bret | Deposits related to degradation processes on Piton des Neiges volcano (Reunion Island): overview and geological hazard[END_REF][START_REF] Fèvre | Mécanismes et vitesses d'érosion à l'échelle géologique sur une île volcanique jeune à relief élevé -La Réunion (Océan Indien)[END_REF][START_REF] Oehler | Landslides and spreading of oceanic hot-spot and arc shield volcanoes on Low Strength Layers (LSLs): an analogue modeling approach[END_REF]. Piton des Neiges started to grow more than 2.17 Ma ago, and has been inactive for at least 0.012 Ma [START_REF] Deniel | New Th-230-U-238 and C-14 age determinations from Piton des Neiges volcano, Reunion-A revised chronology for the Differentiated Series[END_REF][START_REF] Mcdougall | The geochronology and evolution of the young volcanic island of Réunion, Indian Ocean[END_REF][START_REF] Quidelleur | New K-Ar ages from La Montagne massif, Reunion Island (Indian Ocean), supporting two geomagnetic events in the time period 2,2-2,0 Ma[END_REF][START_REF] Smietana | Heterogeneity in the Mantle Source of La Réunion Island, Goldschmidt 2010[END_REF]. Its subaerial shield-building stage, known as the Oceanite Series, extended from 2 Ma to approximately 430 ka [START_REF] Deniel | New Th-230-U-238 and C-14 age determinations from Piton des Neiges volcano, Reunion-A revised chronology for the Differentiated Series[END_REF][START_REF] Mcdougall | The geochronology and evolution of the young volcanic island of Réunion, Indian Ocean[END_REF][START_REF] Upton | The basalt of Réunion Island, Indian Ocean[END_REF]. The second stage, known as the Differentiated Series, occurred between 350 and 12 ka [START_REF] Deniel | New Th-230-U-238 and C-14 age determinations from Piton des Neiges volcano, Reunion-A revised chronology for the Differentiated Series[END_REF][START_REF] Gillot | K-Ar chronology of the ultimate activity of Piton des Neiges volcano, Reunion Island, Indian Ocean[END_REF][START_REF] Mcdougall | The geochronology and evolution of the young volcanic island of Réunion, Indian Ocean[END_REF], with the end of the main effusive activity at about 30 ka [START_REF] Gillot | K-Ar chronology of the ultimate activity of Piton des Neiges volcano, Reunion Island, Indian Ocean[END_REF]. The second stage can be divided into three sub-stages. The first, between 350 and 180 ka, corresponded to the beginning of Piton des Neiges magmatic differentiation. This effusive activity produced lava flows that covered most of the edifice, filling existing valleys [START_REF] Gillot | K-Ar chronology of the ultimate activity of Piton des Neiges volcano, Reunion Island, Indian Ocean[END_REF].

The second sub-stage corresponded to a low effusive activity period lasting 40 ka [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF], with mainly explosive activity [START_REF] Kieffer | Grand traits morphologiques de l'île de la Réunion (Océan Indien)[END_REF]. [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF] suggested that this was a major erosional period corresponding to the formation of large and deep depressions: the cirques. A second period of effusive activity took place between 140 and 30 ka.

Local climate

La Réunion Island is located in the subtropical zone, where it is influenced by the South Equatorial Current and subjected to a prevailing southeasterly trade-wind regime. Trade winds from the east induce highly variable precipitation regimes in time and space, which lead to the island having a wet windward side (East) and a dry leeward side (West). Rainfall also varies according to the elevation (dry coast -wetter upland), with maximum rain in the mid-slope area. Average annual rainfall varies from over 12 000 mm per year between 1300 and 2000 m altitude on windward slopes, to as low as 600 mm near the leeward coast.

The late Quaternary climate of La Réunion Island is largely unknown, as no data are available for this area.

Drainage basins and rivers

On La Réunion Island, the wet tropical climate and basaltic terrains cause high erosion rates, which are amplified by seasonal cyclonic conditions [START_REF] Louvat | Present denudation rates on the island of Reunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions[END_REF][START_REF] Rad | Hidden erosion on volcanic islands[END_REF]. These erosion rates are close to those estimated in active orogenic areas, with values ranging between 0.47 -3.4 m.kyr -1 . They result in a dense hydrographic network with more than 750 gullies and rivers on the island, concentrated in drainage basins like the cirques and the main valleys. These drainage basins are located between the planeze areas [START_REF] Ollier | Volcanic geomorphology of northern Viti Levu, Fiji[END_REF], which are relatively unaffected by erosion because water penetrates rather than forming surface runoff [START_REF] Fèvre | Mécanismes et vitesses d'érosion à l'échelle géologique sur une île volcanique jeune à relief élevé -La Réunion (Océan Indien)[END_REF]. In the drainage basins, the rivers are torrential with mechanical erosion rates among the highest measured in the world, ranging from 1200 to 9100 t/km 2 /yr [START_REF] Louvat | Present denudation rates on the island of Reunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions[END_REF].

One of the major rivers of the island is the "Rivière Saint-Etienne". It is a torrential river formed by the junction of the "Bras de Cilaos", which drains the inner part of the Cirque of Cilaos, and the "Bras de la Plaine", which drains the outer eastern part (Fig. 1). The Saint-Etienne River has a drainage basin of about 200 km 2 reaching altitudes of 2500 to 3000 m. Its mean fluvial solid load is estimated around 470 000 m 3 /yr; up to 1-2 million m 3 /yr during large floods (SOGREAH, 1998). In addition to this drainage basin, the outer western part of the Cirque of Cilaos is incised by many gullies (Fig. 1). The resulting drainage basin for the Cirque of Cilaos ("Rivière Saint-Etienne" and gullies) is about 360 km 2 .

The transition between the subaerial and submarine environments is marked by a narrow shelf. The local absence of this shelf and the presence of steep submarine slopes around the island imply a rapid transfer of sediment from the coast toward the base of the volcanic edifice, allowing the formation of a volcaniclastic deep sea fan.

The Cilaos turbidite system

Recent oceanographic cruises over the submarine flanks of La Réunion Island and the surrounding oceanic plate led to the discovery of five volcaniclastic turbidite systems (Fig. 1) extending over 200 km away from the island [START_REF] Saint-Ange | A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): gradualism versus catastrophism[END_REF][START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF].

On land, these systems are related to major erosional features that constitute the main drainage area of the island. In each case, submarine canyons are directly connected to the main river mouths. The Mafate fan is connected to the Cirque of Mafate, and the Saint-Joseph fan is the only system connected to the Piton de la Fournaise volcano. The latter is considered an embryonic stage fan. Finally, the Cilaos fan is the widest fan and is connected to the Cirque of Cilaos.

The Cilaos turbidite system is located to the southwest of La Réunion Island. It is more than 250 km long and covers an area of about 15 000 km 2 . This sedimentary system extends from the Saint-Etienne river mouth to the Mahanoro fracture zone (Fig. 1). It starts at the coast, with a 70 km long bypass area that directly feeds a deep-sea fan developing at about 4500 m water depth. The Cilaos fan extends over a complex abyssal plain composed of NE-SW volcanic ridges [START_REF] Saint-Ange | A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): gradualism versus catastrophism[END_REF].

The canyon area (Fig. 1; outlined in blue) is composed of two main canyons, Saint-Etienne and Pierrefonds. The Pierrefonds canyon is located in front of the older Saint-Etienne River delta and is connected to the shelf by many tributary canyons. The Saint-Etienne canyon is 4 km wide and is directly connected to the present day Saint-Etienne River. Both canyons merge into the single wide Cilaos canyon, which feeds the main deep sea fan body.

The fan can be divided into two parts: the proximal fan (Fig. 1; outlined in red) and distal fan (Fig. 1; outlined in yellow). The proximal fan is broad, with a maximum width of 120 km and low reflectivity of backscatter data. It is composed of elongated bodies, interpreted as small lobes. The sedimentation in the proximal fan is characterized by coarse sandy turbidites [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. The distal fan is divided into three parts, western, central and eastern, by pre-existing volcanic ridges. The distal prolongation of the turbidite system is visible at the ends of the western and central parts. It is characterized by elongated structures, extending via narrow channels from the proximal fan. The sediments of the distal fan are composed of a succession of fine sandy turbidites covered by a thick clay layer (about 3 m in thickness; Figs.

2 and 3).

Materials and Methods

In this paper, we used seven Küllenberg piston cores taken around La Réunion Island during the oceanographic cruises ERODER 1, onboard the BHO Beautemps-Beaupré in 2006;

FOREVER, onboard the R/V Atalante in 2006; and ERODER2, onboard the R/V Meteor in January 2008 (Fig. 1, Table 1). Five cores were taken from locations in the Cilaos fan (KERO-09, KERO-16, KERO-12, KERO-15 and FOR-C1). Additional cores from the Mafate fan (KERO-07, Fig. 1) and the Saint-Joseph fan (KERO-08, Fig. 1) were used to build a regional age model. All the cores were situated and correlated using Parasound and 3.5 kHz echosounder profiles acquired during the FOREVER and ERODER2 cruises (Fig. 3).

Sedimentary descriptions were made of all the cores, with a particular emphasis on sediment color, visual grain size and turbidite/hemipelagite/pelagite differentiation. Two main types of sediment were distinguished: volcaniclastic sandy turbidites and hemipelagic sediments.

A series of 1-cm-thick sediment slabs were collected from each split core section and examined by X-radiography using a SCOPIX digital X-ray imaging system [START_REF] Migeon | SCOPIX: A new X-ray imaging system for core analysis[END_REF]. Digital images were acquired to provide a precise identification of the sedimentary structures. Sediment cores were sampled for grain-size analyses using a Coulter laser microgranulometer (LS130). The variation of Ca through each of the cores was measured with an Avaatech XRF Core-Scanner equipped with a variable optical system allowing measurements at resolutions between 10 and 0.1 mm. The selected measurement area was 8 mm and the step-size was set at 1 cm.

Oxygen isotope analyses were conducted on small batches of Globigerinoides rubber, the monospecific planktonic foraminifer that calcifies in the surface mixed layer, from cores KERO-07, KERO-08, KERO-09 and KERO-16. Samples were collected at hemipelagic intervals, representing intervals of continuous sedimentation, excluding turbidites. Cores were sub-sampled with a sample spacing of 5 to 20 cm. On average, 15 specimens were picked out from the >150 mm fraction. Using a common 100% phosphoric acid bath at 90°C, 20-50 µg of sample were reacted and analyzed using a GV Isoprime isotope ratio mass spectrometer at University of Pierre & Marie Curie (Paris). Isotope values are given in delta notation relative to Vienna Peedee belemnite. Repeated analyses of a marble working standard (calibrated against the international standard NBS-19) indicate an accuracy and precision of 0.1‰ (1σ).

In core KERO-16, the last occurrence of pink-pigmented G. ruber indicates the transition between Marine Isotopic Stage (MIS) 6 and MIS 5 [START_REF] Thompson | Disappearance of pinkpigmented Globigerinoides ruber at 120,000 yr BP in the Indian and Pacific Oceans[END_REF].

Nine AMS radiocarbon dates were obtained on the cores (Table 2). For each sample, about 10 mg of G. ruber and G. sacculifer specimens were picked out from the >150 mm fraction, washed in an ultrasonic bath with distilled water, and dried. These samples were then analyzed at the Poznan Radiocarbon Lab., Poland, and at the "Laboratoire de Mesure du

Carbone 14" at Saclay, France. Reported radiocarbon ages were corrected for a marine reservoir effect of 400 years and converted to calendar years using CALIB Rev 6.0 [START_REF] Reimer | Intcal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP[END_REF]. Calibrated kilo years before the present will be referred as ka.

The preservation of the test surface of the foraminifer G. ruber was examined by Scanning Electron Microscopy (SEM, Philips XL30). The analysis was performed on G. ruber from seven samples studied for  18 O measurements (three in core KERO-09 and four in core . Foraminifera were placed on adhesive carbon tabs and coated with gold. The observation of the test surfaces was done in the secondary electron mode at 10 kV voltage and at a distance of 10 mm.

In core KERO-09, five representative samples associated with particular sedimentary facies were also analyzed for calcareous nannofossil biostratigraphy (Table 3). Smear slides were made directly from unprocessed samples and were examined with a polarized light microscope at a magnification of 1000×.

Results

Lithology and echosounding facies

Based on the grain-size characteristics, internal sedimentary structures, erosive contacts with underlying sediments and the abundance of glass shards and volcanic crystals, all the sandy beds in the studied cores were interpreted as volcaniclastic turbidites [START_REF] Saint-Ange | A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): gradualism versus catastrophism[END_REF][START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. These turbidite units ranged from a few centimeters up to 20 cm in thickness (Fig. 2).

Cores KERO-09 and KERO-12, taken in the western part of the Cilaos distal fan at about 215 km from the island, were 6.27 and 6.40 m long, respectively (Fig. 1). The lower parts of these cores are characterized by a succession of four sandy units of 30 to 50 cm thick, composed of well-sorted fine sand showing typical Bouma Tb to Te sequences [START_REF] Bouma | Sedimentology of some flysh deposits: a graphic approach to facies interpretation[END_REF]) (Fig. 3). On the echosounder profiles, this lowest unit corresponds to a stratified unit named U1 (Fig. 3). A thick layer of clay (about 3 m thick), showing an alternation of light brown clay and darker brown clay, overlies this unit. The light brown clay is dominated by calcareous sediment (composed of nannoplankton and foraminifera), while the dark brown clay mainly contains siliceous organisms (radiolarians and diatoms). On the echosounder profiles, it corresponds to a semi-transparent unit, named U2 (Fig. 3).

Core KERO-15 was taken in the most distal part of the Cilaos distal fan at about 280 km from the island (Fig. 1). This 6.68 m-long core shows a sedimentary pattern similar to that observed in cores KERO-09 and KERO-12. The base of the core shows a succession of seven fine-sandy layers (20 to 80 cm thick) with typical Bouma Tb to Te sequences, and belongs to unit U1 on the echosounder profiles (Fig. 3). The upper part of the core is composed of a clay layer of about 2 m thick showing an alternation of light brown clay and darker brown clay, corresponding to the unit U2 on the echosounder profiles (Fig. 3).

Core KERO-16 (4.95 m) was taken at a water depth of 4340 m, in the central part of the Cilaos distal fan, on the right side of a channel. Clay layers (alternation of light brown clay and darker brown clay) dominate the lithological succession in the lower part of the core, which locally includes small bioturbation features. This unit corresponds to unit U2 on the echosounder profiles. At the top of the core (0-1.4 meters below seafloor, mbsf), fine-grained turbidite deposits of few centimeters thickness are visible (Fig. 2). They were interpreted as overflow deposits. On the echosounder profiles, they correspond to the upper part of the profiles, characterized by a stratified unit named U3.

Core FOR-C1 was taken at the top of a sedimentary ridge in the central part of the distal fan at a water depth of 4074 m (Fig. 1). The 2.50 meters forming the base of the core are composed of an alternation of clay and thin sandy turbidites (less than 10 cm thick). This unit corresponds to the unit U1 on the echosounder profiles (Fig. 3). It is overlain by 1.50 m of clay (alternation of light brown clay and darker brown clay) corresponding to the semitransparent unit U2 on the echosounder profiles (Fig. 3). The top 0.54 cm are composed of clay interbedded with thin sandy layers (1 cm thick), interpreted as overflow deposits and corresponding to the stratified unit U3 on the echosounder profiles (Fig. 3).

Core KERO-07 was taken in the Mafate fan at a water depth of 791 m. It is a 3.40 mlong core from the left side of the canyon area (Fig. 1). The lower two thirds of the core correspond to a succession of sandy and silty turbidite sequences that are variable in thickness, while the upper third is composed of silty-clay (Fig. 2).

Core KERO-08 was taken near the volcanic ridge R4 at a water depth of 4126 m, in the distal part of the Saint-Joseph fan. Its lithological succession is composed of a succession of sandy and silty turbidites whose thicknesses range from 2 to 15 cm. These well-sorted turbidites show normal grading and horizontal laminations. In the dark sandy layers, laminations are underlined by white laminae characterized by abundant foraminifera and bioclasts. Some clay and silty clay layers are highly bioturbated.

Chronostratigraphic framework

The chronostratigraphic framework of cores KERO-09, KERO-16, KERO-07 and KERO-08 was established through integration of radiocarbon dating, planktonic oxygen isotope, biostratigraphic markers and XRF records. et al., 1993). However, this chronostratigraphy was supported by the study of biostratigraphic markers. The nannofossil assemblage in core KERO-09 contains abundant E. huxleyi at 65 cm below seafloor (bsf), suggesting an age younger than 75-90 ka [START_REF] Berggren | Late Neogene chronology: New perspectives in high-resolution stratigraphy[END_REF]. Samples from 115-117 and 361 cm bsf show abundant well preserved nannofossils. The occurrence of E. huxleyi and the absence of P. lacunosa suggest that these samples are younger than 260 ka [START_REF] Berggren | Late Neogene chronology: New perspectives in high-resolution stratigraphy[END_REF]. The sample from 608 cm bsf has abundant but a poorly preserved nannofossils. Gephyrocapsa spp cf caribbeanica is dominant and two P. lacunosa are present, suggesting an age younger than 460 ka [START_REF] Berggren | Late Neogene chronology: New perspectives in high-resolution stratigraphy[END_REF]. The last occurrence of pink pigmented G. ruber is also observed in core KERO-16 at 4.70 m bsf, suggesting that the upper 4.70 m of KERO-16 is aged 120 ka [START_REF] Thompson | Disappearance of pinkpigmented Globigerinoides ruber at 120,000 yr BP in the Indian and Pacific Oceans[END_REF].

All these data provide a consistent age model around La Réunion Island. This age model was extended to other cores of the Cilaos fan (KERO-12, KERO-15 and FOR-C1) using the XRF records. The age model of cores FOR-C1, KERO-12 and KERO-15 was established by correlating the Ca variation of cores KERO-09 and KERO-16 (Fig. 7). In all the cores, light brown clays, corresponding to high XRF Ca values, allow a reliable core-to-core correlation.

Lithostratigraphy

The lithostratigraphy in the Cilaos fan was established with cores KERO-09, KERO-12, KERO-15, KERO-16 and FOR-C1. Based on the age model, the five cores retrieved from the Cilaos fan extend from 10 ka to 130 ka, with the Holocene period only being recorded for KERO-16 (Fig. 4). Although distances of tens of kilometers separate them, the variation of the calcium XRF correlates well between the five cores through the last glacial-interglacial cycle (Fig. 7). The cores exhibit a common sediment pattern and a fairly similar sedimentation rate. They are all composed of a succession of turbidites covered by a thick clay layer (Fig. 2). The sedimentation rate in the hemipelagic layer ranges between 1.8 and 5.2 cm/ka (Fig. 8). These results are comparable to the minimum sedimentation rate of 1.9 cm/ka observed by [START_REF] Ollier | Deep-sea volcaniclastic sedimentary systems: an example from La Fournaise volcano, Reunion Island, Indian Ocean[END_REF], based on micropaleontological analysis. They also correlate with the sedimentation rates measured by [START_REF] Fretzdorff | Explosive activity of the Reunion Island volcanoes through the past 260,000 years as recorded in deep-sea sediments[END_REF] in core S 17-666, near the Mafate fan (Fig. 1), based on a  18 O stratigraphy (Fig. 4). In this core (S17-666), three stages can be observed in the sedimentation: 1) between 128 and 186 ka, with a sedimentation rate of 4.14 cm/ka; 2) between 26 and 128 ka, with an average rate of sedimentation of 2.27 cm/ka; and 3) between 14.5 and 26 ka, with a sedimentation rate of 4.35 cm/ka. If we calculate the mean of the sedimentation rates in our cores for each period, we obtain sedimentation rates of about 4.9 cm/ka for the first stage, 2.73 cm/ka for the second stage and 3.02 cm/ka for the third. The sedimentation rates lie between those of the Mafate and Cilaos systems, with two stages of relatively high sedimentation rate interrupted by a period of low sedimentation.

These three distinct phases of sedimentation correlate with the three sedimentary units identified in the cores of the Cilaos turbidite system. The first unit corresponds to the turbidite activity visible in the lower part of cores KERO-09, KERO-12 KERO-15 and FOR-C1, characterized by sandy turbidites of 30 to 50 cm thickness (Fig. 3) and corresponding to the stratified unit U1 on the echosounder profiles (Fig. 3). This first stage is characterized by turbidity currents that spread over the entire fan [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. This first documented phase of turbidite activity would have deposited until the end of MIS5 (Fig. 8). Therefore, according to the age model of figure 8, the top of unit U1 -marking the interruption of this first phase of turbidite activity -corresponds to an age of 125-127 ka.

The second phase is characterized by a thick clay layer observed in all the cores (Fig. 2) and by the absence of major turbidite activity. Only a few thin sandy layers are observed in cores FOR-C1 and KERO-16 (Fig. 8). On the echosounder profiles, this unit coincides with the semi transparent unit U2, visible over the entire fan except in the more proximal part [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. The timing of clay deposition ranged from MIS3 to MIS5 (Fig. 8). The top of unit U2, visible in the upper part of cores KERO-16 and FOR-C1, lies within the lower part of MIS2 at about 30 ka (Fig. 8).

The third phase is characterized by thin sandy layers observed in cores FOR-C1 and KERO-16 (Fig. 3), with sediments coarser than in the older phase of turbidite activity. These turbidite events were observed on the proximal fan and into the channels of the distal fan [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. They are characterized, on the echosounder profiles, by a stratified unit (U3) visible in the upper part of the profiles (Fig. 3). This phase corresponds to the most recent activity of the Cilaos fan and was deposited during MIS1 and MIS2 until 30 ka.

Discussion

This discussion is based on cores KERO-09, KERO-12, KERO-15, KERO-16 and FOR-C1.

Cores KERO-07 and KERO-08 were used to build a consistent regional 

18

O stratigraphy around La Réunion Island.

Sedimentation in the Cilaos fan over the last 140 ka

Three distinct episodes of sedimentation correlate with three sedimentary units identified in the cores of the Cilaos turbidite system. The first unit corresponds to the oldest turbidite activity, visible in the lower part of cores KERO-09, KERO-12 KERO-15 and FOR-C1. This first unit, spreading over the entire fan [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF], is characterized by sandy turbidites of 30 to 50 cm thick (Fig. 3) and corresponds to the stratified unit U1 on the echosounder profiles (Fig. 3). According to our age model (Fig. 8), the first period of turbidite activity began before 140 ka and ended at ~ 125-127 ka (which coincides with the MIS5 highstand).

The second phase is characterized by a thick clay layer observed in all the cores (Fig. 2) which illustrates the absence of major turbidite activity. Only a few thin sandy layers are observed in cores FOR-C1 and KERO-16 (Fig. 8). On the echosounder profiles, this unit coincides with the semi transparent unit U2, visible over the entire fan except in the more proximal part [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. The timing of clay deposition ranged from MIS3 to MIS5 (Fig. 8). The top of unit U2, visible in the upper part of cores KERO-16 and FOR-C1, lies within the lower part of MIS2 at about 30 ka (Fig. 8).

The third phase is marked by the deposition of thin sandy layers as observed in cores FOR-C1

and KERO-16 (Fig. 3), with sediments coarser than those deposited during the older U1 unit.

These turbidite events were active in the proximal fan and into the channels of the distal fan [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. They are characterized, on the echosounder profiles, by a stratified unit (U3) visible in the upper part of the profiles (Fig. 3). This phase corresponds to the most recent activity of the Cilaos turbiditic fan since 30 ka until Holocene.

A significant difference in sedimentation rate is observed between core KERO-16 (located near the main channel of the central part of the Cilaos fan, showing the highest sedimentation rate) and the other cores KERO-09, KERO-15, KERO-12 and FOR-C1 (Fig. 9). The location of cores KERO-09, KERO-15 and KERO-12 at the termination of the distal fan might explain a lower sediment supply compared to the upper/central part of the Cilaos fan. The location of core FOR-C1 on top of a sedimentary ridge (about 200 m-high) explain sedimentation rates significantly lower than those observed in KERO-16.

In the distal part of the Cilaos fan, sedimentation rate is rather homogeneous until ~ 60 ka and decreases from ~ 60 ka to ~ 10 ka (Fig. 9), which corresponds to the progressive abandon of the turbiditic sedimentation in the western distal part of the Cilaos fan since ~ 60 ka [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. The increase of sedimentation rate from ~ 100 ka to ~ 45 ka in cores and FOR-C1 reflects a turbiditic activity restricted to the central part of the fan.

From ~ 45 ka to ~ 30 ka, decrease in sedimentation rates in all cores can be interpreted as a major change in the sediment supply at the scale of the entire Cilaos deep-sea fan (Fig. 9).

This period coincides with the youngest phase of effusive volcanic activity of the Piton des Neiges [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF][START_REF] Salvany | Geomorphic evolution of the Piton des Neiges volcano (Réunion Island, Indian Ocean): Competition between volcanic construction and erosion since 1.4 Ma[END_REF], which has probably contributed to fill the cirques and the fluvial valleys. At ~ 30 ka, the major increase in sedimentation rates detected in core KERO-16 corresponds to a new episode of turbidite activity (unit U3, Fig. 9) restricted to the central part of the fan [START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF].

Unravelling the forcing factors of turbidite sedimentation in the Cilaos fan over the last 140 ka

The three main forcing factors controlling sediment supply and transport offshore volcanic islands are volcanic activity, climate and sea-level [START_REF] Krastel | Formation of submarine canyons on the flanks of the Canary Islands[END_REF][START_REF] Quidelleur | Causal link between Quaternary paleoclimatic changes and volcanic islands evolution[END_REF]. Recent works on the morphology and sedimentary architecture of the Cilaos fan show that the sedimentary processes involved in the feeding of the Cilaos turbidite systems are direct feeding by rivers and submarine slope instabilities [START_REF] Saint-Ange | A volcaniclastic deep-sea fan off La Réunion Island (Indian Ocean): gradualism versus catastrophism[END_REF][START_REF] Sisavath | Morphology and sedimentary architecture of a modern volcaniclastic turbidite system: The Cilaos fan, offshore La Réunion Island[END_REF]. The detailed stratigraphy of the Cilaos fan obtained in the present study allows us to test this assumption over the last climatic cycle.

Turbidites in relation to climate and sea-level fluctuations

Offshore La Réunion Island, the two main intervals of turbidite activity coincide with the transition from glacial lowstand to highstand condition. The first phase of turbidite deposits coincided with lowstand and rising sea-level, at about 137 ka and between 137 and 130 ka, respectively [START_REF] Rohling | Antarctic temperature and global sea level closely coupled over the past five glacial cycles[END_REF]Fig. 8). The last recurrence of turbidite deposition in the Cilaos system (unit U3) also coincided with such a sea-level pattern, the turbidite activity is visible during the LGM lowstand (26 to 19.5 ka, [START_REF] Clark | The last glacial maximum[END_REF] and continuing until the next highstand conditions. The intervening period did not show any turbiditic conditions.

A link between sea-level change and large scale landsliding is suggested by some authors (e.

g., [START_REF] Mcmurtry | Giant landslides, mega-tsunamis, and paleo-sea level in the Hawaiian Islands[END_REF][START_REF] Quidelleur | Causal link between Quaternary paleoclimatic changes and volcanic islands evolution[END_REF]. Sea-level variations can change the pore pressure conditions, which are related to the location of the aquifer on top of a hydrothermal unit [START_REF] Join | Aquifers and groundwater within active shield volcanoes. Evolution of conceptual models in the Piton de la Fournaise volcano[END_REF] , or influence the submarine and coastal boundary conditions that control groundwater flow in the volcanic edifice. In his study of factors that could induce large flank destabilization on shield volcanoes, [START_REF] Iverson | Can magma-injection and groundwater forces cause massive landslides on Hawaiian volcanoes?[END_REF] had already concluded to the minor role played by sea level changes. When the sea-level drops, the mechanical resistance of the hydrothermal unit decreases as the pressure exerted by the water table at the base of edifice increases. The mechanical resistance decreases until the rupture threshold is reached, inducing a rapid lateral sliding of the volcano flank. This link between sea-level change and large scale landsliding is not observed for the Cilaos fan because no flank collapses have destabilized the studied area for 1 Ma [START_REF] Oehler | Growth and collapse of the Reunion Island volcanoes[END_REF]. Moreover, turbidite activity is visible during lowstand and highstand conditions, suggesting that sea-level variation has little influence on the development of turbidites in the Cilaos deep-sea fan.

The lack of palaeoclimatic records from La Réunion Island preclude the direct correlation of the turbidite activity in the Cilaos Fan with climate changes. An alternative is to examine the palaeoclimatic reconstructions from southern Africa. Intense debate persists about the climatic mechanisms governing hydrologic changes in this area (e.g., Schefuß et al., 2012).

However, recent results suggest that mean summer insolation controls the atmospheric convection, with higher insolation leading to higher rainfall (Schefuß et al., 2012). By considering this orbital forcing over a geological timescale, this implies that glacialinterglacial transitions in the southern African tropics were characterized by significant changes in rainfall level, from wet to dry conditions. Such a pattern has been demonstrated for the last Termination, through the runoff of the Zambezi river (Schefuß et al., 2012), and corroborates previous rainfall reconstructions from South Africa and Madagascar over the last 150-200 ka (Partridge et al., 1997;[START_REF] Gasse | Late Quaternary environmental changes from a pollen and diatom record in the southern tropics (Lake Tritrivakely, Madagascar)[END_REF].

Based on a geomorphological approach, Saint-Ange et al. (2011) showed that the Cilaos Fan is directly connected to the Saint-Etienne river mouth. This implies that river runoff is a major forcing factor on sediment delivery to the Cilaos Fan, and that a high rainfall period in the southern African tropics would increase sediment supply to the deep-sea fan. A sediment delivery process of this type to a deep-depositional system has been demonstrated from short, mountainous river systems (e.g., Makran margin, Southern California; [START_REF] Bourget | Hyperpycnal-fed turbidite lobe architecture and recent sedimentary processes: A case study from the Al Batha turbidite system, Oman margin[END_REF][START_REF] Covault | Submarine fans at all sea-level stands: Tectonomorphologic and climatic controls on terrigenous sediment delivery to the deep sea[END_REF] et al., 1997;[START_REF] Gasse | Late Quaternary environmental changes from a pollen and diatom record in the southern tropics (Lake Tritrivakely, Madagascar)[END_REF].

To understand the impact of climate on the development of turbidite in the Cilaos fan, it would be necessary to obtain more accurate data about the climatic variations in La Réunion Island but, according to current knowledge, the results presented here call into question the impact of rainfall level alone on sediment delivery to the Cilaos basin.

Turbidites in relation to volcanic activity

The two main periods of turbidite activity in the Cilaos system coincided with periods of low effusive volcanic activity at La Réunion. Indeed, the first phase of turbidite activity (unit U1) can be associated with the low effusive activity identified between 180 and 140 ka at Piton des Neiges [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF][START_REF] Salvany | Geomorphic evolution of the Piton des Neiges volcano (Réunion Island, Indian Ocean): Competition between volcanic construction and erosion since 1.4 Ma[END_REF]. The second period of turbidite activity (Unit U3) began at the end of the effusive activity of the Piton des Neiges volcano, dated at about 30 ka by [START_REF] Gillot | K-Ar chronology of the ultimate activity of Piton des Neiges volcano, Reunion Island, Indian Ocean[END_REF]. These periods correlate with the major erosional episodes interpreted by [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF] and [START_REF] Salvany | Geomorphic evolution of the Piton des Neiges volcano (Réunion Island, Indian Ocean): Competition between volcanic construction and erosion since 1.4 Ma[END_REF] (Fig. 10). The decrease of volcanic production coincided with the erosional formation of the cirques [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF][START_REF] Salvany | Geomorphic evolution of the Piton des Neiges volcano (Réunion Island, Indian Ocean): Competition between volcanic construction and erosion since 1.4 Ma[END_REF]. This strongly suggests that the low effusive activity contributed to increasing the sediment input to the submarine flank and the deep Cilaos basin surrounding La Réunion Island. Conversely, the interruption of turbidite activity (between 127 ka and 30 ka) coincided with a resumption of the effusive and explosive activity of the Piton des Neiges volcano, with large lava flows that filled the cirques and their drainage valleys (Fig. 10) [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF][START_REF] Salvany | Geomorphic evolution of the Piton des Neiges volcano (Réunion Island, Indian Ocean): Competition between volcanic construction and erosion since 1.4 Ma[END_REF]. In the cirque of Cilaos, the first lava flows related to this new volcanic event are estimated to have appeared at 130 ka [START_REF] Kluska | Evolution magmatique et morpho-structurale du Piton des Neiges au cours des derniers 500 000 ans[END_REF]. The filling of the drainage basin by lava flows probably obstructed the pathways for sediment transfers to the deep marine environment.

In the Canary Islands, for comparison, a clear relationship is observed between the timing of emplacement of volcanic-rich turbidites and the period of explosive volcanic activity [START_REF] Alibés | Determination of sediment volumes, accumulation rates and turbidite emplacement frequencies on the Madeira Abyssal Plain (NE Atlantic): a correlation between seismic and borehole data[END_REF][START_REF] Frenz | Provenance and pathways of late Quaternary turbidites in the deepwater Agadir Basin, northwest African margin[END_REF]. During volcanically active stages, an increase of sediment transport from the volcanic island is observed, with major flank collapses feeding turbidite currents [START_REF] Frenz | Provenance and pathways of late Quaternary turbidites in the deepwater Agadir Basin, northwest African margin[END_REF][START_REF] Schneider | Epiclastic sedimentation during the upper Miocene-lower Pliocene volcanic hiatus of Gran Canaria: evidence from sites 953 and 954[END_REF]. [START_REF] Schneider | Epiclastic sedimentation during the upper Miocene-lower Pliocene volcanic hiatus of Gran Canaria: evidence from sites 953 and 954[END_REF] also observed increases of turbidite activity during a non-eruptive period on Gran Canaria, suggesting that sediments were transported by low-density turbidity currents with some turbidites related to the dynamics of the fluvial system. [START_REF] Funck | Growth and destruction of Gran Canaria deduced from seismic reflection and bathymetric data[END_REF] showed that many of the submarine canyons of the Canary Islands are the continuation of onshore canyons. [START_REF] Mitchell | Erosion of the submarine flanks of the Canary Islands[END_REF] and Krastell et al. ( 2001) concluded that the dominant process feeding these canyons was hyperpycnal flow.

For the Cilaos fan, the occurrence of turbidite activity during periods of low effusive volcanic activity suggests that the fan was mainly fed by river sediment load. This means that the turbidite activity occurred when erosional processes dominated, allowing a vast transport of sediment over the submarine flanks of the volcano. These periods also correspond to phases of explosive activity of Piton des Neiges, which could also have produced a large amount of volcaniclastic material during eruption and a rapid transfer of sediments down to the adjacent slope and basin, in a similar way to what was suggested for the Canary Islands (Schminke and Sumita, 1998).

As a result, the volcanic activity appears to be a major controlling factor influencing turbidite development of the Cilaos deep-sea fan.

Conclusions

New stratigraphical data on the deep-sea Cilaos sedimentary system allow us to define the

  Processes controlling a volcaniclastic turbiditic system during the last climatic cycle: Example of the Cilaos deep-sea fan, offshore La Réunion Island Emmanuelle Sisavath, Aude Mazuel, Stéphan J. Jorry, Nathalie Babonneau, Patrick Bachèlery, Béatrice de Voogd, Marie Salpin, Laurent Emmanuel, Luc L Beaufort, Samuel Toucanne To cite this version: Emmanuelle Sisavath, Aude Mazuel, Stéphan J. Jorry, Nathalie Babonneau, Patrick Bachèlery, et al.. Processes controlling a volcaniclastic turbiditic system during the last climatic cycle: Example of the Cilaos deep-sea fan, offshore La Réunion Island. Sedimentary Geology

The  18 O

 18 curves show similar trends between all cores (Figs 5 and 6). The upper parts of the cores show light  18 O values (down to -2.0 ‰ in core KERO-07). A rapid increase of the  18 O signal is observed thereafter, and heavier  18 O values (from -1.0 to 0 ‰) characterized the lower parts of the cores (Fig. 4). Peaks and troughs recognized in the oxygen isotope records were correlated with the reference isotopic signal published by Fretzdorff et al. (2000) from La Réunion area, in agreement with the trends of the  18 O benthic stack record of Lisieky and Raymo (2005) (Figs. 4 and 5). The light  18 O values observed in the upper part of the cores correspond to the Holocene (<11 ka). MIS2 is characterized by values of  18 O between 0 and -0.5 ‰. A general decrease of the  18 O values from 0 to -1 ‰ is clearly shown in MIS3 (Fig. 5). It is followed by an increase of  18 O values identified as the MIS3/MIS4 transition. The relatively light  18 O values (>-1‰) observed in the lower part of cores from the Cilaos fan correspond to the last interglacial. The oxygen isotope stratigraphy of all these cores provides a regional record of the last climatic cycle around La Réunion Island. In cores of the Cilaos fan (KERO-09 and KERO-16), MIS-5 is characterized by  18 O values ranging between -1 ‰ and 0 ‰, which are unusually low for the last interglacial compared with those published by Fretzdorff et al. (2000) (Fig. 5). SEM observations of the test surface of G. ruber in cores KERO-09 and KERO-16 reveal some dissolution pockets and recrystallized areas (Fig. 6), which could explain these inconsistent  18 O values. In addition, periods of high carbonate dissolution have been identified in the western part of the Indian ocean, mainly during interglacials (Divakar

  . Based on the rainfall reconstructions of Partridge et al. (1997) (Fig. 8), one would expect the turbidite sedimentation in the Cilaos Fan to increase during southern hemisphere summer insolation maxima. However, no turbiditic activity was recorded in the Cilaos Fan at the time of glacial rainfall maxima (e.g., ca 90, 70 or 50 ka; Partridge et al., 1997). Moreover, the turbidite activity off the St Etienne river, centred on Termination II and I, began at a time of wet (MIS 2) to very wet conditions (MIS 6) in the southern African tropics and continued during both the climatic transition (i.e., Terminations) and the following dry conditions (i.e., MIS 5 and MIS 1) (Partridge

  timing of turbidite activity, which appears to have occurred close to the last two climatic terminations. A first turbidite activity period occurred around 127 ka and a second one started at 30 ka, which has continued until the present. The two main phases of turbidite activity coincide with the last two transitions from glacial lowstands to subsequent sea level rises.Nevertheless, our study demonstrates that sea level change played a minor role in the triggering of turbidites in the Cilaos fan. On the other hand, the synchronicity between intense turbidite deposition in the Cilaos deep-sea fan and periods of low effusive volcanic activity of the Piton des Neiges volcano is clearly seen. We suggest that high erosional rates, identified in the cirques during periods of low effusive activity, would mainly have contributed to increase the seaward sediment transport. Conversely, the onset of volcanic activity would have prevented intense erosion in the cirques, with the decrease of gravity deposits in the Cilaos deep-sea fan resulting from the consequent low sediment transfer by rivers. Compared with other volcaniclastic systems, explosive events of the Piton des Neiges might have also contributed to feeding the Cilaos deep-sea fan.

Figure captions Fig. 1 :

 1 Figure captions

Fig. 2 :

 2 Fig. 2: Lithological logs, fluctuations of Calcium XRF, and AMS 14 C dates (cal ka BP) of

Fig

  Fig. 5:  18 O curves of cores KERO-07, KERO-08, KERO-09 and KERO-16 compared with

Fig. 6 :

 6 Fig. 6: SEM microphotographs of G. ruber from core KERO-16. Recrystallization and

Fig. 8 :

 8 Fig. 8: Timing of turbidite deposition in the Cilaos deep-sea fan. Blue areas indicate periods

Acknowledgements

The authors thank the crew and scientific teams for the high-quality data recovery during the 2006 ERODER1 cruise aboard the BHO Beautemps-Beaupré and the 2008 ERODER 2 cruise aboard the RV Meteor. Seven radiocarbon dates presented in this paper were acquired with the Artemis program (supported by the CNRS). We are also grateful to Nathalie Labourdette (Université Pierre & Marie Curie) who ran the oxygen isotope analyses and to Tomasz Goslar who managed additional radiocarbon dating at the Poznan Radiocarbon Laboratory (Poland).

Financial support was provided by the "Conseil Régional de La Réunion", by the Institut Universitaire Européen de la Mer (Brest), and by IFREMER (Project "French Territories -Indian Ocean"). The authors thank Marie-France Loutre for the insolation data and the two reviewers Dr. Francky Saint-Ange and Dr. Neil C. Mitchell whose comments and suggestions contributed to improve the original manuscript.

 [START_REF] Berggren | Late Neogene chronology: New perspectives in high-resolution stratigraphy[END_REF].


Pascale Diener

Pierre Rodiere

Thierry Klein

C Marcenat

Jozef Kacmarcik

Zuzana Pribulova

D J Jang

H S Lee

H G Lee

S I Lee

P Rodière

H S Lee

H G Lee

s-wave superconductivity probed by measuring magnetic penetration depth and lower critical field of MgCNi3 single crystals

Keywords: numbers: 74.25.Nf, 74.25.Op, 74.70.Dd

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The interplay between magnetism and superconductivity is currently a subject of great interest. In the UGe 2 and URhGe uranium compounds, for instance, a long range ferromagnetic ordered phase coexists with the superconducting phase and a mechanism of spin fluctuations (SF) could be at the origin of the Cooper pair formation [1,2]. The recent discovery of high temperature superconductivity in oxypnictides also rapidly became the topic of a tremendous number of both experimental and theoretical works. The parent undoped LnO-FeAs (where Ln=La,Sm,...) compound is here close to itinerant magnetism due to the presence of a high density of Fe d states at the Fermi level [3], leading to competing ferromagnetic and antiferromagnetic fluctuations. Similarly in the cubic (anti-)perovskite MgCNi 3 compound [4], the presence of a strong Van Hove singularity in the density of Ni states slightly below the Fermi level also leads to strong ferromagnetic fluctuations [5][6][7][8][9]. These two systems have also a Fermi surface composed of both electron and hole pockets (3D sheets in MgCNi 3 as compared to quasi-cylindrical sheets in oxypnictides).

Despite these striking similarities in their electronic and magnetic properties, spin fluctuations lead to very different effects in those systems. On the one hand, ab-initio calculations rapidly showed that the electronphonon coupling constant (λ e-ph ∼ 0.2) is far too low to account for the high critical temperatures observed in oxypnictides (up to ∼ 55 K) and an unconventional mechanism mediated by the SF associated with a sign reversal of the (s-wave) order parameter between electrons and holes sheets of the Fermi surface has been proposed [10]. On the other hand, it has been suggested that the narrow van Hove singularity could be responsible for a nearly unstable phonon mode in MgCNi 3 inducing a large, although reduced by SF, λ e-ph [11,12] in agreement with experiments which yield an average electronphonon coupling constant in the order of 1.7 [11,13,14]. The interplay between electron-phonon coupling and SF is further emphasized in this system by the existence of a large isotopic effect [15] which has been suggested to be enhanced by the strong SF [16].

In this context, the nature of the superconducting order parameter rapidly became a crucial issue. In MgCNi 3 , the experimental results still remain controversial : on the one hand, penetration depth measurements (in polycrystalline samples) showed a quadratic, i.e. non s-wave, temperature dependence suggesting a nodal order parameter [17], whereas specific heat measurements clearly indicate that the superconducting gap (∆) is fully open, with a ∆/k B T c ratio ranging from 1.9 to 2.1 [13,18,19] i.e. well above the BCS weak coupling 1.76 value.

In this paper, we present high precision magnetic penetration depth and lower critical field measurements performed in the same MgCNi 3 single crystals. We show that λ(T ) clearly follows an exponential temperature dependence for T < T c /3 showing that the gap is fully open on the whole Fermi surface. A zero temperature λ 0 value of 230 nm, i.e. well above the London clean limit BCS value (∼ 60 nm) has been deduced from first penetration field measurements, clearly suggesting the presence of strong mass renormalization and/or impurity scattering effects. Introducing this value into the TDO data however leads to a temperature dependence of the normalized superfluid density ρS (T ) ρS (0) = [ 1 1+δλ(T )/λ0 ] 2 which is different from the one directly deduced from the lower critical field (H c1 ∝ ln(κ)/λ 2 where κ = λ/ξ). Possible reasons for this discrepancy are discussed.

Single crystals were grown in a high pressure furnace as described elsewhere [20]. AC specific heat have been performed on several samples of the same batch [21]. All the measured crystals show sharp superconducting transitions (∆T c ∼ 0.2 K) emphasizing the excellent bulk homogeneity of each crystal. We observed however a large dispersion of critical temperatures from sample to sample, between approximately 5.9 to 7.6 K, probably due to a slight Ni deficiency in the MgCNi 3 structure [20]. Three single crystals with a thickness of 0.1mm but different shapes and critical temperatures have been selected. Sample #A can be approximated by a disk with a diameter of 0.3mm. Samples #B and #C have a rectangular shape of 0.21×0.15 mm 2 and 0.24×0.36 mm 2 respectively. Samples #A and #B both present a bulk T c of 6.9 K and exhibit exactly the same behavior by TDO and HPM, whereas sample #C has the highest T c at 7.6 K.

The magnetic penetration depth has been measured with a high stability LC oscillator operating at 14 MHz, driven by a Tunnel Diode [22,23]. The AC excitation field is below 1 µT and the DC earth magnetic field is screened by a demagnetized weak ferromagnet amorphous ribbon, ensured to work well below H c1 . The sample stage, placed at the bottom of a home-made He 3 refrigerator, is regulated between 0.5 K and 10 K, whereas the LC oscillator remains at fixed temperature. The superconducting sample is glued with vacuum grease at the bottom of a sapphire cold finger, which can be extracted in-situ [24]. The small filling factor of the excitation coil by the superconducting sample (∼ 0.01%) ensures a small perturbation of the circuit and the frequency shift δf divided by the one induced by the extraction of the superconducting sample, ∆f 0 , is then proportional to the imaginary part of the surface impedance and hence to the magnetic penetration depth [25]. As shown in the inset of Fig. 1, all samples present a sharp superconducting transition at a critical temperature T f c (defined by the onset of the frequency shift change) equal to 6.9 K (resp. 7.6 K) for sample #A (resp. sample #C) in good agreement with the C p measurements.

Fig. 1 displays the temperature dependence of the frequency shift, proportional to δλ(T ), compared to the results previously reported in powders [17]. The amplitude of the shift is 10 times larger in the case of the powder reflecting the fact that the surface on which the supercurrents are flowing is much larger in powders than in single crystals (for the same sample volume). It is important to note that the temperature dependence of λ is strikingly different in single crystals than in powder for which a T 2 power law has been reported below 1.8 K. Such a dependence has been interpreted as an evidence for unconventional superconductivity [17] but our measurements do not support this scenario as a T 2 power law only very poorly describes the experimental data (see dashed line in Fig. 1).

A very good fit to our data is actually obtained assuming the low temperature approximation for clean type II [17] divided by a factor 10. The dashed line is the T 2 law below 1.8 K reported for the powder. In the case of the single crystals, a better fit is obtained with an exponential law (solid line). Inset: frequency shift at the critical temperature for both single crystals.

superconductors with a fully open gap : λ(T ) ∝ ∆/k B T e -∆/kB T . This expression is valid for k B T < ∆/5, and leads to ∆/k B = 11.6(1) K for sample #A (and B) and ∆/k B = 12.3(1) K for sample #C. Note that this fitting procedure can lead to a slightly overestimated ∆ value (up to 10%, depending on the range of the fit and the ratio between ∆ and T c ) but unambiguously shows that the gap is fully open in good agreement with previous tunneling spectroscopy [26,27], NMR [8], and specific heat measurements which led to ∆/k B ≈13.0(2) K, 10.5 K and 13 K, respectively. However, fitting the low temperature data only leads to the size of the minimum superconducting gap. To unambiguously exclude the presence of any other gaps (and/or other gap symmetries) it is necessary to analyze the full temperature dependence of the normalized superfluid density ρ S (T ) ∝ 1/λ(T ) 2 up to T c . This superfluid density can be deduced :

-either from the temperature dependence of the lower critical field :

H c1 = Φ 2 0 /(4πλ 2 )(Ln(κ) + c(κ))
where κ = λ/ξ (ξ being the coherence length) and c(κ) a κ dependent function tending towards ∼ 0.5 for large κ values. As κ is almost temperature independent (being in the order of 40), H c1 (T ) is directly proportional to the superfluid density which we will call ρ Hc1 S . -or by introducing the absolute value of the penetration depth at T=0 K (λ 0 ) into the TDO data : 2 where R is a geometrical factor [28].

ρ T DO S (T ) ∝ [ 1 1+δλ(T )/λ0 ] 2 = [ 1 1+δf (T )/∆f0×R/λ0 ]
The local magnetic induction has been measured with a miniature 16 × 16 µm 2 Hall probe and the first penetration field H p has been deduced by measuring the remanent field (B rem ) in the sample after applying an external field H a and sweeping the field back to zero. For H a < H p (i.e. in the Meissner state) no vortices penetrate the sample and the remanent field remains equal to zero (actually close to zero due to partial penetration through the sample corners). H a is then progressively increased until a finite remanent field is obtained (see Fig. 2). Indeed, since vortices remain pinned in the sample, B rem rapidly increases for H a > H p , varying as (H a -H p ) α with α = 0.4 ± 0.1 (solid lines in Fig. 2 for α = 0.5). We get H p ∼ 50 ± 5 G, ∼ 55 ± 5 G and ∼ 70 ± 10 G for sample #A, #B and #C, respectively. In samples with rectangular cross sections, H p is then related to H c1 through H c1 ≈ H p /tanh( αd/2w) where α varies from 0.36 in strips to 0.67 in disks [29,30]. Taking an average α value ∼ 0.5 we hence get H c c1 (0) ∼ 125 ± 15 G and correspondingly λ 0 = 230 ± 15 nm (introducing H c2 (0) = Φ 0 /2πξ(0) 2 = 9.5 T [21]).

This value is in good agreement with the value deduced from muon spin relaxation data in ceramics [31], previous lower critical field measurements in powder [32] as well as the value calculated from the thermodynamic critical field deduced from specific heat measurements by Wälte et al.

Note that, as pointed out by Wälte et al., this value is much larger than the London clean limit BCS value λ L (0) = c/ω p ∼ 60 nm (ω p being the plasma frequency ∼ 3.2 eV [13]). In the presence of strong mass renormalization and/or impurity scattering effects λ 0 = λ L (0) 1 + λ e-ph 1 + ξ 0 /l where l is the mean free path and ξ 0 ∼ vF π∆0 (v F being the bare Fermi velocity). Introducing λ e-ph ∼ 1.8 and v F ∼ 2.1 × 10 5 m.s -1 [13], one obtains l ∼ ξ 0 /4 ∼ 10 nm hence confirm that both strong coupling and strong impurity diffusion are present. Note that this l value corresponds to a resistivity ρ = v F /ǫ 0 lω 2 p ∼ 10 µΩcm i.e. slightly lower than the residual resistivity measured in similar crystals ρ ∼ 30 µΩcm [20]. However, since ρ is expected to be in the order of λ 2 0 µ 0 π∆/ (1 + λ e-ph ) (dirty limit), a residual resistivity of 30 µΩcm value would thus require that λ e-ph ≪ 1 in striking contrast with reported values. For an isotropic superconducting gap, the BCS superfluid density ρ S (T ) reduced by thermally activated excitations is expected to be given by :

ρ S (T ) = 1 - ∂f ∂E E E 2 -∆ 2 (T ) (1)
where f is the Fermi Dirac distribution, E the energy above the Fermi energy, ∆(T ) the value of the superconducting gap at the temperature T . As shown in Fig. 3 (solid lines) very good fits to the ρ Hc1 S data are obtained using an alpha model in which the temperature dependence of the superconducting gap (normalized to its T =0 K value) has been assumed to be equal to the reduced BCS weak coupling value calculated from the gap equation [33] and taking ∆(0) = 2k B T c . Note that a superconducting gap equal to its weak coupling theory value (∆(0) = 1.76k B T c ) only leads to a poor fit of the data, confirming the large value of the ∆(0)/k B T c ratio previously obtained by bulk probes such as specific heat measurements.

On the other hand, ρ T DO S displays a strong downward curvature at low temperature followed by a clear upward curvature as the superfluid density drops below 0.5 (i.e. for λ(T ) > 1.4λ 0 ). As pointed out above one has to determine the R/λ 0 ratio in order to deduce ρ T DO S from the δf /∆f 0 data. The R value has been calculated from the aspect ratio using the formula introduced by Prozorov [28]. The validity of this procedure has been checked on Pb samples. Moreover, different AC magnetic field orientations on the same single crystal of MgCNi 3 (but different R) show the same quantitative temperature dependence of λ(T ), consistently with an isotropic cubic system. A possible explanation would hence be an underestimation of λ 0 . The influence of λ 0 is displayed in the inset of Fig. 3. As shown taking λ 0 ≃ 700 nm instead of 230 nm leads to a temperature dependence for ρ T DO S similar to the one obtained for ρ Hc1 S . This value is however well above our error bars on λ 0 and would correspond to µ 0 H c1 (0) ∼ 15 G i.e. even smaller than our first penetration field values (∼ 55 G). Note that strong bulk pinning could lead to an overestimation of H p , if measured in the center of the sample (see for instance [34]) but we checked that very similar H p values are obtained for several probe positions by placing the sample on an array of 11 miniature (10 × 10 µm 2 ) probes : as shown in the left inset of Fig. 2, the field distribution clearly presents the Vshape profile characteristic of a strong bulk pinning. Even though those profiles confirm the good homogeneity of the sample, one can not exclude the presence of a strong disorder at the surface of the samples leading to a surface penetration field much larger than the bulk value. However, the λ 0 =700 nm value would require an extremely small mean free path (∼ 1 nm, see discussion above). A possible difference between the mixed state and Meissner state penetration depth values associated either to a Doppler shift induced by the supercurrents on the excitation spectra [35,36] or to a strong field dependence of λ in the mixed state (see for instance [37]) due to multiband effects can be excluded in our isotropic, fully gapped system.

Another explanation could be a difference between bulk and surface critical temperature. Indeed, at low temperature TDO measurements only probe the sample on a typical depth in the order of λ 0 ∼ 0.2 µm, i.e. roughly 0.4% of the total volume (for a Volume to Surface ratio of 50 µm). In the presence of a weak coupling superconducting gap, this volume only increases to about 20% of the sample volume for T → T c /2 and the bulk of the sample is only probed close to T c as the magnetic penetration depth finally diverges for T → T c . On the other hand, the Hall probe has been placed close to the center of the sample in the HPM measurements and is hence sensitive to the bulk of the sample. In the case of MgCNi 3 , it is known that the critical temperature has a surprising high sensitivity to a very small change in the C or Ni stoechiometry [4,20] and also surface stress [14,38]. Assuming that the critical temperature of the surface is 20 % smaller than the bulk value, very good fit to the data could be obtained for ρ T DO S using Eq.1 for T < 3 4 T c (still taking ∆(0) = 2k B T c , see solid lines in Fig. 3). Note that a large dispersion of the T c values in powder might explain the anomalous temperature dependence observed in previous λ measurements. Clear deviations from the standard BCS theory (Eq.1) have been observed in systems like MgB 2 [39] or more recently in pnictides [40] but in our case those deviations in ρ T DO S are due to surface inhomogeneities (disorder and/or T c ) and our measurements emphasize the importance of coupling complementary experimental probes in order to unambiguously address this issue.

To conclude, we have shown that the temperature dependence of the magnetic penetration depth is exponential in MgCNi 3 single crystals signalling the presence of a fully open superconducting gap. A drastically different behavior has systematically been observed between the superfluid density extracted from the lower critical field and TDO measurements performed on the same sample, which are most probably due to surface disorder and/or a depletion of 20% of the critical temperature at the surface.

Figure 1 :

 1 Figure1: Low temperature TDO frequency shift δf normalized by the frequency shift for a total extraction, obtained for single crystals #A and #C. Open circles correspond to previous results on polycrystalline powders[17] divided by a factor 10. The dashed line is the T 2 law below 1.8 K reported for the powder. In the case of the single crystals, a better fit is obtained with an exponential law (solid line). Inset: frequency shift at the critical temperature for both single crystals.

Figure 2 :

 2 Figure 2: Remanent field Brem in flux quantum units (Φ0/S, S being the active area of the probe) as a function of the applied field Ha in sample #A showing that Brem remains close to zero up to Ha = Hp (see text for details). Left inset: Local induction at T = 4.2 K as a function of the applied field for several probe positions (see right inset) showing that, even the penetration is much stronger close to the edges (probe 4), the same first penetration field (∼35 G) can be obtained on all of the probes. Right inset: Field profiles at T = 4.2 K for different values of the applied field (measured on probe 2) clearly showing the Bean profile characteristic of bulk pinning. Probe 8 is located close to the center of the sample and probe 4 close to the sample edge. The spacing between probes is 20µm.

  symbols) and ρ T DO S (open symbols) are displayed in Fig. 3 for sample #A (squares) and #C (circles). The two techniques lead to strikingly different temperature dependence for the superfluid density.

Figure 3 :

 3 Figure 3: (Color online) Normalized superfluid density deduced from Hc1 measurements (full symbols) and TDO measurements with λ0 =230 nm (open symbols) for samples #A (blue squares) and #C (red circles). The solid lines are the fit for a superconducting gap ∆ = 2 kBTC with Tc =4.8, 5.6, 6.2 and 7 K (see text). Inset: influence of the λ0 value used to deduce ρS from TDO measurements.

  We are most obliged to V. Mosser of ITRON, Montrouge, for the development of the Hall sensors used in this study. This work was partially supported by the Slovak R&D Agency under Contracts No. VVCE-0058-07, No. APVV-0346-07, and No. LPP-0101-06.