Jérémie Aubineau
email: jeremie.aubineau@univ-poitiers.fr

Abderrazak El Albani

Ernest Chi Fru

Michael A Kipp

Julie Ngwal'

Ghoubou Ikouanga

Andrey Bekker

Abderrazak El

Julie Ngwalghoubou Ikouanga

Benthic redox conditions and nutrient dynamics in the ca. 2.1 Ga Franceville sub-basin

Keywords: Mat-related structures, Paleoproterozoic, nitrogen isotopes, paleoredox, basinal structure, Francevillian biota

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

Introduction

The Paleoproterozoic Era, 2.5-1.6 billion years ago (Ga), was a time of profound change in atmosphere-ocean chemistry, including the so-called "Great Oxidation Event" (GOE; [START_REF] Holland | Volcanic gases, black smokers, and the Great Oxidation Event[END_REF]Bekker et al., 2004) that promoted the global emergence of oxidizing conditions at Earth's surface over an interval of 300 million years [START_REF] Bekker | Oxygen overshoot and recovery during the early Paleoproterozoic[END_REF][START_REF] Lyons | The rise of oxygen in Earth's early ocean and atmosphere[END_REF]. The onset of the GOE, constrained by the disappearance of massindependent fractionation in sulfur isotopes at ca. 2.46-2.43 Ga [START_REF] Gumsley | Timing and tempo of the Great Oxidation Event[END_REF][START_REF] Warke | The Great Oxidation Event preceded a Paleoproterozoic "snowball Earth[END_REF], records a rise in atmospheric oxygen above 10 -5 times present atmospheric level [START_REF] Farquhar | Observation of wavelengthsensitive mass-independent sulfur isotope effects during SO2 photolysis: Implications for the early atmosphere[END_REF][START_REF] Pavlov | Mass-independent fractionation of sulfur isotopes in Archean sediments: Strong evidence for an anoxic Archean atmosphere[END_REF]. Evidence for oxygenated surface environments comes from sedimentary and geochemical proxies including the disappearance of redox-sensitive detrital minerals, the appearance of red beds and sulfate evaporites, and enrichment of redox-sensitive trace elements in iron formations and organicrich shales [START_REF] Bankole | Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization[END_REF][START_REF] Bekker | Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America[END_REF][START_REF] Fru | The rise of oxygen-driven arsenic cycling at ca. 2.48 Ga[END_REF], 2016;Kipp et al., 2017;[START_REF] Konhauser | Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event[END_REF][START_REF] Melezhik | Palaeoproterozoic evaporites in Fennoscandia: Implications for seawater sulphate, the rise of atmospheric oxygen and local amplification of the δ 13 C excursion[END_REF][START_REF] Partin | Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales[END_REF][START_REF] Rasmussen | Redox state of the Archean atmosphere: Evidence from detrital heavy minerals in ca. 3250-2750 Ma sandstones from the Pilbara craton, Australia[END_REF][START_REF] Schröder | Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: Evidence from sulphate evaporites in the ∼2.2-2.1 Gyr shallow-marine Lucknow Formation, South Africa[END_REF][START_REF] Scott | Tracing the stepwise oxygenation of the Proterozoic ocean[END_REF].

The redox-sensitivity of the biogeochemical cycle of nitrogen (N) -an essential nutrient needed for the construction of biomolecules in all known living organisms -has resulted in dramatic redox transformation of the N cycle as the redox state of the Earth's surface evolved through time [START_REF] Stüeken | The evolution of Earth's biogeochemical nitrogen cycle[END_REF]. The primary source of nitrogen to the biosphere is assimilation of dinitrogen (N2) into biomass by nitrogen-fixing organisms (diazotrophs). In oxygen-depleted settings, biologically mediated degradation of diazotrophic biomass produces bioavailable ammonium (NH4 + ). In oxygen-rich waters, NH4 + is rapidly oxidized to nitrite (NO2 -) and nitrate (NO3 -) via the stepwise process of bacterial nitrification. These bioavailable N forms can be assimilated directly or returned to the atmosphere as N2 via denitrification (i.e., reduction of NO3 -to N2) and anaerobic ammonium oxidation (anammox; NH4 + oxidation coupled to NO2 -reduction). These microbial N removal pathways are prevalent in modern oxygen-minimum zones [START_REF] Dalsgaard | Anaerobic ammonium oxidation (anammox) in the marine environment[END_REF][START_REF] Sigman | Ocean process tracers: Nitrogen isotopes in the ocean[END_REF], meaning that oxygen-deficient waters are likely to become depleted in dissolved N.

Thus, studying the response of the N cycle to Earth's oxygenation can not only yield more precise constraints on water column redox conditions, but also can be informative about N availability for early marine ecosystems.

Because the stable isotopes of N are fractionated during the redox transformations mentioned above, many studies leverage N isotope ratios in ancient marine sedimentary rocks as a means to study the N cycle in deep time (reviewed in [START_REF] Ader | Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives[END_REF]. This includes recent studies that have used N isotopes to infer that aerobic N cycling emerged in a stepwise fashion during the GOE [START_REF] Cheng | Nitrogen isotope evidence for stepwise oxygenation of the ocean during the Great Oxidation Event[END_REF][START_REF] Kump | Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event[END_REF][START_REF] Luo | Nitrogen fixation sustained productivity in the wake of the Palaeoproterozoic Great Oxygenation Event[END_REF][START_REF] Zerkle | Onset of the aerobic nitrogen cycle during the Great Oxidation Event[END_REF] and was pervasive in the surface oceans during much of the Paleoproterozoic [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. These studies focused on the global-scale evolution of the marine N cycle across the GOE; however, the basin-scale impact of redox fluctuations, linked to extant N fixing and assimilating communities, as well as episodic incursions of anoxic deep waters, has not yet been studied for the late stage or the immediate aftermath of the GOE. Here, we explore N redox dynamics in the middle Paleoproterozoic Franceville sub-basin of Gabon with a particular focus on a sedimentary interval that bears large macrofossils and microbial mat-related structures (MRS). This allowed us to explore the relationships among redox structure, nutrient cycling, and the early evolution of complex life in this pivotal interval.

The timing of deposition in the Franceville sub-basin corresponds in age to the end of the Lomagundi Event (LE) (∼2.22-2.06 Ga; [START_REF] Karhu | Carbon isotopes and the rise of atmospheric oxygen[END_REF][START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF].

This event represents the longest-lived positive carbon isotope excursion in Earth's history, during which atmospheric oxygen levels rose considerably [START_REF] Bekker | Oxygen overshoot and recovery during the early Paleoproterozoic[END_REF][START_REF] Planavsky | Sulfur record of rising and falling marine oxygen and sulfate levels during the Lomagundi event[END_REF]. The ca. 2.1 Ga Franceville sub-basin records frequent eustatic changes and episodic upwelling of deep, anoxic waters into shallow-marine, oxic settings (Gauthier-Lafaye and [START_REF] Gauthier-Lafaye | Natural nuclear fission reactors: Time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere[END_REF][START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF][START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF] that could have influenced the spatiotemporal dynamics of the N cycle. As only one stratigraphic unit of the Francevillian Group (FC1 Member) has been the focus of N isotope study so far [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF], there remains a considerable gap in our understanding of N cycling throughout the whole Francevillian Group sedimentary record. Furthermore, the Francevillian Group sedimentary rocks have revealed tantalizing evidence that the earliest potential multicellular life [START_REF] El Albani | The 2.1 Ga old Francevillian biota: Biogenicity, taphonomy and biodiversity[END_REF], 2010) capable of locomotion [START_REF] El Albani | Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago[END_REF] flourished next to benthic microbial mats [START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF][START_REF] El Albani | Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago[END_REF][START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF]. The microbial features, preserved as MRS, display a wide range of surface morphologies with mat-growth and mat-protected patterns. The former, including "elephantskin" textures (EST), domal buildups, and discoidal microbial colonies, consist of the mat layer itself generated through mat propagation, while the latter, such as wrinkle and "Kinneyia" structures, arose from structures that relied on the mat communities for their preservation/protection. Individual MRS vary between 0.2 and 3 mm in thickness; this heterogeneity has been attributed to changes in physical and chemical environmental factors [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF]. According to their sulfur content (0.15 to 29.37 wt. %), the Francevillian MRS are further divided into unpyritized EST (0.15 to 0.36 wt. %), poorly pyritized MRS (0.19 to 4.16 wt. %), and pyritized MRS (6.46 to 29.37 wt. %) (cf. [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF]. Detailed descriptions of textural diversity and elemental geochemistry of MRS are provided in [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF][START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF].

Morphological and microtextural observations have shown that the MRS were likely dominated by cyanobacterial communities [START_REF] Aubineau | Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth[END_REF][START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF]. In modern environments, similar microbial mat communities are capable of "fixing" atmospheric nitrogen into bioavailable nitrogen that is subsequently utilized by microorganisms [START_REF] Bebout | Nitrogen cycling in microbial mat communities: The quantitative importance of N-fixation and other sources of N for primary productivity[END_REF][START_REF] Herbert | Nitrogen cycling in coastal marine ecosystems[END_REF]. Nitrogen fixation fulfills the N requirement for primary production, thus sustaining mat growth. Cyanobacterial mats exhibit high annual N fixation activity in the range of 0.8 to 76 g N m -2 year -1 , fueling high rates of CO2 reduction during photosynthesis [START_REF] Herbert | Nitrogen cycling in coastal marine ecosystems[END_REF][START_REF] Woebken | Revisiting N2 fixation in Guerrero Negro intertidal microbial mats with a functional single-cell approach[END_REF]. This bioavailable nitrogen produced by diazotrophs is entirely consumed by the wide array of microorganisms existing within the mat [START_REF] Bebout | Nitrogen cycling in microbial mat communities: The quantitative importance of N-fixation and other sources of N for primary productivity[END_REF]. Hence, the Francevillian Group mats could show a similar pattern of N cycling as there is no a priori reason to believe that Precambrian mats behaved differently from their modern counterparts.

In order to provide further insight into N cycling and microbial biogeochemistry at the end of the LE, this study focused on the MRS and their associated host sediments of the Francevillian Group FB2 Member. Both have been previously characterized for their mineralogy and elemental composition [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF][START_REF] Aubineau | Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth[END_REF]. Here, we use ironbased redox analyses and redox-sensitive element concentrations to constrain local redox conditions in the depositional setting. We additionally measured organic carbon (δ 13 Corg) and whole-rock plus kerogen-bound nitrogen (δ 15 N) isotope compositions of both the microbial structures and host rocks to highlight potential differences related to microbial ecology (i.e., benthic vs. planktonic communities) among the diverse lithologies. In conjunction with previously published data [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF][START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF], we explore the spatial and temporal trends in C and N isotope records of the Upper Francevillian Group to constrain the basin-scale trajectory of C and N cycling at the end of the LE, and implications for the unique macrofossil record of the Franceville sub-basin.

Geological setting

The Francevillian Group in southeastern Gabon comprises five sedimentary formations (FA to FE) deposited during the middle Paleoproterozoic (Fig. 1a). This 1.0 to 2.5 km-thick sedimentary succession rests unconformably on the Archean crystalline basement (Fig. 1b; [START_REF] Weber | Une série précambrienne du Gabon : le Francevillien. Sédimentologie, géochimie, relations avec les gîtes minéraux associés[END_REF]. The basal FA Formation, consisting mainly of fluvial to fluvio-deltaic sandstones accumulated during a progressive basin opening to marine conditions [START_REF] Bankole | Textural and paleofluid flow control on diagenesis in the Paleoproterozoic Franceville Basin, South Eastern, Gabon[END_REF][START_REF] Gauthier-Lafaye | The Francevillian (Lower Proterozoic) uranium ore deposits of Gabon[END_REF], is widely known for post-depositional uranium ore deposits hosted in the Upper FA Formation [START_REF] Bankole | Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization[END_REF]Gauthier-Lafaye andWeber, 2003, 1989). After a period of tectonic subsidence and deepening in the foreland basin (cf. [START_REF] Bankole | Earth's oldest preserved K-bentonites in the ca. 2.1 Ga Francevillian Basin, Gabon[END_REF][START_REF] Ossa Ossa | Constraining provenance for the uraniferous Paleoproterozoic Francevillian Group sediments (Gabon) with detrital zircon geochronology and geochemistry[END_REF], the FA formation gave way to a predominantly marine depositional phase, starting with the FB Formation. The latter is lithostratigraphically subdivided into FB1 (including a, b, and c units) and FB2 (a and b units) members [START_REF] Azziley Azzibrouck | Sédimentologie et géochimie du Francevillien B (protérozoïque inférieur). Métallogénie des gisements de manganèse de Moanda[END_REF][START_REF] Weber | Une série précambrienne du Gabon : le Francevillien. Sédimentologie, géochimie, relations avec les gîtes minéraux associés[END_REF].

The FB1a unit consists entirely of green shales, while the FB1b unit contains both black shales and heterolithic beds, with the latter characterized by rhythmic couplets of greyish shale and dolomite-cemented siltstones [START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF]. Following an episode of sea level rise, deposition continued with the FB1c unit characterized by black shales, the upper part of which contains iron-and manganese-rich sediments (Gauthier-Lafaye and [START_REF] Gauthier-Lafaye | Natural nuclear fission reactors: Time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere[END_REF]. Episodic submarine volcanism and hydrothermal activity have been inferred as the potential sources of aqueous Fe(II) and Mn(II), transported from deep anoxic settings onto the basin margin by upwelling to form the Fe and Mn deposits [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF].

These upwelling events brought reductants and nutrients onto the oxic, shallow-marine basin shelf (Gauthier-Lafaye and [START_REF] Gauthier-Lafaye | Natural nuclear fission reactors: Time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere[END_REF], which would have driven oxygen consumption, contributing to the first of two steps in seawater deoxygenation in the Franceville sub-basin at the end of the LE [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF]. The FB2a unit, formed during sea level fall [START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF], is characterized by massive sandstones with black shale interbeds and overlain by laminated black shales of the FB2b unit, frequently intercalated with cm-thick siltstone layers. The FB2 Member was deposited in a mud-dominated setting with high-density sand flows [START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF].

The multiple sulfur isotope composition of authigenic pyrite in these units indicates that the seawater column during their deposition was relatively oxygenated with a sizable sulfate reservoir [START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF][START_REF] El Albani | Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago[END_REF][START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF]. In addition, the black shales of the FB2b unit host well-preserved complex macrofossils and delicate MRS [START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF][START_REF] El Albani | Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago[END_REF], 2014, 2010).

The overlying FC Formation, deposited mainly in a tidal-flat setting, is further divided into the FC1 and FC2 members based on lithological composition [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF].

The Lower FC Formation consists of massive dolostones and black shales [START_REF] Préat | Paleoproterozoic high δ 13 C dolomites from the Lastoursville and Franceville basins (SE Gabon): Stratigraphic and synsedimentary subsidence implications[END_REF][START_REF] Weber | Une série précambrienne du Gabon : le Francevillien. Sédimentologie, géochimie, relations avec les gîtes minéraux associés[END_REF] with manganese-rich sediments at the top [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF], while the Upper FC Formation is dominated by stromatolitic chert associated with the oldest Gunflint-type assemblage of microfossils that includes filamentous cyanobacteria [START_REF] Bertrand-Sarfati | Microfossiliferous cherty stromatolites in the 2000 Ma Franceville group, Gabon[END_REF][START_REF] Baghekema | Nanoscale analysis of preservation of ca. 2.1 Ga old Francevillian microfossils, Gabon[END_REF]. The FD Formation contains marine fine-to medium-grained sedimentary rocks alternating with volcanic tuffs [START_REF] Thiéblemont | Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern Gabon (Western Central Africa)[END_REF], above which the sandstones of the FE Formation were deposited (Gauthier-Lafaye and [START_REF] Gauthier-Lafaye | The Francevillian (Lower Proterozoic) uranium ore deposits of Gabon[END_REF][START_REF] Thiéblemont | Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern Gabon (Western Central Africa)[END_REF].

Although the depositional age is still controversial and poorly resolved (Fig. 1b) (e.g., [START_REF] Bros | Sm-Nd isotopic dating of Proterozoic clay material: An example from the Francevillian sedimentary series, Gabon[END_REF], precise U-Pb zircon ages of 2083 ± 6 Ma and 2072 ± 29 Ma were reported from ignimbrite tuffs and sandstones near the top of the FD Formation, respectively [START_REF] Horie | U-Pb geochronology and geochemistry of zircon from the Franceville series at Bidoudouma, Gabon[END_REF][START_REF] Thiéblemont | Notice explicative de la carte géologique et des ressources minérales de la République gabonaise à 1/1[END_REF]. Based on the falling limb of the large-magnitude positive excursion in δ 13 C recorded in marine carbonates (δ 13 Ccarb) of the FB and lower FC formations, these units are thought to have been deposited at the end of the LE [START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF][START_REF] El Albani | Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago[END_REF][START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF][START_REF] Préat | Paleoproterozoic high δ 13 C dolomites from the Lastoursville and Franceville basins (SE Gabon): Stratigraphic and synsedimentary subsidence implications[END_REF]. The similar end to the positive δ 13 Ccarb excursion observed in the lowermost part of the ca. 2.0 Ga Zaonega Formation (ZF; Karelia Craton, NW Russia; [START_REF] Hannah | Re-Os geochronology of a 2.05 Ga fossil oil field near Shunga[END_REF][START_REF] Martin | Multiple Palaeoproterozoic carbon burial episodes and excursions[END_REF], together with similar large negative excursions large in δ 13 Corg values below 40‰ in both the FD Formation and ZF, have led to the interpretation that these formations might have been deposited contemporaneously [START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF][START_REF] Kump | Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event[END_REF]. Therefore, deposition of both the FD Formation and ZF most likely corresponds to the aftermath of the LE.

A different view was recently presented by [START_REF] Mayika | The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event[END_REF], who argued that carbon isotope composition of marine carbonates in the Franceville sub-basin reflects basinal stratification with waters in shallow-marine settings having highly positive C isotope values, while waters in deep-marine settings having near-to-zero C isotope values with the difference approaching 10‰. [START_REF] Mayika | The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event[END_REF] related highly positive C isotope values in shallow-water carbonates in the Franceville sub-basin to either high degree of evaporation or high primary productivity in these settings. In other words, this study speculated that the carbon cycle variations in the Franceville sub-basin reflect only regional, basin-scale conditions. However, this extreme level of C-isotope fractionation between shallow and deep waters and deposits is not observed in modern or ancient open-marine settings nor predicted on the theoretical grounds for the Paleoproterozoic pCO2 [START_REF] Bjerrum | New insights into the burial history of organic carbon on the early Earth[END_REF][START_REF] Hotinski | The effectiveness of the Paleoproterozoic biological pump: A δ 13 C gradient from platform carbonates of the Pethei Group (Great Slave Lake Supergroup, NWT)[END_REF][START_REF] Kroopnick | The distribution of 13 C of ΣCO2 in the world oceans[END_REF][START_REF] Kroopnick | Correlations between 13 C and ΣCO2 in surface waters and atmospheric CO2[END_REF]. In contrast, modern, stratified basins with restricted communication with the ocean (e.g., Black Sea and Kyllaren Fjord) develop similar gradients, but with highly negative C isotope values in deep waters, rather than highly positive C isotope values in shallow-water settings [START_REF] Fry | Stable isotope studies of the carbon, nitrogen and sulfur cycles in the Black Sea and the Cariaco Trench[END_REF][START_REF] Smittenberg | A 400-year record of environmental change in an euxinic fjord as revealed by the sedimentary biomarker record[END_REF]. [START_REF] Mayika | The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event[END_REF] related highly positive C isotope values in shallow-water carbonates in the Franceville sub-basin to either high degree of evaporation or high primary productivity in these settings. The FC Member also does not show extensive development of evaporites (Bouton et al., 2009a) nor does it show any evidence for enhanced productivity (such as a signal of nitrogen fixation; e.g., [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. In contrast, mid-Proterozoic carbonate successions with a strong evaporite signal or a signal for enhanced primary productivity do not show high 13 C-enrichments [START_REF] Ingall | Geochemistry of a 1.2 Ga carbonate-evaporite succession, northern Baffin and Bylot Islands: implications for Mesoproterozoic marine evolution[END_REF][START_REF] Papineau | High primary productivity and nitrogen cycling after the Paleoproterozoic phosphogenic event in the Aravalli Supergroup, India[END_REF]. Furthermore, in the case of the [START_REF] Mayika | The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event[END_REF] study, carbon isotope values started to decrease ~10 meters below the transgressive surface, which is inconsistent with the proposed basinal stratification and agrees with a global, secular control. We thus favor the view that the FC Formation records the end of the LE at ca. 2.11-2.06 Ga (see [START_REF] Bekker | The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event: comment[END_REF] for further discussion of [START_REF] Mayika | The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event[END_REF]).

Methods

Sampling. All of our samples (both MRS and host sediments) came from outcrops in the Moulendé Quarry (Fig. 1a). Before collecting outcrop material, the weathered outermost surfaces were removed. We carefully separated the mat laminae from its host rock material with a stainless steel razor blade -avoiding as much as possible contamination -for geochemical analyses. The host sediments were sampled directly below the MRS with a hammer. We made a special effort to exclude contribution of MRS to the host rocks during sample preparation. We cannot entirely rule out small contribution of the underlying sediment to sampled MRS, however, our sampling procedure did not result in significant contamination as far as distinct clay minerals and geochemical compositions were observed for samples of the MRS and their host sediments [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF][START_REF] Aubineau | Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth[END_REF].

Scanning electron microscopy. Pyrite morphology of MRS and black shales was studied in carbon-coated and polished slab sections, using a FEI Quanta 200 scanning electron microscope (SEM) equipped with an energy-dispersive X-Ray Spectrometer (EDX) at the University of Lille. The images of pyrite texture were acquired in back-scattered electron (BSE) mode operated at accelerating voltage of 15 kV, 1 nA beam current, and a working distance of 10.5 mm. Pyrite was specifically targeted for textural analysis to evaluate its morphology and preservation, as it can be readily affected by oxidative weathering.

Whole-rock analysis. Major and trace element concentrations in powdered samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) at Service d'Analyse des Roches et Minéraux (SARM) of the Centre de Recherches Pétrographiques et Géochimiques (CRPG), Nancy, France. The extended protocol for major and trace element measurements at SARM-CRPG is described by [START_REF] Carignan | Routine analyses of trace elements in geological samples using flow injection and low pressure on-line liquid chromatography coupled to ICP-MS: a study of geochemical reference materials BR, DR-N, UB-N, AN-G and GH[END_REF]. Specifically, powdered samples were decomposed using alkali fusion. They were fused with 900 mg of ultra-pure lithium metaborate at 980°C to form a glass. Then, the glass was dissolved in a mixture of 1 M nitric acid, 0.5% hydrogen peroxide, and 10% glycerol. The uncertainty for major elements was determined to be better than 5%, with the exception of Ca (10%) and P (>25%), while that for trace elements was lower than 5% for concentrations >10 ppm and 15% for concentrations between 10 and 1 ppm and >25% for elemental concentrations near the detection limit, as checked with international standards and analysis of replicate samples (cf. [START_REF] Carignan | Routine analyses of trace elements in geological samples using flow injection and low pressure on-line liquid chromatography coupled to ICP-MS: a study of geochemical reference materials BR, DR-N, UB-N, AN-G and GH[END_REF].

Enrichment factors (EF) were calculated for Mn, P, Co, Cu, Mo, Ni, U, and V following the approach described by [START_REF] Tribovillard | Trace metals as paleoredox and paleoproductivity proxies: An update[END_REF]. EF represents the trace metal (TM) excess in the sediments, which is assessed by dividing TM/Al or TM/Ti ratios by that for the upper crustal average [START_REF] Rudnick | Composition of the Continental Crust[END_REF]. We present in Table S1 ratios of P/Ti and P/Al as detrital tracers for P, following previous studies (e.g., [START_REF] Filippelli | Productivity records from the Southern Ocean and the equatorial Pacific Ocean: Testing the glacial Shelf-Nutrient Hypothesis[END_REF][START_REF] Filippelli | A sediment-nutrient-oxygen feedback responsible for productivity variations in Late Miocene sapropel sequences of the western Mediterranean[END_REF][START_REF] Latimer | Terrigenous input and paleoproductivity in the Southern Ocean[END_REF][START_REF] Tribovillard | Trace metals as paleoredox and paleoproductivity proxies: An update[END_REF] Iron speciation analysis. Iron speciation analysis was performed at Cardiff University, according to the method of [START_REF] Poulton | Development of a sequential extraction procedure for iron: Implications for iron partitioning in continentally derived particulates[END_REF], which is reviewed elsewhere [START_REF] Poulton | Ferruginous conditions: A dominant feature of the ocean through Earth's history[END_REF][START_REF] Raiswell | The iron paleoredox proxies: A guide to the pitfalls, problems and proper practice[END_REF]. This method targets iron that is "highly reactive" (FeHR) towards hydrogen sulfide in the form of (oxyhydr)oxides (FeMag and FeOx), carbonates (Fecarb), and pyrite iron (Fepy) relative to total iron (FeT), including poorly reactive Fe (FePRS) and unreactive Fe (FeU). First, FeCarb was extracted with a sodium acetate buffer at pH 4.5 and 50°C for 48 hours, followed by FeOx extraction using sodium dithionite at pH 4.8 and room temperature for 2 hours, and finally FeMag extraction using ammonium oxalate at pH 3.2 and room temperature for 6 hours. FePRS was then determined by boiling in 12 M HCl for 2 minutes. FePy was measured by weighing a CuS precipitate after HCl and chromous chloride distillation, with Fe calculated assuming FeS2 stoichiometry [START_REF] Canfield | The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales[END_REF]. Replicate analyses between runs (n = 5) gave an average standard deviation of 0.11 wt.% for FeCarb, 0.19 wt.% for FeOx, 0.004 wt.% for FeMag, 0.07 wt.% for FePRS, and 0.06 wt.% for FePy. Samples containing <0.5 wt.% Fe were excluded from the iron speciation analysis. This threshold is recommended to avoid samples that are at the resolution limit of the iron-based redox proxy (see [START_REF] Clarkson | Assessing the utility of Fe/Al and Fe-speciation to record water column redox conditions in carbonate-rich sediments[END_REF]Raiswell et al., 2018 for reviews).

Kerogen extraction. Kerogen was isolated from selected whole-rock powders (n = 10) at the University of Washington using the published protocol from [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. In brief, sample powders were treated with a 1:1 mixture of concentrated HF and DI-H2O in order to dissolve silicate minerals. The resulting solutions were centrifuged and the supernatant was decanted. Residual fluoride minerals were then dissolved with a BF3 solution, and the supernatant decanted after centrifugation. The remaining kerogen was then rinsed with three iterations of DI-H2O, and finally placed into a freeze-drier to remove all moisture prior to analysis.

Carbon and nitrogen isotope analyses. Carbon isotope ratios for organic matter (δ 13 Corg) and whole-rock N isotope values (δ 15 NWR) were analyzed for all lithologies (n = 34). The isotope data were measured by flash combustion on a Thermo Scientific DELTA V Advantage isotope ratio mass spectrometer (IRMS), operated under a continuous helium flow, and coupled to a COSTECH 4010 elemental analyzer (EA) at the University of California, Riverside. For δ 13 Corg analysis, ∼200 mg of whole-rock powders were decarbonized with 6 N HCl for one hour at 70°C. The solid residue was thoroughly rinsed in deionized water and dried in a clean hood overnight. Small aliquots of decarbonated residues (mostly less than 10 mg) were weighed into tin cups. For δ 15 NWR analysis, 20 to 50 mg of powdered whole-rock samples were weighed into tin cups with a blank placed after each sample. Combustion at 1020° utilized ~10 mL of O2. All isotope data are reported in standard δ-notation relative to V-PDB and Air-N2 for δ 13 C and δ 15 N, respectively. The isotope measurements were standardized against three in-house standards: Acetanilide from two different batches (δ 13 C = -27.9‰, δ 15 N = -0.8‰ and δ 13 C = -33.7‰, δ 15 N = -0.8‰) and Hawaiian Glycine (δ 13 C = -36.6‰, δ 15 N = 11.3‰), which were calibrated to international reference materials SDO-1 (δ 13 C = -30.0‰, δ 15 N = -0.8‰; [START_REF] Dennen | δ 15 N and noncarbonate δ 13 C values for two petroleum source rock reference materials and a marine sediment reference material[END_REF] and SGR (δ 13 C = -29.3‰, δ 15 N = 17.4‰; [START_REF] Dennen | δ 15 N and noncarbonate δ 13 C values for two petroleum source rock reference materials and a marine sediment reference material[END_REF]. Our samples are fully bracketed by the isotopic compositions of the standards (Fig. S1). Replicate analyses for δ 13 Corg (n = 15) and δ 15 NWR (n = 22) yielded an average standard deviation (1σ) better than 0.13 and 0.15‰, respectively. Total organic carbon (TOC) and total nitrogen (TN) abundances were calculated from CO2 and N2 peak areas, respectively. The δ 15 NWR values are reported only for samples with TN concentrations well above the detection limit. Atomic C:N ratios were calculated by dividing mass ratios (wt.%) by molar masses of 12.01 g/mol for carbon and 14.01 g/mol for nitrogen such that: C:NWR (atomic) = (wt.% C/12.01)/(wt.% N/14.01).

For δ 15 Nkerogen analysis, a few mg of kerogen were weighed into tin cups and analyzed on a Costech ECS 4010 Elemental Analyzer coupled to a ThermoFinnigan MAT 253 continuous-flow isotope ratio mass spectrometer housed in IsoLab at the Department of Earth & Space Sciences, University of Washington. Combustion utilized 20 mL of O2 at 1000°C. Analytical blanks were measured and subtracted from data. The resulting isotope data were standardized using in-house standards "GA1" (glutamic acid, δ 15 N = -4.6‰), "GA2" (glutamic acid, δ 15 N = -5.7‰), and "SA" (dried salmon, δ 15 N = +11.3‰) that were calibrated to international reference materials USGS40 and USGS41. The precision and accuracy of Nkerogen isotope analysis were better than 0.2‰, based on replicate runs (n = 10).

Results

Redox reconstruction

Iron speciation analysis of shale lithofacies is one of the most widely used methods to evaluate local modern and ancient water-column redox conditions [START_REF] Fru | Sedimentary mechanisms of a modern banded iron formation on Milos Island, Greece[END_REF][START_REF] Planavsky | Widespread iron-rich conditions in the mid-Proterozoic ocean[END_REF][START_REF] Poulton | Ferruginous conditions: A dominant feature of the ocean through Earth's history[END_REF][START_REF] Poulton | The low-temperature geochemical cycle of iron: From continental fluxes to marine sediment deposition[END_REF][START_REF] Sperling | A basin redox transect at the dawn of animal life[END_REF]. However, formation of iron monosulfides (AVS, pyrrhotite) from pyrite or a conversion of unsulfidized FeHR to FePRS during sediment diagenesis and metamorphic reactions can significantly alter Fe speciation data [START_REF] Slotznick | The effects of metamorphism on iron mineralogy and the iron speciation redox proxy[END_REF]. Iron sulfides in the Francevillian Group MRS and host sediments are predominantly primary pyrite (Aubineau et al., 2019). The studied samples possess mean FePRS/FeT ratios of 0.07 ± 0.01 wt.% (Table S2), which is below the average Paleozoic shales (0.39 ± 0.11 wt%; [START_REF] Raiswell | Turbidite depositional influences on the diagenesis of Beecher's trilobite bed and the hunsrück slate; sites of soft tissue pyritization[END_REF].

Combined, these data suggest that diagenetic and metamorphic effects are unlikely to have biased our iron speciation results. Petrographic examinations also revealed that the sulfidebearing sedimentary beds in the FB2 Member lacked an oxidized iron residue, which is common to weathered horizons (cf. [START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF][START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF]Fig. 2). Our observations thus suggest that Fe speciation systematics were not appreciably compromised by secondary oxidative weathering, thermal breakdown of pyrite, or late diagenetic processes.

Sediments deposited under anoxic conditions are typically characterized by FeHR/FeT ratios >0.38, while FeHR/FeT ratios less than 0.22 are typical for sediments deposited under oxic conditions. Values between 0.22 and 0.38 are equivocal for interpretation of local redox conditions as they could reflect depositional conditions when sedimentation rate was high or when the transfer of unsulfidized FeHR to less reactive clay minerals occurred during early to late diagenesis [START_REF] Poulton | Spatial variability in oceanic redox structure 1.8 billion years ago[END_REF]. Both mechanisms may mask Fe enrichment under anoxic water-column conditions. If reactive Fe exceeds 38% of the FeT and if the FePy/FeHR values are below 0.7-0.8, deposition in ferruginous environment is inferred [START_REF] Poulton | Ferruginous conditions: A dominant feature of the ocean through Earth's history[END_REF]. Values for FePy/FeHR ≥0.7-0.8 typically indicate sulfide-rich (euxinic) watercolumn conditions. In addition, the FeT/Al proxy can also identify Fe enrichments that discriminate between oxic and anoxic depositional environments [START_REF] Clarkson | Assessing the utility of Fe/Al and Fe-speciation to record water column redox conditions in carbonate-rich sediments[END_REF][START_REF] Lyons | A critical look at iron paleoredox proxies: New insights from modern euxinic marine basins[END_REF]. This ratio is not affected by a transfer of unsulfidized FeHR to FePRS during diagenesis or metamorphism, meaning that the paired use of FeT/Al and iron speciation can robustly identify primary redox conditions [START_REF] Partin | Euxinic conditions recorded in the ca. 1.93 Ga Bravo Lake Formation, Nunavut (Canada): Implications for oceanic redox evolution[END_REF][START_REF] Raiswell | The iron paleoredox proxies: A guide to the pitfalls, problems and proper practice[END_REF]. Modern marine sediments deposited under oxic conditions have FeT/Al ratio below that of the average upper continental crust (0.55 ± 0.11), and this value can be applied as a threshold for redox evaluation of siliciclastic rocks [START_REF] Clarkson | Assessing the utility of Fe/Al and Fe-speciation to record water column redox conditions in carbonate-rich sediments[END_REF]. Thus, FeT/Al values >0.66 (with a confidence threshold of 1σ) suggest local Fe enrichments either under anoxic conditions or due to hydrothermal Fe input. Strong Fe enrichments (FeT/Al > 2) are often derived from an input of hydrothermal fluids to the water column [START_REF] Raiswell | The iron paleoredox proxies: A guide to the pitfalls, problems and proper practice[END_REF].

The poorly pyritized and EST samples are predominantly characterized by sub-crustal FeT/Al and low FePy/FeHR ratios, with an upsection increase in FeHR (Fig. 3; Tables S1,S2). In contrast, the pyritized MRS have supra-crustal FeT/Al ratios, and high FeHR/FeT and FePy/FeHR values. Both host sandstones and black shales have low FeT/Al ratios, whereas the black shales show FeHR/FeT ratios covering the full range of redox conditions, but FePy/FeHR ratios <0.7. The absence of significant, positive Eu anomalies in REE composition of the MRS and black shales indicates that hydrothermal circulation has not strongly affected the depositional site [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF]. Instead, variable redox conditions, including oxic, ferruginous, and euxinic, developed during deposition of the MRS and black shales are consistent with localized expression of dissimilatory iron reduction (DIR) and microbial sulfate reduction (MSR) during early diagenesis [START_REF] Aubineau | Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth[END_REF][START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF] and indicate deposition close to a fluctuating redoxcline.

Redox-sensitive metals

The average Mn concentration in the Francevillian Group FB2 Member is 0.26 wt.% (Fig. 4, Table S1), more than three times the Mn content of 0.078 wt.% in the upper continental crust (UCC; [START_REF] Rudnick | Composition of the Continental Crust[END_REF]. The Mn/Al ratios and EFMn in both the MRS and host sediments moderately increase upsection (Fig. 4; Table S1). On the other hand, the U/Al, V/Al, and Mo/Al ratios are low for most samples, with values being even lower than those for the UCC. Specifically, the redox-sensitive elements are consistently depleted relative to the UCC in the FB2 Member host sediments, with the exception of one sandstone sample (mean EFMo = 0.7 ± 0.2; EFU and EFV = 0.4 ± 0.1). The pyrite-bearing MRS display enrichment in Mo due to microbially induced sulfidic conditions that enhanced incorporation of chalcophilic elements [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF].

In the black shale samples, some elements that are known to be associated with organic matter (e.g., Co, Cu, and Ni;[START_REF] Tribovillard | Trace metals as paleoredox and paleoproductivity proxies: An update[END_REF] do not show high enrichment relative to the UCC (Table S1). Cobalt is moderately depleted (mean EFCo = 0.6 ± 0.8), while Cu and Ni show only slight mean enrichments (EFCu = 1.6 ± 0.5; EFNi = 1.0 ± 0.6, respectively).

Carbon and nitrogen isotope systematics, and phosphorus enrichment

The studied stratigraphic interval shows δ 13 Corg values similar to the previously published data for the FB2 Member [START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF][START_REF] Gauthier-Lafaye | Natural nuclear fission reactors: Time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere[END_REF], but our whole-rock δ 15 N values (Fig. 3; Table S3) differ from those recently reported for the overlying FC Formation by [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. The average TOC content is 0.28 wt.% for sandstones and 2.21 wt.% for black shales. The TN content for sandstones is close to the detection limit (∼0.01 wt.%) and thus must be treated with caution, while that for black shales is nearly constant, with an average of 0.07 wt.%. This difference is accompanied by a change in mean δ 13 Corg values that average -31.5 ± 1.2‰ (1σ, n = 6) for sandstones and -35.1 ± 0.8‰ (1σ, n = 9) for black shales. Thus, the transition from the FB2a to FB2b unit is marked by a shift towards more negative δ 13 Corg values. The mean δ 15 NWR value for the fine-grained sediments is -0.2 ± 0.4‰ (1σ, n = 8; ranging from -0.8 to 0.6‰), and lacks a distinct stratigraphic trend.

Finally, the mean δ 15 Nkerogen value for the black shales is 1.4 ± 1.0‰ (1σ, n = 5; ranging from -0.2 to +2.3‰).

Carbon and nitrogen contents and isotope values of the different MRS (Fig. 3, Fig. S2; Table S3) reveal that TOC content is high in the poorly pyritized MRS and EST, averaging 6.85 wt.% and 9.78 wt.%, respectively, and decreases, by up to 11 times, to below 0.87 wt.% in the pyritized MRS. Uniform mean TN values of 0.13 and 0.12 wt.% characterize the poorly pyritized MRS and EST, respectively, but TN content of the pyritized MRS is twice lower, with a mean of 0.06 wt.%. The δ 13 Corg values average -33.9 ± 0.9‰ (1σ, n = 9), -30.5 ± 0.4‰ (1σ, n = 3), and -33.7 ± 0.5‰ (1σ, n = 7) for the poorly pyritized MRS, EST, and pyritized MRS, respectively. The mean δ 13 Corg values differ between the EST and the pyritecontaining MRS (i.e., poorly pyritized and pyritized MRS) by up to 3.4‰, but without a representative sample size, the significance level of this isotopic heterogeneity remains uncertain. The MRS samples are marked by similar mean δ 15 NWR compositions of 0.8 ± 0.6‰ (1σ, n = 9; ranging from -0.1 to +1.9‰), -1.2 ± 2.0‰ (1σ, n = 3; ranging from -3.5 to +0.3‰), and -1.5 ± 1.4‰ (1σ, n = 5; ranging from -3.3 to +0.3‰) for the poorly pyritized MRS, EST, and pyritized MRS, respectively. The δ 15 Nkerogen values average 2.3 ± 1.0‰ (1σ, n = 5; ranging from +0.9 to +3.6‰) for the pyrite-poor MRS.

The section is characterized by mean P/Ti ratios (ppm/wt.%) of 0.16 and 0.31 for nonpyritized MRS and black shales, respectively (Fig. 3; Table S1). Notably, P is slightly enriched in the FB2b unit (mean EFP = 1.8 ± 0.2) relative to the UCC.

Discussion

Ocean chemistry during deposition of the Francevillian Group FB2 Member

To evaluate the redox state of ancient seawater robustly, Fe-based redox proxies should be coupled with other available redox proxies. The sedimentary enrichment of redox-sensitive trace metals is one source of such information, with the potential to reconstruct bottom-water redox conditions. For example, enrichments in U and V preferentially occur under anoxic (euxinic and ferruginous) conditions, while Mo is readily scavenged in anoxic, sulfide-rich (euxinic) waters [START_REF] Scott | Tracing the stepwise oxygenation of the Proterozoic ocean[END_REF][START_REF] Scott | Contrasting molybdenum cycling and isotopic properties in euxinic versus non-euxinic sediments and sedimentary rocks: Refining the paleoproxies[END_REF]e.g., Tribovillard et al., 2006).

The Fe-and Mn-rich stratigraphic interval in the upper part of the FB1 Member has been related to a progressive oxidation of Fe(II) and Mn(II) delivered with deep, anoxic waters upwelled across the chemocline towards shallow, oxygenated waters (Fig. 5a; Ossa [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF]. The Fe-and Mn-rich deposits are thought to be derived from precipitation of Fe(III) and Mn(IV) (oxyhydr)oxides in the water column, followed by conversion to carbonate minerals through early diagenetic dissimilatory reduction of these compounds by microorganisms in association with organic matter oxidation. The moderate Mn enrichment in the FB2 Member, stratigraphically above the Mn ores of the Upper FB1 Member, likely indicates a similar process where aqueous Mn(II) was continuously supplied by upwelling deep anoxic waters to the depositional site. Consistent with this interpretation, the FB2 Member sediments shows iron enrichment in carbonate minerals with Fecarb fraction contributing up to 41% to the total FeHR reservoir (Table S2). Since the proportion of FePRS/FeT in studied samples is much lower than the Phanerozoic average, it is unlikely that iron of the carbonate fraction was post-depositionally transferred to the silicate pool, which supports preservation of the original FeHR/FeT signal. In addition, relatively high Mn content in the FB2 Member could also reflect deposition near the redox boundary [START_REF] Calvert | Geochemistry of Recent oxic and anoxic marine sediments: Implications for the geological record[END_REF]. While the seafloor colonization by mat-building cyanobacteria in the MRS and large sulfur isotope fractionations have been used to infer an oxygenated, shallowmarine depositional environment in the photic zone for the FB2 Member [START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF][START_REF] El Albani | Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago[END_REF][START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF], our Fe-speciation data for this interval show evidence for intermittently oxic to anoxic bottom-water conditions likely in proximity to the chemocline (Fig. 5b). Weak to moderate correlations are observed between P and both Ti (R 2 = 0.17, p < 0.08) and Al (R 2 = 0.31, p < 0.02) in the studied host sediments (Fig. S3), suggesting that detrital P delivery was not significant, as Ti and Al are usually taken to be detrital tracers [START_REF] Filippelli | A sediment-nutrient-oxygen feedback responsible for productivity variations in Late Miocene sapropel sequences of the western Mediterranean[END_REF][START_REF] Tribovillard | Trace metals as paleoredox and paleoproductivity proxies: An update[END_REF]. Moreover, O2-depleted bottomwaters may limit P burial through reductive dissolution and recycling of primary Fe(III)(oxyhydr)oxides and remineralization of organic matter in sulfate-rich water column and pore-waters [START_REF] Ingall | Evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters[END_REF]. Hence, P input from upwelling waters (Fig. 5b) might be responsible for the minor P enrichment in our black shale samples. Considering the absence of relationship between the sedimentation rate and P and TOC concentrations in modern environments [START_REF] Ingall | Geochemistry of a 1.2 Ga carbonate-evaporite succession, northern Baffin and Bylot Islands: implications for Mesoproterozoic marine evolution[END_REF], the elevated TOC content in the black shales of the FB2 Member could reflect high rates of primary organic productivity stimulated by this P input.

While the absence of Mo enrichment is consistent with the Fe-speciation data arguing against persistently euxinic conditions, anoxia cannot account for the lack of U and V enrichment because their accumulation is expected under oxygen-deficient depositional conditions [START_REF] Tribovillard | Trace metals as paleoredox and paleoproductivity proxies: An update[END_REF]. In addition, the absence of Co, Cu, and Ni enrichment despite appreciable amounts of organic carbon in the black shales is unusual since these elements are usually enriched in sedimentary rocks in association with organic matter as organometallic complexes [START_REF] Tribovillard | Trace metals as paleoredox and paleoproductivity proxies: An update[END_REF]. One possible explanation is that the basinal or global inventory of dissolved redox-sensitive trace metals was depleted.

Considering that trace metal reservoir size exerts a first-order control on the magnitude of enrichment [START_REF] Partin | Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales[END_REF][START_REF] Scott | Tracing the stepwise oxygenation of the Proterozoic ocean[END_REF][START_REF] Tribovillard | Paleodepositional conditions in the Orca Basin as inferred from organic matter and trace metal contents[END_REF], limited metal availability could have played an important role in the absence of Co, Cu, and Ni accumulation. Indeed, trace metals are drawn down almost quantitatively even in some modern anoxic marine environments (e.g., the Black Sea; Algeo & Lyons, 2006), and the same process has been invoked to explain limited trace metal enrichment in the roughly coeval ca. 2.0 Ga Zaonega Formation [START_REF] Kipp | Redox fluctuations, trace metal enrichment and phosphogenesis in the ~2.0 Ga Zaonega Formation[END_REF]. In addition to the redox control on trace metal depletion, basin restriction from the global ocean could have also facilitated depletion of the basinal reservoirs of U, V, Co, Cu, and Ni. The tectonic setting of the Francevillian basin could be invoked to explain basinal restriction from the open ocean or at least to limited connection to the global ocean inventory, especially behind the forebulge (cf. [START_REF] Bankole | Earth's oldest preserved K-bentonites in the ca. 2.1 Ga Francevillian Basin, Gabon[END_REF][START_REF] Ossa Ossa | Constraining provenance for the uraniferous Paleoproterozoic Francevillian Group sediments (Gabon) with detrital zircon geochronology and geochemistry[END_REF]. Similarly, the absence of wave-related features or storm beds in the FB2 Member sandstones may indicate that the depositional setting was sheltered by the fault-bounded compartmentalization of the Francevillian basin [START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF]. It is thus reasonable to assume that geochemical conditions in the Upper FB Formation reflect basin-scale processes. Furthermore, high sedimentation rate of siliciclastic deposits in the FB2 Member [START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF] may have diluted the elemental enrichments [START_REF] Algeo | Mo-total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions[END_REF]. However, MRS indicate periods with slow sedimentation rate in the basin and yet show no significant enrichment in U and V. Along these lines, we envisage that these data might reflect benthic productivity and limited export of organic matter with associated trace elements from the photic zone. In light of these considerations, we interpret the observed trends in major and trace element concentrations to record upwelling, anoxic deep waters enriched in Fe, Mn, and P.

C and N isotopic fidelity

The C and N isotope composition of sedimentary rocks can be affected by three main stages of post-depositional alteration, including early diagenesis, late-stage burial diagenesis, and metamorphism [START_REF] Ader | Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives[END_REF]. Therefore, before evaluating organic δ 13 C and whole-rock δ 15 N signals, we examined whether post-depositional alteration could have overprinted primary signals.

While it has been shown that during the transfer of organic matter from water column to surface sediments the δ 15 N signal of primary producers might be altered, settings with high sedimentation and burial rates, short oxygen-exposure time, and high export production tend to faithfully record the isotopic composition of planktonic organisms [START_REF] Robinson | A review of nitrogen isotopic alteration in marine sediments[END_REF][START_REF] Tesdal | Nitrogen isotopes in bulk marine sediment: linking seafloor observations with subseafloor records[END_REF]. These characteristics match those inferred for the depositional site of the FB2 Member, suggesting that the observed N isotope ratios might be reflective of watercolumn processes; however, a detailed investigation of possible diagenetic effects on C and N isotope ratios is still warranted.

One way to gauge diagenetic reworking of sedimentary organic matter is with C:N ratios. Molar C:N values in modern oceanic phytoplankton range from about 4 to 10 [START_REF] Ader | Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives[END_REF][START_REF] Gruber | An Earth-system perspective of the global nitrogen cycle[END_REF], and the preferential oxidation of labile, N-rich components of marine biomass during its sinking through water column and in sediments can increase the C:N ratio in sedimentary rocks. The analyzed MRS and host sediment samples predominantly show C:N values between 5 and 100 (Fig. 6). These values are similar to those from other Precambrian sedimentary successions that experienced moderate post-depositional thermal alteration (i.e. below greenschist facies; [START_REF] Koehler | Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum[END_REF][START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF][START_REF] Chang | Nitrogen isotope evidence for an oligotrophic shallow ocean during the Cambrian Stage 4[END_REF], and are consistent with a moderate diagenetic increase in C:N ratios.

Anoxic diagenetic conditions during deposition of the FB2 Member are suggested by the occurrence of Fe-and Mn-bearing carbonate minerals formed by microbial reduction of Fe-and Mn-oxyhydroxides [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF] as well as by the abundance of pyrite with early diagenetic S isotope signature (highly negative δ 34 S values). Both early and late stage diagenesis under anoxic conditions could slightly modify the δ 15 NWR values [START_REF] Ader | Interpretation of the nitrogen isotopic composition of Precambrian sedimentary rocks: Assumptions and perspectives[END_REF]. The first step in this process is the degradation of organic-bound N (i.e., remineralization), which releases N as NH4 + to pore-waters below the sediment-water interface. Then, NH4 + can be incorporated into the interlayers of illite via substitution for potassium cations [START_REF] Schroeder | Illite-smectites and the influence of burial diagenesis on the geochemical cycling of nitrogen[END_REF][START_REF] Williams | Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration[END_REF][START_REF] Williams | Ammonium substitution in illite during maturation of organic Mmatter[END_REF]. The phyllosilicate-bound N phases represent up to half of TN content in modern sediments [START_REF] Müller | C/N ratios in Pacific deep-sea sediments: Effect of inorganic ammonium and organic nitrogen compounds sorbed by clays[END_REF]. Our dataset shows a strong positive co-variation (R 2 = 0.74, p < 2 x 10 -9 ) between TN and K (wt.%), supporting NH4 + fixation in K-rich clays during the breakdown of organic matter (Fig. S4a). Nonetheless, there is no apparent correlation between δ 15 NWR and K (wt.%) (R 2 = 0.11, p < 0.12), which suggests that this diagenetic pathway did not affect the whole-rock δ 15 N signal.

Another post-depositional process that elevates C:N ratios and can alter whole-rock C and N isotopic signals is thermal maturation. Specifically, diagenesis and metamorphism can lead to enrichment in heavy C and N isotopes in the remaining reservoir [START_REF] Bebout | Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history[END_REF]Stüeken et al., 2017a), although the δ 15 NWR composition may not significantly change at metamorphic grades up to lower greenschist facies [START_REF] Ader | Nitrogen isotopic evolution of carbonaceous matter during metamorphism: Methodology and preliminary results[END_REF][START_REF] Ader | Isotope study on organic nitrogen of Westphalian anthracites from the Western Middle field of Pennsylvania (U.S.A.) and from the Bramsche Massif (Germany)[END_REF][START_REF] Boudou | Organic nitrogen chemistry during low-grade metamorphism[END_REF]Stüeken et al., 2017a). Thermal alteration of organic molecules also drives the depletion of N relative to C [START_REF] Ader | Isotope study on organic nitrogen of Westphalian anthracites from the Western Middle field of Pennsylvania (U.S.A.) and from the Bramsche Massif (Germany)[END_REF][START_REF] Bebout | Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history[END_REF]. Combined, these processes should result in correlation between δ 13 Corg and TOC as well as δ 13 Corg and C:NWR.

With increasing burial depth and metamorphism, thermal alteration of organic matter could shift δ 15 NWR values up by <1‰ in lower greenschist facies, 1 to 2‰ in greenschist facies, and up to 4‰ in amphibolite facies [START_REF] Bebout | Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history[END_REF]Stüeken et al., 2017a). The Franceville sub-basin was not affected by metamorphism (Gauthier-Lafaye and [START_REF] Gauthier-Lafaye | The Francevillian (Lower Proterozoic) uranium ore deposits of Gabon[END_REF][START_REF] Ngombi-Pemba | From detrital heritage to diagenetic transformations, the message of clay minerals contained within shales of the Palaeoproterozoic Francevillian basin (Gabon)[END_REF], suggesting a maximum increase in whole-rock δ 15 N values by less than 1‰ due to burial. Typically, strong co-variation among δ 15 NWR and TN, δ 15 NWR and C:N, δ 13 Corg and TOC as well as δ 13 Corg and C:NWR are taken to indicate metamorphic alteration, as light isotopes are more mobile than heavier ones and N is more mobile than C [START_REF] Bebout | Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history[END_REF]Stüeken et al., 2017a). In contrast, these correlations are not observed across our Francevillian Group FB2 Member MRS and host-sediment samples (Fig. S4b), suggesting that thermal maturation did not substantially alter whole-rock C and N isotope signatures. In addition, the δ 15 Nkerogen values are slightly enriched (on average by +1.6‰) relative to corresponding δ 15 NWR values in our samples (Table S3), which is in good agreement with the pattern shown by most of the unmetamorphosed rocks studied by Stüeken et al. (2017a). The partitioning between kerogen-and mineral-bound nitrogen within the whole rock under progressive metamorphism is marked by the increase of the isotopic offset between kerogen and whole-rock δ 15 N values (Stüeken et al., 2017a;[START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF], with kerogen becoming lighter than the whole-rock, which is not observed in our Francevillian Group FB2 Member samples, consistent with the unmetamorphosed facies. Thus, we take our C and N isotope data as minimally affected by post-depositional isotopic fractionation.

Interpretation of C and N isotope data for the FB2 Member

Carbon cycling

Autotrophic microorganisms preferentially incorporate 12 C from CO2, CH4, or other carbonbearing organic molecules into their biomass as a function of their carbon fixation pathway, resulting in fractionation of carbon isotopes [START_REF] Zerkle | Biogeochemical signatures through time as inferred from whole microbial genomes[END_REF]. Carbon isotope fractionation during primary productivity is also affected by a combination of other factors, including growth rate, dissolved CO2 level, and the ratio of volume to surface area [START_REF] Hayes | The abundance of 13 C in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma[END_REF].

For the MRS and host sediments, the fractionation of C isotopes from the contemporaneous dissolved inorganic carbon (∆ 13 C) was on average 35 to 44‰ (1σ confidence interval), using the δ 13 Ccarb within the black shales of the FB Formation with the average of 6.6 ± 1.2‰ (1σ, n = 10; ranging from 5.5 to 9.6‰; El [START_REF] El Albani | Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago[END_REF]. The latter δ 13 Ccarb values are similar to those of marine carbonates deposited during the LE [START_REF] Bekker | Fractionation between inorganic and organic carbon during the Lomagundi (2.22-2.1 Ga) carbon isotope excursion[END_REF].

The δ 13 Corg values of the FB2 Member host sediments are unlikely to reflect the activity of photoautotrophic fixation alone. Instead, they may be pointing to the contribution of organic matter derived from multiple carbon fixation pathways, assuming that the 13 C/ 12 C ratio of atmospheric CO2 has not changed significantly through time. Indeed, modern photosynthesizers that assimilate dissolved inorganic carbon in equilibrium with atmospheric CO2 (with a composition of -7‰) fractionate carbon isotopes by up to about 30‰ through the Calvin-Benson-Bassham cycle [START_REF] Karhu | Carbon Isotopes in the Solar System[END_REF][START_REF] Schidlowski | Application of stable carbon isotopes to early biochemical evolution on Earth[END_REF][START_REF] Zerkle | Biogeochemical signatures through time as inferred from whole microbial genomes[END_REF]. The substantially larger fractionations between organic and inorganic carbon as recorded by the black shale samples likely reflect secondary productivity in the water column and near the sediment-water interface (cf. [START_REF] Hayes | The abundance of 13 C in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma[END_REF]. Specifically, carbon fixation through the reductive acetyl-coenzyme A (=acetyl-CoA) pathway of strictly anaerobic microorganisms, including autotrophic and acetoclastic methanogens, and some sulfate reducers, induces fractionations as large as 40‰, and thus contributing strongly depleted 13 Corg to the sedimentary rock record [START_REF] Schidlowski | Application of stable carbon isotopes to early biochemical evolution on Earth[END_REF][START_REF] Zerkle | Biogeochemical signatures through time as inferred from whole microbial genomes[END_REF]Stüeken et al., 2017b). However, carbon fixation pathways that fractionate biomass by more than about 40‰ are unknown [START_REF] Karhu | Carbon Isotopes in the Solar System[END_REF][START_REF] Schidlowski | Application of stable carbon isotopes to early biochemical evolution on Earth[END_REF][START_REF] Zerkle | Biogeochemical signatures through time as inferred from whole microbial genomes[END_REF].

Accordingly, the incorporation of isotopically "superlight" organic carbon into biomass as observed in some of our samples could only be explained by the involvement of secondary productivity, e.g. methanotrophy, believed to have been prominent during the deposition of the FB Member (Gauthier-Lafaye and [START_REF] Gauthier-Lafaye | Natural nuclear fission reactors: Time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere[END_REF].

Such microbial activity relies on methane oxidation during which variable electron acceptors are used; both aerobic and anaerobic processes may promote methane oxidation.

The large seawater sulfate reservoir at the time of deposition of the Francevillian Group [START_REF] Bekker | Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America[END_REF][START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF][START_REF] Schröder | Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: Evidence from sulphate evaporites in the ∼2.2-2.1 Gyr shallow-marine Lucknow Formation, South Africa[END_REF] suggests that the anaerobic oxidation of methane could have occurred with sulfate as an oxidant (i.e., anaerobic oxidation of methane coupled to sulfate reduction). However, we cannot rule out the possibility of aerobic methane oxidation occurring in this setting as well, considering Fespeciation evidence for intermittently oxic to anoxic bottom-water conditions in the likely proximity of the chemocline. Particularly, Cu is an essential cofactor in the methane monooxygenase, the most active enzyme involved during aerobic biological methane oxidation [START_REF] Knapp | Methane monooxygenase gene expression mediated by methanobactin in the presence of mineral copper sources[END_REF]. Given the large amount of land-derived Cu supplied to the oceans during both the GOE and LE [START_REF] Fru | Cu isotopes in marine black shales record the Great Oxidation Event[END_REF], it is likely that Cu was available to support aerobic methanotrophy at the chemocline (Chi Fru, 2011;Chi Fru et al., 2011).

Collectively, highly negative δ 13 Corg values beyond the range of phototroph-derived organic matter in the studied sediments argue for the addition of isotopically light biomass recording biological methane cycling below or at the redoxcline (Fig. 5b).

The presence of EST, "fairy ring" structures, tufted morphology, and structures interpreted as gas bubbles related to oxygen production in microbial mats suggest that filamentous cyanobacteria were responsible for the development of the Francevillian Group MRS [START_REF] Aubineau | Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth[END_REF][START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF]. However, carbon isotope fractionation (>40‰) in the MRS also points to methane-based metabolism as explained above (Fig. 5b). In this context, the whole range of δ 13 Corg values in different MRS morphotypes likely reflect the extent to which CO2 fixation, methane production, and methanotrophy operated in this sedimentary setting.

The absence of extensive pyritization in the pyrite-poor MRS, relative to pyritized MRS, is counterintuitive because a large amount of organic matter is still preserved (Tables S2,S3). However, this could be explained by a change in physical and chemical environmental factors at the FB2a-FB2b unit boundary where high-energy conditions and rapid burial in the depositional setting of the pyrite-poor MRS would have limited the extension of sulfidic pore waters [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF]. In addition, the mat thickness seems to play a role in the microbial community assemblage [START_REF] Suarez | Thickness determines microbial community structure and function in nitrifying biofilms via deterministic assembly[END_REF], although external environmental parameters (e.g., hydrodynamic regime) also influence the microbial architecture [START_REF] Battin | Effects of current velocity on the nascent architecture of stream microbial biofilms[END_REF]. The mat thickness allows the expression of distinct redox gradients and internal carbon recycling styles whereby thick mats develop a wide anoxic zone [START_REF] Suarez | Thickness determines microbial community structure and function in nitrifying biofilms via deterministic assembly[END_REF]. This is consistent with our observations that the poorly pyritized MRS are <0.5 cm thick, whereas the thicker pyritized MRS [START_REF] Aubineau | Trace element perspective into the ca. 2.1-billionyear-old shallow-marine microbial mats from the Francevillian Group, Gabon[END_REF] promoted the development of the anoxic/sulfidic conditions required to enable strong pyritization.

Collectively, the high-energy depositional environment and mat thickness might have been the dominant control on pyrite formation in the FB2 Member MRS.

Nitrogen cycling

The geochemical data described above indicate the development of intermittently oxic to anoxic bottom-water conditions in likely proximity to the redoxcline during deposition of the FB2 Member. Well-oxygenated conditions were inferred for the depositional setting of the overlying FC1 Member [START_REF] Canfield | Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere[END_REF], in which a pervasive aerobic N cycle has been recognized [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. The latter is characterized by positive δ 15 N values (from +4‰ to +7‰) that have been linked to rapid nitrification followed by non-quantitative denitrification in the water column. Such values clearly indicate the predominance of nitrate-using ecosystem that may have comprised both prokaryotes and eukaryotes.

In contrast, the N isotope composition of our samples is consistent with the development of an anaerobic N cycle. δ 15 N values between -2‰ and +1‰ usually reflect biological N2 fixation by cyanobacteria using Mo-bearing nitrogenase [START_REF] Bauersachs | Nitrogen isotopic fractionation associated with growth on dinitrogen gas and nitrate by cyanobacteria[END_REF][START_REF] Carpenter | Biogeochemical tracers of the marine cyanobacterium Trichodesmium[END_REF][START_REF] Minagawa | Nitrogen isotope ratios of red tide organisms in the East China Sea: A characterization of biological nitrogen fixation[END_REF][START_REF] Zhang | Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia[END_REF]. At high Fe 2+ concentrations (>50 nM), fractionations driven by Mo-based diazotrophy can be as large as -4‰ [START_REF] Zerkle | Production of 15 N-depleted biomass during cyanobacterial N2-fixation at high Fe concentrations[END_REF]. The very high Fe/Al ratios in the pyritized MRS horizons point to localized Fe-rich water-column conditions during either the mat growth or early diagenesis where DIR and MSR occurred, which may explain their slightly more negative δ 15 N values (down to -3.3‰; Fig. 3). Alternative nitrogenase enzymes, in which Fe or V replace Mo at the active site, generate even more 15 N-depleted biomass with δ 15 N values as low as -8‰ [START_REF] Stüeken | The evolution of Earth's biogeochemical nitrogen cycle[END_REF][START_REF] Zhang | Nitrogen isotope fractionation by alternative nitrogenases and past ocean anoxia[END_REF]. The invariable N isotope composition of the atmosphere over the last 3 Ga (δ 15 Nair ≈ 0‰; [START_REF] Sano | Nitrogen isotopes and N2/Ar ratios in chests: An attempt to measure time evolution of atmospheric 6 15 N value[END_REF][START_REF] Marty | Nitrogen Isotopic Composition and Density of the Archean Atmosphere[END_REF], together with minimal post-depositional alteration, suggest that the δ 15 NWR values for the MRS and black shales correspond to the microbial N2 fixation by Mo-nitrogenase-utilizing cyanobacteria. The N2 fixation signal recorded by both the MRS and black shales implicates two possible scenarios: the expression of diazotrophy in benthic habitats and in the water column, and/or the contribution of benthically derived biomass (i.e. from MRS) to the organic matter content of the black shales (Fig. 5b). The high TOC, TN, and weakly enriched P contents, at least in the pyrite-poor MRS, suggest that productivity was high, which is consistent with nutrient supply with upwelling, anoxic deep-waters, and/or high burial efficiency due to benthic productivity in microbial mats and anoxic water-column conditions.

In contrast to other middle Paleoproterozoic depositional settings, including the broadly correlative Zaonega Formation of Karelia, Russia [START_REF] Kump | Isotopic evidence for massive oxidation of organic matter following the Great Oxidation Event[END_REF] and the overlying FC1 Member of the Franceville sub-basin [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF], the FB2 Member records diazotrophdominated ecosystem in the coastal area of the upwelling zone where deep, anoxic waters delivered nutrients. Notably, evidence for N2-fixing cyanobacteria, similar to that producing modern microbial mats [START_REF] Herbert | Nitrogen cycling in coastal marine ecosystems[END_REF][START_REF] Woebken | Revisiting N2 fixation in Guerrero Negro intertidal microbial mats with a functional single-cell approach[END_REF], points to processes in the Paleoproterozoic Franceville sub-basin that are reminiscent of modern coastal settings in an oxygen-minimum zone.

One possibility is that the black shales contain organic matter derived from the microbial mats, such that the mat-derived isotopic signatures of N2 fixation were imprinted onto the black shale facies. The contribution of microbial mats would result in advanced stages of illitization, induced by a biologically mediated K-enrichment in the living mats [START_REF] Aubineau | Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth[END_REF]; this signature was not observed in the FB2b unit black shales. Furthermore, the petrographically distinct morphological features of microbial mat structures were also not observed in the FB2 Member black shales [START_REF] Aubineau | Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota[END_REF].

Observations from modern sediments have shown that the contribution of isotopically light biomass from in situ microbial growth can decrease sedimentary δ 15 N values by 1 to 2‰ during anoxic diagenesis [START_REF] Lehmann | Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis[END_REF]. Such a process could potentially explain the near-zero d 15 N values in the black shales. However, the petrographic and mineralogical evidence seem to suggest that benthically produced organic matter was not a significant contributor to the studied black shales, which instead potentially implicates two distinct organic matter sources for black shales and microbial mats. Hence, the δ 15 NWR values of the FB2b unit black shales may reflect the N isotope composition of organic matter delivered from the water column to the sediments. Modern anoxic analogues of a water-column ecosystem dominated by diazotrophs in an otherwise oxygenated and nitrate-replete oceans are known in the Black Sea and Cariaco Basin [START_REF] Fulton | Black Sea nitrogen cycling and the preservation of phytoplankton δ 15 N signals during the Holocene[END_REF][START_REF] Thunell | Nitrogen isotope dynamics of the Cariaco Basin, Venezuela[END_REF]. These environments have limited connection to the global ocean, resulting in nitrate depletion. By contrast, the large seawater sulfate reservoir with marine connection in the Francevillian Group FB2 Member suggests that the depositional environment was not severely restricted with respect to the sulfate reservoir. While it is conceivable that the basin was semirestricted, leading to depletion of the nitrate reservoir, but not sulfate reservoir due to the high sulfate level at the time of deposition [START_REF] Bekker | Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America[END_REF][START_REF] Schröder | Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: Evidence from sulphate evaporites in the ∼2.2-2.1 Gyr shallow-marine Lucknow Formation, South Africa[END_REF], allowing negative δ 34 S values in pyrite, as is observed in the Black Sea [START_REF] Lyons | Sulfur isotopic trends and pathways of iron sulfide formation in upper Holocene sediments of the anoxic Black Sea[END_REF] -it is also possible that another mechanism contributed to the expression of an anaerobic system dominated by biological N2 fixation.

Nitrogen isotopes along Mesoproterozoic cross-basinal transects have been shown to record aerobic N cycling shoreward, while anaerobic N pathways were restricted to the anoxic deep-water part of the basins [START_REF] Koehler | Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum[END_REF][START_REF] Stüeken | A test of the nitrogen-limitation hypothesis for retarded eukaryote radiation: Nitrogen isotopes across a Mesoproterozoic basinal profile[END_REF]. By contrast, the shallow-water depositional setting of the FB2 Member shows evidence of anaerobic N cycling. Given the high metabolic cost of biological N2 fixation (16 ATP molecules per molecule of N2 fixed; [START_REF] Herbert | Nitrogen cycling in coastal marine ecosystems[END_REF], the δ 15 N values near 0‰ hint that bioavailable nitrogen was indeed scarce in this environment. Considering that the redox potential of Mn(II) oxidation is higher than that for NH4 + oxidation, nitrifying microbes could have promoted oxidation of NH4 + to NO2 -and NO3 -since the relevant redox threshold was crossed, as indicated by the extensive Mn enrichment in the FB Formation. Our data rather suggest that the buildup of a nitrate reservoir was not strong enough to leave a positive isotopic fingerprint (i.e., isotopically heavy residual NO3 -) via non-quantitative denitrification. In the modern ocean, bioavailable P, N, and Fe are the main nutrient controls on biological productivity in coastal upwelling systems [START_REF] Capone | Microbial biogeochemistry of coastal upwelling regimes in a changing ocean[END_REF]. Therefore, P or Fe scarcity could have been the main limiting factor in the production of bioavailable nitrogen. Under widespread anoxic conditions as herein inferred to develop at least episodically in the Franceville sub-basin, it has been hypothesized that phosphorus availability would be restricted via adsorption onto Fe minerals [START_REF] Reinhard | Evolution of the global phosphorus cycle[END_REF] and high organic carbon burial efficiency because of a scarcity of oxidants for organic remineralization (Kipp and Stüeken, 2017), hampering rates of primary productivity. In contrast, the sizeable seawater sulfate reservoir during deposition of the FB2 Member sediments would not have limited the P regeneration. We also observe high to moderate TOC concentrations and weak P enrichment in sediments, which contrast with expectations for oligotrophic environments. In modern surface waters of upwelling zones, a shortage of iron induces low N2 fixation rates [START_REF] Sohm | Emerging patterns of marine nitrogen fixation[END_REF]. Petrographic and geochemical data, combined with Fe speciation results, argue against a significant Fe limitation in the basin during deposition of the FB2 Member. Molybdenum, one of the essential constituents of the nitrogenase enzyme [START_REF] Williams | Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration[END_REF], could have been a limiting nutrient for biological activity throughout the mid-Proterozoic [START_REF] Anbar | Proterozoic ocean chemistry and evolution: A bioinorganic bridge?[END_REF] as it is scavenged from euxinic seawater that episodically and locally developed in upwelling zones in the oceans along continental margins and in intracratonic basins during the mid-Proterozoic. We cannot completely rule out the possibility of Mo limitation during deposition of the FB2 Member, since low Mo content has been inferred for the Proterozoic oceans [START_REF] Reinhard | Proterozoic ocean redox and biogeochemical stasis[END_REF][START_REF] Scott | Tracing the stepwise oxygenation of the Proterozoic ocean[END_REF]. However, given that Mo-nitrogenase has been operating at Earth's surface since ~3.2 Ga [START_REF] Stüeken | Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2 Gyr[END_REF], when Mo was likely more scarce in the marine environment, it is perhaps unlikely that widespread Mo limitation occurred in the mid-Paleoproterozoic, particularly during the "oxygen overshoot" interval during which marine trace metal inventories are thought to have expanded [START_REF] Bekker | Oxygen overshoot and recovery during the early Paleoproterozoic[END_REF]Kipp et al., 2017;[START_REF] Partin | Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales[END_REF][START_REF] Scott | Tracing the stepwise oxygenation of the Proterozoic ocean[END_REF].

The threshold at which NH4 + is rapidly oxidized via bacterial nitrification is modeled at low micromolar (>1.8 to 11 µM) dissolved oxygen concentrations [START_REF] Cheng | Nitrogen isotope evidence for stepwise oxygenation of the ocean during the Great Oxidation Event[END_REF][START_REF] Fennel | The co-evolution of the nitrogen, carbon and oxygen cycles in the Proterozoic ocean[END_REF][START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. Estimates of absolute atmospheric O2 level in the Paleoproterozoic remain elusive, however, a massive increase in atmospheric O2 content has been inferred for the LE [START_REF] Bekker | Oxygen overshoot and recovery during the early Paleoproterozoic[END_REF][START_REF] Karhu | Carbon isotopes and the rise of atmospheric oxygen[END_REF]. It seems likely that atmospheric oxygen was maintained at levels high enough to sustain widespread nitrification in the oceans and a substantial dissolved nitrate reservoir during deposition of the FB2 Member. Thus, if the observed δ 15 N values are indicative of locally developed nitrate limitation, this would likely have been a transient state that ultimately yielded to more oxygenated and nitrate-replete conditions.

In this regard, we suggest that the observed nitrate limitation was likely modulated by the basin-scale redox structure in the Franceville sub-basin (Fig. 5). Specifically, the flux of hydrothermally sourced reductants and nutrients may have controlled the redox state at the depositional site for the upper part of the FB1 Member (Fig. 5a) [START_REF] Ossa Ossa | Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event[END_REF], involving the upwelling of deep anoxic waters into oxic, shallow-water coastal settings. The large flux of reductants (e.g., Mn and Fe) and nutrients (e.g., P) could have transiently shifted redox conditions to be oxygen-depleted and, thus, triggered denitrification. In modern and ancient coastal upwelling systems with strong inputs of P, intense denitrification and anammox result in a rapid N removal in oxygen-minimum zones, which in turn triggers the surface activity of N2-fixing photoautotrophs [START_REF] Capone | An extensive bloom of the N2-fixing cyanobacterium Trichodesmium erythraeum in the central Arabian Sea[END_REF][START_REF] Deutsch | Spatial coupling of nitrogen inputs and losses in the ocean[END_REF][START_REF] Wang | Coupling of ocean redox and animal evolution during the Ediacaran-Cambrian transition[END_REF]. Due to the scarcity of NO3 -relative to PO4 3-, the N:P ratio of the water column in these settings reaches below the Redfield ratio of 16:1 in phytoplankton biomass [START_REF] Redfield | The biological control of chemical factors in the environment[END_REF]. The nitrate limitation in surface waters along upwelling transects appears as a natural consequence of high primary productivity. Furthermore, the loss of fixed nitrogen from the ocean has been proposed as a signature of the oxic-anoxic transition [START_REF] Fennel | The co-evolution of the nitrogen, carbon and oxygen cycles in the Proterozoic ocean[END_REF]. This has also been highlighted in the Ediacaran-Phanerozoic settings whereby enhanced denitrification/anammox and sometimes phosphate addition, together with eustatic fluctuations, would have allowed for the expression of N2 fixation in shallow depositional environments [START_REF] Chang | Nitrogen isotope evidence for an oligotrophic shallow ocean during the Cambrian Stage 4[END_REF][START_REF] Cremonese | Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran-Cambrian transition in South China[END_REF][START_REF] Wang | Nitrogen isotope evidence for redox variations at the Ediacaran-Cambrian transition in South China[END_REF]. Further, the nitrate reservoir may be severely depleted if the chemocline was relatively shallow, particularly within the photic zone [START_REF] Ader | Ocean redox structure across the Late Neoproterozoic Oxygenation Event: A nitrogen isotope perspective[END_REF][START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF]. Accordingly, introduction of deep anoxic waters developed a redoxcline in the Franceville sub-basin during deposition of the upper part of the FB1 Member (Fig. 5a). Although the overlying water column should have been more oxygenated with sea-level fall leading to deposition of the FB2 Member, nitrate limitation developed in surface waters due to enhanced flux of nutrients (e.g., P) with upwelling of anoxic deep-waters in a pattern similar to modern oxygenminimum zones along continental margins (Fig. 5b) and restricted basins. These conditions ultimately led to the drawdown of the reservoir of bioavailable nitrogen due to high rates of denitrification and anammox activity. The lack of evidence for Mo and P biolimitation, combined with N isotope data indicative of an aerobic N cycle during deposition of the overlying FC1 Member [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF], argue for a local, short-lived N scarcity (Figs. 5b,c).

Implications for primary productivity in the Franceville sub-basin

Our results provide evidence for N2 fixation both in the MRS and water column in the Franceville sub-basin during deposition of the FB2 Member. N2 fixation is commonly observed in microbial mats, however, benthic communities should not significantly contribute to the N budget of the overlying water column [START_REF] Bebout | Nitrogen cycling in microbial mat communities: The quantitative importance of N-fixation and other sources of N for primary productivity[END_REF][START_REF] Herbert | Nitrogen cycling in coastal marine ecosystems[END_REF]. Rather, cyanobacterial blooms in the water column strive to replenish the bioavailable N pool [START_REF] Tyrrell | The relative infuences of nitrogen and phosphorus on oceanic primary production[END_REF]. The pervasive episodic input of phosphate during deposition of the FB2 Member suggests that N2 fixation sustained primary productivity during deposition of the FB2 Member.

Modelling indicates that a high phosphate flux would increase N2 fixation and bioavailable nitrogen, which in turn invigorates oxygen production and accumulation [START_REF] Fennel | The co-evolution of the nitrogen, carbon and oxygen cycles in the Proterozoic ocean[END_REF].

Although aerobic N cycling during the GOE may have been pervasive as previously suggested [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF], our data suggest that regional redox fluctuations -perhaps related to the degree of basinal restriction from the open ocean -controlled the N dynamics in the Franceville sub-basin, which provides insight into heterogeneity of the marine N cycle in the wake of the LE.

Conclusion

The combined study of bottom-water redox proxies and C and N isotopes in the Francevillian Group FB2 Member MRS and host sediments allowed us to better constrain local redox conditions and microbial biogeochemical cycling at the end of the Lomagundi Event. Our results, summarized in Figure 5, suggest a local, short-lived nitrate scarcity in the water column as well as N2 fixation by benthic and water-column microbial communities.

Importantly, the expression of multiple metabolic C pathways and N2 fixation in this middle Paleoproterozoic coastal setting points to the existence of biogeochemical C and N cycling in patterns similar to an oxygen-minimum zone developed in a modern coastal upwelling system.

Anoxic deep waters were likely upwelled into oxic, shallow coastal settings across the redoxcline during deposition of the upper part of the FB1 Member (Fig. 5a). The subsequent sea-level fall and a decrease in a flux of reductants, combined with a continued supply of nutrients, led to increased primary productivity and nitrate limitation in the water column as recorded by the FB2 Member (Fig. 5b). In the face of this nitrate limitation, diazotrophs sustained primary productivity in surface waters, replenishing the bioavailable N pool (Figs. 5b,c). These redox conditions were superseded by a return to local oxic water-column conditions that allowed the expansion of nitrate-assimilating organisms during deposition of the overlying FC1 Member. The transient interval dominated by N2 fixation was thus controlled by the basinal redox structure and flux of reductants, and, to some extent, by the degree of isolation from the global ocean. Although globally oxic conditions prevailed at this time, our results show that regional redox fluctuations led to local nitrate limitation during the LE. Future work should further elucidate the relationship between these local redox conditions, microbial N2 fixation, and proliferation of the earliest motile macroorganisms. bottom seawater rich in Fe, Mn, and P. Phosphorous availability in surface waters along with limitation of bioavailable nitrate resulted in high primary productivity fueled by diazotrophs, which partially replenished bioavailable nitrogen. Microbial mats flourished and also fixed nitrogen. Sandstones were delivered with high-density gravity currents to the mud-rich depositional environment [START_REF] Reynaud | Depositional setting of the 2.1 Ga Francevillian macrobiota (Gabon): Rapid mud settling in a shallow basin swept by highdensity sand flows[END_REF]. The FB2 Member is, therefore, interpreted as a forced regressive system tract. c -Recovery in nitrate availability during deposition of the lower part of FC1 Member (cf. [START_REF] Kipp | Pervasive aerobic nitrogen cycling in the surface ocean across the Paleoproterozoic Era[END_REF] indicates the transient nature of nitrate limitation during deposition of the FB2 Member that was modulated by the redox structure of the Francevillian basin, degree of isolation from the ocean, and flux of reductants. FWWB, fair-weather wave base; SWB, storm wave base. Purple and green arrows indicate the sediment supply and the upwelling system, respectively. Processes were less intense when arrows are dotted.

FiguresFigure 1 :

 1 Figures

Figure 2 :

 2 Figure 2: SEM images in backscatter mode of pyrite crystals showing the absence of alteration and recrystallization. a and b, Pyritized MRS. c, Poorly pyritized MRS. d, Black shales.

Figure 3 :

 3 Figure 3: Geochemical data for the MRS and host sediments plotted along the lithostratigraphic profile of the studied sequence of the FB2 Member. Grey, black, and purple vertical lines represent redox boundaries as in Raiswell et al. (2018), the UCC value from Rudnick & Gao (2003), and the EFP for the UCC is 1, respectively. Fe/Al for the UCC is shown with grey dash line.

Figure 4 :

 4 Figure 4: Data for redox-sensitive metals from the MRS and host sediments plotted along the lithostratigraphic profile of the studied sequence of the FB2 Member. Orange and green vertical lines correspond to the UCC ratio from Rudnick & Gao (2003) and the EF for the UCC of 1, respectively. The Mn/Al, V/Al, and Mo/Al ratios are expressed in ppm/wt.%.

Figure 5 :

 5 Figure 5: Proposed reconstruction of redox conditions during deposition of the Upper Francevillian Group at the end of the LE. a -During deposition of the upper part of the FB1 Member, upwelling of reducing fluids into oxic, shallow coastal area (Ossa Ossa et al., 2018) likely led to the first step of deoxygenation and decrease in bioavailable nitrate. b -During deposition of the FB2 Member, bioavailable nitrate was scarce. Upwelling brought anoxic

Acknowledgments

We acknowledge the support of the Gabonese Government, CENAREST, General Direction of Mines and Geology, and Agence Nationale des Parcs Nationaux of Gabon. This work was supported by La Région Nouvelle Aquitaine, the University of Poitiers, Théodore Monod Doctoral School, and the French Embassy in Libreville, Gabon. We are grateful to Prof. P. Mouguiama Daouda, J.C. Balloche, L. White, and R. Oslisly for their support during field trips. For assistance, we acknowledge C. Laforest, C. Lebailly, Y. Lin, L. Magad-Weiss, A.

Meunier, A. Oldroyd, P. Recourt, C. Reedman, and X. Tang. J.A. is particularly thankful to T.

Lyons for scientific discussions.