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Nitrogen isotope compositions (δ15N) in sedimentary rocks are extensively used to investigate the biogeochemical nitrogen cycle through geological times. This use relies on the observation that, in modern continental platforms and anoxic basins, surface sediments faithfully record the δ15N of primary producers, assuming that it was similar in the past. Over Earth’s history, however, surface environments experienced profound changes, including the transition of ammonium-dominated to nitrate-dominated waters and the transition from exclusively microbial ecosystems to ecosystems including multicellularity, which make modern environments significantly different compared to earlier ones, potentially invalidating the fundamental assumption that surface sediments faithfully record the δ15N of primary producers. In order to improve our understanding of the nitrogen isotopic information contained in the early Earth’s sedimentary rock record, we investigate here the nitrogen isotope systematics in a microbial, nitrate free and ammonium-rich modern system, the Dziani Dzaha Lake. In this modern system, the δ15N of the reduced dissolved inorganic nitrogen (i.e., NH4+ and NH3) in the water column is close to ∼7‰ . δ15N of suspended particulate matter (SPM) show a similar average value in surface waters (i.e., where SPM is massively composed of active primary producers), but increases up to 14‰ in the deeper part of the water column during periods when it is enriched in dissolved reduced species (i.e., CH4, H2S/HS− and NH4+/NH3). Surface sediments δ15N, with values comprised between 10 and 14 ‰, seem to preferentially record these positive isotopic signatures, rather than those of active primary producers. We propose here that the observed isotopic pattern is mainly linked to the assimilation of ammonium strongly enriched in 15N by isotope exchange with ammonia under basic conditions. Although ammonium assimilation seems here to be responsible for a significant isotopic enrichment due to the basic conditions, in neutral anoxic environments inhabited by similar microbial ecosystems, this process may also significantly impact the δ15N of primary producers towards more negative values. This would have strong implications for our interpretation of the Precambrian sedimentary record as this finding challenges one the fundamental hypotheses underlying the use of sedimentary δ15N in paleo-oceanographic reconstructions, i.e. that surface sediments faithfully record the δ15N of active primary producers in the photic zone.

24 pages; International audience; The Earth’s oxygenation represents one of the most important environmental drivers of life’s evolution, with the first rise, known as ‘the Great Oxidation Event’ (GOE), corresponding to unpreceded accumulation of atmospheric O2, changes in the flux of marine nutrients and possibly global glaciations. However, the detailed evolution of the GOE is still debated, as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation. Here, we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from two drill cores (T2 and T3) in the early Paleoproterozoic Turee Creek Group, Western Australia, to track the redox evolution of marine conditions during the GOE. T2 core samples of the Kungarra Formation, which consists of clastic sedimentary rocks overlaid by the glaciogenic Meteorite Bore Member, were deposited ∼2.31 Ga ago. T3 core intercepts, from bottom to top, quartzite of the Koolbye Formation, and shales and stromatolitic carbonates of the Kazput Formation, which were deposited around ∼2.25 Ga. Samples from T2 show minor variations of δ13Corg (avg. −34.5 ± 1.7‰, n = 30), with no significant difference between siliciclastic and glaciogenic sedimentary rocks. In contrast, T3 samples display an increase in δ13Corg from −32.0 to −24.8‰ (n = 54) from shales to carbonates. In both T2 and T3 cores, δ13Corg values are inversely correlated with Al2O3, suggesting a strong petrological control on δ13Corg values, inferred here as resulting from variable contributions of detrital organic matter. Bulk N contents are low, from 13.5 to 56.7 ppm and 15.7 to 53.4 ppm in T2 and T3 samples, respectively. The δ15N values show a bimodal distribution, with one mode at +2.6‰ in T2 and another at +8.8‰ in T3, independent from lithological variations. This δ15N values shift between T2 and T3 is interpreted as reflecting a change from dominating N2-fixers to NO3-assimilating organisms. This implies an increase of NO3− availability, and thus of O2 concentration, during the time interval separating the deposition of T2 and T3 sediments. Dissolved NO3− and O2 concentrations of the Turee Creek marine basin are estimated from two models using N isotope data. The dissolved NO3− concentration has an upper limit ranging from 1.91 to 3.04 µM, about one order of magnitude below the average value of modern oceans. The lower limit for dissolved oxygen concentration ranges from 1.8 to 4.4 µM, which is two orders of magnitude lower than modern oceans. Together with previous studies, the present data place quantitative constraints on the redox changes associated with the Great Oxidation Event and illustrate a stepwise increase of NO3− bioavailability between 2.31 to 2.25 Ga, in relation with increasing O2 level.

Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale., Key Points The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurementsMeasurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishableWe propose new consensus values for a set of seven carbonate reference materials and updated guidelines to report clumped isotope measurements

The Proterozoic Era records two periods of abundant positive carbon isotope excursions (CIEs), conventionally interpreted as resulting from increased organic carbon burial and leading to Earth’s surface oxygenation. As strong spatial variations in the amplitude and duration of these excursions are uncovered, this interpretation is challenged. Here, by studying the carbon cycle in the Dziani Dzaha Lake, we propose that they could be due to regionally variable methane emissions to the atmosphere. This lake presents carbon isotope signatures deviated by ~ + 12‰ compared to the modern ocean and shares a unique combination of analogies with putative Proterozoic lakes, interior seas or restricted epireic seas. A simple box model of its Carbon cycle demonstrates that its current isotopic signatures are due to high primary productivity, efficiently mineralized by methanogenesis, and to subsequent methane emissions to the atmosphere. By analogy, these results might allow the reinterpretation of some positive CIEs as at least partly due to regionally large methane emissions. This supports the view that methane may have been a major greenhouse gas during the Proterozoic Era, keeping the Earth from major glaciations, especially during periods of positive CIEs, when increased organic carbon burial would have drowned down atmospheric CO2.

Carbonates constitute a significant proportion of the soil mass in many semi-arid soils. Due to their solubility, they can be affected by changes in the soil water regime. This needs to be taken into account when assessing the environmental and agronomical impacts of the adoption of irrigation. To gain knowledge on the importance of the effect of irrigation on carbonates dynamics in the tilled layer (0–20 cm) of agricultural soils, we conducted a two-step study embracing field observations and numerical simulation. First, carbonates storage and their size distribution were quantified for two different situations (irrigation and non-irrigation) in three irrigation districts with different time under irrigation (Valtierra (19 years), Miranda (5 years) and Funes (12 years)) in Navarre (Spain). Soil sampling was designed to ensure homogeneous comparisons in the most characteristic soil types at each site (Xeric Haplocalcid, Typic Calcixerept and Xeric Haplocalcid, respectively). Carbonates concentration was systematically lower with irrigation in the finest (< 50 μm) soil fraction: 22.2 ± 1.4 g carbonates 100 g− 1 fraction without irrigation for 16.1 ± 0.9 with irrigation in Valtierra, 26.7 ± 1.1 for 19.1 ± 3.8 in Miranda and 27.8 ± 2.9 for 22.7 ± 1.5 at Funes1. However, the net annual balance of total carbonates-C between irrigated and non-irrigated condition was neutral at the three sites. This can be explained by agricultural management affecting carbonates in the sand-size fraction, including the addition of carbonates with fertilizers, and coarse carbonate particles brought to the surface by tillage. In a second step, a simplified model was developed for use with easily available data in most irrigation districts, and numerical simulations of the geochemical interactions between the soil, the soil solution and irrigation water were run using actual soil and soil solution data from the tilled layer of another pair of plots in Funes (Funes2). Sensitive analysis was also conducted to investigate the potential impact of water quality and crop types as sources of variability on the model outputs. The modelling results showed annual values of carbonates-C loss in the range between 13.52 and 12.06 g m− 2 year− 1 in the studied depth under irrigation, depending on the quality of irrigation water, for 0.46 g m− 2 without irrigation. These data were within the range of carbonates budgets found in the literature but one order of magnitude lower than the observed results in the fine fraction in the field. Overall, our results showed that irrigation can significantly alter carbonates dynamics in semi-arid Mediterranean land, which implies that human use can significantly alter the mineral phase of these soils in a relatively short time lapse. Simple geochemical models can be a useful approach to evaluate changes in the carbonates balance at the local and regional scale when irrigation is applied, although they have to be improved to account for other factors related to agricultural management and local geochemical conditions.

Carbon and oxygen stable isotope compositions of carbonates are widely used to retrieve paleoenvironmental information. However, bias may exist in such reconstructions as carbonate precipitation is often associated with biological activity. Several skeleton-forming eukaryotes have been shown to precipitate carbonates with significant offsets from isotopic equilibrium with water. Although poorly understood, the origin of these biologically-induced isotopic shifts in biogenic carbonates, commonly referred to as “vital effects”, could be related to metabolic effects that may not be restricted to mineralizing eukaryotes. The aim of our study was to determine whether microbially-mediated carbonate precipitation can also produce offsets from equilibrium for oxygen isotopes. We present here δ18O values of calcium carbonates formed by the activity of Sporosarcina pasteurii, a carbonatogenic bacterium whose ureolytic activity produces ammonia (thus increasing pH) and dissolved inorganic carbon (DIC) that precipitates as solid carbonates in the presence of Ca2+. We show that the 1000 ln α CaCO 3 - H 2 O values for these bacterially-precipitated carbonates are up to 24.7‰ smaller than those expected for precipitation at isotopic equilibrium. A similar experiment run in the presence of carbonic anhydrase (an enzyme able to accelerate oxygen isotope equilibration between DIC and water) resulted in δ18O values of microbial carbonates in line with values expected at isotopic equilibrium with water. These results demonstrate for the first time that bacteria can induce calcium carbonate precipitation in strong oxygen isotope disequilibrium with water, similarly to what is observed for eukaryotes. This disequilibrium effect can be unambiguously ascribed to oxygen isotope disequilibrium between DIC and water inherited from the oxygen isotope composition of the ureolytically produced CO2, probably combined with a kinetic isotope effect during CO2 hydration/hydroxylation. The fact that both disequilibrium effects are triggered by the metabolic production of CO2, which is common in many microbially-mediated carbonation processes, leads us to propose that metabolically-induced offsets from isotopic equilibrium in microbial carbonates may be more common than previously considered. Therefore, precaution should be taken when using the oxygen isotope signature of microbial carbonates for diagenetic and paleoenvironmental reconstructions.

19 pages; International audience; The volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l−1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the

Alteration of ultramafic rocks plays a major role in the production of hydrocarbons and organic compounds via abiotic processes on Earth and beyond and contributes to the redistribution of C between solid and fluid reservoirs over geological cycles. Abiotic methanogenesis in ultramafic rocks is well documented at shallow conditions, whereas natural evidence at greater depths is scarce. Here we provide evidence for intense high-pressure abiotic methanogenesis by reduction of subducted ophicarbonates. Protracted (≥0.5–1 Ma), probably episodic infiltration of reduced fluids in the ophicarbonates and methanogenesis occurred from at least ∼40 km depth to ∼15–20 km depth. Textural, petrological and isotopic data indicate that methane reached saturation triggering the precipitation of graphitic C accompanied by dissolution of the precursor antigorite. Continuous infiltration of external reducing fluids caused additional methane production by interaction with the newly formed graphite. Alteration of high-pressure carbonate-bearing ultramafic rocks may represent an important source of abiotic methane, with strong implications for the mobility of deep C reservoirs., Alteration of ultramafic rocks plays a role in hydrocarbon production, but little is known about this process at depth. Here, the authors provide evidence that alteration of carbonated ultramafic rocks at high-pressures are an important source of abiotic methanogenesis with implications for deep C mobility.

We acknowledge the financial support from the UnivEarths Labex program of Sorbonne Paris Cite (ANR 11-IDEX-00005-02). CH and AL acknowledge support from the Max Planck Society. EES and RB were funded by the NASA Exobiology grant NNX16AI37G and the Virtual Planetary Laboratory of the NASA Astrobiology Institute and were technically assisted by Andy Schauer. Interpreting the organic carbon content (TOC) and stable carbon isotopic composition (δ13C) of organic matter in the sedimentary rock record depends on our capability to accurately measure them, while excluding sources of contamination. This however becomes increasingly problematic as we analyze samples with ever-lower organic carbon content. Accordingly, organic solvents are sometimes used to remove contaminating traces of modern organic matter from ancient rock samples. However, especially for very low TOC samples, traces of solvents or their impurities remaining in the sample may contribute a significant organic contamination that can impact the bulk measurements of both TOC and δ13C values. This study, including three independent investigations performed in different laboratories, is the first detailed examination of the effect of cleaning treatments on the reliability of TOC and δ13C values in a range of natural rock samples and synthetic materials with low TOC content from below detection limit to 3330 ppm. We investigated the four most common organic solvents used to remove modern organic matter: dichloromethane (DCM), n-hexane, methanol and ethanol, and evaluated the effect of grain size and mineralogy. We find that (i) cleaning treatments with methanol, n-hexane and dichloromethane contaminate rock samples when used directly on sample powder, regardless of the grain size; (ii) this pollution buffers the natural variability and homogenizes the δ13C values of samples around the isotopic composition of the solvent, i.e. between −27 and −29‰; (iii) the extent of contamination depends on the solvent used, DCM being the most contaminating (up to 6000 ppm) and ethanol the only solvent that does not seem to contaminate rock samples above our detection limit; (iv) sample mineralogy also exerts an influence on the extent of contamination, clay minerals being more prone to adsorb contaminants. We conclude that the response of carbon concentrations and the stable carbon isotopic composition of organic matter in geological samples to cleaning treatments is neither negligible nor systematic when investigating samples with low carbon content. Postprint

In sedimentary basins, the precise temperature and timing at which relatively shallow (

International audience; The interplay between the processes controlling the mobility of H2O and C-bearing species during subduction zone metamorphism exerts a critical control on plate tectonics and global volatile recycling. Here we present the first study on fresh, carbonate-bearing, lawsonite eclogite-facies metabasalts from Alpine Corsica, France, which reached the critical depths at which important devolatilization reactions occur in subducting slabs. The studied samples indicate that the evolution of oceanic crustal sequences subducted under present-day thermal regimes is dominated by localized fluid-rock interactions that are strongly controlled by the nature and extent of inherited (sub)seafloor hydrothermal processes, and by the possibility of deep fluids to be channelized along inherited or newly-formed discontinuities. Fluid channelization along inherited discontinuities controlled local rehydration and dehydration/decarbonation reactions and the stability of carbonate and silicate minerals at the blueschist-eclogite transition. Fluid-mediated decarbonation was driven by upward, up-temperature fluid flow in the inverted geothermal gradient of a subducting oceanic slab, a process that has not been documented in natural samples to date. We estimate that the observed fluid-rock reactions released 20–60 kg CO2 per m3 of rock (i.e. ~0.7–2.1 wt% CO2), which is in line with the values predicted from decarbonation of metabasalts in open systems at these depths. Conversely, the estimated time-integrated fluid fluxes (20–50 t/m2) indicate that the amount of carbon transported by channelized fluid flow within the volcanic part of subducting oceanic plates is potentially much higher than previous numerical estimates, testifying to the percolation of C-bearing fluids resulting from devolatilization/dissolution processes operative in large reservoirs.

International audience; Diagenetic minerals may provide information about the burial history of geological units and can have practical applications, for instance, for reconstructing the geochemical and thermal histories of sedimentary basins. Clumped isotope, or Δ47, thermometry on carbonates opens a new avenue for interpreting carbonate formation temperature and thermal history of rocks. Yet, most of current knowledge on Δ47 systematics has been acquired via theoretical or experimental studies with only limited validation by the rock-record at geological conditions/timescales. Here, we investigate calcitic and dolomitic cements representative of three genetically different cementation phases from a well-documented mineral paragenesis of a carbonate unit (Middle Jurassic, Paris basin, France). We compare Δ47 with fluid inclusions microthermometry (FIM) data that were independently obtained from the same calcite and dolomite crystal specimens. The range of homogenization temperatures (Th) found for Cal1, Cal2 and Dol1 fluid inclusions fit remarkably well (i.e. within < 5 °C) with the temperatures determined from the Δ47 measurements (TΔ47), for a temperature range between 60 and 100 °C and salinities between 0 and 15 wt% NaCl eq. This provides a consistent rock-based validation of the experimentally determined Δ47 calibration with formation temperature for both calcite and dolomite mineralogy. Such findings also confirm the applicability of Δ47 thermometry in low temperature diagenetic environments (i.e., below circa. 100 °C), which provides higher precision than FIM measurements (i.e., typical uncertainties of ± 6 °C with three Δ47 measurements) though significantly less time-consuming. Importantly, this study underlines how the coupling of both techniques can help to evaluate the degree of preservation of the original temperature information captured by either fluid inclusions or Δ47 compositions, and interpret each proxy as confidently/accurately as possible. Moreover, because both FIM and Δ47 measurements can provide independent constraints on the geochemistry of diagenetic paleofluids (via their salinity and δ18O composition), this study also highlights the benefits of coupling both techniques to further unravel the nature of paleofuids. Finally, we propose a practical guideline as a basis for future applications of combined FIM and Δ47 thermometry.

International audience; Carbonate clumped isotope thermometry is based on the temperature-dependent formation of 13C18O16O22− ion groups within the lattice of solid carbonate minerals. At low temperatures the bonds between rare, heavy 13C and 18O isotopes are thermodynamically favored, and thus at equilibrium they are present in higher than random abundances. Here we calibrate the use of this temperature proxy in a previously uncalibrated carbonate phase — dolomite [CaMg(CO3)2] — over a temperature range that extends to conditions typical of shallow crustal environments, by determining the Δ47 values of CO2 extracted from synthetic or natural (proto)dolomites grown at known temperatures from 25 to 350 °C and analyzed in two different laboratories using different procedures for sample analysis, purification and post-measurement data treatment. We found that the Δ47-1/T2 dependence for (proto)dolomite is linear between 25 and 350 °C, independent of their Mg/Ca compositions or cation order (or the laboratory in which they were analyzed), and offset from, but parallel to, the theoretical predictions of the Δ63 dependence to temperature of the abundance of the 13C18O16O2 isotopologue inside the dolomite and calcite mineral lattices as expected from ab-initio calculations (Schauble et al., 2006). This suggests that neither the equilibrium constant for 13C–18O clumping in (proto)dolomite lattice, nor the experimental fractionation associated with acid digestion to produce CO2, depend on their formation mechanism, degree of cation order and/or stoichiometry (i.e., Mg/Ca ratio) and/or δ18O and δ13C compositions (at least over the range we explored). Thus, we suggest the following single Δ47-1/T2 linear regression for describing all dolomite minerals:Δ47CDES90=0.0428(±0.0033)∗106/T2+0.1174(±0.0248)(r2=0.997),Δ47CDES90=0.0428(±0.0033)∗106/T2+0.1174(±0.0248)(r2=0.997),Turn MathJax onwith T in kelvin and Δ47 in the Carbon Dioxide Equilibrium Scale (CDES) of Dennis et al. (2011) and referring to CO2 extracted by phosphoric acid digestion at 90 °C. The listed uncertainties on slope and intercept are 95% confidence intervals. The temperature sensitivity (slope) of this relation is lower than those based on low temperature acid digestion of carbonates, but comparable to most of those based on high temperature acid digestion (with however significantly better constraints on the slope and intercept values, notably due to the large range in temperature investigated here and the large number of Δ47 measurements performed, n = 67). We also use this dataset to empirically determine that the acid fractionation factor associated with phosphoric acid digestion of dolomite at 90 °C (Δ*dolomite90) is about +0.176‰. This is comparable to the Δ*calcite90 experimentally obtained for calcite (Guo et al., 2009), suggesting that the acid fractionation Δ* for acid digestion of dolomite and calcite are the same within error of measurement, with apparently no influence of the cation identity. This hypothesis is also supported by the fact that our dolomite calibration data are indistinguishable from published calibration data for calcite, aragonite and siderite generated in similar analytical conditions (i.e., carbonate digested at T ⩾ 70 °C and directly referenced into CDES), demonstrating excellent consistency among the four (Ca, Mg, Fe)CO3 mineral phases analyzed in seven different laboratories (this represents a total of 103 mean Δ47 values resulting from more than 331 Δ47 measurements). These data are used to calculate a robust composite Δ47-T universal relation for those carbonate minerals of geological interest, for temperatures between −1 and 300 °C (that is statistically indistinguishable from the one described by dolomite only):Δ47CDES90=0.0422(±0.0019)∗106/T2+0.1262(±0.0207)(r2=0.985)Δ47CDES90=0.0422(±0.0019)∗106/T2+0.1262(±0.0207)(r2=0.985)Turn MathJax onThus, in order to standardize the temperature estimates out of different laboratories running high temperature digestion of (Ca, Mg, Fe)CO3 carbonate minerals, we recommend the use of this single Δ47-T calibration to convert Δ47CDES data into accurate and precise temperature estimates. More widely, this study extends the use of the Δ47 thermometry to studies of diagenesis and low-grade metamorphism of carbonates with unprecedented precision on temperature estimates based on Δ47 measurements — environments where many other thermometers are generally empirical or semi-quantitative.

International audience; One of the most serious issues with deciphering the evolution of organisms and their biogeochemical environments from the ancient rock record is the difficulty in obtaining well-preserved samples. Although not much can be done to avoid diagenetic and metamorphic alteration when they have occurred, alteration due to weathering can be avoided by working on drill core samples. This implies however that the amount of sample is limited, which may in turn restrain the number of possible chemical and isotopic analyses that can be performed. In order to save sample we show here that the chemical protocol used for the sulfur sulfide extraction (for later sulfur isotope analyses) is also suitable to decarbonate samples (for later organic carbon isotope analyses). In the case of carbonated rocks, both sulfur sulfide extraction and decarbonation require high amounts of sample so that coupling them may save a significant amount of sample and time. In addition it allows both organic carbon (TOC and δ 13 C) and sulfur isotope composition measurements to be performed on the exact same powder , which is essential when trying to understand couplings between S and C cycles in heterogeneous samples. We thus tested the efficiency of the acidic chromium solution, commonly used to extract sulfur from sulfide, for sample decarbonation on various Archean rocks. Our results show that no significant carbon isotope fraction-ation is caused by this new decarbonation protocol, even for the samples with low organic carbon content. The chromium solution seems to be perfectly adapted for the analysis of organic matter in the ancient rock record, at least when the rock samples have experienced low greenschist facies metamorphism. Further tests will be needed to verify if this protocol can also be used for less mature organic matter.

article i nfo The measurement of stable carbon (C) and oxygen (O) isotope compositions in siderite from sediments and soils can be useful to constrain carbonate genesis processes and/or to reconstruct paleoclimates. In order to evaluate our ability to determine C and O isotope compositions of siderite in modern sediments and soils containing immature organicmatter, we prepared andanalyzedsyntheticsamples made of variableproportionsof puresid- erite and yeast (selected for representing an immature organic matter because of its potential high reactivity to H3PO4 digestion). Replicate analyses of CO2 produced by phosphoric acid (H3PO4) digestion of our pure siderite standard at 130 °C provided δ 13 Csid and δ 18

Mangroves are the major ecosystems of tropical and subtropical coastlines. They are considered as a sink for atmospheric CO2 because they are characterized both by high net primary production, and by low rates of organic matter decomposition. However, a recent reassessment of the global mangrove budget suggests that organic carbon sinks have been underestimated, notably CO2 efflux from sediments and creek waters, and tidal export of dissolved inorganic carbon. Our objective was to understand the influence of mangrove zonation on the magnitude of CO2 fluxes at the sediment–air interface. Transparent and opaque dynamic closed chamber systems, coupled with an infra-red gas analyzer were used to measure CO2 fluxes. In addition, the physico-chemical properties (salinity, redox potential) of pore waters were determined, as well as the carbon content and the origin of surface sediments (Chlorophyll-a and δ13C). Depending on the type of measurement (in the dark with or without biofilm, in the light with biofilm) and mangrove stand (saltflat, Avicennia sp., or Rhizophora spp.), mean surface sediment CO2 fluxes ranged between 40 ± 56 and 199 ± 95 mmol·m− 2·d− 1. We suggest that these differences mainly result both from the organic content and the redox conditions of the sediments, which are influenced by the physiological activities of the root system, and by the position and the elevation of the stand in the intertidal zone. In addition, the quality and abundance of biofilm, which also vary with the mangrove stand, also appear to strongly affect sediment CO2 fluxes as a result of chemical (metabolism) and also physical (barrier) processes.

In the tropical zone, small watersheds are affected by intense meteorological events. These events play an important role in the erosion of soils and therefore on the sources of organic carbon in small tropical rivers. We studied the geochemistry of two soils on Basse-Terre Island (French West Indies, FWI): ferralitic soil and Andosol. The two studied soils are very similar in terms of soil organic matter (SOM) and soil solution parameters. The total organic carbon (TOC) and total nitrogen (TN) contents vary between 1.7 and 92 g kg‐1 and between 0.1 and 5.5 g kg‐1, respectively, with the highest concentrations observed in the topsoil. The C/N ratios are relatively constant throughout the soil profiles (ca. 12). The carbon isotopic composition of SOM varies between ‐27.3‰ and ‐22.7‰ and presents an enrichment with increasing depth of soil profiles. Dissolved organic carbon (DOC) concentrations in soil solutions, varying from 3.2 to 91.3 mg L‐1, are similar for the both extraction used in lab (with milliQ water and Ca(NO3)2) but are higher than those measured in soil solutions sampled from lysimeters (0.65–1.46 mg L‐1). The isotopic compositions of DOC obtained by extractions and SOM are comparable, with δ13C values ranging from ‐28.6‰ to ‐25.8‰. The DOC sampled from lysimeters is systematically depleted in 13C compared to DOC obtained by extractions, with δ13C values of ‐33.8‰ to ‐30.6‰. The enrichment of δ13C of SOM through the soil profiles is either consistent with the carbon isotopic fractionation of SOM by decomposing organisms, or the differential mineralization of both labile and stable carbon stocks in soils. DOC concentrations in stream waters vary between 0.46 and 5.75 mg L‐1, and are generally lower during low water level than floods. The isotopic compositions of DOC in the rivers range from ‐38.9‰ to ‐27.2‰, with δ13C values, which are more depleted in 13C during low water level than flood events. The δ13CDOC of water river samples and soil solutions obtained by extraction and collected with lysimeters demonstrates that the DOC in rivers derives essentially from both the lixiviation of the soil surface layers during floods and groundwater flow during low water levels. Lixiviation of soil surface layers can be boosted by significant increases of intensity and duration of meteorological events and can strongly favor the release of surface soil organic matter in rivers and the impoverishment in nutrients of soil surface layers.

International audience; Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon, regulating its fluxes between shallow and deep reservoirs. Recent estimates predict that almost all subducted carbon is transferred into the crust and lithospheric mantle during subduction metamorphism via decarbonation and dissolution reactions at high-pressure conditions. Here we report the occurrence of eclogite-facies marbles associated with metasomatic systems in Alpine Corsica (France). The occurrence of these marbles along major fluid-conduits as well as textural, geochemical and isotopic data indicating fluid–mineral reactions are compelling evidence for the precipitation of these carbonate-rich assemblages from carbonic fluids during metamorphism. The discovery of metasomatic marbles brings new insights into the fate of carbonic fluids formed in subducting slabs. We infer that rock carbonation can occur at high-pressure conditions by either vein-injection or chemical replacement mechanisms. This indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but can interact with slab and mantle-forming rocks. Rock-carbonation by fluid–rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Furthermore, carbonation may modulate the emission of CO2 at volcanic arcs over geological time scales.

En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/2009/publication-6165.pdf; International audience; In Marennes-Oleacuteron Bay, a macro-tidal bay located on the French Atlantic coast, kilometer-scale acoustic turbidity reveals an accumulation of free gas in the sediment. Large concentrations of organic matter and rapid sedimentation rates provide ideal settings for biogenic methane cycling. We integrate seismic, sedimentologic, biogeochemical and molecular genetic approaches to determine whether microbial methane cycling is involved in this process. Here we show that the acoustic turbidity upper boundary matched with X-ray facies displaying fissures with the highest methane concentrations, demonstrating the existence of methane bubbles in the sediment. 16S rRNA and mcrA gene clone libraries were dominated by sequences affiliated to the three known ANME lineages and to putative methanogens. Sequences related to the marine benthic group B (MBG-B) and miscellaneous crenarchaeotal group (MCG) were also detected. However, the highest methane concentration facies was the only section where active Archaea were detected, using reverse-transcribed rRNA, indicating that these communities were involved either directly or indirectly in the methane cycling process. Moreover, three metabolically active novel uncultivated lineages, related to putative methane cycling Archaea, could be specifically associated to these methane bearing sediments. As methane cycling Archaea are commonly retrieved from deep subseafloor and methane seep sediment, the study of coastal gassy sediments, could therefore help to define the biogeochemical habitats of deep biosphere communities.

Owing to their inertness and contrasted composition in the various earth reservoirs, helium isotopes are powerful tracers of a number of processes pertaining to geophysics and geochemistry. Because sediments cover a large portion of the earth's surface, helium isotope geochemistry of sediment pore-waters is of particular interest. In spite of this potential, its development has been hampered by the difficulty of collecting samples without gas loss and/or contamination problems. We developed a new method for the sampling and the quantitative extraction of dissolved helium from sediment pore-waters, leading to the determination of 3He and 4He concentration profiles. Core sampling is non-destructive (no squeezing). The principle of the method is to use standard copper tubes (1.2 cm OD/25 cm in length), subsequently sealed with clamps, to take mini-cores along the sediment core immediately following its retrieval. In the lab, the sediment is transferred from the copper tube to a noble gas extraction line by applying pressurized helium-free water at one end of the copper tube. This technique allows dissolved helium to be recovered and analyzed using standard procedures for water samples. Tests were carried out successfully on an artificial core equilibrated with air to check the extraction efficiency in the same conditions as for real cores. The validity of the method was further confirmed by acquiring a vertical helium profile from a real marine core from the Zaire deep-sea fan, illustrating some possible applications.

International audience; Carbon is transported from Earth's surface into its interior at subduction zones. Carbonates in sediments overlying hydrothermally altered rocks (including serpentinites) within the subducted slab are the main carriers of this carbon1. Part of the carbon is recycled back to the surface by volcanism, but some is transferred to the deep Earth1, 2. Redox transformations during shallow subduction control the transfer and long-term fate of carbon, but are poorly explored1, 3. Here we use carbon stable isotopes and Raman spectroscopy to analyse the reduction of carbonate in an exhumed serpentinite-sediment contact in Alpine Corsica, France. We find that highly crystalline graphite was formed during subduction metamorphism and was concentrated in the sediment, within a reaction zone in direct contact with the serpentinite. The graphite in this reaction zone has a carbon isotopic signature (δ13C) of up to 0.8±0.1‰, similar to that of the original calcite that composed the sediments, and is texturally associated with the calcium-bearing mineral wollastonite that is also formed in the process. We use mass-balance calculations to show that about 9% of the total carbonaceous matter in the sedimentary unit results from complete calcite reduction in the reaction zone. We conclude that graphite formation, under reducing and low-temperature conditions, provides a mechanism to retain carbon in a subducting slab, aiding transport of carbon into the deeper Earth.

International audience; The main role of INERIS (French National Institute for Industrial Environment and Risks) is to assess and avert accidental and chronic risks to both people and the environment linked to industrial installations, chemical substances and underground operations. INERIS is thus involved in research consortiums for underground storages of wastes, hydrocarbons or carbon dioxide. Concerning carbon dioxide storage, INERIS works for many years on defining and testing monitoring methods. As a French expert, the institute also performs integrated risk assessment studies. This paper focuses on the last improvements concerning the monitoring methods developed or patented by INERIS. Because the institute was created from the research centre of the former French national coal mining company, it has a well-established know-how in monitoring gas atmospheres. Having developed monitoring methods for mining contexts, INERIS has tools to constrain gas migrations in subsurface: 1. to determine gas flux between soil and atmosphere with dynamic accumulation chambers; 2. to determine gas concentrations in unsaturated zones through integrated gas sensor systems linked to subsurface boreholes (from 0 to about 300 meters depth). Initially designed for mining context, these two methods have been tested and proven for the monitoring of CO2 geological storage contexts. For example between 2005 and 2007, INERIS was one of the five partners involved in the 'GeoCarbone MONITORING' research project. This project was funded by the French Research Agency and aims at defining methods to monitor CO2 storage sites. Today the institute is involved in other projects studying pilot sites. In this paper we present results collected in analog contexts to CO2 storage sites. We will discuss in which conditions the carbon isotopic signature can help to determine the possible origins of the gas analyzed in our devices and to better understand the physical and chemical processes which can have led to the studied gas compositions. We will also highlight the fact that in some cases, there is a real need of using isotopic tracing methods otherwise the identification of these different processes cannot be done easily.

The definitive version is available at ww3.interscience.wiley.com. En libre-accès sur Archimer : http://archimer.ifremer.fr/doc/2009/publication-6801.pdf; International audience; The distribution of the archaeal communities in deep subseafloor sediments [0-36 m below the seafloor (mbsf)] from the New Caledonia and Fairway Basins was investigated using DNA- and RNA-derived 16S rRNA clone libraries, functional genes and denaturing gradient gel electrophoresis (DGGE). A new method, Co-Migration DGGE (CM-DGGE), was developed to access selectively the active archaeal diversity. Prokaryotic cell abundances at the open-ocean sites were on average approximately 3.5 times lower than at a site under terrestrial influence. The sediment surface archaeal community (0-1.5 mbsf) was characterized by active Marine Group 1 (MG-1) Archaea that co-occurred with ammonia monooxygenase gene (amoA) sequences affiliated to a group of uncultured sedimentary Crenarchaeota. However, the anoxic subsurface methane-poor sediments (below 1.5 mbsf) were dominated by less active archaeal communities, such as the Thermoplasmatales, Marine Benthic Group D and other lineages probably involved in the methane cycle (Methanosarcinales, ANME-2 and DSAG/MBG-B). Moreover, the archaeal diversity of some sediment layers was restricted to only one lineage (Uncultured Euryarchaeota, DHVE6, MBG-B, MG-1 and SAGMEG). Sequences forming two clusters within the Thermococcales order were also present in these cold subseafloor sediments, suggesting that these uncultured putative thermophilic archaeal communities might have originated from a different environment. This study shows a transition between surface and subsurface sediment archaeal communities.

During the ZaiRov2 cruise of the ZaiAngo project (1998–2000) on the passive Congo-Angola margin, several gravity cores were analyzed for helium isotopic composition of sedimentary pore waters in two cold fluid seepage zones: the Astrid slide area and the Regab giant pockmark. Gas concentration and isotopic composition are presented along with thermal data in terms of the origin and circulation of fluids. Helium isotope data lie on a mixing line between bottom seawater and an almost pure radiogenic. Helium and temperature vertical profiles are well described by the classic diffusionadvection equation. On the basis of He profiles, we estimate the advection rate on the rim of the pockmark between 1.2 and 2.3 mm/a. The He flux derived for a pure diffusive regime (2.4 × 10 −8 mol/m 2 /a) can favorably be compared to literature data and contrasts with the flux computed close to the pockmark center (1.9 × 10 −7 mol/m 2 /a). Helium depth profiles turned to be more sensitive to advection rate than temperature profiles are.