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Background: Editorial board members of academic journals are often considered gatekeepers of knowledge and role models for the community. Editorial boards should have sufficiently diverse backgrounds to facilitate the publication of manuscripts with a wide range of research paradigms, methods, and cultural perspectives.Objectives: This study critically evaluates changes in the representation of binary gender and geographic diversity over time for the editorial boards of Chemical Geology and Geochimica et Cosmochimica Acta. These are the two flagship geochemistry journals from the European Association of Geochemistry and the Geochemical Society, respectively.Methods: Composition of editorial boards was taken from the first issue of the year in question and editorial board members were coded for country of affiliation and binary gender. Results: Gender parity, limited to men and women, and geographic representation of the editorial boards of Chemical Geology and Geochimica et Cosmochimica Acta, have steadily increased between the late 1980’s and 2021. However, geographic distribution remains largely dominated by affiliations from North America and Western Europe. The Editor-in-Chief or board of editors have a significant impact on the level of diversity of the editorial board. With nearly every newly appointed editor, both geographic and gender diversity may evolve. However, the persisting substantial underrepresentation of editorial board members from outside of North America and Europe is of concern and needs to be the focus of active recruitment and ongoing monitoring. This approach will ensure that traditionally low levels of geographic diversity are mitigated and representation of our global communities is improved and maintained in the future. Conclusions: Improving diversity and inclusion among editorial boards as well as strengthening journal and disciplinary reputations will reinforce one another. Instituting a rotating editorship with an emphasis on embedding broader geographic networks and more equitable international recruitment could ensure sustained and improved geographic, gender, and wider representation, which, as a consequence, may lift scientific originality and the quality of published research.

Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.

International audience; Constraining the timing of eukaryogenesis and the divergence of eukaryotic clades is a major challenge in evolutionary biology. Here, we present trace metal concentration and zinc isotope data for c. 2.1 billion-year-old Francevillian Group pyritized structures, previously described as putative remnants of the first colonial multicellular organisms, and their host black shales. Relative to the host rocks, pyritized structures are strongly enriched in zinc, cobalt and nickel, by at least one order of magnitude, with markedly lighter zinc isotope compositions. A metabolic demand for high concentrations of aqueous zinc, cobalt, and nickel combined with preferential uptake of lighter zinc isotopes may indicate metalloenzyme utilization by eukaryotes in marine habitats c. 2.1 billion years ago. Once confirmed, this would provide a critical calibration point for eukaryogenesis, suggesting that this major evolutionary innovation may have happened contemporaneously with elevated atmospheric oxygen levels during the latter part of the Great Oxidation Event, some 400 million years earlier than is currently widely accepted.

International audience; Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as fossilized microbial mats developed in terrestrial to marine transitional environments. Petrological and geochemical characteristics of a ~ 350 m thick Moodies succession in the Saddleback Syncline in the central BGB were analyzed to reconstruct the post-depositional history of the sediments and to better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz, potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low current velocity allowed the repeated deposition of thin (

This review addresses the role of nitrogen (N) in vine balance and grape composition. It offers an integrative approach to managing grapevine N nutrition. Keeping in mind that N excess is just as detrimental to wine quality as N depletion, the control of grapevine N status, and ultimately must N composition, is critical for high-quality grape production. N fertilisation has been intensively used in the past century, despite plants absorbing only 30 to 40 % of applied N. By adapting plant material, soil management and vine balance to environmental conditions, it would be possible for grape growers to improve plant N use efficiency and minimise N input in the vineyard. Vineyard N management is a complex exercise involving a search for a balance between controlling vigour, optimising grape composition, regulating production costs and limiting pollution. The first part of this review describes grapevine N metabolism from root N uptake to vine development and grape ripening, including the formation of grape aroma compounds. The advantages and limits of methods available for measuring plant N status are addressed. The second part focuses on the parameters that influence grapevine N metabolism, distinguishing the impacts of environmental factors from those of vineyard management practices. Areas for further research are also identified.

Exopolymeric Substances (EPS) form biofilms in which the vast majority of prokaryotic organisms develop and thrive. They are ubiquitous, harbor metal and chemical species binding properties and can be the matrix for bio/orgamineralizations. Because of these properties, EPS have also been proposed as some of the most ancient traces of microbial life on Earth. They form sedimentary structures diagnostic of biological activity in some of the most ancient sedimentary rocks of the Archean. However, the metabolisms hosted by such ecosystems remain poorly understood given poor preservation and specificity of the available molecular, isotopic or fossilized signatures. Deep time paleobiological research therefore needs for new ways to unlock the history of the rare and variably preserved sedimentary rocks of these ages, by looking for new proxies that could help characterizing microbial ecosystems and better understand the co-evolution of the geosphere and the biosphere.We here attempt to describe trace metal signatures of a modern, arsenic-rich lacustrine microbial mat fueled by oxygenic and anoxygenic photosynthesis as an analog to better understand how microbial mats and their sedimentary and chemical signatures can be preserved in the Archean sedimentary record. We coupled in-situ imaging of a microbial mat from modern Dead Sea shore with SEM, Raman spectroscopy, and X-ray mapping, to (meta)genomics data and chemical analyses. Arsenic enrichments in the anoxygenic photosynthetic layer of the mat reached a 10’000-fold level, and was associated to Mg-Si-rich EPS. The latter ultimately mineralized into aragonite clusters with a co-enrichment of Sr, Mn and Mo. At the mat scale, the mineralized zone (rich in Fe, Sr and Ca from authigenic calcium carbonates and detrital clay) is clearly located above the As-enrichment layer, in association with Mn. These data support a chemically dynamic microbial mat where microbial activity, EPS chemical affinity and environmental processes lead to specific organic and mineralized chemical signatures linked to metabolic activity. Metagenomics and synchrotron-based speciation analyses shall confirm the links between elemental enrichments and the microbial metabolic pathways.We parallel this study to a well characterized microbial system of the Archean, the stromatolitic units of the Tumbiana Lake (2.72 Ga, Pilbara, Western Australia). In this environment, microbial mat accretionary behaviour has formed limestone stromatolites harboring layered nanopyrites embedded in carbonaceous material and chlorite (Marin-Carbonne et al., 2018). C, N and S isotopes of mineral fractions have suggested a connection of photosynthetic activity, sulfate reduction and methane cycling, potentially influenced by arsenotrophy (e.g. Thomazo et al., 2009; Sforna et al., 2014; Lepot et al., 2019). This is likely the most diverse undisputed microbial environment of the Archean. Our trace-elemental mapping using PIXE suggests co-enrichment of Mo and As in association to the nanopyrite-OM layer, that could be attributed to arsenotrophic and anoxygenic photosynthetic activity, in a similar fashion than argued for in the Dead Sea mat. Simulated diagenesis experiments are planned and should be able to provide chemical insights into the transformation of microbial mats to their fossilized counterparts at the microscale, to further validate the promises of metal biosignatures for reconstructing Archean ecosystems.

Diversity, at every step along the scientific path, drives innovative research. Scientific societies, like the Geochemical Society (GS) and the European Association of Geochemistry (EAG), have a significant influence on which innovators are celebrated. Such choices have the consequence of shaping the future of research, and so are responsible for the evolution of our discipline and its relationship to the global community. These professional bodies are uniquely positioned to define and promote the success of all scientists, including those from under-represented groups, through proactive advocacy, inclusive mentorship, awards, and leadership. At present, the only data available to examine the distribution of under-represented groups between memberships, awardees leadership are those of gender. To assess gender diversity in the geochemistry and cosmochemistry community, we thus introspectively review available records of GS and EAG membership through Goldschmidt Conference attendees, awardees, leadership, and editorial boards. This work identifies areas for growth and begins a dialogue about how the society and its members can work together to better reflect and progress our community. Our examination of the record spanning the last decade demonstrates that leadership positions, awards, and honors have continued to be disproportionately given to white men, to the exclusion of women. The GS and EAG have recently taken positive steps towards becoming more inclusive; however, much more work is needed. In order for both communities to become diverse, equitable and inclusive, where all scientists flourish, we offer suggestions for swift steps that the GS and EAG and their members can pursue. The suggested structural improvements will require ongoing analysis and reforms, which must be shared by all of us, to create a sustainable legacy that we can be proud of.

Co-auteur étranger; International audience; This study reports the first Si isotope data measured in zircon using nanosecond laser ablation multiple collector inductively coupled plasma mass spectrometry. Long-term (>2 years) external reproducibility obtained on the 91500 zircon standard is 0.13‰ and 0.21‰ (2SD) for δ29Si and δ30Si (per mil deviation from the quartz reference NBS 28), respectively, and typical precision on a single run is on the order of 0.10‰ and 0.12‰ (2SE) for δ29Si and δ30Si, respectively. Our results show good consistency between laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) and solution multiple collector inductively coupled plasma mass spectrometry (S-MC-ICP-MS)/secondary ionization mass spectrometry (SIMS) data demonstrating that the Si isotopic composition of zircon can be accurately measured by LA-MC-ICP-MS. Obtained δ30Si values on natural zircon standards range from −0.32 ± 0.23‰ (2SD) for 91500 to −0.47 ± 0.17‰ for MudTank, whereas the MUN (synthetic) zircon standards have lower δ30Si values around −2.0‰ (e.g. −2.08 ± 0.17‰ for MUN-1-2b). Silicon isotope measurements by LA-MC-ICP-MS open new possibilities for studying zircon formation in various geological contexts.

International audience; Iron isotope compositions of sedimentary pyrites (FeS2) are used to constrain the redox evolution of the Precambrian ocean and early Fe-based metabolisms such as Dissimilatory Iron Reduction (DIR). Sedimentary pyrites can record biotic and abiotic iron reduction, which have similar ranges of Fe isotopic fractionation, as well as post-depositional histories and metamorphic overprints that can modify Fe isotope compositions. However, some exceptionally well-preserved sedimentary records, such as the stromatolite-bearing Tumbiana Formation (ca. 2.7 Ga, Western Australia) have been proven to retain primary information on Early Neoarchean microbial ecosystems and associated metabolic pathways. Here, we present in situ Fe isotope measurements of micropyrites included in four stromatolites from the Tumbiana Formation in order to assess iron respiration metabolism using Fe isotope signatures. A set of 142 micropyrites has been analyzed in three lamina types, i.e. micritic, organic-rich and fenestral laminae, by Secondary Ion Mass Spectrometry (SIMS), using a Hyperion radio-frequency plasma source. The diversity of laminae is attributed to specific depositional environments, leading to the formation of Type 1 (micritic laminae) and Type 2 (organic-rich laminae) and early diagenetic effects (Type 3, fenestral laminae). Type 1 and 2 laminae preserved comparable δ56Fe ranges, respectively from −1.76‰ to +4.15‰ and from −1.54‰ to +4.44‰. Type 3 laminae recorded a similar range, although slightly more negative δ56Fe values between −2.20‰ and +2.65‰. Globally, our data show a large range of δ56Fe values, from −2.20‰ to +4.44‰, with a unimodal distribution that differs from the bimodal distribution previously reported in the Tumbiana stromatolites. Such a large range and unimodal distribution cannot be explained by a unique process (e.g., biotic/abiotic Fe reduction or pyrite formation only controlled by the precipitation rate). It rather could reflect a two-step iron cycling process in the sediment pore water including i) partial Fe oxidation forming Fe(OH)3 with positive δ56Fe values followed by ii) partial, possibly microbially induced, Fe reduction leading to Fe2+ availability for pyrite formation by sulfate reducers carrying both negative δ56Fe and δ34S signatures. In this model, the buildup and subsequent reduction through time of a residual Fe(OH)3 reservoir arising from the activity of methanotrophs, can explain the strongly positive δ56FeFe(OH)3 values up to 4‰. These results indicate that Archean microbial mats have been the site of the interaction of several closely linked biogeochemical cycles involving Fe, S and C.

Progressive reforms and community kindness are encouraged; terminology common among DEI discussions is explored here.

This presentation addresses the actual concerns in viticulture regarding grapevine nitrogen (N) metabolism in the context of reducing both inputs and environmental pollution, while optimizing the balance between yield and wine quality. By adapting agronomical practices to the environmental conditions (i.e. soil and climate), it is possible to optimise both plant N use efficiency (NUE) and crop quality, while reducing N input in the vineyard. The present trial demonstrates the potential of crop-load limiting (via bunch thinning) to fine-tune plant NUE and optimise grape N composition at harvest. These results improve the comprehension of the seasonal plant N cycle in perennial crops and it contributes to the implementation of sustainable practices in vineyards and potentially in other crops.Over the past decades, N supply in vineyards has been reduced with the aim of adjusting vigour and yield. Moreover, the development of cover cropping has led to increased competition for N resources in vineyards, which can, in some cases, be detrimental to both yield and quality of the crop. This evolution of management practices – without considering the environmental conditions – has led to situations with major grape N deficiencies, being detrimental to fermentation kinetics, yield and possibly wine quality. Given the major role of N in plant physiology, an integrative approach to managing grapevine N nutrition from soil to crop – in accordance with the environmental conditions – represents a sustainable solution for high-quality grape production.In this trial on white cv. Chasselas (Vitis vinifera L.), plant N partitioning and grape composition were monitored over two years, in relation to both crop load and fertilisation. These aims were accomplished by testing a large crop load gradient (via bunch thinning, resulting in 0.7–5.2 kg per plant) and by using a 15N-labelling method (fertilization with 10 atom % 15N foliar urea). The results indicate that the mobilisation of root N reserves plays a major role in the balance of fruit N content. Carry-over effects to the next year were highlighted. N uptake and assimilation appeared to be strongly stimulated by high-yield conditions. Fertilisation largely contributed to fulfilling the high fruit N demand while limiting the mobilisation of root N reserves under high-yield conditions. Plants were able to modulate both root N reserve mobilisation and N uptake as a function of crop load, thus maintaining a relatively uniform N concentration in fruits. However, the fruit free amino N profile was modified, which potentially affected aromas in grapes and wines. A modelling of the seasonal plant N cycle (i.e. uptake and efflux) is also proposed.Key words: Nitrogen metabolism, 15N-isotope labelling, crop load, grape composition, wine quality

The Black Lives Matter movement[1] and the global COVID-19 pandemic have highlighted inequalities in opportunities and vastly different lived experiences. This pandemic has exacerbated professional challenges among disproportionately affected and historically minoritized and marginalised people. Excellence in diversity, equity, and inclusion (DEI) is foundational for scientists to thrive and reach their full potential. Together, these factors have prompted the European Association of Geochemistry (EAG) to reflect on how to better DEI across all sectors, levels of employ, and positions of community leadership. It is with increased urgency that the EAG is acting to raise awareness of persisting challenges; creating new initiatives to reform practices that marginalise talented people, while building kind and understanding community cultures that are actively anti-racist and anti-discriminatory. In 2020, the EAG’s newly-constituted DEI Committee was charged with challenging the status quo and addressing system-wide barriers to participation in geochemistry and cosmochemistry[2]. Key to this committee’s work is the development of a DEI strategic plan and recommendations, informed by tangible data, for EAG Council. It is well documented that the wider geosciences continue to be gated, and is one of the least diverse STEM fields. Lack of diversity is detrimental for representation, but also critically limits scientific excellence[3]. This knowledge leads us to ask: does geochemistry and cosmochemistry - having particular field, laboratory, and instrument needs - foster less diverse communities relative to other fields within and beyond geosciences? If so, what are the procedural barriers and processes preventing underrepresented groups from thriving? We have prepared a survey to circulate among the membership (on an anonymous basis) after this Goldschmidt Conference to constrain the make up of our community. This survey will provide the first demographic data specific to our community, thereby benchmarking both our present mix and the condition of our culture. We appeal to the membership to engage and come together to help identify priority challenges and help us address them. 1Cooperdock et al., 2020. Black Lives Matter...Elements. http://elementsmagazine.org/2020/08/01/black-lives-matter-promoting-diversity-equity-and-inclusion-in-geochemistry/ 2Riches et al., 2021. Uniting to Advance Diversity, Equity, and Inclusion. https://blog.eag.eu.com/advance-dei-pandemic-post-pandemic/ 3Bernard and Cooperdock, 2018. No progress on diversity in 40 years. Nature Geosci. https://doi.org/10.1038/s41561-018-0116-6

This review addresses the role of nitrogen (N) in vine balance and grape composition. It offers an integrative approach to managing grapevine N nutrition. Keeping in mind that N excess is just as detrimental to wine quality as N depletion, the control of grapevine N status, and ultimately must N composition, is critical for high-quality grape production. N fertilisation has been intensively used in the past century, despite plants absorbing only 30 to 40 % of applied N. By adapting plant material, soil management and vine balance to environmental conditions, it would be possible for grape growers to improve plant N use efficiency and minimise N input in the vineyard. Vineyard N management is a complex exercise involving a search for a balance between controlling vigour, optimising grape composition, regulating production costs and limiting pollution. The first part of this review describes grapevine N metabolism from root N uptake to vine development and grape ripening, including the formation of grape aroma compounds. The advantages and limits of methods available for measuring plant N status are addressed. The second part focuses on the parameters that influence grapevine N metabolism, distinguishing the impacts of environmental factors from those of vineyard management practices. Areas for further research are also identified.

Co-auteur étranger; International audience; Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as fossilized microbial mats developed in terrestrial to marine transitional environments. Petrological and geochemical characteristics of a ~ 350 m thick Moodies succession in the Saddleback Syncline in the central BGB were analyzed to reconstruct thepost-depositional history of the sediments and to better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz, potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low current velocity allowed the repeated deposition of thin (

Significance Until now, all of the ca. 1,800 known modern scleractinian coral species were thought to produce skeletons exclusively of aragonite. Asymbiotic Paraconotrochus antarcticus living in the Southern Ocean is the first example of an extant scleractinian that forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. This discovery adds support to the notion that the coral skeletal formation process is strongly biologically controlled. Mitophylogenomic analysis shows that P. antarcticus represents an ancient scleractinian clade, suggesting that skeletal mineralogy/polymorph of a taxon, once established, is a trait conserved throughout the evolution of that clade., One of the most conserved traits in the evolution of biomineralizing organisms is the taxon-specific selection of skeletal minerals. All modern scleractinian corals are thought to produce skeletons exclusively of the calcium-carbonate polymorph aragonite. Despite strong fluctuations in ocean chemistry (notably the Mg/Ca ratio), this feature is believed to be conserved throughout the coral fossil record, spanning more than 240 million years. Only one example, the Cretaceous scleractinian coral Coelosmilia (ca. 70 to 65 Ma), is thought to have produced a calcitic skeleton. Here, we report that the modern asymbiotic scleractinian coral Paraconotrochus antarcticus living in the Southern Ocean forms a two-component carbonate skeleton, with an inner structure made of high-Mg calcite and an outer structure composed of aragonite. P. antarcticus and Cretaceous Coelosmilia skeletons share a unique microstructure indicating a close phylogenetic relationship, consistent with the early divergence of P. antarcticus within the Vacatina (i.e., Robusta) clade, estimated to have occurred in the Mesozoic (ca. 116 Mya). Scleractinian corals thus join the group of marine organisms capable of forming bimineralic structures, which requires a highly controlled biomineralization mechanism; this capability dates back at least 100 My. Due to its relatively prolonged isolation, the Southern Ocean stands out as a repository for extant marine organisms with ancient traits.

RATIONALE Iron isotopic signatures in pyrites are considered as a good proxy for reconstructing paleoenvironmental and local redox conditions. However, the investigation of micro-pyrites less than 20 μm in size has been limited by the available analytical techniques. The development of a new brighter radio-frequency plasma ion source (Hyperion-II source) enhances the spatial resolution by increasing the beam density 10 times compared with the Duoplasmatron source. METHODS Here we present high-spatial-resolution measurements of iron isotopes in pyrites using a 3 nA-3 μm primary 16 O- beam on two Cameca IMS 1280-HR2 ion microprobe instruments equipped with Hyperion sources at CRPG-IPNT (France) and at SwissSIMS (Switzerland). We tested analytical effects, such as topography and crystal orientation, that could induce analytical biases perceptible through variations of the instrumental mass fractionation (IMF). RESULTS The δ56 Fe reproducibility for the Balmat pyrite standard is ±0.25‰ (2 standard deviations) and the typical individual internal error is ±0.10‰ (2 standard errors). The sensitivity on 56 Fe+ was 1.2 × 107 cps/nA/ppm or better. Tests on Balmat pyrites revealed that neither the crystal orientation nor channeling effects seem to significantly influence the IMF. Different pyrite standards (Balmat and SpainCR) were used to test the accuracy of the measurements. Indium mounts must be carefully prepared with a sample topography less than 2 μm, which was checked using an interferometric microscope. Such a topography is negligible for introducing change in the IMF. This new source increases the spatial resolution while maintaining the high precision of analyses and the overall stability of the measurements compared with the previous Duoplasmatron source. CONCLUSIONS A reliable method was developed for performing accurate and high-resolution measurements of micrometric pyrites. The investigation of sedimentary micro-pyrites will improve our understanding of the processes and environmental conditions during pyrite precipitation, including the contribution of primary (microbial activities or abiotic reactions) and secondary (diagenesis and/or hydrothermal fluid circulation) signatures.

The Moodies Group, the uppermost unit in the Barberton Greenstone Belt (BGB) in South Africa, is a ~3.7-km-thick coarse clastic succession accumulated on terrestrial-to-shallow marine settings at around 3.22 Ga. The multiple sulfur isotopic composition of pyrite of Moodies intervals was newly obtained to examine the influence of these depositional settings on the sulfur isotope record. Conglomerate and sandstone rocks were collected from three synclines north of the Inyoka Fault of the central BGB, namely, the Eureka, Dycedale, and Saddleback synclines. The sulfur isotopic composition of pyrite was analyzed by Secondary Ion Mass Spectrometry (SIMS) for 6 samples from the three synclines and by Isotope Ratio Mass Spectrometry (IR-MS) for 17 samples from a stratigraphic section in the Saddleback Syncline. The present results show a signal of mass-independent fractionation of sulfur isotopes (S-MIF), although t-tests statistically demonstrated that the Moodies S-MIF signals (mostly 0&permil, &lt, ∆33S &lt, +0.5&permil, ) are significantly small compared to the signal of the older Paleoarchean (3.6&ndash, 3.2 Ga) records. These peculiar signatures might be related to initial deposition of detrital pyrite of juvenile origin from the surrounding intrusive (tonalite&ndash, trondhjemite&ndash, granodiorite, TTG) and felsic volcanic rocks, and/or to secondary addition of hydrothermal sulfur during late metasomatism. Moreover, fast accumulation (~0.1&ndash, 1 mm/year) of the Moodies sediments might have led to a reduced accumulation of sulfur derived from an atmospheric source during their deposition. As a result, the sulfur isotopic composition of the sediments may have become susceptible to the secondary addition of metasomatic sulfur on a mass balance point of view. The sulfur isotopic composition of Moodies pyrite is similar to the composition of sulfides from nearby gold mines. It suggests that, after the Moodies deposition, metasomatic pyrite formation commonly occurred north of the Inyoka Fault in the central BGB at 3.1&ndash, 3.0 Ga.

5 pages; International audience; Stable isotope signatures of elements related to life such as carbon and nitrogen can be powerful biomarkers that provide key information on the biological origin of organic remains and their paleoenvironments. Marked advances have been achieved in the last decade in our understanding of the coupled evolution of biological carbon and nitrogen cycling and the chemical evolution of the early Earth thanks, in part, to isotopic signatures preserved in fossilized microbial mats and organic matter of marine origin. However, the geologic record of the early continental biosphere, as well as its evolution and biosignatures, is still poorly constrained. Following a recent report of direct fossil evidence of life on land at 3.22 Ga, we compare here the carbon and nitrogen isotopic signals of this continental Archean biosphere with biosignatures of cyanobacteria biological soil crusts (cyanoBSCs) colonizing modern arid environments. We report the first extended δ13C and δ15N data set from modern cyanoBSCs and show that these modern communities harbor specific isotopic biosignatures that compare well with continental Archean organic remains. We therefore suggest that cyanoBSCs are likely relevant analogs for the earliest continental ecosystems. As such, they can provide key information on the timing, extent, and possibly mechanism of colonization of the early Earth's emergent landmasses.

Oxygen, magnesium, and silicon isotopic compositions in the mineralogically complex, ultrarefractory (UR) calcium-aluminum-rich inclusion (CAI) E101.1 from the reduced CV3 chondrite Efremovka confirm that E101.1 is a compound CAI composed of several lithological units that were once individual CAIs, free-floating in the solar protoplanetary disk. Each precursor unit was found to have had its own thermal history prior to being captured and incorporated into the partially molten host CAI. Four major lithological units can be distinguished on the basis of their isotopic compositions. (1) Al-diopside-rich sinuous fragments, hereafter sinuous pyroxene, are 16O-rich (Δ17O ≤ −20‰) and have light Mg and Si isotopic compositions with mass fractionation down to −3.5‰/amu for both isotopic systems. We attribute these peculiar isotopic compositions to kinetic effects during condensation out of thermal equilibrium. (2) Spinel clusters are 16O-rich (Δ17O ∼ −22‰) and have Mg isotope systematics consistent with extensive equilibration with the host melt. This includes (i) δ25Mg values varying between + 2.6‰ and + 6.5‰ close to the typical value of host melilite at ∼+5‰, and (ii) evidence for exchange of radiogenic 26Mg with adjacent melilite as indicated by Al/Mg systematics. The spinel clusters may represent fine-grained spinel-rich proto-CAIs captured, partially melted, and recrystallized in the host melt. Al/Mg systematics indicate that both the sinuous pyroxene fragments and spinel clusters probably had canonical or near-canonical 26Al contents before partial equilibration. (3) The main CAI host (Δ17O ≤ −2‰) had a complex thermal history partially obscured by subsequent capture and assimilation events. Its formation, referred to as the “cryptic” stage, could have resulted from the partial melting and crystallization of a 16O-rich precursor that underwent 16O-depletion and a massive evaporation event characteristic of F and FUN CAIs (Fractionated with Unknown Nuclear effects). Alternatively, a 16O-rich UR precursor may have coagulated with a 16O-poor FUN CAI having 48Ca anomalies, as indicated by perovskite, before subsequent extensive melting. The Al/Mg systematics (2.4 × 10−5 ≤ ( 26 Al / 27 Al ) 0 ′ ≤ 5.4 × 10−5, where ( 26 Al / 27 Al ) 0 ′ is a model initial 26Al/27Al ratio per analysis spot) are best understood if the FUN component was 26Al-poor, as are many FUN CAIs. (4) A complete Wark-Lovering rim (WLR) surrounds E101.1. Its Mg and Si isotopic compositions indicate that it formed by interaction of the evaporated interior CAI with an unfractionated 16O-rich condensate component. Heterogeneities in 26Al content in WLR spinels (3.7 × 10−5 ≤ ( 26 Al / 27 Al ) 0 ′ ≤ 5.7 × 10−5) suggest that the previously reported age difference of as much as 300,000 years between interior CAIs and their WLRs may be an artifact resulting from Mg isotopic perturbations, possibly by solid state diffusion or mixing between the interior and condensate components. The isotopic systematics of E101.1 imply that 16O-rich and 16O-poor reservoirs co-existed in the earliest solar protoplanetary disk and that igneous CAIs experienced a 16O-depletion in an early high temperature stage. The coagulation of various lithological units in E101.1 and their partial assimilation supports models of CAI growth by competing fragmentation and coagulation in a partially molten state. Our results suggest that chemical and isotopic heterogeneities of unclear origin in regular CAIs may result from such a complex aggregation history masked by subsequent melting and recrystallization.

This GGR biennial critical review covers developments and innovations in key analytical methods published since January 2014, relevant to the chemical, isotopic and crystallographic characterisation of geological and environmental materials. In nine selected analytical fields, publications considered to be of wide significance are summarised, background information is provided and their importance evaluated. In addition to instrumental technologies, this review also presents a summary of new developments in the preparation and characterisation of rock, microanalytical and isotopic reference materials, including a précis of recent changes and revisions to ISO guidelines for reference material characterisation and reporting. Selected reports are provided of isotope ratio analyses by both solution-nebulisation MC-ICP-MS and laser ablation-ICP-MS, as well as of radioactive isotope geochronology by LA-ICP-MS. Most of the analytical techniques elaborated continue to provide new applications for geochemical analysis, however it is noted that instrumental neutron activation analysis has become less popular in recent years, mostly due to the reduced availability of nuclear reactors to act as a neutron source. Many of the newer applications reported here provide analysis at increasingly finer resolution. Examples include atom probe tomography, a very sensitive method providing atomic scale information, nanoscale SIMS, for isotopic imaging of geological and biological samples, and micro-XRF, which has a spatial resolution many orders of magnitude smaller than conventional XRF.

The Barberton Greenstone Belt hosts abundant structurally controlled gold mineralisation of Mesoarchaean age. More than 300 gold occurrences have been reported, although most of the gold production so far (>350 tonnes Au) has come from a handful of deposits located along the northern margin of the greenstone belt. Most deposits are hosted by greenschist-facies metasedimentary and metamafic rocks, with the notable exception of the amphibolite-facies rocks at New Consort mine. Mineralisation is associated with quartz–carbonate veins that truncate major compressional structures at the greenstone belt scale. The age of mineralisation is loosely constrained at circa 3080–3030 Ma, based on U–Pb dating of hydrothermal rutile and titanite. In greenschist-facies deposits, the ore assemblage is dominated by pyrite and arsenopyrite, which contain up to thousands of ppm of ‘invisible’ gold, Ni–As–Sb sulphides and native gold. At New Consort mine, mineralisation includes massive replacement-style ore and vein-hosted or disseminated types. Both structural studies in the field and microstructural observation point to a multistage ore deposition process, which is reflected in the re-activation of brittle to ductile structures and the overprinting of sulphide assemblages. The presence of mass-independently fractionated S isotopes (Δ33S = –0.6 to +1.0‰) in pyrite from Sheba and Fairview mines suggests that hydrothermal fluids mobilised S from volcanic and sedimentary rocks of the greenstone belt and places constraints on the origin of the Au itself.

18 pages; International audience; Microbial sulfate reduction (MSR) is thought to have operated very early on Earth and is often invoked to explain the occurrence of sedimentary sulfides in the rock record. Sedimentary sulfides can also form from sulfides produced abiotically during late diagenesis or metamorphism. As both biotic and abiotic processes contribute to the bulk of sedimentary sulfides, tracing back the original microbial signature from the earliest Earth record is challenging. We present in situ sulfur isotope data from nanopyrites occurring in carbonaceous remains lining the domical shape of stromatolite knobs of the 2.7-Gyr-old Tumbiana Formation (Western Australia). The analyzed nanopyrites show a large range of δ34S values of about 84‰ (from -33.7‰ to +50.4‰). The recognition that a large δ34S range of 80‰ is found in individual carbonaceous-rich layers support the interpretation that the nanopyrites were formed in microbial mats through MSR by a Rayleigh distillation process during early diagenesis. An active microbial cycling of sulfur during formation of the stromatolite may have facilitated the mixing of different sulfur pools (atmospheric and hydrothermal) and explain the weak mass independent signature (MIF-S) recorded in the Tumbiana Formation. These results confirm that MSR participated actively to the biogeochemical cycling of sulfur during the Neoarchean and support previous models suggesting anaerobic oxidation of methane using sulfate in the Tumbiana environment.

The significant degradation that fossilized biomolecules may experience during burial makes it challenging to assess the biogenicity of organic microstructures in ancient rocks. Here we investigate the molecular signatures of 1.88 Ga Gunflint organic microfossils as a function of their diagenetic history. Synchrotron-based XANES data collected in situ on individual microfossils, at the submicrometre scale, are compared with data collected on modern microorganisms. Despite diagenetic temperatures of ∼150–170 °C deduced from Raman data, the molecular signatures of some Gunflint organic microfossils have been exceptionally well preserved. Remarkably, amide groups derived from protein compounds can still be detected. We also demonstrate that an additional increase of diagenetic temperature of only 50 °C and the nanoscale association with carbonate minerals have significantly altered the molecular signatures of Gunflint organic microfossils from other localities. Altogether, the present study provides key insights for eventually decoding the earliest fossil record., Thermal diagenesis is generally seen as detrimental to the preservation of organic biosignatures. Using synchrotron-based XANES data, Alleon et al. find preservation of the molecular signatures of organic microfossils from the 1.88 Ga Gunflint cherts.

We report in situ and bulk S-isotope analyses of the 3.2 Ga old barite deposit of the Sargur Group, Dharwar Craton, India. Compared with the two other coeval barite deposits in Western Australia (Pilbara Craton, Warrawoona and Sulfur Springs Group) and South Africa (Kaapvaal Craton, Onverwacht and Fig Tree Group), the rocks of the Sargur Group have experienced pervasive ductile deformation and equilibrated under high-grade metamorphic conditions, which could have caused partial S-isotope re-equilibration. However, in situ techniques revealed high spatial heterogeneities in δ 34 S of up to ∼18‰ in barite-bearing pyrites, as well as similar Δ 33 S anomalies compared to the associated sulfate indicating bacterial sulfate reduction isotope effects and a minor impact of the pervasive ductile deformation and metamorphism on the primary/diagenetic signatures. Our data in associated silicate rocks define consistent trends in Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S reflecting metamorphic reworking of primary photolytic signatures with sulfide deriving from thermal reduction of the barite sulfate at >400 °C. Our results match the previous data obtained on other Early Archean barite deposits. This strongly suggests that similar sulfur sources were at play over the three continents during the production and preservation of atmospheric sulfate and elemental sulfur aerosols. This also confirms that the different Early Archean barite deposits known on Earth formed during periods of intense subaerial volcanism and record volcanic photolytic signatures different from those common in the Archean.

International audience; The coupled O and Si isotope variations of Precambrian cherts show secular and correlated variations that have been interpreted as a progressive cooling of the oceans on Earth. However, this reconstruction has been challenged because cherts can have various origins (hydrothermal, sedimentary, volcanic silicification) and their isotopic compositions might have been reset by metamorphic fluid circulation. Even pristine samples show evidence of fluid circulation by the presence of various quartz veins. Here, we have determined the effect of fluid circulation on the isotopic composition of the microquartz chert matrix by studying the mineralogical relationships between quartz veins and microquartz in two distinctive chert samples. These samples show different quartz habit and texture in their quartz veins, which imply various fluid system conditions. Therefore the presence of quartz spherulites in the quartz veins indicates a more moderate fluid circulation than the presence of columnar quartz. These observations were also confirmed by the oxygen isotope compositions of both microquartz matrix chert and quartz veins. We have demonstrated that in some conditions, fluid circulations do not modify the oxygen isotope composition of the chert matrix and thus the microquartz can still record paleo-environmental conditions. The petrography of quartz vein associated with in situ oxygen isotope composition provides one criterion to characterize the state of preservation of the microquartz.

The Earth, which is characterized by an active mantle convection coupled to plate tectonics, is unique in the Solar System. Such mecanisms obscure our understanding of the Earth’s early history 4.5 Ga ago. The evolution of our planet depends on the nature of the parent bodies that have accreted to form the Earth. Then the Earth has differentiated to form the crust, mantle and the core, as well as its outer envelopes (hydrosphere and atmosphere). It is the interactions between these different envelopes that have led and controlled the emergence of life, but also its evolution to the present day. In this contribution we will retrace the history of the Earth from its formation to the development of the first environments favorable to life. This story is told through research conducted at the Laboratoire Magmas et Volcans of the “Université Blaise Pascal” and the CNRS (Centre National de la Recherche Scientifique)., La Terre se distingue des autres corps planétaires par une dynamique mantellique active couplée à une tectonique des plaques. Ces processus rendent particulièrement difficile l’étude des premiers stades d’évolution de notre planète formée il y a plus de 4,5 milliards d’années. L’évolution de notre planète dépend de la nature des corps qui se sont accrétés pour la former puis des processus qui ont permis sa structuration chimique. La Terre s’est rapidement différenciée pour former la croûte en surface, le manteau et le noyau au centre, ainsi que ses enveloppes externes (hydrosphère et atmosphère). Ce sont les interactions entre ces différentes enveloppes qui ont conduit et contrôlé l’émergence de la vie, mais aussi son maintien et son évolution jusqu’à nos jours. Dans cette contribution nous allons retracer l’histoire de la Terre depuis sa formation jusqu’au développement des premiers environnements favorables à la vie. Cette histoire est racontée via les recherches menées au Laboratoire Magmas et Volcans de l’université Clermont Auvergne et du CNRS (Centre National de la Recherche Scientifique).

International audience; The occurrence of Early Archean barite deposits is intriguing since this type of sediment requires high availability of dissolved sulfate (SO42-), the oxidized form of sulfur, although most authors argued that the Archean eon was dominated by reducing conditions, with low oceanic sulfate concentration (56Fe values varying between -2.04‰ in Fe sulfide-dominated barite beds, to +2.14‰ in Fe oxide-bearing cherts. δ34S values of sulfides vary between -10.84 and +3.56‰, with Δ33S in a range comprised between -0.35 and +2.55‰, thus supporting an O2-depleted atmosphere (-5 PAL). Iron isotope variations together with major element correlations show that, although the sediments experienced a pervasive stage of hydrothermal alteration, the rocks preserved a primary/authigenic signature predating subsequent hydrothermal stage. Highly positive δ56Fe values recorded in primary Fe-oxides from ferruginous cherts support partial Fe oxidation in a reducing oceanic environment (O2 < 10-4 μM), but are incompatible with a model of complete oxidation at the redox boundary of a stratified water column. Iron oxide precipitation under low O2 levels was likely mediated by anoxygenic photosynthesis, and/or abiotic photo-oxidation processes. Our results are consistent with global anoxic conditions in the 3.2 Ga-old sediments, implying that the barite deposits were most likely sourced by atmospheric photolysis of S gases produced by large subaerial volcanic events, and possibly SO42- produced by magmatic SO2 disproportionation in hydrothermal systems.

Oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the Precambrian. These reconstructions have been challenged because cherts can have various origins (hydrothermal, sedimentary, volcanic silicification) and their isotopic compositions might have been reset by metamorphic fluid circulation. Existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography (criterion #1: chert is composed mostly of microquartz); on the bulk oxygen isotopic composition (criterion #2: bulk δ 18 O has to be close enough to the maximum δ 18 O value previously measured in other cherts of the same age); and on the presence of a large δ 18 O range at the micrometer scale (criterion #3: δ 18 O range of ∼10‰ at ∼2 μm). However, these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts. We report in situ Si and O isotope compositions and trace element concentrations in seven chert samples ranging from 1.88 to 3.5 Ga in age. Correlations between δ 30 Si and Al 2 O 3 (and K 2 O, TiO 2 ) reveal that microquartz is of three different origins, i.e. diagenetic, hydrothermal or silicification. Moreover, chert samples composed mostly of diagenetic microquartz show a large range of δ 30 Si at the micrometer scale (1.7–4.5‰), consistent with the large range of δ 18 O previously found in the Gunflint diagenetic cherts. We propose two further quantitative criteria to assess the origin, state of preservation and diagenetic history of cherts. Criterion #4 uses trace element concentrations coupled with δ 30 Si to ascribe the origin of cherts among three possible end-members (diagenetic, hydrothermal, and silicified). Criterion #5 is the presence of a large range of δ 30 Si in pure diagenetic microquartz. In the seven samples analyzed in this study, only one (from the Gunflint Iron formation at 1.88 Ga) passes all the criteria assessed here and can be used for paleo-temperature reconstructions. The criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo-temperature reconstructions by taking into account the effect of diagenesis.

An analytical procedure is described for in situ measurements of iron isotope compositions of different sulfides and oxides with a spatial resolution of 15–20 μm using a Cameca IMS 1270 ion microprobe (SIMS). The 54 Fe and 56 Fe isotopes were measured on two Faraday cups. The isobaric 54 Cr interference on 54 Fe was corrected by measuring 53 Cr and 52 Cr intensities. For magnetite crystals, which contain some chromium, this correction is consistent at 0.11 ± 0.06‰. A SIMS instrumental mass fractionation was observed between oxides and sulfides, and this instrumental mass fractionation decreases with the oxidation state. The iron isotope composition of various reference materials, including hematite, pyrite, siderite and magnetite, were intercalibrated with those obtained by MC-ICP-MS. The SIMS δ 56 Fe reproducibility obtained on these various reference materials is typically 0.2–0.3‰ at 2σ; the MC-ICP-MS δ 56 Fe reproducibility is about 0.1‰ (2σ). This analytical procedure was applied to a Precambrian magnetite grain from the Gunflint Iron formation (1.88 Ga, Canada). This magnetite shows a large range of iron isotope compositions of about 4‰ between the rim and the core. We have also measured the oxygen isotope composition of this magnetite with the new IMS 1280 HR2 (CRPG–CNRS, Nancy), and observed no relation between δ 56 Fe and δ 18 O. The magnetite rim exhibits some extreme δ 56 Fe values of + 5.2‰ never reported before. This result emphasizes the crucial interest of in situ-measurements for studying the processes of Fe isotopic fractionation in depositional environments.

International audience; The Barberton Greenstone Belt of southern Africa hosts several Mesoarchaean gold deposits. The ores were mostly formed in greenschist facies conditions, and occur as hydrothermal alteration zones around extensional faults that truncate and post-date the main compressional structures of the greenstone belt. Ore deposition was accompanied by the intrusion of porphyries, which has led to the hypothesis that gold may have been sourced from magmas. Because the transport of Au in the hydrothermal fluids is widely believed to have involved S complexes, tracing the origin of S may place strong constraints on the origin of Au. We measured multiple S isotopes in sulfide ore from Sheba and Fairview mines of the Barberton Greenstone Belt to distinguish ‘‘deep” S sources (e.g. magmas) from ‘‘surface” S sources (i.e. rocks of the volcano-sedimentary succession that contain S processed in the atmosphere preserved as sulfide and sulfate minerals). Ion probe (SIMS) analyses of pyrite from ore zones indicate mass-independent fractionation of S isotopes (D33S = 0.6‰ to +1.0‰) and the distribution of the analyses in the D33S–d34S space matches the distribution peak of previously published analyses of pyrite from the entire volcano-sedimentary succession. Notwithstanding that the H2O–CO2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks, the fact that S bears an atmosphericsignature suggests the hypothesis that the source of Au should also be identified in the supracrustal succession of the greenstone belt. Our findings differ from conclusions of previous studies of other Archaean shear-hosted Au deposits based on mineralogical and isotopic evidence, which suggested a magmatic or mantle source for Au, and imply that there is no single model that can be applied to this type of mineralisation in the Archaean.

Concentrically laminated pyrite is a relatively common, although volumetrically minor, component of auriferous conglomerates in the Archaean (ca. 3.0–2.7 Ga) Witwatersrand Basin of South Africa. This type of pyrite contains high amounts (several tens of ppm) of Au, but the origin of the pyrite is debated, and the timing of Au deposition in these grains is not known. In order to constrain the formation of pyrite, we have studied concentrically laminated pyrite and other coexisting types of pyrite (inclusion-rich, massive pyrite) by analysing the contents and distribution of Au and other trace elements by laser ablation ICP-MS, the S and Fe isotope composition by SIMS, and the mineral inclusions by scanning electron microscope and laser Raman spectroscopy. Trace element maps indicate that concentrically laminated pyrite is enriched in Sb, Mn, Au, Ag, Tl, Cu, Mo, Mn, and contains two types of gold: finely dispersed Au (“invisible gold”, with Au/Ag ~ 0.1 and likely of primary origin) and Au inclusions with Au/Ag ~ 10 of secondary origin. The study of mineral inclusions revealed the presence of muscovite, chlorite, fine-grained carbonaceous matter, monazite, Ti-oxides, and quartz. Iron and multiple S isotopes suggest that concentrically laminated pyrite and inclusion-rich pyrite were formed from two separate pools of S and Fe with different isotope characteristics. Sulfur was derived from atmospheric S that had undergone mass-independent isotope fractionation to form SO 4 2− with negative Δ 33 S that constituted concentrically laminated pyrite, and elemental S with positive Δ 33 S that formed inclusion-rich pyrite. Iron pools were derived from partial oxidation of Fe 2 + , so that concentrically laminated pyrite formed from a low-δ 56 Fe residual Fe 2 + (average + 0.2‰) and inclusion-rich pyrite formed from a high-δ 56 Fe Fe 3 + pool (average + 2.7‰). Biological activity may have been involved in the reduction of SO 4 2− , causing a wide spread of δ 34 S values (~ 25‰, S reducing microorganisms), as well as in the partial oxidation of Fe 2 + (anaerobic photosynthetic Fe reducers or photosynthetic O 2 producers), and in the formation of pyrite from Fe 3 + (dissimilatory Fe reducers). We propose that concurrent biogenically-mediated pyrite formation and Au trapping suggest that microbial activity was responsible for the accumulation of Au and other trace elements (e.g. Sb, Mn, Ag, Tl, Cu, Mo, Mn) which are commonly enriched in organic matter-rich sediments.

International audience; Oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperatures during the Precambrian. During the past decade, the advance of in situ analysis of Si isotopes has enhanced the interest on cherts as a paleo-environmental proxy. The coupled O and Si isotope composition variations show secular and correlated trends that have been interpreted as a progressive cooling of the ocean. However, this reconstruction has been challenged because cherts can have various origins (hydrothermal, sedimentary, volcanic silicification) and their isotopic compositions might have been reset by metamorphic fluid circulation. In this case, the secular oxygen and silicon isotope variation are considered as reflecting a mixing between seawater and hydrothermal sources. A key point in this discussion deals with the origin of cherts: sedimentary, hydrothermal or chemically silicified? Therefore, several petrographical and geochemical criteria are proposed to recognize the pristine sedimentary origin of a chert. Namely they are: (1) the occurrence of microquartz, (2) a 18O-rich bulk oxygen isotopic composition, (3) the occurrence of large δ18O ranges at a micrometer scale, (4) variation of trace element compositions coupled with δ30Si, (5) the occurrence of large ranges of δ30Si in pure microquartz. These criteria should be regarded as guides to the identification of pristine diagenetic cherts in order to better constrain seawater paleo-temperature reconstructions by taking into account the effect of diagenesis. This article will review the different interpretations about O and Si isotope variation and propose a model of formation based on ancient and modern chert studies.

Iron and sulfur isotope compositions recorded in ancient rocks and minerals such as pyrite (FeS2) have been widely used as a proxy for early microbial metabolisms and redox evolution of the oceans. However, most previous studies focused on only one of these isotopic systems. Herein, we illustrate the importance of in-situ and coupled study of Fe and S isotopes on two pyrite nodules in a c. 2.7 Ga shale from the Bubi Greenstone Belt (Zimbabwe). Fe and S isotope compositions were measured both by bulk-sample mass spectrometry techniques and by ion microprobe in-situ methods (Secondary Ion Mass Spectrometry, SIMS). Spatially-resolved analysis across the nodules shows a large range of variations at micrometer-scale for both Fe and S isotope compositions, with δ56Feδ56Fe and δ34Sδ34S values from −2.1 to +0.7‰+0.7‰ and from −0.5 to +8.2‰+8.2‰, respectively, and Δ33SΔ33S values from −1.6 to +2.9‰+2.9‰. The Fe and S isotope variations in these nodules cannot be explained by tandem operation of Dissimilatory Iron Reduction (DIR) and Bacterial Sulfate Reduction (BSR) as was previously proposed, but rather they reflect the contributions of different Fe and S sources during a complex diagenetic history. Pyrite formed from two different mineral precursors: (1) mackinawite precipitated in the water column, and (2) greigite formed in the sediment during early diagenesis. The in-situ analytical approach reveals a complex history of the pyrite nodule growth and allows us to better constrain environmental conditions during the Archean.