Corrigendum to “Formation pathways of Precambrian sedimentary pyrite: Insights from in situ Fe isotopes” [Earth Planet. Sci. Lett. 609 (2023) 118070]

International audienceThe authors regret that some of the data published in the original article were recently found to be incorrect. 101 measurements over 1999 are shifted. The authors state that this shift has no effect on the figures or the discussion. These data originate from the Francevillan Formation (2.1 Ga) samples

Intense biogeochemical iron cycling revealed in Neoarchean micropyrites from stromatolites

International audienceIron 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

High‐spatial‐resolution measurements of iron isotopes in pyrites by secondary ion mass spectrometry using the new Hyperion‐II radio‐frequency plasma source

International audienceIron isotopic signatures in pyrites are considered as a good proxy to reconstruct paleoenvironmental and local redox conditions. However, the investigation of micro-pyrites less than 20µm in size has been limited by the evaluable analytical techniques. The development of the 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.Here we present high-spatial-resolution measurements of iron isotopes in pyrites using a 3 nA–3 μm primary 16O− 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 δ56Fe 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 56Fe+ 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 highresolution 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

BIOLOGICAL VS. DIAGENETIC CONTROLS IN MICROBIALITES FROM SPATIALLY RESOLVED IRON ISOTOPES IN PYRITE

Comprendre l’apparition et le développement de la vie nécessite d’étudier des roches très anciennes, vieilles de plusieurs milliards d’années. Cette quête des origines de la vie est extrêmement difficile pour deux raisons : (1) les premières traces de vie sur Terre sont microbiennes, donc extrêmement petites et (2) ces roches ont une histoire complexe, impliquant des processus qui ont modifié leurs apparences et parfois leurs compositions chimiques initiales. Heureusement, il existe encore sur Terre des roches sédimentaires laminées formées grâce à l’activité d’organismes microbiens. Ces dernières sont appelées stromatolites. Certains stromatolites sont reconnus depuis l’Archéen, soit il y a près de 3,5 Ga (pour le plus vieux spécimen découvert à ce jour), alors que d’autres sont toujours en cours de formation, par exemple dans les milieux marins peu profonds des Bahamas, de la Baie des Requins en Australie ou dans certains lacs volcaniques mexicains. Reconnaitre l’origine biologique (biogénicité) de ces stromatolites anciens est un défi pour la communauté scientifique puisqu’ils ne préservent a priori pas de microorganismes fossilisés. De plus, la structure laminée qui les rend facilement reconnaissable ne peut pas être utilisée seule comme critère de biogénicité, puisque qu’elle peut également résulter de procédés abiotiques (absence d’organismes vivants). Toutefois, les stromatolites contiennent des sulfures de fer (FeS2) micrométriques, connus sous le nom de pyrite. L’intérêt de ces pyrites réside dans leur potentiel d’enregistrer des processus de respiration microbienne à travers leurs compositions isotopiques en fer et/ou en soufre. En effet, les microorganismes ont tendance à mieux assimiler les isotopes légers (54Fe ou 32S) par rapport aux isotopes lourds (56Fe ou 34S), entrainant des différences de masse spécifiques aux différents processus microbiens. Comme le fer est un élément sensible aux réactions d’oxydation et de réduction (réactions redox), la géochimie du fer est couramment utilisée pour tracer des changements redox de l’environnement et/ou l’activité microbienne. Cette thèse se propose d’explorer la variabilité de la composition isotopique du fer des pyrites contenues dans les stromatolites à différentes périodes géologiques, afin de déterminer (1) si les pyrites peuvent être utilisées comme biosignatures, (2) l’influence et l’évolution des métabolismes microbiens utilisant le fer dans des environnements différents, (3) la capacité des compositions isotopiques en fer à renseigner des changements redox globaux comme l’oxygénation de l’atmosphère il y a 2.4 Ga et/ou des variations de l’oxygénation de l’océan pendant des crises d’extinction des espèces (exemple avec la crise du Smithien-Spathien). Pour répondre à ces questions, une comparaison d’échantillons anciens archéens (Formation de Tumbiana, 2,7 Ga) et phanérozoïques (bassin de Sonoma, 251 Ma) a été réalisée avec des microbialites modernes provenant de Cayo Coco (Cuba) et du lac Atexcac (Mexique). Dans toutes ces formations, les pyrites ont enregistré une très grande variabilité des compositions isotopiques du fer. Dans les microbialites modernes, les compositions isotopiques du fer reflètent des processus de réduction des oxydes de fer contrôlés par des microorganismes ferri-réducteurs indépendamment des conditions chimiques de l’environnement. Les compositions isotopiques mesurées dans les sédiments du Phanérozoïque montrent un contrôle de l’environnement de dépôt (différents degrés de remobilisation des sédiments) et de la nature des dépôts (i.e. différentes signatures selon la présence ou l’absence des dépôts microbiens). Dans les échantillons archéens, la large gamme isotopique mesurée est interprétée comme résultant de procédés d’oxydation et de réduction complexes, contrôlés par l’activité des microorganismes. Cette thèse démontre l’importance de processus locaux dans la formation des pyrites préservées dans les stromatolites, comme l’influence de gradient redox à l’échelle du sédiment ou du biofilm et des différents métabolismes microbiens qui composent le biofilm. Ainsi, les pyrites associées à ces dépôts microbiens ne semblent pas permettre de reconstruire les signatures de l’environnement global. En revanche, ces pyrites peuvent être utilisées comme des biosignatures, à conditions de mener des études détaillées combinant l’isotopie du Fe, du S et minéralogie. -- Recognition of fossils in the Archean sedimentary rocks is essential to constraining when and how life evolved, and the nature of the microbial metabolisms present on early Earth. Unfortunately, preservation of microorganisms is very limited in the Archean rock record. Direct observations of Archean microfossils are not convincing, yet, indirect traces of metabolic activity are described as early as ~3.5 Ga in form of stromatolites. Stromatolites are laminated organo-sedimentary benthic structures formed by the activity of microbial communities. They represent the oldest archives of life on Earth. However, laminated structures have also been reproduced by abiotic experiments, undermining the biological origin of the ancient stromatolite specimens. This thesis work focuses on refining geochemical and isotope proxies that can be used to assess the stromatolite biogenicity. I investigated pyrite, a mineral that is ubiquitous in the stromatolite record. It is well demonstrated that in modern sediments specific microorganisms produce Fe2+ and H2S that ultimately lead to the formation of micrometric pyrite. Over the course of microbial activity and mineral precipitation, both sulfur and iron exhibit large isotope fractionations. Iron is transformed to pyrite through various aqueous and mineral species in the environment through redox-sensitive processes. Therefore Fe isotopes are used in reconstructing paleoredox conditions, diagenetic processes and/or metabolic signatures. Consequently, this thesis (1) tests if iron isotope compositions of micrometric pyrite can be used as a biosignature and (2) assesses sensitivity of Fe isotopes in pyrite with respect to global redox changes. I used a spatially resolved secondary ion mass spectrometry technique (SIMS) to develop a new analytical protocol to investigate the Fe isotope variability in pyrite smaller than 10 µm. In this thesis, samples of different age (modern, Mesozoic and Archean) have been selected to reconstruct the iron isotope variations through time and to differentiate the global versus local environmental influences on the pyrite isotope compositions. Modern samples are two microbialites collected from two different environments. Spatially resolved S isotope analyses is employed via nanoscale secondary ion mass spectrometry (NanoSIMS) to document the large isotope ranges and its relationship to the pyrite morphology and the activity of sulfate- reducing bacteria. As one of the main findings of the thesis, Fe isotope compositions (from -3.5 to +3.5‰) measured on a micrometer scale are consistent with a microbially-mediated Fe-oxide reduction by Fe-reducing organisms. The studied here Mesozoic samples were deposited during the Smithian-Spathian boundary (SSB, ~251 Ma), an interval post-dating the end-Permian mass extinction event. According to multiple lines of evidence, the oceans experienced abrupt swings in redox state and temperature, all of which leading to a major biotic diversity crisis. During this period of major ecological stresses, microbial communities fluorished leading to deposition of a rich stromatolite record. I measured eight samples deposited along a ramp system which revealed a wide Fe isotope range (i.e. ~7‰). The δ56Fe values show a clear influence of the depositional environment and the nature of deposit, i.e. the presence of microbialite. The Fe isotope compositions collected on the Archean Tumbiana stromatolites, displayed the widest range of δ56Fe values measured in the entire Archean sedimentary pyrite record (i.e. -2.2 to +4.4‰). This exceptionally large isotope range is interpreted as the result of an intense local iron cycling within the microbial mat, including repeated cycles of partial oxidation and microbially- mediated reduction processes, related to biogeochemical carbon and sulfur cycles. All together, Fe isotope compositions of micrometric pyrite grains are likely to record synsedimentary and early diagenetic processes that occur within the sediment or in the biofilm. Importantly, the seawater column has a limited influence on the final δ56Fe values of pyrite. The δ56Fe values measured in pyrite highlight the intimate interaction between the local pools of Fe, O, C and S. The speciation and isotope compositions of these elements are affected by the microbially mediated cycling as well as the redox gradients created abiotically. Therefore, to better understand the conditions of microbialite formation through geological time, it is critical to coup e the Fe- and S-isotope measurements with detailed sedimentological and petrological studies

Formation pathways of Precambrian sedimentary pyrite: Insights from in situ Fe isotopes

International audienceIron isotope compositions (expressed as δ 56 Fe) in sedimentary pyrite have been widely used as tracers of redox and chemical evolution of the ocean through geological time. Previous studies mostly built on the mechanical extraction of sulfides from bulk rock samples, and focused on visible macroscopic pyrites, which may introduce a sampling bias. In situ analyses of micropyrite grains can provide new insights into the processes of pyrite formation and their time evolution. Here, we compile ca. 2000 in situ iron isotope compositions of Archean to Paleoproterozoic sedimentary pyrite, from previous literature as well as new data. Contrasting with bulk analyses, micropyrite displays a large and constant range of δ 56 Fe values, from-4 to +4 , through time. Micropyrite δ 56 Fe values are not significantly influenced by metamorphic grade. A bimodal distribution of positive versus negative δ 56 Fe values can be attributed to two different processes of pyrite formation, Fe (oxyhydr)oxide sulfidation, versus kinetic and possibly microbially mediated pyrite precipitation. These processes are tightly related to rock lithology and thus to sedimentary conditions, and have existed since 3.8 Ga

Pyrite iron isotope compositions track local sedimentation conditions through the Smithian-Spathian transition (Early Triassic, Utah, USA)

International audienceThe late Smithian and the Smithian-Spathian boundary (SSB) are associated with harsh environmental conditions, including abrupt temperature changes, oceanic acidification and oxygen deficiency causing an additional marked loss of biotic diversity in the aftermath of the end-Permian mass extinction. Such environmental disturbances are documented worldwide through large fluctuations of the C, O, S and N biogeochemical cycles. This study presents secondary ion mass spectrometry pyrite Fe isotope analyses from the Lower Weber Canyon (LWC) section (Utah, USA) combined with bulk rock δ34Spy and δ34SCAS analyses in order to better understand the redox changes in different environmental settings along a ramp depositional system through the SSB. δ56Fe analyses show a large variability along the studied ramp system of ∼7‰ (from −1.99 to +5.39‰), over a set of 350 microscale analyses. Bulk sulfide sulfur isotope analyses, performed on 30 samples, show δ34Spy varying from −20.5 to +16.3‰. The inner ramp domain is characterized by a mean negative δ34Spy values of −11.4‰. A progressive 34S-enrichment (up to +16.3‰) is recorded in pyrite from mid and outer ramp settings. Carbonate associated sulfate (CAS) sulfur isotope analyses, performed on 5 samples, show relatively steady δ34Scas of +30.2 ± 2.2‰. Variations in δ34Spy are interpreted as reflecting the degree of connection between sediment porewaters and the overlying water column. Multiple lines of evidence point to a fully oxygenated water column and thus restricts pyrite formation to the sediments. Both the sedimentary environment and the nature of deposits seem to control δ56Fepy. In the inner ramp, high δ56Fepy values averaging +2.05‰ are only observed in microbially induced sedimentary structures (MISS), which record partial Fe-oxide reduction and oxidation reactions occurring at biofilms scale. In the absence of MISS, δ56Fepy inner ramp values are lighter (δ56Femean = +0.90‰) and reflect total reduction of Fe-oxides. In more distal and deeper mid and outer ramp settings, Fe isotope compositions are controlled by microbially-produced H2S that scavenged iron into sulfides. This study unravels local redox state changes in the upper part of some marine sediments by coupling Fe and S isotope systematics. It demonstrates that pyrite grains, and their sulfur and iron isotopic compositions, formed throughout the SSB should be used with caution to infer the redox state of the ocean after the Permian-Triassic biotic crisis

Corrigendum to “Pyrite iron isotope compositions track local sedimentation conditions through the Smithian-Spathian transition (Early Triassic, Utah, USA)” [Palaeogeography, Palaeoclimatology, Palaeoecology, 617 (2023), 1–16].

International audienceBalmat pyrite standard (Whitehouse and Fedo, 2007; Marin-Carbonne et al., 2011) is known to have variable δ56Fe values (Xu et al., 2022). We have recently reanalyzed the batch of Balmat standard from UNIL (hereafter called Balmat-UNIL) at the University of Chicago (courtesy of Dr. Rego and Prof. Dauphas, unpublished data). The δ56Fe bulk value of Balmat-UNIL pyrite used in this study is −1.459 ± 0.024 ‰, compared to the previous published value of −0.399 ‰ (Whitehouse and Fedo, 2007). As all the SIMS data are standardized to the Balmat-UNIL pyrite, the correction with this new value led to a shift of 1.06 ‰ of the entire published dataset. This corrigendum presents the corrected pyrite Fe isotope data, now displaying variations between −3.05 ‰ and + 4.33 ‰. Most of the figures (Fig. 3, Fig. 4, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11), and the supplementary data (Figs. S3, S5, S6 and Table S9) need to be modified as presented below. As a result of this shift, we also propose hereafter an alternative model to explain the δ56Fe signal of pyrite precipitated in the inner ramp system (section 4.3.1 - discussion), combining petrographic observations and both pyrite S and newly corrected Fe isotope results.The authors would like to apologize for this mistake