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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 (

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.

The Precambrian rock record contains numerous examples of microscopic organic filaments and spheres, commonly interpreted as fossil microorganisms. Microfossils are among the oldest traces of life on Earth, making their correct identification crucial to our understanding of early evolution. Yet, spherical and filamentous microscopic objects composed of organic carbon and sulfur can form in the abiogenic reaction of sulfide with organic compounds. Termed organic biomorphs, these objects form under geochemical conditions relevant to the sulfidic environments of early Earth. Furthermore, they adopt a diversity of morphologies that closely mimic a number of microfossil examples from the Precambrian record. Here, we tested the potential for organic biomorphs to be preserved in cherts; i.e., siliceous rocks hosting abundant microbial fossils. We performed experimental silicification of the biomorphs along with the sulfur bacterium Thiothrix. We show that the original morphologies of the biomorphs are well preserved through encrustation by nano-colloidal silica, while the shapes of Thiothrix cells degrade. Sulfur diffuses from the interior of both biomorphs and Thiothrix during silicification, leaving behind empty organic envelopes. Although the organic composition of the biomorphs differs from that of Thiothrix cells, both types of objects present similar nitrogen/carbon ratios after silicification. During silicification, sulfur accumulates along the organic envelopes of the biomorphs, which may promote sulfurization and preservation through diagenesis. Organic biomorphs possessing morphological and chemical characteristics of microfossils may thus be an important component in Precambrian cherts, challenging our understanding of the early life record.

International audience; Widespread preservation of fossilized biomolecules in many fossil animals has recently been reported in six studies, based on Raman microspectroscopy. Here, we show that the putative Raman signatures of organic compounds in these fossils are actually instrumental artefacts resulting from intense background luminescence. Raman spectroscopy is based on the detection of photons scattered inelastically by matter upon its interaction with a laser beam. For many natural materials, this interaction also generates a luminescence signal that is often orders of magnitude more intense than the light produced by Raman scattering. Such luminescence, coupled with the transmission properties of the spectrometer, induced quasi-periodic ripples in the measured spectra that have been incorrectly interpreted as Raman signatures of organic molecules. Although several analytical strategies have been developed to overcome this common issue, Raman microspectroscopy as used in the studies questioned here cannot be used to identify fossil biomolecules.

The search for fossil biomolecules often requires combining many techniques to properly characterize their chemical composition. Recently, it was suggested in 6 papers that conventional Raman spectroscopy (i.e., equipped with a 532 nm laser as the excitation source under continuous illumination) can be used alone to identify diverse remnants or derivatives of biomolecules in animal fossils, with important implications for both evolutionary events and fossilization processes. Unfortunately, the reported claims are not supported by the spectroscopy data provided, which actually result from instrumental artefacts and data processing. Here, we outline the limitations of Raman spectroscopy with respect to the identification of biomolecules in fossil materials, and then describe in detail the origin of the misinterpreted signal. Conventional Raman spectroscopy alone cannot be used to identify fossil biomolecules. Instead, non-conventional Raman spectroscopy, mass spectrometry and infrared and X-ray absorption spectroscopy techniques, are successfully used by paleontologists to identify fossil biomolecules that help understanding both the history of life and the mechanisms of fossilization.

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 (

Claims for the widespread preservation of fossilized biomolecules in many fossil animals have recently been reported in six studies, based on Raman microspectroscopy. Here, we show that the putative Raman signatures of organic compounds in these fossils are actually instrumental artefacts resulting from intense background luminescence. Raman spectroscopy relies upon the detection of photons scattered inelastically by matter as a result of its interaction with a laser beam. For many natural materials, this interaction also generates a luminescence signal that is often orders of magnitude more intense than the light produced by Raman scattering. Such luminescence, coupled with the transmission properties of the spectrometer, induced quasi-periodic ripples in the measured spectra that have been incorrectly interpreted as Raman signatures of organic molecules. Although several analytical strategies have been developed to overcome this common issue, Raman microspectroscopy as used in the studies questioned here cannot be used to identify fossil biomolecules.

Hydrothermal and metamorphic processes could have abiotically produced organo-mineral associations displaying morphological and isotopic characteristics similar to those of fossilized microorganisms in ancient rocks, thereby leaving false-positive evidence for early life in the geological record. Recent studies revealed that geologically-induced alteration processes do not always completely obliterate all molecular information about the original organic precursors of ancient microfossils. Here, we report the molecular, geochemical, and mineralogical composition of organo-mineral associations in a chert sample from the ca. 3.47 billion-year-old (Ga) Mount Ada Basalt, in the Pilbara Craton, Western Australia. Our observations indicate that the molecular characteristics of carbonaceous matter are consistent with hydrothermally altered biological organics, although significantly distinct from that of organic microfossils discovered in a chert sample from the ca. 3.43 Ga Strelley Pool Formation in the same area. Alternatively, the presence of native metal alloys in the chert, previously believed to be unstable in such hydrothermally influenced environments, indicates strongly reducing conditions that were favorable for the abiotic formation of organic matter. Drawing definitive conclusions about the origin of most Paleoarchean organo-mineral associations therefore requires further characterization of a range of natural samples together with experimental simulations to constrain the molecular composition and geological fate of hydrothermally-generated condensed organics.

© 2019 The Authors Here we discuss the early geological record of preserved organic carbon and the criteria that must be applied to distinguish biological from non-biological origins. Sedimentary graphite, irrespective of its isotopic composition, does not constitute a reliable biosignature because the rocks in which it is found are generally metamorphosed to the point where convincing signs of life have been erased. Rather, multiple lines of evidence, including sedimentary textures, microfossils, large accumulations of organic matter and isotopic data for co-existing carbon, nitrogen and sulfur are required before biological origin can be convincingly demonstrated.

International audience; Most ancient organic microfossils delicately preserved in 3D have been found in cherts. Although entombment within silica has been shown to promote morphological preservation, the impact of early silicification on the molecular evolution of fossilized microorganisms during burial remains poorly constrained. Here, we report results of advanced fossilization experiments performed under pressure (250 bars) and temperature (250 °C) conditions typical of sub-greenschist facies metamorphism for different durations up to 100 days on microorganisms experimentally entombed (or not) within a silica gel. The experimental residues have been characterized using XRD and XANES spectroscopy. The present study demonstrates that entombment within silica limits the degradation of microorganism molecular signatures, likely through specific chemical interactions, despite the progressive conversion of silica into quartz during the experiments. Extrapolation of the present results suggests that such protection may persist during geological timescales. The present experimental study provides molecular evidence that, in addition to morphologies, cherts may support the chemical preservation of remains of ancient life. The present results thus constitute a step forward towards the reconstruction of the original chemistry of pu-tative fossilized microorganisms.

International audience; The biogenicity of putative traces of life found in early-Archean rocks is strongly debated. To date, only equivocal lines of evidence have been reported, which has prevented a full consensus from emerging. Here we report elemental and molecular data from individual organic microfossils preserved within the 3.4 billion-year-old cherts of the Strelley Pool Formation, Western Australia. The present results support the growing body of evidence advocating their biogenicity, promoting them as the oldest known authentic organic microfossils. These microfossils consist of nitrogen- and oxygen- rich organic molecules that have been only slightly degraded despite experiencing temperatures of ~300 °C. Such molecular preservation emphasises the palaeobiological potential of the Earth’s oldest geological record, whilst providing a promising window into the early biosphere.