Frédéric Delarue
email: frederic.delarue@upmc.fr

Sylvain Bernard

Kenichiro Sugitani

François Robert

Romain Tartèse

Sonja-Verena Albers

Rémi Duhamel

Sylvain Pont

Sylvie Derenne

Affiliations

Microfossils with tail-like structures in the 3.4 Gyr old Strelley Pool Formation Authors

Keywords:

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Introduction

Archean carbonaceous microfossils testify for the widespread presence of life on Earth as early as ca. 3.4 billion years ago [START_REF] Westall | Implications of a 3.472-3.333 Gyr-old subaerial microbial mat from the Barberton greenstone belt, South Africa for the UV environmental conditions on the early Earth[END_REF][START_REF] Sugitani | Biogenicity of Morphologically Diverse Carbonaceous Microstructures from the ca. 3400 Ma Strelley Pool Formation, in the Pilbara Craton, Western Australia[END_REF][START_REF] Wacey | Microfossils of sulphurmetabolizing cells in 3.4-billion-year-old rocks of Western Australia[END_REF]Alleon et al., 2018). However, the interpretation of the Archean palaeobiological record is fraught with difficulties pertaining to fossilization and burial-induced degradation processes, as exemplified by intense debates over the past couple of decades (e.g., [START_REF] Schopf | Laser-Raman imagery of Earth's earliest fossils[END_REF][START_REF] Brasier | Questioning the evidence for Earth's oldest fossils[END_REF][START_REF] Wacey | 3.46 Ga Apex chert 'microfossils' reinterpreted as mineral artefacts produced during phyllosilicate exfoliation[END_REF]. Remnants of early life forms have experienced burial and thermal alteration for billions of years, which led to the degradation of many pristine biological traits [START_REF] Javaux | Challenges in evidencing the earliest traces of life[END_REF]. Archean putative microfossils tend to exhibit simple morphological shapes at the micrometric scale (e.g., spheroidal, filamentous, film, and lenticular forms) that can also be abiotically produced [START_REF] Garcia-Ruiz | Self-Assembled Silica-Carbonate Structures and Detection of Ancient Microfossils[END_REF][START_REF] Cosmidis | Self-assembly of biomorphic carbon/sulfur microstructures in sulfidic environments[END_REF], precluding, in turn, any simple morphological distinction between genuine biological remnants and mineral/organic biomorphs. Because of the lack of taxonomically informative features [START_REF] Javaux | Challenges in evidencing the earliest traces of life[END_REF], morphological criteria alone are generally considered as insufficient to assess the biological nature of ancient traces of life in the Archean geological record [START_REF] Brasier | A fresh look at the fossil evidence for early Archaean cellular life[END_REF]. As a result, the ancient fossil record has not yet conveyed a complete picture of ancient biodiversity.

Here, we report the discovery of 3.4 billion years old organic microfossils from the Strelley Pool Formation (SPF) exhibiting exceptionally preserved morphological traits possibly indicative of active motility.

Material and methods

Studied sample

For this study, we selected a 3.4 billion year-old black chert sampled from the Panorama locality (PANX1-1) situated in the SPF (Western Australia), and which displays abundant microfossils and microscopically identifiable parallel laminations (see [START_REF] Sugitani | Biogenicity of Morphologically Diverse Carbonaceous Microstructures from the ca. 3400 Ma Strelley Pool Formation, in the Pilbara Craton, Western Australia[END_REF][START_REF] Sugitani | Microfossil assemblage from the 3400Ma Strelley Pool Formation in the Pilbara Craton, Western Australia: Results form a new locality[END_REF][START_REF] Sugitani | Early evolution of large micro-organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic-walled microfossils[END_REF] for detailed description of the geological background).

Chemical isolation of microfossils

Organic-walled microfossils were isolated from the SPF carbonaceous black chert sample using a modified version of the classical acid maceration procedure (Delarue et al., 2020). A 'soft' acid maceration procedure was applied in order to minimize both potential physical and chemical degradation of organic microstructures. Prior to acid maceration, about 30 g of rock samples were fragmented into ~3 g rock chips rather than crushed into finer grains. Rock chips were cleaned using ultrapure water and a mixture of dichloromethane/methanol (v/v: 2/1), and were then directly placed in a Teflon vessel filled with a mixture of HF (40%, reagent grade) / HCl (37%; reagent grade; v/v: 9/1) at room temperature. After 48 hours, successive centrifugation and rinsing steps using ultrapure water were performed until reaching neutrality.

The residual material was suspended in ethanol and filtered on polycarbonate filters (pore Ø = 10 µm). After ethanol evaporation, polycarbonate filters were fixed on carbon tape and coated with 20 nm of gold to prevent further contamination by atmospheric deposits and for further analyses.

Scanning electron microscopy and Energy Dispersive X-Ray Spectroscopy (SEM-EDXS)

SEM-EDXS imaging and analysis were performed on gold-coated filters using a TESCAN VEGA II at the French National Museum of Natural History (MNHN) operated with an accelerating voltage of 15 kV.

Raman spectroscopy

Raman microspectroscopy was carried out using a Renishaw InVIA microspectrometer equipped with a 532 nm green laser. The laser was focused on the sample by using a DMLM Leica microscope with a 50× objective. The spectrometer was first calibrated with a silicon standard before the analytical session (matching at 520.5 cm -1 ). For each target, we determined the Raman shift intensity in the 1000 to 2000 cm -1 spectral window that includes the first-order defect (D) and graphite (G) peaks. A laser power below 1 mW was used to prevent any thermal alteration during spectrum acquisition. Spectrum acquisition was achieved after three iterations using a time exposure of 10 seconds (spectral resolution of 1.5 cm -1 ). Raman microspectroscopy was performed on gold-coated organic surfaces, implying a slight lowering of the relative intensity of the D band with respect to the G one (Delarue et al. 2020).

Nanoscale secondary ion mass spectrometry

Isolated microfossils were analyzed using a CAMECA NanoSIMS 50 ion probe using a Cs + primary ion beam. Before measurements, pre-analysis sputtering was performed over 30 × 30 µm 2 areas for ca. 8 minutes using a 500 pA primary current (750 µm aperture diaphragm) to remove surficial contamination, and achieve Cs + saturation fluence and constant secondary ion count rates. Analyses were then carried out using a 10 pA primary current (200 µm aperture diaphragm) on smaller areas to avoid pre-analysis sputtering edge artifacts. The secondary molecular species 12 C 14 N -and 31 P -were collected simultaneously in electron multipliers. The NanoSIMS raw data were corrected for a 44 ns dead time on each electron multiplier and processed using the Limage software.

Focused ion beam (FIB)

FIB ultrathin sections were extracted from the organic microfossils using an FEI Strata DB 235 (IEMN, Lille, France). Milling at low gallium ion currents minimizes common artefacts, including local gallium implantation, mixing of components, creation of vacancies or interstitials, creation of amorphous layers, redeposition of the sputtered material on the sample surface, and significant changes in the speciation of carbon-based polymers.

Scanning transmission X-ray microscopy (STXM)

X-ray Absorption Near Edge Structure (XANES) investigations were conducted using the HERMES STXM beamline at the synchrotron SOLEIL (Gif-sur-Yvette, France). Carbon contamination on beamline optics was constantly removed thanks to a continuous flow of pure O2. The well-resolved 3p Rydberg peak of gaseous CO2 at 294.96 eV was used for energy calibration. Collecting image stacks at energy increments of 0.1 eV with a dwell time of ≤ 1 ms per pixel prevented irradiation damage. The estimations of N/C values and the normalization of the C-XANES spectra shown here were done using QUANTORXS (Le [START_REF] Guillou | XANES-Based Quantification of Carbon Functional Group Concentrations[END_REF].

Results and Discussion

Microscope observations of the studied SPF sample thin sections revealed the presence of organic-walled microfossils exhibiting a tail-like structure (Fig. 1a,b). These microfossils are exclusively observed within the siliceous sedimentary matrix, precluding their introduction during hydrothermal fluid circulation post 3.4 Ga. Raman spectra of specimens exhibiting a tail-like structure chemically isolated from the siliceous matrix are typical of those of disordered carbonaceous materials having undergone a low-grade metamorphism (Fig. 2a; [START_REF] Pasteris | Necessary, but Not Sufficient: Raman Identification of Disordered Carbon as a Signature of Ancient Life[END_REF]. Their Raman line shapes suggest that these microfossils experienced hydrothermal and/or diagenetic peak temperatures of approximately 250-300 °C [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland): RSCM in low-grade metasediments[END_REF]Kouketsu et al., 2014). Raman first-order spectra of the studied SPF microfossils exhibiting a tail-like structure are similar to those previously determined on syngenetic microfossils from the same geological formation observed in thin sections [START_REF] Lepot | Texture-specific isotopic compositions in 3.4Gyr old organic matter support selective preservation in cell-like structures[END_REF][START_REF] Sugitani | Microfossil assemblage from the 3400Ma Strelley Pool Formation in the Pilbara Craton, Western Australia: Results form a new locality[END_REF], on freshly fractured faces (Alleon et al., 2018), and in acid maceration residue (Delarue et al., 2020). Therefore, these organic-walled microfossils exhibiting a tail-like structure should be regarded as syngenetic as they were subjected to the maximum metamorphic temperature registered by their host rock.

Although Raman spectroscopy is a useful tool to assess syngeneity, it is not sufficient to determine the biogenicity of putative remains of ancient life [START_REF] Pasteris | Necessary, but Not Sufficient: Raman Identification of Disordered Carbon as a Signature of Ancient Life[END_REF].

Energy-dispersive X-ray spectroscopy data show that the studied specimens essentially contain C and O (Fig. 2b), confirming their organic nature, while nanoscale secondary ion mass spectrometry reveals the presence of nitrogen and, in one specimen, phosphorus (Figs. 2d, f-g).

The presence of these key elements of cell walls, proteins, and nucleic acids are consistent with a biological origin. image illustrates the presence of phosphorus. No 31 P -was recorded on the second specimen shown in panel c. Variations in 31 P -emission intensity between the smooth (e.g., microfossil itself) and rough (e.g., coating by amorphous submicrometric organic matter) surfaces cannot be used to depict any P enrichment because of microtopographic features biasing ion emissions (Delarue et al., 2017).

Spatially resolved chemical investigations exploiting X-ray absorption confirm the heterogeneous chemical nature of the investigated organic-walled microfossils: at least three different types of chemical structures could be distinguished within a given specimen (Fig. 3). Specimens contain some highly graphitic organic materials with almost no nitrogen as revealed by X-ray absorption spectra exhibiting a broad peak of conjugated aromatic groups (285.5 eV) and the excitonic absorption feature of planar domains of highly conjugated π systems (291.7 eV;Bernard et al., 2010). Closely associated are N-poor materials with XANES spectra similar to those of thermally-altered kerogen with an intense absorption peak at 285 eV (aromatic or olefinic groups), a relatively broad absorption feature at 287.5 eV (aliphatic carbons), and an absorption feature at 286.6 eV (imine, nitrile, carbonyl and/or phenol groups; Bernard et al., 2010;[START_REF] Guillou | XANES-Based Quantification of Carbon Functional Group Concentrations[END_REF]. Specimens also contain N-rich compounds (N/C ~ 0.22) with XANES spectra that exhibit clear contributions of quinones or cyclic amides (284.5 eV), aromatic or olefinic carbons (285.1 eV), imine, nitrile, carbonyl and/or phenol groups (286.6 eV), aliphatics (287.7 eV) and amides (288.2 eV). Altogether, the chemical structure of the SPF specimen investigated here is consistent with the preservation of partially degraded biomolecules. From a morphological point of view, the organic-walled microfossils are leaf-shaped cells ranging from 30 to 84 µm in length and from 16 to 37 µm in width (Fig. 1). They exhibit classic taphonomical degradation features, including folds and tears (Figs. 1c-g). The preparation of ultrathin foils using focused ion beam illustrates their relative limited thickness, ranging from 200 to 500 nm (Fig. 3). Four specimens also exhibit a specific morphological feature: a tail-like appendage protruding from the leaf-shaped cell (Figs. 1c,d).

From the comparison with modern microorganisms, we can assume that this tail-like appendage is a remnant of an ancient prostheca or of a locomotory organelle. The tail-like appendages observed in SPF microfossils are between 0.7 and 1.2 µm in diameter, which is, by far, larger than those reported for modern archaella, flagella, and cilia, reaching ca. 10, 20, and 200 nm, respectively (Jarell and McBride, 2008;Beeby et al., 2020). This would be forgetting that Precambrian organic-walled microfossils exhibit very large cell dimensions (Ø > 10 µm) compared to modern microorganisms [START_REF] Javaux | Organic-walled microfossils in 3.2-billion-yearold shallow-marine siliciclastic deposits[END_REF][START_REF] Sugitani | Biogenicity of Morphologically Diverse Carbonaceous Microstructures from the ca. 3400 Ma Strelley Pool Formation, in the Pilbara Craton, Western Australia[END_REF]2015;Balidukay et al., 2016;[START_REF] Loron | Organic-walled microfossils from the late Mesoproterozoic to early Neoproterozoic lower Shaler Supergroup (Arctic Canada): Diversity and biostratigraphic significance[END_REF] In order to take this difference into account, we propose to use the Appendage Shape Index (ASI), which is based on the ratio between the width of the tail-like appendage and that of the parent cell (Fig. 4). Compilation of morphometric data from extant microorganisms shows that prosthecae and locomotory organelles are characterized by different ASI values. Prosthecae display ASI values ranging from 13 to 39 % while they range from 1 to 10 % for modern archaella, flagella, and cilia (Fig. 4; see supplementary information for detailed values). In the present study, the tail-like appendages are characterized by ASI values ranging from 2 to 6 %, that is values falling within the domain of modern archaella, flagella, and cilia (Fig. 4).

Involved in anchoring cells to organic and mineral surfaces, in nutrient uptake, or in asexual reproduction by budding at its tip [START_REF] Curtis | Stalk formation of Brevundimonas and how it compares to Caulobacter crescentus[END_REF], a prostheca is a micrometric tube-like appendage consisting in an extension of the cellular membrane. This implies a structural continuity between the microorganism body and the base of the prostheca [START_REF] Javaux | Recognizing and interpreting the fossils of early eukaryotes[END_REF].

Here, the SPF specimens investigated exhibit an anchoring attachment point and a filament-like appendage, indicating two distinct structural subunits (Fig. 1) at odds with any extension of the cellular membrane. Because of their ASI values and their two structural subunits, the tail-like appendages observed in SPF microfossils cannot be considered as remnants of prosthecae.

As far as we are aware, occurrence of distinct external and functional subunits can only be assigned to locomotory organelles, in good agreement with ASI values. However, these subunits do not meet standard structural features (for instance, a curved hook connecting the filament to the basal body in flagella) observed on locomotory organelles from any organism of the three extant domains of life (see [START_REF] Khan | Assembly, Functions and Evolution of Archaella, Flagella and Cilia[END_REF] implying in turn, that the observed structural features do not allow to depict the biological affinity of these remnants of tailed microorganisms.. on SPF organic-walled microfossils. ASI ranges from 4.8 to 5.8 % and from 2.2 to 3.3 % in organic-walled microfossil observed in thin sections (n = 2) and in the acid maceration residue (n = 2), respectively. ASI is likely overestimated in thin sections as a consequence of shadows occurring at the edge of microfossils.

Previous reports of 3.4-3.0 Ga lenticular microfossils exhibiting a flange were interpreted as demonstrating passive motility of microbial planktons drifting depending on their surrounding environment [START_REF] House | Carbon isotopic analyses of ca. 3.0 Ga microstructures imply planktonic autotrophs inhabited Earth's early oceans[END_REF][START_REF] Sugitani | Early evolution of large micro-organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic-walled microfossils[END_REF][START_REF] Oehler | Large and robust lenticular microorganisms on the young Earth[END_REF][START_REF] Kozawa | Early Archean planktonic mode of life: Implications from fluid dynamics of lenticular microfossils[END_REF].

To date, the oldest evidence for active motility was recorded as tubular sedimentary structures in 2.1 Ga Francevillian sedimentary series in Gabon [START_REF] El Albani | Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago[END_REF]. The preservation of tail-like structures by some SPF microfossils suggests that some microorganisms could have been capable of active motility -a mechanism whereby microorganisms can direct their movement -as early as 3.4 Gyr ago. Since it likely provided them with the ability to move in the water column or at the surface of organic and/or mineral surfaces, this finding suggests that microorganisms were possibly able to escape harsh environments, adapt their feeding strategies moving towards more favorable nutrient sources, and colonize new ecological niches less than a billion years after the Earth became habitable. The chemical composition of ancient organic matter (OM) attests to turning points in the evolution of life during the history of the Earth [START_REF] Summons | Distinctive hydrocarbon biomarkers from fossiliferous sediment of the Late Proterozoic Walcott Member, Chuar Group, Grand Canyon, Arizona[END_REF][START_REF] Edwards | Flash pyrolysis of the outer cortical tissues in Lower Devonian Psilophyton dawsonii[END_REF][START_REF] Love | Analysis of molecular biomarkers covalently bound within Neoproterozoic sedimentary kerogen[END_REF][START_REF] Duda | Opening up a window into ecosystems with Ediacara-type organisms: preservation of molecular fossils in the Khatyspyt Lagerstätte (Arctic Siberia)[END_REF][START_REF] Nguyen | Absence of biomarker evidence for early eukaryotic life from the Mesoproterozoic Roper Group: Searching across a marine redox gradient in mid-Proterozoic habitability[END_REF][START_REF] Love | Sources of C30 steroid biomarkers in Neoproterozoic-Cambrian rocks and oils[END_REF]. However, investigating the structural and chemical compositions of ancient OM is still a challenging issue as thermal alteration during geological times results in losses of pristine molecular and elemental content. Since ancient sediments and metasediments are generally depleted in OM, isolation procedures have typically been employed before applying analytical techniques to investigate the chemical composition of ancient OM, notably at the molecular scale.

Historically, in the field of organic geochemistry, isolation of organic-insoluble and mineralfree OM is performed through a standard acid maceration procedure involving maceration followed by successive organic solvent extractions (e.g., dichloromethane [DCM] and methanol [MeOH]) and demineralisation with acid (e.g., hydrochloric [HCl] and hydrofluoric [HF]). This physical and chemical maceration procedure (HMM) yields kerogen, which is defined as insoluble macromolecular OM [START_REF] Durand | Kerogen, Insoluble Organic Matter from Sedimentary Rocks[END_REF][START_REF] Vandenbroucke | Kerogen origin, evolution and structure[END_REF]. Several investigations have suggested that acid maceration does not significantly modify the structure of kerogens from ancient rocks [START_REF] Larsen | Effect of demineralization on the macromolecular structure of coals[END_REF][START_REF] Vandenbroucke | Kerogen origin, evolution and structure[END_REF][START_REF] Aboulkas | Effects of acid treatments on Moroccan Tarfaya oil shale and pyrolysis of oil shale and their kerogen[END_REF]. However, there is contrasting evidence that acid maceration of coals can lead to either a rise in carbon structural order [START_REF] Zhang | Effect of acid treatment on the characteristics and structures of high-sulfur bituminous coal[END_REF] or a decrease in hydrocarbon content [START_REF] Tekely | Chemical structure changes in coals after low-temperature oxidation and demineralization by acid treatment as revealed by high resolution solid state 13 C NMR[END_REF]. Moreover, [START_REF] Kebukawa | Comparison of FT-IR spectra of bulk and acid insoluble organic matter in chondritic meteorites: An implication for missing carbon during demineralization[END_REF] demonstrated the contrasting effects of acid maceration by comparing the chemical structure of bulk chondrites to their corresponding insoluble OM fractions. Insoluble OM from type 3 chondrite was depleted in aliphatic moieties in comparison to the bulk type 3 chondrite starting material. However, the effect of acid maceration varied among the different chondrite groups studied [START_REF] Kebukawa | Comparison of FT-IR spectra of bulk and acid insoluble organic matter in chondritic meteorites: An implication for missing carbon during demineralization[END_REF]. Thus, according to current literature, it therefore seems that the chemical stability of thermally altered OM to standard acid maceration cannot simply be assumed.

Among ancient sedimentary rocks, silicified sediments are of interest as they are essential geological archives of biological evolution throughout Earth's history. However, there is often very little OM remaining in these sediments and acid maceration procedures are required to concentrate it before studying its molecular content. Occurring prior to cell lysis and early degradation of organic matter, rapid silicification leads to a closed chemical system by reducing sediment porosity [START_REF] Boyce | Organic chemical differentiation within fossil plant cell walls detected with X-ray spectromicroscopy[END_REF][START_REF] Ledevin | Silica precipitation triggered by clastic sedimentation in the Archean: New petrographic evidence from cherts of the Kromberg type section, South Africa[END_REF], which favours preservation of organic remnants against thermal alteration [START_REF] Alleon | Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis[END_REF]. Using geochemical micrometre-scale analysis tools, it was demonstrated that some silicified Precambrian organic-walled microfossils are composed of significant amounts of carbonyl, phenolic, carboxylic, hydroxyl, and amide functional groups, despite being subjected to peak temperatures of up to 300 °C [START_REF] Alleon | Early entombment within silica minimizes the molecular degradation of microorganisms during advanced diagenesis[END_REF]. However, standard acid maceration procedures applied to ancient silicified sediments have the potential to degrade syngenetic OM through solvent extraction and acid hydrolysis of bitumens and chemically labile organic moieties, respectively. Such degradation, if it occurs, implies that a substantial amount of the initial molecular content depicting the evolution of life, past environmental conditions, and/or thermal maturation may be lost in the process. Further investigations on the effect of standard acid maceration on the chemical structure of ancient thermally altered silicified OM are therefore required.

Our aim in this study was to investigate the effect of the standard acid maceration procedure on the chemical structure of ancient silicified OM. We selected the iconic Lower Devonian Rhynie chert, the earliest preserved terrestrial ecosystem owing to a rapid and complete silicification through siliceous hot-spring deposits [START_REF] Trewin | History of research on the geology and palaeontology of the Rhynie area, Aberdeenshire, Scotland[END_REF][START_REF] Preston | The Rhynie Chert, Scotland, and the search for life on Mars[END_REF]. Despite the exceptional preservation of fossil plants, fungi, insects, and other organisms, the overall organic carbon content of Rhynie chert is very low [START_REF] Summons | Lipid biomarkers for bacterial ecosystems: studies of cultured organisms, hydrothermal environments and ancient sediments[END_REF]. It therefore typically requires acid maceration to access the molecular signature(s). The Rhynie chert therefore constitutes a case study to evaluate the impact of standard acid maceration on the chemical structure of ancient thermally altered silicified OM. To this end, OM was isolated according to two acid maceration procedures: (1) a high-manipulation maceration (HMM), which followed the standard treatments generally applied to geological samples, and (2) a low-manipulation maceration (LMM), which minimized chemical degradation by avoiding organic solvents and HCl. Rock-Eval pyrolysis and Raman spectroscopy were used to assess the chemical structure of the OM isolated by each procedure. Comparison of the so-obtained chemical structures allowed us to estimate the minimum amount of chemical degradation imparted by HMM maceration.

Material and methods

The Rhynie chert

Situated in Aberdeenshire (Scotland), the Rhynie chert is a Konservat-Lagerstätte. It is hosted by the Dryden Flags Formation, which is characterised by a succession of Lower Devonian sedimentary and volcanic rocks [START_REF] Rice | Geological setting of the Early Devonian Rhynie cherts, Aberdeenshire, Scotland: an early terrestrial hot spring system[END_REF]. Pragian-earliest Emsian in age (407.1 ± 2.2 Ma; [START_REF] Mark | 40 Ar/ 39 Ar dating of hydrothermal activity, biota and gold mineralization in the Rhynie hot-spring system, Aberdeenshire, Scotland[END_REF], the Rhynie chert is composed primarily of microcrystalline silica and was deposited as siliceous sinter from subaerial hot springs systems [START_REF] Rice | A Devonian auriferous hot spring system, Rhynie, Scotland[END_REF].

High-and low-manipulation maceration procedures

HMM and LMM procedures were performed on similar portions of the same rock sample. HMM was performed on ~30 g of crushed rock. Solvent extraction was first performed on rock powder using a mixture of DCM and MeOH (2:1; v:v). Carbonates were then removed at room temperature using HCl (37%; reagent grade) to minimise the formation of fluorides during subsequent HF/HCl maceration. Samples were then centrifuged and washed with ultrapure water until reaching neutrality. Concentration of OM was achieved through acid maceration at room temperature in a mixture of HF (40%, reagent grade) and HCl (2:1, v/v; reagent grade). Samples were centrifuged and washed with ultrapure water to reach neutrality. Finally, HCl (37%; reagent grade) at 60 °C (24 h) was used to degrade neoformed fluorides. After an additional step of solvent extraction using a mixture of DCM and MeOH (2:1, v/v), the isolated OM was again centrifuged/washed with ultrapure water until reaching neutrality. Samples were then air-dried at 60 °C after final rinsing in acetone.

Hereafter, OM isolated by the HMM procedure is referred to as OHMM.

LMM was performed by first fragmenting ~30 g of rock samples into ∼3 g rock chips.

Rock chips were cleaned using ultrapure water and a mixture of DCM and MeOH (2:1, v/v) and were then placed directly in a Teflon vessel filled with HF (40%, reagent grade) at room temperature. After 48 h, successive centrifugation and rinsing steps using ultrapure water were performed until reaching neutrality. Samples were then air-dried at 60 °C. Hereafter, OM isolated by the LMM procedure is referred to as OLMM.

Rock-Eval pyrolysis

OHMM and OLMM were analysed using Rock-Eval 6 (Vinci Technologies) following the standard pyrolysis protocol described in [START_REF] Behar | Rock-Eval 6 Technology: Performances and Developments[END_REF]. Performed in a N2 atmosphere, Rock-Eval pyrolysis comprises two steps: an isothermal phase held for 3 minutes followed by a rise in pyrolysis temperature from 300 to 650 °C at a rate of 25 °C/min. After pyrolysis, the residual material was then heated from 300 °C to 850 °C under purified air in an oxidation oven in order to calculate total organic carbon (TOC) value (see [START_REF] Behar | Rock-Eval 6 Technology: Performances and Developments[END_REF] for further details about calculation procedure). Released hydrocarbons (HC) were continuously quantified by a flame ionisation detector (S1 and S2, for the first and second pyrolysis steps, respectively, in mg HC/g of sample) while released CO and CO2 were continuously and simultaneously monitored by infrared detectors during both pyrolysis (S3CO and S3CO2) and combustion (S4CO and S4CO2). Quantification of the amount of effluents led to the determination of the TOC (wt%), of the Hydrogen Index (HI, defined as S2×100/TOC, in mg HC/g of TOC) and of the Oxygen Index (OI; defined as S3×100/TOC, in mg CO2/g of TOC). The pyrolysis temperature associated with the maximum release of hydrocarbons, called "TpkS2", was also determined.

Raman spectroscopy

Raman spectroscopy analysis was performed using a Renishaw inVia microspectrometer equipped with a 532 nm argon laser. The spectrometer was first calibrated using a silicon standard before each session. For each sample analysis, the laser was focused using a Leica microscope with a ×50 objective and the spectra were recorded in the 1000-1900 cm -1 first order spectral window including the defect (D) and graphite (G) peaks (Fig. 1). The laser power at the sample surface was kept below 1 mW to prevent thermal alteration of isolated OM [START_REF] Everall | The effect of laser-induced heating upon the vibrational Raman spectra of graphites and carbon fibres[END_REF]. Spectra acquisition was achieved after two iterations using a time exposure of 40 s. After linear correction of the baseline between 1000 and 1900 cm -1 and spectra normalization, we identified Raman peaks according to the nomenclature defined in [START_REF] Romero-Sarmiento | Evolution of Barnett Shale organic carbon structure and nanostructure with increasing maturation[END_REF].

The heights of the sub-bands D1 (~1365 cm -1 ), D4 (~1285 cm -1 ), D5 (~1445 cm -1 ) and G+D2 (~1600 cm -1 ; Fig. 1) were then determined to compute the ID1/IG+D2, ID4/IG+D2 and ID5/IG+D2, ratios. Slopes αD4 (determined between 1265 and 1300 cm -1 Raman shift) and αD5 (determined between 1415 and 1445 cm -1 Raman shift) were determined to geometrically evaluate the expression of D4 and D5 shoulders on the D band.

Results and discussion

The TOC of OHMM and OLMM samples was measured to determine the efficiency of rock mineralization through HMM and LMM, respectively. The TOC in all OHMM and OLMM samples was low (25.4% and 27.5% respectively; Table 1) suggesting that the preservation and/or neoformation of substantial mineral content had occurred during both procedures. The OI was approximately 15 mg and 59 mg CO2/g TOC in OHMM and OLMM, respectively (Table 1). The higher OI in OLMM is partially explained by the higher level of CO2 emissions during the first isothermal step, showing that they are mostly related to desorbable OM (Fig. 2). Such a result is in line with previous investigations on coals suggesting that acid maceration can lead to a rise in oxygen content resulting possibly from the neoformation of carboxylic groups replacing carboxylate through ion exchange [START_REF] Larsen | Effect of demineralization on the macromolecular structure of coals[END_REF]. Higher levels of CO2 emissions were also recorded for OLMM during the rise in pyrolysis temperature from 300 to 650 °C (Fig. 2). In this pyrolysis temperature range, CO2 emissions appeared to be independent of hydrocarbon emissions suggesting that they do not correspond to the thermal cracking of the macromolecular network.

OHMM and OLMM isolated from the Rhynie chert were characterised by a TpkS2 of 481 °C and 504 °C, respectively (Table 1). In every type of kerogen, these pyrolysis temperatures are commonly assigned to OM that has been subjected to high thermal alteration. Consistent with high TpkS2 values, pyrolysis of OHMM and of OLMM released low amounts of HC as evidenced by HI of 46 HC/g TOC and 68 mg HC/g TOC, respectively.

Together, high TpkS2 and low HI imply that OM from the Rhynie chert falls at the transition between the oil and gas windows registered by TpkS2 ranging between 470 °C and 505 °C in analogous Type III kerogens [START_REF] Espitalié | Use of Tmax as a maturation index for different types of organic matter: comparison with vitrinite reflectance[END_REF]. Hydrocarbon release during Rock-Eval pyrolysis was higher in OLMM than in OHMM (Table 1). The S1 value for OLMM was about 320% higher than for OHMM (Table 1), which suggests that thermolabile OM was more abundant in OLMM than in OHMM. This is consistent with the fact that OLMM was not subjected to organic solvent extractions, which have been shown to dramatically reduce S1 values [START_REF] Delvaux | Comparative Rock-Eval pyrolysis as an improved tool for sedimentary organic matter analysis[END_REF]. Between 300 and 350 °C, no release of hydrocarbons was observed suggesting a virtual absence of residual heavy oil and/or of pyrobitumen in both OHMM and OLMM (Fig. 2; [START_REF] Clementz | Effect of oil and bitumen saturation on source-rock pyrolysis: Geologic notes[END_REF][START_REF] Sanei | Characterization of organic matter fractions in an unconventional tight gas siltstone reservoir[END_REF]. The S2 curves for OHMM and OLMM differ at high pyrolysis temperature (> 500 °C), at which point a broad shoulder is only observed in OLMM (Fig. 2). The occurrence of this thermorecalcitrant OM in OLMM mostly explains its higher HI, in the absence of solvent extractions and/or hydrochloric acid hydrolysis (Table 1). In turn, this suggests that standard acid maceration can lead to a modification of the macromolecular chemical structure in this ancient silicified OM by promoting a loss of hydrocarbonaceous moieties.

Determined on 15 random targets in both OHMM and OLMM, Raman spectra exhibited two broad D and G+D2 bands at around 1365 cm -1 and 1600 cm -1 in each preparation (Fig. 1). The D band presents a complex structure resulting from the presence of several sub-bands including here, the D4, D1, and D5 bands. In addition to the contribution of amorphous carbon, the D band is dominated by the D1 sub-band corresponding to the breathing mode of the sp 2 aromatic ring within polyaromatic clusters and is attributed to defects in these aromatic structures [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF]. The D2 band corresponds to defects in aromatic structure but in contrast to the D1 band, does not include amorphous carbon. A distinct D2 peak (usually centered at about 1620 cm -1 ) is mainly observed in highly mature OM (Kouketsu et al. 2014) and is not observed in OM studied here. Therefore, the G+D2 band is mostly related to the G band corresponding to in-plane C-C bond stretching in polyaromatic layers from thermally altered materials [START_REF] Marshall | Raman spectroscopy as a screening tool for ancient life detection on Mars[END_REF]. As the height of the D1 band increases with temperature, the ID1/IG+D2 ratio is often used to probe the structural order of OM in the course of carbonization (Table 2; [START_REF] Wopenka | Structural characterization of kerogens to granulite-facies graphite: Applicability of Raman microprobe spectroscopy[END_REF][START_REF] Quirico | Maturation grade of coals as revealed by Raman spectroscopy: Progress and problems[END_REF]Kouketsu et al., 2014;[START_REF] Delarue | The Raman-derived carbonization continuum: A tool to select the best preserved molecular structures in Archean kerogens[END_REF]. In this study, OHMM and OLMM are characterized by ID1/IG+D2 ratios of 0.58 ± 0.02 and 0.52 ± 0.02, respectively (Table 2). This result would imply a higher carbon structural order in OHMM compared to OLMM, which is in agreement with previous findings indicating that standard acid maceration can yield a rise in carbon structural order [START_REF] Zhang | Effect of acid treatment on the characteristics and structures of high-sulfur bituminous coal[END_REF].

The D band also exhibits two shoulders corresponding to the D4 (1285 cm -1 ) and D5 (1445 cm -1 ) bands (Fig. 1). The D4 peak is generally assigned to C-H bonds in aliphatics as C-H in aromatics does not seem to directly contribute to the D4 region [START_REF] Ferralis | Rapid, direct and non-destructive assessment of fossil organic matter via microRaman spectroscopy[END_REF].

The D5 band indicates the presence of hydrocarbons trapped within the organic porosity and has been detected in a few oil and gas shale samples [START_REF] Romero-Sarmiento | Evolution of Barnett Shale organic carbon structure and nanostructure with increasing maturation[END_REF][START_REF] Rouzaud | Carbons at the heart of questions on energy and environment: A nanostructural approach[END_REF]. In the current study, αD4 and αD5 slopes in Raman spectra were lower in OLMM than in OHMM, consistent with the more pronounced D4 and D5 shoulders in the Raman line of OLMM (Fig. 1; Table 2). However, the more pronounced D4 and D5 shoulders in OLMM were not associated with higher ID4/IG+D2 and ID5/IG+D2 ratios (Table 2). This could be explained by the higher carbon structural order in OHMM implying a bias in the determination of ID4/IG+D2 and ID5/IG+D2 ratios. The occurrence of more pronounced D4 and D5 shoulders in OLMM indicate that OLMM is indeed enriched in hydrocarbonaceous moieties, due to the C-H bonds in aliphatics and other entrapped hydrocarbons, relative to OHMM. The presence of entrapped hydrocarbons in the macromolecular network of OLMM likely explains the broad shoulder cracking at high temperature (> 500 °C) as revealed by Rock-Eval pyrolysis (Fig. 2). Both Raman spectroscopy and Rock-Eval pyrolysis suggest that standard acid maceration results in a loss of hydrocarbons in OM isolated from the Rhynie chert. As LMM does not involve the organic solvent extraction used in HMM, part of this hydrocarbon loss is likely due to solvent-extractible OM, which is reflected by the respective S1 values. In contrast to OHMM, OLMM did not involve extraction with HCl, which when mixed with HF has been demonstrated to favour hydrolysis leading to a loss of aliphatic CH and/or CH2 functional groups in coal [START_REF] Tekely | Chemical structure changes in coals after low-temperature oxidation and demineralization by acid treatment as revealed by high resolution solid state 13 C NMR[END_REF]. Even though our experimental design did not allow us to test the singular effect of HCl hydrolysis, such an effect can be hypothesized to partly explain the loss in aliphatic moieties with HMM treatment.

Although the Raman spectroscopy results support the notion that hydrocarbons are lost during HMM, it does not directly allow the extent of this loss to be determined. Following the relationship between H/C and HI [START_REF] Espitalié | Méthode rapide de caractérisation des roches mètres, de leur potentiel pétrolier et de leur degré d'évolution[END_REF], we determined that H/C atomic ratios were 0.53 and 0.57 in OHMM and OLMM, respectively (Table 1). Standard acid maceration therefore led to a reduction of at least about 7-8% of the hydrocarbon content of Rhynie OM. We use "at least" because chemical degradation of hydrocarbons occurring during the isolation of OLMM cannot be straightforwardly excluded. To evaluate this point, Raman spectra were also acquired on in situ OM from thin sections of Rhynie chert (Supplementary information). These in situ Raman spectra were acquired with another Raman spectroscope and on polished thin sections, which can modify the apparent carbon structural order by yielding an increase in the D band [START_REF] Ammar | On a reliable structural characterization of polished carbons in meteorites by Raman microspectroscopy[END_REF][START_REF] Maslova | Determination of crystallite size in polished graphitized carbon by Raman spectroscopy[END_REF]. These issues therefore prevent a straightforward comparison between Raman-derived ratios from in situ OM and from OLMM/OHMM. Nonetheless, the αD4 and αD5 parameters are independent of the height of the D band and can be used to track a potential effect of LMM procedure. In situ OM presents a mean αD4 and αD5 of about 31 ± 4 and -24 ± 23, respectively (n = 10), indicating more pronounced D4 and D5 shoulders in the in situ OM compared to OLMM (Supplementary information). This suggests that simple treatment with HF and water also led to a slight loss in aliphatic content. In addition to these hydrocarbon losses related to acid maceration procedures, the estimated 7-8% loss in hydrocarbons can also be considered as minimum estimation because of thermolabile hydrocarbons (S1 parameter), which are not taken into account in the calculation of HI and were more abundant in OLMM than in OHMM.

Thermolabile hydrocarbons are tightly linked to soluble OM [START_REF] Delvaux | Comparative Rock-Eval pyrolysis as an improved tool for sedimentary organic matter analysis[END_REF].

However, this soluble OM in ancient sediments is often ignored because its syngeneity can be difficult to prove due to the potential for post-deposit contamination (Brocks et al., 2003a(Brocks et al., , 2003b;;[START_REF] Derenne | Molecular evidence for life in the 3.5 billion year old Warrawoona chert[END_REF]. However, silicified sediments can be seen as unconventional dual source/reservoir rocks. Indeed, rapid silicification drastically reduces porosity leading to a closed chemical system [START_REF] Boyce | Organic chemical differentiation within fossil plant cell walls detected with X-ray spectromicroscopy[END_REF][START_REF] Ledevin | Silica precipitation triggered by clastic sedimentation in the Archean: New petrographic evidence from cherts of the Kromberg type section, South Africa[END_REF]. In such source/reservoir rocks, migration of bitumens was very restricted implying that bitumens and/or pyrobitumens coexisted in close proximity with residual kerogens [START_REF] Vandenbroucke | Kerogen origin, evolution and structure[END_REF].

Hence, in ancient silicified sediments, bitumens and pyrobitumens can be considered syngenetic.

In contrast to soluble OM, insoluble OM is often considered less prone to late contamination [START_REF] Derenne | Molecular evidence for life in the 3.5 billion year old Warrawoona chert[END_REF]. Involving no chemical treatment or low-manipulation procedures, numerous studies have investigated the molecular structure of OM -at the scale of plant fossils -to assess the effects of fossilization or to depict plant evolution and affinities [START_REF] Ewbank | Chemical characterization of Lower Devonian vascular plants[END_REF][START_REF] Edwards | Flash pyrolysis of the outer cortical tissues in Lower Devonian Psilophyton dawsonii[END_REF][START_REF] Abbott | Molecular characterization of some enigmatic Lower Devonian fossils[END_REF][START_REF] Czaja | Characterization of permineralized kerogen from an Eocene fossil fern[END_REF][START_REF] Quijada | Protosalvinia revisited, new evidence for a land plant affinity[END_REF]. Because a large proportion of the hydrocarbon content of these samples is preserved, these investigations minimized secondary hydrothermal and endolithic contaminations -which can also yield insoluble OM -by studying insoluble OM directly from plant fossils. To avoid contamination and to optimize the study of syngenetic hydrocarbons that can be degraded in the course of acid maceration, the development of in situ molecular micrometre-scale analyses techniques (e.g., time-of-flight secondary ion mass spectrometry and laser micropyrolysis) are of interest. Such techniques have the potential to directly probe the molecular structure of organic fossils independent of the soluble/insoluble operational definition. In addition to insightfully assessing chemical heterogeneity within macrofossils [START_REF] Boyce | Organic chemical differentiation within fossil plant cell walls detected with X-ray spectromicroscopy[END_REF][START_REF] Abbott | Exploring the geochemical distribution of organic carbon in early land plants: a novel approach[END_REF], in situ molecular micrometre-scale analyses also offers the possibility to study the molecular composition of micrometric to submicrometric diffuse particulate OM [START_REF] Stout | Lasers in organic petrology and organic geochemistry, II. In-situ laser micropyrolysis-GCMS of coal macerals[END_REF][START_REF] Greenwood | In situ analytical pyrolysis of coal macerals and solid bitumens by laser micropyrolysis GC-MS[END_REF][START_REF] Yoshioka | Analysis of organic compounds in coal macerals by infrared laser micropyrolysis[END_REF][START_REF] Silva | Botryococcus braunii versus Gloeocapsomorpha prisca : Chemical composition correlation using laser micropyrolysis-gas chromatography/mass spectrometer (LmPy-GCMSMS)[END_REF]. This includes remnants of microorganisms, such as organicwalled microfossils, which are essential organic components of the early geological record.

In the specific context of silicified rocks -where silicification prevented migration of bitumens and favoured formation of pyrobitumen in the close vicinity of the residual kerogen -the use of in situ molecular micrometre-scale analyses offers promise in the investigation of the molecular composition of early life found within Archean silicified sediments. Indeed, the emergence of micrometre-scale in situ analyses have provided compelling evidence supporting an unexpected and wide chemical heterogeneity among Archean putative organicwalled microfossils (Delarue et al., 2017(Delarue et al., , 2020;;[START_REF] Hickman-Lewis | Diverse communities of Bacteria and Archaea flourished in Palaeoarchaean (3.5-3.3 Ga) microbial mats[END_REF]. Overlooked by bulk extraction approaches, this chemical heterogeneity defines a chemically-well preserved end product among the earliest traces of life. Therefore, the precise study of such geological archives requires further development of in situ molecular micrometre-scale analyses Such techniques may pave the way to document biological and abiotic processes and affinities of the earliest putative remnants of life by: (i) focussing on chemically well preserved specimens, (ii) without the use of chemical extraction and maceration techniques, which can degrade aliphatic moieties preserved in sediments subjected to early and prompt silicification.

Conclusions

In this study, we investigated the impact of standard acid maceration on the chemical structure of silicified OM from the Lower Devonian Rhynie chert. Using Rock-Eval pyrolysis and Raman spectroscopy, we assessed the chemical structure of OM isolated by two different physical and chemical manipulation procedures. Results from Rock-Eval pyrolysis and Raman spectroscopy converged to demonstrate that the standard procedure of OM isolation led to a significant and substantial degradation of hydrocarbons. These chemical alterations indicate that a significant amount of molecular content is lost as a result of standard acid maceration procedures, thus preventing a thorough examination of the molecular content of organic remnants in thermally altered silicified sediments. Table 1: Rock-Eval parameters determined in OHMM and OLMM. *H/C atomic ratio was estimated using the relationship between H/C atomic ratio and HI (H/C = 0.0017 × HI + 0.453) published by [START_REF] Espitalié | Méthode rapide de caractérisation des roches mètres, de leur potentiel pétrolier et de leur degré d'évolution[END_REF].

Figure captions

Figure 1 :

 1 Figure 1: Thin section micrographs and scanning electron microscopy images of organic microfossils exhibiting or not a tail-like structure. (a) Micrograph presenting a microfossil exhibiting a tail-like structure embedded in the main siliceous matrix of the studied SPF chert. (b) Micrograph presenting a microfossil exhibiting no tail-like structure. (c-f) SEM images of organic microfossils exhibiting or not a tail-like structure isolated by acid maceration. (c,d) Exceptionally-well preserved leaf-shaped cells presenting a locomotory organelle composed of an attachment point and of a tail; (e-g) Corresponding degraded organic-walled microfossils lacking a tail but (e, f) exhibiting an attachment point. Comparison between microfossils

Figure 2 :

 2 Figure 2: Raman spectra, energy-dispersive X-ray spectra, and nanoscale secondary ion mass spectrometry images. (a) First-order Raman spectra determined on isolated organic microfossils exhibiting a tail-like structure and (b) corresponding energy-dispersive X-ray spectra. Green and purple lines indicate that spectra were acquired on specimens shown in panels c and e, respectively. (c, e) SEM images of organic-walled microfossils investigated by EDXS, Raman spectroscopy and NanoSIMS. Green and purple squares indicate areas probed by NanoSIMS. (d, f) The 12 C 14 N -ion images illustrate the presence of nitrogen. (g) The 31 P -

Figure 3 :

 3 Figure 3: Scanning transmission X-ray microscopy-based X-ray absorption near edge structure characterization. (a) SEM image of the specimen from which a focused ion beam foil has been extracted (green line). (b) SEM image of the focused ion beam foil evidencing the limited thickness of the specimen. The green square indicates the area investigated using

Figure 4 :

 4 Figure 4: Compilation of Appendage Shape Indices determined on extant microorganisms and on studied microfossils. ASI was computed according to the ratio between the width of appendage (archaellum, flagellum, cilium and prostheca) and that of its parent cell (×100). Each width of appendage and of its parent cell was determined graphically based on micrographs and images published in Southam et al. (1990), Poindexter and Staley (1996), Furuno et al. (1997), Wustman et al. (1997), Quintero et al. (1998), Wang et al. (2000), Miller et al. (2004), Bergholtz et al. (2006), Vasilyeva et al. (2006), Wagner et al. (2006), Kanbe et al. (2007), Abraham et al. (2008), Nge et al. (2008), Pyatibratov et al. (2008), Siano et al. (2009), Craveiro et al. (2010), Wang et al. (2011), Abraham and Rohde (2014), Lim et al. (2014), Albers and Jarrell (2015), Deng et al. (2016), Kinosita and Nishizaka (2016), Sugitomo et al. (2016), Curtis (2017), and Leander et al. (2017; (see supplementary information)). ASI determined on archaella, flagella, and cilia are indicated in green while those determined on prosthecae are indicated in pink. The area delimited by dotted lines indicate ASI determined on four tail-like appendages observed

  Phenylobacterium conjunctum sp. nov., and emendation of the genus Phenylobacterium.International Journal ofSystematic and Evolutionary Microbiology 58, 1939-1949. Abraham, W.-R., Rohde, M., 2014. The Family Hyphomonadaceae, in: Rosenberg, E., DeLong, E.F., Lory, S., Stackebrandt, E., Thompson, F. (Eds.), The Prokaryotes. Springer Berlin Heidelberg, Berlin, Heidelberg, pp. 283-299. Albers, S.-V., Jarrell, K.F., 2015. The archaellum: how archaea swim. Frontiers in Microbiology 6. doi:10.3389/fmicb.2015.00023 Alleon, J., Bernard, S., Le Guillou, C., Beyssac, O., Sugitani, K., Robert, F., 2018. Chemical nature of the 3.4 Ga Strelley Pool microfossils. Geochemical Perspectives Letters 37-42. Baludikay, B.K., Storme, J.-Y., François, C., Baudet, D., Javaux, E.J., 2016. A diverse and exquisitely preserved organic-walled microfossil assemblage from the Meso-Neoproterozoic Mbuji-Mayi Supergroup (Democratic Republic of Congo) and implications for Proterozoic biostratigraphy. Precambrian Research 281, 166-184. Beeby, M., Ferreira, J.L., Tripp, P., Albers, S.-V., Mitchell, D.R., 2020. Propulsive nanomachines: the convergent evolution of archaella, flagella, and cilia. FEMS Microbiology Reviews fuaa006. Bergholtz, T., Daugbjerg, N., Moestrup, O., Fernandez-Tejedor, M., 2006. On the identity of karlodinium veneficum and description of karlodinium armiger sp. nov. (dinophyceae), based on light and electron microscopy, nuclear-encoded lsu rdna, and pigment composition. Journal of Phycology 42, 170-193. Bernard, S., Beyssac, O., Benzerara, K., Findling, N., Tzvetkov, G., Brown, G.E., 2010. XANES, Raman and XRD study of anthracene-based cokes and saccharose-based chars submitted to high-temperature pyrolysis. Carbon 48, 2506-2516. Introduction

Fig. 1 .

 1 Fig. 1. (a) Typical Raman spectra measured on OHMM (black line) and OLMM (grey line).

Fig. 2 .

 2 Fig. 2. Emissions of (a) hydrocarbons and (b) CO2 during Rock-Eval pyrolysis of OHMM

Table 2 :

 2 Raman-derived parameters (mean value ± S.D.) determined in OHMM and OLMM (n= 15; Wilcoxon rank test).

		ID1/IG+D2 ID4/IG+D2 ID5/IG+D2	αD4	αD5
	OHMM 0.58 ± 0.02 0.38 ± 0.02 0.43 ± 0.04 359 ± 35 .-288 ± 39
	OLMM	0.52 ± 0.02 0.36 ± 0.02 0.31 ± 0.01 202 ± 37 .-240 ± 28
	p-value	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001

Acknowledgments

We thank D. Troadec (IEMN) for FIB extraction. We also acknowledge The National NanoSIMS Facility at the MNHN, supported by MNHN, CNRS, Région Ile de France, and Ministère de l'Enseignement Supérieur et de la Recherche. Special thanks go to Stefan Stanescu and Sufal Swaraj for their expert support with the HERMES STXM beamline at SOLEIL. The HERMES beamline (SOLEIL) is supported by the CNRS, the CEA, the Region Ile de France, the Departmental Council of Essonne and the Region Centre. This work was supported by the Programme National de Planétologie (PNP) of CNRS/INSU, co-funded by CNES R.T. also acknowledges the UK Science and Technology Facilities Council (grant ST/P005225/1) for financial support. Authors are grateful to the associate editor and the two anonymous reviewers for their constructive comments.

Acknowledgements

Authors are grateful to Andrew Ross (Department of Natural Sciences, National Museum of Scotland, Edinburgh) for providing the Rhynie chert sample. We also thank F. Savignac (ISTeP) for Rock-Eval pyrolysis and O. Belhadj (CRCC) for Raman spectroscopy.

We also acknowledge K. Liitschwager for proofreading the English writing of this manuscript. J.K. Volkman, G.D. Abbott, anonymous reviewers and C.K. Boyce are acknowledged for their constructive comments on the former versions of the manuscript.