Recent interest in Mg-rich silicate formation stems from their role as valuable paleoclimatic indicators in fluvio-lacustrine environments and their insights into metal geochemical cycling. Traditionally, Mg-silicate genesis in lacustrine contexts is linked to alkaline or saline conditions in closed, evaporitic basins. However, the discovery of interparticle amorphous kerolite-like Mg-silicates in the sediments of Lake Clifton, a currently hypersaline coastal lagoon in Western Australia with circumneutral pH and moderate alkalinity, challenges existing models. In this study, petrographic, hydrochemical, and microbial genomic data from different Lake Clifton sub-environments (episodically submerged and subaerial settings) and substrates (pustular microbial mats and non-lithifying microbial sediments) were integrated with geochemical modelling to quantify the mechanisms underlying the formation of Mg-silicates and aragonite peloids as lake shoreline sediments. Geochemical modelling suggests that neither evaporation-driven alkalinity fluctuations nor mixing of lake water with groundwater can solely explain the kerolite-like/carbonate association observed in lakebed sediments. Kerolite-like phases nucleate in association with twisted microbial extracellular polymeric substances (EPS) and organic-rich bacterial remains; this, combined with the identification of diatom- and cyanobacteria-powered photosynthesis, putative anoxygenic photosynthesis, and sulphate-reducing metabolisms, suggests an intimate link between biologically induced processes and the co-precipitation of aragonite peloids and interparticle kerolite-like phases in the lake. Moreover, the contribution of dead diatom frustule dissolution towards kerolite-like authigenesis was geochemically simulated, revealing that the precipitation of observable amounts of kerolite-like at pH values measured in Lake Clifton waters would prevent the formation of aragonite, questioning the feasibility of a scenario dominated by large inputs of dissolved biogenic silica. Discovery of kerolite-like Mg-silicates in microbial-bearing sediments of a hypersaline coastal lagoon prompts a holistic re-evaluation of the environmental and microbiological factors influencing Mg-silicate-carbonate co-precipitation in lacustrine-peri-marine settings. Studying modern Mg-silicate-bearing lacustrine sediments offer the opportunity to better understand the early diagenetic biotic- abiotic processes that may have had limited petrographic preservation potential in ancient saline lake deposits.

The article from this special issue was previously published in Journal of Chemometrics, Volume 35, Issue 10, 2021. For completeness we are including the title page of the article below. The full text of the article can be read in Issue 35:10 on Wiley Online Library: https://doi.org/10.1002/cem.3370

The Late Archean atmosphere is thought to have been largely reducing as suggested by the preservation of large Mass-Independent fractionations of S isotopes (MIF-S) in sedimentary sulphides and sulphates. However, there is a growing body of evidence for transient oxygen accumulation in the Neoarchean oceans, especially from shallow marine environments. Here Delta 2.52 Ga carbonates in the upper Campbellrand Group of South Africa are investigated from the shallow marine carbonate shelf Kogelbeen Formation to the deeper water basinal Gamohaan Formation. Previous studies of these rocks have found large positive Delta 33S signatures, but with differing interpretations of the controls on, and pathways for, the preservation of Delta 33S. Here bulk S isotope measurements from sedimentary sulphides and carbonate associated sulphate (CAS) are reported, combined with in-situ S isotope measurements by Secondary Ion Mass Spectrometry (SIMS) from well-preserved pyritised microbial mat microtextures of the Gamohaan Formation. Large positive Delta 33S signatures are observed in sulphide grains of the Gamohaan and Kogelbeen formations (max Delta 3SSIMS= 12.6%) confirming the isotopic effects of photolytic reactions in an anoxic atmosphere. Small, early pyrites in fenestrate microbialites record an atmospheric signature (average d34SSIMS= 7.6%, Delta 33S(SIMS)= 8.5%, Delta 36S(SIMS)=-7.6%) reflecting the preservation of atmospherically derived Snparticles in the microbial carbonates. Contorted microbial mat layers, also from the carbonate slope, exhibit large mass-dependent fractionations with d34SSIMSof up to 30.6% plotting off the Archean reference array, suggesting oxygenated seawater on the shelf-slope. Pyrite concretions in organic rich carbonates from deeper water facies show S isotope values (average d34SSIMS= 3.1%, Delta 33SSIMS=- 0.2%, Delta 36SSIMS= 2.5%) indicative of microbial sulphate reduction (MSR). The CAS data show positive Delta 33S with values of up to 8.49% and positive d34S values that are more similar to sulphide data measured from each sample than estimated late-Archean seawater sulphate. This is interpreted to record the early oxidation of sulphides with positive Delta 33S by shallow oxygenated waters in the upper diagenetic zone that moved through the sediment and was then incorporated into the platform carbonates. In this way, we hypothesise that positive Delta 33S can be inherited by the Neoarchean diagenetic sulphate reservoir and preserved in carbonates. (c) 2022 Elsevier B.V. All rights reserved.

Extraction of useful information from unstructured, large and complex mass spectrometric signals is a challenge in many application fields of mass spectrometry. Therefore, new data analysis approaches are required to help uncover the complexity of such signals. In this contribution, we examined the chemical composition of the 1.88 Ga Gunflint chert using the newly developed high mass resolution laser ionization mass spectrometer (fs-LIMS-GT). We report results on the following: 1) mass-spectrometric multi-element imaging of the Gunflint chert sample; and 2) identification of multiple chemical entities from spatial mass spectrometric data utilizing nonlinear dimensionality reduction and spectral similarity networks. The analysis of 40′000 mass spectra reveals the presence of chemical heterogeneity (seven minor compounds) and two large clusters of spectra registered from the organic material and inorganic host mineral. Our results show the utility of fs-LIMS imaging in combination with manifold learning methods in studying chemically diverse samples.

The investigation of chemical composition on planetary bodies without significant sample processing is of importance for nearly every mission aimed at robotic exploration. Moreover, it is a necessary tool to achieve the longstanding goal of finding evidence of life beyond Earth, for example, possibly preserved microbial remains within martian sediments. Our Laser Ablation Ionization Mass Spectrometer (LIMS) is a compact time-of-flight mass spectrometer intended to investigate the elemental, isotope, and molecular composition of a wide range of solid samples, including e.g., low bulk density organic remains in microfossils. Here, we present an overview of the instrument and collected chemical spectrometric data at the micrometer level from a Precambrian chert sample (1.88 Ga Gunflint Formation, Ontario, Canada), which is considered to be a martian analogue. Data were collected from two distinct zones-a silicified host area and a carbon-bearing microfossil assemblage zone. We performed these measurements using an ultrafast pulsed laser system (pulse width of ∼180 fs) with multiple wavelengths (infrared [IR]-775 nm, ultraviolet [UV]-387 nm, UV-258 nm) and using a pulsed high voltage on the mass spectrometer to reveal small organic signals. We investigated (1) the chemical composition of the sample and (2) the different laser wavelengths' performance to provide chemical depth profiles in silicified media. Our key findings are as follows: (1) microfossils from the Gunflint chert reveal a distinct chemical composition compared with the host mineralogy (we report the identification of 24 elements in the microfossils); (2) detection of the pristine composition of microfossils and co-occurring fine chemistry (rare earth elements) requires utilization of the depth profiling measurement protocol; and (3) our results show that, for analysis of heterogeneous material from siliciclastic deposits, siliceous sinters, and cherts, the most suitable wavelength for laser ablation/Ionization is UV-258 nm.

The search for a fossil record of Earth's deep biosphere, partly motivated by potential analogies with subsurface habitats on Mars, has uncovered numerous assemblages of inorganic microfilaments and tubules inside ancient pores and fractures. Although these enigmatic objects are morphologically similar to mineralized microorganisms (and some contain organic carbon), they also resemble some abiotic structures. Palaeobiologists have responded to this ambiguity by evaluating problematic filaments against checklists of “biogenicity criteria”. Here, we describe material that tests the limits of this approach. We sampled Jurassic calcite veins formed through subseafloor serpentinization, a water–rock reaction that can fuel the deep biosphere and is known to have occurred widely on Mars. At two localities ~4 km apart, veins contained curving, branched microfilaments composed of Mg‐silicate and Fe‐oxide minerals. Using a wide range of analytical techniques including synchrotron X‐ray microtomography and scanning transmission electron microscopy, we show that these features meet many published criteria for biogenicity and are comparable to fossilized cryptoendolithic fungi or bacteria. However, we argue that abiotic processes driven by serpentinization could account for the same set of lifelike features, and report a chemical garden experiment that supports this view. These filaments are, therefore, most objectively described as dubiofossils, a designation we here defend from criticism and recommend over alternative approaches, but which nevertheless signifies an impasse. Similar impasses can be anticipated in the future exploration of subsurface palaeo‐habitats on Earth and Mars. To avoid them, further studies are required in biomimetic geochemical self‐organization, microbial taphonomy and micro‐analytical techniques, with a focus on subsurface habitats.

The similar to 1878 Ma Gunflint Formation contains some of the foremost examples of Palaeoproterozoic life but questions remain regarding the potential metabolic and taphonomic pathways within the microfossil assemblage. Here we report on hematite-mineralised examples of the most abundant Gunflint organisms (Gunflintia and Huroniospora), using correlative light-, Raman- and electron-microscopy to document subtle variations in iron oxide mineralisation styles. Data come from two localities where similar stromatolitic facies are found, namely Schreiber Channel and Mink Mountain, Ontario. At Schreiber Channel, rare hematitic microfossils are found at the margins of small pods of pyritised microorganisms, with some individual cell or sheath walls retaining pyrite cores and hematite rims. Microfossil wall hematite grains form anhedral masses, contain large numbers of quartz inclusions, plus a small number of barium-rich and calcium-rich nano-crystals. Hematitic, pyritic and carbonaceous microfossil preservation styles co-occur within a single petrographic thin section. A sub-set of iron-mineralised fossils at Schreiber have thinner walls, comprising tabular hematite grains plus Fe-Al-silicates and traces of organic material. At Mink Mountain, microfossil walls are ubiquitously preserved as hematite but well-preserved examples occur only in isolated zones of particularly dark brown/purple hematite. Here, hematite grains show a cuspate to anhedral habit with embayment and inclusions of quartz plus rare Ti-rich nano-crystals; nano-particles of iron oxide and rare organic material occur as bridges between some grains. The morphological similarity of microfossil assemblages having walls now preserved with different chemical compositions (carbon, pyrite, Fe-Al-silicate and hematite), and across widely-spaced stromatolitic localities, indicates that iron oxide mineralisation was a taphonomic rather than primary metabolic process. The pyrite and Fe-Al-silicates at Schreiber are interpreted as the earliest stages of iron mineralisation, forming under reducing diagenetic conditions in the presence of excess organic material. Partial replacement of these phases by hematite followed later when oxygen bearing fluids penetrated limited zones within the sediments. At Mink Mountain, we infer that zones of well-preserved microfossils were initially mineralised as pyrite, followed by the complete oxidation of pyrite plus all surrounding organic material to hematite. This study demonstrates the importance of localised micro-environmental conditions on taphonomic processes and, in turn, the role that taphonomy can play in the modification of the morphology of fossilised organisms. This has implications both for the study of early life on Earth and for the assessment of any putative microfossils returned from Mars.

In this contribution, we present results of non-linear dimensionality reduction and classification of the fs laser ablation ionization mass spectrometry (LIMS) imaging dataset acquired from the Precambrian Gunflint chert (1.88 Ga) using a miniature time-of-flight mass spectrometer developed for in situ space applications. We discuss the data generation, processing, and analysis pipeline for the classification of the recorded fs-LIMS mass spectra. Further, we define topological biosignatures identified for Precambrian Gunflint microfossils by projecting the recorded fs-LIMS intensity space into low dimensions. Two distinct subtypes of microfossil-related spectra, a layer of organic contamination and inorganic quartz matrix were identified using the fs-LIMS data. The topological analysis applied to the fs-LIMS data allows to gain additional knowledge from large datasets, formulate hypotheses and quickly generate insights from spectral data. Our contribution illustrates the utility of applying spatially resolved mass spectrometry in combination with topology-based analytics in detecting signatures of early (primitive) life. Our results indicate that fs-LIMS, in combination with topological methods, provides a powerful analytical framework and could be applied to the study of other complex mineralogical samples.

Sediments of the Torridonian sequence of the Northwest Scottish Highlands contain a wide array of micro fossils, documenting life in a non-marine setting a billion years ago (1 Ga).(1-4) Phosphate nodules from the Diabaig Formation at Loch Torridon preserve microorganisms with cellular-level fidelity,(5,6) allowing for partial reconstruction of the developmental stages of a new organism, Bicellum brasieri gen. et sp. nov. The mature form of Bicellum consists of a solid, spherical ball of tightly packed cells (a stereoblast) of isodiametric cells enclosed in a monolayer of elongated, sausage-shaped cells. However, two populations of naked stereo blasts show mixed cell shapes, which we infer to indicate incipient development of elongated cells that were migrating to the periphery of the cell mass. These simple morphogenetic movements could be explained by differential cell-cell adhesion.(7,8) In fact, the basic morphology of Bicellum is topologically similar to that of experimentally produced cell masses that were shown to spontaneously segregate into two distinct domains based on differential cadherin-based cell adhesion.(9) The lack of rigid cell walls in the stereoblast renders an algal affinity for Bicellum unlikely: its overall morphology is more consistent with a holozoan origin. Unicellular holozoans are known today to form multicellular stages within complex life cycles,10(-13) so the occurrence of such simple levels of transient multicellularity seen here is consistent with a holozoan affinity. Regardless of precise phylogenetic placement, these fossils demonstrate simple cell differentiation and morphogenic processes that are similar to those seen in some metazoans today.

In this contribution, we investigated the chemical composition of Precambrian microfossils from the Gunflint chert (1.88 Ga) using a miniature laser ablation ionization mass spectrometer (LIMS) developed for in situ space applications. Spatially resolved mass spectrometric imaging (MSI) and depth profiling resulted in the acquisition of 68,500 mass spectra. Using single mass unit spectral decomposition and multivariate data analysis techniques, we identified the location of aggregations of microfossils and surrounding inorganic host mineral. Our results show that microfossils have unique chemical compositions that can be distinguished from the inorganic chert with high fidelity. Chemical depth profiling results also show that with LIMS microprobe data, it is possible to identify chemical differences between individual microfossils, thereby providing new insights about nature of early life. Analysis of LIMS spectra acquired from the individual microfossils reveals complex mineralization, which can reflect the metabolic diversity of the Gunflint microbiome. An intensity-based machine learning model trained on LIMS Gunflint data might be applied for the future investigations of putative microfossils from silicified matrices, where morphological integrity of investigated structures is lost, and potentially in the investigation of rocks acquired from the Martian surface.

This study reports in-situ sulfur isotope analyses (S-32, S-33, S-34 and S-36) of pyrite in strongly sulfidized stromatolites from the -3.48 billion-year-old Dresser Formation, Pilbara Craton, Australia. These data shed light on sulfur reservoirs and sulfide precipitation processes and provide clues for the contribution of sulfur-cycling microbes to sulfidization. Sulfur isotope signatures derived from mass dependent fractionation (MDF; monitored by delta S-34) and mass independent fractionation (MIF; here Delta S-33 and Delta S-36) of pyrite in stromatolites, and of microscopic pyrite within associated barite, allow for the identification of distinctive sulfur sources: i) magmatic-hydrothermal sulfide (H2S) with delta S-34 and Delta S-33 similar to 0%; ii) magmatic-hydrothermal sulfate (SO42-) with a MDF signature (MDF-SO42-; delta S-34 similar to 10 parts per thousand and Delta S-33 similar to 0 parts per thousand; iii) photochemically-derived sulfate with a MIF signature (MIF-SO4; delta S-34 similar to -6 parts per thousand and Delta S-33 similar to 3.0 parts per thousand); iv) photochemically-derived elemental sulfur (S-0) with delta S-34 << 0 and Delta S-33 >> 0 parts per thousand. The sulfur isotope data suggest that sulfidization was largely driven by reduction of intermixed MDF-SO(4)(2-)and MIF-SO42- (bulk signature of delta S-34 similar to 5 parts per thousand and Delta S-33 similar to -1.4 parts per thousand), and dilution of produced H2S (delta S-34 similar to -12 parts per thousand and Delta S-33 similar to -1.4 parts per thousand) by native H2S in magmatic-hydrothermal fluids. The delta S-34 shifts (up to similar to 17 parts per thousand) generated by sulfate reduction are consistent with both thermochemical reactions and influence of sulfate-cycling microbes, the latter which may have facilitated rapid pyrite precipitation and preservation of microbial remains that are entombed within the petrogenetically earliest pyrite generation of stromatolites. Collectively, our data are consistent with ancient stromatolite growth in proximity to shallow marine hydrothermal vents, where hydrothermal fluids contributed to sulfidization that may have been further influenced by sulfur-cycling microbes.

The siliciclastic ~1 Ga-old strata of the Torridon Group, Scotland, contain some of the most exquisitely preserved three-dimensional organic-walled microfossils (OWMs) of the Precambrian. A very diverse microfossil assemblage is hosted in a dominantly phosphatic and clay mineral matrix, within the Diabaig and the Cailleach Head (CH) Formations. In this study, we report on several microfossil taxa within the CH Formation (Leiosphaeridia minutissima, Leiosphaeridia crassa, Synsphaeridium spp. and Myxococcoides spp.) that include populations of cells containing an optically transparent and highly refringent mineral, here identified using electron microscopy as anatase (TiO2 ). Most anatase crystals occur entirely within individual cells, surrounded by unbroken carbonaceous walls. Rarely, an anatase crystal may protrude outside a cell, interpreted to correspond to zones where the cell wall had broken down prior to anatase precipitation. Where an anatase crystal entombs an organic intracellular inclusion (ICI), the ICI is large and well preserved. These combined observations indicate that the intracellular anatase is an authigenic sedimentary phase, making this the first report of in situ precipitated anatase intimately associated with microfossils. The ability of anatase to preserve relatively large volumes of intracellular and cell wall organic material in these cells suggests that the crystallisation of anatase entombed cellular contents particularly quickly, soon after the death of the cell. This is consistent with the strong affinity of Ti for organic material, the low solubility of TiO2 , and reports of Ti occurring in living organisms. With the data currently available, we propose a mineralisation pathway for anatase involving Ti complexation with organic ligands within specific cells, leading to localised post-mortem anatase nucleation inside these cells as the complexes broke down. Further overgrowth of the anatase crystals was likely fuelled by very early diagenetic mobilisation of Ti that had been bound to more labile organic material nearby in the sediments.

Microtextures of titanite (CaTiSiO5 ) in exceptionally preserved Archean pillow lavas have been proposed as the earliest examples of microbial ichnofossils. An origin from microbial tunneling of seafloor volcanic glass that is subsequently chloritized and the tunnels infilled by titanite has been argued to record the activities of subseafloor microbes. We investigate the evidence in pillow lavas of the 3.35 Ga Euro Basalt from the Pilbara Craton, Western Australia, to evaluate the biogenicity of the microtextures. We employ a combination of light microscopy and chlorite mineral chemical analysis by EPMA (electron probe micro-analysis) to document the environment of formation and analyze their ultrastructure using FIB-TEM (focussed ion beam combined with transmission electron microscopy) to investigate their mode of growth. Petrographic study of the original and re-collected material identified an expanded range of titanite morphotypes along with early anatase growth forming chains and aggregates of coalesced crystallites in a sub-greenschist facies assemblage. High-sensitivity mapping of FIB lamellae cut across the microtextures confirm that they are discontinuous chains of coalesced crystallites that are highly variable in cross section and contain abundant chlorite inclusions, excluding an origin from the mineralization of previously hollow microtunnels. Comparison of chlorite mineral compositions to DSDP/IODP data reveals that the Euro Basalt chlorites are similar to recent seafloor chlorites. We advance an abiotic origin for the Euro Basalt microtextures formed by spontaneous nucleation and growth of titanite and/anatase during seafloor-hydrothermal metamorphism. Our findings reveal that the Euro Basalt microtextures are not comparable to microbial ichnofossils from the recent oceanic crust, and we question the evidence for life in these Archean lavas. The metamorphic reactions that give rise to the growth of the Euro Basalt microtextures could be commonplace in Archean pillow lavas and need to be excluded when seeking traces of life in the subseafloor on the early Earth.

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion‐year‐old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro‐mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro‐mineralization within the sulfidized stromatolites generally forms submicron‐scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro‐spherulites show thin outer shells. Within stromatolites, barite micro‐spherulites are intimately associated with petrographically earliest dolomite and nano‐porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr‐rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro‐spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near‐identical morphology and micro‐texture is known from zones of high bio‐productivity under low‐temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro‐spherulitic barite in the organic matter‐bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well‐preserved, and very ancient microbialites.

Stromatolites of the similar to 3.48 billion year old Dresser Formation (Pilbara Craton, Western Australia) provide some of the oldest convincing evidence of life on Earth. Here, we augment previous evidence with a detailed investigation of the concentrations and distributions of various transition metals (Cr, Mn, Co, Ni, Cu, Zn, Mo, Se, Ag, Sn, Au, Hg, and Pb) and metalloids (As, Sb, and Te) in unweathered samples of strongly sulfidized stromatolites from drill cores. High resolution elemental mapping and in situ compositional analysis of sulfides (pyrite and sphalerite) show that these sedimentary and hydrothermally sourced elements are strongly concentrated in texturally distinctive, nano-porous pyrite enriched in autochthonous organic matter, which forms the major, petrogenetically earliest component of wrinkly laminated and digitate growth fabrics within the stromatolites. Repeated cyclic alternations of various transition metals and metalloids (most importantly Ni and Zn), plus the presence of disconformities and overgrowth relationships between wrinkly stromatolite laminae, suggest that these element accumulations were primarily established by depositional processes during continuous stromatolite formation. Because transition metals and metalloids generally have strong affinities for organic matter, and can play active roles in biochemical processes, we interpret these element accumulations in the Dresser Formation stromatolites to be the result of binding to organic matter of living microbial communities and/or dead biomass, and perhaps also microbial utilization. Collectively, our results show that the precise characterization of transition metal-metalloid concentrations and distributions can unveil element enrichment patterns suggestive of biological activity, even in some of Earth's oldest stromatolites.

Stromatolites of the similar to 3.5 billion-year-old Dresser Formation (Pilbara Craton, Western Australia) are considered to be some of Earth's earliest convincing evidence of life. However, uniquely biogenic interpretations based on surface outcrops are precluded by weathering, which has altered primary mineralogy and inhibited the preservation of microbial remains. Here, we report on exceptionally preserved, strongly sulfidized stromatolites obtained by diamond drilling from below the weathering profile. These stromatolites lie within undeformed hydrothermal-sedimentary strata and show textural features that are indicative of biogenic origins, including upward-broadening and/or upward-branching digitate forms, wavy to wrinkly laminae, and finely laminated columns that show a thickening of laminae over flexure crests. High-resolution textural, mineralogical, and chemical analysis reveals that the stromatolites are dominated by petrographically earliest, nano-porous pyrite that contains thermally mature, N-bearing organic matter (OM). This nano-porous pyrite is consistent with a formation via sulfidization of an originally OM-dominated matrix. Evidence for its relationship with microbial communities are entombed OM strands and filaments, whose microtexture and chemistry are consistent with an origin as mineralized biofilm remains, and carbon isotope data of extracted OM (delta C-13(OM) = -29.6 parts per thousand +/- 0.3 parts per thousand VPDB [Vienna Peedee belemnite]), which lie within the range of biological matter. Collectively, our findings provide exceptional evidence for the biogenicity of some of Earth's oldest stromatolites through preservation of OM, including microbial remains, by sulfidization.

Journal Article Correlative Microscopy of Diverse Filamentous Microfossils from 850 Ma Rocks Get access David Wacey, David Wacey Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia Search for other works by this author on: Oxford Academic Google Scholar Martin Saunders, Martin Saunders Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia Corresponding author: Martin.Saunders@uwa.edu.au Search for other works by this author on: Oxford Academic Google Scholar Andrew McPherson, Andrew McPherson Geoscience Australia, Canberra, Australia Search for other works by this author on: Oxford Academic Google Scholar Sarah Gain, Sarah Gain Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia Search for other works by this author on: Oxford Academic Google Scholar Eva Sirantoine, Eva Sirantoine Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia Search for other works by this author on: Oxford Academic Google Scholar Kate Eiloart Kate Eiloart Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, Australia Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 2466–2467, https://doi.org/10.1017/S1431927619013060 Published: 01 August 2019

Filamentous microfossils belonging to Cephalophytarion from the 850 Ma Bitter Springs Group have previously been used as key analogues in support of a biological interpretation for filamentous objects from the 3460 Ma Apex chert. Here we provide a new perspective on this interpretation by combining Raman data with correlative electron microscopy data from both Cephalophytarion and Apex specimens. We show that, when analysed at high spatial resolution, the Apex filaments bear no morphological resemblance to the younger Bitter Springs microfossils. Cephalophytarion filaments are shown to be cylindrical, comprising chains of box-like cells of approximately constant dimensions with lateral kerogenous walls and transverse kerogenous septa. They exhibit taphonomic shrinkage and folding, possess fine cylindrical sheaths and are permineralized by sub-micrometric quartz grains. They fulfil all established biogenicity criteria for trichomic microfossils. In contrast, Apex filaments do not possess lateral cell walls, are not cylindrical in nature, and vary considerably in diameter along their length. Their kerogenous carbon does not have a cell-like distribution and their chemistry is consistent with an origin as exfoliated phyllosilicate grains. This work demonstrates the importance of high-resolution data when interpreting the microstructure, and origins, of putative Precambrian microfossils.

On rocky planets such as Earth and Mars the serpentinization of olivine in ultramafic crust produces hydrogen that can act as a potential energy source for life. Direct evidence of fluid-rock interaction on Mars comes from iddingsite alteration veins found in martian meteorites. In the Yamato 000593 meteorite, putative biosignatures have been reported from altered olivines in the form of microtextures and associated organic material that have been compared to tubular bioalteration textures found in terrestrial sub-seafloor volcanic rocks. Here, we use a suite of correlative, high-sensitivity, in situ chemical, and morphological analyses to characterize and re-evaluate these microalteration textures in Yamato 000593, a clinopyroxenite from the shallow subsurface of Mars. We show that the altered olivine crystals have angular and micro-brecciated margins and are also highly strained due to impact-induced fracturing. The shape of the olivine microalteration textures is in no way comparable to microtunnels of inferred biological origin found in terrestrial volcanic glasses and dunites, and rather we argue that the Yamato 000593 microtextures are abiotic in origin. Vein filling iddingsite extends into the olivine microalteration textures and contains amorphous organic carbon occurring as bands and sub-spherical concentrations <300 nm across. We propose that a martian impact event produced the micro-brecciated olivine crystal margins that reacted with subsurface hydrothermal fluids to form iddingsite containing organic carbon derived from abiotic sources. These new data have implications for how we might seek potential biosignatures in ultramafic rocks and impact craters on both Mars and Earth.