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Ooids are accretionary grains commonly reported from turbulent, shallow‐water environments. They have long been associated with microbially dominated ecosystems and often occur in close proximity to, or embedded within, stromatolites, yet have historically been thought to form solely through physicochemical processes. Numerous studies have revealed both constructive and destructive roles for microbes colonizing the surfaces of modern calcitic and aragonitic ooids, but there has been little evidence for the operation of these processes during the Archean and Proterozoic, when both ooids and microbially dominated ecosystems were more widespread. Recently described carbonate ooids from the 2.9 Ga Pongola Supergroup, South Africa, include well‐preserved examples composed of diagenetic dolomite interpreted to have formed from a high‐Mg‐calcite precursor. Spatial distributions of organic matter and elements associated with metabolic activity (N, S, and P) were interpreted as evidence for a biologically induced origin. Here, we describe exceptionally well‐preserved ooids composed of calcite, collected from Earth's oldest known carbonate lake system, the ~2.72 Ga Meentheena Member (Tumbiana Formation), Fortescue Group, Western Australia. We used optical microscopy, Raman spectroscopy, XRD, SEM‐EDS, LA‐ICP‐MS, EA‐IRMS, and a novel micro‐XRF instrument to investigate an oolite shoal deposited between stromatolites that preserve abundant evidence for microbial activity. We report an extremely fine, radial‐concentric, calcitic microfabric that is similar to the primary and early diagenetic fabrics of calcitic ooids reported from modern temperate lakes. Early diagenetic silica has trapped isotopically light and thermally mature organic matter. The close association of organic matter with mineral phases and microfabrics related to primary and early diagenetic processes suggest incorporation of organic matter occurred during accretion, likely due to the presence of microbial biofilms. We conclude that the oldest known calcitic ooids were likely formed through processes similar to those that mediate the accretion of ooids in similar environments today, including formation within a microbial biosphere. [ABSTRACT FROM AUTHOR]

The large isotopic fractionation of carbon associated with enzymatic carbon assimilation allows evidence for life’s antiquity, and potentially the early operation of several extant metabolic pathways, to be derived from the stable carbon isotope record of sedimentary rocks. Earth’s organic carbon isotope record extends to the Late Eoarchean-Early Paleoarchean: the age of the oldest known sedimentary rocks. However, complementary inorganic carbon reservoirs are poorly represented in the oldest units, and commonly reported bulk organic carbon isotope measurements do not capture the micro-scale isotopic heterogeneities that are increasingly reported from younger rocks. Here, we investigated the isotopic composition of the oldest paired occurrences of sedimentary carbonate and organic matter, which are preserved as dolomite and kerogen within textural biosignatures of the ∼3.43 Ga Strelley Pool Formation. We targeted least-altered carbonate phases in situ using microsampling techniques guided by non-destructive elemental mapping. Organic carbon isotope values were measured by spatially-resolved bulk analyses, and in situ using secondary ion mass spectrometry to target microscale domains of organic material trapped within inorganic carbon matrixes. Total observed fractionation of 13 C ranges from −29 to −45‰. Our data are consistent with studies of younger Archean rocks that host biogenic stromatolites and organic–inorganic carbon pairs showing greater fractionation than expected for Rubisco fixation alone. We conclude that organic matter was fixed and/or remobilized by at least one metabolism in addition to the CBB cycle, possibly by the Wood-Ljungdahl pathway or methanogenesis-methanotrophy, in a shallow-water marine environment during the Paleoarchean. [ABSTRACT FROM AUTHOR]

Highly 13 C-depleted organic matter reported from Neoarchean formations worldwide has led to the concept of a “Global Age of Methanotrophy” (GAM) in the Neoarchean. A temporal peak in the GAM is suggested by values as low as −61‰ that are reported from rocks deposited at ∼2.7 Ga. Here we analyse previously reported values, report new field observations and isotope data, and re-evaluate the depositional settings of several units of this age. We find a statistically significant lowering of δ 13 C org values in units of Neoarchean age compared to values reported from other Precambrian intervals, both older and younger, confirming the existence of the GAM. However, we also report a correlation between very low δ 13 C org values and lacustrine units deposited during the Neoarchean. We hypothesize methanogenesis may have been promoted in some Neoarchean lakes due to local deficiencies of oxidants, specifically Fe 3+ and SO 4 , relative to the Archean oceans. Lower availability of these oxidants could have limited higher energy yield metabolisms such as sulfate and iron reduction and provided an ecological niche for methanogens, ultimately resulting in the local burial of biomass highly depleted in 13 C. We conclude that the exceptionally low δ 13 C org values reported from formations deposited at ∼2.7 Ga could represent the prevalence of closed basin depositional environments preserved in the limited outcrop available, rather than a peak in the global age of methanotrophy at this time. [ABSTRACT FROM AUTHOR]

Abstract: Rare earth elements and selected trace elements were measured in 48 samples of carbonate and chert from stromatolites and associated facies in the 3.45billion year old Strelley Pool Formation, Pilbara Craton, Western Australia. The samples show coherent REE+Y patterns that vary systematically with sedimentary facies. Chert samples from bedded cherts beneath the Strelley Pool Formation and from the upper bedded chert members in the formation show REE+Y patterns consistent with originating by precipitation from hydrothermal and mixed marine-hydrothermal fluids. In contrast, carbonates and cherts from the stromatolitic reef member share the essential shale-normalized characteristics of other Archean marine precipitates (LREE depletion, positive La and Gd anomalies, absence of a negative Ce anomaly and a strongly superchondritic Y/Ho ratio). The close correspondence between REE+Y signatures and independent sedimentary facies interpretations is viewed as strong evidence for the primary nature of REE+Y patterns. They can thus be used as a proxy for the fluids from which sediments precipitated. Mixing hyperbolae can be constructed that reproduce the chemistry of cherts and carbonates by mixing of hydrothermal and marine fluid endmembers throughout the entire vertical succession from beneath the Strelley Pool Formation to the uppermost cherts. The mixing hyperbolae provide semi-quantitative confirmation that the trace element compositions across the suite of cherts represent different mixtures of ambient seawater and hydrothermal fluids. Our results indicate that the Earth''s oldest supracrustal carbonates and associated cherts record important aspects of the REE geochemistry of the waters in which they precipitated, and provide valuable information on possible habitats of some of Earth''s earliest biota. [Copyright &y& Elsevier]

• Claimed ca. 3.7 Ga Isua stromatolites are instead granoblastic quartz + dolomite boudins. • Absence of internal convex upward laminae precludes classification as stromatolites. • 3-D structural analysis reveals the boudins form ridges parallel to regional deformation. • Parallel micro- and macro-structural and chemical analyses are essential to assess biogenicity of any proposed bio-structures in ancient, deformed terranes. The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. The structures in question are found within a suite of multiply deformed greenschist- to amphibolite-facies metamorphic rocks. To assess their premise as primary sedimentary features – as opposed to products of strain localization in layered, variably ductile rocks – we report new field mapping at the appropriate scale and resolution from the original discovery Sites reported by Nutman et al. (2016). Our new map was used to guide micro- and macro-structural investigations and comprehensive geochemical sampling. Here, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they are not deformed conical stromatolites. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron backscatter diffraction (EBSD) patterns collected on fresh surfaces cut perpendicular and parallel to the ridges attest to the lack of any residual sedimentary laminae (e.g., compositional layering) within these structures' cores. Internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceals granoblastic quartz ± dolomite cored boudins that sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. The morphology of boudins reflects viscosity contrasts of the different ductile layers during deformation. Therefore, these features are not of sedimentary origin. Furthermore, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results show that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites. [ABSTRACT FROM AUTHOR]