Stromatolitic microorganisms in and on top of fluid-escape structures of the 3.2 Ga Moodies Group

The global record of early life is only poorly preserved, but has an ark in the 3.2 Ga Moodies Group of the Barberton Greenstone Belt, South Africa and Eswatini. It preserves silicified photosynthetic and sulfate-reducing metabolic signatures in sandstone-dominated, terrestrial to shallow-marine strata. Large fluid-escape structures are common in thick-bedded kerogen-laminated sandstones of (sub-)tidal facies. We document and interpret silicified, massive and laminated carbonate aggregates and beds, both of likely microbial origin, within and on top of these syndepositional and early diagenetic features, not previously described from Archean shallow-water fluid-escape structures. We distinguish three morphotypes: (1) cm-scale, silicified, bulbous aggregates aligned within fluid-escape conduits; (2) up to dm-scale, dolomitized, finely-laminated conical and tabular mounds on top of the conduits; (3) cm-scale, isolated, silicified, finely-laminated, stromatolitic aggregates. In-situ SIMS isotope analyses from traverses across the best-preserved laminae of a mound yielded δ13C(PDB) values relative to a dolomite standard of -2.5 to 0.5‰, and -3.5 to 4.0‰ for δ34S(VCDT) from diagenetic rims of nearby detrital pyrite grains, respectively. Values and ranges are consistent with a near-complete hydrothermal alteration. Facies context, location within and on top of the fluid-escape structures, stromatolitic morphology, and carbonate composition suggest that robust microbial communities utilized one or several carbon-based redox pathways in this siliciclastic tidal setting. Methanogens, methanotrophs, sulfate reducers and photosynthesizers may have colonized these tidal-zone sand volcanoes at 3.2 Ga, collectively forming a diverse microbial community.