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Abstract

Shallow-marine environments are thought to have been pivotal to the spreading, perhaps even the origin, of early life on Earth. The shallow-marine Archean sedimentary record of early life, however, is biased towards carbonates; nearshore siliciclastic environments have not received proportional attention. Here we describe densely laminated, silicified and dolomitized fossil calcareous mounds in tidal-facies sandstones of the Archean Moodies Group (ca. 3.22 Ga) in the Barberton Greenstone Belt, Eswatini. They vary between (1) cm- to dm-scale, isolated, club- to pedestal-shaped, nodular mounds on top of and within the conduits of fluid-escape structures, and (2) mm- to cm-scale, undulatory and wavily laminated structures, interbedded with well-bedded silt- and sandstones. Geochemical indicators of a possible biogenic origin were largely obliterated by local hydrothermal alteration and regional lower-greenschist-facies metamorphism: In situ SIMS δ13Ccarb isotope analyses from several traverses across the best-preserved laminae of a mound and δ34SVCDT values from diagenetic rims of nearby detrital pyrite grains yield ambiguous isotopic evidence about biologic processing; TOC of putative laminae is too low to measure δ13Corg, and Raman spectroscopy of finely dispersed carbonaceous particles and of kerogenous laminae indicate mean maximum metamorphic temperature of ca. 500°C. Textural and regional evidence, however, suggests that the carbonate laminae represent metabolic products of microbial communities that took advantage of sand volcanoes from which nutrient-rich fluids erupted episodically. We base this inference on the habitable depositional setting on a wave- or current-swept photic-zone tidal platform, the stromatolitic morphologies in two and three dimensions, the occurrence of in-situ kerogen, the carbonate mineralogy, and the presence of comparable mound structures elsewhere in the Moodies Group. Although the metabolic strategies utilized by the microorganisms remain unknown, this occurrence places a novel ecologic niche in the Paleoarchean microbial colonization of coastal regions.