First occurrence of stromatolites in the Early Archean Moodies Group (3.22 Ga), Barberton Greenstone Belt, South Africa

• Applicant: Professor Dr. Christoph Heubeck
• Subject Area: Palaeontology
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 447538422

The biosphere has attained near-complete control over the Earth’s surface systems but it remains poorly understood which mechanisms and dominant metabolisms were responsible, at what time and rate this process occurred, and which consequences for Earth’s atmo-, hydro- and geosphere resulted. This is in part due to the restricted and incomplete rock record of life’s earliest traces. We here present preliminary data from shallow-water siliciclastics of the Paleoarchean Moodies Group of the Barberton Greenstone Belt (BGB), one of the earliest well-preserved microbial ecosystems. Kerogenous laminations from tidal-facies sandstones of this unit, strongly resembling modern cyanobacterial mats, are reasonably well studied, but stromatolites have been thought to be absent from this unit, for unknown reasons. In 2019, we discovered calcareous, stromatolitic carbonate mounds cm to dm in size, some of which top unambiguous fluid-escape structures in biolaminated sandstones. How did they form? Aside from abiogenic precipitation of calcite due to pressure and temperature reduction of escaping fluids, mineral precipitation could also reflect the metabolic utilization of gases from the degradation of kerogenous microbial mats, e.g., methanotrophy (using waste O2 from nearby photosynthesizing microbial mats as oxidant), conventional oxygenic photosynthesis exploiting a concentrated CO2 flux, respiration, fermentation, or a combination of these. Euhedral overgrowth rims of detrital pyrite grains interbedded with the carbonate crusts may indicate the former presence of an additional S-based metabolism, likely microbial sulfate reduction (MSR). We suggest to test these hypotheses by measuring δ13C in selected SIMS traverses across the stromatolitic laminations and by measuring δ34S in pyrite grains, by classifying stromatolite morphologies in outcrop and hand sample, and by geologic mapping of relevant sedimentary facies. If SIMS results supported the biogeochemical origin of the carbonate mounds, then they would highlight a novel strategy in the early history of life, possibly document one of the earliest examples of microbial symbiosis in Earth’s history, and potentially even add to the known inventory of microbial communities along Paleoarchean shorelines.

• DFG Programme: Research Grants