EVIDENCE FOR AN EXTENSIVE NON-MARINE BIOSPHERE ~1100 MA AGO

Evidence for Archaean and Proterozoic photosynthetic microbial life is well-known in the marine realm both from stromatolites and carbonates. In contrast, there are only a few reports of Archaean life in the continental realm, in an Archaean pond deposit and in a paleosol. There are no convincing reports of life on land for the Paleoproterozoic and evidence for life on land in the Mesoproterozoic has been limited to rare putative microfossils in alluvial deposits, rare cryptoalgal stromatolites preserved between clasts in alluvial fan deposits, and probable microbially induced sedimentary structures. Here we present sedimentological and carbon isotopic evidence for an extensive terrestrial biosphere from recently identified ~1100 Ma old paleosols and associated sediments from the Keweenawan Rift (USA), at a time when atmospheric CO₂ levels were decreasing to below 10 times pre-industrial levels. Organic matter is preserved throughout a 65m section of intrabasaltic sediments in paleosols and microbial mats, and as detrital carbon in laminated fluvial sediments. δ¹³C_org values range from -29.6 to -25.5‰, suggesting oxygenic photosynthesis. These results establish the presence of extensive microbial life on land ~1100 Ma ago, which could represent an important and overlooked carbon sink in the Mesoproterozoic.