Evidence for microbial life in synsedimentary cavities from 2.75 Ga terrestrial environments

Fluviolacustrine sediments of the 2.75 Ga Hardey Formation (Fortescue Group, Australia) preserve pendant columnar structures with stromatolitic lamination within synsedimentary cavities. The millimeter-sized, finger-like columns strongly resemble microbialites from modern basaltic caves and indicate the likely presence of microbial biofilms. The ancient microbial columns are preserved by chert and locally occur as reworked clasts, indicating a near-depositional age for the structures. Sulfur isotopic analysis of pyrite in the columns and adjacent carbonaceous matrix yields delta S-34(CDT) (CDT-Canyon Diablo troilite) values between -8.5 parts per thousand and +19 parts per thousand, showing significant fractionation characteristic of biological cycling of sulfur. Organic matter in cavity ceilings and shale matrix has delta C-13(PDB) (PDB-Peedee belemnite) values between -55 parts per thousand and -43 parts per thousand, suggesting the presence of methanotrophs. Our results suggest that 2.75 Ga terrestrial environments supported a microbial ecosystem, including microbes that inhabited synsedimentary hollows, extending the fossil record of coelobionts by similar to 1.5 b.y. Subsurface cavities represent a new habitable microenvironment for early life on Earth, and an analogue for ancient life on Mars.