High permeability, meteoric influx, and biomass promote microbial signatures in upper km of sedimentary basins

The few studies of modern or ancient microbial communities beyond ~100 m depth in the crust limit our view into the complex deep terrestrial biosphere. In sedimentary systems, isotopic signatures of by-products of anaerobic microbial activity, such as microbial methane and carbon dioxide, and molecular signatures of oil biodegradation are more widespread. Yet, it’s unclear how these biosignatures in subsurface fluids relate to microbial biomass and their hydrogeologic context. We expect that hydrological parameters, such as meteoric influx, permeability, porosity and pore throat size, as well as the burial and thermal history of sedimentary environments, are key to microbial habitability. Here, we compiled molecular and isotopic datasets of CO2 and CH4 with oil density (API gravity), microbial cell counts, and stable water isotopes of formation waters for sedimentary basins across the United States and Canada to investigate the depth and spatial distribution of biosignatures in produced fluids related to cell abundance, hydrologic properties, and burial/thermal histories. Most robust signatures of microbial activity in CO2, CH4, and oil occur in the upper ~1km of sedimentary basins, co-occurring with relatively high biomass (>104 cells/mL), permeability (>10-16 m2), and meteoric circulation. Our results provide insights into the distribution as well as geologic and hydrologic controls on microbial activity in the terrestrial sedimentary deep biosphere which may have been imprinted in the rock record. The identified regions with more robust biosignatures may warrant more comprehensive fluid, rock and microbial studies to investigate these potential unique extant ecosystems.