Permeability, Pore Throat Size, Meteoric Influx, and Biomass Control the Depth‐Distribution of Microbial Biosignatures in Sedimentary Basin Fluids

Abstract

The few studies of modern or ancient subsurface microbial communities beyond ∼100 m depth limit our view into the vast and complex deep terrestrial biosphere. In sedimentary systems, growing isotopic and molecular data sets of fluids, co‐produced with hydrocarbons, provide an unprecedented opportunity to evaluate the depth‐distribution of biosignatures and relate them to microbial biomass and their hydrogeologic context. Here, we compiled molecular and isotopic data sets of CO ₂ and CH ₄ with oil density (API gravity), microbial cell counts, and stable isotopes of formation waters for sedimentary basins globally to investigate the depth‐distribution of biosignatures in produced fluids related to cell abundance, hydrologic properties, and burial/thermal histories of host rocks. The most robust signatures of microbial activity in CO ₂ , CH ₄ , and oil are apparent in the upper ∼1 km depth of sedimentary basins, co‐occurring with relatively high biomass (>10 ⁴ cells/mL), permeability (>10 ⁻¹⁶  m ² ), and meteoric circulation. Small matrix pore throat sizes of sedimentary rocks beyond ∼1 km depth and in shallower confining units likely limit microbial habitation and transport, while in situ temperatures are cool enough for microbial activity up to several km depth. Our results refine and extend deeper previous estimates of the “biogenic floor,” above which microbial methanogenesis and oil biodegradation are identifiable in bulk isotopic biosignatures. We find significant microbial activity is required to leave biological signatures in the rock record, even where cells are present. We suggest that hydrologic factors (i.e., lower pore throat size and permeability) are inhibiting microbial life below ∼1 km in terrestrial sedimentary systems.