DISTINGUISHING ABIOTIC FROM BIOLOGICAL CARBON SOURCES IN THE DEEP BIOSPHERE

Carbon and sulfur compounds in Earth’s interior are hypothesized to serve as vital energy sources and substrates that sustain microbial life in the largely unexplored deep subsurface biosphere. Deep continental fracture systems and the subseafloor may represent a significant volume of Earth’s habitable space, however little is known regarding the sources of carbon compounds needed to sustain these aqueous ecosystems. Compound specific stable isotopes can be used in tandem with chemical and thermodynamic analysis to elucidate the origin of carbon compounds, which can be generated by microbial metabolic activity, thermogenic processes in which biomass is degraded, and purely abiotic processes occurring without interaction with life.

Volatile fatty acids (VFAs) may support chemosynthetic communities in terrestrial and deep-sea hot springs, and make up the dominant form of dissolved organic carbon in ancient fracture waters sequestered for >1 Ga within the Precambrian Shield in Canada. Due to their high dissolved H₂ contents, near-neutral pH, and relatively low temperatures, all thermodynamic drive is toward the synthesis of organic species in these fluids, and is on par with, if not greater than submarine hydrothermal vent systems. Chemical and δ¹³C analysis of VFAs in a spectrum of fracture waters reveal that elevated levels of acetate at Birchtree and Thompson mine may be generated by microbial fermentation. In contrast, notably higher acetate abundances and formate species at Kidd Creek mine suggest possible abiogenic production of VFAs. VFAs represent a potential key substrate for life in these systems, and their abiotic generation could carry implications for the origin of life on Earth.