oa Geosphere-Biosphere Interactions in Geothermally Heated Subseafloor Sediments

Several expeditions have explored the limits of life beneath the seafloor and found microbial life at temperatures of ∼120 °C ( Heuer et al., 2020 ; Beulig et al., 2022 ). Questions were raised: Can we expect to find life >120 °C? What energy sources may support these hottest members of the deep biosphere? At elevated temperature and pressure, sedimentary organic matter (OM) is altered and may provide substrates for (hyper)thermophilic microbes. But rates and mechanisms of the reactions producing these metabolic substrates remain unclear.

Low-molecular weight substrates at high temperatures were examined conducting hydrous pyrolysis experiments with deeply buried Nankai Trough sediments from IODP Site C0023 off Japan. In these experiments, temperatures ranged from 150-250 °C at a pressure of 25 MPa.

We measured several C-compound concentrations to examine the rates and yields of these products of OM breakdown. Additionally, we analyzed the isotopic compositions of the carbon compounds to construct Rayleigh fraction models for potential formation pathways. DIC is of particular interest here, as it is distinctly enriched in 13C with d13CDIC values exceeding 10 ‰. The combination of thermodynamic calculations and Rayleigh fractionation models result in the development of a model for carbon cycling under specific pressure and temperature conditions.