Microbial activity at different types of mud volcanos in Japan: an insight from methane clumped isotope signatures

Recently developed methane clumped isotope analytical technique, which focuses on the abundances of paired mass-18 isotopologues of methane (12CH2D2 and 13CH3D), provides new insights in understanding the methane cycle in nature1-3. However, the discrepancies in methane clumped isotope signatures of microbial methanogenesis between laboratorial cultivation experiment and natural samples, as well as the post-alteration controlled by aerobic and anerobic methanotrophy, have not been fully understood4-6. Investigating the relationships between methane clumped isotope signatures and the in-situ microbial activities in natural ecosystems can benefit future application of this novel approach in constraining the microbial contributions in atmospheric methane.

In this study, we focus on gas samples collected from different types of mud volcanos located in central and northern Japan, where methane was considered to originate from the thermal-cracking of organics and/or the mantle source. Secondary microbial methane associated with biodegradation of hydrocarbons has been suggested in some mud volcanos which may contribute to the methane concentration and its δ13C value7. By combination of multiple approaches, such as bulk carbon and hydrogen isotopes of hydrocarbons, position-specific isotope analysis of propane, microbial density and diversity, we will discuss (1) the sources of hydrocarbons; (2) to which extent microbial activity alters the clumped isotope signatures of methane in these mud volcanoes.

References: [1] Stolper et al., 2014, Geochim. Cosmochim. Acta.; [2] Stolper et al., 2014, Science; [3] Young et al., 2017, Geochim. Cosmochim. Acta.; [4] Ash et al., 2019, Geochem. Perspect. Lett.; [5] Ono et al., 2021, Geochim. Cosmochim. Acta; [6] Zhang et al., 2021, Geochim. Cosmochim. Acta; [7] Gilbert et al., 2022, Microorganisms.