[M-IS20] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS20-P06] Potential methane production rates of methanogen cultures and deep subseafloor sediments assessed with ¹³C stable isotope tracers

*Yusuke Miyajima¹, Hideyoshi Yoshioka¹, Taiki Katayama¹, Toshinori Imai², Than Tin Aung² (1.Research Institute for Geo-Resources and Environment, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.Japan Organization for Metals and Energy Security (JOGMEC))

Keywords:microbial methanogenesis, methanogenic archaea, sediment incubation, stable isotope tracer

Natural gases and gas hydrates in subseafloor sediments are primarily sourced from microbial methanogenesis (Milkov, 2005). The activity of methanogenic archaea (methanogens) is thus an essential factor in evaluating the formation process of fuel resources. Sediment incubation with ¹⁴C radiotracers is a common method for assessing methanogenic activity. Due to a trace background level of ¹⁴C, the ¹⁴C tracer method can detect extremely low methane production rates (e.g., Yoshioka et al., 2009). However, the limited availability of facilities capable of handling radioactive ¹⁴C tracers impedes the reuse of ¹⁴C-amended samples for subsequent analyses. This study applies ¹³C stable isotope tracers to assess the methanogenic activity (methane production rates). Pure cultures of methanogens were supplemented with ¹³C-labeled substrates (bicarbonate, acetate, or methanol) to evaluate the applicability of the ¹³C tracer method to the three major methanogenic pathways. The carbon isotope composition (δ¹³C value) of methane increased continuously during the incubation, enabling the calculation of methane production rates. We then used the ¹³C-labeled substrates to incubate subseafloor core sediments retrieved from Hyuganada Sea, western Nankai Trough, using the drilling vessel Chikyu. Potential methane production rates of the sediments were estimated to be 10⁻² to 10¹ pmol/cm³/d, mostly below the detection limit. We observed a significant correlation between methane production rates estimated using ¹³C-bicarbonate and those using ¹⁴C-bicarbonate, indicating that ¹³C tracers effectively evaluate the relative methanogenic activity among samples. Samples amended with ¹³C tracers are not subject to legal regulations and can be broadly utilized for additional microbiological and geochemical analyses. This study was conducted as part of activities of the MH21-S R&D consortium (MH21-S) as supported by the Ministry of Economy, Trade and Industry (METI), Japan.