[M-IS22] Interactions of Geosphere-Hydrosphere-Biosphere and Deep-sea Methane Environments

[MIS22-P07] The sulfur isotope fingerprint of the active methane seepage events in the Pleistocene sediments collected from the northeastern Japan Sea

*Yuki Ota¹, Yusuke Miyajima¹, Satoshi Furota¹, Eri Ikeuchi¹, Kenta Asahina¹, Masanori Kaneko¹, Koki Tokuhiro², Shinsuke Aoki³, Kiyofumi Suzuki¹, Mikio Satoh¹ (1.National Institute of Advanced Industrial Science and Technology, 2.Marine Ecology Research Institute, 3.Faculty of Agriculture, Kagawa University)

Keywords:Japan Sea, Marine sediment, Methane hydrate, Anaerobic oxidation of methane, Sulfur isotope

Tracing the active paleo-methane seepage events and understanding the associated environmental changes at the seafloor in methane hydrate-bearing sites in geological history play an important role in the assessment of the environmental impacts of the potential methane flux variations induced by future environmental changes. A sediment core was recovered from site RC2101A in Sakata Knoll offshore Sakata City during the PS21 cruise of R/V Poseidon-1 in August 2021, and two drilling cores were obtained from sites C8007A in Joetsu Knoll and C8008A in Umitaka Spur offshore Joetsu City during Expedition 809 of D/V Chikyu in September 2022. This study reports age models based on tephrochronology, diatom biostratigraphy, and radiocarbon age determinations, and geochemical composition of sediments including sulfur stable isotope ratios of acid insoluble sulfur (δ³⁴S) for these drilling cores collected from methane hydrate-bearing areas in the northeastern Japan Sea. Sediment ages at the core bottoms were calculated by linear interpolation of the dated sediment layers as ~750 kyr before present (BP) at ~37 m below seafloor (bsf), ~450 kyr BP at ~98 mbsf, and ~260 kyr BP at ~87 mbsf at sites RC2101A, C8007A, and C8008A, respectively. The sedimentation rates at sites RC2101A, C8007A, and C8008A were between 0.8–20.8, 18.8–137.3, and 10.8–81.8 cm/kyr, respectively. The δ³⁴S values were relatively stable and negative (–47‰ to –21‰) in the entire core at site RC2101A, which are consistent with the values of sulfide and secondary sulfur compounds in marine sediments formed by organoclastic sulfate reduction (OSR) (–50‰ to –10‰, Liu et al., 2022). In contrast, the δ³⁴S values display large variabilities at sites C8007A and C8008A, with values ranging from –35‰ to +14‰ and from –47‰ to +10‰, respectively, and are characterized by positive anomalies during ~70, 250, and 360 kyr BP. When sulfate ion is rapidly consumed by not only OSR but also sulfate-driven anaerobic oxidation of methane (AOM), such as the marine sediments with strong upwelling methane flux, the sulfide δ³⁴S values are increased relative to their values in OSR and tend to be close to the original δ³⁴S values of seawater sulfate (+21.24‰, Tostevin et al., 2014). Therefore, ³⁴S enrichment has been observed in sulfur compounds in methane seepage areas (δ³⁴S = +10‰ to +40‰) (e.g., Liu et al., 2022). The ³⁴S enrichments in cores at sites C8007A and C8008A may indicate the occurrences of sulfate-driven AOM due to the active methane seepage events during these periods off Joetsu. In future studies, we try to better understand the effects of methane seepage on the environments at Sakata Knoll, Joetsu Knoll, and Umitaka Spur by detailed investigation of other biogeochemical data in the cores. This study was conducted as a part of the methane hydrate research project funded by METI (the Ministry of Economy, Trade and Industry, Japan).