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

[MIS20-09] Impacts of sedimentary structures on methane seep-related geochemical properties in the Kujukuri coastal area, eastern Japan

*Yuki Ota¹, Masahiro Suzumura¹, Ayumi Tsukasaki¹, Atsushi Suzuki¹, Kyoko Yamaoka¹, Masayuki Nagao¹, Satoru Takahashi¹, Akira Iguchi¹, Miyuki Nishijima¹, Kodai Gibu¹, Toru Tamura^1,2, Koji Seike^1,2, Shinsuke Aoki³, Haruka NAKANO⁴, Hiroshi Ishida⁵, Takeshi Yoshida⁶ (1.National Institute of Advanced Industrial Science and Technology (AIST), 2.Graduate School of Frontier Sciences, The University of Tokyo, 3.Faculty of Agriculture, Kagawa University, 4.Research Institute for Applied Mechanics, Kyushu University, 5.The Marine Ecology Research Institute, 6.Research Institute of Environmental Geology, Chiba Prefectural Environmental Research Center)

Keywords:Anaerobic oxidation of methane, Marine ecosystems, Geochemical cycles, Kujukuri coastal plain

Microbially driven anaerobic oxidation of methane (AOM) coupled with sulfate reduction constrains modern methane emissions from the ocean to atmosphere. However, our understanding of the environmental parameters that influence the AOM-related geochemical process and microbial community compositions remains limited. Here, to assess the AOM activity to differences in sedimentary structures at a methane seep site SP in coastal waters off Kujukuri, eastern Japan (Fig. 1), we collected sediment cores LC1 and LC3 composed by only of sand and core LC2 composed by sandy sediments containing a mud layer (Fig. 2). For these cores, mud fraction content, sedimentary total inorganic carbon (TIC) and total sulfur (TS) contents, δ¹³C of bulk carbonate minerals (δ¹³C_TIC), total sulfur δ³⁴S, and 16S rRNA gene sequences were analyzed. In the cores LC1 and LC3, the high abundance of 16S rRNA genes of ANME were observed in the almost entire core, indicating the occurrence of AOM in these sediments. However, remarkable anomalies of geochemical data indicating a noticeable accumlation of authigenic sulfide compounds and carbonate mineral associated with AOM were absent in these cores. These results suggest that sedimentary conditions with high permeability and sedimentation rate may have caused active diffusion of AOM-generated sulfide and bicarbonate ions in these cores. In contrast, in cores composed of sandy sediments with a mud layer, TIC and TS contents were high, δ¹³C_TIC values were extremely negative (approximately -20‰), and δ³⁴S values were high (approximately +15‰) just above the mud layer. These geochemical anomalies suggested that authigenic carbonate minerals and sulfide compounds had accumulated in the sediment immediately overlying the mud layer as a result of high AOM activity over a long period. This activity could occur there because the low-permeability mud layer has trapped the upward methane flow, leading to a high methane concentration around this layer over a long period, and sulfate ions in interstitial water have easily diffused downward to that level through the overlying sandy sediment. In future studies, the comparison between spatial distributions of muddy sediment and calculated seafloor methane emission offshore of Kujukuri will clarify more details on the influence of different sediment structure on the methane cycling in the coastal environment.