3:30 PM - 3:45 PM
[MIS20-06] Emission of dissolved organic carbon from submarine mud volcanoes along the Ryukyu Trench
Keywords:DOC, 14C, FDOM, mud volcano
Submarine mud volcanoes are small mounds formed by the eruption of deep sediments with high pore water pressure, transporting materials such as methane (CH4) generated in the deep subsurface to the seafloor. Dissolved organic carbon (DOC) in the deep ocean is a large carbon reservoir with a long residence time of about 4000-6000 years because of its refractory. Recent studies suggest that in methane seep areas, DOC is produced as a byproduct of methane oxidation by microbes in the surface sediments and may contribute to the marine carbon cycle when released into the ocean. Mud volcanoes are one such methane seep area, and the release of DOC is possible. However, there are few observations of DOC released from mud volcanoes into seawater, and there have been few extensive investigations.
This study aimed to clarify whether DOC is released from the mud volcanoes by analyzing DOC in seawater above the summits of submarine mud volcanoes located in the northern Ryukyu Trench. The mud volcanoes studied are located at 7 sites off Kikai Island, 8 sites off Tanegashima, and 4 sites in the Hyuganada region, for a total of 19 sites. At each mud volcano, seawater was collected from 8 to 12 layers above the summits of mud volcanoes using a CTD rosette sampling system.
The results of the DOC concentration analysis showed that at two sites off Kikai Island, six sites off Tanegashima, and one site in Hyuganada, the DOC concentration just above the mud volcano was 2 to 5 µM higher than the background concentration (~39 µM), suggesting DOC emission from the mud volcanoes. In particular, at the HyMV9-2 mud volcano in Hyuganada, a high concentration peak in CH4 (82 nM) was observed at 20 m above the seafloor, suggesting a large release of CH4. A peak in DOC concentration was also observed at the same depth as the CH4 peak. Fluorescent dissolved organic matter (FDOM) analysis revealed a decrease in semi-labile protein-like fluorescent components at the depths where the DOC concentration was maximal. This suggests that high-concentration DOC was not directly released from mud volcano sediments, but rather, microbes in the seawater may have decomposed protein-like fluorescent components, generating non-fluorescent DOC. Moreover, radiocarbon analysis of DOC showed the presence of high-concentration, aged DOC near the peak DOC concentrations, suggesting that aged carbon released from the mud volcano may be a source of DOC carbon.
Based on these results, it is suggested that microbes in the seawater may be using deep-origin carbon such as CH4, CO2, and old organic matter to generate DOC, leading to localized increases in DOC concentration directly above the mud volcanoes.
This study aimed to clarify whether DOC is released from the mud volcanoes by analyzing DOC in seawater above the summits of submarine mud volcanoes located in the northern Ryukyu Trench. The mud volcanoes studied are located at 7 sites off Kikai Island, 8 sites off Tanegashima, and 4 sites in the Hyuganada region, for a total of 19 sites. At each mud volcano, seawater was collected from 8 to 12 layers above the summits of mud volcanoes using a CTD rosette sampling system.
The results of the DOC concentration analysis showed that at two sites off Kikai Island, six sites off Tanegashima, and one site in Hyuganada, the DOC concentration just above the mud volcano was 2 to 5 µM higher than the background concentration (~39 µM), suggesting DOC emission from the mud volcanoes. In particular, at the HyMV9-2 mud volcano in Hyuganada, a high concentration peak in CH4 (82 nM) was observed at 20 m above the seafloor, suggesting a large release of CH4. A peak in DOC concentration was also observed at the same depth as the CH4 peak. Fluorescent dissolved organic matter (FDOM) analysis revealed a decrease in semi-labile protein-like fluorescent components at the depths where the DOC concentration was maximal. This suggests that high-concentration DOC was not directly released from mud volcano sediments, but rather, microbes in the seawater may have decomposed protein-like fluorescent components, generating non-fluorescent DOC. Moreover, radiocarbon analysis of DOC showed the presence of high-concentration, aged DOC near the peak DOC concentrations, suggesting that aged carbon released from the mud volcano may be a source of DOC carbon.
Based on these results, it is suggested that microbes in the seawater may be using deep-origin carbon such as CH4, CO2, and old organic matter to generate DOC, leading to localized increases in DOC concentration directly above the mud volcanoes.