11:30 AM - 11:45 AM
[MIS19-04] Release of dissolved organic carbon from submarine mud volcanoes off Tanegashima Island, Japan
Keywords:Submarine mud volcanoes, Dissolved organic carbon, Dissolved methane
Submarine mud volcanoes (MVs) are cone-like structures formed when a high pore pressure sediments at deep subsurface ascend as mud diapirs and erupt onto the seafloor. They are an emission source of deep subsurface materials such as methane. Dissolved organic carbon (DOC) in deep ocean is thought to be derived largely from marine organic matter produced in the upper photic ocean. However, recent studies also suggest that DOC is produced as an additional product associated with anoxic methane oxidation in the surface sediments at methane seeps, and contributes to the ocean carbon cycle. Since MVs are one of the methane seeps, it is possible that they also release DOC. However, the release of DOC from MVs to water column has not been observed. In this study, we investigate whether DOC is released from MVs.
We surveyed MVs off Tanegashima Island, and obtained seawater and sediment samples at the mud volcanoes, MV#08 (30°45'N, 131°36'E), MV#10 (30°23'N, 131°24'E) and MV#15 (31°03'N, 131°4'E). We analyzed DOC concentrations in those. We also analyzed dissolved CH4 concentrations and the carbon isotopic compositions of CH4 in seawater samples.
The DOC concentrations in seawater ware highest near the seafloor and decreased toward shallower depths at the two sampling sites in MV#08 (M8-1 [just above the summit] and M8-2 [about 320 m southwest of the summit]), indicating the DOC release from the seafloor. Assuming steady-state eddy diffusion, the vertical DOC fluxes for M8-1 and M8-2 estimated from the DOC concentrations gradient were 42×103 μmol m-2 d-1 and 20×103 μmol m-2 d-1, respectively. Assuming that DOC seeped with uniform flux from a circular area (0.32 km2) centered on the summit (M8-1) and having the distance to M8-2 as its radius, the DOC release was estimated to be 1.2×102 kg C d-1. The value is comparable to that in the methane seep off Vancouver (5.6×102 kg C d-1), which has the highest DOC flux from marine sediments. On the other hand, no obvious increase in DOC concentrations was observed in MV#10 and MV#15.
The DOC concentrations in pore water were lowest near the seafloor and increased with depth for all MVs studied, indicating that DOC diffuses from deep mud volcano sediments to the seafloor. Assuming steady-state molecular diffusion in the sediment, the DOC fluxes for MV#08, MV#10, and MV#15 estimated from the concentration gradient of DOC were 36 µmol m-2 d-1, 182 µmol m-2 d-1, and 21 µmol m-2 d-1, respectively. These fluxes are comparable to those from ordinary marine sediments reported for other areas.
The concentrations of dissolved CH4 in seawater showed no change near the seafloor at MV#08, while MV#10 and MV#15 showed concentrations maxima near the seafloor, suggesting the little release of CH4 from MV#08 and a substantial CH4 release from MV#10 and MV#15.
The results of DOC analysis of pore water samples obtained from MVs showed the DOC diffusion toward to seafloor in all MVs, and DOC flux from MV#10 was the highest flux of three. In contrast, seawater samples showed DOC release only from MV#08. These differences imply that the DOC released from the sediments of MV#10 and MV#15 may be labile, and immediately degraded in the surface sediments or in bottom water, and thus not detected in seawater sample. On the other hand, the CH4 release was not observed from MV#08, but was observed from MV#10 and MV#15, indicating that the CH4 release does not related to the DOC release.
In conclusion, we revealed the release of DOC for the first time from the mud volcano to the ocean based on the analysis of seawater samples obtained from water column overlying the mud volcano. The DOC fluxes estimated from sediment samples of each mud volcano were not corresponding to those estimated from seawater samples, suggesting the difference of degradability of DOC in each mud volcano.
We surveyed MVs off Tanegashima Island, and obtained seawater and sediment samples at the mud volcanoes, MV#08 (30°45'N, 131°36'E), MV#10 (30°23'N, 131°24'E) and MV#15 (31°03'N, 131°4'E). We analyzed DOC concentrations in those. We also analyzed dissolved CH4 concentrations and the carbon isotopic compositions of CH4 in seawater samples.
The DOC concentrations in seawater ware highest near the seafloor and decreased toward shallower depths at the two sampling sites in MV#08 (M8-1 [just above the summit] and M8-2 [about 320 m southwest of the summit]), indicating the DOC release from the seafloor. Assuming steady-state eddy diffusion, the vertical DOC fluxes for M8-1 and M8-2 estimated from the DOC concentrations gradient were 42×103 μmol m-2 d-1 and 20×103 μmol m-2 d-1, respectively. Assuming that DOC seeped with uniform flux from a circular area (0.32 km2) centered on the summit (M8-1) and having the distance to M8-2 as its radius, the DOC release was estimated to be 1.2×102 kg C d-1. The value is comparable to that in the methane seep off Vancouver (5.6×102 kg C d-1), which has the highest DOC flux from marine sediments. On the other hand, no obvious increase in DOC concentrations was observed in MV#10 and MV#15.
The DOC concentrations in pore water were lowest near the seafloor and increased with depth for all MVs studied, indicating that DOC diffuses from deep mud volcano sediments to the seafloor. Assuming steady-state molecular diffusion in the sediment, the DOC fluxes for MV#08, MV#10, and MV#15 estimated from the concentration gradient of DOC were 36 µmol m-2 d-1, 182 µmol m-2 d-1, and 21 µmol m-2 d-1, respectively. These fluxes are comparable to those from ordinary marine sediments reported for other areas.
The concentrations of dissolved CH4 in seawater showed no change near the seafloor at MV#08, while MV#10 and MV#15 showed concentrations maxima near the seafloor, suggesting the little release of CH4 from MV#08 and a substantial CH4 release from MV#10 and MV#15.
The results of DOC analysis of pore water samples obtained from MVs showed the DOC diffusion toward to seafloor in all MVs, and DOC flux from MV#10 was the highest flux of three. In contrast, seawater samples showed DOC release only from MV#08. These differences imply that the DOC released from the sediments of MV#10 and MV#15 may be labile, and immediately degraded in the surface sediments or in bottom water, and thus not detected in seawater sample. On the other hand, the CH4 release was not observed from MV#08, but was observed from MV#10 and MV#15, indicating that the CH4 release does not related to the DOC release.
In conclusion, we revealed the release of DOC for the first time from the mud volcano to the ocean based on the analysis of seawater samples obtained from water column overlying the mud volcano. The DOC fluxes estimated from sediment samples of each mud volcano were not corresponding to those estimated from seawater samples, suggesting the difference of degradability of DOC in each mud volcano.