5:15 PM - 6:45 PM
[MIS22-P12] Emission of dissolved organic carbon and nutrients from submarine mud volcanoes.
Keywords:submarine mud volcanoes, dissolved organic carbon, nutrients
Submarine mud volcanoes (SMVs) 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 methane. Recent studies have indicated that SMVs release materials other than methane, originating from deep subseafloor. For example, recent studies suggest that DOC is produced as a byproduct associated with anoxic methane oxidation in the surface sediments at methane seeps, and contributes to the ocean carbon cycle. Since SMVs are one of the methane seeps, it is possible that they also release DOC to the ocean. However, there have been only a few reports for DOC release from SMVs to the ocean. Furthermore, recent studies indicated that SMVs release microbial cells in subseafloor with methane into the ocean.
In this study, we investigated whether materials other than methane, originating from deep subseafloor, are released from SMVs. During KH-23-4 cruise in August 2023 by R/V Hakuho-Maru, we surveyed SMVs located from Ryukyu Trench to the western end of the Nankai Trough, off Kikaijima Island Tanagashima Island, and Hyuga-nada. We obtained seawater samples at the mud volcanoes, from a total of 14 volcanoes across 33 sites—7 off Kikai Island, 3 off Tanegashima Island, and 4 in the Hyuga-nada—using a CTD rosette sampling system. At some sites, sea water was sampled 1–2 m above the seafloor using a sampler attached to the multiple corer. We analyzed DOC and nutrients (NO2, NO3, NH4, SiO2, PO4) concentrations in those water samples.
The NH4 concentrations in seawater increased approximately 1.5–5 times near the seafloor compared to the background concentration of 1–2 μM at five mud volcanoes off Kikai Island (KSK2, KSK3, KSK4, KSK5, KNK1), and two in the Hyuga-nada (HyMV24, HyMV9). The NO2 concentrations increased from 0 µM to 0.46 µM from 50 m to 10 m above the summit at MV3 (31º02.6N, 131º40.90′E, 1200 m). Since NH4 and NO2 are produced in reductive environment, the increase in their concentrations above the mud volcanoes summits suggests that these nutrients, produced in subseafloor, are released into the ocean.
Although measurements of DOC concentration are not yet completed, so far, an increase in the DOC concentration above the summits of mud volcanoes (2-3 μM higher than the background levels) were observed, indicating the release of DOC from these mud volcanoes, KNK1 (28º25.37′N, 130º11.55′E, 976 m) and KNK2 (28º33.48′N, 130º3.78′E, 868 m) off Kikai Island.
These observations suggest that SMVs have released not only methane but also DOC and nutrients produced in the subseafloor.
In this study, we investigated whether materials other than methane, originating from deep subseafloor, are released from SMVs. During KH-23-4 cruise in August 2023 by R/V Hakuho-Maru, we surveyed SMVs located from Ryukyu Trench to the western end of the Nankai Trough, off Kikaijima Island Tanagashima Island, and Hyuga-nada. We obtained seawater samples at the mud volcanoes, from a total of 14 volcanoes across 33 sites—7 off Kikai Island, 3 off Tanegashima Island, and 4 in the Hyuga-nada—using a CTD rosette sampling system. At some sites, sea water was sampled 1–2 m above the seafloor using a sampler attached to the multiple corer. We analyzed DOC and nutrients (NO2, NO3, NH4, SiO2, PO4) concentrations in those water samples.
The NH4 concentrations in seawater increased approximately 1.5–5 times near the seafloor compared to the background concentration of 1–2 μM at five mud volcanoes off Kikai Island (KSK2, KSK3, KSK4, KSK5, KNK1), and two in the Hyuga-nada (HyMV24, HyMV9). The NO2 concentrations increased from 0 µM to 0.46 µM from 50 m to 10 m above the summit at MV3 (31º02.6N, 131º40.90′E, 1200 m). Since NH4 and NO2 are produced in reductive environment, the increase in their concentrations above the mud volcanoes summits suggests that these nutrients, produced in subseafloor, are released into the ocean.
Although measurements of DOC concentration are not yet completed, so far, an increase in the DOC concentration above the summits of mud volcanoes (2-3 μM higher than the background levels) were observed, indicating the release of DOC from these mud volcanoes, KNK1 (28º25.37′N, 130º11.55′E, 976 m) and KNK2 (28º33.48′N, 130º3.78′E, 868 m) off Kikai Island.
These observations suggest that SMVs have released not only methane but also DOC and nutrients produced in the subseafloor.