10:45 AM - 12:15 PM
[MIS19-P01] Origin of methane hydrate in submarine mud volcano No.15 off Tanegashima Island
Keywords:submarine mud volcano, methane hydrate, pore water, Tanegashima
Submarine mud volcanoes occur along the margins of convergent plates and are formed by the vertical intrusion of low density, deformable sediments from the deep subsurface to the seafloor. These migration pathways supply subsurface-derived water, gaseous compounds such as methane to the overlying hydrosphere. In particular, methane emission from mud volcanoes is a significant source of fossil methane to atmosphere or hydrosphere. Around Japan, the Kumano forearc basin and off Tanegashima Island have been known as mud volcano fields. Several tens of mud volcanoes have been found off Tanegashima Island along the Ryukyu Trench. We surveyed three mud volcanoes, MV#8 (30º45´N, 131º36´E), MV#10 (30º23´N, 131º24´E), and MV#15 (31º03´N, 131º40´E), in this area during KS-21-27 cruise from December 2021 to January 2022 by R/V Shinsei-Maru. At MV#15, we found and collected methane hydrate in the sediment core sample.
At MV#15, the Cl–concentration in pore water showed local decrease (271-340 mM) at 104-143 cm below seafloor (cmbsf). The stable oxygen and hydrogen isotopic compositions (δD and δ18O) of pore water exhibit 18O-enriched and D-enriched isotopic values in proportion to the depletion of the Cl– concentration, indicating the addition of water from the decomposition of methane hydrate. Based on the δD and δ18O of hydrate water, and isotopic fractionation of oxygen and hydrogen in water caused by gas hydrate formation, the original δD and δ18O of pore water before the methane hydrate formation was estimated to be -4.3– +2.5‰ and +0.8– +1.8‰, respectively. These values suggest the fluid composing the methane hydrate was originally derived from clay mineral dehydration.
The methane hydrate saturation, that is, the volume ratio of gas hydrate to the pore space, estimated from the low Cl- anomaly ranged from 42 to 55%. The amount of methane in the sediment contained as methane hydrate was estimated to be 98×10-3 m3 CH4 /m3 (STP). Assuming that the sediments contain this methane volume are uniformly distributed over the summit of MV#15 (350 m diameter), the methane content of MV#15 is estimated to be about 1.9×106 m3.
The δ13C (-54– -47‰) and δD (-205– -171‰) of the methane obtained by melting the methane hydrate indicate that the methane was derived thermogenic decomposition of organic matter. The abundance of 13CH3D, a multiply substituted “clumped” isotopologue indicates apparent equilibrium temperatures (Δ13CH3D temperatures) of 141 +39/-32ºC (Δ13CH3D: 3.15±0.52‰), suggesting that the methane is mostly composed of thermogenic methane.
At MV#10, the Cl- concentration decreased with increasing depth. The stable isotopic compositions of pore water exhibit 18O-enriched and D-depleted isotopic values in proportion to the depletion of the Cl– concentration, indicating the addition of water from the decomposition of methane hydrate, indicating the addition of water from the dehydration of clay minerals that typically occur in the temperature range from 60 to 160℃.
The Cl- concentration and the stable isotopic compositions of the pore water in MV#08 showed no obvious change from seawater values, suggesting that MV#08 is dormant mud volcano.
In conclusion, we revealed that the methane hydrate in MV#15 is mostly composed of thermogenic methane, and water in the methane hydrate was originally derived from clay mineral dehydration. The observations indicate that fluid experienced high temperature, and were derived from deeply buried sediments (>several kms below seafloor), because the formation of thermogenic methane and clay mineral dehydration occur at >60ºC.
At MV#15, the Cl–concentration in pore water showed local decrease (271-340 mM) at 104-143 cm below seafloor (cmbsf). The stable oxygen and hydrogen isotopic compositions (δD and δ18O) of pore water exhibit 18O-enriched and D-enriched isotopic values in proportion to the depletion of the Cl– concentration, indicating the addition of water from the decomposition of methane hydrate. Based on the δD and δ18O of hydrate water, and isotopic fractionation of oxygen and hydrogen in water caused by gas hydrate formation, the original δD and δ18O of pore water before the methane hydrate formation was estimated to be -4.3– +2.5‰ and +0.8– +1.8‰, respectively. These values suggest the fluid composing the methane hydrate was originally derived from clay mineral dehydration.
The methane hydrate saturation, that is, the volume ratio of gas hydrate to the pore space, estimated from the low Cl- anomaly ranged from 42 to 55%. The amount of methane in the sediment contained as methane hydrate was estimated to be 98×10-3 m3 CH4 /m3 (STP). Assuming that the sediments contain this methane volume are uniformly distributed over the summit of MV#15 (350 m diameter), the methane content of MV#15 is estimated to be about 1.9×106 m3.
The δ13C (-54– -47‰) and δD (-205– -171‰) of the methane obtained by melting the methane hydrate indicate that the methane was derived thermogenic decomposition of organic matter. The abundance of 13CH3D, a multiply substituted “clumped” isotopologue indicates apparent equilibrium temperatures (Δ13CH3D temperatures) of 141 +39/-32ºC (Δ13CH3D: 3.15±0.52‰), suggesting that the methane is mostly composed of thermogenic methane.
At MV#10, the Cl- concentration decreased with increasing depth. The stable isotopic compositions of pore water exhibit 18O-enriched and D-depleted isotopic values in proportion to the depletion of the Cl– concentration, indicating the addition of water from the decomposition of methane hydrate, indicating the addition of water from the dehydration of clay minerals that typically occur in the temperature range from 60 to 160℃.
The Cl- concentration and the stable isotopic compositions of the pore water in MV#08 showed no obvious change from seawater values, suggesting that MV#08 is dormant mud volcano.
In conclusion, we revealed that the methane hydrate in MV#15 is mostly composed of thermogenic methane, and water in the methane hydrate was originally derived from clay mineral dehydration. The observations indicate that fluid experienced high temperature, and were derived from deeply buried sediments (>several kms below seafloor), because the formation of thermogenic methane and clay mineral dehydration occur at >60ºC.