5:15 PM - 6:30 PM
[SGL40-P02] Origins of water and methane in submarine mud volcanoes off Tanegashima
Keywords:dehydration from clay mineral, methane, off Tanegashima
At the MV#1, the chloride (Cl–) concentration linearly decreased from 550 mM near the sediment surface to 220 mM at 250 cmbsf. Below 248 cm to core bottom, the concentration was constant at ~220 mM. The stable isotopic compositions of pore waters exhibit 18O-enriched and D-depleted isotopic values in proportion to the depletion of the Cl– concentration, indicating the addition of water from the dehydration of clay minerals that typically occur in the temperature range from 60℃ to 160℃. Generally low concentration ratios of methane to ethane (C1/C2: ~ 30) and the stable carbon and hydrogen isotopic compositions of methane (δ13C: ~ –45‰; δD: ~ –120‰) consistently indicate that the hydrocarbon gases are derived from thermal decompositions of organic matter in deep sediments where the in situ temperature is >80℃. In contrast to the MV#1, at the MV#14, the Cl– concentration only slightly decreased from 556 mM near the sediment surface to 490 mM at core bottom, indicating slow fluid advection. This sugggests that the activity of MV#14 is lower than the MV#1. The C1/C2 ratios were high as 700-4000, and δ13C and δD values of methane were –75‰ and –150‰, respectively. These data strongly indicate that most methane is microbially produced via hydrogenotrophic methanogenesis.