Ryo Matsumoto, University of Tokyo (Japan)
Rika Takeuchi, University of Tokyo (Japan)
Hitoshi Tomaru, Kitami Institute of Technology (Japan)
Akihiro Hiruta, University of Tokyo (Japan)
Risa Sanno, University of Tokyo (Japan)
3D mapping, drillings and submersible dives have revealed two contrasting mode of occurrence of marine gas hydrates. One is a pore-filling type, concentrated in sandy layers near the base of gas hydrate stability [P-type]. The other accumulates as dense and massive deposits near the seafloor [M-type]. Response of marine gas hydrates to environmental perturbations is different between these two types. Temperature increase was believed to have caused massive dissociation of P-type deposits, however, due to low heat-propagation through low conductive soft sediments, temperature increase of sea waters is not likely to instantaneously destabilize subsurface gas hydrates, whereas M-type deposits are to be dissociated. To the contrary, sea-level fall during the glacial should promptly depressurize subsurface deposits and substantial amount of gas hydrate should be dissociated to release methane. Nankai trough is dominated by P-type deposits with little M-type. Dissociation of subsurface gas hydrate due to the sea level fall during the glacial episodes is indicated by double BSRs at around 300 mbsf.
Eastern margin of Japan Sea is an active zone of gas hydrate accumulation represented by M- and P-type deposits and methane plumes. M-type deposits concentrate in gas chimneys, 500 m in diameter, whereas P-type occurs along the BGHS at about 180 mbsf. High amplitude BSRs appear within gas chimneys, suggesting dense accumulation of P-type and even M-type hydrates. In case of Japan Sea, sea level fall during the LGM is believed to have caused dissociation of P-type deposits, resulting in a buildup of shallow gas pools within the chimneys. Gas buildups eventually caused a collapse of shallow M-type deposits (hydrate caps) in hydrate mounds, and large amount of methane and gas hydrates rose up to the sea surface. The collapse of hydrate caps formed large pockmarks, approximately 500 m in diameter, and methane expulsion precipitated C-13 depleted carbonate concretions. U-Th ages of carbonate concretions center around 15-22 ka around the MIS stage 2. Carbon isotopic composition of benthic foraminifers of 19-22 ka exhibit sharp negative excursions, suggesting enhanced methane flux at LGM. Recently, we identified C-13 depleted carbonates at the transition from the deep-sea mudstone to shallow calcareous sandstone and reef limestone in Ryukyus. Rapid uplift of the Ryukyu Islands may have caused massive dissociation of gas hydrates. In conclusion, (1) subsurface P-type hydrate is not likely to respond instantaneously to the warming, and (2) sea-level fall due to glacial eustasy or tectonic uplift effectively destabilize subsurface and even seafloor gas hydrates.