CHANGE OF GAS HYDRATE STABILITY ZONE IN THE NORTHEASTERN CONTINENTAL SLOPE OF SAKHALINS ISLAND, SEA OF OKHOTSK AND ITS IMPLICATIONS FOR SLOPE FAILURE

Abstract

The sudden dissociation of gas hydrate within sediments at continental margin due to ocean warming and/or sea level drop has been suggested as a possible cause of global climate change as well as extensive slope failure. In the northeastern continental slope of the Sakhalin Island (Sea of Okhotsk), numerous gas hydrate-related manifestations in addition to gas hydrates have been reported, which include hydroacoustic anomaly through the water column, pockmarks and mounds on the seafloor, seepage structure and bottom-simulating reflectors (BSRs) in the sediment. The gas hydrate found at 385 mbsl is the shallowest occurrence ever reported in the Sea of Okhotsk. BSR depths matches well with the base of gas hydrate stability zone (BGHSZ) estimated under the current environmental conditions such as gas composition, water temperature and the background geothermal gradient. In terms of thermal structure, an important distinction can be made between seafloors containing the seepage structure or none. We explore the timing of a large slope occurred in the study area using new stratigraphic evidence from subbottom profiles. The timing of the failure seems to be much younger, 20 ka roughly corresponding to the late stage of the Last Glacial Maximum, than the previous estimate, older than 350 ka. The extent of the slope failure occurs up to much shallower depth than the intersection depth of BGHSZ with the seafloor at 20 ka, possibly indicating complexity of natural landslides. Furthermore, this region has witnessed a rapid sea water temperature increase in the last 50 years. If such a trend continues, additional slope failure can be expected in the near future, considering that the region is not far from a transform plate boundary where shallow seismicity occurs. It is noted that the contents consist of the published material [1] together with newly-added discussion on the possibility of future slope failure in the study area.