Abstract
This paper discusses effects of methane hydrate (MH) formation and dissociation on water content of the sediment in the eastern Nankai Trough area. Sediment can be divided into three types. The Type 1 sediments indicate coarser than 20μm in median with inverse and normal grading structures, and the Type 2 sediments are finer than 20μm in median with normal grading structure. Both sediments are interpreted as turbidite deposits. The Type 3 sediments show constant grain size, almost 10μm in median, and this fact implies this type is of hemipelagic mud. As to the Type 1 sediments after MH dissociation, relationship between water content below 10% and clay-slit-sand contents differs from that in water saturated sediments. This fact implies that MH accumulated in the sediments with water contents below 10%, and dehydrating action occurred during dissociation of MH. In this study, water content decreased at most 15% after MH dissociation. Low water content sediments owing to selective dehydration can provide one of the criteria for identifying the MH-bearing strata in past times.
