Water temperature variation and thermal conductivity in the Kuroshima Knoll area, southwestern part of the Ryukyu Arc

Abstract

The Kuroshima Knoll, located in the southwestern part of the Ryukyu Arc, is one of the best fields for the study of gas hydrate dissociation, because the Pressure-Temperature condition at the top of the knoll is close to the phase boundary between gas and hydrate. In order to understand the geothermal structure around the Kuroshima Knoll, heat flow and thermal conductivity measurements were carried out around the knoll during the R/V Kaiyo KY03-09 Cruise. During heat flow measurements we found that the vertical variation of water temperature was as large as 1.2 degC just above the seafloor at Sites A and B on top of the knoll. This can either be temporal or spatial variation, and it disturbs the subbottom temperature field significantly, possibly affecting the state of hydrate. The heat flow value is 42 mW/m 2 and thermal conductivity is 1.0 W/m/K at the Site 2 in the Yaeyama Basin. The thermal conductivity value in the Yaeyama Basin istypical for the ordinary marine sediment. On the other hand, the thermal conductivity is much higher (1.8 Wm -1 K -1 ) at the Calyptogena colony site (Site A) on top of the knoll.