5:15 PM - 6:45 PM
[SCG48-P30] Chemical and isotopic characteristics of interstitial water in surface sediments in the southern Okinawa Trough back-arc basin
Keywords:chemical composition, isotopic composition, interstitial water, southern Okinawa Trough, back-arc basin
The Okinawa Trough is a rifting zone related to the subduction of Philippine plate, where multiple grabens are formed in echelon in the mid-axis, and submarine hydrothermal activity has been discovered in the axial part of the axis. However, the surrounding ground temperature is extremely low, and the geothermal structure is extremely complex, although it is still unclear whether this is because seawater is recharged into the hydrothermal circulation or because the plate itself is cold. The purpose of this study is to investigate the chemical composition of the porewater in the surface sediments and to evaluate the influence of submarine hydrothermal activity, which could be the most important factor influencing the geothermal structure.
In KH-23-11, sediment cores were collected from four points, the axial and western ends of the Yonaguni Graben and two points from the axial part of the Yaeyama Graben, located in the southern Okinawa Trough, using a 4 m long piston corer, and several thermometers were attached to measure temperature. Porewater was extracted from the sediment using a hydraulic press on board the vessel, and pH and ammonia concentrations were measured on board. On land, concentrations of dissolved inorganic carbon (DIC), anions (chloride, bromide, and sulfate), and major elements (Na, Mg, Ca, and K) were measured.
The chemical composition of the porewater samples at all sites was similar to that of ordinary seawater. The ammonia concentration tended to increase with depth, but the DIC did not change with depth or location. On the other hand, sub-seafloor ground temperatures were extremely high at the western end of the Yonaguni Graben.
At the sites surveyed in this study, no decrease in sulfate was detected in the depth direction, suggesting that sulfate reduction by organic matter did not even occur. This suggests that the survey site is located at the bottom of the trough, and the content of organic matter in the sediment is extremely low. These findings suggest that methane is not being supplied from the great depths beneath the seafloor, and it is thought that there is no effect of hydrothermal activity at all. On the other hand, the high ground temperature in some areas may indicate that although heat supply has begun, there is a time lag before the formation of hydrothermal fluids due to the reaction between rocks and seawater.
In the future, we would like to further investigate the western end of the Yonaguni Graben and clarify the cause of the high temperature.
In KH-23-11, sediment cores were collected from four points, the axial and western ends of the Yonaguni Graben and two points from the axial part of the Yaeyama Graben, located in the southern Okinawa Trough, using a 4 m long piston corer, and several thermometers were attached to measure temperature. Porewater was extracted from the sediment using a hydraulic press on board the vessel, and pH and ammonia concentrations were measured on board. On land, concentrations of dissolved inorganic carbon (DIC), anions (chloride, bromide, and sulfate), and major elements (Na, Mg, Ca, and K) were measured.
The chemical composition of the porewater samples at all sites was similar to that of ordinary seawater. The ammonia concentration tended to increase with depth, but the DIC did not change with depth or location. On the other hand, sub-seafloor ground temperatures were extremely high at the western end of the Yonaguni Graben.
At the sites surveyed in this study, no decrease in sulfate was detected in the depth direction, suggesting that sulfate reduction by organic matter did not even occur. This suggests that the survey site is located at the bottom of the trough, and the content of organic matter in the sediment is extremely low. These findings suggest that methane is not being supplied from the great depths beneath the seafloor, and it is thought that there is no effect of hydrothermal activity at all. On the other hand, the high ground temperature in some areas may indicate that although heat supply has begun, there is a time lag before the formation of hydrothermal fluids due to the reaction between rocks and seawater.
In the future, we would like to further investigate the western end of the Yonaguni Graben and clarify the cause of the high temperature.