The Study of Gas Hydrate Bearing-Sediments from Joetsu Area
Antonio F. Freire1, Akinori Nagasaka2, and Ryo Matsumoto2
1Natural Environmental Studies, University of Tokyo, Kashiwa, Japan
2Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan
Two research-cruises occurred in 2007 on the east part of Japan Sea off Joetsu. A total of 192 samples were collected and analyzed in order to obtain TOC content of these sediments, mineral composition by XRD measurements, and d13C isotope concentration by mass spectrometry. The intention is to identify environmental changes occurred in the last 30.000 years, because the behavior of total organic Carbon and mineral composition follows the characteristics of the environment that surrounds the study area and at the depositional time. At Last Glacial Maximum (LGM) time, about 20 Ka B.P., the sea-level dropped around 90 meters than the actual sea-level. The rivers were more close to the edge of the platform and, because this, closer to slope and basin sites. That rivers mouth location put more organic matter and fine sediments on suspension and this material precipitated to the basin-seafloor. On the other hand, at Holocene time, the rise of sea-level puts the rivers mouth far from the slope by the retraction of the coastal line. For this reason, theoretically, it is expected that less organic matter will appears at Holocene time than at Pleistocene time. Nevertheless, in the study area, it is possible to see a TOC increased upward. So, at Holocene time, the production of organic carbon was bigger than at Pleistocene time, in opposition to the thinking above. A possible reason for this is that the organic matter production was from marine phytoplankton and not delivered from land, according to the heavy d13C isotope concentration increasing to the top. On the other hand, piston-cores located over Umitaka Spur and Joetsu Knoll shows different TOC value patterns. Gas Hydrate occurrence can explain it because part is thermogenic and part is biogenic and the organic carbon could be used on gas hydrate formation.
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