11:45 〜 12:00
[MIS15-10] 日本海北部IODP Site U1422堆積物に記録された40万年前以降の東アジア冬モンスーン強化
キーワード：IODP U1422地点、海氷運搬礫、鉱物組成
The Japan Sea oceanography has been highly affected by both summer and winter monsoons in east Asia as well as the sea level due to its shallow sill depth. Since the productivity in the Japan Sea is maintained by the input of nutrient-rich East China Sea Coastal Water (ECSCW) through the Tsushima Strait, summer monsoon precipitation is essential to supply nutrient to ECSCW through the Yangtze discharge as well as the surface water input by the Tsushima Warm Current. East Asian winter monsoon (EAWM) promotes the sea-ice expansion in the northern Japan Sea which could supply coarse clastic materials to the sediments. EAWM also makes the surface water density higher by cooling and brine ejection during the sea-ice formation, which leads to the strong ventilation of the deeper water column of the Japan Sea. Therefore, the preservation of organic matter and carbonate in the deeper part of the Japan Sea is considered to be controlled by the balance between the productivity promoted by summer nutrient input and the organic matter decomposition / carbonate dissolution by winter ventilation. In order to monitor a long term evolution of EAWM during the late Pleistocene, we tried to reconstruct the sea-ice expansion and the preservation of organic matter and carbonates using the sediments from the northern Japan Sea.
IODP Site U1422 is located at 43°45.99′N, 138°49.99′E, and 3429 m water depth in the northern Japan Sea. The Pleistocene sediments at Site U1422 is characterized by centimeter to meter scale alternation of dark and light silty clay rarely yielding biogenic opal, carbonate, and sand grains. The sediment ages are constrained well by the basin wide correlation of dark-light cycles of the sediment sequence and the orbitally tuned age model established at IODP Site U1424 in the Japan Sea (Tada et al., 2018). We selected 216 samples for the last 684 kyr from the spliced sequence of Site U1422, and determined the contents of detrital sand as ice-rafted detritus (IRD) and mineral composition of the mud fraction using a powder X-ray diffraction analysis (XRD). We also referred to RGB values of core photographs as organic matter proxy because the darker the sediments, the more organic carbon were contained.
IRD contents are higher during glacial times and lower during interglacial times. The amplitude of IRD variability increases after 400 ka. Calcite peak height (content) detected by XRD shows large variability before 400 ka, and significantly reduced after that. In association of these changes at 400 ka, minima of RGB (maxima of organic carbon content) show larger values (less organic carbon) after 400 ka compared to those before. These lines of evidence suggest that sea-ice expansion was more frequent after 400 ka, which was associated with the stronger ventilation of the deeper water column leading to enhanced decomposition of organic matter and dissolution of carbonate due to shallowed carbonate compensation depth, which could be the results of an intensified EAWM at that time.
IODP Site U1422 is located at 43°45.99′N, 138°49.99′E, and 3429 m water depth in the northern Japan Sea. The Pleistocene sediments at Site U1422 is characterized by centimeter to meter scale alternation of dark and light silty clay rarely yielding biogenic opal, carbonate, and sand grains. The sediment ages are constrained well by the basin wide correlation of dark-light cycles of the sediment sequence and the orbitally tuned age model established at IODP Site U1424 in the Japan Sea (Tada et al., 2018). We selected 216 samples for the last 684 kyr from the spliced sequence of Site U1422, and determined the contents of detrital sand as ice-rafted detritus (IRD) and mineral composition of the mud fraction using a powder X-ray diffraction analysis (XRD). We also referred to RGB values of core photographs as organic matter proxy because the darker the sediments, the more organic carbon were contained.
IRD contents are higher during glacial times and lower during interglacial times. The amplitude of IRD variability increases after 400 ka. Calcite peak height (content) detected by XRD shows large variability before 400 ka, and significantly reduced after that. In association of these changes at 400 ka, minima of RGB (maxima of organic carbon content) show larger values (less organic carbon) after 400 ka compared to those before. These lines of evidence suggest that sea-ice expansion was more frequent after 400 ka, which was associated with the stronger ventilation of the deeper water column leading to enhanced decomposition of organic matter and dissolution of carbonate due to shallowed carbonate compensation depth, which could be the results of an intensified EAWM at that time.