地球環境変化とヒマラヤ・チベット山塊の役割 中国内陸部のレス・古土壌堆積物と日本海堆物の粘土鉱物からみた東アジアにおける過去２４０万年間のモンスーン・偏西風変動

Abstract

Activities of Asian monsoon and westerlies during the last 2.4Ma were determined through the investigation of sedimentary features in the Chinese loess-paleosol sequences and the deep-sea sediments of the Sea of Japan. It was already known that sequential changes of magnetic susceptibility in the loess-paleosol sequences of the Chinese Loess Plateau were closely concerned with the glacial-interglacial cycles. Also, sequential changes of illite crystallinity, expressing as full width of half maximum of 10 A peaks by X-ray diffraction patterns in late Quaternary fine-grained sediments at Site 795A of Ocean Drilling Program in northeastern part of the Sea of Japan, were well correlated to standard records of oxygen isotope changes by SPECMAP. The samples of this research were derived from the Luochuan of northwestern China and from deep-sea sediments in ODP site 795A. This research is dealing with the changes of magnetic susceptibility and/or clay mineralogy of the loess-paleosol sequences during the last 2.4 Ma.The results are summarized as follows:1) Illite is one of the main detrital minerals in the loess-paleosol sequences, and illite concentrations in loess are higher than those concentrations found in paleosol. Therefore, illite crystallinity and concentrations are useful indicators to detect concentrations of eolian dust deriving from the Chinese Loess Plateau.2) During interglacial periods, wet atmospheres drifted from the Eastern China Sea into the inland area of the Asian Continent. This increase of moisture has promoted the hydration and decomposition of illite crystals in paleosols.3) According to the investigation of sedimentary features, it is obvious that there was little supply of eolian dust origin minerals transported from the arid area of western China to the Chinese Loess Plateau. This fact cannot always support the formation process of the loess-paleosol sequences, as it was interpreted by Kukla et al.(1988).4) The large amplitudes of Magnetic susceptibility in the loess-paleosol sequences during the last 0.6 Ma and there was a high degree of illite crystallinity in the deep-sea sediments during the last 0.8 Ma appeared. These facts suggest that changes of atmospheric circulations were caused by the uplift of the Himalayan/Tibetan Plateau about 0.6-0.8 Ma ago.