レス・風成塵からみた最終氷期のモンスーンアジアの古環境とヒマラヤ・チベット高原の役割

抄録

Loess deposits spread widely in central and East Asia, where the main area of loess deposition is on the Chinese Loess Plateau. Climatic conditions in central and east Asia allowed a substantial loess accumulation since 2, 400ka., while in South and Southeast Asia, the presence of loess is recognized locally in a periphery of Thar Desert and on the terraces of South India and Thailand. A weakened monsoon during glacial phases seems to diminish a loess accumulation in these regions. Most areas of East Asia such as the Japanese Islands, Korea and Taiwan were affected the depositionof eolian dust originated in the drier area of Asian continent, especially during the Last Glacial time. A depositional rate of loess and eolian dust in the glacial period was higher than in the interglacial. Fine silt production in deserts and glacierized areas seems to have increased in glacial period. On the Chinese Loess Plateau and western Japan, a depositional rate of loess and dust in MIS 2 was 1.3 times higher than in MIS 4. On the contrary, in northern Japan, a dust deposition rate of MIS 2 was 2.5 times higher than in MIS 4. The difference of rates between the western and the northern Japan seems to reflect a paleoenvironmental change of the source areas of eolian dust: a desert condition might have increased more in the northern Asia from which the eolian dust mainly came to the northern Japan, because of a drier condition in winter by a decrease of snowfall.
ESR intensity signal of fine quartz (finer than 20μm) during MIS 2 in the Japanese Islands varies in different localities. Under the assumption that the values of ESR intensity signal of eolian fine quartz reflect those of the quartz at the source area of eolian dust, the source area and trajectory of eolian dust are tentatively reconstructed. The northern Japan where the ESR intensity signal is high (10-12), the eolian dust seems to be supplied from Siberia and Mongolia where the Precambrian rocks with a high ESR intensity signal are widely exposed. The central and southern Japan where the ESR intensity signal is medium (5.8-8.7), the eolian dust seems to be supplied from central Asia where the Paleozoic-Mesozoic rocks with a medium ESR intensity signal are widely exposed. The southernmost islands of Japan where the ESR intensity signal increases again, seem to have an eolian dust supply from South China and India where the Precambrian rocks are widely exposed. On the basis of ESR intensity signals, three major courses of eolian dust transport in MIS 2 are proposed: the winter monsoon in the northern Japan, the summer subtropical jet in the central and southern Japan, and the winter subtropical jet in the southernmost islands of Japan.
The uplift of the Tibetan Plateau and Himalayan Range seems to enhance a production and supply of Asian loess, by the intensification of Siberian high and Asian monsoon system. Glacier extension on the Tibetan Plateau and Himalayan Range, which is related to the uplift of these areas, also enhanced the production of loess material in the source area. The detail study which correlates the glaciation with the loess deposition in central Asia is much more required.