北九州および与那国島のレス--後期更新世の風成塵の意義

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The eolian dust comes from China to Japan, flying frequently in spring. The eolian contribution to surface sediments cannot be ignored, because Japan is exposed to the westerly wind from China. The magnitude of the quantities of eolian dust that had been transported to Japan in the last glacial period may be greater than that in the Holocene. The purposes of this paper are to elucidate the depositional sequences, chemical properties, clay minerals of loesses, and to point out the significant contribution of eolian dust to accumulate on the surface sediments in Japan in the late Pleistocene. The results from our research are summarized as follows : 1) The twenty Japanese samples were collected from the outcrops of Karatsu city, Genkai town, Iki Island, Yonaguni Island. The analyses of three Chinese loess samples are quoted from Inoue and YOSHIDA (1978) and RYU and CHAN (1962) in China, the analyses of two eolian dust samples are quoted from Jyoetsu in 1966 (HASEGAWA 1967) and MORIOKA in 1977 (INOUE and YOSHIDA 1978) in Japan. The loess deposits consist of mixture of two component parts : silt and clay fractions and sand fraction. Most of silt and clay fractions is considered to be eolian materials which were originated in eolian dust from China, while sand fraction is eolian sand which was blown out from dried sea bottom in the W&uum l;rm glacial age. A judgement based on analyses of size distribution, we can conclude that eolian dust is mixed much in paleosol horizons in Karatsu city. 2) Loess including paleosol is considerably subjected to a weathering action. They are weakly acidified in soil reaction, and the amount of their exchangeable bases is relatively low except Mg2+. Loess and eolian sand beds are characterized by low fluoride pH and low phosphate adsorption. They are distinguished from volcanic ash beds, which are characterized by high fluoride pH and high phosphate adsorption. According to the amount of SiO2, Al2O3 and ±H2O in the < 0.02mm fraction of twenty five samples, they are distinctly divided into four groups ; Chinese loess, eolian dust and loess including paleosol, eolian sand, and volcanic ash. The relative orders of the SiO2, Al2O3 and H2O contents are in the following order on the basis of the diagram of three components distribution : SiO2 : Chinese loess> eolian dust, loess including paleosol> eolian sand> volcanic ash Al2O3 : eolian sand, volcanic ash> eolian dust, loess including paleosol>Chinese loess H2O : volcanic ash> eolian sand> eolian dust, loess including paleosol> Chinese loess An eolian dust in the late Pleistocene, which is originated from the arid regions in China, had gradually accumulated on the paleosol more than the eolian sand bed. 3) The clay fraction of loess including paleosol is characterized by predominance of 14 Å minerals, illite, kaolinite, and quartz. The 14 Å minerals are composed mainly by vermiculite, chlorite, and their intergrades. On the contrary, volcanic ash beds are characterized by predominance of allophane, imogolite, and amount of layer silicates, and they also contain a great amount of gibbsite concretion. The eolian contribution is primarily reflected in the distribution of illite, kaolinite and quartz in the loess deposits. The clay mineral composition of loess including paleosol resembles nearly to that of Chinese loess in China and eolian dust which failed in Japan in 1966 and 1977. 4) Depositional rate of loess is calculated 2.1-3.3g / cm2 / 1, 000 years by the analyses of thickness and silt and clay contents of the loess including paleosols in Karatsu in Würm glacial age. The results of the study in Karatsu may indicate the important contribution of eolian dust to the last glacial period sediments.

北九州および与那国島のレス--後期更新世の風成塵の意義

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