火山灰土壌中の14Å鉱物の起源 : 風成塵の意義

抄録

Generally, the essential clay mineral of the volcanic ash soils is characterized by a predominance of allophane, imogolite, halloysite, and gibbsite. The 14Å minerals also are, however, extensively identified in the volcanic ash soils, but there is a divergence of opinion as to their origin. The occurrence of 14Å minerals in the volcanic ash soils in the Honshu and Hokkaido districts of Japan is considered to have relations with a snowfall. The 14Å minerals in these areas are probably provided as an ice nuclei of snow from eolian dusts in the atmosphere, which are dominantly originated from China. The eolian dusts may directly fall on the ground in the Kyushu district. From the amounts of eolian dusts in the palaeosols and peat soils, the accumulation rates of eolian dusts in last glacial intervals and Holocene are estimated to be 2.1〜3.3 and 0.11〜0.80 g/cm^2/1,000 yr, respectively. The clay mineral of eolian dusts is characterized by a predominance of 14Å minerals, illite, kaolinite, and quartz. The 14Å minerals are dominant in montrnorillonite, vermiculite, and intergrades, and their contents depend on the degree of weathering. The composition of crystalline clay minerals in the volcanic ash soils resembles nearly to that of Chinese loess, eolian dusts, and the North Pacific marine sediments, although the latter contains much more illite than the 14Å minerals in comparison with the former. Probably the dominating weathering sequence of eolian dusts deposited in the soils is considered to be as follows : [chemical formula] From these results, the contribution of eolian dusts is primarily reflected in the 14Å minerals, illite, kaolinite, and quartz in the volcanic ash soils in Japan.