A Long-range Transported Eolian Dust Found in Thphra of the Iwate Volcano after the Fall of the Toya Ash.

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  • 洞爺火山灰以降に堆積した岩手火山テフラ層中の広域風成塵
  • トウヤ カザンバイ イコウ ニ タイセキシタ イワテ カザン テフラソウチュウ

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Abstract

Physical, chemical, clay mineralogical, and geochemical analyses of the tephra at the foot of Mt. Iwate after the fall of the Toya Ash (90, 000-130, 000yrs BP) revealed the accumulation of a long-range transported eolian dust in the tephra layer. The tephra deposits are characterized by the predominance of allophane and imogolite or halloysite (10Å and 7Å) and by a small amount of silt-sized cristobalite. However, the soil of the Shibutami Crack horizon in the tephra is characterized by low pH (NaF) and phosphate retention values, high silt and clay contents, and high bulk densities. Their dominant clay minerals are vermiculite and vermiculite-chlorite intergrades, which are weathering products from muscovite, and kaolinite. The soil also contains a considerable amount of silt-sized quartz. The oxygen isotopic ratios of 1-10μm quartz purified from two soils of Shibutami Crack horizons are +16.2 and +17.0‰, and similar to those of quartz from loess in China, loess-derived soils in China, Korea, and Japan, and modern tropospheric eolian dust in Japan. The experimental data indicate that vermiculite, vermiculite-chlorite intergrades, kaolinite, and quartz in these soils are not weathering products from tephra deposits but originated from a long-range transported eolian dust from China. The tephra stratigraphy indicates that the eolian dust was mainly deposited 20, 000-34, 000yrs BP and 50, 000-70, 000yrs BP. This study suggests that the crack horizon in the tephra section found in northern Tohoku and southern Hokkaido districts could be a key bed indicating a long-range transport of the eolian dust from China during the Last Glacial age. The flux of eolian dust onto tephra deposits at the foot of Mt. Iwate during the late period of the Last Glacial age is estimated to be 48kg/m2/1, 000 years or 4cm/1, 000 years.

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