Aquatic environments reconstructed since the Last Glacial based on fossil diatom assemblages from the northern Aso Caldera, Central Kyushu, Japan

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  • 珪藻化石群集に基づく阿蘇カルデラ北部の最終氷期以降の水域環境
  • ケイソウ カセキ グンシュウ ニ モトズク アソカルデラ ホクブ ノ サイシュウヒョウキ イコウ ノ スイイキ カンキョウ

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Abstract

Aso Caldera in Central Kyushu is a large depression formed after the huge pyroclastic eruption of Aso Volcano. In the northern half of the depression called Asodani (Aso valley), sediments have been deposited in lake, river, swamp and fan conditions. We analyzed the fossil diatom of samples taken from cores and an outcrop of nine sites in the northern Aso Caldera area in order to reconstruct the aquatic environments since the Last Glacial. A fluvial condition assumed to be coarse sand and gravel in the western area of Asodani, was deposited in the latter part of the Last Glacial. Above it, the diatom assemblages become dominated by planktonic genera such as Auracoseira, Cyclotella and Stephanodiscus, showing that a lake condition started around 21,000yBP and lasted to 8,800yBP at the Hosenbashi site, 8,200yBP at the Kario site and about 7,900yBP at the Senchomuta site. Then, the gradual shift in the diatom assemblage in the overlying horizons indicates that the lake condition changed to the present-day condition. After a period of ongoing decrease in depth, the lake disappeared. The diatom assemblages of such sediments are composed of epithyte genera, such as Fragilaria, Synedra and Cocconeis, and benthic genera such as Navicula, Gomphonema, Nitzchia and Neidium. Whereas in eastern Asodani, a fan condition has continued since the Last Glacial, as indicated by dominance of the epiphyte and benthic diatom genera. On the rim plain, environmental conditions were mainly eolian, but sediments from the Sanno site, contain the diatom assemblages of some epiphyte and benthic genera showing river and/or swamp conditions. At the Zogahana site, swamp conditions have existed since 1,000years ago.

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