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Major element and REE compositions of Pliocene sediments in southwest Japan; Implications for paleoweathering and paleoclimate

西南日本における鮮新統の主成分元素および希土類元素組成; 古風化および古気候への意義

葉田野 希*; 吉田 孝紀*; 森 沙織*; 笹尾 英嗣

Hatano, Nozomi*; Yoshida, Koki*; Mori, Saori*; Sasao, Eiji

東アジアモンスーンの履歴や、モンスーン気候の発達とテクトニクス、地球規模の気候との関係は複雑で議論が多い。本研究では、西南日本の湖成堆積物の主成分元素と希土類元素の組成に基づいて化学風化条件を検討した。その結果、380$$sim$$340万年前には主成分元素組成から推定される変質強度の指標や希土類元素組成,試料の鉱物組成から、化学風化が強まったことがわかった。既往研究成果との比較から、西南日本におけるこの時期の強風化条件は、黒潮の流入による温暖湿潤気候と東アジア夏季モンスーンに起因する可能性が考えられた。

The history of the East Asian monsoon and the relationship between the development of the monsoon climate, tectonics and global climate are complicated and controversial. The present study clarifies the chemical weathering conditions in southwest Japan based on the concentrations of major elements and rare earth elements (REEs) in lacustrine muddy sediments. Between 3.8 and 3.4 Ma, chemical weathering significantly intensified, as indicated by the high values of the chemical index of alteration as well as the high concentrations of REEs and light REEs against heavy REEs and kaolinite-rich clay mineral compositions. The intense chemical weathering on land in southwest Japan from 3.8 to 3.4 Ma may have been regulated by alternating periods of warm and humid climate brought by the invasion of the Kuroshio Current and the intensification of the East Asian summer monsoon.

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