Chemical weathering of the Himalayan river system in Bangladesh: a multi-isotope approach

2019 Fiscal Year Final Research Report

• Project/Area Number: 16H05883
• Research Category: Grant-in-Aid for Young Scientists (A)
• Allocation Type: Single-year Grants
• Research Field: Environmental dynamic analysis
• Research Institution: Japan Agency for Marine-Earth Science and Technology (2017-2019); The University of Tokyo (2016)
• Principal Investigator: YOSHIMURA Toshihiro 国立研究開発法人海洋研究開発機構, 海洋機能利用部門(生物地球化学プログラム), 研究員 (90710070)
• Project Period (FY): 2016-04-01 – 2020-03-31
• Keywords: 化学風化 / ヒマラヤ / 同位体

Outline of Final Research Achievements

This study investigated spatial isotopic variations of Sr, Li, Mg, Ca, O and H in dry- and wet-season river waters from the Ganges, Brahmaputra, and Meghna river system and groundwater in the Bengal Plain, Bangladesh. The sediment load in low-altitude plains is derived from rapid denudation of high mountains, where physical erosion rate is high and hence material is incompletely weathered (i.e., weathering-limited regime). The river samples collected in dry and wet seasons were within the analytical error of our analyses, thus indicating that relative inputs from the distinct the sources were effectively the same at a given sampling site. For the Bengal Plain, chemical weathering at greater sediment depths is more congruent. The Himalayan-derived material continue to be actively weathered during their transit through the floodplain, which is an important knowledge for climate-weathering feedback.

Free Research Field

地球化学

Academic Significance and Societal Importance of the Research Achievements

ベンガル平野の河川水と地下水の同位体組成から、その溶質が上流の高山地帯からに由来するものではなく、多くは降雨と堆積物が反応することでもたらされたことが判明した。50m以深の地下水では化学組成がケイ酸塩鉱物の溶解反応の影響を多く受ける。本研究地域から海洋にもたらされる各種元素の同位体フラックスは非常に大きいため、長期的な海水の同位体組成に影響を及ぼす。地下水の同位体組成の平均値はガンジス・ブラマプトラ川の平均値と誤差の範囲で一致することから、化学風化が上流部と下流部で一致する定常状態にあることを示した。風化の気候影響を知る上で重要な知見である。