Impact of estuaries on heavy metal isotope ratios of river water: Implications for redox proxy

2023 Fiscal Year Final Research Report

• Project/Area Number: 21K17882
• Research Category: Grant-in-Aid for Early-Career Scientists
• Allocation Type: Multi-year Fund
• Review Section: Basic Section 63010:Environmental dynamic analysis-related
• Research Institution: Kyushu University (2022-2023); Kwansei Gakuin University (2021)
• Principal Investigator: Ito Akane 九州大学, 工学研究院, 助教 (30844659)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: 河川水 / 汽水 / クロム / 重元素同位体 / 化学種 / 有機物 / 物質循環 / 酸化還元指標

Outline of Final Research Achievements

This study focused on the chromium isotope ratio, an important redox proxy for reconstructing ancient environments, and tested the hypothesis that the chromium isotope ratio preserved in marine sediments suggests the extent of past oxidation. Specifically, we examined the "preservation of chromium isotope ratio from land to sea" in two rivers in Kochi Prefecture, Japan, investigating variations in chromium concentration, chemical species, and isotope ratios. As a result, it became evident that hexavalent chromium is reduced in freshwater areas, while in brackish waters, the isotope ratio undergoes significant changes due to mixing with seawater and release from riverbed sediments. Further research is needed to understand the preservation of chromium isotope ratios from land to sea, but our study suggests the importance of evaluating the influence of riverbed sediments based on our findings.

Free Research Field

環境動態解析

Academic Significance and Societal Importance of the Research Achievements

本研究において存在が示唆されたクロムの溶存有機錯体は, 地球表層におけるクロムの物質循環の議論において, 未だ情報が少なく空白となっている領域である. 本研究で開発した有機-無機の化学種一斉分析法を発展させることで, その生成機構や環境条件の理解が深まることが期待される. また, これらの情報の拡充により, 陸域から海洋間のクロム同位体比の保存性に関するより良い制約が得られ, 堆積岩中のクロム同位体比から表層の酸化還元環境の変遷を復元することが可能となり, 縞状鉄鉱床などの表層大気の進化に伴い形成された金属鉱床の新規発見など, 資源開発の発展にもつながる可能性がある.