土壤和河流沉积物吸附三价铬过程中的铬同位素分馏  

Chromium isotope fractionation during adsorption of chromium(Ⅲ)by soils and river sediments

作　　者：方子遥 何小庆 俞樨 秦礼萍[1,2] Ziyao Fang;Xiaoqing He;Xi Yu;and Liping Qin(CAS Key Laboratory of Crust-Mantle Materials and Environments,University of Science and Technology of China,Hefei 230026,China;CAS Center for Excellence in Comparative Planetology,Hefei 230026,China)

机构地区：[1]中国科学技术大学中科院壳幔物质与环境重点实验室,安徽合肥230026 [2]中国科学院比较行星学卓越创新中心,安徽合肥230026

出　　处：《中国科学技术大学学报》2023年第5期13-21,I0002,I0008,共11页JUSTC

基　　金：supported by the National Key R&D Program of China (2021YFA0718200);the National Natural Science Foundation of China (42103006, 42103007);the Pre-research Project on Civil Aerospace Technologies (D020202) of the Chinese National Space Administration;the Natural Science Foundation of Anhui Province (2108085QD163);the Fundamental Research Funds for the Central Universities of China (WK3410000019, WK2080000152, WK2080000154)。

摘　　要：沉积岩的铬同位素组成已被广泛应用于重建地质历史时期大气氧气含量的波动。这一指标的一个基本前提是自然环境中铬同位素分馏只发生在六价铬与三价铬的氧化还原转化过程中。但是,对非氧化还原过程中的铬同位素分馏行为目前仍然不清楚。本文通过模拟实验发现三价铬被土壤和河流沉积物吸附这一非氧化还原过程也会造成明显的铬同位素分馏。这一过程的铬同位素分馏系数(-0.06‰至-0.95‰,表示为103lnα)要小于氧化还原过程中的。结合前人对于有机配体溶解三价铬过程伴随的非氧化还原铬同位素分馏的研究结果,本文认为沉积岩中观测到的系统性的非常偏正的铬同位素记录可以被解释为大气氧化事件,但是在某些地质时代观测到的较弱的偏正铬同位素记录可能是由于非氧化还原过程造成的,应谨慎解译数据。Chromium(Cr)isotope compositions of sedimentary rocks have been widely used to unravel fluctuations in atmospheric oxygen levels during geologic history.A fundamental framework of this application is that any Cr isotope fractionation in natural environments should be related to the redox transformation of Cr species[Cr(Ⅵ)and Cr(Ⅲ)].However,the behavior of Cr isotopes during non-redox Cr cycling is not yet well understood.Here,we present laboratory experimental results which show that redox-independent adsorption of Cr(Ⅲ)by natural river sediments and soils can be accompanied by obvious Cr isotope fractionation.The observed Cr isotope fractionation factors(−0.06‰–−0.95‰,expressed as 103lnα)are much smaller than those caused by redox processes.Combined with previous studies on redoxindependent Cr isotope fractionation induced by ligand-promoted dissolution,we suggest that the systematic shift to highly fractionated Cr isotope compositions of sedimentary rocks is likely to represent atmospheric oxygenation,but muted signals observed in some geologic periods may be attributed to non-redox Cr cycling and should be interpreted with caution.

关 键 词：铬同位素 吸附 非氧化还原铬循环 大气氧含量 

分 类 号：P597[天文地球—地球化学]