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Contrasting Mg isotopic compositions between Fe-Mn nodules and surrounding soils: Accumulation of light Mg isotopes by Mg-depleted clay minerals and Fe oxides
Ting Gao;  Shan Ke;  Shui-Jiong Wang;  Fangbai Li;  Chengshuai Liu;  Jing Lei;  Changzhong Liao;  Fei Wu
2018
发表期刊Geochimica Et Cosmochimica Acta
卷号237页码:205-222
摘要

Magnesium isotopic systematics has been increasingly used to trace the biogeochemical cycle of Mg in soil systems, and Fe oxides are the critical soil components that affect the geochemical behaviours of elements in soils. The role of Fe oxides in fractionating Mg isotopes, however, remains unclear. Here, Mg isotopic compositions are reported for typical Fe-Mn nodules (FMNs), surrounding soils, soil waters, and soil surface waters for a paddy soil profile, and stream waters, and rainwaters in southwestern China to improve our understanding of the processes that control the Mg isotopic compositions in soil systems. Further sequential extraction experiments are conducted to separate two pools of Mg in the FMNs and soils: structural Mg and exchange Mg. The FMNs (-1.39 to -1.58 parts per thousand) are isotopically lighter than surrounding soils (-0.59 to -0.85 parts per thousand) but heavier than soil waters (-1.59 parts per thousand), and surrounding soils are isotopically lighter than parent granite (-0.25 parts per thousand). The difference in Mg isotopic compositions between FMNs and surrounding soils reflects different sources of Mg in the mineral crystal structures. Structural Mg in surrounding soils is mainly from the chemical weathering of parent granite. By contrast, structural Mg in FMNs is from soil waters because of the frequently repeated dissolution and precipitation of Fe oxides under alternating redox conditions. Enrichment of heavy Mg isotopes in the FMNs relative to soil waters results from preferential incorporation of 26 Mg via Mg2+ substitution for Fe3+ in goethite. Given that the exchangeable Mg (-1.62 to -1.91 parts per thousand) is significantly enriched in light Mg isotopes, the lighter Mg isotopic compositions in surrounding soils relative to their parent granite can be explained by the retention of light Mg isotopes in exchangeable sites of Mg-depleted minerals (kaolinite). Exchangeable Mg in FMNs (-1.79 to -2.15 parts per thousand) is also shown to be enriched in light Mg isotopes. These light isotopic compositions of exchangeable Mg can be explained by a combination of carbonate contribution and isotope fractionation processes on the soil exchange fractions. Ion-exchange processes preferentially remove heavy Mg isotopes from soil minerals, leaving soil exchange fractions hosting light Mg isotopes. Additionally, river waters draining carbonates contributes light Mg isotopes to soil exchangeable fractions. Our study demonstrates that the development of Mg-depleted clay minerals and Fe oxides can considerably lower the soil delta Mg-26 values, highlighting the major roles of these two soil minerals in controlling soil Mg isotopic compositions. 

关键词Mg Isotopes Soils Fe Oxides Mg-depleted Clay Mineral Exchangeable Mg
收录类别SCI
语种英语
文献类型期刊论文
条目标识符http://ir.gyig.ac.cn/handle/42920512-1/8837
专题环境地球化学国家重点实验室
作者单位1.State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, PR China
2.Guangdong Key Laboratory of Integrated Agro-Environmental Pollution Control and Management, Guangdong Institute of Eco-Environmental Sciences & Technology, Guangzhou 510650, PR China
3.University of Chinese Academy of Sciences, Beijing 100049, PR China
4.State Key Laboratory of Geological Process and Mineral Resources, School of Earth Science and Resources, China University of Geosciences, Beijing 100083, PR China
5.College of Agriculture, Guangxi University, Nanning 530005, PR China
推荐引用方式
GB/T 7714
Ting Gao;Shan Ke;Shui-Jiong Wang;Fangbai Li;Chengshuai Liu;Jing Lei;Changzhong Liao;Fei Wu. Contrasting Mg isotopic compositions between Fe-Mn nodules and surrounding soils: Accumulation of light Mg isotopes by Mg-depleted clay minerals and Fe oxides[J]. Geochimica Et Cosmochimica Acta,2018,237:205-222.
APA Ting Gao;Shan Ke;Shui-Jiong Wang;Fangbai Li;Chengshuai Liu;Jing Lei;Changzhong Liao;Fei Wu.(2018).Contrasting Mg isotopic compositions between Fe-Mn nodules and surrounding soils: Accumulation of light Mg isotopes by Mg-depleted clay minerals and Fe oxides.Geochimica Et Cosmochimica Acta,237,205-222.
MLA Ting Gao;Shan Ke;Shui-Jiong Wang;Fangbai Li;Chengshuai Liu;Jing Lei;Changzhong Liao;Fei Wu."Contrasting Mg isotopic compositions between Fe-Mn nodules and surrounding soils: Accumulation of light Mg isotopes by Mg-depleted clay minerals and Fe oxides".Geochimica Et Cosmochimica Acta 237(2018):205-222.
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