Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; University of Chinese Academy of Sciences, Beijing 100049, China; Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin 300387, China.
Center for Environmental Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China.
Sci Total Environ. 2022 Apr 15;817:152609. doi: 10.1016/j.scitotenv.2021.152609. Epub 2021 Dec 25.
The chemical composition and redox conditions of the Precambrian ocean are key factors for reconstructing the temporal evolution of atmospheric oxygen through time. In particular, the isotopic composition of iron are useful proxies for reconstructing paleo-ocean environments. Yet, respective processes and related signatures are poorly constrained, hindering the reconstruction of iron redox mechanisms in the Archean ocean. This study centers on Sihailongwan Lake, a stratified water body with a euxinic lower water column considered as an Archean ocean analogue. Results show that the anaerobic oxidation layer is so different from other similar lakes in which dissolved Fe oxidation is present in redoxcline layer. And the fractionation factor between ferrous Fe and iron hydroxide observed in nature water body of Sihailongwan Lake reaches to 2.6‰, which would benefit the production of the oxidations of BIF in sediment. By the spatial distribution of Fe isotope, the benthic water in autumn and the hypolimnetic anoxic water in spring has been identified as iron sulfide zone, where iron isotopic fractionation factor during iron sulfide formation is 1.16‰, accounting for partial scavenging of dissolved Fe(II) with an associated isotopic fractionation. However, pyrite in the sediment records the iron isotopic signal from the redoxcline but not in the iron sulfide or oxide zones of the water column. Above findings indicate that neither the iron isotope fractionation during partial transfer of ferrous iron to iron sulfide nor the partial oxidation of ferrous iron are recorded as pyrite in sedimentary rock. Importantly, the signal of Fe isotopic fractionation in water was archived in the suspended particulate matter and transferred into the sediment, rather than via ferrous iron directly deposited in the sediment. This study reveals that Fe isotopes from modern natural environments are useful proxies for reconstructing iron oxidation-reduction process during Earth's early history.
前寒武纪海洋的化学成分和氧化还原条件是重建大气氧随时间演变的关键因素。特别是，铁的同位素组成是重建古海洋环境的有用示踪剂。然而，相关过程和相关特征还没有得到很好的约束，这阻碍了太古代海洋中铁的氧化还原机制的重建。本研究以扇积龙洼湖为中心，扇积龙洼湖是一个分层水体，其下部水体缺氧，被认为是太古代海洋的类似物。结果表明，厌氧氧化层与其他类似湖泊不同，在其他类似湖泊中，溶解的 Fe 氧化存在于氧化还原层中。在扇积龙洼湖天然水体中观察到的亚铁 Fe 和氢氧化铁之间的分馏因子达到 2.6‰，这有利于 BIF 在沉积物中的氧化。根据 Fe 同位素的空间分布，秋季的底水和春季的缺氧下底层水已被确定为铁硫化物区，在此期间，铁硫化物形成过程中的铁同位素分馏因子为 1.16‰，这部分解释了溶解 Fe(II)的部分消耗及其相关的同位素分馏。然而，沉积物中的黄铁矿记录了来自氧化还原层的铁同位素信号，但没有记录在水柱的铁硫化物或氧化物区。上述发现表明，亚铁铁向铁硫化物的部分转移过程中的铁同位素分馏，以及亚铁铁的部分氧化，都没有记录在沉积岩中的黄铁矿中。重要的是，水中 Fe 同位素分馏的信号被保存在悬浮颗粒物中，并转移到沉积物中，而不是通过亚铁铁直接沉积在沉积物中。本研究表明，现代自然环境中的 Fe 同位素是重建地球早期铁氧化还原过程的有用示踪剂。
