海相沉积岩铁同位素体系及其古环境指示

Iron isotopic systematics of marine sedimentary rocks and its implications on paleo-environment

铁(Fe)作为主要造岩元素及生命必需元素,广泛参与地球表层圈层所涉及的物理、化学和生物作用,在调节海洋初级生产力和驱动海洋生物地球化学循环过程扮演重要角色。Fe同位素作为较早开发的非传统稳定同位素体系,目前已成为示踪生物地球化学循环过程及古海洋环境演变的有效手段。本文在对全球海洋Fe循环和表生地质过程Fe同位素分馏机理进行系统总结的基础上,统计了地质历史时期不同类型海相沉积岩的Fe同位素组成,并通过实例探讨了海相沉积岩Fe同位素体系在研究重大地质事件环境演变的潜力,最后对海相沉积岩Fe同位素体系的未来发展方向进行了展望。

Background,aim,and scope As an essential nutrient and major rock-forming element,iron (Fe) plays an important role in regulating marine primary productivity and driving biogeochemical cycles in both the modern and ancient oceans.Fe isotopes are significantly fractionated during marine processes and thus have been widely applied to investigate the marine biogeochemical cycles and redox evolution of paleo-ocean through Earth’s history.To understand Fe isotopic systematics of marine sedimentary rocks and its implications on paleoenvironment,this paper reviewed current major developments,problems and challenges in Fe isotopes of marine sedimentary rocks.Materials and methods Review and prospect are made based on the domestic and international investigations of Fe isotopic behavior during the supergene process and potential implications on biogeochemical cycle and ocean redox in the past two decades.Results The marine sedimentary rocks and minerals,especially pyrite and banded iron formation (BIF),have large Fe isotopic range (- 3.52‰ to 3.91‰).In terms of ages,pyrite has more negative δ;Fe values in Archean.In the Proterozoic,δ;Fe values of pyrite became gradually higher,and δ;Fe values of BIF vary from - 1‰ to 1‰.The Archaean shale has relatively lower δ^(56)Fe values than the Proterozoic shale,which are slightly higher than that of contemporaneous pyrite.The Phanerozoic iron oxide has lighter Fe isotopic composition than that of Precambrian.Discussion Fe isotopes are significantly fractionated during the processes of dissolution,adsorption,dissimilatory iron reduction,and bacterial iron oxidation.In this case,Fe isotope systematics in marine sedimentary rocks is a potential tracer of major geological events,including Great Oxidation Events,Neoproterozoic Oxygenation,and Cretaceous Oceanic Anoxic Events.Conclusions Different sources of Fe and redox processes have great influence on the variation of Fe isotopic composition.How to separate diverse minerals and measure individual Fe speciation could provide significant approach to explain the mechanism of Fe isotope fractionation.Besides,more attention should be focused on distinguishing the isotopic signatures between biotic and abiotic processes.Our study highlights the potential and uniqueness of Fe isotope systematics in tracing paleo-ocean environment and marine biogeochemical cycles.Recommendations and perspectives Further investigation can be strengthened from the following several aspects:(1) Exploring the mechanism of Fe isotopic fractionation on the basis of effectively distinguishing different iron-bearing minerals.(2) In-situ measurement and high-dimensional theory should be developed and improved.(3) The quantitative numerical models can quantify the effects of Fe isotopic fractionation on different end-members of geological processes.