沉积岩汞同位素的古环境指示意义

Mercury isotopes in sedimentary rocks as a paleoenvironmental proxy

环境与生命的协同演化是地球科学的核心问题之一,而古环境的精确重建对于认识生命演化规律、机制以及预测地球宜居性的未来发展方向具有至关重要的意义.近年来,沉积岩中的汞及其同位素组成在重建地球古环境演化方面展现出巨大的潜力,获得了广泛的关注和应用,已发展成一个新的研究方向.其主要原因是汞及其同位素组成在记录古代火山活动和海洋氧化还原状态方面具有不可替代的优势,能为研究上述过程与重大环境和生命演化事件的关系提供新的证据.本文系统分析了古代沉积岩的汞同位素数据及其意义,总结了大型火山活动和海洋氧化还原状态改变引起汞同位素变化的特征与机制,深入探讨了火山类型及释放汞的方式、大气和海洋汞循环过程、沉积环境、岩性、成岩作用、汞赋存形态等因素对沉积岩汞同位素组成的影响,并对该领域当前存在的问题和发展前景作出了评述和展望.总体而言,这一研究领域仍处于起步阶段,对古环境演化事件中汞同位素组成的变化规律和分馏机理还缺乏系统和定量的认识,亟须深入研究.未来需要揭示古环境汞循环过程中的同位素分馏机理,进一步明确汞同位素指示古环境演化的原理和影响因素,拓展其在环境和生命协同演化领域的应用.

The co-evolution of Earth’s environment and life is one of the most fundamental questions in Earth science.Precise paleoenvironmental reconstruction is the key to understanding the evolution of life,which help predict the future prospect of Earth’s habitability.In recent years,mercury(Hg)and its stable isotopes in ancient sedimentary rocks have emerged as irreplaceable tools to reconstruct Earth’s paleoenvironment,opening a new frontier in Earth science.This study summaries the current state of Hg isotope geochemistry in paleoenvironmental research,and reviews the status and prospects of this promising research field.Mercury and its stable isotopes have been primarily applied as proxies for large-scale volcanic activities(e.g.,large igneous provinces,LIPs),terrestrial soil erosion,and oceanic redox conditions,facilitating the evaluation of the roles of these processes in critical environmental and biotic crises,such as mass extinctions.For example,LIPs are able to emit large quantities of Hg that are readily dispersed globally through the atmosphere,leading to widespread Hg enrichment in marine and terrestrial sediments.Mercury isotopes can distinguish between volcanic and non-volcanic Hg sources,owing to the unique mass-independent fractionation(MIF,represented by Δ^(199)Hg)that this element undergoes.During LIP events,the Δ^(199)Hg value can display either positive or negative shifts relative to the pre-volcanic values,or shifts towards the MIF value of direct volcanic emission(Δ^(199)Hg≈0).A positive shift is typically observed in sections where Hg emitted by volcanism has undergone long-distance transport before deposition,because the photochemical transformations during atmospheric dispersion typically result in positive Δ^(199)Hg in oxidized Hg species that deposit more easily to ocean.A negative shift is typically explained as the result of enhanced inputs of terrestrial Hg associated with soils and plants,which are characterized by negativeΔ^(199)Hg in modern environments.For example,most marine and terrestrial sections of the end-Permian mass extinction(EPME,~252 Ma)display significant negative Δ^(199)Hg shifts(up to-0.20‰)during the extinction interval.It is hypothesized that the eruptions of Siberian Traps LIP and large-scale volcanism in South China led to abrupt climate and environmental changes that enhanced terrestrial weathering and soil erosion.The Δ^(199)Hg value can also shift towards the signature of direct volcanic emission if local volcanism is the dominant source of Hg.Mercury from local volcanism has not undergone long-range atmospheric transport and thus is more likely to retain the original MIF signals of volcanic sources.However,the variations of Hg MIF during volcanic events do not always follow the above framework.Significant differences in the direction and degree of the Δ^(199)Hg shifts can be found either between different volcanic events or between different sedimentary environments.Thus,the mechanism controlling the variations of Hg isotopes during different volcanic events requires further study.In addition to being a proxy for volcanism,Hg isotopes were also recently recognized as a promising proxy for photic zone euxinia(PZE),the enrichment of toxic H_(2)S in the marine photic zone.PZE is particularly detrimental to shallow marine inhabitants because the photic zone is the critical zone that supports marine primary productivity and hosts the majority of marine life.Therefore,the occurrence of PZE has been considered as a potent kill mechanism during almost all the Phanerozoic mass extinction events.A previous pilot study found that ancient marine sedimentary rocks deposited under PZE exhibit more negative odd-MIF and more positive MDF than those formed under non-PZE conditions.This observation was explained by two possible mechanisms:photoreduction of Hg(Ⅱ)complexed by reduced sulfur in a sulfide-rich photic zone,and enhanced sequestration of atmospheric Hg(0)by sulfidic surface water.However,application of Hg isotopes in this aspect is still relatively limited.Overall,as an emerging paleoenvironmental proxy,Hg isotopes have shown great potential in reconstructing the importance of ancient volcanism and ocean redox evolution.However,this field is still at a beginning stage.It faces various challenges,notably the lack of a mechanistic,coherent,and quantitative understanding on how Hg isotopes indicate volcanism and oceanic redox variations.For example,various factors could affect the interpretation of Hg MIF values in sedimentary rock,including the type of volcanism(e.g.,local vs.global,subaerial vs.submarine),the transformation processes of Hg during atmospheric and marine cycling,the lithology and Hg speciation,transformation and migration of Hg during diagenesis.However,these influences have received little investigation.Future research should focus on revealing the underlining principle of Hg isotopes as a paleoenvironmental proxy by clarifying its fractionation mechanisms and calibrating its use in various depositional settings,thus advancing this new research field.