I/(Ca+Mg)作为指示碳酸盐沉积氧化还原条件的重要指标: 研究进展与问题评述*

海水氧化还原条件显著影响真核生物的起源与早期演化,但以往有关早期海水氧化还原条件研究的对象,主要依赖相对深水的细粒碎屑岩沉积(如黑色页岩),而对真核生物集中分布的浅水环境中的碳酸盐岩关注不够且手段缺乏。这显著制约了对真核生物起源与早期演化机理的认识。近年来,有学者提出碳酸盐岩的I/(Ca+Mg)值可作为反映海洋氧化还原条件的重要指标,并将其广泛应用于海相碳酸盐岩的古氧相研究中。该指标的提出主要基于对现代海洋碘组分的观测以及室内方解石合成实验结果: 观测结果表明,海水中的碘主要以氧化态(IO₃^-)和还原态(I^-)2种形式存在,随着氧含量的下降(如在氧极小带),氧化态的碘被逐步转换为还原态的碘,且海水中的IO₃^-浓度与海水氧含量大体呈正相关。实验研究证明,IO₃^-可按一定的分配系数进入到碳酸盐矿物晶格中,但I^-则不能。由于IO₃^-/I^-的还原势能与O₂/H₂O的还原势能接近,因此I/(Ca+Mg)值是最早响应海洋氧含量下降的指标之一,可用于表征深时(如前寒武纪)次氧化环境中表层海水的氧含量波动。此外,学者们也尝试建立I/(Ca+Mg)值与氧含量之间的半定量关系,如I/(Ca+Mg)值大于0和2.5μmol/mol这两个临界值所对应的海水氧含量。结合大量现代缺氧水体和氧极小带中碘组分与溶解氧浓度相关关系的研究,作者提出I/(Ca+Mg)=1.5μmol/mol为重要的临界值之一,可用于限定初级生产力在表层海水中所能产生的最大氧浓度值(~10 μM),并能进一步区分海水和大气的氧化。此外,对I/(Ca+Mg)值的应用进展及潜在问题进行评述,并对可能的发展方向进行展望。

I/(Ca+Mg)as an important redox proxy for carbonate sedimentary environments: Progress and problems

The redox conditions of seawater play a pivotal role in influencing the origin and early evolution of eukaryotes. However,previous studies regarding ocean redox conditions mainly focus on fine-grained siliciclastic rocks(e.g.,black shale)deposited in relatively deep seawater,rather than carbonates formed in eukaryote-concentrated shallow seawater,due largely to a lack of valid method,significantly limiting our understanding of the mechanisms concerning the origin and early evolution of eukaryotes. In recent years,I/(Ca+Mg)was proposed as a proxy for redox conditions of seawater,and has been widely employed in carbonates to analysis seawater redox conditions. The proposal of this proxy is mainly based on measurements of iodine speciation in modern oceans and experiments of calcite synthesis in laboratory. The measurements demonstrate that marine iodine composition mainly occur in two states, namely, Oxidized-state iodate(IO₃^-)and reduced-state iodide(I^-). With the decrease of oxygen concentration(such as in an oxygen minimum zone,OMZ),the oxidized-state iodate,which is proportional to the oxygen concentration,would be gradually reduced into reduced-state iodide. The experiments confirm that only IO₃^- could be incorporated into the lattices of carbonate minerals with a fixed distribution coefficient,but I^-would be excluded. Because of the high redox potential of IO₃^-/I^-,which is close to that of O₂/H₂O,I/(Ca+Mg) is one of the proxies earliest responding to the decrease of ocean oxygen concentration. I/(Ca+Mg) is therefore sensitive to the variation of oxygen concentrations in weakly oxidized surface seawaters in deep time(e.g.,Precambrian). Furthermore,some scholars attempted to establish semiquantitative relationships of I/(Ca+Mg)values to oxygen concentrations,and two threshold values of I/(Ca+Mg)>0 and 2.5μmol/mol have been proposed as the semiquantitative constraints for the oxygen concentrations in ancient ocean waters. In addition,in the light of the study of iodine speciation and dissolved oxygen concentrations in modern anoxic basins and water columns within OMZs,our results suggest that I/(Ca+Mg)=1.5μmol/mol could be used as the threshold between atmosphere and surface seawater. This threshold value may be used to reflect that the oxygen concentration of surface ocean is up to 10 μM,which is the maximum oxygen concentration increased by the primary productivity,and therefore to distinguish the potential variations of oxygen concentration between atmosphere and surface seawaters. In this paper,some of the recent progress and potential problems in redox analysis using I/(Ca+Mg)in ancient carbonates were briefly reviewed,and some tentative suggestions for future study were also put forward.