I/(Ca+Mg)as an important redox proxy for carbonate sedimentary environments: Progress and problems
Shang Mo-Han1,2, Tang Dong-Jie1,3, Shi Xiao-Ying1,2, Wei Hao-Ming2, Liu An-Qi3
1 State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences(Beijing),Beijing 100083; 2 School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083; 3 Institute of Earth Sciences,China University of Geosciences(Beijing),Beijing 100083
Abstract：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.
Shang Mo-Han,Tang Dong-Jie,Shi Xiao-Ying et al. I/(Ca+Mg)as an important redox proxy for carbonate sedimentary environments: Progress and problems[J]. JOPC, 2018, 20(4): 651-664.
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