High oxygen concentration in shallow seawater during a transient mid-Proterozoic oxygenation event revealed by iodine content in water-column precipitated calcite
Carbonate I/(Ca+Mg) ratio has been used as a proxy to track shallow-seawater oxygen levels through Earth’s history. However, due to potential loss of iodine during diagenetic transformation (e.g., dolomitization) and homogenization of iodine in carbonate minerals formed in the redox-stratified water column or porewater, bulk-rock carbonate I/(Ca+Mg) values―and thus the oxygen levels in Precambrian shallow seawater―could have been significantly underestimated. Here, we report a mineralogy-based sequential dissolution method to obtain I/(Ca+Mg) values from water-column precipitated calcites in mid-Proterozoic dolomitic limestones from North China. Petrographic observations indicate that these dolomitic limestones consist of both calcite precipitated from the water column and dolomite that formed in porewater. Using dilute acid (0.1% v/v and 0.03% v/v nitric acid) for stepwise carbonate dissolution showed progressive decrease in I/(Ca+Mg) values and increase in Mg/Ca molar ratios. Comparison of the data obtained from the dissolution steps, using 0.03% v/v nitric acid to dissolve 5% of dolomitic limestones most effectively captured the I/(Ca+Mg) values of the initial water-column precipitated calcites, which are significantly higher than those of the corresponding bulk-rock. Application of this method to the ~1.57 Ga oxygenation event suggests initial I/(Ca+Mg) ratios up to ~11 μmol/mol. These I/(Ca+Mg) values indicate that shallow seawater O2 levels, which may have exceeded 125 μM, could have been underestimated by previous bulk-rock analyses that indicated I/(Ca+Mg) up to 4.1 μmol/mol, corresponding to O2 levels > 20 μM. These results suggest that fluctuating O2 concentrations in mid-Proterozoic (1.8–0.8 Ga) shallow seawater were likely high enough to support the respiratory requirements of eukaryotes including animals, at least during transient oxygenation events. This data set contains the duplicate tests of iodine in dolostone standard GSR-12, and major elements in JDo-1 (Table S1 and S2). The data set also shows the detailed dissolution data of a stepwise dissolution protocol (Table S3 and S4), and its application on the ~1.57-Ga Gaoyuzhuang Formation samples (Table S5). We use dilute nitric acid to partially dissolve the dolomitic limestone samples to acquire the I/(Ca+Mg) values from water-column precipitated carbonate mud. The results show significantly high I/(Ca+Mg) values than those acquired with bulk-rock dissolution methods.