Mineralogy-based stepwise dissolution of dolomitic limestone reveals high iodine content in water-column precipitated calcite and well-oxygenated shallow seawater during the ~1.57 Ga oxygenation event
Description
Carbonate I/(Ca+Mg) has been used as a proxy to track shallow-seawater oxygen levels through Earth’s history. However, due to diagenetic alteration and homogenization of iodine in carbonates formed in a redox-stratified water column or in porewater, bulk-rock 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 using dilute nitric acid (0.03% v/v) to obtain I/(Ca+Mg) values of water-column precipitated calcite during the ~1.57 Ga oxygenation event in North China. The results show that at the peak of the oxygenation event, the I/(Ca+Mg) ratios of primary calcites are up to ~11 μmol/mol, which are significantly higher than the bulk-rock I/(Ca+Mg) values (up to ~4 μmol/mol). The new data imply that local shallow seawater O2 concentrations at ~1.57 Ga were higher than previously estimated and sufficient to support the respiratory needs of eukaryotes including animals. The delay of complex eukaryote and ecosystem evolution during the mid-Proterozoic (1.8–0.8 Ga) was not due to the lack of local oxic niches for eukaryotes but a consequence of temporal and spatial redox instability in shallow-marine environments. 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.