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

Abstract

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.