Redbed formation in the redox-stratified mid-Proterozoic ocean

Abstract

Deposition of iron-enriched redbeds occurred episodically on different continents throughout the mid-Proterozoic, but their origin remains unclear. Since these sediments host abundant Fe oxides, their occurrence potentially represents changes in ocean-redox state, a hypothesis having significant implications for the evolution of early aerobic eukaryotes. In the present study, we report Fe speciation and isotopes, major and trace elements, organic carbon contents, and mineralogy of the 1.6-Ga Cuizhuang and 1.4-Ga Xiamaling redbeds and intercalated non-redbeds in North China to investigate their depositional environments. As expected, the redbeds show significantly higher total Fe contents (FeT), FeT/Al, and FeHR/FeT ratios (FeHR: highly reactive Fe) than the adjacent non-redbeds (mean FeT: 5.0 % vs. 2.9 %), with 73–97 % of the FeHR in the form of Fe oxides (Feox), accounting for the difference in FeHR/FeT ratios. However, almost all other major (e.g., Al and Ti, typically associated with the detrital fraction) and trace elements show no obvious difference in concentrations between the redbeds and non-redbeds, supporting a single, shared detrital source. The FeT/Al and FeHR/FeT ratios of the redbeds are higher than their respective thresholds for anoxic conditions, whereas the non-redbeds show the opposite. We thus infer that the non-redbeds were deposited in a shallow oxic setting and the redbeds in a deep anoxic setting, separated by a pronounced Fe-redoxcline. The iron was mainly sourced from seafloor hydrothermal fluids and then partially oxidized and precipitated in shallow environments, resulting in positive Fe-isotope values (+0.24 to +0.57‰). Preservation of large amounts of Fe-(hydr)oxides within an anoxic setting can be explained through low-productivity conditions, as inferred from low TOC (mean 0.1 %) and P (mean 452 ppm) contents in both redbeds and non-redbeds. Based on this scenario, modeling indicates that the degree of fractionation of FeHR isotopes (+0.36 to +1.38‰) in the redbeds is consistent with oxidation of 66–98 % of the hydrothermally supplied Fe in waters with an average photic-zone O2 concentration of <0.1 μmol/L (equivalent to atmospheric pO2 < 0.1 % PAL), providing further evidence for extremely low O2 levels during the mid-Proterozoic.