- © 2006 Geological Society of South Africa
Comparison of shallow-water limestone and silicified dolomite from the Mid-Paleoproterozoic Mooidraai Formation, Transvaal Supergroup, South Africa, shows that the primary rare earth element (REE) distribution of these pure marine sedimentary carbonates has been preserved during dolomitization and silicification. Both lithologies display REE (and Y) patterns closely resembling those of present-day seawater, i.e. they show enrichment of the heavy relative to the light REE, positive anomalies of La, Gd and Lu, and super-chondritic Y/Ho ratios. However, these shallow-water carbonates lack any Ce anomalies, indicating that in the Mid-Paleoproterozoic the redox-level of surface water in the Griqualand-West sub-basin of the Kaapvaal Craton did not allow for oxidation of Ce(III). The absence of Ce anomalies from shallow water Mooidraai carbonates indicates a return to marine anoxia immediately after large amounts of marine sedimentary Mn oxides had been deposited in a highly oxygenated marine environment in the underlying Hotazel Formation. This suggests that the “Great Oxidation Event” in the Paleoproterozoic was a transition period characterized by strong fluctuations of the redox level of the Earth’s surface ocean.