Calcium isotope constraints on Mesoarchean seawater

The cause of the Great Oxidation Event ~2.4 billion-years-ago (Ga) is hotly debated. Recent models favor the emergence of continents as driving the event. However, we suggest that extensive shallow-marine carbonate platforms existed in the Mesoarchean. This conclusion is based on Ca isotopes from 2.8 Ga carbonate rocks, that constrains the Ca isotope value of Mesoarchean seawater to -0.5‰ relative to present day values. This estimate is strikingly similar to pre-Mesozoic values, suggesting that continental freeboard and the area of carbonate platforms was relatively consistent through most of Earth history. Shallow-marine environments were not only extensive in the Mesoarchean, but carbonate Ce anomalies and Mn concentrations indicate that sedimentary conditions for organic carbon burial existed prior to the rise of atmospheric oxygen.