As it ever was: Calcium isotope constraints on Mesoarchean seawater chemistry

Earth’s long-term climate is closely tied to the formation of carbonate sediments in the ocean, driving the ocean’s ability to moderate atmospheric CO2 levels. It has been hypothesized that in the Precambrian, this process was controlled by a “hidden” deep-sea carbonate sink. However, our results indicate that shallow-marine platforms have been the dominant mode of carbonate formation since the Mesoarchean. This conclusion is based on Ca isotopes from 2.8 Ga carbonates, which constrain the Ca isotope value of Mesoarchean seawater to −0.5‰ relative to modern values, strikingly similar to pre-Mesozoic values and suggesting that carbonate deposition mainly was confined to shallow-water platforms until the evolution of pelagic calcifiers in the Mesozoic.