Numerous investigators have sought to identify the perturbations to the global carbon cycle that fueled Earth system change during the Neoproterozoic Era. Nevertheless, a lack of constraints on ocean-atmosphere carbon chemistry has precluded efforts to link biology, climate, and the lithosphere. We combined field and petrographic observations with experimental and theoretical geochemistry to show that early Neoproterozoic seawater featured elevated alkalinity in the presence of high atmospheric pCO2, which sustained remarkable marine CaCO3 supersaturation (Ωcalcite). Without pelagic calcification, Neoproterozoic marine Ωcalcite and pCO2 would have been mediated principally by CaCO3 precipitation kinetics; thus, secular changes in kinetic inhibitors to CaCO3 nucleation may have destabilized the global carbon cycle.

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