CaCO3 precipitation kinetics and polymorphism in ferruginous seawater

Secular trends in Precambrian calcium carbonate (CaCO₃) sediments that nucleated within the water column or precipitated directly on the seafloor reflect evolving chemical controls on their formation. Previous work has hypothesized that kinetic inhibitor species like ferrous iron (Fe²⁺) significantly raised the free energy barrier to CaCO₃ nucleation and drove alternative mineralization pathways. However, direct evidence for Fe²⁺-inhibition on CaCO₃ precipitation in anoxic seawater is limited. Here we present experimental results on the nucleation of carbonate minerals in simulated anoxic Archean-Paleoproterozoic seawater across a range of ferrous iron concentrations ([Fe²⁺] = 0.05 to 5.0 mmol/kg) and calcite saturation states. With increasing [Fe²⁺] the rate of CaCO₃ precipitation decreased, aragonite increased in abundance over calcite, and calcite crystallites exhibited marked morphological modification. We observed no concentration of ferrous iron at which calcite nucleation is fully suppressed, but at [Fe²⁺] > 1 mmol/kg noted aggregates of variably Ca-rich, Fe(II)-bearing carbonate which may have crystallized from an amorphous Ca-Fe carbonate precursor. Our results suggest that it is unlikely that dissolved iron was present in high enough concentrations to independently control secular trends in Precambrian carbonate sedimentation, but that Fe²⁺-inhibition contributed to maintaining supersaturation with respect to calcite. Furthermore, our results imply that the nucleation of ferroan calcite and/or amorphous Ca-Fe carbonate from supersaturated ferruginous solutions served as an additional and potentially significant sink for iron from the Precambrian oceans.