Experimental constraints on nonskeletal CaCO₃ precipitation from Proterozoic seawater

Precambrian carbonates record secular variations in the style of CaCO₃ nucleation and growth, yet the geochemical conditions recorded by some enigmatic textures remain poorly quantified. Here, we performed CaCO₃ nucleation experiments in synthetic seawater in order to constrain the mineralization pathways of synsedimentary calcite microspar cement, a prolific component of Proterozoic carbonates. We found that dissolved PO₄ above ~12 μmol/L (μM) inhibits the nucleation of aragonite and calcite and permits the formation of an amorphous Ca-Mg carbonate (ACMC) precursor once CaCO₃ supersaturation (Ω_cal) is ≥ 45. Depending on seawater Mg/Ca, ACMC then rapidly recrystallizes to monohydrocalcite and/or calcite. This precipitation mechanism is consistent with sedimentological, petrographic, and geochemical characteristics of Proterozoic synsedimentary calcite microspar cement, and it suggests that kinetic interactions among common seawater ions may open nontraditional CaCO₃ mineralization pathways and sustain high CaCO₃ supersaturation.