Evaporation of seawater produces amorphous calcium-magnesium carbonate when aragonite precipitation is inhibited

As a metastable precursor to crystalline carbonate minerals, amorphous calcium-magnesium carbonate (ACMC) has been implicated in the formation of enigmatic fabrics and minerals, such as calcite microspar, fibrous cements and fabric-retentive dolomite, that characterise both modern and ancient carbonate systems (e.g. Wang et al., 2012). The detection of nanocrystals within ancient dolomicrite, using high-resolution transmission electron microscopy, strengthens this hypothesis (Meister Frisia, 2019). However, it has remained unclear whether natural, abiotic processes could produce ACMC, because of a requirement for extreme carbonate supersaturation. This study tests the hypothesis that in the presence of micromolar concentrations of aqueous phosphate, which can inhibit aragonite precipitation (Roest-Ellis et al., 2021) the evaporation of Neoproterozoic seawater may have increased alkalinity, pH, and carbonate saturation enough to precipitate ACMC in shallow-water settings. We conducted evaporation experiments using phosphate-free and phosphate-bearing (PO4Total = 50 mol/kg) synthetic seawater with Tonian composition. Solution samples, pH and alkalinity measurements were collected at regular intervals over 9-14 days. Final solids were collected and analysed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Experimental data show that phosphate-bearing seawater undergoing evaporative concentration reaches increasingly high alkalinity, pH and carbonate saturation until the first solid phase forms. Analytical data indicate that ACMC precipitated during evaporation of phosphate-bearing seawater, whereas aragonite dominated in phosphate-free systems. When evaporating the water more slowly, the ACMC is observed to recrystallise into other metastable carbonate minerals either fibrous monohydrocalcite or needles of hydromagnesite, depending on the solutions initial Mg/Ca ratio. These results suggest that low concentrations of species which inhibit crystalline carbonate precipitation allow extreme carbonate supersaturation to be reached upon evaporation. We speculate that this pathway may have facilitated platform-scale production of metastable precursors to syndepositional and early diagenetic dolomite in shallow-water late Proterozoic carbonate sediments, consistent with sedimentological and stratigraphic evidence for evaporation.