Geochemistry of ikaite formation at Mono Lake, California: Implications for the origin of tufa mounds

The mineral ikaite (CaCO₃ ⋅ 6H₂O), not previously observed in lake environments, precipitates seasonally along the shore of Mono Lake, California, where Ca-HCO₃ spring water mixes with cold Na-CO₃ lake water. During the winter, cold water temperatures and high concentrations of PO₄^3- and organic carbon inhibit calcite precipitation, allowing the metastable ikaite to form. During the spring warming, however, ikaite decomposes to form calcium carbonate and water, occasionally leaving pseudomorphs of the primary precipitate. The identification of modern ikaite suggests that both Pleistocene and Holocene tufas in the Mono basin originally precipitated as ikaite. This mineral may also form in other lake environments, but rapid recrystallization after warming destroys the physical, chemical, and isotopic evidence of formation, and alters the geochemical record.