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Geologists have documented at least 14 occurrences of "giant ooids," a geologically rare type of carbonate allochem, in Neoproterozoic successions at low paleolatitudes. Recent experiments and modeling demonstrated that ooid size reflects an equilibrium between precipitation and abrasion rates, such that ooid size could be used as a geological proxy for CaCO3 mineral saturation state (Omega). Here, the documented sizes of Neoproterozoic giant ooids were applied to estimate seawater Omega, which provided a novel approach to constraining temperature, partial pressure of CO2, and alkalinity preceding Neoproterozoic glaciations. The results suggest that giant ooid formation was most plausible with seawater alkalinity elevated over its present value by at least a factor of 2, and either much warmer (40 degrees C) or much colder (0 degrees C) climate than modern tropical carbonate platforms, which have important and divergent implications for climate states and ecosystem responses prior to the initiation of each Neoproterozoic glaciation.

Plain Language Summary Ooids are a type of calcium carbonate sediment grain composed of a set of concentric layers formed around a small particle. Although most ooids are sand-size grains (<2 mm in diameter), rare cases, referred to as "giant ooids," are much larger, with some >1 cm in diameter. Geologists have suggested that these giant ooids reflected unusual seawater chemistry, but the exact conditions required for their formation remained unknown. Although giant ooids are geologically rare, a surprising number of occurrences have been described from Neoproterozoic rocks (1,000-541 million years old) that underlie sedimentary layers deposited by low paleolatitude glaciations (i.e., "Snowball Earth" events). This study used the grain diameters of Neoproterozoic ooids to estimate the temperature and composition of seawater when they formed. The results showed that Neoproterozoic seawater must have been either very hot or very cold just prior to these glaciations, a finding that challenges either climate models of this era or conceptual models of common modes of carbonate sediment formation and deposition.