Geologists have documented at least fourteen occurrences of “giant
ooids”, a geologically rare type of carbonate allochem, in
Neoproterozoic successions at low paleo-latitudes. 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 (Ω). Here, the
documented sizes of Neoproterozoic giant ooids were applied to estimate
seawater , which provided a novel constraint on 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 two, and either much warmer (40C) or much colder (0C) 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.