The Enigma of Neoproterozoic Giant Ooids—Fingerprints of Extreme Climate?

Abstract

AbstractGeologists 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 (Ω). Here, the documented sizes of Neoproterozoic giant ooids were applied to estimate seawater Ω, 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 °C) or much colder (0 °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.