Two or four Neoproterozoic glaciations?

A thick Neoproterozoic carbonate and glaciogenic succession of the southern Congo craton has yielded δ¹³C and ⁸⁷Sr/⁸⁶Sr records through the later Cryogenian (ca. 750-600 Ma) and earlier part of the Terminal Proterozoic (ca. 600-570 Ma). Sizeable negative δ¹³C excursions (to less than -5‰) occur above each of two glacial intervals and the ⁸⁷Sr/⁸⁶Sr values of marine carbonates shift from ~0.7072 to ~0.7079 at the upper glacial level. These geochemical constraints provide a Marinoan (younger Varanger) age for the upper glacial interval, previously regarded as a second phase of the Sturtian glaciation. The δ¹³C record from the Congo craton is therefore incompatible with recent global δ¹³C syntheses that have identified four or more separate ice ages during the Neoproterozoic. A cladistic analysis of geologic and geochemical characters of 12 Neoproterozoic glacial deposits identifies two distinct groups that are found in a consistent stratigraphic order whenever two glacial units occur within a single succession. We use δ¹³C and ⁸⁷Sr/⁸⁶Sr records from the Congo craton and other key successions to test the null hypothesis that there were only two global glaciations (Sturtian and Marinoan) during the Neoproterozoic. Placing the GSSP (global stratotype section and point) for the base of the Terminal Proterozoic within or just above a cap carbonate of the younger (Marinoan) glaciogenic succession would confine all known Neoproterozoic glaciations to the Cryogenian. The rapid shift in marine ⁸⁷Sr/⁸⁶Sr to more radiogenic values during the Marinoan glaciation is opposite that predicted by the snowball Earth scenario which calls for continental runoff to cease during glaciation, resulting in a shift to less radiogenic values.