Two or four Neoproterozoic glaciations?

A thick Neoproterozoic carbonate and glaciogenic succession of the southern Congo craton has yielded delta(13)C and Sr-87/Sr-86 records through the la ter Cryogenian (ca. 750-600 Ma) and earlier part of the Terminal Proterozoi c (ca. 600-570 Ma). Sizeable negative delta(13)C excursions (to less than - 5 parts per thousand) occur above each of two glacial intervals and the Sr- 87/Sr-86 values of marine carbonates shift from similar to 0.7072 to simila r to 0.7079 at the upper glacial level. These geochemical constraints provi de a Marinoan (younger Varanger) age for the upper glacial interval, previo usly regarded as a second phase of the Sturtian glaciation. The delta(13)C record from the Congo craton is therefore incompatible with recent global d elta(13)C syntheses that have identified four or more separate ice ages dur ing the Neoproterozoic. A cladistic analysis of geologic and geochemical ch aracters of 12 Neoproterozoic glacial deposits identifies two distinct grou ps that are found in a consistent stratigraphic order whenever two glacial units occur within a single succession. We use delta(13)C and Sr-87/Sr-86 r ecords from the Congo craton and other key successions to test the null hyp othesis 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 a ll known Neoproterozoic glaciations to the Cryogenian. The rapid shift in m arine Sr-87/Sr-86 to more radiogenic values during the Marinoan glaciation is opposite that predicted by the snowball Earth scenario which calls for c ontinental runoff to cease during glaciation, resulting in a shift to less radiogenic values.