THE AFTERMATH OF NEOPROTEROZOIC GLACIATIONS

Of the many geological features that record Earth system changes associated with Neoproterozoic glaciations, the most perplexing is the genesis of cap carbonates. The seeming consistency of the d¹³C trends of the younger cap carbonate in the Otavi succession of northwestern Namibia forms the cornerstone of the nonuniformitarian snowball Earth hypothesis. The hypothesis infers that epochal-duration global sea-ice cover resulted in sterilisation of Earth's oceans to a more-or-less mantle-derived -5‰ 'stable' state at the time of cap precipitation. Within the snowball Earth model these conditions are envisaged to be global and temporally repetitive and, given that cap precipitation is considered to have occurred geologically instantaneously, then all Neoproterozoic cap carbonates should display reasonably similar trends. We have examined in detail the younger and older cap carbonates on three cratons (southeastern Congo, northwestern Kalahari and southwestern Laurentia) and find a surprising variability in d¹³C trends (as much as 10‰). Isotopic signatures show striking differences for different palaeogeographic settings (shelf, slope, basinal) as well as between the older and younger caps. Evidence favours synchronous cap deposition so that the wide variability in d¹³C trends indicates that Earth's oceans in the immediate aftermath of Neoproterozoic glaciations were not sterile, -5‰ realms, but rather a richly isotopically varied world whose local diversity was captured in the rapidly precipitating cap carbonates.