Secular changes in the importance of neritic carbonate deposition as a control on the magnitude and stability of Neoproterozoic ice ages

We hypothesize that secular evolution in the control of calcium carbonate deposition dictated the severity of Neoproterozoic ice ages. In the modern ocean, reduction in carbonate deposition on the continental shelves can be compensated for by the increased preservation in deep sea sediments of biogenic carbonate originating from planktic calcifiers living in the open ocean. The result is that ocean carbonate chemistry is strongly buffered and the carbon-climate system relatively stable. However, before the advent of metazoan biomineralization in the Cambrian and proliferation of calcareous plankton during the Mesozoic, carbonate deposition would have been largely restricted to shallow water photic environments in the Neoproterozoic. A fall in sea level acting to restrict the photosynthetic area within Precambrian seas would necessarily initially decrease the global rate of carbonate deposition without being compensated by the typical Phanerozoic deep sea sedimentary `buffer'. Ultimately, carbonate precipitation would reestablish itself at a greater local rate throughout the more areally restricted seas. The resulting higher degree of ocean saturation translates to substantially lower atmospheric CO<sub>2</sub> and colder terrestrial conditions. Ice ages of near-global extent and multi million-year duration can thus be understood as a direct consequence of the weak `buffering' of the Precambrian carbon cycle, amplified by feedbacks involving CO₂ and climate. Both the widespread occurrence and observed thickness of `cap' (dolostone) carbonate deposited during postglacial flooding of the shelves are explicit predictions of this hypothesis, and record the rapid removal from a highly oversaturated ocean of excess alkalinity accumulated during the glacial.