SECULAR CHANGES IN THE MODES OF CARBONATE DEPOSITION AND THE STABILITY OF THE CLIMATE SYSTEM

In the late Phanerozoic, 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 of this is that ocean carbonate chemistry is strongly buffered and the carbon-climate system relatively stable against perturbation of sea level. However, before the advent of biogenic carbonate precipitation by pelagic calcareous plankton and benthic metzoa, carbonate deposition would have been largely restricted to shallow water photic environments. Neoproterozoic 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.

We hypothesize that the evolution of calcifying coccolithophores and foraminifera during the early Phanerozoic and associated secular change in the controls on carbonate deposition and ocean chemistry led to a fundamental change in how the Earth’s climate system operated. This is arguably the first clear case in which biological evolution has led to increased climate stability – the central tenet of the ‘Gaia hypothesis’. It would appear that a relatively small proportion of all planktic taxa are thus directly responsible for the relative stability of the modern climate system.