Caldeira Lab Research:Paleoclimate and geochemical cycles

Carbonate deposition, climate stability, and Neoproterozoic ice ages

Andy J. Ridgwell, Martin J. Kennedy & Ken Caldeira

In the Neoproterozoic, climate was greatly influenced by sea level change. A reduction in sea level would have dramatically increased carbonate ion concentration in the ocean by reducing shallow water deposition areas, which would have in turn reduced atmospheric CO2 and lowered temperature. This effect is not seen in modern climate systems due to the rise of planktic calcifiers that allow for carbonate buildup in the deep sea. It is suggested here that a drastic lowering of sea level could have contributed to the Neoproterozoic ice ages, which lasted for millions of years.

Ridgwell, A,J,, M.J. Kennedy, and K. Caldeira, Carbonate deposition, climate stability, and Neoproterozoic ice ages, Science 302, 859–862, 2003.

The ocean system in the Precambrian and the present: In the modern ocean (A and B), a drop in sea level and a loss of space for carbonate deposition in shallow water would eventually be compensated for by larger amounts of deposition in deep water. In the precambrian system (C and D), there was nowhere to store carbonate other than shallow water deposition sites. A reduction in sea level would reduce the size of these deposition sites and raise the ocean's carbonate ion concentration.

Sea level and its effect on atmospheric carbon: In the model, a large reduction in sea level quickly led to highly reduced atmospheric CO2 concentration. This level of reduction would have caused a global decrease in temperature and could have been the cause of ice ages.


The evolutionary success of planktic calcifiers during the Phanerozoic stabilized the climate system by introducing a new mechanism that acts to buffer ocean carbonate-ion concentration: the saturation-dependent preservation of carbonate in sea-floor sediments. Before this, buffering was primarily accomplished by adjustment of shallow-water carbonate deposition to balance oceanic inputs from weathering on land. Neoproterozoic ice ages of near-global extent and multimillion-year duration and the formation of distinctive sedimentary (cap) carbonates can thus be understood in terms of the greater sensitivity of the Precambrian carbon cycle to the loss of shallow-water environments and CO2-climate feedback on ice-sheet growth.