labs_title

Caldeira Lab Research:Paleoclimate and geochemical cycles

Aftermath of the end-Cretaceous mass extinction -- possible biogeochemical stabilization of the carbon cycle and climate

Ken Caldeira & Michael R. Rampino

Following the K/T boundary event 65 million years ago, carbonate productivity in the ocean was reduced by a factor greater than 3. This would generally imply that following the boundary event, there would be a large buildup of carbonate and eventually a calcite saturated ocean. However, since there is little evidence to suggest this, there must have been a change in ocean chemistry by a separate process(es). Following is a study of what ocean process prevented the expected carbonate buildup.


Caldeira, K. and M.R. Rampino, Aftermath of the end-Cretaceous mass extinction — possible biogeochemical stabilization of the carbon cycle and climate, Paleoceanography 8, 515-525, 1993.

Removal of pelagic productivity's effect on carbonate-ion concentration in the ocean: In the model, immediate removal of pelagic productivity first lowered carbonate-ion concentration which was then followed by a sharp rise. In the mixed layer, reduction in pelagic production caused it to begin outgassing CO2, which eventually led to lower concentrations. Following the end of this period of outgassing (around 103 years after cessation), there was a flux of CO2 and alkalinity into the ocean, sharply increasing carbonate-ion concentrations in the absence of pelagic productivity.

Atmospheric CO2 and a removal of pelagic productivity: Following the end of productivity, atmospheric CO2 first rose (due to oceanic degassing) and then began to fall as outgassing ceased.

Abstract

In the aftermath of the Cretaceous/Tertiary (K/T) boundary event (~65 Myr ago), pelagic carbonate productivity was greatly reduced for several hundred thousand years. A decrease in carbonate productivity by a factor greater than 3, in the absence of some mechanism to remove excess carbonate from the ocean, should have resulted in the accumulation of carbon and alkalinity in the oceans. This would cause the atmospheric partial pressure of CO2 to fall dramatically and the deep ocean to become fully saturated with respect to calcite. Evidence of such a period of highly calcite-saturated oceans with low atmospheric pCO2 in the earliest Tertiary is lacking, suggesting that ocean chemistry may have been buffered by some process or processes. Shallow-water carbonate accumulation rates may depend, in part, on carbonate ion concentrations, and thus shallow-water carbonate deposition might act to stabilize ocean chemistry in the face of a dramatic reduction in pelagic productivity. In our four-box ocean model, as the oceanic carbonate ion concentration rises in the face of diminished pelagic carbonate accumulation, the shallow-water carbonate accumulation rate increases, compensating for the reduction in pelagic carbonate accumulation. These model results indicate that the carbonate-ion feedback on shallow-water carbonate sedimentation may have acted to balance oceanic carbon and alkalinity budgets at the K/T boundary, and, furthermore, may have been a primary mechanism maintaining high shallow-water carbonate accumulation rates prior to the Jurassic onset of widespread pelagic carbonate accumulation.