In the aftermath of the Cretaceous/Tertiary (K/T) boundary event (∼65 m.y. 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 CO2to 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 atmosphericpCO2in 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 carbo
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