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

Ken Caldeira and Michael R. Rampino · Paleoceanography 8, 515-525 · 1993

Key finding. A more-than-threefold decrease in pelagic carbonate productivity after the K/T boundary should have caused atmospheric CO2 to fall dramatically and the deep ocean to become fully calcite-saturated, but no such interval is recorded — indicating ocean chemistry was buffered, plausibly by shallow-water carbonate deposition responding to rising carbonate ion concentration.

Two stacked line plots of modelled atmospheric partial pressure of carbon dioxide, from 350 to 550 parts per million, against time since the cessation of pelagic productivity. In panel a, on a linear axis to one million years, the curve spikes briefly at the start and then settles into a slow decline from about 400 to about 365 parts per million. In panel b, the same result on a logarithmic axis from 1 to 10 million years, showing a rise to a peak near 505 parts per million at about 1,000 years, then a decline back to about 365 parts per million by a million years.
The logarithmic axis (panel b) separates what the linear one hides. Losing the biological carbon pump first raises atmospheric carbon dioxide, as the deep ocean degasses to an atmosphere no longer separated from it by a strong gradient; only later does carbonate accumulation draw it back down. The whole excursion stays within about 150 parts per million — not the dramatic collapse the loss of pelagic carbonate production would otherwise imply. Figure 4 from Caldeira and Rampino (1993), Paleoceanography 8, 515-525. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

For several hundred thousand years after the Cretaceous/Tertiary boundary event about 65 million years ago, pelagic carbonate productivity was greatly reduced. Organisms that had been removing carbonate from the ocean largely stopped doing so.

Arithmetically that should have caused carbon and alkalinity to accumulate in the oceans, atmospheric CO2 to fall dramatically, and the deep ocean to become fully saturated with respect to calcite. The geological record shows no such episode. This paper asked what prevented it.

What did we find?

The mismatch is the starting point. Pelagic carbonate productivity fell by a factor greater than three, and absent some compensating removal of carbonate, carbon and alkalinity would have accumulated in the ocean.

The predicted consequences — a dramatic fall in atmospheric partial pressure of CO2 and a fully calcite-saturated deep ocean in the earliest Tertiary — are not evident in the record.

A four-box ocean model shows how the discrepancy can be resolved. If shallow-water carbonate accumulation rates depend in part on carbonate ion concentration, then as carbonate ion rises in response to diminished pelagic productivity, shallow-water deposition increases and removes the excess.

That makes shallow-water carbonate deposition a stabilizing feedback on ocean chemistry, capable of absorbing a very large disruption to the biological carbon pump.

Why does it matter?

The paper is an early example of using a mass extinction as a natural experiment on the carbon cycle. A perturbation far larger than anything observable today reveals which feedbacks are strong enough to matter.

The feedback identified here also sets expectations for the present. Carbonate chemistry has mechanisms that stabilize it over hundreds of thousands of years, which is precisely why they offer no protection against a disturbance delivered over two centuries.

Citation

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

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