Severity of ocean acidification following the end-Cretaceous asteroid impact

Most paleo-episodes of ocean acidification (OA) were either too slow or too small to be instructive in predicting near-future impacts. The end-Cretaceous event (66 Mya) is intriguing in this regard, both because of its rapid onset and also because many pelagic calcifying species (including 100% of ammonites and more than 90% of calcareous nannoplankton and foraminifera) went extinct at this time. Here we evaluate whether extinction-level OA could feasibly have been produced by the asteroid impact. Carbon cycle box models were used to estimate OA consequences of (i) vaporization of up to 60 × 10¹⁵ mol of sulfur from gypsum rocks at the point of impact; (ii) generation of up to 5 × 10¹⁵ mol of NO_x by the impact pressure wave and other sources; (iii) release of up to 6,500 Pg C as CO₂ from vaporization of carbonate rocks, wildfires, and soil carbon decay; and (iv) ocean overturn bringing high-CO₂ water to the surface. We find that the acidification produced by most processes is too weak to explain calcifier extinctions. Sulfuric acid additions could have made the surface ocean extremely undersaturated (Ω_calcite <0.5), but only if they reached the ocean very rapidly (over a few days) and if the quantity added was at the top end of literature estimates. We therefore conclude that severe ocean acidification might have been, but most likely was not, responsible for the great extinctions of planktonic calcifiers and ammonites at the end of the Cretaceous.From preindustrial times up to 2008, ca. 530 Pg of carbon were added to the atmosphere through burning of fossil fuels and deforestation (1). This has led to an increase in atmospheric CO₂ of 40% (from 280 in 1750 to 400 ppm today). Simultaneously, about 160 Pg C has been taken up by the ocean, causing ocean acidification (OA) (2).OA is of particular concern for calcifying organisms, because it leads to lower CO₃²⁻ concentrations and hence lower seawater saturation states with respect to CaCO₃ (Ω). In theory, lower Ω should make it energetically more costly for organisms to synthesize CaCO₃ shells and skeletons and, subsequently, if Ω falls below 1.0, to maintain them against dissolution. A large variety of short-term experiments have been carried out to test for such consequences (2). It is widely recognized, however, that one aspect that these experiments generally do not address is the degree to which organisms can evolve in response to the changing carbonate chemistry and thereby become more tolerant of the new conditions. As a result, there is a need for approaches that reveal the long-term response to OA with evolutionary adaptation factored in.