Severity of ocean acidification following the end-Cretaceous asteroid impact

Abstract

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 × 10(15) mol of sulfur from gypsum rocks at the point of impact; (ii) generation of up to 5 × 10(15) mol of NOx 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 (Ωcalcite <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.

Keywords: K/Pg boundary; asteroid impact; mass extinction; ocean acidification.

Fig. 1.

Impacts of different scenarios of environmental change on surface ocean chemistry: (A–C) pH and (D–F) saturation state for calcite (Ω_calcite). Each column shows the acidification impacts for a different type of forcing (same vertical axis scale for each) when the forcing is applied with an e-folding time of 6 mo. Minimum Ω_calcite values for each model run are shown in the boxes. The color of each line indicates the magnitude of the forcing for that run.

Fig. 2.

Impacts of very rapid additions (e-folding time of 10 h, ref. 14) of H₂SO₄ (A and D), CO₂ (B and E), and HNO₃ (C and F) on pH (A–C) and saturation state for calcite (D–F). Minimum Ω_calcite values for each model run are shown in the boxes. The color of each line indicates the magnitude of the forcing for that run. The axis scale for pH differs from that in Fig. 1.