Model Calculations of Ocean Acidification at the End Cretaceous

Abstract:

Most episodes of ocean acidification (OA) in Earth’s past were either too slow or too minor to provide useful lessons for understanding the present. The end-Cretaceous event (66 Mya) is special in this sense, both because of its rapid onset and also because many calcifying species (including 100% of ammonites and >95% of calcareous nannoplankton and planktonic foraminifera) went extinct at this time. We used box models of the ocean carbon cycle to evaluate whether impact-generated OA could feasibly have been responsible for the calcifier mass extinctions. We simulated several proposed consequences of the asteroid impact: (1) vaporisation of gypsum (CaSO₄) and carbonate (CaCO₃) rocks at the point of impact, producing sulphuric acid and CO₂ respectively; (2) generation of NO_x by the impact pressure wave and other sources, producing nitric acid; (3) release of CO₂ from wildfires, biomass decay and disinterring of fossil organic carbon and hydrocarbons; and (4) ocean stirring leading to introduction into the surface layer of deep water with elevated CO₂. We simulated additions over: (A) a few years (e-folding time of 6 months), and also (B) a few days (e-folding time of 10 hours) for SO₄ and NO_x, as recently proposed by Ohno et al (2014. Nature Geoscience, 7:279-282). Sulphuric acid as a consequence of gypsum vaporisation was found to be the most important acidifying process. Results will also be presented of the amounts of SO₄ required to make the surface ocean become extremely undersaturated (Ω_calcite<0.5) for different e-folding times and combinations of processes. These will be compared to estimates in the literature of how much SO₄ was actually released.