Terrestrial Acidification at the K/T Boundary

Abstract

We focus here on geochemically constraining the production of acid during the KT impact. The potential importance of acid formation at the Cretaceous/Tertiary (K/T) boundary has been discussed by many authors (e.g., [2–7]). Fossil evidence (i.e., the survival of some species of calcareous plankton) for the lack of acidification of ocean surface waters has been cited by D’Hondt et al. [6], thus placing an important constraint on the total acid deposited in the oceans. Global surface cooling due to a decrease in solar insolation by an ejectaderived stratospheric dust layer, an idea originally proposed by Alvarez et al. [8], may have occurred on a decadal timescale as stratospheric SO₂ was slowly converted to sulfuric acid aerosol [9–11]. Evaporite deposits at Chicxulub (e.g., [12]) provided the SO₂ via devolatilization of anhydrite and gypsum. Numerical model estimates of the amount of SO₂ liberated, based on laboratory hyper-velocity impact studies of anhydrite [13, 14], predict ~ 1 × 10¹⁶−1 × 10¹⁷ moles S were released [7]. Assuming complete oxidation of SO₂ to sulfuric acid, this corresponds to ~12−20 × 10¹⁶ equivalents (eq) of acid. Zahnle [5] has estimated that ∼ 1 × 10¹⁵ moles of nitric acid were produced by shock heating of air during bolide and ejecta passage through the atmosphere, making nitric acid only 1–10 % of total strong acid production. Morgan et al. [15], by determining the diameter of the transient impact crater at Chicxulub, estimated that the bolide was the equivalent of the impact of a 12 km asteroid and that ~1 × 10¹⁶ mol SO₂ were produced by the event.