Bolide impacts, acid rain, and biospheric traumas at the Cretaceous-Tertiary boundary

Large projectiles impacting the Earth will cause severe shock heating and chemical reprocessing of the Earth's atmosphere. This occurs during atmospheric entry and, more importantly, as a result of the supersonic plume ejected on impact. In particular, very large amounts of nitric oxide would result from the impact of the putative Cretaceous-Tertiary bolide(s). We present models of the shock chemistry, the conversion of NO to NO₂ and thence to nitrous and nitric acids, and the global dispersion of the NO₂ and acids. Two plausible projectiles are considered: an ice-rich long-period comet and a much smaller rock-metal asteroid. The comet produces semi-global atmospheric NO₂ mixing ratios of 0.1% while the asteroid produces these high mixing ratios only in the immediate region of the impact. The comet produces concentrated nitrous and nitric acid rain with apH ≈ 0–1.5 globally. The asteroid produces rain with apH ≈ 0–1 near the impact and≈ 4–5 globally. Immediate environmental effects of these nitrogen species include inhibition of photosynthesis due to extinction of solar radiation by NO₂, foliage damage due to exposure to NO₂ and HNO₃, toxicosis resulting from massive mobilization of soil trace metals, and respiratory damage due to exposure to NO₂. The acid rain decreases the pH of the oceanic mixed layer affecting the stability of calcite with important implications for the survival of calcareous-shelled organisms and for exhalation of CO₂ from the mixed layer to the atmosphere. Longer-term environmental effects perhaps≈ 1000years in duration include a global warming due to the sudden CO₂ injection and the simultaneous extinction of the ocean micro-organisms which normally help remove CO₂ from the atmosphere-mixed layer system. Havens for survival include carbonate-buffered lakes and burrows. This acid rain theory therefore serves to explain the peculiar selectivity of the extinctions seen at the Cretaceous-Tertiary boundary. The first few years of acid rain will lead to massive weathering rates of continental soils characterized by large ratios of the relatively insoluble metals (e.g. Be²⁺, Al³⁺, Hg²⁺, Cu⁺, Fe²⁺, Fe³⁺, Tl³⁺, Pb²⁺, Cd²⁺, Mn²⁺, Sr²⁺) to the more soluble metals (Ca²⁺, Mg²⁺) which should have left a clear signal in the fossils of neritic organisms and in unperturbed neritic sediments near river deltas if such sediments still exist for the Cretaceous-Tertiary time frame.