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 NO 2 and thence to nitrous and nitric acids, and the global dispersion of the NO 2 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 NO 2 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 NO 2, foliage damage due to exposure to NO 2 and HNO 3, toxicosis resulting from massive mobilization of soil trace metals, and respiratory damage due to exposure to NO 2. 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 CO 2 from the mixed layer to the atmosphere. Longer-term environmental effects perhaps≈ 1000years in duration include a global warming due to the sudden CO 2 injection and the simultaneous extinction of the ocean micro-organisms which normally help remove CO 2 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. Be 2+, Al 3+, Hg 2+, Cu +, Fe 2+, Fe 3+, Tl 3+, Pb 2+, Cd 2+, Mn 2+, Sr 2+) to the more soluble metals (Ca 2+, Mg 2+) 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.