The Cretaceous–Palaeogene mass extinction

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The extinction that removed the non-avian dinosaurs, the ammonites, the large marine reptiles and, on the usual estimate, around three quarters of species. It is the best-understood of the five because it has a physical cause that can be pointed at: a global iridium-rich boundary layer, shocked quartz, impact spherules, and a crater of the right size and age at Chicxulub. High-precision argon-40/argon-39 dating published in 2013 placed the boundary at 66.043 ± 0.043 million years and showed the impact and the extinction horizon to be indistinguishable in time. The kill mechanism is generally attributed less to the impact itself than to what followed — sulfate aerosols and soot causing months to years of darkness and cooling, shutting down photosynthesis on land and in the surface ocean. A serious alternative and complementary hypothesis points to the Deccan Traps in India, erupting across the same interval; the current mainstream position, argued in a large multi-author 2010 review, is that the impact was the proximate cause while accepting that Deccan volcanism was already stressing the biosphere.

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