NO_xproduction and rainout from Chicxulub impact ejecta reentry

Abstract

Abstract The Chicxulub impact 66.0 Ma ago initiated the second biggest extinction in the Phanerozoic Eon. The cause of the concurrent oceanic nitrogen isotopic anomaly, however, remains elusive. The Chicxulub impactor struck the Yucatán peninsula, ejecting 2 × 10 15 kg of molten and vaporized rock that reentered globally as approximately 10 23 microscopic spherules. Here we report that modern techniques indicate that this ejecta generates 1.5 × 10 14 moles of NO x , which is enough to cause the observed nitrogen enrichment of the basal layer. Additionally, reentry‐based NO production would explain the anomalously heavy isotopic composition of the observed nitrogen. We include N, O, N 2 , O 2 , and NO species in simulations of nonequilibrium chemically reacting flow around a reentering spherule. We then determine the net production of NO from all the spherules and use turbulence models to determine how quickly this yield diffuses through the atmosphere. Upon reaching the stratosphere and troposphere, cloud moisture absorbs the NO x and forms nitric acid. We model this process and determine the acidity of the resulting precipitation, which peaks about 1 year after the impact. The precipitation ultimately reaches the upper ocean, where we assume that the well‐mixed surface layer is 100 m deep. We then model the naturally occurring carbonate/bicarbonate buffer and determine the net pH. We find that insufficient NO x reaches the ocean to directly cause the observed end‐Cretaceous oceanic extinction via acidification and buffer removal. However, the resulting nitrates are sufficient to explain the concurrent nitrogen isotopic anomaly and facilitate an end‐Cretaceous algae bloom.