Two large meteorite impacts at the Cretaceous-Paleogene boundary

Abstract

The end-Cretaceous mass extinction has been attributed by most to a single asteroid impact at Chicxulub on the Yucatán Peninsula, Mexico. The discovery of a second smaller crater with a similar age at Boltysh in the Ukraine has raised the possibility that a shower of asteroids or comets impacted Earth close to the Cretaceous-Paleogene (K-Pg) boundary. Here we present palynological and δ 13 C evidence from crater-fi ll sediments in the Boltysh impact crater. Our analyses demonstrate that a post-impact fl ora, formed on the ejecta layer, was in turn devastated by the K-Pg event. The sequence of fl oral recovery from the K-Pg event is directly comparable with that in middle North America. We conclude that the Boltysh crater predated Chicxulub by ~2-5 k.y., a time scale that constrains the likely origin of the bodies that formed the two known K-Pg craters.

Environmental change prior to the K–T boundary inferred from temporal variation in the morphology of cheilostome bryozoans

“…Whether pre-boundary environmental perturbations (either global or regional) had a progressive effect on the reduction of biotic diversity is debatable (Jolley et al, 2010). Declining taxonomic diversity is apparent throughout the Maastrichtian in many taxonomic groups (e.g.…”

“…These contrasting views are likely to reflect idiosyncrasies common to interpreting data from the fossil record (Keller and Abramovich, 2009) and more subtle effects exerted by preconceived conclusions (Valen, 1984). Identifying a single cause for the K-T extinction has therefore been challenging, and a multicausal scenario remains a viable alternative (Birkelund and Håkansson, 1982;Keller et al, 2003;Archibald and Fastovsky, 2004;Jolley et al, 2010).…”

Giant comets and mass extinctions of life

“…Although originally ascribed to an asteroid impact, iridium and energy constraints yield a best estimate that the impactor was a comet 10-80 km in diameter (Durand-Manterola & Codero-Tercero 2014). However a detailed stratigraphic study of the Ukrainian Boltysh crater by Jolley et al (2010) has shown that it was formed only 2000-2500 years before the Chicxulub impact in the Gulf of Mexico (the radiometric age given in Table1 has standard deviation ±0.6 Myr). This short timescale is consistent with the breakup of a large short-period comet yielding N Earthcrossing fragments with dynamical or physical lifetimes L yr such that N L > ∼ C, where C represents the mean lifetime of a fragment before collision with the Earth.…”

The Geochronology of Impact Craters

“…It is thus not possible to use radioisotope dating to prove beyond doubt that two impacts were simultaneous. In fact, recent research on sediments from the Boltysh Crater shows that it formed as little as 2000 years before Chicxulub (Jolley et al, 2010). The conclusion is derived from an estimate based upon the flora in sediments recovered from a drill core of the crater fill which indicate that the local area of the ejecta blanket became colonized by earlysequence plants after the impact, but that this ecosystem was in turn suddenly devastated by a second event, believed to be the result of the Chicxulub impact.…”