Palynology of the Cretaceous-Tertiary boundary in central Montana: Evidence for extraterrestrial impact as a cause of the terminal Cretaceous extinctions

Abstract

This report details a statistical analysis of palynological change across iridium-enriched claystones defining the Cretaceous-Tertiary (K-T) boundary in the Cretaceous Hell Creek Formation and Tertiary Tullock Member of the Fort Union Formation, eastern Montana, United States. The results strongly support the bolide impact model of terminal Cretaceous extinctions. Chi-square analysis of presence and abundance below and above the iridium datum was used to define characteristic Cretaceous and Tertiary palynomorph species. There is no statistical support for gradual decline in Cretaceous species below the iridium datum, although the presence and abundance of certain Cretaceous species decline somewhat, and the presence of Tertiary species increases slightly, 0-3 m below the iridium datum. An estimated 15%-30% of the Hell Creek palynoflora disappear 0-2 cm above the iridium datum, and another 20%-30% undergoes a significant decline in abundance. Palynoflorules directly overlying the iridium datum are characteristically depauperate. Partial recovery of the palynoflora occurs within 10-20 cm of the boundary, but species richness remains depressed throughout the lower Tullock Member compared to the Hell Creek Formation. Palynoflorules from rare Hell Creek coals contain significantly fewer Cretaceous and significantly more Tertiary species, relative to those of flood-plain deposits, suggesting that perhaps 30%-40% of affected Hell Creek species disappear or decline due to facies change. Greenhouse warming from impact-generated atmospheric CO 2 and consequent increased soil water saturation, combined with still uncertain mass-kill mechanisms, best fit the available data to explain the pattern of change in vegetation at the K-T boundary throughout the Western Interior.