Paleobotanical Evidence for Climatic Change across the Cretaceous-Tertiary Boundary, North America: Twenty Years after Wolfe and Upchurch (original) (raw)

A b s t r a c t ) provided the first micropaleontological evidence for terrestrial mass-mortality at the Cretaceous-Tertiary (or K-T) boundary associated with the impact of a large bolide on Earth. Five years later published corroborative evidence from leaf fossils and proposed that long-term increases in precipitation, and possibly temperature, were also associated with the K-T boundary impact event. Over the subsequent two decades numerous palynological and paleobotanical studies have corroborated early evidence for mass mortality at the K-T boundary in North America and other regions of the world, refined interpretations, and suggested additional long-term environmental changes associated with the terminal Cretaceous event. In this paper we review what we consider to be some of the more important paleobotanical and palynological literature for the K-T boundary of North America and evaluate hypothesized climatic changes. Patterns of change in species diversity and patterns of life form dominance in the pollen and leaf records are consistent with a reduction in solar radiation over a period of one year to more than a century by a global cloud of dust and/or sulfate aerosols resulting from an impact at the K-T boundary. However, limited evidence exists for associated freezing temperatures or global wildfires. Changes in the physiognomy of leaf fossils, plant communities, sedimentary facies, and paleosols provide evidence for up to a four-fold increase in mean annual precipitation after the K-T boundary that may have lasted for >1 million years. Evidence from leaf margin analysis for an early Paleocene cooling of ~5 °C is restricted to the northern Western Interior and is contradicted by evidence for no temperature change or a temperature increase. Multivariate analysis of foliar physiognomy (CLAMP) suggests a temperature increase of as much as 9 °C. Preliminary evidence for a four-fold or greater increase in atmospheric pCO 2 across the K-T boundary (to >2300 ppm) is consistent with a proposed warming of 9 °C during the early Paleocene, based on the range of sensitivity of Earth-system models to increased pCO 2 . Biogeochemical modeling indicates that a one-year period of darkness, with the lower temperatures and partial biomass burning proposed by some authors, would be sufficient to shut down photosynthesis and significantly reduce terrestrial biomass. Long lasting (10 2 to 10 6 yr) increases in precipitation, temperature, and pCO 2 created favorable environmental conditions for the recovery of net primary productivity and terrestrial biomass. This may help explain why plants at the K-T boundary, while experiencing high extinction at the level of species regionally, experienced relatively little extinction at the level of genus and family, and why the K-T mass extinction event is more pronounced in the marine realm.

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