Prolonged climatic stress main reason for mass extinction 65 million years ago, paleontologist says

Prauss thus challenges the almost 30-year-old theory that a meteorite impact at the Mexican Yucatan peninsula was the single cause for one of the five largest mass extinctions in Earth history, which has most recently been reiterated in a publication in the journal Science. According to Prauss, the impact was only one in a chain of catastrophic events that caused substantial environmental perturbations, probably largely controlled by the intermittent activity of the Deccan volcanism near the then-Indian continent, that continued over several million years and peaked at the Cretaceous-Paleogen boundary.

"The resulting chronic stress, to which of course the meteorite impact was a contributing factor, is likely to have been fundamental to the crisis in the biosphere and finally the mass extinction," says Michael Prauss. In a research project funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and in collaboration with Prof. Dr. Gerta Keller, paleontologist at Princeton University, U.S.A., Prauss analyzed several drill cores and rock sections covering the extended Cretaceous-Paleogene boundary interval at Brazos River, Texas, USA. The investigated region is located about 1000 km northwest of the Chicxulub impact crater and is well known among geologic scientists for its exceptionally complete preservation of Upper Cretaceous sediments.

Based on an analysis of the appearance and distribution of organic-walled microfossils such as Algal cysts, pollen, and spores of terrestrial plants, Prauss shows that significant and persistent variations in the ecosystem of the Upper Cretaceous started long before the meteorite impact. Among others, these are reflected by fluctuations in sea-level and marine algae productivity.

Prauss also considers it highly problematic to equate the meteorite impact with the position of the Cretaceous-Paleogene boundary: "The actual impact took place well before the geochemically and micropaleontologically defined Cretaceous Paleogene boundary." He supports his assertion with the position of the so-called fern spike, an episodic, significant increase in the proportion of fern spores caused by the pioneering phase of ferns in repopulating landscapes of destroyed ecosystems. In all sections of the investigated area the fern spike occurs well before important stratigraphic evidence for the Paleogene.

The new results contradict a publication by Schulte et al. (2010) in the March 5 issue of Science. Schulte et al. summarize the Cretaceous-Paleogene issue only to arrive at the 30-year-old theory of the impact as the sole cause of mass extinction. The occurrence of substantial fluctuations within the ecosystem of the Upper Cretaceous before the impact is disputed and the impact event is equated in time with the biostratigraphic Cretaceous-Paleogene boundary. "In the light of the new data, both of these points have to be refuted," says Prauss.