160-2 Insights into the K-Pg boundary from studies of outcrops on the U.S. Gulf and Atlantic Coastal Plain

Abstract:

The Cretaceous-Paleogene (K-Pg) asteroid impact hypothesis—the first credibly suggesting that mass extinction events could occur extremely rapidly—is now widely accepted, and it has evolved over time from near total rejection, to testing and re-examination, to regular appearance in textbooks and popular culture. This is a remarkable transition, for both paleontology and Earth science in general, comparable to plate tectonics in its scope and significance for our understanding of the history of life and the planet.

While research on the K-Pg event has been global, a significant dataset has been accumulating on the US Gulf and Atlantic coastal plains (GACP), where the number of known K-Pg sections has increased from fewer than a dozen to more than 30. These sections provide some of the most detailed records anywhere of what happened just before, during, and immediately after the impact.

The high temporal resolution and outstanding preservation of sediments and fossils in these sections have taught us things about the K-Pg that we could not have learned from other sections: they show us details of what happened in shallow shelf environments geographically proximal to the impact; they reveal the sedimentological chaos resulting from the impact, suggesting reinterpretations of some sections elsewhere in the world; they confirm that ammonites survived for a brief period after the impact, and that evolutionary rates immediately post-K-Pg may have been extremely rapid (e.g., foraminifera).

More broadly, the wide acceptance of the impact hypothesis and the extraordinary level of temporal resolution that is now possible in GACP and other boundary sections support a robust assessment of the role of this event in Earth’s biological history. The event abruptly pruned branches of the evolutionary tree. The lineages that survived did not do so because they were better adapted to existing conditions than those that experienced extinction, but because of idiosyncratic, non-adaptive characters such as geographic and environmental distribution or having a dormancy state in ontogeny. The evolutionary results of this pruning were growth and diversification along new and unpredictable trajectories, with contingency and exaptations playing major roles. The consequent changes in life were irreversible. But within a few hundred thousand years at most, the planet had physically returned to pre-impact state. Sea floor spreading, global climate change, and sea level were unaffected.

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