TESTING LATE DEVONIAN EXTINCTION HYPOTHESES

Modeling the conditions of extinction hypotheses can lead to the discernment of the critical observations that must be made in order to definitively test those hypotheses. Models considered here are based upon the beginning assumption that the end-Frasnian biodiversity crisis was triggered in large part by pulses of global cooling. The Devonian is a critical period in the history of the Earth's climate. During the Devonian the global climate switched from the hot greenhouse phase of the earlier Paleozoic to the cold icehouse phase of the later Paleozoic, a phase that would lead to continental glaciations in the end-Famennian and Carboniferous. However, the rapidity of the late Frasnian cooling has a quite different signature than that of the gradual pattern of cooling that triggered the end-Famennian glaciations.

Both classes of models are catastrophic: large-igneous province volcanism and asteroid or comet impact. A single impact model can be ruled out, as its predicted effects do not match those of the extinction pulses seen in the late Frasnian. An episodic global-winter model would, but those effects can be produced by both the catastrophic volcanism and multiple impacts models. Alternative models suggest that both catastrophic volcanism and multiple impacts may trigger a global greenhouse-induced temperature increase, a predicted effect that does not match the empirical data. However, if both of those mechanisms are shifted back in time to produce lag-time models of short-term global temperature increase and then rapid temperature drops as the anomalous greenhouse interval collapsed, then both produce effects that do match the empirical data.

To definitively choose among the several model predictions two critical data are needed: (1) an accurate Late Devonian temperature curve, and (2) an accurate radiometric date on the Frasnian-Famennian boundary.