Anoxia was Gradual in Appalachian During Frasnian–Famennian Kellwasser Event

THE LAST GASP: TRACE FOSSILS TRACK DEOXYGENATION LEADING INTO THE FRASNIAN–FAMENNIAN EXTINCTION EVENT

Authors:

Boyer et al

Abstract:

The Frasnian–Famennian boundary is correlated with one of several Late Devonian extinction pulses that resulted in a significant decrease in diversity as well as ecological restructuring. This event is recognized within the globally correlated Upper Kellwasser interval that is well exposed and biostratigraphically well constrained in shale units of western New York State. The ichnological and geochemical signals of the interval stratigraphically below the Upper Kellwasser event at these localities provides insight into the onset of this important extinction event. Detailed analysis of ichnogeneric composition, relative size of burrow populations, amount of bioturbation, and trace metal concentrations vary in concert. Deep-penetrating, pyritized Skolithos burrows terminate abruptly at a thin, laminated black shale interval with enriched Mo levels, up to 31 ppm, and are overlain by an interval of gray-green bioturbated shales dominated by Chondrites. These textural and chemical shifts reveal that bottom-water oxygen levels decrease rapidly below the base of the Upper Kellwasser interval. Relative oxygen levels are interpreted to remain low through the Chondrites-dominated interval, with protracted stressed conditions followed by a gradual decrease to anoxic conditions within the Upper Kellwasser interval. These results suggest that, at least locally in the Appalachian Basin, bottom-water oxygen stress and/or fluctuating oxygen conditions were present leading up to the extinction event. This evidence does not support an instantaneous onset of anoxia as causal mechanism for extinction.