Discovery of fish mortality horizon at the K-T boundary on Seymour Island: Re-evaluation of events at the end of the Cretaceous

Journal of Paleontology, May 1998 by Zinsmeister, William J

ABSTRACT-The discovery of a fish bone layer immediately overlying the K-T iridium anomaly on Seymour Island, Antarctic Peninsula, which may represent the first documented mass kill associated with the impact event, together with new faunal data across the boundary has provided new insight into events at the end of the Cretaceous. The utilization of a geographical approach and a new graphical representation of range data has revealed that events at the end of the Cretaceous were not instantaneous, but occurred over a finite period of time. Although the fish bone layer may contain victims of the impact event, the absence of ammonites in either the iridiumbearing layer or the overlying fish layer suggests that the extinction event at the end of the Cretaceous was the culmination of several processes beginning in the late Campanian. The impact was the proverbial "straw that broke the camel's back," leading to the extinction of many others forms of life that might have survived the period of global biotic stress during the waning stages of the Mesozoic if there had not been an impact. The absence of mass extinction following catastrophic geologic events in a biotic robust world, such as the Middle Ordovician Millbrig-Big Bentonite volcanic event suggests that the biosphere is remarkably resilient to major geologic catastrophes with mass extinction events occurring only when there is a conjunction of geologic events none of which might be capable of producing a global mass extinction by itself.

INTRODUCTION

MORTALITY HORIZONS produced by mass kill events have been recognized in the fossil record throughout the Phanerozoic. Although the causes of mass kills vary, each resulted in the formation of a extirpation layer containing the victims of the event. A classic example is the "camposaurs bone bed" containing remains of thousands of individuals of the dinosaur, Maiasaurus, at Egg Mountain, Montana which was produced as a consequence of a vast herd of maiasaurs being overwhelmed by gases and ash from a nearby volcanic eruption (Horner and Gorman, 1988). One of the puzzling questions associated with the K-T extinction event is the absence of any type of extirpation layer at any of the well-studied boundary sections (Zinsmeister, 1993). The principal biotic data used to support the K-T extinction event is "species disappearance," which constitutes negative evidence (Zinsmeister, 1993). Although the absence of an extirpation horizon has not been addressed by proponents of the impact extinction hypothesis until recently (Cutler and Behrensmeyer, 1996), there are several possible explanations for the absence of a mortality layer at the boundary. Since most of the K-T boundary sections are finite localities with little or no significant geographic component, the chances of finding such evidence of a mass kill at any given boundary would be remote. Cutler and Behrensmeyer (1996, p. 375) have suggested that " . . much of the terrestrial fossil record is time-averaged, any peak in bone abundance resulting from a mortality pulse is likely to be obscured by background attritional mortality.... No special explanation is needed for the absence of bones beds at the K/T boundary." To dismiss the absence of a mortality horizon at the K-T boundary to "time-averaging" ignores the existence of numerous examples of mass mortality horizons throughout the Phanerozoic which clearly demonstrates that "time-averaging" does not preclude the preservation of catastrophic kill events. Alternatively, the absence of a mortality horizon at the K-T boundary may reflect that events leading to the mass extinction at the end of the Cretaceous were not instantaneous, but occurred over a longer period of time, and, as a consequence, no single global extirpation horizon would have formed. The discovery of a layer of fish debris in close association with the iridium anomaly horizon on Seymour Island, northeast tip of the Antarctic Peninsula (Fig. 1), during the austral summer of 1994-95, may represent the first documented, direct evidence of a kill event associated with the bolide impact 65 m.y. ago (Zinsmeister, 1995).

The purpose of this paper is 1) to present a detailed description of the microstratigraphy five meter interval crossing the KT boundary and detail geologic map of the boundary sequence using the recently completed USGS topographic map (1:10,000 with 5 m contour interval) of Seymour Island, 2) to discuss the conditions leading to the formation of the fish bone layers immediately above the iridium anomaly, and lastly to examine biotic changes across the K-T boundary using spatial biostratigraphy, a new methodology for the analysis and graphical representation of biostratigraphic data. The sequential nature of the disappearance of the ammonites, iridium anomaly, and fish kill horizon on Seymour Island provides new insight into the temporal sequence of events at the end of the Cretaceous and the effect of major catastrophes on the Earth's biosphere.