THE LAST GASP: DEVONIAN PYRITIZED BURROWS PRESERVE RAPID OXIC-ANOXIC TRANSITION BEFORE MASS EXTINCTION

The Upper Kellwasser Event, globally associated with a major biotic turnover, is preserved in a black shale interval in the Hanover Formation exposed at numerous localities in western New York. Within this black shale interval, the Frasnian-Famennian boundary has been precisely correlated using conodont data. At Eighteenmile Creek in Eden, NY, abundant pyritized burrows are preserved in bioturbated green-gray shales directly underlying the black shale associated with the Upper Kellwasser Event. Thousands of individual burrows were measured and described based on size, orientation, and horizon of origination. At least two distinct ichnogenera are identified. Vertical pyritized “Skolithos” type burrows are the most common and restricted to a 15 cm interval just below the Upper Kellwasser Event bed. These vertical burrows, preserved within bioturbated sediments, were open to the overlying water column and, therefore, the preserved pyrite infilling is interpreted to record rapid environmental shifts in bottom water oxygen conditions from oxic to anoxic. Because several different horizons of origination are identified, it is likely that conditions were fluctuating between oxic and anoxic before becoming more persistently anoxic, as inferred from the overlying black shales.