HOW DYSAEROBIC IS THE DYSAEROBIC ZONE?

The dysaerobic biozone is broadly defined by deposition under conditions of less than normal oxygen levels. Although precise oxygen levels are not typically quantified from the rock record, reduced oxygen conditions are largely recognized by the preserved biological signal, as metazoan life is very sensitive to fluctuations in bottom water oxygen levels. Dysoxic, or reduced oxygen conditions are common in the rock record, but potentially represent a large range of relative oxygen levels between anoxic and fully oxygenated, as well as a broad spectrum of depositional settings. Devonian aged black shales from localities across the Appalachian Basin representing deposition in a broad shallow epeiric sea were sampled at high resolution to investigate the dynamics of the dysaerobic zone. Sampled intervals were selected to represent a range of relative oxygen levels through the dysaerobic zone. Using a combined trace and body fossil approach, including relative amount of bioturbation, burrow size, and species richness, variations in relative oxygen are recognized on the sub-cm scale. These results reveal that intervals broadly defined as dysaerobic based on low macrofaunal and trace fossil diversity, and low to moderate bioturbation are not temporally stable. No intervals greater than 3 cm in thickness that recorded stable dysoxic conditions were identified. It is likely that the dysaerobic zone represents conditions of changing bottom water oxygen levels, commonly fluctuating rapidly between anoxic and dysoxic, rather than persistent dysoxic conditions. These findings have important implications for understanding oceanographic dynamics of ancient epeiric seaways.