The Ichnological Signal of Paleozoic Black Shales: A Tool for Delineating The Dysaerobic Zone

Boyer, Diana L.¹, Mary L. Droser¹ (1) University of California, Riverside, Riverside, CA

Fine-grained, organic-rich sediments deposited under reduced but nonzero oxygen conditions are widely described as black shales. In modern environments the dysaerobic zone extends from the edge of metazoan habitability to near normal marine conditions within a broad range of physical and oceanographic conditions. Trace fossil models utilizing the appearance and disappearance of specific ichnogenera are typically used to interpret variations in relative oxygen levels in Mesozoic and younger strata (Savrda and Bottjer, 1986). These methods have not been tested in Paleozoic strata despite evidence of variability in the nature of trace fossil record through the Phanerozoic. Middle Devonian aged black to gray shales from the Hamilton Group outcropping in central New York were sampled on a microstratigraphic scale to test for a tractable gradient through the dysaerobic zone preserved as an ichnological signal. Data reveal that Devonian dysaerobic trace fossils are consistently very small in size and lack the ichnogeneric diversity to effectively utilize established trace fossil models. Planolites and Chondrites are the only trace fossils present within beds through most of the dysaerobic zone and ichnogeneric diversity only increases within well bioturbated beds. Therefore, maximum burrow diameter, based on fundamental physiological oxygen demands, is the most consistent measure of variation in relative oxygenation within these units and provides the finest resolution through the dysaerobic zone. Further, the reduced size of burrow penetration allows for a taphonomic window as the depth of overprinting and destruction of older sedimentary signals is reduced.