UC Riverside

The deposition of the Upper Kellwasser black shale at the Frasnian-Famennian stage boundary in the Late Devonian coincided with one of the most enigmatic diversity crises and ecological perturbations in the Phanerozoic. Because of the ubiquity of organic-rich sediments deposited in Devonian epeiric seaways, it is commonly assumed that anoxia was the main driver of biological turnover at this time. Little is understood, however, about the actual mechanisms of extrinsically-forced ecological change in the Late Devonian, in part because of an incomplete understanding of the paleoenvironmental conditions of the habitats in which these changes took place. Specifically, the duration and extent of anoxia and euxinia during the deposition of the Upper Kellwasser has not been well-constrained globally. Previous studies have focused on the presence or absence of these conditions, especially in the European type sections, even though it is recognized that organic-rich sediments can capture rapid fluctuations in ocean redox chemistry and that oxygen profiles vary between basins.

Here I present new data integrating trace fossils, lipid biomarkers, trace metals, stable isotopes, and elements which track temporal and spatial changes in the marine redox structure and primary producer assemblages leading up to and through the Upper Kellwasser in the Appalachian Basin, North America and the Madre de Dios Basin, South America. Highly variable dissolved oxygen levels were reconstructed for the Appalachian Basin at an unprecedented high stratigraphic resolution. The ichnological and trace metal data show that environmental perturbation in association with the Frasnian-Famennian extinction event was concurrent with bottom-water oxygen conditions that fluctuated rapidly through suboxia to anoxia preceding and during the deposition of the Upper Kellwasser; lipid biomarker data support this basinal model characterized by neither permanent nor pervasive anoxia and with only sporadic sulfidic conditions in the photic zone. These results challenge the assumption of widespread and persistent anoxia and euxinia at the Frasnian-Famennian boundary. Integrating biomarker parameters with nutrient profiles also facilitated an inter-latitudinal comparison of phytoplanktonic ecology: hopane and sterane ratios suggest eukaryotes, and specifically prasinophytes at high latitudes, thrived during the deposition of the Upper Kellwasser in Laurentia and Gondwana.