NUTRIENTS AND DIVERSITY: δ¹⁵N AND δ¹³C_ORG COVARIATION ACROSS THE FRASNIAN/FAMMENIAN BOUNDARY IN THE APPALACHIAN BASIN OF NORTH AMERICA

The extended biotic crisis of the Late Devonian in the Appalachian Basin is characterized by diminished marine diversity, especially among low latitude and shallow water fauna. In this region, the Late Devonian is also marked by a significant advance of terrestrial forest ecosystems and intensified pedogenesis. An increased flux of terrestrially-derived fixed nitrogen into N-limited, oligotrophic basins, such as the Appalachian Basin, likely resulted in episodic increases in primary production and consequent oxygen depletion in the water column. Globally correlated positive δ¹³C_org excursions (Upper and Lower Kellwasser Events) mark distinct extinction horizons and periods of increased productivity leading to enhanced organic matter preservation. Corresponding δ¹⁵N and δ¹³C_org variations from locations within the Appalachian Basin possibly indicate an increasing terrestrial organic matter flux along with increased rates of microbially-mediated denitrification or a diminished rates of marine nitrogen fixation. Taken together, δ¹³C_org and δ¹⁵N patterns of covariation may serve to clarify the relationship among increasingly eutrophic marine ecosystems, increasing terrestrial and marine biomass, and suppression of generic origination rates in the Late Devonian.