MASS EXTINCTION AS REGIME SHIFT WRIT LARGE: GEOCHEMICAL AND PALYNOLOGICAL EVIDENCE FROM THE FRASNIAN/FAMENNIAN BOUNDARY

The cause of the mass extinction at the Late Devonian Frasnian/Famennian boundary has been ascribed to, among others, warming, cooling, bolide impact, volcanism, and eutrophication. Although fossil evidence records the decline in faunal diversity primarily in shallow, basin margin environments, deeper basinal settings preserve a record of coeval ecological change that may serve to constrain the possible causes of the global biotic crisis. This study examines geochemical and palynological evidence from the Frasnian/Famennian boundary interval within the New Albany Shale of the Illinois Basin in order to contrast ecosystem dynamics above and below the boundary. Element ratio (C/N/P/S) and carbon and nitrogen isotope abundance data suggest distinct biogeochemical regimes before and after the extinction event. The Frasnian regime is characterized by high coefficients of variance in all geochemical parameters relative to the overlying Famennian interval. The Famennian regime is characterized by low coefficients of variance and high total organic carbon content. Degree of pyritization and sulfur isotope abundance data suggest variable redox conditions below the boundary and persistent euxinic conditions above. Detailed palynological examination of the boundary interval reveals that the mean volume of Tasmanites increases by two orders of magnitude across the boundary, suggesting a dramatic increase in the biomass yield of each green algal reproductive cyst with potentially profound consequences for trophic relationships. The relative abundance of Tasmanites compared to other algal palynomorphs also increases dramatically across the boundary. The increase in the variance of Tasmanites abundance in the uppermost ~30cm of the Frasnian may be a critical indicator of the impending regime shift at the extinction boundary. If the regime shift were prompted by an external driver acting over an extended time interval, then it is likely that within this basinal setting the ecological state change at the extinction boundary was a function of nonlinear system dynamics rather than a singular catastrophic event.