SILURIAN-DEVONIAN ENVIRONMENTAL AND BIOTIC CHANGE RECAST IN A NEW TIME SCALE: EVIDENCE OF CHANGING VOLATILITY FROM THE APPALACHIAN BASIN

High precision U-Pb zircon dates for the Silurian and Devonian Periods provide an unprecedented chronostratigraphic framework within which to evaluate the timing of physical and biotic events. Reassessment of classic sections within the Appalachian Basin reveals major discrepancies in the frequency and amplitude of eustatic, isotopic, and biotic fluctuations through various parts of the Silurian-Devonian and demonstrates a bias toward relatively short “event” intervals. The record of this interval in the Appalachian Basin and elsewhere provides strong evidence that increases in environmental volatility and bioevents are linked. For instance, the long Telychian (5-6 my) and Emsian (~17 my) stages have generally low sedimentation rates, show few and relatively low amplitude sea level, carbon isotopic events, or bioevents, and contain few microbialites or bioherms. These patterns are in sharp contrast to the high frequency fluctuations of the relatively thick and cyclic Wenlock, Eifelian and Givetian intervals, which also contain abundant microbialites/bioherms. Recent stable isotope studies show roughly parallel patterns of low and high volatility. Moreover, the rising phases of positive δ¹³C excursions appear to be associated with bioevents, e.g., Wenlock Ireviken and Mulde events, late Eifelian Kacak and late Givetian Taghanic events, which bound ecological-evolutionary subunits; epiboles and Lazurus taxa are also unusually abundant during volatile intervals. Together, the changes in environmental and biotic volatility in the Appalachian Basin reflect the signature of oscillations between greenhouse and icehouse conditions. Conodont apatite δ¹⁸O values corroborate other evidence for variable temperatures during times of volatility; a series of superimposed δ¹⁸O lows, reflecting warming spikes, are associated with events of hypoxia, sea level rise, and global extinction. Regardless of mechanism, there appear to be prolonged intervals characterized by weak fluctuations in sea level, oxygenation, carbon isotopes, and biotic change. Surprisingly, the intervals of much stronger environmental and biotic volatility that dominate the record are relatively short-lived. It is during these intervals that much net evolutionary and ecological change appears to take place.