INSIGHTS INTO THE PATTERN AND TIMING OF THE FRASNIAN-FAMENNIAN BIOTIC CRISIS IN WESTERN CANADA FROM GEOCHEMISTRY AND ASTRONOMICAL TUNING

Astronomical tuning of magnetic susceptibility data and stable isotopic and geochemical oceanographic proxy data from western Canada provide insight into the pace and timing of the Late Devonian, Frasnian-Famennian (F-F) biotic crisis. Two organic-rich shales that, in much of the world, display geochemical anomalies indicating low oxygen conditions and carbon burial, characterize the F-F event. These events, commonly referred to as the Lower and Upper Kellwasser events (LKE & UKE), have been linked to the expansion of deeply rooted terrestrial forests and associated changes in soil development, chemical weathering and Late Devonian climate. Our geochemical data from the events record relatively high levels of redox sensitive trace metals (Mo, U, V), proxies for biological productivity (Ba, Cu, Ni, Zn), and detrital input (Al, Si, Ti, Zr). δ¹³C data generated from organic matter records a 3-4‰ positive excursion during each event. These data corroborate hypotheses about enhanced biological productivity, driven by terrigenous input, leading to low oxygen conditions and decreased biotic diversity during relatively low stands of Late Devonian sea level.

Age dating of events in deep time is problematic due to insufficient biochronologic control. Each event falls within one conodont zone, with durations on the order of 0.5-1.0 Ma. The LKE occurs near the base of Frasnian Montagne Noir Zone 13A while the UKE begins at the F-F boundary. Time series analysis of high-resolution magnetic susceptibility data identified 16 long eccentricity cycles (405 ky) during the Frasnian and one in the earliest Famennian. The geochemical anomalies associated with the LKE and UKE in western Canada are recorded over 7 and 14 m of stratigraphic section respectively. These strata represent only a portion of a 405 ky long eccentricity cycle and astronomical tuning implies that the LKE likely occurred during a single short eccentricity cycle (100 ky) while the UKE was more protracted, lasting approximately two short eccentricity cycles (200 ky). The events are separated by 1.5-2 long eccentricity cycles indicating that they occurred between 600 and 800 kyr apart. This work demonstrates the utility of long time series of magnetic susceptibility data used in conjunction with other multi-proxy data to provide insight into events in geologic time.