DIFFERENTIAL GEOCHEMICAL RESPONSE DURING TWO LATE DEVONIAN MASS EXTINCTION PULSES: IMPLICATIONS FOR GLOBAL CHANGE

The Late Devonian Lower and Upper Kellwasser mass extinction events (LKE & UKE) are linked to episodes of global change and episodic low O₂ conditions. We report the results of facies, major and trace element, and organic carbon and nitrogen stable isotopic analyses of rocks from western Canada that provide insight into paleoenvironmental change during these events. These data are evaluated within a sequence stratigraphic framework which indicates that the LKE and UKE intervals were both deposited during lowstand and early marine transgression. They, however, display markedly different geochemical signatures. Ni, Cu, and Zn, are micronutrients used by marine primary producers and are at background levels in our study sections except within the LKE and UKE. Si, Ti, and Zr are useful indicators of terrigenous influx or dilution and display increasing values during the events. Mo, U, and C_org:P are useful redox proxies and all are enriched within the event intervals. The events are both associated with positive δ¹³C_org excursions (~3-4‰). The δ¹⁵N_org signature covaries with δ¹³C_org and generally increases during the events.

The above proxies display several abrupt fluctuations during the LKE while the UKE records a single long-term cycle. Generally the LKE records increasing proxy levels upsection with Ni, Cu, Zn, and Ba indicating heightened productivity, Si, Ti, and Zr signifying an increase in terrigenous influx, U, Mo, and C_org:P ratios implying decreasing bottom water O₂ levels, positive δ¹³C_org excursions implying increased carbon burial, and δ¹⁵N_org changing from lowstand (-2-0‰) to transgression (>6‰) suggesting a change from nitrogen fixation to denitrification. These patterns are similar to those observed during recent and Carboniferous interglacial to glacial cycles. The combination of increased continental weathering during sea level drawdown and higher wind stress and eolian transport (due to a greater pole-to-equator temperature gradient) ultimately resulted in higher productivity (fueled by terrigenous nutrient delivery), carbon burial, atmospheric CO2 drawdown, cooling, and episodic suboxic conditions. The pattern of rapid change recorded during the LKE may mark the beginning-of-the end of the Late Devonian greenhouse world and the initiation of small-scale continental glaciers.