MERCURY AND MICROFOSSIL TRENDS DURING END-DEVONIAN EXTINCTION EVENTS

The cause or causes of the late Devonian extinctions remain unclear. The sheer number of proposed trigger and kill mechanisms paired with the spatial and temporal heterogeneity in the extinction itself–as well as in proxies–makes finding one all-encompassing cause difficult. However, one commonly described trigger, volcanism, has recently garnered more support due to Hg anomalies identified in Morocco, Germany, and Russia during the late Devonian Frasnian-Fammenian Kellwasser events (Racki et al. 2018). Here, we expand this record by documenting Hg records for the Lower and Upper Kellwasser events in Upstate New York as well as the latest Devonian Hangenberg event in Ohio. We couple these Hg records with microfossil analyses including abundance and diversity. We normalize both microfossil abundance and Hg to total organic carbon (TOC) to account for biases in preservation of both proxies.

Our findings show high mercury signals (3x median) immediately preceding the Lower and Upper Kellwasser in multiple localities, results which may support the findings of Racki et al. Furthermore, high Hg at the end of the Hangenberg event supports evidence of volcanism in the late Hangenberg Black Shale in Europe. However, all of these sites have a strong correlation between Hg and TOC, indicating that Hg trends may, in part, be driven by fluctuations in TOC and related redox variability. The exception to this trend is in site TGB where we do see a significant Hg spike not associated with TOC in the middle of the Lower Kellwasser event. While our evidence does not strongly support volcanism as a proximal trigger of the late Devonian extinction events, we do draw trends between Hg, microfossil abundance, and microfossil diversity. In our shallowest sections, we find a positive correlation between mercury and microfossil abundance, while in our deeper water sections we see a negative correlation between the two proxies, indicating that one or both signals are influenced by water depth and/or correlated factors such as bottom water dysoxia. Integrating high resolution Hg records with coeval microfossil records of abundance and diversity thus reveals new intricacies of the Late Devonian extinction events in North America.