Appalachian Basin mercury enrichments during the Late Devonian Kellwasser Events and comparison to global records

Abstract

The Late Devonian Kellwasser Events are two major biodiversity crises recorded at and just below the Frasnian-Famennian stage boundary for which the ultimate cause or causes of diversity depletion remain unclear. The discovery of anomalous mercury (Hg) enrichments in Upper Kellwasser Event (UKE) strata has led to increased interest in a potential volcanic trigger for one or both events. However, UKE Hg anomalies are found mainly on the continental shelves of the Palaeotethys Ocean, and Hg isotope values from at least one distal basin indicate a non-volcanic Hg source. These findings suggest volcanism during the UKE may have had regional rather than global impacts. In comparison to the UKE, few works have investigated the sedimentary Hg record of the Lower Kellwasser Event (LKE). Here, we contribute new Hg data from the LKE of the Appalachian Basin and compare this to a compilation of global Hg data from both events. The Appalachian Basin contains no sedimentary Hg enrichments consistent with massive volcanism during the LKE, and Hg concentrations are strongly correlated with total organic carbon. We argue that organic matter drawdown exerted a primary control upon Hg concentration in the Appalachian Basin. While basins more proximal to the Palaeotethys Ocean preserve potentially volcanogenic LKE Hg anomalies, the absence of such anomalies here reflects volcanism during the LKE that was at most only regionally significant, as hypothesized for the UKE. Volcanism is therefore an unlikely trigger of the global Kellwasser diversity depletions. Broader sampling of the Kellwasser record will assist in the development of an extinction model that accounts for variations in global Hg concentrations, as the depressed origination rates, prolonged background environmental stress, and variations in intra-basinal expression that characterize both events may indicate that the trigger of diversity loss in this interval is unique among the “Big Five” mass extinctions.