MERCURY CONCENTRATIONS IN THE APPALACHIAN BASIN DURING THE LATE DEVONIAN KELLWASSER EVENTS SHOW NO EVIDENCE OF VOLCANISM

The paired Lower and Upper Kellwasser Events at the Frasnian-Famennian (F-F) stage boundary together represent one of the largest recognized biodiversity crises in Earth’s history. Multiple triggers have been proposed as causes of one or both of these events, including large-scale volcanism; in particular, the emplacement of a large igneous province has been associated with the diversity depletions of the F-F boundary. Anomalous mercury enrichments in some Kellwasser-aged horizons in Morocco, Germany, and Russia have provided support for a volcanic trigger. Here, we expand the F-F mercury (Hg) record with geochemical data from six Kellwasser successions in the Appalachian Basin, a siliciclastic-dominated foreland basin that in the Late Devonian contained a shallow, epicontinental seaway. Our study sites reflect a gradient of paleoenvironments and lithologies, from shallow, near-terrestrial mixed silty shale and sandstone to deeper-water black and gray shale. At each locality, we present measurements of Hg concentration, as well as normalizations to total organic carbon (TOC), sulfur, and other trace metals (Al, Fe, Mo, U) to avoid common biases, including the tendency of Hg to complex with organic matter, sulphide and clay minerals, and/or hydrous Fe oxides.

We find no anomalous Hg spikes associated with the onset of either Kellwasser event in the Appalachian Basin. Hg concentrations were generally very low, with most samples < 50 ppb. Though our dataset covers a wide array of TOC values (< 0.1 – 10 wt%) and paleoenvironments, we observed a strong correlation between Hg concentration and TOC at most sites, indicating that Hg concentration of sediments deposited in the basin was generally controlled by local TOC and redox variability. The Appalachian Basin Hg record therefore does not support volcanism as a cause of the Kellwasser diversity depletions. We suggest that a re-evaluation of Hg anomalies in other localities may be necessary, since volcanism of the scale necessary to cause massive diversity depletions is likely to have left a record within the Appalachian Basin. Alternately, some of these anomalies might have resulted from regional volcanic activity or from the localized tectonic remobilization of older volcanogenic material.