THE LATE DEVONIAN KELLWASSER EVENTS IN THE APPALACHIAN BASIN: A SYNTHESIS OF MICROFOSSIL AND GEOCHEMICAL DATA THROUGH SPACE AND TIME

The Late Devonian mass extinction is the only “Big Five” mass extinction with a yet-unknown cause. In the Appalachian Basin, the first two pulses of the extinction are expressed in the Lower and Upper Kellwasser Event black shale horizons (ca. 372 Ma). While considerable work has been done on these events, the black shale strata themselves have been less well studied by paleontologists because macrofossils are not present during the extinction interval. However, organic-walled microfossils (OWMs) are present in these strata before, during, and after the extinction horizons and provide a rare glimpse into this enigmatic biodiversity crisis. Here, we present our findings from detailed studies of 7 Kellwasser Appalachian Basin sites that span the basin before, during, and after both Kellwasser Events. We present integrated data from both fossils and geochemical proxies including: 1) OWM absolute and relative abundance, 2) single-fossil OWM and bulk rock organic δ¹³C, 3) fossil taxonomy, 4) fossil taphonomy, 5) bulk Hg, TOC, and additional trace metal redox proxies.

Our conclusions from this multi-proxy analyses include the following: 1) During the extinction, OWMs increased in absolute abundance, indicating they may represent disaster taxa that flourished as other clades perished. 2) These OWMs show unusual taphonomic trends that correlate roughly with TOC which may reflect a difference in cell wall composition compared with earlier OWMs. 3) Geobiological and geochemical data support paleogeographic reconstructions that the epicontinental sea was shallower to the east, and there may have been significant riverine input to proximal sites. 4) Hg analysis does not support the hypothesis of massive volcanism as an extinction trigger, nor does it correlate with any other proxies or fossil data and 5) There are distinct differences in microfossil assemblage taxonomic richness and composition across the basin, with higher richness in shallower water localities. Together, these conclusions support evidence for low-oxygen conditions caused by eutrophication or increased weathering/nutrient input as a driver of the Late Devonian extinction in the Appalachian Basin and also emphasize the utility of broad spatial and temporal analyses of multiple proxies when investigating the end Devonian extinction event.