SMALL BUT MIGHTY: MICROFOSSIL-BASED INSIGHTS INTO THE LATE DEVONIAN LOWER KELLWASSER EVENT

The Late Devonian mass extinction is the only “Big Five” mass extinction with a yet-unknown cause. In the Appalachian Basin, the most significant extinction pulse is during the Lower Kellwasser Event. Here, we compile and synthesize microfossil data from five Lower Kellwasser sites in the Appalachian Basin including absolute and relative abundance and category-level diversity and evenness. We compare these results with those from macrofossil data and interpret them in light of proxies including TOC, redox-sensitive trace metal concentrations, and bulk carbon isotopes.

Major observations of this synthesis include the following 1) We observe no significant difference in overall microfossil richness, diversity, or evenness between beds representing the extinction interval and those before and after the extinction. This stands in contrast to the brachiopod record for some of these same strata, which shows a 55% diversity loss (Pier et al. 2021). 2) We observe higher absolute abundance of microfossils scaled to TOC in beds representing the extinction interval as compared to those before and after the extinction. 3) We observe an increase in richness, diversity, and evenness in nearshore environments as compared to deep-water environments. 4) Absolute abundance scaled to TOC shows a similar trend as diversity, with higher microfossil abundance in shallow-water settings. Our data on lower diversity and TOC-scaled abundance in deeper water settings, coupled with redox proxies that show increased anoxia in these sections, supports the idea that anoxia and dysoxia played a role in ecosystem stress in the Lower Kellwasser Event in the Appalachian Basin. However, our finding that within individual stratigraphic sections microfossil diversity metrics are invariable between extinction and non-extinction beds complicates this narrative and suggests that water depth, rather than any single extinction driver, may be the most significant variable influencing microfossil diversity in these strata. In addition, our finding that microfossil abundance is higher in extinction beds than non-extinction beds supports hypotheses that some components of the microfossil assemblage may represent bloom or disaster taxa.