98-7 Plankton rise up! The Late Devonian Lower Kellwasser Event in the Appalachian Basin was a benthic extinction

Session: Linking Biodiversity Loss to Environmental Stressors Through Integrated Approaches

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The Late Devonian mass extinction, notably the Lower Kellwasser Event (LKE; ~372 Ma), represents the most significant biotic crisis within the Paleozoic Appalachian Basin. Despite extensive documentation of macrofossil loss—especially among benthic marine invertebrates—the ecological dynamics of the surface ocean during this interval remain poorly understood. We present an updated synthesis of organic-walled microfossil (OWM) data coupled with bulk and single-fossil organic carbon isotopic data from stratigraphic sections on a proximal-to-distal transect in the Appalachian Basin which show that in the Appalachian Basin the LKE was a benthic-selective extinction. New data on benthic microfossils (scolecodonts), combined with bioturbation indices and benthic marine invertebrate records show clear cross-basinal trends with higher benthic metazoan extinction/extirpation in deeper portions of the basin. In contrast, microfossil diversity and evenness remained stable across the LKE in all environments, despite clear evidence of increased anoxia in deeper water settings. Distal sections show increased microfossil abundance during the LKE, which correspond to depleted δ¹³C values attributed to upwelling of nutrient-rich and isotopically light deep waters. Nearshore environments host more taxonomically diverse and even microfossil assemblages throughout, possibly due to greater nutrient availability from terrestrial weathering and less oxygen stress due to water depth. Overall, we find that planktonic communities are largely unaffected or even thriving, likely in response to terrestrial and deep water nutrient influxes - the same nutrients that may be responsible for benthic dysoxia and metazoan extinction. By integrating pelagic and benthic fossil records with organic geochemistry records this study emphasizes the critical role of microfossils in reconstructing extinction dynamics in ancient marine systems.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10401

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