Paleobathymetric trends in Northern Hemisphere calcareous nannoplankton “boom-bust” successions following the Cretaceous/Paleogene (K/Pg) mass extinction

Heather Jones , Lorna Kearns, Julio Sepúlveda, Christopher Michael Lowery, Laia Alegret, Thomas Westerhold, M. Hedi Negra, Mark E. Patzkowsky, Timothy J Bralower

The Cretaceous/Paleogene (K/Pg) bolide impact ~66 Ma caused the near-demise of calcareous nannoplankton – key primary producers and major components of the biological pump that exports organic and inorganic carbon to the deep sea. Despite their ecological significance, resolving the exact impact of nannoplankton’s mass extinction on ecosystem structure and function is complicated by marked global heterogeneity in paleoecological and biogeochemical responses. To explore this further, we generated a ~3.7 Myr nannofossil record from sediment cores at the Global Stratotype Section and Point for the basal Danian near El Kef, Tunisia, representing an outer continental shelf setting in the peri-Tethys Ocean. Our data reveal a series of low-diversity, high-abundance assemblages that persisted throughout the study interval. Although such boom-bust successions typify Northern Hemisphere nannoplankton recovery, statistical comparison with published datasets indicates that their duration – and the sequence of taxa comprising them – were location-specific and predominantly controlled by paleobathymetry, namely a site’s position along a shelf-to-open-ocean transect. We hypothesize that this spatial variability reflects faster restoration of biological pump efficiency in open-ocean versus continental shelf settings, which in turn led to a quicker reduction in surface ocean nutrient availability. This would have encouraged the earlier proliferation of specialist, low-nutrient-adapted nannoplankton taxa with larger average cell volumes in open-ocean environments, further boosting pump efficiency via organic matter ballasting. Therefore, future research should prioritize disentangling the causal relationships between carbon cycling and nannoplankton recovery following the K/Pg extinction through rigorous integration of high-resolution biogeochemical time series and abundance-weighted nannofossil cell-volume data.