Calcareous nannofossil assemblages from El Kef (Tunisia) reveal strong bathymetric controls on Northern Hemisphere recovery patterns following the Cretaceous/Paleogene (K/Pg) mass extinction

The Cretaceous/Paleogene (K/Pg) bolide impact ~66 million years ago caused the near-demise of calcareous nannoplankton (coccolithophores): key primary producers and major contributors to the biological pump that exports organic carbon from the surface ocean to the deep sea. Although their mass extinction likely had a profound impact on ecosystem structure and function, the interpretation of early Danian recovery patterns remains challenging due to global heterogeneity in both nannoplankton community dynamics and carbon-cycling responses. To explore this further, we generated a ~3.65 Myr nannofossil record from sediment cores recovered near the classic El Kef (Tunisia) K/Pg boundary section, representing an outer continental shelf/upper slope setting in the peri-Tethys Ocean. Our data revealed four distinct nannoplankton acmes, with transitions between them likely influenced by the progressive restoration of biological pump efficiency. Comparison with previously published datasets from multiple ocean basins and paleoenvironmental settings indicates that the sequence of taxa comprising Northern Hemisphere acme successions were predominantly controlled by paleobathymetry at both a regional and global scale. We speculate that these bathymetric controls on nannoplankton acme characteristics were intrinsically linked to global heterogeneity in the restoration of biological pump efficiency. In particular, nannoplankton acmes marked by extremely small average cell volumes persisted for more than two million years at continental shelf sites, whereas contemporaneous open-ocean sites were dominated by taxa with average cell volumes an order of magnitude larger. Because coccolith calcite enhances organic matter ballasting, we suggest that biological pump efficiency was restored faster in open-ocean settings than in continental shelf environments.