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

The Cretaceous-Paleogene (K-Pg) mass extinction event ~66.0 million years ago (Ma), led to the elimination of over 75% of species on Earth and drove large-scale ecological reorganization in both the marine and terrestrial realms. In particular, the nearly complete extinction of calcareous nannoplankton-the most dominant phytoplankton group at the end of the Cretaceous- led to a shift in community structure within marine primary producers, which had a profound effect on trophic interactions and the marine carbon cycle. Earliest Paleocene nannoplankton assemblages were comprised of a series of short-lived, opportunistic taxa which we term “boom-bust” successions. Although these high dominance, low diversity assemblages have been observed worldwide, the series of taxa that comprise boom-bust successions, as well as the timing of the switchovers between dominant taxa, are globally heterogeneous. Here, we use the expanded and well-preserved fossil record of calcareous nannoplankton from the Global Stratotype Section and Point (GSSP) for the K-Pg boundary near El Kef, Tunisia to better understand the environmental and ecological conditions that drove boom-bust successions. We then incorporate our data into a statistical meta-analysis that includes nannoplankton records from eight other K-Pg sites in the literature, allowing us to better understand the significance of boom-bust successions in a global context. Our results support earlier findings that indicate that changes in nutrient availability, caused by the gradual restoration of biological pump efficiency following the K-Pg impact, was the main driver of boom-bust successions in the earliest Paleocene. Thus, the series of taxa that comprise boom-bust successions are geographically distinct due to differences in the timing of biological pump recovery, the type of marine environment examined (e.g., neritic vs. pelagic settings), and the influence of regional, or even local, controls on ecology.