Significance of Morphological Change in Braarudosphaera Following the K-Pg Mass Extinction Event at "Ground Zero"

The K-Pg mass extinction (66.0 Ma) was caused by a meteorite impact off the Yucatán Peninsula in the Gulf of Mexico, which eliminated 75% of species on Earth. Among the biggest casualties were calcareous nannoplankton: unicellular, photosynthetic, marine algae. In the Northern Hemisphere, one of the only survivors of the mass extinction was Braarudosphaera: an enigmatic nannoplankton which forms pentagonal calcite plates (pentaliths). This taxon bloomed at many shelf sites almost immediately after the mass extinction but was largely absent in pelagic environments, indicating that specific environmental or ecological requirements were essential for its success. Braarudosphaera also forms short-lived, monospecific, and geographically restricted blooms during other periods of Earth's history from the Cenomanian to modern. Although it has been linked to environmental stress, low salinity, and high nutrient conditions, its ecological preferences are still uncertain.

Results from the peak ring of the Chicxulub Impact Crater (IODP-ICDP 364) indicate that Braarudosphaera bloomed for 1 Myr post-impact and remained unusually common until the end-Danian at "ground zero". Furthermore, the size of Braarudosphaera pentaliths appears to increase and become less variable through time. It is likely that pentaliths of different sizes represent morphotypes or pseudo-cryptic species which each had unique environmental or ecological preferences. This may explain how Braarudosphaera was able to survive and adapt to volatile conditions following the K-Pg boundary. However, Braarudosphaera morphology has rarely been studied in the fossil record, and size has never been correlated to a specific environmental variable.

Here we present results showing how Braarudosphaera morphology (including pentalith side length, radius, internal angle, and area) changed during the recovery interval of the K-Pg mass extinction event at "ground zero". These data will be correlated to geochemical records at the same site (e.g. δ¹⁸O, δ¹³C, & B/Ca), to determine whether morphotypic variation was controlled by a specific environmental variable. To test whether changes in Braaurudosphaera morphology were local or global, the results from Chicxulub will be compared to data from other K-Pg sites with Braarudosphaera blooms.