Last Abundant Appearance Datum of Hemidiscus karstenii driven by climate change

The Last Abundant Appearance Datum of the endemic Southern Ocean diatom Hemisdiscus karstenii Jousé is dated at around 190,000 years Before Present, which provides a robust and convenient biostratigraphic marker to Late Pleistocene paleoceanographic studies. However, no prior studies have sought to understand what processes may have driven H. karstenii to extinction. We here present the first morphometric investigation of H. karstenii over Marine Isotope Stage (MIS) 10 to 6 (355,000 years to 150,000 years). The combination of these data to records of H. karstenii absolute abundances, Fragilariopsis kerguelensis biometry and absolute abundances, and southern high-latitude air and ocean temperatures over the past 355,000 years suggests that H. karstenii populations reduced their mean size and productivity in response to the rapid drop in temperatures across the MIS 7 to MIS 6 glacial inception. This prolonged cooling event may have pushed H. karstenii to its adaptive limit until it could not sustain physiological viability. The repetitive occurrence of very cold events during the early MIS 6 may have additionally prevented surviving populations of H. karstenii from re-establishing abundant communities leading to extinction late in MIS 6.