Abrupt turnover in calcareous-nannoplankton assemblages across the Paleocene/Eocene Thermal Maximum: implications for surface-water oligotrophy over the Kerguelen Plateau, Southern Indian Ocean

Ocean Drilling Program (ODP) Core Section 183-1135A-25R-4 from the Kerguelen Plateau in the Indian
Ocean sector of the Southern Ocean represents only the second complete, expanded sequence through the
Paleocene/Eocene Thermal Maximum (PETM; ~55 Ma) recovered from Antarctic waters. Calcareous nannoplankton at
this site underwent an abrupt, fundamental turnover across the PETM as defined by a carbon isotope excursion.
Although Chiasmolithus, Discoaster, and Fasciculithus exponentially increase in abundance at the onset, the former
abruptly drops but then rapidly recovers, whereas the latter two taxa show opposite trends due to surface-water
oligotrophy. These observations confirm previous results from ODP Site 690 on Maud Rise. The elevated pCO₂ that
accompanied the PETM caused a shoaling of the lysocline and carbonate compensation depth, leading to intensive
dissolution of susceptible holococcoliths and poor preservation of the assemblages. Similarities and contrasts between
the results of this study and previous work from open-ocean sites and shelf margins further demonstrate that the
response to the PETM was consistent in open-ocean environments, but could be localized on continental shelves where
nutrient regimes depend on the local geologic setting and oceanographic conditions.