SOME LIKE IT COOL: DEEP-SEA BENTHIC FORAMINIFERAL RESPONSE TO PALEOGENE WARMING EVENTS

Paleogene long-term climate trends were interrupted by short-term perturbations of the carbon cycle, global warming and ecosystem disturbance. Earth's climate state and its perturbations are reflected in temperatures of the deep ocean, Earth's largest habitat, thus by deep-sea biota. We assessed deep-sea benthic foraminiferal diversity across Paleogene short-term warming events of different magnitude superimposed on different climate states: the Latest Danian Event (LDE), the Middle Paleocene Biotic Event (MPBE), the Paleocene-Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM2), the H2 event, and Eocene Thermal Maximum 3 (ETM3), occurring during a greenhouse climate, and the Middle Eocene Climatic Optimum (MECO), the C21r-H6 event and the Late Lutetian Thermal Maximum (LLTM), punctuating middle-late Eocene cooling.

We evaluated benthic foraminiferal assemblage diversity (Fisher-a index, calculated in MATLAB; standardized for all sites) across these warming events at ocean drilling sites and land sections by applying the non-parametric Epps-Singleton and Fligner-Killeen tests. Most Paleocene-early Eocene cases show a decrease in diversity and an increase in the coefficient of variation (standard deviation/mean value) during warming events, with major declines in diversity during the most severe ones, but changes are not statistically significant for all lesser events. In contrast, diversity did not decline significantly during the middle Eocene warming events, and became more stable after those events. Cooler background conditions from the middle Eocene on thus may have supported relatively high diversity and assemblage stability.

The decline in diversity may be due to combined stress conditions (high temperatures, changes in carbonate corrosivity and/or oxygenation), and the higher coefficients of variation may be due to unstable environmental conditions. Regression analyses for Site 1262 (Walvis Ridge, SE Atlantic) show that only about half of the variability in diversity is correlated to δ¹³C and δ¹⁸O changes, indicating that diversity was not exclusively determined by the magnitudes of carbon emission and warming. Feedback, e.g., changes in phytoplankton communities, thus type and/or continuity of food supply may have been important.