Some like it cool: Benthic foraminiferal response to Paleogene warming events

Abstract

Long-term climatic trends of the Paleogene were interrupted by global perturbations of the carbon cycle, commonly associated with warming of surface and bottom waters and ecosystem disturbance. Most of these perturbations occurred in the Paleocene - Eocene greenhouse climate, but others were superimposed on the transition of greenhouse-to- icehouse conditions during the middle to late Eocene. Here we analyze deep-sea benthic foraminifera to focus on the impact of Paleogene warming events on ocean bottom water ecosystems. We compare changes in assemblage diversity (Fisher-α index) across nine warming events at 16 locations (ocean drilling sites and land sections; 25 case studies) by applying the Epps-Singleton and Fligner-Killeen non-parametric statistical tests. Generally, diversity decreased whereas the coefficient of variation increased during warming events, but not all changes were statistically significant. The declines in diversity may have been due to adaptation to stressed conditions, whereas the higher coefficients of variation may have been caused by unstable environmental conditions. The most severe events, the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) and Eocene Thermal Maximum 2 (ETM2, ~53.7 Ma), had a higher impact on bottom water ecosystems, as shown in major drops in diversity. Regression analysis of data for ODP Sites 1262 and 1263 (SE Atlantic) shows that only up to 62% of the variability of benthic foraminiferal diversity at these sites depends on δ 13 C and δ 18 O values, indicating that changes in diversity were not exclusively determined by the magnitude of the events as defined by the magnitude of changes in carbon isotope excursion and temperature. No major changes in diversity were identified during middle Eocene warming events, and post-event diversity became significantly more stable, suggesting that the overall cooler middle Eocene conditions supported higher faunal diversity and stability of benthic foraminiferal assemblages. • Declined diversity at warming events correlated to magnitude and local factors. • Background climate conditions influenced the impact of warming on marine biota. • Cooler middle Eocene conditions promoted higher diversity of benthic foraminifera.