Climate Control on Paleogene Benthic Foraminiferal Assemblages: Analysis from a Different Point of View

The geological record of the Paleogene reflects the transition from a greenhouse to an icehouse world, with short-term rapid warming events superimposed on this gradual trend. We propose the use of accumulated heterogeneity (Haccum) to identify benthic foraminiferal biofacies over time, to evaluate the impact of global climate changes on deep-sea ecosystems. We thereby focus on the continuity of assemblages through time considering their structure rather than faunal composition, and interruption of continuity denotes a change of biofacies. We applied this method to benthic foraminiferal data from Ocean Drilling Program Site 1263 (SE Atlantic), covering distinct Eocene intervals that include rapid warming events such as the Paleocene-Eocene Thermal Maximum (PETM), Eocene Thermal Maximum-2 (ETM2), or the Eocene Thermal Maximum-3 (ETM3), as well as the Middle Eocene Climatic Optimum (MECO), the Ypresian-Lutetian boundary (CP12a-CP12b biozones), and the gradual cooling across the Eocene-Oligocene Transition (Priabonian-Rupelian; CNE20-CNO2 biozones). We show an abrupt change in biofacies at the beginning of the PETM, the most extreme, short-term warming event, in accordance with the Benthic Foraminiferal Extinction. The “PETM biofacies” displays a negative stable trend that extends above the event interval, indicating that conditions remained unfavourable for the deep-sea benthos for ~70 kyr after the end of the PETM. In contrast, during ETM2, there were multiple shifting biofacies. These reiterated changes suggest highly perturbed and unstable environmental conditions, but insufficient to cause a drastic drop in Haccum. Other events, however, show a minor, negative impact on benthic foraminiferal assemblages. Two biofacies were seen during the ETM3: the first indicates a slight decrease in Haccum, the second, starting at the middle of the event, an increasing trend in Haccum. This pattern is repeated during the MECO, with an enhanced decreasing trend in Haccum at the base of the event. Biofacies across the Ypresian-Lutetian boundary point to optimum environmental conditions towards and after the Early Eocene Climatic Optimum (EECO). However, a marked change in biofacies occurred across the Priabonian-Rupelian boundary, reflecting a turnover in the Haccum trend, from increasing values during the late Eocene to decreasing ones into the Oligocene. These results allow us to identify patterns not always evidenced by “simple heterogeneity”, i.e., heterogeneity calculated separately for each sample. For instance, heterogeneity values fluctuate across the ETM3, the MECO and the Ypresian-Lutetian transition without a clear trend, and they show an increasing trend during the Oligocene, opposite to the Haccum record. By assessing the Haccum, we identified a significant, strong correlation with benthic foraminiferal stable isotope values (δ13C and δ18O), not only across rapid warming events (hyperthermals) but also through more gradual climate changes (e.g., EECO, and the Eocene-Oligocene transition), across which there is no significant correlation when looking at simple heterogeneity. Therefore, these findings suggest a stronger influence than previously assumed of climate conditions on benthic foraminiferal assemblages on a variety of timescales, improving our understanding of the impact of Paleogene climate variability on deep-sea ecosystems.