RESPONSE OF HIGH-LATITUDE PLANKTIC FORAMINIFER COMMUNITIES TO THE PALEOCENE-EOCENE THERMAL MAXIMUM: INSIGHTS FROM THE SOUTHERN INDIAN OCEAN (ODP SITE 1135)

Marine microfossil records provide a wealth of information on the biotic effects of abrupt climate change. Yet the diminutive sizes of microfossils make them susceptible to sediment reworking, which can mask primary biotic signals in the deep-sea sedimentary record. Here we use a novel method, isotopic filtering, to deconvolve the effects of sediment mixing on planktic foraminifer assemblages from a deep-sea record of the Paleocene Eocene thermal maximum (PETM) recovered at austral Ocean Drilling Program (ODP) Site 1135. The PETM was an ancient (ca. 56 Ma) global warming event associated with a rapid perturbation of the global carbon cycle, making it an informative analogue for gauging the biotic effects of modern climate change. Earlier studies employing standard point-counting methods have shown that the onset of PETM conditions is marked by the arrival of warm-water taxa and an overall increase in planktic foraminifer diversity throughout the circum-Antarctic region. However, the unprecedented resolution afforded by isotopic filtering reveals a transient but significant decrease in planktic foraminifer diversity subsumed within the early stages of the overall community response to the PETM. This previously unrecognized loss of biodiversity is roughly correlative with the onset of sustained declines in both coccolithophore accumulation rates and abundances of benthic foraminifers at Site 1135. Although ocean warming and acidification acted as first-order stressors, the oligotaxic composition of this distinctive fauna suggests that varying levels of primary production with attendant deoxygenation also affected planktic foraminifer communities at the onset of the PETM. The results of this study further demonstrate the utility of isotopic filtering for deconvolving the time-averaging effects of sediment mixing on microfossil assemblages associated with paleoclimate events signaled by abrupt biogeochemical change such as the globally recognized carbon isotope excursion marking the PETM in the geological record.