ISOTOPIC FILTERING REVEALS HIGH SENSITIVITY OF PLANKTIC FORAMINIFERS TO PALEOCENE EOCENE THERMAL MAXIMUM OCEAN ACIDIFICATION AND WARMING

Planktic foraminifers are prolific calcifiers in the global ocean, making them prime candidates for studying the effects of ocean acidification and warming on pelagic ecosystems. Here we reexamine the response of tropical planktic foraminifers to an ancient (ca. 56 Ma) episode of ocean acidification and warming that occurred during the Paleocene-Eocene thermal maximum (PETM) as recorded at Ocean Drilling Program Site 865 in the central Pacific Ocean. A hallmark of the PETM is a negative carbon isotope excursion (CIE) signaling the rapid release of massive quantities of δ¹³C-depleted carbon into the ocean-atmosphere system. Previous studies have used standard point-counting methods to document changes in species relative abundances and diversity amongst planktic foraminifer assemblages. However, this standard practice likely underestimates the true biotic response because pervasive sediment mixing smooths the micropaleontological record by blending microfossils of differing ages into aggregate assemblages. In this study, we use the δ¹³C signatures of individual planktic foraminifer shells to determine the relative proportions of pre-CIE specimens reworked into the CIE interval of the Site 865 section. Armed with this information, we corrected the assemblage counts for the effects of sediment mixing. Contrary to previous interpretations, application of this “isotopic filter” reveals that a transient but significant decrease in planktic foraminifer diversity coincided with the CIE onset. This sharp decline in diversity reflects the extra-tropical migration of several key taxa. The local extirpation of such shallow-dwelling (oceanic mixed-layer) taxa as Acarinina soldadoensis, Morozovella aequa, and M. subbotinae as well as all species belonging to the deep-dwelling (thermocline) genus Subbotina indicates that a wide range of depth ecologies were adversely affected. In addition, one of the holdover lineages (M. acuta/velascoensis/allisonensis) that persisted in the study area over the course of the PETM shows signs of an ecophenotypic response to attendant carbonate undersaturation. Although they did not suffer an extinction event like their benthic counterparts, our results show that planktic foraminifers were more sensitive to stressful PETM conditions than previously perceived.