RESPONSE OF AUSTRAL, HIGH-LATITUDE (ODP SITE 1135) PLANKTIC FORAMINIFERA TO THE PALEOCENE-EOCENE THERMAL MAXIMUM AS REVEALED BY STABLE ISOTOPE SIGNALS FROM INDIVIDUAL SHELLS

We report preliminary results of an ongoing study focused on the response of high-latitude planktic foraminifera to an ancient (~55.8 Ma) global warming event and future climate analog, the Paleocene-Eocene thermal maximum (PETM). Published records indicate that PETM warmth was amplified at the high latitudes, and that this transient (~170 k.y.) global warming event was closely associated with the massive input of isotopically light carbon into the ocean-atmosphere system. This negative carbon isotope excursion (CIE) is documented in both marine and terrestrial global PETM records. The targeted PETM section (ODP Site 1135) from atop the Kerguelen Plateau (59°42’S, 84°16’E, water depth ~1 km) in the Indian sector of the Southern Ocean, is one of the few PETM records from the climatically sensitive circum-Antarctic region. Examination of planktic foraminiferal faunas from the base of the Site 1135 PETM section reveals the presence of Morozovella aequa and M. subbotinae, subtropical to temperate water taxa. To reduce effects of sediment mixing and elucidate faunal response, stable isotope analyses were performed on individual shells of shallow-dwelling morozovellids and the deep-dwelling genus Subbotina. Owing to the magnitude of the CIE, “isotopic fingerprinting” of individual specimens makes it possible to distinguish between shells calcified post-carbon input with low carbon isotope ratios and reworked shells (contaminants) calcified pre-carbon input with distinctly higher carbon isotope ratios. Results show that morozovellids populated this area only during the early PETM stages, with all specimens analyzed (n=36) registering low CIE values, while only 12 of the 47 subbotinids analyzed registered low CIE values. Thus, the relative abundances of morozovellids and subbotinids are underestimated and overestimated, respectively, in conventional census counts. This finding also reveals a temporary local subbotinid population crash during the early PETM stages. Two important corollaries are (1) the initial transition between pre-CIE and CIE conditions appears to be missing in this record due to a coring gap at the base of the section and (2) population dynamics of local foraminiferal faunas should be considered when reconstructing carbon exchange pathways using species of differing depth ecologies.