COUPLED ISOTOPIC AND TRACE METAL EVIDENCE FOR A SIGNIFICANT RISE IN TROPICAL SEA SURFACE TEMPERATURE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to an increase in greenhouse gas levels, possibly through the dissociation of > 2000 Gt of marine clathrate. In theory, if driven by greenhouse forcing, warming should occur at all latitudes though amplified toward the poles. Available data shows as much as 10°C of warming in the high-latitudes, and 5°C in the deep sea during the PETM. The character of the tropical sea surface temperature (SST) response, however, remains unconstrained. The few marine sections recovered from the tropics have been either unconformable across the P-E boundary, significantly disturbed by the coring process, or diagenetically compromised. Moreover, interpretation of the most commonly used temperature proxy, d¹⁸O, has been somewhat ambiguous because of potential salinity effects.

These deficiencies were largely addressed by ODP Leg 198 which recovered several stratigraphically continuous pelagic P-E boundary sections in 4 Sites on Shatsky Rise. With samples from the PETM interval of one Site, 1209, we determined changes in SST by measuring both the oxygen isotope and Mg/Ca ratios of mixed-layer planktonic foraminifera Morozovella velascoensis and Acarinina soldadoensis, both of which harbored photosymbionts. We also measured several indicies of preservation. Based on calibrations for modern species, the Mg/Ca data for both species indicate a 5°C rise in tropical (~5°N) SST during the PETM, while the oxygen isotope data indicate a 2.5°C rise. The discrepancy in the O-isotope data can be reconciled with ~1-2 ppt rise in local/regional sea surface salinity (and d¹⁸O). These results, when considered with the SST changes recorded in the high-latitudes, are consistent with the SST and hydrologic response predicted by models in conjuction with a doubling of atmospheric pCO₂ (in the absence of sea-ice).