LATE PALEOCENE TO EOCENE PALEOCEANOGRAPHY OF THE EQUATORIAL PACIFIC-OCEAN - STABLE ISOTOPES RECORDED AT OCEAN DRILLING PROGRAM SITE-865, ALLISON-GUYOT

An expanded and largely complete upper Paleocene to upper Eocene secti on was recovered from the pelagic cap overlying Allison Guyot, Mid-Pac ific Mountains at Ocean Drilling Program (ODP) Site 865 (18 degrees 26 'N, 179 degrees 33'W; paleodepth 1300-1500 m). Reconstructions show th at the site was within a few degrees of the equator during the Paleoge ne. Because no other Paleogene sections have been recovered in the Pac ific Ocean at such a low latitude, Site 865 provides a unique record o f equatorial Pacific paleoceanography. Detailed stable isotopic invest igations were conducted on three planktonic foraminiferal taxa (specie s of Acarinina, Morozovella, and Subbotina). We studied benthic forami niferal isotopes at much lower resolution on species of Cibicidoides a nd Lenticulina, Nuttallides truempyi and Gavelinella beccariiformis, b ecause of their exceptional rarity. The delta(18)O and delta(13)C stra tigraphies from Site 865 are generally similar to those derived from o ther Paleocene and Eocene sections. The planktonic foraminiferal recor ds at Site 865, however, include significantly less short-term, single -sample variability than those from higher-latitude sites, indicating that this tropical, oligotrophic location had a comparatively stable w ater column structure with a deep mixed layer and less seasonal variab ility. Low-amplitude (0.1-0.8 parts per thousand) oscillations on time scales of 250,000 to 300,000 years correlate between the delta(13)C re cords of all planktonic taxa and may represent fluctuations in the mix ing intensity of surface waters. Peak sea surface temperatures of 24 d egrees-25 degrees C occurred in the earliest Eocene, followed by a rap id cooling of 3-6 degrees C in the late early Eocene. Temperatures rem ained cool and stable through the middle Eocene. In the late Eocene, s urface water temperatures decreased further. Vertical temperature grad ients decreased dramatically in the late Paleocene and were relatively constant through much of the Eocene but increased markedly in the lat e Eocene. Intermediate waters warmed through the late Paleocene, reach ing a maximum temperature of 10 degrees C in the early Eocene. Cooling in the middle and late Eocene paralleled that of surface waters, with latest Eocene temperatures below 5 degrees C. Extinction patterns of benthic foraminifera in the latest Paleocene were similar to those obs erved at other Pacific sites and were coeval with a short-term, very r apid negative excursion in delta(13)C values in planktonic and benthic taxa as at other sites. During this excursion, benthic foraminiferal delta(18)O values decreased markedly, indicating warming of 4 to 6 deg rees C for tropical intermediate waters, while planktonic taxa show sl ight warming (1 degrees C) followed by 2 degrees C of cooling. Converg ence of delta(18)O values of planktonic and benthic foraminifera sugge sts that thermal gradients in the water column in this tropical locati on collapsed during the excursion. These data are consistent with the hypothesis that equatorial Pacific surface waters were a potential sou rce of warm, higher salinity waters which filled portions of the deep ocean in the latest Paleocene. Oxygen isotopic data indicate that equa tor to high southern latitude sea surface thermal gradients decreased to as little as 4 degrees C at the peak of the excursion, suggesting s ome fundamental change in global heat transport.