INTRODUCTION

One of the goals of Ocean Drilling Program (ODP) Leg 199 was to study the changes in ocean circulation in the Pacific Basin during the Paleogene. Stable isotopes are useful proxies for this purpose because the temperature of water masses can be deduced from ¹⁸O records, whereas ¹³C can be used as a nutrient tracer for ocean circulation. We present oxygen and carbon isotope values from bulk carbonate and benthic foraminifers across the Paleocene/Eocene (P/E) boundary in Holes 1220B and 1221C in the equatorial Pacific Ocean. The P/E boundary is characterized by an abrupt negative excursion in both oxygen and carbon isotopes, which is an indication of transient changes in temperature and the global carbon cycle at this time. Here, high-resolution sampling (3–5 k.y. in the core of the carbon isotope excursion) focused on the P/E transition to capture a detailed record of the carbon and oxygen isotope excursions in the deep equatorial Pacific.

Both sites are assumed to have had a paleodepth of ~2500–2600 m at the time of the P/E boundary. It is important to recognize that there may be significant uncertainty in the actual paleodepth for both sites, which is estimated by assuming that both sites were deposited near the crest of the newly formed oceanic ridge. Both sites were drilled on 56-Ma crust (considered to have formed near 2500 m water depth) but then had perhaps 500,000 to 1 million years to subside before deposition of the P/E sequence. Hence, it is possible that the sites actually reached depths of ~2580–2660 m prior to deposition of the P/E section, depending upon what age is assumed for the P/E boundary and assuming constant thermotectonic subsidence of the ocean crust. In addition, bottom roughness around the sites could account for perhaps up to an additional ±100 m of depth uncertainty. We assume an age for the P/E boundary of ~55.2 Ma, suggesting that the sites may both have reached depths of as much as ~2660–2760 m by the time the carbonate sequence began to accumulate.