During Ocean Drilling Program (ODP) Leg 199, we drilled a transect (Fig. F1) across the position of the equator at 56 Ma to study the ocean dynamics of the Eocene equatorial Pacific. In addition, one site (Site 1218) was drilled on 42-Ma crust at the paleoequatorial position to document the middle-late Eocene and the Eocene-Oligocene transition in more detail. The Leg 199 transect extends from a paleolatitude of ~11°N to ~5°S and encompasses a relatively thick early Eocene sediment section, perhaps 8°N of the paleoequator (see "Leg 199 Summary" chapter).
The early Paleogene (~60-45 Ma) witnessed the warmest global climates recorded on Earth in the entire Cenozoic. Deepwater temperatures, as well as subtropical and temperate faunas at high latitudes, support the idea that polar regions were much warmer than modern conditions, whereas debate continues over the sea-surface temperatures that prevailed in the tropics (Bralower et al., 1995; Andreasson and Schmitz, 1998; Pearson et al., 2001).
There can be little doubt that latitudinal temperature gradients during the early Paleogene were substantially smaller than today (Crowley and Zachos, 2000). This observation raises an intriguing paleoclimate problem because if warmer high-latitude climates depend on enhanced wind-driven ocean currents or wind-carried heat and moisture to transport heat to the poles, it is difficult to explain how this transport was maintained under the weaker pole-to-equator thermal gradients. Instead, weaker latitudinal temperature gradients should give rise to weaker winds and diminished wind-driven transport. This apparent paradox is a persistent problem in numerical general circulation-model reconstructions of warm paleoclimates (Barron and Washington, 1984; Manabe and Bryan, 1985; Sloan and Huber, 2001)
Because the Pacific plate drifted northward through Cenozoic time, it has transported Paleogene biogenic sediments deposited under the high-productivity equatorial belt into a zone of extremely slow sediment (red clay) accumulation. Thus, the central tropical North Pacific Ocean is an ideal region in which to sample shallowly buried Paleogene sequences of equatorially deposited biogenic sediments. The thin Neogene cover of red clay in the area means that the sediments are typically unconsolidated and the entire Paleogene sediment section can be drilled by ODP advanced piston corer and extended core barrel methods. These drilling techniques allow for construction of the most detailed paleoceanographic reconstructions.