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The circulation of the equatorial surface ocean is inescapably linked to the trade wind system. The equatorial Pacific is the classic "world ocean" example of this linkage. It is dominated by wind-driven circulation and is largely unfettered by ocean boundaries. Here, the Equator itself is characterized by a narrow zone of divergence that results from the change in the sign of the Coriolis effect and gives rise to upwelling of subsurface waters and a band of high biologic productivity. The strength of the equatorial circulation and this divergence is linked to the strength of the trade winds, which are in turn strongly tied to the global climate system. Variations in global climate, interhemispheric differences in temperature gradients, and marked changes in the ocean boundaries are all imprinted on the biogenic-rich sediments that are accumulating in the equatorial zone. The PEAT science program is designed to provide an understanding of equatorial Pacific circulation, carbonate production, deposition, and dissolution for the last 55 m.y. at a scale where orbital forcing can be resolved. Combined with seismic reflection data following in the vein of Mitchell et al. (2003) and synthesized with earlier drilling (e.g., Moore et al., 2002, 2004) we can reconstruct equatorial Pacific history with high confidence and substantially improve upon work from the early stages of Deep Sea Drilling Project (DSDP) and recent Ocean Drilling Program (ODP) legs.
Deciphering the sedimentary history of the equatorial Pacific has been greatly simplified by favorable motion of the Pacific plate. Throughout the Cenozoic, movement of the Pacific plate has had a northward component of ~0.25°/m.y. This northward movement transports the equatorial sediments gradually out from under the zone of highest sediment delivery, resulting in a broad mound of biogenic sediments (Fig. F1). This transport prevents the older equatorial sections from being buried deeply beneath the younger sections as the crust moves northward. The northward displacement, however, is not so large that the tectonic traverse of the equatorial zone (within 2° latitude of the Equator) was too rapid to record a reasonable period of equatorial ocean history. Drill sites typically remain within the equatorial zone for 10–20 m.y. before passing beyond the northern edge of high biogenic sedimentation. Older equatorial sections are thus buried beneath a thin veneer of younger sediments as the crust moves northwestward. The resulting diminished overburden minimizes burial diagenesis of the biogenic debris. It also allows advanced piston corer (APC) piston coring of much of this section with the right strategy for locating the drill sites (Lyle et al., 2002).
In their summary of DSDP results in the equatorial Pacific, van Andel et al. (1975) give a general view of the development of the equatorial mound of sediments in the Pacific Ocean, based mostly upon three early DSDP legs (5, 8, and 16). They showed how both temporal and spatial variation in sediment accumulation rates resulted from plate movement, varying biologic productivity at the equatorial divergence, and carbonate preservation. The buildup of the equatorial Pacific mound of sediment has been more recently documented and discussed by Mitchell (1998) and Mitchell et al. (2003) (Fig. F1).
Drilling across the equatorial Pacific mound occurred decadally after the van Andel et al. (1975) compilation, first with DSDP Leg 85, then when an equatorial latitudinal transect along 10 Ma crust was drilled during ODP Leg 138, and finally when a similar transect along 56 Ma crust was conducted during ODP Leg 199. The newer drilling, coupled with major advances in geochronology, has documented the remarkable correlation of paleoceanographic events over thousands of kilometers in the equatorial Pacific, caused by the large scale of equatorial Pacific circulation (Fig. F2). It is possible, with the addition of a relatively small number of new sites, to build detailed reconstructions of equatorial Pacific circulation through the Cenozoic.
Early drilling missed most of this detail because of the lack of important drilling technologies such as APC coring, multisensor track correlation, and core-log integration that now allow collection of relatively undisturbed sediments, rebuilding of a continuous sediment column from individual cores, and correlation to seismic reflection data. Together with an improved knowledge of the plate tectonic regime, these advances will allow us to locate areas of enhanced depositional rates associated with the paleoequator. Combining multiple sites along the Equator will result in a detailed record from the Pleistocene to the Paleocene. These records will also be invaluable for the continued development of the Cenozoic timescale.
During ODP Legs 138 and 199, excellent sections were recovered that have provided detailed orbital tuning of the geologic timescale. These sections give a much clearer picture of variations in sedimentation rates, isotopic evolution of the oceans, biologic evolution and zoological provenance, variations in carbonate preservation, and variations in geochemical fluxes that result from paleoceanographic and paleoclimate changes. There are, however, still parts of the Cenozoic timescale that require further refinement and verification of the proposed orbital tuning. The timescale older than the late Eocene has not yet been calibrated sufficiently, even though there is evidence of orbital frequencies in parts of the records recovered from this older interval (e.g., Norris and Röhl, 1999; Röhl et al., 2001).