BACKGROUNDThe complex system of equatorial currents is one of the most persistent and clear traces of wind-driven circulation in the oceans. In the Neogene, the unequal hemispheric thermal gradients have pushed the Intertropical Convergence Zone (ITCZ) north of the equator and given rise to a narrow band of equatorial upwelling as well as an equatorially assymetric zonal current system. The high productivity associated with the equatorial upwelling results in a high rain of biogenic debris to the seafloor within 1.5°-2° of the geographic equator, with peak values restricted to an even narrower zone. In the Pacific Ocean, this biogenic rain has built a mound over geologic time of almost pure calcareous and siliceous sediments stretching along the equatorial region and reaching a thickness of over 500 m (Fig. 2).
The central equatorial Pacific is unique in the world's oceans because the path of plate motion carries this linear trace of equatorial upwelling and productivity northward with time (van Andel, 1974). There are two clear implications of this northward plate motion: (1) the thickest part of the equatorial mound of biogenic sediment is displaced several degrees to the north of the equator and (2) sediments deposited a few tens of millions of years ago have moved completely out of the region of high sediment flux. This movement into regions of very low sediment accumulation (or even erosion) puts Paleogene equatorial sediments within the reach of the Ocean Drilling Program's (ODP) advanced piston corer (APC)/extended core barrel (XCB) technology. For the most part, the sediments have never been subject to strong burial diagenesis and can be cored easily with little disturbance by APC. Time intervals notorious for extensive chert formation (e.g., the middle Eocene) are more likely to contain only oozes because they have never been deeply buried.
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