Biogenic sedimentation at ODP Site 138-847

Site 847, located on the equator beneath the region of divergent-driven upwelling in the eastern tropical Pacific Ocean, provides a continuous record of biogenic sedimentation spanning the past 3 m.y. Sediments at the site are primarily composed of carbonate and opal microfossils, with secondary amounts of detrital material that was transported to the site by means of winds. The cyclic changes in the relative abundance of carbonate and opal during the past 1 m.y. exhibit a strong 100-k.y. variability, which generally corresponds to late Pleistocene climatic oscillations. A distinct, carbonate-poor interval is evident from the last major interglacial and has been attributed to a decrease in production rather than to dissolution. The long-term changes in CaCO3 mass accumulation rates (MARs) also are driven by production rather than dissolution. High rates near 2.8 Ma are followed by a distinct minima near 1.8 Ma. CaCO3 MARs increase to a maxima near 0.4 Ma, followed by the decrease in production that occurred during the last interglacial period. Opal accumulations exhibit distinct maxima between 1.7 and 2.0 Ma, 1.1 and 1.3 Ma, and during glacial stages 6 and 2. These increases by more than a factor of 2 in opal accumulation tentatively have been attributed to opal production.
Site 847 provides the eastern anchor of three sites studied from the central (near 140°W) to eastern (near 95°W) equatorial Pacific Ocean to evaluate spatial changes in carbonate and opal sedimentation in an area of strong divergence during the past 3 m.y. Modern Oceanographic studies show a west to east gradient in surface productivity across this region. Analysis of the carbonate sedimentation for the past 3 m.y. and the opal sedimentation for the past 1 m.y. at the three sites reveals distinct differences in the respective gradients of accumulation. The west to east gradient in carbonate accumulation is small despite the fact that the eastern site is shallower and has better carbonate preservation. However, the gradient in opal accumulation is steep and shows a four- to fivefold increase from the central to the eastern equatorial Pacific sites. Thus, the surface gradient in primary production is reflected by bulk opal accumulation, not by carbonate accumulation.