Site 1238 (proposed Site CAR-2C) is located at 1°52.310´S, 82°46.934´W ~200 km off the coast of Ecuador (Fig. F1). On the southern flank of Carnegie Ridge at 2203 m water depth, the site is situated roughly in the middle of a bench that slopes gently to the south (Fig. F2). This portion of Carnegie Ridge is just outboard of the Peru-Chile Trench, where subduction is consuming the volcanic seamounts produced by the Galapagos hotspot (Lonsdale and Klitgord, 1978). From south of Sarmiento Ridge (~7°S), where water depths are >4000 m, the seafloor steps up in a series of southwest-trending ridges and reaches the main scarp of Carnegie Ridge ~20 km north of Site 1238. Basement at Site 1238 likely consists of basalt formed at the Galapagos hotspot at ~11-13 Ma, but it might also be oceanic crust formed at the Galapagos Rift at ~16-18 Ma (Lonsdale and Klitgord, 1978).
The region of Site 1238 has a relatively uniform pelagic sediment drape of 400-500 m on gently sloping topography. Seismic profiles (Fig. F3) reveal well-stratified sediments on reflective crust. Expected sediments are diatom-nannofossil ooze, with some intervals of diatom ooze (Kemp et al., 1995) and ash layers (Ledbetter, 1985). The steep basaltic flanks of Carnegie Ridge are mostly bare rock, suggesting some downslope transport of sediment (Fig. F2). The southern end of the sedimented ramp reveals rough bathymetry just to the north of a prominent seamount, indicative of erosion, slumping, or other deformation of the sediment column. At the top of the ramp, a disturbed zone, perhaps a detachment feature, is present near the volcanic scarp. These features may suggest that a giant sediment package including the area of Site 1238 was displaced by slumping or creeping downslope.
A tectonic backtrack path on the Nazca plate (Pisias et al., 1995) moves Site 1238 about 600 km westward and slightly to the south relative to a fixed South America (and probably to shallower depths—note possible sea level terraces on the ridge north of Site 1238 in Fig. F2) to a position just south of the Galapagos hotspot ~12 m.y. ago (Fig. F4).
Today, Site 1238 is situated under the eastern reaches of the equatorial cool tongue in an open-ocean upwelling system near the equator (Fig. F5). Nutrient-rich waters of the Equatorial Undercurrent (EUC) surface here and along the coast of Peru and Ecuador (Ocean Climate Laboratory, 1999), and nitrate and phosphate are not fully utilized by the phytoplankton in spite of high production (Fig. F6). Here, a limited supply of micronutrients such as iron, for which the EUC is a major source, may play an important role as a regulator of production (Murray et al., 1994).
The site is likely to record changes in upwelling and biological production, as well as long-term changes in upper-ocean temperature and pycnocline depth. The surface-ocean properties of the eastern equatorial Pacific are sensitive to interannual to decadal oscillations that are strongly controlled by El Niņo Southern Oscillations (ENSO) (Cane, 1986) as well as to Pleistocene climate fluctuations (Pisias and Mix, 1997; Mix et al., 1999; Lea et al., 2000; Beaufort et al., 2001). Site 1238's tectonic backtrack to the west is subparallel to the equator, so it is likely that the site resided within the highly productive and climatically sensitive region south of the equator throughout its history.
Plate tectonic backtrack locations can be used to predict general features of paleoceanographic change at Site 1238, under the assumptions that the modern overall conditions in the region remain constant through time and that the only change in the system is drift of the site location relative to this fixed oceanographic background (Fig. F7). In this analysis, we ignore changes in the position of the continental margin through time, which may be significant in the region of northern South America and the Central American Isthmus. Sampling of modern oceanographic atlas values at the paleosite locations suggests that from 2 to 3 Ma, sea-surface temperatures at Site 1238 may have been slightly (~1°C) cooler than today, but that at greater ages the site warms gradually as it backtracks across the South Equatorial Current. Salinities increase monotonically at greater ages, as the site backtracks gradually away from the equator and the eastern boundary. Pycnocline depth is relatively stable along the backtrack path, at values of 20-25 m. Sea-surface nutrient concentrations of silicate, phosphate, and nitrate all increase significantly from 0 to 2 Ma but are relatively stable on the Site 1238 backtrack path prior to 2 Ma. In contrast to nutrients, primary productivity is highest within the past 2 m.y. and declines gradually at greater ages as the site backtracks away from the eastern margin. In the absence of other regional changes, we would expect that both biogenic and terrigenous sediment accumulation rates at Site 1238 would be relatively high at younger ages, when the site was located closer to both the eastern boundary and the equator. Significant deviations from these general trends, if detected in the sediment cores, would imply changes in regional oceanographic conditions, or errors in the tectonic backtrack model.
The modern water depth of Site 1238 is ideal to monitor Pacific Central Water (PCW) south of Carnegie Ridge, roughly at the sill depth of the Panama Basin (Tsuchiya and Talley, 1998). The water mass enters the Peru Basin from the west, around the Galapagos platform and through fracture zones in the East Pacific Rise, and is characterized by relatively low oxygen and salinity and high nutrients (Fig. F8). Near the equator, the EUC brings nutrient-rich waters at a few hundred meters depth from the western Pacific to the eastern boundary region, where they are upwelled to the surface. Oxygen-depleted intermediate waters (remnants of North Pacific Intermediate Water) enter the region from the north at depths of a few hundred to 1000 m, but they are mostly limited to the Northern Hemisphere. In the Southern Hemisphere, remnants of low-salinity Antarctic Intermediate Water are present in the upper 1000 m but are not detected in oxygen concentrations because of strong biological overprints. Below 3 km depth, and thus below the sill depth of the Panama Basin (~2000 m), is Peru Basin Water. This water mass enters the region carrying a weak influence of Circumpolar Deep Water as a relatively low-nutrient, high-oxygen water mass.
The primary objectives at Site 1238 were to provide a continuous sedimentary sequence of Quaternary to Miocene age (as old as ~12-18 Ma) to
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