Site 1237 (proposed Site NAZCA-17A) is located at 16°0.421´S, 76°22.685´W, ~140 km off the coast of Peru (Fig. F1A). The site lies on a relatively flat bench (Fig. F2) on the easternmost flank of Nazca Ridge at a water depth of 3212 m. Nazca Ridge, a fossil hotspot track with its modern expression at Easter Island, terminates just outboard of the Peru-Chile Trench, where it is deformed and subducted beneath Peru. About 19 km west of Site 1237, an abrupt scarp (probably an active normal fault) rises to the summit of Nazca Ridge.
Eastern Nazca Ridge is covered by a thick drape of pelagic sediment to its shallowest reaches. The total thickness of the sedimentary section at Site 1237 is estimated at 280-300 m based on site survey seismic profiles (Fig. F3). The seismic record reveals well-stratified reflective layers, which clearly drape the underlying bathymetry from the sediment surface to acoustic basement, which may be either basalt or lithified sediment.
Basement ages are expected to be between 40 and 45 Ma, based on magnetic anomalies of the surrounding oceanic crust (Cande and Haxby, 1991). If we assume that the Nazca Ridge was formed at the Easter Island hotspot, then the age difference between the basement of the ridge and adjoining plate may only be a few million years. Therefore, triple coring with the advanced piston corer (APC) at Site 1237 had the potential of providing a continuous pelagic record that spans much of Neogene time and a potentially into the Paleogene.
The tectonic backtrack path on the Nazca plate moves Site 1237 about 20° westward relative to South America over the past 42 m.y. (Fig. F4). Removal of thermal subsidence effects would predict that the site was at shallower water depths early in its history. Although backtracking water depths is difficult on oceanic rises, we expect that the site may have risen to within a few kilometers of sea level in its early history. Uncertainties in this calculation include effects of faulting during the approach of the site to the Peru-Chile Trench, the inferred scarp just west of Site 1237 has a bathymetric expression of ~600 m, and an unknown history of movement.
Today, Site 1237 is situated near the eastern edge of the northward-flowing Peru-Chile Current (Fig. F1B), a major conduit of cool-water transport from high to low latitudes. The site's position near the productive upwelling systems of Peru suggests that it may record changes in upwelling and biological production in this eastern boundary current setting (Fig. F5). Plate tectonic backtrack locations can be used to predict general features of paleoceanographic change at Site 1237 under the assumptions that overall conditions in the region remained constant over time and that the only change in the system is drift of the site location relative to this fixed oceanographic background (Fig. F6). In this analysis, we ignore the relatively small changes in the position of the South American margin through time. Sampling of modern oceanographic atlas values at the paleosite locations suggests that 25-30 m.y. ago, sea-surface temperatures at Site 1237 should have been ~2.5°C higher (and salinities 0.7 higher) than today. Pycnocline depth changes significantly along the backtrack path, starting at values of 70-80 m prior to 20 Ma and shallowing in steps to values of 25 m in the past 1 m.y. Sea-surface nutrient concentrations of silicate, phosphate, and nitrate would have been generally lower than at present with a pronounced increase after ~8 Ma. Modern primary productivity roughly follows sea-surface temperature trends along the backtrack path and implies that 30 m.y. ago primary productivity would have been less than one-third of its present value at the site. Thus we expect lower biogenic sedimentation rates and relatively minor amounts of terrigenous sediment with greater age at Site 1237. 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. In its older intervals, the geologic record at Site 1237 is likely to resemble that of Site 1236, although Site 1237 probably resided at somewhat greater water depths than Site 1236 throughout its history.
The modern water depth of Site 1237 reflects the transition zone between the relatively oxygen rich (nutrient depleted) remnants of Circumpolar Deep Water (CPDW) that enter the Peru Basin as bottom water through the Peru-Chile Trench (Lonsdale, 1976) and the relatively oxygen-depleted (nutrient rich) Pacific Central Water (PCW) (Tsuchiya and Talley, 1998). In the Pliocene-Pleistocene interval, Site 1237 may record variations in carbonate preservation associated with changes in deepwater masses and global carbonate budgets. A tectonic backtrack to shallower depths suggests that it may have occupied depths of the modern PCW (or its paleoequivalent) during its early history (Fig. F7). The apparent fault scarp to the west (Fig. F3) suggests a step up to paleodepths perhaps 600 m shallower than the present location within the past few million years.
The primary objective at Site 1237 is to provide a continuous sedimentary sequence of the Neogene and Quaternary and as complete as possible a record of older intervals to
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