Site 1236 (proposed Site NAZCA-10A) is located atop Nazca Ridge at 21°21.539´S, 81°26.165´W at a water depth of 1323 m. (Fig. F1A). This aseismic ridge, a fossil hotspot track with its modern expression at Easter Island, is especially well developed and shallow here at its intersection with the Nazca Fracture Zone.
The site occupies the center of a ~20-km-wide plateau that is punctuated by small volcanic spires that rise a few hundred meters above the relatively flat bathymetric surface (Fig. F2). These abrupt features likely represent small eroded remnants of an archipelago of volcanic islands that subsided below sea level after the site moved eastward over the active hotspot relatively early in its history. About 20 km southwest of Site 1236, a flat-topped guyot rises to within 350 m of present sea level. The presence of this relatively well-preserved, shallow bathymetric feature adjacent to the deeper, more eroded surface suggests multiple episodes of volcanism or the presence of a younger carbonate reef system that grew atop the volcanic edifice.
Nazca Ridge is covered with sediment to its shallowest reaches. The total thickness of the sedimentary section at Site 1236 is estimated to be 200 m based on site-survey seismic profiles (Fig. F3). The pelagic sequence reveals well-stratified strong reflective layers from the sediment surface to ~60 meters below seafloor (mbsf) and moderate reflective layers from ~60 to ~120 mbsf. Farther below to ~200 mbsf, deformed but almost continuous layers of moderate reflection are believed to smooth the relief of the acoustic basement.
Basement ages are expected to be between 30 and 35 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 plateau and adjoining plate may only be a few million years. Triple coring at Site 1236 with the advanced piston corer (APC) therefore had the potential of providing a continuous pelagic record of 200 m that spans the entire Neogene and possibly the uppermost part of the upper Oligocene.
The tectonic backtrack path on the Nazca plate moves Site 1236 ~20° westward, approximately parallel to its modern latitudinal position (Fig. F4). Thermal subsidence would predict that the site was at shallower water depths during the early Miocene and perhaps was exposed above sea level very early in its history. Such subsidence is consistent with bathymetric evidence for erosion of the plateau surface. The implications of a late stage of volcanism (as suggested by the presence of shallower well-preserved guyots in the region) to paleobathymetric history are unknown.
Today, Site 1236 is situated near the western edge of the northward-flowing Peru-Chile Current (Fig. F1B). Its position relatively far from the productive upwelling systems places it in a modern oligotrophic region of the subtropical gyre. Plate tectonic backtrack locations can be used to predict general features of paleoceanographic change at Site 1236 under assumption that the modern overall conditions in the region remained almost constant through time and the only change in the system is drift of the site location (Fig. F5) relative to this fixed oceanographic background. In this analysis we ignore the relatively small changes in the position of the South American margin through time. Sampling of modern oceanographic atlas (Ocean Climate Laboratory, 1999) values at the paleosite locations suggests that 25 m.y. ago, sea-surface temperatures at Site 1236 should have been ~3°C higher (and salinities 0.6 psu higher) than today. Sea-surface nutrient concentrations would have been generally lower at the site location 25 m.y. ago than today, although it appears that the relative concentrations of silicate, nitrate, and phosphate at the sea surface vary somewhat differently along the backtrack path. Modern primary productivity roughly follows sea-surface silicate concentrations along this path and implies that primary productivity would have been ~50% lower 25 m.y. ago than at present. Thus, we expect lower biogenic sedimentation rates and relatively minor amounts of terrigenous sediment with greater age at Site 1236, with the exception of volcanics or shallow-water sediments early in the site's history. 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 1236 reflects the transition zone between the northernmost extension of oxygen-enriched (nutrient depleted) Antarctic Intermediate Water (AAIW) and the deeper oxygen-depleted (nutrient rich) Pacific Central Water (PCW). Its tectonic backtrack to shallower depths suggests that it may have been bathed in AAIW (or its paleoequivalent) during much of Miocene time (Fig. F6).
The primary objectives at Site 1236 were to provide a continuous sedimentary sequence of the Neogene and Quaternary to
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