Site 1234 (proposed Site SEPAC-13B) is located at 36°13.153´S, 73°40.902´W on a relatively flat bench in the middle of the continental slope, at 1015 m water depth, ~60 km shoreward of the Peru-Chile Trench and ~65 km offshore (Fig. F1). Basement is likely continental crust. The continental shelf here is ~60 km wide. Predrilling surveys (970312 Revelle, Mix et al., 1998; SO161-5 Sonne, Wiedecke-Hombach et al., 2002), indicate mostly hemipelagic sedimentation at the site. The uppermost part of the seismic profile shows flat-lying reflectors with deformed but continuous layers farther below, possibly reflecting the original relief of the acoustic basement (Fig. F2). Disrupted reflectors only occur at the base of the seismic profile. This site was chosen to take advantage of the high sedimentation rates to reconstruct continental climate and oceanography on millennial to centennial timescales for the late Quaternary. Canyons to the north and south of the site appear to channel most turbidity currents away from this shallow basin.
Site 1234 is ~7 km southwest of Site 1235 but is located at greater depth (Fig. F1). These two sites are expected to be influenced by similar surface water conditions so that major differences in physical, chemical, and paleontological properties can be attributed to depth-related effects.
The regional surface circulation at Site 1234 is marked by the northward-flowing Peru-Chile Current (PCC) and the e Coastal Current (CC), which are separated by the poleward-flowing Peru-Chile Counter Current (PCCC) (Strub et al., 1998) (Fig. F3A). The PCC and the CC transport cold, nutrient-enriched subpolar water masses northward, whereas of the CC is significantly affected by admixture of low-salinity waters from the Chile fjord region (Fig. F3B). The PCCC stretches from 100 to 300 km offshore and transports subtropical surface water to the south. Beneath these surface currents, at a depth of 100-400 m, the poleward-flowing Gunther Undercurrent transports relatively low-oxygen and high-salinity water masses southward along the shelf edge. These nutrient-rich water masses provide the source of nearshore upwelled waters (Fonseca, 1989). At Site 1234, seasonal upwelling favors high biogenic productivity during the southern summer as long as wind directions from south to south-southwest dominate. During winter, dominant wind directions from the north significantly reduce coastal upwelling (Strub et al., 1998).
Deeper currents include the northward-flowing Antarctic Intermediate Water (AAIW) at a depth between 400 and 1000 m. This water mass is relatively high in oxygen and low in salinity and overlies the southward-flowing nutrient-rich Pacific Central Water (PCW). We expect that benthic proxies at Site 1234 will reflect temporal and vertical changes of these water masses because Site 1234 is positioned in the modern transition zone between AAIW and PCW (Fig. F4).
The continental climate of southern Chile constitutes a transition zone between summer-dry Mediterranean climates to the north and heavy year-round rainfall to the south of Site 1234, due to the influence of westerly winds. Interannual rainfall in this region of Chile is thought to reflect the influence of El Niño Southern Oscillation (ENSO) events emanating from the tropics (Hebbeln et al., 2000; Dettinger et al., 2001). Modern sediments on the southern Chile continental slope are primarily provided by rivers (Lamy et al., 2002). Thus we expect variations in terrigenous sediment composition and mineralogy at Site 1234 to reflect changes in the latitudinal position of the westerly winds through time.
Primary goals of this site were to
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