Site 1235 (proposed Site SEPAC-14A) is located at 36°9.594´S, 73°33.983´W on a gently sloping terrace on the upper continental slope, at 489 m water depth, ~65 km shoreward of the Peru-Chile Trench and ~60 km offshore (Fig. F1). Basement is likely continental crust. The continental shelf here is ~60 km wide. Predrilling surveys indicate hemipelagic sedimentation at the site as indicated by gravity cores and seismic data (970312 Revelle, Mix et al., 1998; SO161-5, Sonne, Wiedicke-Hombach et al., 2002). The uppermost part of the seismic profile shows well-stratified, flat-lying, strong reflectors (Fig. F2). Farther below, deformed but continuous layers of moderate reflection are believed to smooth the original relief of the acoustic basement. This site was chosen to take advantage of the expected high sedimentation rates to decipher the late Quaternary history of continental climate and southeast Pacific oceanography on millennial to centennial timescales. Small canyons to the north and south of the site appear to channel most turbidity currents away from this shallow basin.
Site 1235 is ~7 km northeast of Site 1234 (1015 m water depth) but is located upslope in shallower waters (Fig. F1). These two sites are expected to be influenced by similar surface water conditions, so major differences in physical, chemical, and paleontological properties can be attributed to depth-related effects.
The regional surface circulation at both sites is marked by the northward-flowing Peru-Chile Current (PCC) and the 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 the CCC is significantly affected by admixture of low-salinity waters from the Chilean 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 (GUC) transports relatively low-oxygen and high-salinity water masses near the shelf edge. These nutrient-rich water masses provide the source of nearshore upwelled waters (Fonseca, 1989). At both sites, seasonal upwelling favors a high biogenic productivity during southern summer as long as wind directions from south to south-southwest dominate. During extreme upwelling events, upwelling of very low oxygen water from the GUC has caused fish kills (Ahumada, 1989). During winter, dominant wind directions from the north significantly reduce coastal upwelling (Strub et al., 1998).
The water depth of 489 m at Site 1235 reflects the modern transition zone between the relatively low-oxygen, high-salinity, nutrient-rich GUC and the relatively high-oxygen, low-salinity, nutrient-poor Antarctic Intermediate Water (AAIW) (Fig. F4). We expect that benthic proxies will reflect temporal and vertical changes of these water masses on millennial to centennial timescales throughout the late Quaternary.
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 1235, 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). We expect variations in terrigenous sediment composition and mineralogy at Site 1235 to reflect changes in the latitudinal position of the westerly winds through time.
Primary goals of Site 1235 were to
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