4. Site 12331

Shipboard Scientific Party2


Site 1233 (proposed Site SEPAC-19A) is located at 41°0.005´S, 74°26.992´W in a small forearc basin on the continental slope, at a water depth of 838 m, shoreward of the Chile Trench. Basement is likely continental crust. The continental shelf here is 30-60 km wide. Predrilling survey data include high-resolution Parasound profiles that image the site to 80 m below seafloor (mbsf) (Fig. F1A, F1B) (Hebbeln et al., 1995, 2001) and an 8-m sediment core (GeoB3313-3), which suggests continuous sedimentation at the site at rates of ~100 cm/k.y. within Holocene time (Lamy et al., 2001). During our approach to Site 1233, acquisition of 3.5-kHz seismic reflection data revealed well-stratified reflective layers. Based on these new data, drilling was approved to 110 mbsf (Fig. F1C). The intent of drilling here was to recover a very high resolution record of late Quaternary sediments. Although lower on the slope thick turbidites bury the Peru-Chile Trench as a bathymetric feature, most turbidity currents appear to be channeled away from this shallow basin.

The large-scale oceanic circulation off southern Chile includes the southward-flowing Cape Horn Current, a part of the subpolar system, and the northward-flowing Peru-Chile Current, which forms the eastern boundary of the subtropical gyre of the South Pacific (Strub et al., 1998) (Fig. F2A). The transition zone between these systems is currently between 40° and 45°S, coincident with the northernmost reaches of the southern westerlies. Thus, surface-water proxies at Site 1233 will be sensitive to the latitudinal position of the westerly winds.

North of Site 1233, Mediterranean (dry summer) climates prevail on the continent. To the south, heavy year-round precipitation results in alpine glaciation and rapid erosion and transport of sediment from the continent to the ocean. Interannual rainfall in this region of Chile is thought to reflect the influence of El Niņo Southern Oscillation (ENSO) events emanating from the tropics (Dettinger et al., 2001). Thus, we expect variations in terrigenous sediment composition and mineralogy at Site 1233 to reflect changes in the latitudinal position of the westerly winds through time as well as possible linkages to climate changes in the tropical Pacific. Heavy rainfall and runoff in the south results in seasonal transport of relatively low salinity surface waters, known as Chilean Fjord Waters, to the north (Fig. F2). The position of Site 1233 offers an excellent opportunity to monitor sea-surface salinity anomalies caused by variations in river runoff or by melting pulses associated with episodes of net glacier retreat.

Subsurface currents offshore include the southward-flowing Gunther Undercurrent, which can be detected as a relatively low oxygen and high salinity water mass near the shelf edge, at depths of 100-300 m (Fonseca, 1989). This current transports waters of the oxygen minimum zone, which is strongly expressed in northern Chile and Peru, as far as southern Chile. Between ~400 and 1000 m, Antarctic Intermediate Water (AAIW) enters the Pacific from the south. This water mass is relatively high in oxygen and low in salinity. We expect that benthic proxies at Site 1233 will reflect temporal changes in the intensity or character of Antarctic Intermediate Water (Fig. F3).

Primary goals for this site were to

  1. Provide a high sedimentation rate site off southern Chile for study of millennial-scale (and perhaps even century scale) changes of climate related to the latitudinal position of the westerly winds. For example, geochemical and isotopic tracers of paleosalinity in near-surface waters will provide information on rainfall, runoff, and meltwater pulses; terrigenous sediments will provide information on source areas and intensity of erosion on the continent and geochemical and isotopic tracers of changes in near-surface paleotemperatures and nutrients will register changes in coastal upwelling in the transition zone between the subpolar and subtropical climate systems.
  2. Provide suitable materials to monitor changes in AAIW. For example, species of benthic foraminifers and their chemical and isotopic composition may help to characterize changes in the nutrient, temperature, and, perhaps, salinity of this water mass as it enters the Pacific.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 202IR-104