3. Site 12321

Shipboard Scientific Party2

INTRODUCTION

Site 1232 (proposed Site SEPAC-9A) is located at 39°53.45´, 75°54.08´W in the Chile Basin between the Mocha and Valdivia Fracture Zones, ~50 km west of the Chile Trench (Fig. F1). The water depth of 4072 m is ideal to monitor the influx of Circumpolar Deep Water into the Chile Basin. The crustal age is not well defined but was expected to be between 19 and 29 Ma as derived from magnetic lineations in the region.

The tectonic backtrack path of Site 1232 on the Nazca plate is approximately parallel to its latitudinal position and to surface-ocean gradients such as sea-surface temperatures in the subpolar transition zone. Thus, Site 1232 should have been situated close to the northernmost reaches of the westerly winds and the Antarctic Circumpolar Current during the Neogene. Today, Site 1232 lies ~300 m below the lysocline depth.

The total thickness of the sedimentary section was estimated at ~470 m based on site survey seismic profiles (80-in3 water gun at 30-400 Hz and 3.5-kHz shallow profiler) (Mix et al., 1997, 1998). The seismic data reveal well-stratified, flat-laying, moderately reflective layers (Fig. F2). Basement appears as a highly reflective surface that outcrops or is thinly covered with sediment on abyssal hills to the north and south of the basin. North of Site 1232, a Parasound profile indicated turbidite deposits in this terrain, as the upper 30 m of seismic penetration were marked by diffuse reflection. A 10-m-long piston core from the location of Site 1232 (RR9702A-26PC; Oregon State University) taken during the site survey cruise recovered hemipelagic sediments containing only a few centimeter-scale turbidite deposits.

The primary objectives at Site 1232 were to

  1. Examine the long-term Neogene response of the southeast Pacific to major tectonic and climatic events, such as the opening of the Drake Passage (creating the Antarctic Circumpolar Current), the closing of the Isthmus of Panama (terminating the Atlantic-Pacific exchange of water masses), the uplift of the Andes (modifying the atmospheric circulation pattern) and the glaciation of the Antarctic;
  2. Assess short-term variations in the character and expansion of Antarctic Circumpolar Deep Water on orbital timescales (Fig. F3); and
  3. Monitor variations in the southernmost reaches of the northward-flowing Peru-Chile Current, based on planktonic fauna and flora, allowing comparisons to other sites of the latitudinal transect (Fig. F1).

Site 1232 revealed a dramatically different sequence than expected. The sediment record consists of an expanded Pleistocene section (<0.8 Ma) of silty clay (inferred turbidites and hemipelagic sediments) interbedded with thin beds of sandy silt (basal parts of distal turbidites), considerably limiting the paleoceanographic opportunity to fully achieve the outlined objectives.

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-103

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