Survey and Drilling Strategies | Table of Contents


Table 1 and Table 2 list approved primary and alternate sites, respectively.

Chile Margin (Primary Sites SEPAC-13B, SEPAC-14A; Alternate Site SEPAC-19A)
These sites form a depth transect to monitor AAIW (with higher oxygen content than overlying waters of the GU and deeper PCW). Sedimentation rates are expected to be high (>20 cm/k.y.) and, thus, these sites will provide a high-resolution record of climate change off central Chile. Sites SEPAC-13A and SEPAC-14A are within a highly productive coastal upwelling area near Concepcion. To the south, Site SEPAC-19A lies in the transition zone between the subtropical and subpolar gyres and thus may help to monitor latitudinal translations of the northern boundary of the westerly wind belt on millennial scales.

Chile Basin (Primary Site SEPAC-9A; Alternate Site SEPAC-10A)
The drill sites in the Chile Basin appear to be on relatively old oceanic crust (Cande and Haxby, 1991) and will provide records as old as 50 Ma (Table 1, Table 2), suitable for testing the subtropical oceanographic effects of opening Drake Passage, as well as for reconstruction of younger events. Although sedimentation rates are probably low (~1-2 cm/k.y.) and carbonate preservation poor, these sites provide an important location to monitor CPDW, if benthic foraminifers are consistently present.

Chile Rise (Alternate Site SEPAC-5A)
The Chile Rise provides a good location to monitor the depth and chemistry of CPDW at relatively low latitudes. At its shallowest, this active spreading ridge is young and sediment cover is thin. Previous ODP drilling on the eastern Chile Rise area during Leg 141 concentrated on tectonic objectives in the zone of deformation where the Chile Rise interacts with the subduction zone. Excellent information on regional magnetic anomalies and tectonics that led to Leg 141 (Herron et al., 1981; Cande and Leslie, 1986; Cande et al., 1987), however, provide good control on basement ages of the sites proposed here. Alternate Site SEPAC-5A is south of the Chile Rise axis on the Antarctic plate. Because the crust here spreads westward relative to the eastward propagating ridge axis, this site will maintain a relatively steady geographic position near the South American margin throughout its history. Site SEPAC-5A is considered to be an alternate site for Leg 202 because of the significant transit time to the site, the relatively young crust that limits the length of record, and because the cores taken during the site survey cruise are barren of microfossils in many intervals.

Nazca Ridge (Primary Sites NAZCA-10A, 17A; Alternate Sites NAZCA-14A, 16A)
The Nazca Ridge provides sites to monitor a latitude band where the near-surface cool tongue of the Peru-Chile Current is well developed. This ancient aseismic ridge, a fossil hotspot track, is sediment covered to its shallowest reaches. Basement ages are estimated based on the 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 and adjoining plate may only be a few million years. If so, the drill sites on the Nazca Ridge have the potential of providing pelagic records spanning the entire Neogene and possibly as old as the late Oligocene time. Site NAZCA-10 will help to monitor the boundary between PCW and AAIW.

Cocos and Carnegie Ridges (Primary Sites CAR-1C, CAR-2C, PAN-2A; Alternate Sites COC-2A, COC-3A, COC-4A)
In the regions proposed for coring, the Cocos and Carnegie Ridges are fossil traces of volcanism at the Galapagos hotspot split by seafloor spreading at the Galapagos Rift. Shallow sites of the ridge crests appear to be drowned volcanic seamount tops. Both ridges are heavily sedimented. Sediments here are typically comprised of nannofossil-foraminifer ooze, with diatoms and radiolarians. In the site documentation, basement ages are based on the tectonic reconstructions of Hey et al., (1977) and Lonsdale and Klitgord (1978) extrapolated from regional magnetic anomalies in normal oceanic crust. The age of basement sampled at DSDP Site 158, cored on the north flank of Cocos Ridge, is ~12 to 14 Ma. The ridges, although basically aseismic, retain some seismicity associated with interaction of the ridges with subduction zones off Ecuador (Carnegie Ridge) and Costa Rica (Cocos Ridge). To the south of Cocos Ridge and west of Carnegie Ridge, the Galapagos Islands are volcanically active. Near the equator, the Cocos and Carnegie Ridges, which form the boundaries of the Panama Basin, provide an excellent opportunity for a depth transect from <1000 to ~4000 m. Water chemistry at the seafloor here reflects the southward flow of high-nutrient, low oxygen North Pacific water, but may also respond to high equatorial and eastern boundary productivity. Sites CAR-1C and CAR-2C will monitor intensity of the equatorial Pacific cold tongue at its eastern boundary over the last ~15 m.y. Site PAN-2A will provide a shorter record, perhaps 5 m.y., but at relatively high sedimentation rates (10-15 cm/k.y.) suitable for analysis of millennial-scale climate changes. Sites on the Cocos Rise (Alternate Sites COC-2A, 3A, and 4A) underlie the low-salinity surface ocean layer west of the Panama Isthmus and thus help monitor long-term changes in cross-isthmus transport of fresh water vapor across the Panama Isthmus.

Survey and Drilling Strategies | Table of Contents