The overdeepened Antarctic inner continental shelves, with water depths often exceeding 1000 m, create deep repositories that efficiently trap the products of near-shore productivity during interglacials (Domack et al., 1991b; Leventer et al., 1996). In contrast to outward-sloping, low- to middle-latitude continental shelves, the sedimentary record is preserved because biogenic particles are not easily removed. Periodically, the advance of a grounded ice sheet across the continental shelf removes by basal erosion all or most soft sediments deposited during the previous interglacial. The record in the inner-shelf basins, therefore, is believed to be no older than the LGM. Palmer Deep, south of Anvers Island, is one such basin.
The following sites aimed at ultrahigh-resolution late Quaternary sedimentary records of paleoproductivity have been successfully drilled by the Ocean Drilling Program in areas where local geological conditions (tectonically overdeepened marginal basins with suboxic or anoxic conditions, or northern mid-latitude anoxic fjords) have allowed their preservation:
Palmer Deep is the first such site in the Southern Hemisphere high latitudes, in an area directly influenced by ice-cover fluctuations and widely recognized as highly sensitive to climatic change, at least in historical time (Doake and Vaughan, 1991; Fraser et al., 1992; Jacobs and Comiso, 1993; Ridley, 1993; Skvarca, 1993; Smith, 1994; Domack et al., 1995). Palmer Deep, just south of Anvers Island (Fig. F5), is probably the best known of the overdeepened inner-shelf basins on the Antarctic margin: the site is a rimmed seafloor depression with maximum water depth of 1400 m, in which lie three flat-floored sub-basins (Kirby et al., in press). It has been intensively investigated with high-resolution deep-tow and seismic reflection techniques (Kirby et al., in press; Rebesco et al., 1998) and with piston coring (Kirby et al., in press; Leventer et al., 1996). Oceanographic measurements and sediment trap deployments are under way (E. Domack, pers. comm., 1997).
Sediments in Palmer Deep are alternations of laminated siliceous ooze and hemipelagic mud, with minor locally derived turbidites. Analy-ses of piston cores have outlined downcore variations of magnetic susceptibility, grain size, organic carbon, bulk density, and diatom and foraminiferal assemblages and abundances. These demonstrate short-term (~200 yr) and longer term (~2500 yr) cycles, most probably related to global climatic fluctuations (Leventer et al., 1996; Fig. F16), at least down to 3700 yr, according to radiocarbon dating. The 200-yr cycles are thought to reflect alternations of periods of high productivity, driven by higher atmospheric and seawater temperature and greater flux of meltwater from the ice fronts; and those of lower productivity, resulting from colder temperatures, more storms, and better mixing in the water column. These 200-yr cycles, possibly driven by variations in solar activity, have been identified in other terrestrial records such as tree rings (Wigley, 1988; Scuderi, 1993), lake sediments (Anderson, 1992), and ice-core records (Raisbeck et al., 1990). They appear to have global significance.
For the Antarctic, the possibility of correlating the ice and the marine sediment records at a decadal and centennial scale is important because it would provide a link between paleo-atmospheric conditions, directly recorded in the ice cores; and paleoceanographic conditions, directly recorded in the sediment cores. Both will contribute to the reconstruction of a global paleoclimate record since the LGM.
The strategy of drilling in Palmer Deep was developed in two stages. First, from the deep-tow-boomer and piston-core records, it was known that Basin I, the smallest and shallowest of the three Palmer Deep sub-basins, contained 40-50 m of sediment, possibly turbidite-free. From downward extrapolation of the sedimentation rate (0.39 cm/yr) obtained by radiocarbon dating of the upper 10 m, this section might extend the sedimentary record back to about 13ka at the basin base. Basins II and III, the deeper and larger basins (1400 m water depth), were thought to contain a similar sediment thickness but to include a higher proportion of turbidites. Sites in these basins were proposed as alternates to Basin I in case of adverse ice conditions.
Second, an air gun survey of the area (February 1997), requested by the ODP Site-Survey Panel to improve seismic cover around the sites, revealed that Basins II and III contained a greater sediment thickness, reaching about 270 m in the center of Basin III. Estimates of the age of the deeper sediments in Basin III, aided by seismic stratigraphic correlation, suggested that, despite the higher sedimentation rate (caused by turbidites), the lower sediments in Basin III could be older than in Basin I, thus possibly extending the record to before the LGM. An addendum to the original proposal for drilling Basin I was then written (August 1997) to propose Basin III as an additional target, although the higher priority was retained for Basin I drilling.
Drilling in Palmer Deep as a site of opportunity has been supported by the newly formed ANTIME in the Antarctic program, which aims to reconstruct the glacial cycle of Antarctic history as part of the SCAR contribution to the IMAGES, part of the IGBP. The drilling also fits the objectives of the (mainly U.S.) West Antarctic Ice Sheet program and is located in the only marine area of the Long-Term Ecological Research program.