7. Palmer Deep (Sites 1098 and 1099)¹

Shipboard Scientific Party²

BACKGROUND AND SCIENTIFIC OBJECTIVES

Sites 1098 and 1099 are located in Palmer Deep (Fig. F1; Leventer et al., 1996), a glacially overdeepened basin on the inner shelf of the Pacific margin of the Antarctic Peninsula, southwest of Anvers Island.

Site 1098 lies at a water depth of 1012 m in Basin I (Kirby, 1993), the shallowest of the three ponded sub-basins aligned in an approximately southwest-northeast direction that comprise Palmer Deep (Fig. F2). The basin floor is ~4 km long and 1 km wide and is surrounded by steep slopes (16º to 26º). High-resolution acoustic surveys of the basins suggested that a sediment fill ~50 m thick rests on a highly reflective, probably hard-rock basement (Rebesco et al., 1998; Fig. F3; also see "Appendix," and Fig. AF1, both in the "Leg 178 Summary" chapter). Piston cores collected from this basin (Leventer et al., 1996) indicate that laminated sediments composed of alternations of biogenic siliceous and hemipelagic muds have accumulated at extremely high rates (260-390 cm/k.y.). This means that an ultrahigh-resolution Holocene sedimentary record is preserved, possibly extending back to the Last Glacial Maximum (LGM).

Site 1098 is located at the southwest end of Basin I, where the sediment fill is draped, contrasting with the overall ponded geometry of the rest of the basin. In this location, Core PD92-30 (Leventer et al., 1996) indicates only minor turbiditic deposition in the upper 9 m and a linear trend of radiocarbon ages.

Site 1099 lies in 1400 m of water in Basin III, the largest and deepest of the three sub-basins comprising Palmer Deep (Fig. F2). Basin III is more than 6 km long, 3-4 km wide, and elongated markedly northeast-southwest. It has a thicker sediment fill than Basin I; in the center of the basin, it reaches 260 m (310 ms two-way traveltime [TWT] [Figs. F4, F5; also see "Appendix," and Fig. AF1, both in the "Leg 178 Summary" chapter]). Normal faulting is suggested by stratigraphic growth in the sediment fill on the northwest side of the basin, implying that overdeepening of the basin may be related not only to glacial erosion but also to neotectonic subsidence. Of the three major acoustic units present in Basin III, the uppermost (97 ms TWT, equivalent to ~70 m) shows character similar to the fill of Basin I: low reflectivity at the seafloor and acoustic transparency. In Basin III, a continuous reflector (mid-basin reflector [MBR]) within the uppermost transparent unit was considered to be a mass-flow deposit. The two underlying units are highly reflective, with subparallel reflectors showing a moderately divergent pattern suggestive of turbiditic deposition from the east or northeast.

Sedimentation in Basin III was recognized as more affected by mass flow than in Basin I. Additional stratigraphic data derived from piston cores, including radiocarbon dating from the upper unit in Basin III (Kirby et al., 1998), suggest that the base of the unit, as well as the deeper units, may predate the most recent glaciation on the shelf (LGM). The seismic units below the MBR were therefore considered to be older than the basin-fill unit of Basin I.

Because Basin III is sufficiently wide, Site 1099, selected originally at the crossing of two site-survey seismic profiles ~275 m from Core PD92-63 in the center of the basin floor, could be occupied using predetermined Global Positioning System (GPS) coordinates without the aid of an additional survey (Fig. F6).

The aim of drilling in Palmer Deep (Sites 1098 and 1099) was to reconstruct the most recent glacial history of the Antarctic Peninsula, through recovery of a Holocene, and possibly latest Pleistocene, paleoproductivity record representative of regional climate at a decadal and millennial scale. Such a record might be compared with those of low-latitude regions and with that recorded in ice cores. In addition, Site 1099 would provide a potential test of the hypothesis that Palmer Deep was a subglacial lake during the most recent episode of ice grounding on the Antarctic Peninsula continental shelf.