PROPOSED SITESThe primary sites for Leg 188 are (Table 1; Figs. 13, 14)
The Mesozoic and Paleogene sediments beneath outer Prydz Bay form a series of seaward dipping sequences (Figs. 6, 14). Cooper et al. (1991a) recognized sequence PS.2A as Cenozoic glacial sediments and the underlying Sequence PS.2B as preglacial, mostly nonmarine Mesozoic sediment. Site 742 reached middle Eocene-lower Oligocene glacial sediments in Sequence PS.2A without reaching the base of the sequence or nonglacial facies (Hambrey et al., 1991). Hambrey et al. (1991) suggest that the base of the glacial interval was close because they interpreted preglacial alluvial sediment mixed into the lowermost unit in the hole by subglacial deformation.
Proposed Site PBS-2A was chosen to recover core from the Cenozoic sediments below the horizon reached in Site 742 into the top of Sequence PS.2B, the uppermost Mesozoic sediments identified at Site 741. This site should provide an age for the arrival of glaciers in Prydz Bay, a record of changes in depositional environments with the onset of glaciation, and an indication of changes in biota. Site PBS-2A is the primary site because of its thin Quaternary diamict section and a slightly thicker Paleogene section than at Site 742 and should be a more complete record. Site PBS-2A is expected to encounter a thin Quaternary section of diamict and mud overlying an interval of upper Eocene-lower Oligocene stratified diamictite with sandstone and mudstone interbeds similar to sediments in Site 742 below 173 mbsf. Below the interval equivalent to the lower part of Site 742, the lithologies are unknown, but the presence of reworked dinoflagelates and pebbles of ferruginous marine marl, both of Eocene age, at ODP Sites 739 and 742 (Jenkins and Alibert, 1991; Truswell, 1991) suggests a section containing shallow marine sediments. This site should reach Cretaceous nonmarine sediments.
Drilling beneath the Antarctic Peninsula continental rise during Leg 178 showed that the drifts are likely to provide fine-grained equivalents to continental slope and rise sediments. They should also have microfossils for age estimates and interpretation of paleoceanography. Site PBD-12B was selected to acquire a reasonably complete section of the drift record that includes Oligocene and younger sediments. The site is located on the western flank of the Wild Drift (Fig. 14) where Surface P3, which is thought to represent the arrival of grounded ice at the shelf edge during the late Oligocene to early Miocene?, can be reached at about 1000 mbsf. It will also intersect Surface PP.15 that marks the major late Miocene?-late Pliocene? change in glaciation on the shelf and the base of the Prydz Channel Fan. The sediments expected at Site PBD-12B are fine grained hemipelagic clays and distal turbidites with a higher biogenic component in intervals deposited in interglacial periods compared to glacial intervals. Ice-rafted detritus should also be present.
The slope site is aimed at the clinoforms of the Prydz Channel Trough Mouth Fan (Fig. 14). Construction of the fan started in the late Miocene to middle Pliocene when the Lambert Glacier formed a fast-flowing ice stream on the western side of Prydz Bay. The fan grew most during episodes when the Lambert Glacier grounded at the shelf edge, delivering basal debris to the fan apex. This material was then redistributed by sediment gravity flows and meltwater plumes. Between such ice advances, the fan surface has received hemipelagic sediment. Thus, the alternation of facies recovered from the fan should reflect the number of times the East Antarctic Ice sheet has expanded to the shelf edge since late Miocene time. The age of inception of the trough mouth fan will also indicate the time of a major change in glaciation style, probably related to a major increase in the importance of coastal ice masses caused by cooling.
Gravity cores from the fan surface indicate that the section should consist of mud, turbidites, and some debris flows deposited during glacial advances separated by finer, more biogenic intervals. Microfossils present in gravity cores are diatoms and planktonic foraminifers. The optimum position for drilling is in the mid fan, where mapped sequences are all present but not excessively thick. Proposed Site PBF-6A is the primary site in this area.
The development of climatic models and the prediction of future short-term climate changes requires detailed records of past climates to determine what processes operated in the oceans and atmosphere and to document the natural variability of global climate for the Holocene. Ice cores have been the only source of such detailed information. Recent ODP drilling in Saanich Inlet, the Santa Barbara Basin, Cariaco Basin, and Palmer Deep have yielded sedimentary sections with annual resolution comparable to ice cores. Other sites around Antarctica have the potential to provide similar sections. Having more than one sedimentary section from Antarctica would provide the opportunity to study decadal and interannual variability of such climatic features as the Circumpolar Wave and El Niño-Southern Oscillation. Deep valleys on the Mac Robertson Shelf west of Prydz Bay (Fig. 11) contain thick Holocene ooze deposits that probably have such resolution (Fig. 12). Therefore, if time permits, we will try to recover a high-resolution Holocene environmental record by coring biogenic deposits on the outer Mac Robertson Shelf (proposed Site PBS-3A).
The Holocene sequence in Iceberg Alley north of Mawson Station (Site PBS-3A) consists of a flat lying fill within a U-shaped valley cut in older sediments and metamorphic basement (Harris and O'Brien, 1996). Holocene sediments have been imaged by 3.5-kHz echo sounder and intermediate resolution seismic and sampled by gravity coring (Fig. 12). The gravity cores recovered laminate biosiliceous ooze showing laminations, some of which represent monospecific algal or resting spore concentrations. Taylor (1999) interpreted down-core changes in diatom floras as reflecting decadal-scale changes in productivity and water column characteristics during the Holocene. Seismic data indicate that the ooze is about 50 m thick along the valley axis so that cores extending through the full thickness of the deposit should have close to annual resolution. Site PBS-3A is the primary contingency site, but there are a number of locations along the valley axis that contain similar sediments (Fig. 12).
To 188 Drilling Strategy
To 188 Table of Contents