INTRODUCTIONThe Antarctic Ice Sheet and the ocean surrounding it are key components in the global climate regime from the early Cenozoic to the present. The steep climatic gradient generates the vigorous atmospheric and oceanic circulation of the Southern Hemisphere, and sea ice formation ventilates and provides nutrients to much of the world's oceans. Deep water mass formation around Antarctica is also a major sink for atmospheric carbon dioxide. The ice sheet is also the most likely governor of rapid eustatic sea-level change (Barrett, 1999). As part of the effort to understand the mechanics of Antarctic climate and its likely response to change, the Scientific Committee for Antarctic Research (SCAR) Working Groups on Geology and Solid Earth Geophysics have supported the Antarctic Offshore Stratigraphy Project (ANTOSTRAT), which has developed a series of Ocean Drilling Program (ODP) proposals for drilling on the Antarctic margin.
Because of its long history of sedimentation, Prydz Bay was seen as a place to investigate the long-term record of Antarctic glaciation. Also, the size of the Lambert Glacier-Amery Ice Shelf system that enters the bay in relation to the total drainage from East Antarctica (~20%) makes it a potential indicator of the state of the East Antarctic interior. It complements other ANTOSTRAT-identified regions that hold records of fast-responding coastal ice centers (Antarctic Peninsula), the West Antarctic Ice Sheet (Ross Sea), and coastal East Antarctica (Wilkes Land) (Barker et al., 1998). Leg 188 was designed to build on the results of Leg 119 by investigating the history of the Lambert-Amery system during key periods of the Cenozoic. In particular, the proposal aimed at dating the earliest arrival of glacier ice on the shelf, documenting the number and timing of late Neogene expansions of the ice to the shelf edge, and extracting a climate record from the interbedding of continent-derived siliciclastics and marine biogenic sediments on the continental rise.
Investigations of Antarctic Cenozoic Climate History
Antarctic climate history has largely been deduced from oceanic proxy records, from fragmentary onshore outcrops and from several drilling campaigns around the margin. Modeling of ice sheet development has also been used to extend this fragmentary record and to provide a guide for further work, but it needs continual calibration with the geological record.
The main proxy records from marine sediments that have been used so far are eustatic sea level curves, Oxygen isotope (18O) records and ice-rafted debris (IRD) records. Sea-level curves are probably the most controversial; some workers claim a record of ice expansion extending back to the Mesozoic (Miller et al., 1999), whereas others doubt the validity of global correlation based on eustacy because of regional tectonic influences and a lack of precision in correlation (e.g., Miall, 1986). Short-term global eustatic changes during the Pleistocene are generally recognized as driven by ice volume changes, but the role of the Antarctic Ice Sheets even for the Pleistocene is not clear. The initial assumption that Antarctica ice extended to the shelf edge during the last glacial maximum (LGM) (Denton and Hughes, 1981) used in modeling postglacial sea-level rise has been shown not to be accurate in the Ross Sea (Licht et al., 1996), some coastal oases (Goodwin, 1992), and Prydz Bay (Domack et al., 1998).
Ice-rafted debris indicates the presence of floating debris-charged icebergs at a particular site, but its interpretation in terms of climate and glacial history is extremely complex because of the many poorly understood factors influencing the discharge and dispersal of debris-rich icebergs. IRD records can set a minimum age for the arrival of calving glaciers at the coast and the increase of icebergs associated with climatic cooling.
Oxygen isotope variations are the only measurement of global ice volume that is relatively continuous for the Cenozoic, with curves published and interpreted back to the Paleocene (e.g., Abreu and Anderson, 1998; Flower, 1999). Some researchers have used isotope curves to argue for the onset of glaciation in Antarctica as early as the Paleocene (Denton et al., 1991), whereas others argue for glacial onset in the early to middle Eocene (Abreu and Anderson, 1998). Major ice expansions have been inferred at 33.6 Ma, (Eocene/Oligocene boundary), 23.7 Ma (Oligocene Miocene), 12 to 16 Ma (middle Miocene), and 2.7 Ma (late Pliocene) that marked the onset of Northern Hemisphere glaciation (Flower, 1999). For all the value of 18O curves, they are a function of both temperature and global ice volumes, making for ambiguity in the detailed interpretation of the record (Wise et al., 1992; Barker et al., 1999). The ability of isotope curves to resolve short-term changes is strongly dependent on finding sedimentary sections with sufficient resolution to avoid aliasing the signal and so becomes more difficult with increasing age of the section. Also, isotope curves are a measure of global ice volume and cannot provide information on the distribution of ice between the continents during periods of bipolar glaciation. Neither can they indicate the distribution and interplay of ice and the ocean around different parts of Antarctica, information necessary for calibrating models of glaciation, or details of the Antarctic environment.
Deposits and landforms on the Antarctic continent provide a direct window into Antarctic climate history that can be very detailed (e.g., Quilty, 1991) but, by their nature, are fragmentary and difficult to correlate. Difficulties in the correlation of sediments in the Transantarctic Mountains have led to major disagreements over the extent of ice retreat during the Pliocene (Webb et al., 1984; Denton et al., 1991; Warnke et al., 1996). However, outcrops and landforms also indicate the possible complexities of responses by the ice to climate change with out-of-phase expansion of valley glaciers and the ice sheet first reported by Scott (1905).
Drilling of continental shelf sediments can provide a record of ice expansion onto the shelf and the evolution of shelf environments but must be considered in the light of both (1) depositional models for the facies present and (2) sequence stratigraphic models of facies stacking patterns and preservation (Fielding et al., 1998). Such concepts need to be applied to drilling of sections on the continental shelf and slope because of the strongly reciprocal nature of glacial-interglacial sedimentation on continental margins (Boulton, 1990). During major ice advances, the shelf tends to be eroded followed by the deposition of compact till, a difficult stratigraphic record to sample and interpret (Barron et al., 1991). At the same time, significant deposits are forming on the upper slope in trough mouth fans (Vorren and Laberg, 1997) or sediment transfers to the continental rise as turbidite deposits or contourite drifts (Rebesco et al., 1997).
A more complete picture of glacial history can be obtained by linking onshore outcrop data to the deep sea record through drilling key locations on the Antarctic shelf, slope and rise. Leg 188 was designed to provide such a transect, building on the work of Leg 119 (Barron, Larsen, et al., 1991) and linking studies of Cenozoic glacial sediments in the Prince Charles Mountains (Hambrey and McKelvey, in press) and around Prydz Bay (Quilty, 1991) to the oceanic record. Linking these data sources has the potential to provide a transect extending ~1000 km from the interior of East Antarctica to the continental rise along a single ice drainage system.
Aims of the Leg
Onset of Glaciation
Prydz Bay is at the downstream end of a drainage system that rises in the Gamburtsev Mountains in central East Antarctica (Fig. 1). Modeling studies of ice sheet development indicate that these mountains, if present in the Paleogene, would be the first area to develop extensive ice cover. Thus, Prydz Bay could contain the first sedimentary evidence of ice in East Antarctica. Evidence on the early development of the Antarctic Ice Sheet has been interpreted as indicating initiation of ice sheet growth from as early as the early middle Eocene to the early Oligocene (Abreu and Anderson, 1998; Barron et al., 1991). As yet, there has been no section drilled in the Antarctic that clearly spans the transition period from preglacial to glacial conditions. ODP Site 742 in eastern Prydz Bay reached glacial deposits interpreted by Barron et al. (1991) as late middle Eocene in age. Cooper et al. (1991) estimated that these sediments continued for 100 m below the total depth of Site 742, resting on an erosion surface on older sediments. A hole that drills this lower 100 m as well as the underlying sediments could establish the lower age limit for large-scale glaciation of Prydz Bay. Any preglacial Cenozoic sediments would contain paleontological and sedimentological evidence of the preglacial environment.
Oxygen isotope records have been used to infer episodes of increased ice volume at 33.6 Ma, (Eocene/Oligocene boundary), 23.7 Ma (Oligocene/Miocene boundary), 12 to 16 Ma (middle Miocene) and 2.7 Ma (late Pliocene) that marked the onset of Northern Hemisphere glaciation (Flower, 1999). In Prydz Bay, an unconformity identified during Leg 119 (Solheim et al., 1991) may have formed in the late Miocene by a major ice expansion (Barron et al., 1991). Kuvaas and Leitchenkov (1992) studied the seismic facies of the Prydz Bay continental rise and slope and identified the initiation of drift sedimentation, which they suggested could have been related to initiation of the Antarctic Circumpolar Current after the opening of Drake Passage around the Oligocene/Miocene boundary or may relate to a major ice expansion during the Oligocene or Miocene. A hole drilled on the continental rise to intersect the base of the thick drift section could date the onset of drift formation and potentially record changes in rise sedimentation controlled by currents and continental sediment supply. The drift sediments should also contain a record of climate fluctuations reflected in the interplay of siliciclastic fed by the Lambert Glacier drainage system and biogenic sediment.
Late Neogene Fluctuations
A major change in Prydz Bay shelf progradation took place in the late Miocene to mid-Pliocene when a fast-flowing ice stream developed and excavated a channel across the shelf on the western side of Prydz Bay (Harris and O'Brien, 1996). Basal debris carried to the shelf edge was then deposited in a trough mouth fan on the upper slope (Fig. 2). This trough mouth fan probably contains a reasonably complete record of glacial history because it received siliciclastic sediment when the shelf eroded during major ice advances and hemipelagic material during interglacials and smaller glaciations (Vorren and Laberg, 1997). A history of major ice advances for the Antarctic is only just developing and, at present, relies on outcrop studies that have poor time control (Denton et al., 1991) and on seismic stratigraphic studies in which erosion surfaces are identified but the associated sediments have been sparsely sampled (e.g., Alonso et al., 1992). Identification of LGM grounding lines well in from the shelf edge in the Ross Sea (Licht et al., 1996) and Prydz Bay (Domack et al., 1998) means that not every glacial episode sees the full advance of the ice. A section drilled through the Prydz Channel Fan will record the episodes that did produce a major advance and so give insight into the mechanisms of ice sheet growth through multiple glacial cycles.
Regional Setting of Prydz Bay | Table of Contents