CONCLUSIONS

Leg 188 drilling (O'Brien, Cooper, Richter, et al., 2001) has added much proximal evidence to that collected during prior Leg 119 drilling in Prydz Bay (Barron, Larsen, et al., 1989, 1991). These drilling legs have established that changes in ice volume/extent affected sediment erosion and redistribution, which resulted in cyclic patterns observed in the drilled Cenozoic sections. These effects are more prominent since the early Miocene than before. Leg 188 drilling in Prydz Bay has augmented the initial discoveries of Leg 119 to identify and/or better define key paleoenvironmental transitions in the region since the late Mesozoic (Fig. F15). The transitions outline a general climate cooling since the late Eocene, increasing ice cover to today's polar setting, a trend that is consistent with that illustrated by global oxygen isotopic curves showing cooling of the world's oceans (e.g., Zachos et al., 2001). In detail, the transitions exhibit systematic variability over times of millions to tens of thousands of years, suggesting also that regional- to local-scale processes have been active. Leg 188 drilling provides the first complete transect of drill sites across the Antarctic margin including shelf, slope, and rise sections, to yield new detailed data on these depositional paleoenvironments.

Key transitions in the region deciphered from Leg 188 drilling studies include the following:

The Prydz Bay shelf evolved from a largely low-lying pre-Oligocene alluvial to deltaic system to a shallow-marine setting until about the middle Miocene, when shelf progradation to aggradation coeval with erosional shelf overdeepening commenced.
Vegetation onshore changed from temperate conifer woodland to cool Nothofagus scrub by the late Eocene with Nothofagus possibly lasting into the early Miocene before the loss of vegetation in the late Neogene.
The first evidence for ice near the shelf is in the late Eocene, with grounded ice there in the early Oligocene, some floating ice in the early Miocene, and ice buildup in the middle Miocene, based on glacial sediments and IRD from the continental shelf and rise. A change from distributed to focused ice flow on the shelf occurred in the late Miocene to early Pliocene with the carving of Prydz Trough and others. No cores exist in Prydz Bay for the early Oligocene to early Miocene to decipher ice volumes for this period.
On the continental rise and slope, pre-Miocene canyons and channel/levy systems were enhanced by large sediment drifts initiated in the late Oligocene to early Miocene, and canyons were covered by thick slope-fan deposits starting in the middle to late Miocene. Sedimentation rates on the rise decreased gradually by ~10-fold from the early Miocene to Quaternary, with the most rapid decline since 14–16 Ma (middle Miocene) during a period when IRD increased.
On the slope, abrupt shifts in lonestone compositions, clay contents, and other core properties as well as shifts in regional depositional patterns heralded changes in ice/sediment source areas in Pleistocene times.
Leg 188 Site 1165 shows evidence of relatively rapid fluctuations between times of principally biogenic and principally terrigenous sedimentation with a glacial component, and in some cores with adequate age control the fluctuations occur at or near Milankovitch periodicities.

The combination of seismic reflection, drilling, and onshore geologic records from Prydz Bay continental margin illustrate that the Cenozoic morphologic and paleoenvironmental histories were influenced by sedimentologic and oceanographic processes such as sea level changes and others common to low-latitude nonpolar margins in preglacial and earliest-glacial times (i.e., pre-early Oligocene) and by those such as subglacial erosion common to high-latitude polar margins in full-glacial periods (i.e., middle to late Miocene and younger).

The intermediate history is mostly unknown for lack of cores, but where known in the early to middle Miocene at Site 1165 and onshore back into Oligocene time in the Prince Charles Mountains, the record is one of transitions with both long-term components (e.g., shift in depocenters, reorganization of ocean currents, and increase in onshore ice) and short-period ones (e.g., ice volume and subglacial water fluctuations and biogenic productivity variations). Perhaps during the early to middle Miocene, the margin of Prydz Bay was similar to that of east Greenland today, as suggested also by Hambrey and McKelvey (2000a, 2000b). Regardless, large volumes of glacially influenced sediment were eroded from Prydz Bay onshore and shelf regions and redistributed down canyon systems into deepwater areas during this period. These post-early Miocene transitions point to increasing ice, decreasing onshore erosion, relative increases in biogenic productivity, and increasing erosion of the shelf, leading to today's polar environment.

A major feature of the Prydz Bay record that emerges from the new drilling and other studies is the effect of the progressive long-term cooling trend on the geological record. The record on the continental shelf suggests the first signs of Cenozoic glaciation were stunted vegetation and glacially abraded sand grains, followed by ice-rafted clasts in the marine realm. Glaciomarine deposition and erosion and subglacial deposition then followed. Early Miocene glaciation produced huge quantities of fine sediments that bypassed the shelf and were deposited in drifts on the rise. We suggest that this was because of abundant meltwater outflow from the early Miocene ice sheets. Progressive cooling during the middle Miocene (14 Ma) (Florindo et al., 2003a) led to expansion of the ice that started eroding the sedimentary basins of the shelf and Lambert Graben. Cooler ice produced less meltwater and less suspended sediment delivery to the rise, but ice at sea level produced more icebergs and, hence, more ice rafting of debris. This trend continued through the late Miocene and Pliocene. Further cooling shifted the locus of maximum snow accumulation from the interior of the continent to the coastal fringe, so, during the early Pliocene, coastally sourced ice deflected the Lambert Glacier westward, forming the Prydz Channel Ice Stream. The combination of low precipitation, cold ice, and deep shelf erosion eventually led to the cessation of advances to the shelf edge by the Lambert–Amery Ice Stream during the middle Pleistocene (O'Brien et al., this volume).

This overall trend to cold, less active ice sheets through Neogene was punctuated by interludes of warm conditions that included extreme advances of the Lambert Glacier–Amery Ice Shelf system during the Pliocene.