The Polar lee Sheet Late Miocene Pleistocene

In the early Pliocene, ice flow regimes changed and ice was focused into an ice stream on the western side of the bay, cutting a cross-shelf trough, the Prydz Channel. The ice stream delivered basal debris to the shelf edge, where the debris built a trough mouth fan on the upper continental slope (Prydz Channel Fan, O'Brien and Harris, 1996; O'Brien and Leitchenkov, 1997; O'Brien et al., 2004). On the banks adjacent to Prydz Channel, vertical aggradation of subglacial debris produced tabular units while glacial erosion overdeepened the inner shelf.

Two ODP holes were drilled into the continental slope. ODP Site 743 was drilled to 98 mbsf into the eastern, steep part of the slope, and recovered diamict. ODP Site 1167 was drilled to 447.5 mbsf into the Prydz Fan, and recovered muddy, pebbly sands and diamicts deposited by slumping of subglacial debris interpreted to have originated at the ice grounding line at the shelf edge (Foldout PB-2, O'Brien et al., 2001; Passchier et al., 2003). ODP Site 1167 also recovered thin mudstone units deposited during periods of reduced ice extent (Shipboard Scientific Party, 2001b, Passchier et al.. 2003). More than 90% of the fan was deposited before the mid Pleistocene, and there were only three advances of the Amery Ice Shelf to the shelf edge in the late Pleistocene (O'Brien et al., 2004). Clay mineralogy, magnetic properties and clast composition at ODP Site 1167 show changes suggesting that the Pleistocene peak of erosion and ice volume in the Lambert-Amery drainage system occurred in the early Pleistocene (O'Brien et al., 2004). Oxygen isotope measurements on foraminifera from ODP Site 1167 also suggest that sedimentation was reduced after the mid-Pleistocene, with the last ice advance to the shelf edge at about Marine Isotope Stage 16 (612-698 ka; Lisiecki and Raymo, 2005). However, the stratal record is fragmentary because hiatuses are common, which leads to a tentative identification of isotope stages (Theissen et al., 2003). During the mid-to-late Pleistocene, ice advances were less extensive. During the last glacial cycle, the Amery Ice Shelf grounded only 100 km north of the present ice shelf edge and far from the continental shelf edge (Domack et al., 1998; O'Brien et al., 1999) (Fig. PB-1).

On the continental rise, sedimentation rates decreased through the Pliocene and Pleistocene because less detritus was eroded from the continent and because sediment was deposited on the upper slope in front of the Prydz Channel. The inferred early Pliocene base of the Prydz Channel Fan is the prominent unconformity mapped by Mizukoshi et al. (1986, Reflector A) and O'Brien et al. (2004, Reflector PP-12).

Drilling and seismic evidence indicate that glaciers advanced to the edge of the Prydz Bay shelf in cold episodes during the Pliocene and early Pleistocene, yet evidence of warm episodes also exists. Sediments in the Prince Charles Mountains indicate open-water fjordal environments in the Miocene to Pliocene (Hambrey and McKelvey, 2000; Whitehead et al., 2003. 2004). Lower Pliocene marine diatomite in the Vestfold Hills, on the eastern side of Prydz Bay, contains evidence of temperatures 4°C warmer than today (Whitehead et al., 2001). ODP Site 1167 includes a thin mudstone horizon at 217 mbsf with calcareous nannoplankton not presently found in Prydz Bay (Shipboard Scientific Party, 2001b), suggesting warmer conditions at about 1.1 Ma (Pospical 2004; Lavelle, personal communication, 2001). These occurrences indicate warmer episodes when the Amery Ice Shelf edge retreated several hundred kilometres inland from its present position, and warmer water intruded Prydz Bay.

5.4.5. Prydz Bay Summary

Seismic interpretation and drilling data reveal that the glaciation of Prydz Bay started in the latest Eocene. At that time, Prydz Bay was occupied by a fluvio-deltaic plain covered with stunted cool-temperate vegetation. Rivers flowing through the Lambert Graben were fed by glaciers in the hinterland. The sea transgressed across the plain and floating ice delivered dropstones to the shallow embayment. The embayment became glaciated in the early Oligocene, with ice-sheet-scale glaciers depositing subglacial till, and glacimarine diamicts when the ice was not grounded. The ice was probably wet-based. In the early Miocene, a large temperate to polythermal ice sheet advanced and retreated across the embayment, supplying large quantities of detritus to the continental rise, where the detritus was deposited in large mounds.

The mid-Miocene was marked by the start of a cooling trend and the development of a thicker, colder and more erosive ice sheet. Shelf over-deepening began, but progressively less detritus was delivered to the continental rise. In the early Pliocene, ice flow became concentrated on the western side of the bay in an ice stream that deposited sediment in a trough mouth fan. During warm phases, open-water extended landward as far as the northern Prince Charles Mountains. Ice volumes and depths of erosion reached a peak in the mid Pleistocene and the cold, polar ice sheet was established. The Amery Ice Shelf no longer grounded at the shelf edge in Prydz Channel during glacial episodes.

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