Antarctic Peninsula

Poor core recovery from shelf sites during Leg 178 drilling on the Pacific margin of the Antarctic Peninsula resulted in virtually no sedimentary record (Barker et al., 1999) of the progradational wedges that are inferred to have built the continental shelf during Quaternary oscillations of the WAIS (Larter and Barker, 1989; Larter and Cunningham, 1993). Notwithstanding this, the characteristics of the Pleistocene diamicts and glacimarine sediments that were recovered from the shelf sites were consistent with sedimentation on the margin since the Last Glacial Maximum (LGM). This observation led Barker et al. (1999) to propose that the topset deposits known as Antarctic Margin seismic sequence unit S1 were deposited under the influence of an expanded and periodically grounded WAIS.

Rebesco et al. (2006) correlated a grid of seismic reflection data collected on the Pacific shelf margin after Leg 178 into the more complete and betterdated, continental rise Site 1101. They showed a transition across the seismic sequence unit S2-S1 boundary marked by a change from deep erosion and debris flows to the progradation of relatively stable wedges on the continental slope occurred ~ 3.0 Ma. The timing is coincident with a significant reduction in sedimentation rate over the distal margin area consistent with observations on the other side of the Antarctic Peninsula in the Weddell Sea (Kennett and Barker, 1990). Both studies argued that the change in depositional style corresponds to major ice sheet expansion onto the continental shelf associated with reduced sediment supply and meltwater volume as the Antarctic Ice Sheets attained their present cold polar state.

Drilling of continental rise sediment drifts at sites 1095 and 1096 produced a more continuous record of Pleistocene G-I variability expressed as lithologic alternations between interglacial biosiliceous silts and glacial non-fossiliferous, terrigenous laminated clays and silts. Intriguingly, time-series analysis of the properties of G-I cycles at sites 1095 and 1096 shows no periodicity in the orbital frequency bands. However, iceberg-rafted debris (IRD) mass accumulation rates in the Pliocene-Pleistocene drift record at Site 1101 show strong covariance with the ~40kyr, obliquity frequency between 3 and 1 Ma, after which 100 kyr frequencies dominate (Cowan, 2002). Two large spikes in IRD abundance at 2.8 and 0.88 Ma have been interpreted as corresponding to Late Pliocene cooling and ice sheet expansion onto the shelf, and a second phase of cooling and ice expansion across the MPT (see below). The occurrence of orbitally influenced ice rafting in records around the Antarctic Peninsula from ~ 2.8 Ma has led a number of workers to suggest a NH eustatic influence on the Quaternary stability of the Antarctic Ice Sheet margin (e.g. Barker et al., 1999; Cowan, 2002; Hillenbrand and Ehrmann, 2005).

11.2.3. Prydz Bay

Drilling was undertaken at five sites (739-743, Fig. 11.1) during ODP Leg 119 on a transect across the continental shelf of Prydz Bay to elucidate the long-term glacial history and evolution of the EAIS (Barron et al., 1989). Although the late Pliocene to Pleistocene interval is limited by poor core recovery and inadequate age control, a cyclic record of tills and ice proximal, glacimarine sediments implies a succession of advances and retreats by the EAIS across the region (Hambrey et al., 1991).

During Leg 188, two sites sampled Late Pliocene-Pleistocene strata (O'Brien et al., 2001). Site 1167 was the first hole to be drilled in an Antarctic margin trough mouth fan, and the core provides an expanded view of glacier fluctuations in the Prydz Bay region during the last 2 Ma. The sedimentary record is dominated by several metres thick diamictons deposited as debrites separated by thin mud units (O'Brien et al., 2007). The debrites represent slumping and remobilisation of glacial outwash deposits and tills when the EAIS grounding line is near the shelf edge. Intervening interglacials are represented by glacimarine muds deposited by the retreating ice sheet. While the diamicton-mud cycles maybe of orbital duration, hiatuses within the section and insufficient age control preclude the duration of cycles from being established. O'Brien et al. (2007) also pointed out that the trend to expanded ice in the Prydz Bay region during the Late Pliocene maybe associated with the movement of interior precipitation to the coast, either from changes in storm tracks or the increase in ice around the EAIS margin. Moreover, they argue that this process could have caused the change to more laterally confined ice streams since the early Pliocene.

A significant change in sedimentation rate and sediment composition suggests a colder and less erosive ice sheet developed following the MPT at about 0.78 Ma. Three of the sedimentary cycles above the Bruhnes-Matuyama (B-M) magnetic reversal boundary (0.78 Ma) correspond to three cycles in a d18O record from Neogloboquadrina pachyderma at Site 1167 that are tentatively correlated with Marine Isotope Stages 21-16 and indicate orbital control on ice sheet fluctuations (Thiessen et al., 2003).

Site 1165 sampled sediment drift deposits on the continental rise and comprises a strongly cyclic Pleistocene interval in its upper 50 m in particle size, grey-scale and clay mineral records (O'Brien et al., 2007). A recent revision of the Pleistocene bio- and magnetostratigraphy of the sites show an incursion of calcareous nannofossils in the Site 1165 core at around 1 Ma (Villa et al., 2007), coincident with a decrease in 818O values in records from both 1167 and 1165 (Thiessen et al., 2003).

Late Pliocene-Early Pleistocene glacimarine deposits are described on land from the uplifted sequences of the Bardin Bluffs Formation (uppermost formation of the Pagodroma Group) in the Prince Charles Mountains (e.g. Whitehead and McKelvey, 2001). While the formations within this group are geographically separated they have been correlated into a composite stratigraphic section on the basis of diatom biostratigraphy (e.g. McKelvey et al., 2001; Whitehead et al., 2004). Bardin Bluffs Formation sediments mark a dramatic change from older warmer intervals of diatom-bearing glacimarine sediments to coarse-grained, ice-proximal diamicts overlying a striated erosion surface above the Amery Group.

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