Late Neogene ice drainage changes in Prydz Bay, East Antarctica and the interaction of Antarctic ice sheet evolution and climate

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Abstract

During the late Neogene, the Lambert Glacier–Amery Ice Shelf drainage system flowed across Prydz Bay and showed several changes in flow pattern. In the Early Pliocene, the Lambert Glacier ice stream reached the shelf edge and built a trough mouth fan on the upper continental slope. This was associated with an increase in ice discharge from the Princess Elizabeth Land coast into Prydz Bay. The trough mouth fan consists mostly of debris flow deposits derived from the melting out of subglacial debris at the grounding line at the continental shelf edge. The composition of debris changes at around 1.1 Ma BP from material derived from erosion of the Lambert Graben and Prydz Bay Basin to mostly basement derived material. This probably results from a reduction in the depth of erosion and hence the volume of ice in the system. In the trough mouth fan, debris flow intervals are separated by thin mudstone horizons deposited when the ice had retreated from the shelf edge. Age control in an Ocean Drilling Program hole indicates that most of the trough mouth fan was deposited prior to the Brunhes–Matuyama Boundary (780 ka BP). This stratigraphy indicates that extreme ice advances in Prydz Bay were rare after the mid Pleistocene, and that ice discharge from Princess Elizabeth Land became more dominant than the Lambert Glacier ice in shelf grounding episodes, since the mid Pleistocene. Mechanisms that might have produced this change are extreme inner shelf erosion and/or decreasing ice accumulation in the interior of East Antarctica. We interpret this pattern as reflecting the increasing elevation of coastal ice through time and the increasing continentality of the interior of the East Antarctic Ice Sheet. The mid Pleistocene change to 100 ka climatic and sea level cycles may also have affected the critical relationship between ice dynamics and the symmetry or asymmetry of the interglacial/glacial climate cycle duration.

Introduction

The East Antarctic Ice Sheet is presently the largest and longest-lived ice mass on earth (Barron et al., 1991, O'Brien et al., 2001). Since its inception, it has played a central role in global climate and in higher order sea level change. Although global ice volume/temperature changes can be estimated using low latitude δ18O records in ocean sediments, the distribution of ice on the continent and the detailed evolution of the Antarctic Ice Sheet require direct evidence from Antarctica (Barker et al., 1998). The understanding of regional changes in ice distribution and behaviour may provide evidence for the processes that have contributed to major climate changes.

Obtaining records of glacial history from the Antarctic continental margin is complicated by the extensive erosion of the shelf by Cenozoic ice advances (Barron et al., 1991). While subglacial and interglacial sediments are preserved in places, erosion has removed large amounts of sedimentary section (Solheim et al., 1991, Forsberg et al., 2001). However, erosion features can indicate palaeoflow directions and erosion surfaces can be traced into their correlative conformities on the continental slope, where the debris carried by the grounded ice has been deposited (Cooper et al., 1991, Kuvaas and Leitchenkov, 1992, Bart et al., 2000). More complete sedimentary records are possible in trough mouth fans formed where fast flowing ice streams reach the shelf edge during episodes of extreme ice extent. These fans consist of mass flow deposits formed from debris released from the basal ice at the shelf break, interbedded with thin hemipelagic and pelagic intervals deposited when the ice was shoreward of the shelf edge (Boulton, 1990, Vorren and Laberg, 1997). The East Antarctic margin in Prydz Bay provides an important opportunity to understand the behaviour of the East Antarctic Ice Sheet. Seismic data collected since 1982 and cores holes drilled on ODP Legs 119 and 188 drilled in Prydz Bay, provide a record of the Lambert Glacier–Amery Ice Shelf drainage system that flows from deep in the interior of the East Antarctic Ice Sheet. Neogene sediments are also represented in outcrops in the Prince Charles Mountains (PCMs) south of Prydz Bay (Hambrey and McKelvey, 2000, Whitehead and Boharty, 2003) providing samples of depositional conditions extending nearly 700 km into the interior of East Antarctica.

Section snippets

Regional setting

Prydz Bay forms the terminus of the Lambert Glacier–Amery Ice Shelf ice drainage system, which drains about 16% of the grounded East Antarctic Ice Sheet (Allison, 1979, Fricker et al., 2000). Most of the ice in the Lambert Glacier–Amery Ice Shelf system accumulates in the interior of East Antarctica (Allison, 1979, Fricker et al., 2000) so it responds to interior mass balance fluctuations. The dynamical response is recorded in the sediments of Prydz Bay and the adjacent slope and rise (Fig. 1,

Pre Neogene

The broad pattern of ice and sediment movement in the region has long been controlled by the Lambert Graben. This feature has been a major influence on drainage since even the Carboniferous (Arne, 1994). During the Palaeogene, the Lambert Graben–Prydz Bay Basin would have trained the fluvial systems that formed the delta plain that deposited poorly sorted sands of Late Eocene age intersected by ODP Site 1166 (Macphail and Truswell, 2004, Cooper and O'Brien, 2004). The first signs of glaciation

Early Pliocene

The north-west orientation of Prydz Channel relative to the underlying structural grain of Prydz Bay suggests that the change in drainage in the early Pliocene results from a change in the balance of ice discharge to Prydz Bay, as the relative ice volume discharged from the Ingrid Christensen Coast increased sufficiently to deflect the main Lambert Glacier axial drainage. Both geological and climatic processes are considered to have contributed to this change in ice discharge. Erosion of the

Discussion

These observations pose several questions: why should some major East Antarctic ice streams not ground at the shelf edge during the last glacial cycle, and why did extreme advances cease in the Late Pleistocene when global ice volumes were apparently higher than during the Early-mid Pleistocene? There is also the question as to what climatic conditions produced the combination of open water, relatively warm conditions existing in areas now occupied by the Amery Ice Shelf during warm phases,

Conclusions

The Lambert Glacier–Amery Ice Shelf system experienced several reorganisations of ice flow and sedimentation during the Neogene. In the late Miocene, ice advanced evenly across Prydz Bay. Then, in the Early Pliocene (3.6–3.9 Ma.), an increase in the importance of ice discharged from coastal Princess Elizabeth Land relative to the interior drainage basin produced extra outflow from the southeast of Prydz Bay. Combined with erosion of the inner shelf, this flow deflected the main axial drainage

Acknowledgments

We thank all those who supported ODP Leg 188, especially the captain, crew and technical staff of the JOIDES Resolution. German Leitchenkov, Takemai Ishihara and Manabu Tanahashi contributed data and advice on drill sites. Reviews by Alix Post, Mark Hemer, Damian Gore and an anonymous reviewer assisted in improving the paper. Philip O'Brien publishes with the permission of the Chief Executive Officer, Geoscience Australia.

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