Late Neogene Ice Drainage Changes in Prydz Bay, East Antarctica and the Interaction of Antarctic Ice Sheet Evolution and Climate ⁎ P.E
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Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 390–410 www.elsevier.com/locate/palaeo Late Neogene ice drainage changes in Prydz Bay, East Antarctica and the interaction of Antarctic ice sheet evolution and climate ⁎ P.E. O'Brien a, , I. Goodwin b, C.-F. Forsberg c, A.K. Cooper d, J. Whitehead e a Geoscience Australia, GPO Box 378, Canberra Australia 2904 b Environmental and Climate Change Group, School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW Australia 2308 c Norwegian Geotechnical Institute, P.O. Box 3930, Ulleval Hageby, N-0806 Oslo, Norway d Department of Geological and Environmental Sciences, Stanford University, Stanford California USA e Institute of Antarctic and Southern Ocean Studies, University of Tasmania, Australia Received 18 January 2006; received in revised form 1 September 2006; accepted 5 September 2006 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. © 2006 Elsevier B.V. All rights reserved. Keywords: East Antarctica; Extreme ice advances; Pliocene; Pleistocene; Trough mouth fan; Precipitation; Amery Ice Shelf 1. Introduction O'Brien et al., 2001). Since its inception, it has played a central role in global climate and in higher order sea The East Antarctic Ice Sheet is presently the largest level change. Although global ice volume/temperature and longest-lived ice mass on earth (Barron et al., 1991; changes can be estimated using low latitude δ18O re- cords in ocean sediments, the distribution of ice on the ⁎ Corresponding author. Tel.: +61 2 62499409; fax: +61 2 62499920. continent and the detailed evolution of the Antarctic Ice E-mail address: [email protected] (P.E. O'Brien). Sheet require direct evidence from Antarctica (Barker 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.09.002 P.E. O'Brien et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 390–410 391 et al., 1998). The understanding of regional changes ice shelf grounding zone well inland (484 km) from Prydz in ice distribution and behaviour may provide evidence Bay (Fricker et al., 2002). On the eastern side of the bay, for the processes that have contributed to major climate the sea floor shoals to depths of 100–200 m on Four changes. Ladies Bank which is separated from the Ingrid Obtaining records of glacial history from the Antarc- Christensen Coast by a series of connected troughs and tic continental margin is complicated by the extensive saddles known as the Svenner Channel. The Amery erosion of the shelf by Cenozoic ice advances (Barron Depression is linked to the shelf edge by Prydz Channel et al., 1991). While subglacial and interglacial sediments which occupies the western part of the bay and has depths are preserved in places, erosion has removed large from 700 m below sea level at its inshore end to about amounts of sedimentary section (Solheim et al., 1991; 500 m below sea level at the shelf edge. Forsberg et al., 2001). However, erosion features can Prydz Bay lies at the seaward end of the Lambert-rift indicate palaeoflow directions and erosion surfaces can Graben that extends about 500 km south of Prydz Bay. be traced into their correlative conformities on the con- The adjacent rift-flank mountains (Prince Charles tinental slope, where the debris carried by the grounded Mountains) are mostly ice-covered and have a visible ice has been deposited (Cooper et al., 1991; Kuvaas and relief of up to 3500 m. Most of the basement rocks of the Leitchenkov, 1992; Bart et al., 2000). More complete region are high-grade Precambrian metamorphic and sedimentary records are possible in trough mouth fans intrusive rocks (Mikhalsky et al., 2001) with isolated formed where fast flowing ice streams reach the shelf exposures of Paleozoic granite, mafic dykes, Permian edge during episodes of extreme ice extent. These fans sediments, Paleogene volcanics, and Oligocene and consist of mass flow deposits formed from debris re- younger glacial sediments (Tingey, 1991; Hambrey and leased from the basal ice at the shelf break, interbedded McKelvey, 2000). The offshore area is underlain by the with thin hemipelagic and pelagic intervals deposited Prydz Bay sedimentary basin, with up to 12 km of when the ice was shoreward of the shelf edge (Boulton, sediment (Cooper et al., 1991), which is part of the an 1990; Vorren and Laberg, 1997). The East Antarctic extensive rift system that also includes the Lambert margin in Prydz Bay provides an important opportunity Graben. The rifting history extends back to at least to understand the behaviour of the East Antarctic Ice Cretaceous (Truswell, 1991) and probably back to the Sheet. Seismic data collected since 1982 and cores holes Permo-carboniferous (Arne, 1994). drilled on ODP Legs 119 and 188 drilled in Prydz Bay, provide a record of the Lambert Glacier–Amery Ice 2.1. Glaciology Shelf drainage system that flows from deep in the interior of the East Antarctic Ice Sheet. Neogene sediments are Prydz Bay and the shelf offshore receives ice from also represented in outcrops in the Prince Charles three regions; the Lambert Glacier–Amery Ice Shelf Mountains (PCMs) south of Prydz Bay (Hambrey and system, Princess Elizabeth Land and Mac.Robertson McKelvey, 2000; Whitehead and Boharty, 2003) Land, with four discrete ice drainage catchments (Fig. 2). providing samples of depositional conditions extending The main drainage into the Amery Ice Shelf is fed by nearly 700 km into the interior of East Antarctica. glaciers that break through the barrier of the PCMs along its length. At the southern end, the major glaciers are the 2. Regional setting Lambert, Mellor and Fisher Glaciers that flow into the graben through the southern group of nunataks and form Prydz Bay forms the terminus of the Lambert Glacier– the main axial ice drainage. They drain a catchment area Amery Ice Shelf ice drainage system, which drains about of 1,481,000 km2 (excluding the floating Amery Ice 16% of the grounded East Antarctic Ice Sheet (Allison, Shelf, Allison, 1979; Fricker et al., 2000). Examination 1979; Fricker et al., 2000). Most of the ice in the Lambert of a digital terrain model derived from satellite altimetry Glacier–Amery Ice Shelf system accumulates in the (Liu et al., 2001) indicates that four other ice drainage interior of East Antarctica (Allison, 1979; Fricker et al., basins discharge to the Amery Ice Shelf and Prydz Bay. 2000) so it responds to interior mass balance fluctuations. Ice flowing from Mount Creswell to the Northern Prince The dynamical response is recorded in the sediments of Charles Mountains along the western side of the Amery Prydz Bay and the adjacent slope and rise (Figs. 1 and 2). Ice Shelf drains about 303,710 km2. The largest glaciers Prydz Bay is typical of the Antarctic shelf with the deepest in this area are the Charybdis and Scylla Glaciers. Other areas (∼1200 m) inshore, near the front of the Amery Ice smaller ice streams feed into the Amery Ice shelf from Shelf in the Amery Depression (Fig. 1). Greater depths Mac.Robertson Land via re-entrants in the grounding (N2200 m, Fricker et al., 2001) are found near the present line, draining only about 113,200 km2. 392 P.E. O'Brien et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 245 (2007) 390–410 Fig. 1. Location of Prydz Bay with named bathymetric features and Ocean Drilling Program sites. The Lambert–Amery System receives ice from a Ice Shelf Glacier. At the eastern edge of this region is catchment area of 635,500 km2 along its eastern side the West Ice Shelf (Fig. 2). There is a general between the Mawson Escarpment and edge of the asymmetry in the ice accumulation distribution across Amery Ice Shelf. North-east of the modern edge of the the Lambert–Amery System, with higher ice accumu- Amery Ice Shelf, Prydz Bay receives ice from the lation along the eastern side of the basin up to 2500 m Ingrid Christensen Coast (375,100 km2 in area).