Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments

Global and Planetary Change 45 (2005) 51 – 81 www.elsevier.com/locate/gloplacha

Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments C. Escutiaa,*, L. De Santisb, F. Dondab, R.B. Dunbarc, A.K. Cooperc,d, G. Brancolinib, S.L. Eittreimd a

Istituto Andaluz de Ciencias de la Tierra, CSIC/Universidad de Granada, Facultad de Ciencias, 18002, Granada, Spain Instituto Nazionale di Oceanografia e Geofisica Sperimentale, Borgo Grotta Gigante 421c, Sgonico, Trieste 34010, Italy c Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA d US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA

b

Received 4 December 2003; accepted 28 September 2004

Abstract The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene bclimatic optimumQ (14–10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14–10 Ma). Sequence WL-S9 forms under a polar regime during the Pliocene–Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope-forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability.

* Corresponding author. Tel.: +34 958 24 0504; fax: +34 958 24 3384. E-mail address: [email protected] (C. Escutia). 0921-8181/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2004.09.010

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Validation of our inferences about the nature and timing of Wilkes Land glacial sequences can be achieved by deep sampling (i.e., using IODP-type techniques). The most complete record of the long-term history of glaciation in this margin can be obtained by sampling both (1) the shelf, which contains the direct (presence or no presence of ice) but low-resolution record of glaciation, and (2) the rise, which contains the distal (cold vs. warm) but more complete record of glaciation. The Wilkes Land margin is the only known Antarctic margin where the presumed bonsetQ of glaciation unconformity (WL-U3) can be traced from shelf to the abyssal plain, allowing links between the proximal and the distal records of glaciation to be established. Additionally, the eastern segment of the Wilkes Land margin may be more sensitive to climate change because the East Antarctic Ice Sheet (EAIS) is grounded below sea level. Therefore, the Wilkes Land margin is not only an ideal location to obtain the long-term EAIS history but also to obtain the shorter-term record of ice sheet fluctuations at times that the East Antarctic Ice Sheet is thought to have been more stable (after 15 Ma-recent). D 2004 Elsevier B.V. All rights reserved. Keywords: Wilkes Land; Cenozoic; East Antarctic Ice Sheet; Glacial evolution

1. Introduction Cyclic global sea level change, ocean circulation, and the oxygen isotopic record, among others, have been influenced by the Antarctic glaciation since the formation of the first continental ice sheet around 34 Ma (Wise et al., 1991; Zachos et al., 1997; Barker et al., 1998; Fig. 1). Past drilling into continental shelf basins in Prydz Bay and the Ross Sea [i.e., Ocean Drilling Program (ODP) Legs 119 and 188, Deep Sea Drilling Program (DSDP) Leg 28 and Cape Roberts Drilling] indicates that, in the 20 my that followed inception, the ice cover reached continental dimensions, although it grew and collapsed many times in a cyclic fashion (Zachos et al., 1997). From around 14 Ma to the present day, the deep-sea isotope record (Kennett, 1978; Flower and Kennett, 1994) indicates that the Antarctic Ice Sheet has been a permanent feature of the continent, bringing with it a polar climate (Fig. 1). Such an ice sheet may have varied considerably in size perhaps by as much as 25 m of Sea Level Equivalent (SLE; Kennett and Hodell, 1993). The West Antarctic Ice Sheet (WAIS) is mainly marine-based and thus is less stable. The East Antarctic Ice Sheet (EAIS), which covers a landscape that is largely above sea level, is considered stable and likely to respond slowly to changes in climate. Reports of beach gravel deposited 20 m above sea level in Bermuda and the Bahamas from 420 to 360 ky indicate, however, the collapse of not only of WAIS (6 m of SLE) and the Greenland Ice Sheet (6 m of SLE) but likely also 8 m of SLE from East Antarctic ice (Hearty et al., 1999). The eastern sector

of the Wilkes Land margin is located at the seaward termination of the largest East Antarctic subglacial basin, the Wilkes Basin (Drewry, 1983; Ferraccioli et al., 2001). The base of the portion of the EAIS draining through the Wilkes Basin is largely below sea level, which suggests that this portion of the EAIS may be less stable than the rest of the EAIS. Numerical models of ice sheet behavior (e.g., Huybrechts, 1993; DeConto and Pollard, 2003) provide an understanding of the significance of events in a particular region for the history of the entire ice sheet. In these models, glaciation is believed to have begun in the East Antarctic interior, discharging mainly via the Lambert Graben to Prydz Bay. These models imply that the EAIS did not reach sea level in the Wilkes Land margin until a later stage (Fig. 2). Models of past events need to be validated through concrete evidence such as is the sedimentary record. Sediments from Prydz Bay drilled during ODP Leg 188 (O’Brien et al., 2001) contained the record of the first arrival of ice sheets to the margin, which in this paper we refer to as the onset of glaciation, dated late Eocene–Oligocene. The time of the onset of glaciation in the Wilkes Land continental margin is unknown at present but is essential for providing age constraints for the models of EAIS development and changes in its volume. The record of Antarctic glaciation, from the time of first ice sheet inception through the significant periods of climate change during the Cenozoic (i.e., Oligocene–Miocene boundary (Mi-1 glaciation), middle–late Miocene cooling, Pliocene warm periods, Pliocene–Pleistocene cooling, Quaternary periods of unusual warmth and extreme cold, etc.) is not only of

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Fig. 1. Earth’s temperature variations during the last 80 my based on reconstructions from deep-marine oxygen isotope records. Future temperature scenarios based on IPCC greenhouse trace gas projections are also shown. Climatic threshold events in the history of the Antarctic and Northern Hemisphere ice sheets are labeled. Modified from Barrett (1996).

scientific interest but also is of great importance for society. The ice sheet models by DeConto and Pollard (2003) suggest that the main triggering mechanism for initial inception and development of the Antarctic Ice Sheet were the decreasing levels of CO2 concentration in the atmosphere, and that the opening of Southern Ocean gateways played a secondary role. At present, increasing of CO2 and other greenhouse gases in the atmosphere (being now 30% above the preindustrial level) causes land surface and ocean temperatures to rise. If the concentration of CO2 continues to increase at the present rate, global temperature projections produced by the Intergovernmental Panel in Climate Change (IPCC, 2001) are expected to be the highest for the past 20 Ma, believed to be before the Antarctic Ice Sheet became a permanent feature and before the northern hemisphere ice sheets were established (Fig. 1). From the above, it is of great relevance to

validate these models because they may be the basis for climate predictions. In this paper, we outline the nature of glacial stratigraphy, sedimentation, and processes in the Wilkes Land margin. We provide with inferences about the nature (i.e., type of glacial sediment, environmental conditions, etc.) and timing of Cenozoic glacial events from the onset of glaciation in this margin to the Holocene. We outline a strategy for deep sampling (i.e., IODP-type techniques) that would ground truth our inferences on the long-term record of multiple growth and collapse of the EAIS and related sea level, paleoceanographic and paleoenvironmental changes from this margin. We also consider the use of deep sampling to examine the ultrahighresolution sedimentary record of Holocene climate variability preserved in the Wilkes Land continental shelf.

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Fig. 2. Simulated initiation of East Antarctic glaciation in the earliest Oligocene, using a coupled GCM-ice sheet model (from DeConto and Pollard, 2003). Note that arrival of ice sheets to the Wilkes Land margin, according to this model, takes place at a later stage than in Prydz Bay.

To achieve our objectives, we integrate seismic stratigraphic and sedimentological data available across the Wilkes Land margin. Previous efforts in the Wilkes Land margin focused on obtaining a better understanding of Cenozoic glacial history by conducting either seismic stratigraphic studies or sedimentological studies. Most of these studies were carried out on specific physiographic areas of the continental margin, such as the shelf and slope (e.g., Domack et al., 1980, 1989; Milam and Anderson, 1981; Domack, 1982, 1987; Anderson et al., 1983; Dunbar et al., 1985; Eittreim et al., 1995; Beaman and Harris, 2003; Harris and Beaman, 2003; Presti et al., 2003) or the continental rise (Escutia et al., 2000, 2002; Busetti et al., 2003; Donda et al., 2003). Only few studies have focused on continental shelf to abyssal plain seismic stratigraphic studies (Eittreim and Smith, 1987; Hampton et al., 1987a; Wannesson,

1991; Tanahashi et al., 1994; Escutia et al., 1997; Barker et al., 1998; De Santis et al., 2003) or the study of glacial sediment character, distribution, and processes across the margin (Payne and Conolly, 1972; Hampton et al., 1987b; Escutia et al., 2003). The data sets integrated for this study comprise (Fig. 3): (1) Multichannel seismic reflection profiles collected during the 1981–1982 austral summer by the Institute Franc¸ais du Pe´trole (IFP); in 1982, 1983, and 1990 by the Japan National Oil Corporation (JNOC); in 1984 by the United States Geological Survey (USGS); and in 2000 during the Italian–Australian WEGA-2000 cruise. The available technical (i.e., source, receiver and recording) parameters for each of the cruises are detailed in Table 1. (2) Sediment cores collected from the Wilkes Land margin during DSDP Leg 28, Site 269 (Hayes et al., 1975), the USNS Eltanin cruise (Payne and Conolly, 1972), the

C. Escutia et al. / Global and Planetary Change 45 (2005) 51–81 Fig. 3. Map showing the location of multichannel seismic reflection profiles and sediment cores considered for this work. The sediment cores referred to in the text have been labeled. Location of the seismic sections shown in Figs. 5–9 are indicated.

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Table 1 Technical parameters of the multichannel data collected from the Wilkes Land margin during the WEGA-2000 cruise (De Santis et al., 2003), the USGS 1984 cruise (Eittreim and Smith, 1987), the JNOC 1982 cruise (Tanahashi et al., 1994), and the IFP 1981–1982 cruise (Wannesson, 1991) Source

Receiver

Recorder

Seismic source firing chamber Operating pressure (psi) Operating depth (m) Shot interval (m) Streamer length (m) No. of channels Group interval No. of phones No. of channels Record length (s) Sampling rate (ms)

WEGA-2000

USGS-1984

JNOC-1982

IFP 1981–1982

6.8l 2000 3 25 500 40 12.5 16 48 7 1

211 1800–2000 10.5 50 2400 24 100 60 24 10 2

9.21 1900 NA 50 600 24 25 32

361 NA NA 50 200 48 NA NA NA NA NA

5 4

NA—not available.

Deep Freeze 79 cruise (Domack, 1982), the USGS 1984 cruise (Hampton et al., 1987b), and the WEGA2000 cruise (Busetti et al., 2003; Harris and Beaman, 2003; Presti et al., 2003). Two bathymetric charts were considered for this study: the more recently published bathymetric chart by Porter-Smith (2003) (Fig. 4) and the bathymetric map of Chase et al. (1987).

2. Geographic and physiographic setting The studied segment of the east Antarctic Wilkes Land margin is located between 1388E and 1458E longitude (Figs. 3 and 4). The Ade´lie and George Vth Coasts of the Wilkes Land margin drain the East Antarctic Ice Sheet with a mostly divergent flow pattern. Ice cliffs and two prominent outlet glaciers, the Mertz and the Ninnis, characterize the present coastline (Fig. 3). These outlet glaciers extend seaward as ice tongues and have an important role in ice drainage and sediment delivery to the ocean (Anderson et al., 1980; Drewry and Cooper, 1981). Drainage velocities in outlet glaciers range from more than 0.5 km/yr to about 3.7 km/yr (Lindstrom and Tyler, 1984; MacDonald et al., 1989), while drainage in the areas in between them, occupied by sea cliffs, may range from few meters to tens of meters every year (Anderson, 1999). The bathymetric charts for this margin (Chase et al., 1987; Porter-Smith, 2003) are still highly interpretative because they are based in few data, some of

it with poor navigation. Despite this limitation, seismic profiles allow us to confirm some of the main physiographic features of this margin. The continental shelf in this segment of the Wilkes Land margin has an average width of 125 km and an average water depth of 450–500 m (Fig. 4). As commonly observed around Antarctica, the shelf exhibits an overdeepened and landward-sloping bathymetric profile that is caused by glacial erosion and sediment loading (Vanney and Johnson, 1979; Ten Brink and Cooper, 1992). The shelf topography is irregular with (1) deep (N1000 m) basins, which occupy the inner-shelf, their continuation, namely, (2) shelf troughs, which become shallower as they traverse from the inner-shelf basins (N1000 m) to the outer shelf (500 m), and flanking them, (3) shallow (200–400 m) outer shelf banks. More than 1000-m deep inner-shelf basins develop at the mouth of the outlet glaciers. The continental shelf troughs are eroded by ice streams during times of glacial advance. Ice streams correspond with areas of extremely rapid ice-flow and their presence identifies a rapidly drained area of an ice sheet. Adjacent to the troughs are the shallow banks, which are areas bypassed by the most recent ice streams and where grounded ice has been slow moving or relatively immobile. The continental slope, which extends from the shelf break to about 2000–2500 m water depth, is steep (typically 88) narrow (15 km average, but up to 25 km) and is incised by submarine canyons (Chase et al., 1987; Escutia et al., 2000; Porter-Smith, 2003; Fig. 4). Seaward of the slope is the continental rise,

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Fig. 4. Wilkes Land bathymetry map modified from Porter-Smith (2003). Location of multichannel seismic profiles and of DSDP Site 269 is shown.

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also relatively steep (average gradients greater than 1:100 in the upper rise to less than 1:150 in the lower rise) and rugged because of (1) the presence of a complex network of tributary-like channels that continue from the slope canyons, (2) high-relief levee systems associated to the channels, and (3) a system of sediment ridges (Escutia et al., 2000; Escutia et al., 2002; De Santis et al., 2003; Donda et al., 2003).

3. Geologic setting The Wilkes Land continental margin is believed to have formed in mid-Jurassic/Cretaceous time (Cande and Mutter, 1982; Veevers, 1987), during an extensional tectonic episode that separated the Antarctic and the Australian continents. The acoustic basement across the margin consists of block-faulted continental crust, thinned and intruded transitional crust, and oceanic crust (Eittreim and Smith, 1987; Eittreim, 1994). Deep marginal rift basins generally characterize the transition zone from continental to oceanic crust (Eittreim, 1994). Maximum sedimentary thickness (about 8 km) has been reported from these marginal rift basins. Based on recently acquired high-resolution multichannel seismic profiles, De Santis et al. (2003) renamed, and in some instances redefined, the seismic sequences (i.e., WL-S1 to WL-S9) lying on top the acoustic basement and the unconformities (i.e., WLU1 to WL-U6) bounding them. Table 2 summarizes

the nomenclature given to the seismic sequences and unconformities in the different studies conducted in the Wilkes Land margin. Eittreim and Smith (1987) interpreted a prominent erosional unconformity that terminates at the southern edge of the oceanic crust as the breakup unconformity. This unconformity separates 3-km-thick synrift sediment below from up to 5km-thick postrift sediment above. Domack et al. (1980) cored fossiliferous Lower Cretaceous siltstones, interpreted to comprise part of an early synrift sequence. The fossil assemblages recovered from this site are diverse and entirely nonmarine in character (Domack et al., 1980). In seismic reflection profiles, synrift strata (WL-S1 sequence) are highly variable in seismic character, not well layered, and have discontinuous, weak reflections that dip to the north and downlap onto the base of the sequence (Eittreim and Smith, 1987; Eittreim, 1994; De Santis et al., 2003). Postrift sediments are well stratified and continuous over the oceanic crust and become less stratified and less continuous in a landward direction (Eittreim and Smith, 1987; Eittreim, 1994; De Santis et al., 2003). Postrift strata are mostly undeformed and are more than 2 km thick across the shelf. A marked intraerosional surface that can be traced across the margin, WL-U3 (WL2 unconformity of Tanahashi et al., 1994 and Eittreim et al., 1995; Table 2), is interpreted as a regressive surface (Eittreim and Smith, 1987; De Santis et al., 2003) and has been inferred to represent the first arrival of an ice sheet on the Wilkes Land continental shelf during the Cenozoic (Tanahashi et al.,

Table 2 Summary of the terminology assigned in previous publications to the inferred Wilkes Land glacial sequences and their bounding unconformities

Unconformities (tied with lines) and sequences (in between these lines) are listed from younger at the top to older.

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1994; Eittreim et al., 1995; Escutia et al., 1997; Barker et al., 1998). From the time of the formation of unconformity WL-U3, the stratigraphic evolution and the sedimentary history of this margin are inferred to be closely related to the glacial history of the adjacent continent.

4. Wilkes Land margin glacial stratigraphy 4.1. Continental shelf glacial stratigraphy On the continental shelf, Eittreim et al. (1995) reported two main regional unconformities: WL2 and WL1, truncating continental shelf postrift Cenozoic strata, which De Santis et al. (2003) renamed WL-U3 and WL-U8, respectively (Fig. 5; Table 2). Both unconformities have been interpreted as regressive surfaces (Eittreim and Smith, 1987; Hampton et al., 1987a; De Santis et al., 2003) and have been thought to represent the erosion caused by grounded ice sheets advancing into the continental shelf (Tanahashi et al., 1994; Eittreim et al., 1995; Escutia et al., 1997; Barker et al., 1998). Eittreim et al. (1995) calculated that the erosional event that formed the unconformity WL-U3 truncated from 300 to 600 m of strata. This event marks the change from dominantly aggradational sequences below to dominantly progradational sequences above. Low-angle seaward deeping foresets characterize the lowermost inferred glacial strata packages above unconformity WL-U3 (Fig. 5). Eittreim et al. (1995) also estimated the erosional event that formed unconformity WL-U8 to have truncated about 350 to 700 m of the sedimentary unit above WL-U3, which forms the lower boundary onto which strata of the outer shelf downlap (Fig. 5). Unconformity WL-U8 marks a change in the geometry of the progradational wedge, from lower angle prograding strata below to steeply-dipping prograding strata above the unconformity (foreset slopes up to about 108; Fig. 5). Line IFP 103 is a true dip line running along one of the Wilkes Land shelf troughs, and thus, the reflector geometries imaged in it best represents the true infill geometries within the troughs (Fig. 5). Between these two major regional unconformities, there are numerous erosional events that cannot be tied through the regional grid of seismic lines, making

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difficult their interpretation in a regional context. Two of the more important episodes are represented by two highly reflective irregular surfaces [Downlap Surfaces (DLS) of Eittreim et al., 1995], part way up each of the prograding sequences above unconformities WLU3 and WL-U8 (Fig. 5). The progression of sedimentary events is similar in each sequence: (1) deposition of low-angle downlapping strata; (2) hiatus represented by the highly reflective surface; and (3) more steeply prograding strata downlapping onto the surface representing the hiatus. Other significant observations regarding the inferred continental shelf glacial stratigraphy on the Wilkes Land are: (1) Along strike seismic profiles show the broad (about 20 km wide at their tops) erosional troughs to be common features in the Wilkes Land continental shelf after the development of unconformity WL-U3 (see fig. 10 of Eittreim et al., 1995). The most recent troughs are broad and shallow when compared with some of the earlier troughs (e.g., fig. 10 of Eittreim et al., 1995). Across the margin profiles show these erosional troughs to be filled with sediments of progradational wedges (Fig. 5). (2) The recent shelf banks are characterized by flatlying strata that, in some cases, are deposited unconformably on top of older prograding strata. (3) Foreset strata in the outer shelf troughs are truncated at or near the seafloor, and thus, topset strata are thin or not resolvable in our seismic data (Fig. 5). Wilkes Land margin prograding wedges lack the thick topset sections characterizing the shelf glacial sequences in the Ross Sea (e.g., Cooper et al., 1991) and Antarctic Peninsula (Barker et al., 1998, 1999) and resemble more those on the Lambert Trough in Prydz Bay (e.g., Cooper et al., 1991; O’Brien et al., 2001). (4) Mounded-like features with disrupted strata are commonly observed at the toes of foreset strata of the paleoslopes (Eittreim et al., 1995; Fig. 5). 4.2. Continental rise glacial stratigraphy Seismic units have been correlated from shelf to rise areas based largely on tracing and projecting unconformities and seismic units. Seismic units above the WL-U8 unconformity downlap and pinch out at the base of the continental slope, but deeper units (i.e., between WL-U8 and WL-U3) continue across the

60 C. Escutia et al. / Global and Planetary Change 45 (2005) 51–81 Fig. 5. Line drawings of multichannel seismic profiles IFP 103 and 107. Inferred major glacial unconformities WL-U3 and WL-U8 are shown. DSL—downlap surfaces referred to in the text. Shaded areas indicate the presence of mounded bodies with chaotic reflectors. Line IFP 103 is a true dip line along the Dumont D’Urville Basin. See Fig. 3 for location of this profiles. Modified from Eittreim et al. (1995).

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margin (Figs. 5 and 6). The principal marker is unconformity WL-U3, which in seismic lines IFP 107 (Fig. 6) and IFP 105 can be traced from the continental shelf, where it is inferred to mark the onset of progradation on the Wilkes Land margin to the lower rise where it appears to correlate with an upsection increase in turbidite (Eittreim and Smith, 1987; Hampton et al., 1987a; Escutia et al., 1997, 2000, 2002; De Santis et al., 2003; Donda et al., 2003; Figs. 7 and 8), and bottom-current deposition (Escutia et al., 2000, 2002; Donda et al., 2003). Continental rise strata above the WL-U3 unconformity are divided by four regional unconformities named from older to younger WL-U4 and WL-U5 (which correspond to unconformities WL1c and WL1b, respectively, of Escutia et al., 1997, 2000, 2002), WL-U6, and WL-U7 (De Santis et al., 2003; Donda et al., 2003; Table 2). These unconformities bound six glacial-related seismic sequences named from older to younger: WL-S4 to WL-S9 (Figs. 6–8). Based on the internal seismic character of continental rise strata, it can be differentiated between an upper rise (Fb3000 m water depth) and a lower rise (FN3000 m water depth). Table 3 summarizes the seismic character of each seismic sequence in the upper and lower rise. Here we explain the main characteristics of the sequences from older to younger: (1) Sequence WL-S4 is characterized by distinct stratification and continuous reflectors that onlap in the base-of-the-slope uppermost rise (Fig. 6). (2) Sequence WL-S5 also pinches out and onlaps the WL-U3 unconformity in the base-of-the-slope (Fig. 6). In the upper and middle rise, sequence WLS5 has irregular thickness caused by the development of channel and levee complexes (Fig. 7). In the lower rise, WL-S5 is characterized by a more tabular sheetlike geometry with continuous and stratified reflectors. Locally, stratified reflectors have a lens-like geometry and are associated with irregular and discontinuous reflectors, which characterize channels, and parallel to subparallel reflectors that thin away from the channels, indicative of levee deposition (Figs. 7 and 8). Towards the lowermost rise, WL-S5 reflectors become subparallel and are of low-amplitude, and locally, they exhibit a subdued wavy geometry (Donda et al., 2003). (3) Seismic sequence WL-S6 thins towards the base-of-slope where in some of the seismic lines

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(e.g., IFP 107) it appears to mostly consist of a mounded body with discontinuous to chaotic internal reflectors. In the upper rise, reflector configurations within WL-S6 are characteristic of widespread channel and levee deposition (Fig. 7). In the lower rise, acoustic signatures of channela˜ levee complexes are also common, although the channelsa˜ levee complexes have less relief (Fig. 8). In general, channelsa˜ levee systems within sequence WL-S6 have higher relief and narrower cross-section profiles than channels and levees in sequence WL-S5 (Figs. 7 and 8). In the lower rise, seismic signatures that characterize sediment waves (i.e., wavy reflectors) are observed (Fig. 8). (4) Sequence WL-S7 both in the upper and the lower rise exhibits a similar character to sequence WL-S6 (Figs. 7 and 8); the main difference is that, in Sequence WL-S7 wavy reflectors that characterize sediment waves are observed across the continental rise. (5) Sequences WL-S8 and WL-S9 are not continuous across the continental rise (De Santis et al., 2003; Donda et al., 2003; Figs. 6–8). Internal reflector configurations within sequence WL-S8 in the upper rise are more continuous when compared to reflectors within sequence WL-S7. Signatures characteristic of channel–levee complexes are less widespread and more subdued (Fig. 7). In the lower rise, sequence WL-S8 is identified filling the previous depressions and consists of horizontally stratified reflectors, which locally exhibit wavy geometries indicative of bottom contour–currents. (6) Sequence WL-S9 in the base of slope and uppermost rise consists of channel and levee complexes (Fig. 6). Towards the lower rise, WL-S9 consists of wavy reflectors that extend to the seafloor. The wavelengths and amplitudes of the undulations decrease up-section (Donda et al., 2003). In the lowermost continental rise–abyssal plain environment, all glacial sequences become thinner above unconformity WL-U3 and consist of parallel and subparallel horizontal reflectors of varying amplitudes (Fig. 9). Other significant observations regarding the inferred glacial stratigraphy in the Wilkes Land continental rise are: (1) Acoustic signatures of channel–levee complexes that characterize turbidite deposition are

62 C. Escutia et al. / Global and Planetary Change 45 (2005) 51–81 Fig. 6. Multichannel seismic line WEGA W21 and line drawings of multichannel seismic profiles IFP 107 and WEGA W35, showing the overall architecture of the Wilkes Land margin from the continental shelf to the continental rise. See Fig. 3 for location of the seismic lines.

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Fig. 7. High-resolution multichannel line WEGA W32 across a more proximal area of the continental rise. Also shown is a line drawing interpretation of this profile after De Santis et al. (2003). Location of seismic line is shown in Fig. 3.

64 C. Escutia et al. / Global and Planetary Change 45 (2005) 51–81 Fig. 8. High-resolution multichannel line WEGA W30 across a more distal area of the continental rise. Also shown is a line drawing interpretation of this profile after De Santis et al. (2003). Location of seismic line is shown in Fig. 3.

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Table 3 Seismic character of sedimentary sequences in the Wilkes Land continental rise Sequences

Upper continental rise (b3000 m water depth)

Lower continental rise (N3000 m water depth)

WL-S9

Wavy reflector geometry with a more uniform reflector character. Undulating reflectors extend to the seafloor. Wavelength and amplitude of sediment waves decreases up-section. Channel–levee complexes within this sequence develop at the base-of-the-slope.

When identified consists of subhorizontal reflectors. Very smooth wavy reflectors are also observed extending to the seafloor.

WL-S8

Cut and fill geometries are more subdued than in sequence WL-S7. Larger channel–levee complexes form only in the uppermost rise.

When identified is similar to WL-S7.

WL-S7

Similar to WL-S6 but in general exhibits smoother channel–levee cross-sectional profiles and wavy geometry.

Horizontally stratified reflectors with uniformly undulating reflectors interpreted as sediment waves.

WL-S6

High-amplitude, low lateral continuity, and high-relief undulations. Cut-and-fill geometries are widespread within this sequence. Channel incisions are narrower and deeper than in sequence WL-S5. Levees are well-developed and asymmetric.

Horizontally stratified reflectors that in a northward direction increase in amplitude and continuity. Includes uniformly undulating reflectors interpreted as sediment waves.

WL-S5

Discontinuous with external lens-like geometries. Localized cut-and-fill geometries that characterize channel–levee complexes.

Subparallel reflectors of low-amplitude with localized subdued wavy geometry.

WL-S4

Continuous parallel and Subparallel reflectors of moderate to low amplitude. External sheet-like geometry.

Continuous parallel and subparallel reflectors of moderate to low amplitude. External sheet-like geometry.

Descriptions are compiled after Escutia et al. (1997, 2000), De Santis et al. (2003), and Donda et al. (2003).

observed above the WL-U4 unconformity starting with deposition of sequence WL-S5 (Figs. 7 and 8). (2) The main development of channel–levee complexes forming sedimentary ridges in the Wilkes Land continental rise takes place above the WL-U5 unconformity, during deposition of sequence WL-S6 (Phase 1 of Escutia et al., 1997; Escutia et al., 2002) and WL-S7 (Phase 2 of Escutia et al., 1997, 2002) (De Santis et al., 2003; Donda et al., 2003; Figs. 7 and 8; Table 2). (3) Wavy reflectors interpreted as sediment waves formed by bottom contour–current are apparent since the development of Sequence WL-S6 in the lower rise (Fig. 8) and since Sequence WL-S7 in the upper rise (Fig. 7). (4) Deposition of sequence WL-S8 corresponds with the attenuation stage of De Santis et al. (2003)

and Donda et al. (2003) (Phase 3 of Escutia et al., 1997, 2002; Figs. 7 and 8; Table 2).

5. Wilkes Land margin glacial sedimentation and processes Glacial sedimentary processes in the Wilkes Land margin are inferred to be active from the time of the formation of the WL-U3 unconformity to present. We use surface sediment cores to provide us with a better understanding on the more recent (Quaternary) sediment distribution and processes. We use seismic reflection profiles as a tool to decipher the geometries and seismic facies of earlier (pre-Quaternary) depositional bodies above the WL-U3 unconformity. From these analyses, we can differentiate the following

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Fig. 9. Multichannel seismic profile IFP 103 across the Wilkes Land abyssal plain. Location of seismic line is shown in Fig. 3.

depositional environments within the continental shelf, slope, and rise. 5.1. Continental shelf sedimentation and processes Glacial sedimentation on the Wilkes Land continental shelf shows marked lateral variability which illustrates three depositional environments: inner-shelf basins, shelf troughs, and shelf banks. 5.1.1. Inner-shelf basins Sediment recovered from gravity and piston cores raised from deep (N1000 m) inner-shelf basins during the Deep Freeze 79 (DF-79) and USGS 1984 cruises are characterized by diatomaceous ooze with high (N30%) biogenic silica content (Domack, 1982, 1988; Anderson et al., 1983; Dunbar et al., 1985; Hampton et al., 1987b). Diatom analyses conducted by Escutia et al. (2003) on one of the cores (DF79-13) from the King George Vth Basin yielded an age younger than 18 Ka for the 5.95-m-long core, which provides with minimum Holocene sedimentation rates of 0.33 m/ky for this environment. Quantitative diatom analyses also showed that the diatom species present in the core (i.e., sea-ice marker species: Fragilariopsis curta, F. cylindrus, F.

obliquecostata, F. ritscheri, F. sublinearis, and F. vanheurckii) are indicative of fluctuating sea-ice conditions. Other recent surveys in the inner-shelf basin environment also report thick Holocene sediment sections. Analyses conducted on up to 5.5-m sediment cores collected from the Mertz Drift (Fig. 3), also in the King George Vth Basin, during the WEGA-2000, give an age raging from late Pleistocene (14 Ka) for a basal diamicton to Holocene (Presti et al., 2003). Holocene sedimentation rates in this core are thus of about 0.4 m/ ky More recently, the Ade´lie Drift was discovered in February 2001 during a RVIB Nathaniel Palmer cruise (Leventer et al., 2001). The Ade´lie Drift is located in the Dumont D’Urville Basin at the mouth of the Astroblade Glacier (Fig. 4). The drift consists of about 200 m of inferred Holocene sediment overlaying a glacial diamict (Leventer et al., 2001). The upper 30 m of the section were piston and kasten-cored in 2001 (Leventer et al., 2001). In March 2003 the joint French–Australian Marion Dufresne CADO cruise collected 40- and 60-m sediment cores (Fig. 3). AMS C-14 dating has established that the uppermost 50 m of the drift extend back only ca. 3000 years, which indicate very high sedimentation rates (17 m/ky) and suggests that this site contains an ultrahigh resolution Holocene record (Dunbar et al., 2003, IODP proposal).

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5.1.2. Shelf troughs Sediment surface cores collected from the shelf trough environment contain mostly Pleistocene diamicton locally covered by a thin layer (0 to ~80 cm) of Holocene diatomaceous mud (e.g., Domack, 1982; Domack and Anderson, 1983; Dunbar et al., 1985; Hampton et al., 1987b; Escutia et al., 2003). Diatom and magnetostratigraphic analyses on core USGS A1G1 (2.08-m long; Fig. 3) provided with Pleistocene ages younger than 78 Ka for the diamicton (Escutia et al., 2003). During the Pleistocene glacial, when grounded ice streams extend onto the continental shelf, massive to crudely stratified diamictons are deposited in the Wilkes Land shelf troughs. Diamictons were deposited as tills (imaged in seismic lines as topsets) directly below the grounded ice sheet (Anderson et al., 1980; Domack, 1982) or as glacial open-marine diamictons opposite to the ice stream terminus as the foresets of the prograding wedges also known as btrough mouth fansQ. Pre-Pleistocene prograding wedge sediments have not yet been sampled from the Wilkes Land margin. Changes in the geometry of the prograding wedge from low-angle progradational strata above unconformity WL-U3 to steep foreset strata above WL-U8 (Fig. 5) are likely the result from changes in the glacial processes. The sampled Pleistocene diamictons are sediments from the most recent topset and steep foreset sequence on the continental shelf. The similar acoustic character of these foresets to those forming the prograding wedge between unconformity WL-U8 and the seafloor suggests to us a common set of processes involved in their formation (i.e., sedimentation by grounded ice streams extending onto the shelf during times of glacial maxima). Pre-Pleistocene subtill rocks have been dredged from the eroded flanks of the Mertz–Ninnis Trough. Dredges collected during the R/V Nathaniel Palmer cruise NBP0101 (Leventer et al., 2001) contained, from deeper to shallower materials, (a) crystalline Precambrian rocks of distal (i.e., onshore and beneath the ice sheet) provenance and organic rich mudstones and sandstones. The later are similar to apparently in situ siltstones that were sampled also from the Mertz– Ninnis Trough (i.e., DF79-38) and dated Lower Cretaceous (Aptian; Domack et al., 1980); (b) poorly sorted nonmarine sandstones and siltstones; and (c)

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metamorphic and igneous rocks of Precambrian affinities likely representing an sub-ice source on the Craton. 5.1.3. Shelf banks Surface sediment cores collected from the shelf banks contain overconsolidated diamicton and residual glacial-marine sand and muddy sand with varying concentrations of fine-grained matrix (e.g., Domack, 1982; Dunbar et al., 1985; Hampton et al., 1987b). Diatoms in these cores are mostly fragmented and consist of a mixture of early Pleistocene and Neogene species, tentatively suggesting an early Pleistocene age of subbottom sediments (Escutia et al., 2003). Glacial well-sorted and rounded to angular sand and gravels can be either nongraded or graded and likely represent redistribution of the original glacial diamictons. The common absence of mud in these deposits and their well-sorted nature suggests that the finer component of the original deposit has been likely removed by bottom-currents such as the ones sweeping across today’s continental shelf (Bindoff et al., 2000; Williams and Bindoff, 2003). The coarser component being sorted as the different particle sizes settle down according to the different carrying capacity of the bottom flows. Presence of graded deposits indicates that gravity flows (i.e., turbidity flows and grain flows) are also an important process in resedimentation of the original glacial deposits. Pre-Quaternary strata from the banks have not been sampled. In seismic reflection profiles, flat-lying or very gently seaward-dipping strata characterize the shallow banks (Eittreim et al., 1995; Fig. 5). The present shallow banks on the shelf, however, may not have been permanent features. Buried troughs filled with prograding wedges, similar to the ones developed in front of the present day outer-shelf troughs, have been described in the Wilkes Land continental shelf underlying some of the more recent flat-lying aggraded sediment of the present banks based on seismic lines. This suggests that ice streams may have shifted position during consecutive glacial advances (Eittreim et al., 1995; Escutia et al., 2000). 5.2. Continental slope sedimentation and processes Surface sediments from the slope environment consist of massive sandy mud to fine sand, alternating

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with massive sandy and pebbly deposits (Payne and Conolly, 1972; Domack, 1982; Milam and Anderson, 1981; Anderson et al., 1983; Hampton et al., 1987b; Escutia et al., 2003), which resemble the diamictons of the continental shelf (Escutia et al., 2003). Base-ofslope and upper continental rise sedimentation are characterized by overall finer-grained deposits (i.e., mud and silty to sandy mud; Payne and Conolly, 1972; Hampton et al., 1987b; Escutia et al., 2003). These deposits are characterized by alternating massive intervals with more abundant IRD and crudely stratified to laminated intervals (Escutia et al., 2003). Diatom and magnetostratigraphy analyses reveal the existence of numerous hiatuses in the up to 6-m sediment cores from the slope and the base-of-slope, which are interpreted as repeated erosional episodes by gravity flows (Escutia et al., 2003). As illustrated by Hampton (1972), gravity flow transition from slumps to turbidity flows occurs as water is added to the flow. In the Wilkes Land margin, Escutia et al. (2003) conclude that the composition, sedimentary structures, and textures observed in the cores from the continental slope suggest a downslope transition on gravity flow types. They interpret sediment from the upper slope, with a texture that greatly resembles the diamictons on the shelf, to represent either part of a slump block or the start of a debris flow, and the crudely stratified to laminated intervals in the lower slope and base-of-the-slope to represent the transition between an end member of a debris flow and a turbidity flow. Gravity flows can be generated with minimal triggering on the steep slopes (~88–108) of the Wilkes Land Pleistocene continental slope. Sediment overloading at the shelf break during times of glacial maxima and earthquakes can trigger instability and generate slumps. The thick progradational wedges on the outer continental shelf attests to the large amount of sediment delivered to the Wilkes Land outer shelf and slope during times of glacial maxima (Fig. 5), which can cause overloading at the shelf break. It has also been postulated that, in glaciated margins, such as the Labrador Sea, gravity flows can be generated by direct bedload-rich melt-water discharges from the glacier terminus sides (Hesse et al., 1997). The heads of some of the slope canyons in the Wilkes Land margin appear to originate at each side of the terminus of the shelf troughs (Fig. 4). This suggests that slope

canyon development may have been favored by gravity flows originating from the glacier terminus sides during times of glacial maxima. However, because the present bathymetric maps are based in few data sets, some of them with poor navigation, more precise swath bathymetry is needed to support this observation. Anderson et al. (1979) proposed the isostatic rebound that occurs after the ice sheet retrieves from the shelf as an important mechanism for generation of failures, which would generate gravity flows, in the Antarctic continental margin. Truncation of most of the topset and part of the foresets sequences at or near the seafloor (Fig. 5) and recovery of Pleistocene surface diamictons indicates that grounded ice sheets extended across the continental shelf during the Pleistocene. Isostatic rebound could thus have taken place when ice retreated. No cores longer than 6 m exist from the slope or base-of-the-slope environment. In multichannel reflection profiles, the slope is characterized by a prograding wedge (btrough mouth fanQ), which consists of high-amplitude stratified foreset reflectors (Figs. 5 and 6). Mounded features with disrupted strata are common features at the toes of the successive paleoslope foreset strata (Eittreim et al., 1995), which could be indicative of continued mass wasting activity in this margin. 5.3. Continental rise sedimentation and processes Surface sediment cores from the Wilkes Land continental rise consists of three main components: gravity flow deposits (i.e., debris flows and turbidites), contourites, and hemipelagic deposits (Payne and Conolly, 1972; Hampton et al., 1987b; Escutia et al., 1997, 2002, 2003; Busetti et al., 2003). Upper rise surface sediment consists of mud and silty mud with alternating massive intervals with more abundant IRD, and laminated silty to sandy mud (Payne and Conolly, 1972; Hampton et al., 1987b; Escutia et al., 2002, 2003; Busetti et al., 2003). Massive intervals are bioturbated and contain wellpreserved diatom assemblages deposited in an openmarine environment during interglacial times (Hampton et al., 1987b; Busetti et al., 2003; Escutia et al., 2003). The higher biogenic content, reflected in the higher percent-opal values in the analyzed samples across the slope and rise (Escutia et al., 2003),

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correspond with higher abundance of IRD (Busetti et al., 2003; Escutia et al., 2003). Laminated intervals contain few Pliocene/Pleistocene age fragmented diatoms with reworked Neogene forms deposited during glacial times (Hampton et al., 1987b; Busetti et al., 2003; Escutia et al., 2003). Diatom and paleomagnetic analyses were conducted on some of the continental rise cores (e.g., Cores A8G1 and A10G1; Fig. 3) that are up to 3.15-m long (Escutia et al., 2003). Diatom analyses give Pleistocene ages younger than Marine Isotopic Stage (MIS) 7, and paleomagnetic analyses are consistent with the entire cored section being deposited during Chron C1n (0.0– 0.78 Ma). Based on the composition, texture and structure, laminations in the glacial sediments, they have been interpreted to form from turbidite and bottom contour–current deposition (Hampton et al., 1987b; Escutia et al., 2002, 2003). Pre-Quaternary sediments from the Wilkes Land upper rise have not yet been sampled. The acoustic signature and geometries that characterize channel– levee complexes indicate that gravity flows have been important processes in this margin not only during the Pleistocene but also since the deposition of sequences above the WL-U3 unconformity (Hampton et al., 1987a; Escutia et al., 1997, 2000, 2002; De Santis et al., 2003; Donda et al., 2003; Figs. 7 and 8). Wavy reflectors (Escutia et al., 2002; Donda et al., 2003) and erosional morphologies (Eittreim and Smith, 1987) indicate that erosion and redeposition of fine-grained sediment by bottom contour–currents has been also an important processes on the Wilkes Land continental rise at least since the time of deposition of Sequence WL-S6 (Fig. 8). As seen previously, turbidity currents can originate from slope failure, direct bedload-rich melt-water discharges from the glacier terminus sides (Hesse et al., 1997), and by isostatic rebound. In the Wilkes Land margin, slope canyons are the main conduit for turbidity currents transporting sediment from the outer shelf and slope to the continental rise where they form the channel–levee complexes (Escutia et al., 2000). Antarctic Bottom Water (AABW) flowing from east to west along the Wilkes Land continental slope and rise was first reported by Gordon and Tchernia (1972). Salinity distribution suggested to Gordon and Tchernia (1972), that dense Ross Sea

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water spills into the South Indian Basin along the continental margin at about 1408E, where it flows westward below 2000 m water depth. Polynyas in the Ade´lie Land were also cited as possible regions of high-salinity dense water production (e.g., Zwally et al., 1985; Bindoff et al., 2000). Williams and Bindoff (2003) linked AABW production to the Mertz Polynya and other coastal polynyas west of the Mertz Glacier Tongue. Bindoff et al. (2000) used an acoustic Doppler current-profiler (ADCP) to measure eastward flow velocities with maximum values of 140 cm/s in the vicinity of the shelf break (about 500-m water depth). Eittreim et al. (1971) reported the highest velocities (N15 cm/sec) in the continental slope of the Wilkes Land margin. High-velocities and the shearproducing irregular motions are likely responsible for the turbulence in the bottom waters, which produces a thick (maximum thickness of about 600 m) nepheloid layer about 100 km across in water depths of 3100 m (Eittreim et al., 1971). This nepheloid layer is characterized by high particulate matter content derived from the Antarctic continent (Eittreim et al., 1971). Our observations agree with interpretation of the abyssal plain sediments recovered during DSDP Leg 28 at Site 269 (200 km north from the lowermost rise; Hayes et al., 1975). Leg 28 cored sediments intermittently to a depth of 958 m below seafloor (mbsf). The upper 8 m of the recovered section at Site 269 consisted of Quaternary diatom ooze, silty clays, and very fine sands, and were interpreted as distal abyssal plain sediments with a large percentage of sediment originated by distal turbidity currents (60% turbidite clays, 20% turbidite silt, and 2% of turbidite sand; Piper and Brisco, 1975). Turbidite sands and silts become increasingly common on the proximal parts of the margin (Payne and Conolly, 1972). Pre-Quaternary sediments recovered at Site 269 are indicative of deposition by hemipelagic setting, turbidity currents, and bottom-contour currents, and these processes appear to have been active since at least the early Miocene (Hayes et al., 1975; Piper and Brisco, 1975). Turbidite beds in the lower part of the site consist of silt and clay and become sandier towards the top of the section, which Piper and Brisco (1975) interpreted to result from continental margin progradation. Near the surface (i.e., top 20 m), and within the Quaternary section, they

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report finer-sedimentation more characteristic of a distal setting.

6. The Wilkes Land long-term record of glaciation 6.1. Quaternary record of glaciation 6.1.1. Holocene The Holocene corresponds with a warm period of time that coincides with glacial continental polar conditions in Antarctica. In Wilkes Land, Holocene sedimentary processes are dominated by settling of the products of biogenic activity and by reworking by strong bottom currents. Maximum thicknesses of Holocene diatomaceous ooze are found in deep inner-shelf basins, where sediment accumulates rapidly (i.e., up to F16 m/ky in the Ade´lie Drift), preserving a high to ultrahigh resolution record of varying primary productivity and of fluctuations in the sea-ice cover extent (Dunbar et al., 2003 IODP proposal). Both sea-ice extent and primary productivity are environmental variables strongly related to changes in oceanographic conditions as a function of climate. The laminated Holocene ooze recovered from the Wilkes Land inner-shelf basins resembles the 40 m of oozes recovered from the Palmer Deep west of the Antarctic Peninsula during ODP Leg 178 at Site 1098 (Barker et al., 1999; Escutia et al., 2003). These oozes contain a high-resolution millennial- and centuryscale record of variability in primary production and sea-ice extent (Domack et al., 2001; Barker et al., 2002). The periodicity observed in the Antarctic Peninsula Holocene records correlate with marine and continental records from other areas of the globe (Domack and Mayewski, 1999), which suggest a common driving mechanism for these changes. For example, similar fluctuations, with periodicity of 200– 250 years, from the Bransfield Strait have been considered to result from Holocene neoglacial events (Barcena et al., 1998, 2002) and suggested to Leventer et al. (1996) that solar variability may have a role in driving changes in Holocene climate. The West Antarctic Palmer Deep sediment section serves at present as the Holocene reference section for the Antarctic margin (Domack et al., 2001). The ultrahigh-resolution Holocene record contained in the

Wilkes Land inner-shelf basins (i.e., the Ade´lie Drift) can provide with a unique annual to millennial record of Holocene variability in East Antarctica, which is very relevant to (1) understanding circum-Antarctic response to climate forcing, such as, for example, the role of the El Nin˜o Southern Oscillation (ENSO) on sea-ice extent and productivity, and the ENSO activity during the early and middle Holocene. This record is relevant to test if the physical processes that govern ENSO cycles (i.e., an apparent propagation of sea surface temperature anomalies and atmospheric pressure patterns forced by tropical ocean variability) are the processes connecting the tropics and high latitudes; (2) obtaining the Holocene variability record. Ice cores show the mid-Holocene to have little variability, but the land and some polar marine records show large excursions during the mid-Holocene; (3) assessing the regional or global significance of Holocene events; and (4) understanding the nature an timing of the withdrawal of the East Antarctic Ice Sheet from this margin after the last glacial maximum (Barker et al., 1998). 6.1.2. Pleistocene During the Pleistocene glacial periods of time, grounded ice sheets extend onto the Wilkes Land continental shelf. In the shelf areas occupied by fast moving ice streams, troughs are eroded, and subglacial compacted till is deposited directly below the ice sheet in both troughs and banks. Massive to crudely stratified diamictons are deposited in openmarine conditions in front of the ice streams forming prograding wedges. On the continental slope and rise, downslope gravity flow processes are very active (if not dominant) during the Pleistocene glacial times (Escutia et al., 2003). Hemipelagic diatomaceous mud deposition dominates Pleistocene interglacial sedimentation. On the continental shelf, interglacial mud is not always preserved because oceanic currents sweep it away or it is eroded during successive ice sheet advances. Interglacial Pleistocene sediments are best preserved on the continental rise where they consist of massive bioturbated diatomaceous mud, more IRD-abundant, and with higher percent opal values (Busetti et al., 2003; Escutia et al., 2003). Bottom contour–current erosion and deposition are also active processes on the Wilkes Land continental

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rise during the Pleistocene (Hayes et al., 1975; Hampton et al., 1987b; Escutia et al., 1997, 2000, 2002, 2003; Busetti et al., 2003; Donda et al., 2003). Are bottom contour–current processes active during both glacial and interglacials periods of time? Or are they more dominant/intensified during glacials or during interglacials? In their recent paper, Williams and Bindoff (2003) conclude that, at present (i.e., warm period of time coinciding with polar conditions in Antarctica), the Mertz Polynya and coastal polynyas west of the Mertz Glacier tongue are the source of AABW. They also conclude that if the Mertz Glacier tongue was to break off, the Mertz polynya and the coastal polynyas would become reduced in area, leading to a decrease or even a cessation of the Ade´lie Land Bottom Water (ALBW) production. Based on the above, it is likely that, in the Wilkes Land margin, bottom contour–current processes have been active since the time polar conditions were established in this margin (i.e., 10–14 Ma), and thus, it has been an active process throughout the Pleistocene. Decreased bottom contour–current activity can then be related to either (1) vulnerability of the ice sheet during Pleistocene or (2) masking of bottom contour–current processes by other more dominant processes (e.g., large volumes of sediment delivered by downslope processes). Pleistocene sediments from the Wilkes Land margin can thus provide a record of glacial interglacial cycles. Continental shelf sediments hold the direct record of ice advances and retreats, while continental rise sediments hold the distal record of glaciation. Based on ODP Legs 119 (Hambrey et al., 1991), 178 (Barker et al., 1999) and 188 (O’Brien et al., 2001), the sedimentary record from the continental shelf is expected to be (1) discontinuous because of the repeated erosion by grounded ice sheets advancing onto the shelf during glacial times, and (2) difficult to date because the biogenic sediment deposited during warmer periods of time is removed by bottomcurrents that sweep the continental shelf, such as those active at present (Bindoff et al., 2000; Williams and Bindoff, 2003), and because of erosion during the successive advances of grounded ice sheets onto the shelf. A more continuous and datable Pleistocene section can be obtained by sampling the interbedded glacial laminated and interglacial biogenic massive sediments from the Wilkes Land rise, which hold a record of colder vs. warmer periods of time that are

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linked to the glacial evolution in the Antarctic continent. Sampling the combined Pleistocene record from the Wilkes Land shelf and rise can provide answers to some of the outstanding questions regarding Pleistocene glacial–interglacial cycles and intervals of extreme warmth. For example, (1) the Pleistocene oxygen isotopic record from Prydz Bay indicates as few as three glacial–interglacial cycles during the Pleistocene (Theissen et al., 2003). Pleistocene records from other margins are needed to discern if the Prydz Bay record represents (a) few extreme advances of the ice sheet during the mid-Pleistocene as a result from shelf overdeepening or atmospheric cooling over Antarctica (O’Brien et al., 2004), or (b) the preserved record of advances and retreats of the ice sheet onto the continental shelf with erosion of the sediments containing the record of additional glacial–interglacial cycles. (2) It is not well known at present what is the vulnerability of the ice sheet during Pleistocene climatic optima and its potential impact on (a) global thermohaline circulation as a southern source of melt water discharge (e.g., Mikolajewicz, 1998), and (b) sea level rise well beyond present sea level stand (Rohling et al., 1998). If the WAIS melted, sea level would increase by 5–7 m, having a major impact on life and human activities on Earth. If the less stable portions of the EAIS were to melt in some degree, sea level rise would be even more extensive. Based on isotopic evidence from Prydz Bay, Theissen et al. (2003) report on a period of reduced ice volume and possible warmer conditions during the early to middle Pleistocene. The sedimentary sections from the Wilkes Land, the East Antarctic margin adjacent to a marine based portion of the EAIS, may be ideally located to test the vulnerability to climate change of this sector of the EAIS during the Pleistocene. 6.2. Pre-Quaternary glacial record The pre-Quaternary glacial record encompasses a long time span (i.e., Eocene–Oligocene to present time) that includes the initiation and several important stages in the long-term evolution of the Antarctic Ice Sheet (Figs. 1 and 10). Because pre-Quaternary sediments have not been sampled from the Wilkes Land margin, glacial sedimentation and processes are

72 C. Escutia et al. / Global and Planetary Change 45 (2005) 51–81 Fig. 10. East Antarctic Ice Sheet evolution in the Wilkes Land margin, showing the timing and changes in the glacial regime inferred from continental shelf and rise stratigraphy and sedimentary processes.

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inferred by (1) interpreting the seismic stratigraphy and seismic facies observed in seismic reflection profiles collected from this margin; (2) comparing our interpretations to the closest sediment cores collected during the DSDP Leg 28 at Site 269 on the abyssal plain, taking into consideration that drilling of the Leg 28 sites was not continuous (Hayes et al., 1975); and (3) comparing our seismic observations and interpretations with other Antarctic margins where validation through drilling has taken place such as are the Antarctic Peninsula (ODP Leg 178) and Prydz Bay (ODP Legs 119 and 188). Obtaining the long-term record of glaciation from the Wilkes Land margin, sedimentary sequences have three unique advantages compared to other Antarctic margins: (1) It is the only known margin around Antarctica where an unconformity inferred to represent the onset of glaciation can be traced from the continental shelf to the continental rise deposits, allowing sequences to be linked from shelf to rise (Figs. 5 and 6). (2) Strata below and above the bglacial onsetQ unconformity WL-U3 can be sampled with IODPtype techniques at relative shallow depths in both shelf foreset and lowermost rise–abyssal plain strata (Figs. 5 and 9). Coring of both depositional environments can provide the missing correlations between the proximal and the distal record of glaciation. (3) In the Wilkes Subglacial Basin, the EAIS is grounded below sea level and may have undergone deglaciation during the late Neogene, allowing testing of the vulnerability to climate change of this segment of the EAIS. (4) Most of the sedimentary sequences of the Wilkes Land margin were not affected by tectonic deformation throughout Cenozoic glaciation. Transtensional deformation affected the innermost shelf postrift strata (De Santis et al., 2003), although the age of such a tectonic phase is not known yet. The climate signal contained in the rest of the margin should therefore be relatively simple to interpret as it is in Prydz Bay. (5) The Mertz Polynya and coastal polynyas west of the Mertz Glacier Tongue are the source of AABW (Williams and Bindoff, 2003). The Wilkes Land is thus an ideal location to study global climate changes related to the increase, decrease, or even cessation of AABW production.

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Some of the most relevant unknowns and pending questions from the Eocene–Oligocene transition to the Pliocene that can be addressed by sampling Wilkes Land margin sediments include: (1) Nature and timing of onset of glaciation in this East Antarctic margin sector and ice sheet evolution during the mid-Oligocene. The Eocene and the Oligocene are key time intervals in the development of the Antarctic Ice Sheet. Deep-sea bproxyQ records (e.g., oxygen isotope, global sea level, ice-rafted debris, etc.) indicate the Eocene (~52 to ~34 Ma) to be characterized by a global cooling trend that culminates at the Eocene–Oligocene boundary (~34 Ma), time of development of first Antarctic Ice Sheets (Fig. 1). The mid-Oligocene transition (~30 Ma) represents another major cooling event (Fig. 1), which is associated with a major eustatic fall that likely represents a large Antarctic Ice Sheet expansion. Although it is accepted that the first ice sheet started 34 Ma ago (e.g., Macphail and Truswell, 2004; Barrett, 2003; Cooper and O’Brien, 2004), little is known about (a) the nature and timing of first arrival of ice sheets to the different sectors of the Antarctic continental shelf; and (b) the temporal and spatial evolution of the Antarctic Ice Sheet trough the Oligocene and across the mid-Oligocene transition in each margin sector. (2) Timing and nature of the transition from a dynamic to a more permanent ice sheet and stability of the EAIS during the late Miocene. The late Oligocene to early Miocene transition (at about 24 Ma), recorded as a shift in the benthic isotopic record (Zachos et al., 2001; Fig. 1), resulted in the small dynamic ice sheets of the late Oligocene, rapidly expanding to continental scale in the early Miocene. This ice volume increase is shown in Mg/Ca reconstructions to be equivalent to 90 m of sea level fall. The middle to late Miocene period represents a time of significant expansion in Antarctica with a middle Miocene bclimatic optimumQ at about 15 Ma (Zachos et al., 2001), followed by a cooling trend over the next 6 my (Fig. 1). It is during the middle Miocene interval that EAIS is thought to have evolved into a major and permanent ice sheet. However, there are many questions about the stability of the Antarctic climate and ice during the late Miocene. A variety of indicators from the McMurdo Dry Valleys suggest the maintenance of stable hyper-

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arid cold-desert conditions since 13 Ma (Marchant et al., 2002). Microfossil studies in the Transantarctic Mountains (i.e., glacial deposits in the Sirius Group) and sedimentological work within Antarctic fjords and the continent (i.e., Pagodroma Group) indicate significant climate dynamism extending from the late Miocene through the Pliocene–Pleistocene. (3) Stability or variability of the ice sheet during the Pliocene. Indirect evidence (i.e., sea level changes and ocean floor sediments) suggests that ice volume during the Pliocene was subject to cyclical variability. Since Northern Hemisphere ice sheets were not fully developed, it is thought that sea level changes were driven by fluctuations of the Antarctic Ice Sheet. Many scientists believe the relatively unstable WAIS to have been responsible for these changes, but the role of the much larger and thought to be more stable EAIS remains controversial. The timing of the transition of the EAIS from a polythermal dynamic condition to a predominantly cold stable state is critical to this argument. The long-standing view is that the EAIS became stable in mid-Miocene, but if the EAIS was indeed subject to major fluctuations until Pliocene time, then, taking into account IPCC projections, there is cause for concern about the possibility of the EAIS becoming unstable within the next century. 6.2.1. Oligocene to Pliocene glacial record from the Wilkes Land continental shelf Two main episodes of ice advance with a regional character have been differentiated on the Wilkes Land shelf, represented by unconformities WL-U3 and WLU8. The prograding wedges deposited after each unconformity formed have marked differences in the acoustic geometries of their strata. Low-angle prograding strata between WL-U3 and WL-U8 contrast with the steeply prograding character of strata between unconformity WL-U8 and the seafloor (Fig. 5). Similar geometries of the prograding wedge are reported from Prydz Bay, drilled during ODP Legs 119 (Hambrey et al., 1991) and 188 (O’Brien et al., 2001). The suite of sediments and rocks recovered in Prydz Bay during both legs attest to the general cooling trend in the margin from the late Eocene– Oligocene. Leg 119 recovered reworked preglacial sediment from a section of very gentle foresets,

interpreted by Hambrey et al. (1991) to indicate the history of early glaciation in Prydz Bay during the early Oligocene. Massive and stratified diamicton with some slumping recovered from a section of steeply dipping foresets were interpreted as water-lain till (Cooper et al., 1991) and assigned an age of early Oligocene–middle Miocene. From older to younger, the sediments recovered during Leg 188 are characteristic of a vegetated alluvial plain or braided delta preglacial environment dated late Cretaceous, of an early glacial outwash environment during the late Eocene–early Oligocene, and of a fully glacial environment mostly composed of diamictons deposited by glaciers during the early Pliocene–Holocene (O’Brien et al., 2001; Macphail and Truswell, 2004). Based on the benthic foraminifera oxygen isotopic data, the onset of glaciation in Prydz Bay took place at the beginning of an abrupt cooling trend at the end of the Eocene (34 Ma; O’Brien et al., 2001). Leg 178 shelf drilling in the Antarctic Peninsula margin recovered Pliocene–Pleistocene unsorted and unconsolidated diamicts from the topset strata of a steep prograding wedge, which were interpreted as subglacial deposits (Barker et al., 1999, 2002). With depth, glacial consolidated deposits were obtained from strata imaged in seismic profiles as seaward dipping. The glacial nature and age (late Miocene to earliest Pliocene) of these strata were established with a wide range of depositional environments (subglacial, proglacial, and glaciomarine) represented (Barker et al., 1999). Because the Antarctic Peninsula margin has experienced subsidence until recently, the climatic signal is complicated by the tectonic signal. This has resulted in two different interpretations regarding the depositional environment of the lower consolidated strata: one group considers the sediments recovered to represent deposition in a paleoshelf (topset), with the dip observed being caused by postdepositional subsidence (Barker et al., 2002), the other considers these deposits to form in an upper slope environment (Eyles et al., 2001). Based on the above, the changes in geometry within the Wilkes Land prograding wedge (Fig. 5) likely result from large fluctuations in the ice sheet volume. Unconformity WL-U3 is thought to represent first ice expansion onto the Wilkes Land margin. Ice sheet models (Huybrechts, 1993; DeConto and Pollard, 2003) indicate that the Wilkes Land margin

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becomes glaciated at a later stage than Prydz Bay (Fig. 2). If these simulations hold true, the initiation of glaciation in the Wilkes Land margin should be younger than 34 Ma. This is supported by sediments from ODP Leg 182 Site 1128 in the Great Australian Bight, which show a prominent positive oxygen and carbon isotope shifts in bulk carbonate stable isotope analyses inferred to relate to a major increase in glaciation starting at approximately 33.6–33.42 Ma, with maximum oxygen isotopic values of bulk carbonate at 33.42 Ma (Mallinson et al., 2003). Coincident with the oxygen and carbonate shift, there is a change in clay mineralogy at Site 1128, with smectite as the sole clay mineral until 33.5 Ma, and increase of illite and kaolinite after 33.42 Ma, which Mallinson et al. (2003) infer to correspond to changes in temperature and precipitation in Australia, related to ice sheet expansion in Antarctica. Here we postulate that the expansion of the EAIS with first arrival of the ice sheet to the Wilkes Land margin takes place after 33.42 Ma during the cooling trend in the early Oligocene (~30 Ma), shown in the oxygen isotopic record (Fig. 10), which culminates with a major eustatic fall inferred to result from a large Antarctic ice sheet expansion. After development of WL-U3, early glacial sediments (e.g., outwash deltas) were deposited as lowangle prograding strata under temperate and subpolar conditions with dynamic ice sheets (Fig. 10). The next major change in the regional geometry of the Wilkes Land prograding wedge is across the WL-U8 unconformity, which separates the lower-angle prograding foreset sequences below from steep-angle foreset sequences (Fig. 5). We postulate that the change in geometries of the prograding wedge represents the transition from a regime with dynamic ice sheets to polar conditions with persistent but oscillatory ice sheets sometime at the end of the middle Miocene ¨ (14–10 Ma). We base this hypoth¨climatic optimum esis on the sharp change in the reflector configurations of the progradational wedge, which above the WL-U8 unconformity have similar character to the more recent Pleistocene foresets that we know (i.e., based on surface cores) developed under polar conditions (i.e., by grounded dry-base ice sheets extending to the shelf). We interpret the steep foresets deposited above unconformity WL-U8 to be massive to stratified diamictons of late Miocene to Pleistocene age

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(Fig. 10), similar to those from the thick prograding wedges formed by steep foresets in the Antarctic Peninsula and Prydz Bay (Barker et al., 1999; O’Brien et al., 2001). 6.2.2. Oligocene to Pliocene glacial record from the Wilkes Land continental slope In the continental slope, debris flows, which dominate Quaternary deposition, have been likely an important (if not dominant) process during deposition of pre-Quaternary sequences above unconformity WL-U3. This is suggested by the mounded geometries with chaotic seismic facies reported by Eittreim et al. (1995) at the toes of the successive paleoslopes (Fig. 5). ODP Leg 188 was the first to ever drill on the Antarctic slope btrough mouth fanQ environment to a total depth of 447.5 m below seafloor (mbsf; O’Brien et al., 2001). In multichannel seismic profiles, the continental slope prograding wedge is imaged with stratified continuous reflectors of high-amplitude, which suggest uniform sedimentation. Sediment from the top 5.17 m is younger than 0.66 Ma and is composed of fine-grained hemipelagic sediments. The rest of the 442.3 m sedimentary section consists of clayey silty sands with dispersed rock clasts and minor beds of coarse sands, clays, and sandy clays. These sediments have been interpreted as a series of stacked massive debris flows deposited during glacial times when ice extended to the shelf broke and delivered large volumes of sediment to the upper continental slope (O’Brien et al., 2001). A sample of interbedded hemipelagic material preserved at F215 mbsf has been dated early to middle Pleistocene (O’Brien et al., 2001). The mounded geometries and internal reflector configurations in multichannel seismic profiles from the Wilkes Land slope and base-of-the-slope are indicative of mass wasting processes active in this margin (Figs. 5 and 6). The continental slope in the Wilkes Land margin therefore does not appear to be a good environment for obtaining the record of EAIS glacial advances and retreats. The record from this environment is expected to be of very low-resolution and difficult to interpret because of the low preservation of the datable interglacial sediments and because of the large erosional hiatuses in the sedimentary section.

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6.2.3. Oligocene to Pliocene glacial record from the Wilkes Land continental rise Based on the interpretation of high-resolution and low-resolution multichannel seismic reflection profiles, pre-Quaternary sedimentation on the Wilkes Land continental rise had three main components as it does during the Quaternary: turbidite, contourite, and hemipelagic (Hayes et al., 1975; Piper and Brisco, 1975; Hampton et al., 1987a; Escutia et al., 1997, 2000, 2002; De Santis et al., 2003; Donda et al., 2003). Other Antarctic continental rise settings exhibit similar processes and a similar sequence in the style of deposition to the one observed in the Wilkes Land margin (i.e., an earlier growth stage with high rates of sediment supply and a more recent attenuation stage coinciding with very low sediment supplied to the lower rise). Continental rise sediments were sampled during ODP Leg 178 in the Antarctic Peninsula (Barker et al., 1999, 2002) and Leg 188 in Prydz Bay (O’Brien et al., 2001). Drilling the Antarctic Peninsula drift sites yielded a high-resolution record extending back to 10 my of the general cooling trend in the global climate with a cool late Miocene, a warm early Pliocene, and a cold Pleistocene (Barker et al., 1999). IRD confirmed that the ice sheet was present in the Antarctic Peninsula throughout this period and clay mineralogy shows that it remained sufficiently large for regular grounding line migration to the shelf edge (Barker et al., 2002). Leg 188 drilled a nearly continuous 999-m-thick sedimentary section from the Wild Drift in Prydz Bay dated early Miocene to Pleistocene. This site provided a record of numerous cycles and of significant up-hole changes in the sedimentary record since the earliest Miocene. These up-hole changes include, for example, a decrease in sedimentation rates and an increase in the total clay content (O’Brien et al., 2001). O’Brien et al. (2001) interpret these changes to reflect a shift in the sediment source area to an overdeepened shelf paleoenvironment that produced less terrigenous material and a lower energy current regime starting in middle Miocene time. During the latest Neogene, the Wild Drift received little sediment; most sediment at that time was delivered to the slope as debris flows (O’Brien et al., 2001). The drifts drilled in the Antarctic Peninsula and the Wild Drift in Prydz Bay differ in the apparent dominant process in their

development. In the Antarctic Peninsula, the drifts appear to be dominated by contour–current deposition. In Prydz Bay, the Wild Drift forms by interaction between turbidity and bottom-contour currents. De Santis et al. (2003) discuss in detail the differences and probable causes for the observed differences between the more contouritic depositional systems in the Antarctic Peninsula continental rise and the more turbidite systems in Prydz Bay. Independent of these differences in the sedimentary processes, both margins exhibit a similar sequence in the style of deposition, which suggest the sedimentary architecture of the continental rise results from changes in the glacial state (i.e., temperate vs. polar) of the continent. In the Wilkes Land continental rise and above unconformity WL-U3, the sheeted geometry of continental rise sequence WL-S4 with dominantly stratified reflectors suggests sedimentation of likely distal fine-grained turbidites and hemipelagites. We interpret the acoustic signatures within WL-S5, WLS6, and WL-S7 to record an up-section increase in the proximal character of the turbidite systems. Less numerous and lower relief channel–levee complexes within WL-S5 suggests to us that they represent turbidite deposition in a middle-to-lower-fan environment in contrast with upper fan channel–levee complexes (i.e., more channels with narrower crosssection) and the lobes of the lower fan (i.e., channels are merely small incisions on the seafloor, or paleoseafloor, and overbank deposits blanket the interchannel areas without development of levees). More widespread and higher relief channel–levee complexes within sequences WL-S6 and WL-S7 (Figs. 7 and 8) suggest to us a more complex network of high-relief tributary-like fan channels similar to what Escutia et al. (2000) described from the present day morphology of the Wilkes Land upper-fan systems. We interpret the high sediment supply and the up-section increase in the proximal character of the turbidite systems at the time of deposition of sequences WL-S5, WL-S6, and WL-S7 to result from continuous progradation of the margin during an overall cooling trend (Fig. 10). It is also during deposition of Sequence WL-S6 that we observe for the first time wavy geometries indicative of bottom contour–current deposition. During this time, temperate and subglacial conditions dominates sedimentation on the margin, with dynamic ice sheets conducive to

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the delivery of large sediment loads to the continental rise. At the base of the slope, large mounded and internally chaotic bodies rest on top the WL-U5 unconformity and attest to coeval continental margin instability and delivery of large amounts of sediment to the continental rise at that time (Fig. 6). Reflector configurations within the two uppermost sequences WL-S8 and WL-S9 in the lower rise suggest that there is a decrease in the energy of the depositional environment (Figs. 7 and 8). Channel– levee complexes within sequence WL-S8 in the lower rise exhibit characteristics of a more distal middle to lower submarine fan environment. The more proximal turbidite system signatures within sequences WL-S8 and WL-S9 are found in locations of the upper rise closer to the slope where sequences are thicker (Fig. 6). Turbidite deposition within sequence WL-S9 in the lower rise is mainly observed by overbank sedimentation that onlaps or attenuates previous relief (Figs. 7 and 8). Also, sediment waves within sequence WL-S9 are smaller and more isolated (Figs. 7 and 8). The lower energy depositional environment in the lower rise at the time of deposition of sequences WL-S8 and WL-S9 coincides with a shift in continental rise depocenters to a more proximal environment, as suggested by increased thickness of these units in the upper rise. Decrease in sediment supply to the lower rise during deposition of sequences WL-S8 and WL-S9, corresponds with the time before and after the development of unconformity WL-U8. Earlier, we inferred the WL-U8 unconformity to form in the late Miocene at the end of the middle Miocene bclimatic optimumQ (14–10 Ma) time interval and interpreted the steep foresets above the unconformity to be deposited under polar conditions during the latest Miocene–Pliocene. De Santis et al. (2003) also postulated the decrease in sediment supply to the continental rise during deposition of sequences WL-S8 and WL-U9 to result from the transition to polar conditions with persistent but oscillatory ice sheets (Fig. 10). Here, the decrease in sediment supply during deposition of sequence WL-S8 is inferred to be the response of this margin to increased cooling during the late middle Miocene and thus to record the transition between a dynamic and persistent ice sheets (Fig. 10). Sequence WL-S9 would develop under fully developed polar conditions, as proposed previously by De Santis et al.

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(2003) (Fig. 10). Shelf overdeepening may have also played an important role in decreased delivery of sediments to the continental rise during deposition of the two uppermost sequences, as postulated for the similar decrease in sedimentation rates during the Pliocene–Pleistocene in Prydz Bay (O’Brien et al., 2001, 2004). From the above, sequences WL-S4, WL-S5, WLS6, and WL-S7 are deposited by glacial processes active under temperate to subpolar conditions with dynamic ice sheets between the late Oligocene and the late Miocene (Fig. 10). Based on indirect correlations with DSDP Site 269, we postulate that maximum sediment supply to the continental rise at the time of deposition of sequences WL-S6 and WLS7 may be the response to increased cooling during the middle-Miocene. Sediments from Site 269 show a significant change in sediment composition in the middle Miocene, with older sediments containing sparse nannofossils but no diatoms, and younger sediments containing diatoms and no nannofossils (Hayes et al., 1975). As a result of the general cooling trend, ice sheets become cooler and dryer during the late middle Miocene and late Miocene, a transition that is likely recorded in sequence WL-S8 (Fig. 10). Sequence WL-S9 develops under polar conditions during the latest Miocene, Pliocene, and Pleistocene (Fig. 10). 6.2.4. Oligocene to Pliocene glacial record from the Wilkes Land lowermost rise–abyssal plain Horizontally stratified continuous reflectors that characterize lowermost rise–abyssal plain sedimentation are likely fine-grained sediments such as the ones recovered from DSDP Leg 28 at Site 269 (200 km north of the edge of the continental rise; Hayes et al., 1975). Leg 28 report up-section changes in the lithologies, which is interpreted to reflect substantial progradation of the continental margin (Piper and Brisco, 1975) caused by increasingly cold conditions in the Antarctic continent. Similar sections from the lowermost rise–abyssal plain can provide a distal record of paleoceanographic and paleoenvironmental changes related to the glacial evolution of the continent. Because the bonsetQ of glaciation unconformity can be traced from the continental shelf to this distal setting, and because in both environments this unconformity can be

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reached using IODP-type technologies (Fig. 9), direct links between the shelf and rise records can be established.

7. Conclusions The long-term history of glaciation in the Wilkes Land, from the time of first ice sheet inception through the significant periods of climate change during the Cenozoic (i.e., Oligocene–Miocene boundary [Mi-1 glaciation], middle–late Miocene cooling, Pliocene warm periods, Pliocene–Pleistocene cooling, Quaternary periods of unusual warmth and extreme cold, etc.), is inferred using an integrated geophysical and geological approach. We postulate that the bonsetQ of glaciation in this margin resulted in the development of unconformity WL-U3 between 33.42 and 30 Ma during the early Oligocene cooling climate trend. From the time of the onset, substantial progradation takes place on the shelf with deposition of low-angle foreset strata under temperate to subpolar conditions with dynamic ice sheets. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition at the end of the middle Miocene bclimatic optimumQ (14–10 Ma) to a glacial regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8, likely consist of diamicton deposited by grounded ice sheets extending to the shelf. On the continental rise, progradation of the continental shelf first results in an increase in the energy of the depositional environment (i.e., turbidite seismic facies become more proximal up-sections from WL-S5 to WL-S7) through the deposition of sequences WL-S5, WL-S6, and WLS7, which we infer to be early to middle Miocene in age. Maximum sediment delivery to the continental rise takes place during deposition on sequences WLS6 and WL-S7 during the middle Miocene bclimatic optimumQ (14–10 Ma). A decrease in the sediment supply to the continental rise and a shift in the depocenters to more proximal areas of the margin characterizes sedimentation of the uppermost continental rise sequences WL-S8 and WL-S9. WL-S8 is interpreted to record the final transition between temperate/subpolar and polar conditions in this margin. WL-S9 is interpreted to form under fully developed polar conditions during the Pliocene–

Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope. High sedimentation rates of biogenic sediment characterize the Holocene sediments from inner-shelf basins. These sediments have the potential of providing an ultrahigh record of Holocene climate variability. Our inferences need to be validated by deep sampling (i.e., IODP-type techniques). The most complete record of the history of glaciation in this margin needs to be obtained by sampling both the shelf, which contains the direct (but low resolution) record of glaciation, and the rise, which contains the distal but more complete record of glaciation. The Wilkes Land margin is the only known Antarctic margin where the bonsetQ of glaciation unconformity (WL-U3) can be traced from shelf to the abyssal plain, allowing the links between the proximal and the distal records to be established. The EAIS in the Wilkes Basin is grounded below sea level and thus may have been more sensitive to climate changes in the late Neogene. Thus, the sedimentary sections on the Wilkes Land margin may not only hold the record of the time when the East Antarctic Ice Sheet first reached this margin but also the record of ice sheet fluctuations during the times that the East Antarctic Ice Sheet is thought to be more stable (after 15 Ma-recent). This information is critical for developing reliable models of ice sheet behavior, which may be the basis for future climate predictions.

Acknowledgements This paper has been possible through a coordinated approach to understanding glacial history in the Wilkes Land margin. The approach has depended upon the considerable efforts of coproponents and contributors of site survey data for two IODP drilling proposals, Proposal #482 (Escutia et al.) and an IODP Ancillary Project Letter (Dunbar et al.). Besides the coauthors, coproponents and contributors include, in alphabetical order, Xavier Crosta, Eugene Domack, Takemi Ishihara, Amy Leventer, Rick Murray, Phil O’Brien, and Manabu Tanahashi. This manuscript was greatly improved thanks to the constructive reviews provided by Eugene Domack, German Leitchenkov, and Phil O’Brien.

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