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doi:10.2204/iodp.sp.318.2008

Scientific objectives

The overall aim of drilling the Wilkes Land margin is to obtain a long-term record of Antarctic glaciation and its relationships with global paleoclimatic and paleocenographic changes along the inshore–offshore transect. Of particular interest are testing the sensitivity of the EAIS toward episodes of global warming and the detailed analysis of climatically critical periods in Earth climatic evolution coupled to the Antarctic cryosphere (i.e., the Eocene–Oligocene and Oligocene–Miocene glaciations, upper Miocene, Pliocene, and the last deglaciation) when the Antarctic cryosphere formed in a step-wise fashion, and while waxing and waning evolved to assume its present day configuration, characterized by a relatively stable EAIS.

To attain these objectives we will drill and analyze sedimentary records along the inshore–offshore gradient to constrain the age, nature, and environments of deposition of the previously only seismically inferred glacial sequences in the Wilkes Land continental shelf rise and abyssal plain (Fig. F10). Of particular note are stratigraphic intervals that have the potential of preserving records of the key phases of the evolution of the Antarctic cryosphere in general, and the EAIS in particular, like the Eocene–Oligocene and Oligocene–Miocene transitions, the middle and late Miocene, warm early Pliocene events, and Holocene climate variability (Fig. F11). The expected improved chronostratigraphy and integrated multidisciplinary climatic proxy record-based reconstructions are essential to provide accurate constrains for models of the dynamic development of the Antarctic ice sheet and sensitivity to global climate change (Fig. F2).

Specific scientific objectives

1. Timing and nature of the onset of glaciation at the Wilkes Land margin.

The timing and nature of the first arrival of the ice sheet at the Wilkes Land margin, the so-called "onset" of glaciation, is presently inferred to have occurred during the earliest Oligocene (Fig. F10). The late middle Eocene to early Oligocene is universally regarded to represent a long-term episode of global climatic cooling, some time during which the Antarctic ice sheet developed. Ice sheet development is presumed to have been a response to decreasing atmospheric CO₂ values rather than the opening of Southern Ocean conduits like the Drake and Tasman gateways (e.g., Pagani et al., 2005; DeConto and Pollard, 2003a, 2003b; Huber et al., 2004). For example, earlier ODP drilling around Tasmania (ODP Leg 189) indicated that the opening and deepening of the Tasmanian Gateway are much older than the Eocene/Oligocene boundary (Brinkhuis et al., 2003a, 2003b; Sluijs et al., 2003; Huber et al., 2004; Stickley et al., 2004). The pronounced deep-sea benthic foraminifer oxygen earliest Oi-1 (Miller et al., 1985; Zachos et al., 1997; Coxall et al., 2005; Pälike et al., 2006) is widely regarded to mark the strongest step of rapid continental ice growth on Antarctica with concomitant strong sea level response (Fig. F11). However, recent studies (e.g., Coxall et al., 2005) indicate that this onset was in fact a two-step phased event, in line with model predictions of DeConto and Pollard (2003a, 2003b) that the EAIS was established somewhat later than the WAIS (Fig. F2). Following the initiation of a significant Antarctic cryosphere, indications that it was relatively unstable with cyclic alternations of waxing and waning that show strong orbital forcing components (e.g., O'Brien, Cooper, Richter, et al., 2001; Barrett et al., 2003; Pälike et al., 2006; DeConto et al., 2007) are present. The mid-Oligocene transition (Rupelian/Chattian boundary; ~30 Ma), likely reflected by the Oi-2b isotope event (Van Simaeys et al., 2005; Pälike et al., 2006) represents another strong cooling phase associated with a major eustatic fall that likely represents a large Antarctic ice sheet expansion. Cape Roberts drilling in the Ross Sea and during ODP Leg 188 in Prydz Bay suggests the onset of glaciation at these margins occurred at 34–33 Ma and 35 Ma, respectively (e.g., Macphail and Truswell, 2004; Barrett, 2003; Cooper, O'Brien, and Richter, 2004). Ice sheet development models (Huybrechts, 1993; DeConto and Pollard, 2003a, 2003b; DeConto el al., 2007) suggest that the arrival of the first ice sheet to the Wilkes Land margin should have taken place at a somewhat later time (Fig. F2). Constraints on the age and nature of the onset of glaciation in the Wilkes Land margin expected from continental shelf sites (proposed Site WLSHE-09B or alternates) and abyssal plain proposed Site WLRIS-02A (Fig. F12) are therefore essential to providing age constraints for models of Antarctic ice sheet development.

2. Fluctuations in the glacial regime during the Miocene (?) and transition from wet-based to cold-based glacier regimes (late Miocene–Pliocene?).

The latest Oligocene to middle Miocene was characterized by a wet-based dynamic ice sheet that fluctuated in size. The Oligocene/Miocene boundary (~23 Ma) is marked by a major excursion in benthic isotopes (Mi-1 glaciation) (e.g., Miller et al., 1985; Zachos et al., 2001) (Figs. F1, F11), In the early Miocene, a general trend towards moderately larger ice sheets, tracking global cooling, was interrupted by the middle Miocene "climatic optimum" from ~17 to 14 Ma (Zachos et al., 2001) (Figs. F1, F11). At the mid-Miocene transition (~14 Ma) and shortly afterwards, again tracking apparent renewed global cooling, the benthic oxygen isotopic records imply that the EAIS evolved from a wet-based and dynamic setting into a cold-based semipermanent ice sheet. However, even this aspect is highly controversial because some records from the Antarctic continent and margin indicate the presence of a highly dynamic ice sheet from late Miocene into early Pliocene times (e.g., Hambrey and McKelvey, 2000a, 2000b; Webb et al., 1996; Hambrey et al., 2003; Whitehead and Bohaty, 2003; Whitehead et al., 2003, 2004). The glaciomarine continental shelf deposits expected to be recovered from proposed Site WLSHE-08A or alternate should provide the required chronostratigraphic and paleoenvironmental records to help solve this controversy (Fig. F11).

3. Distal record of climate variability during the late Neogene and the Quaternary.

The record of the middle Miocene climatic optimum and the transition from a dynamic to a persistent ice sheet, inferred to have occurred at the Wilkes Land margin during the late Miocene–Pliocene (Fig. F10), is planned to also be sampled at the continental rise proposed Site WLRIS-04A (or alternate) (Fig. F11). Additionally, at this site we expect to test the stability of the ice sheet during the late Miocene and the extreme warm early Pliocene events, which has been the subject of almost continuous debate for more than two decades (e.g., Hardwood and Webb, 1998; Stroeven et al., 1998). A key question is whether relatively short warm intervals can cause a loss in ice sheet volume once a stable ice sheet is thought to be in place (i.e., since the middle–late Miocene). The marine oxygen isotope record suggests warming in the earliest Pliocene, culminating at ~3 Ma during the mid-Pliocene climate optimum (e.g., Kennett and Hodell, 1995; Zachos et al., 2001). Marine sediments exposed on land show evidence for a dynamic ice sheet during the late Miocene–early Pliocene as well as for early Pliocene warming. The marine record from drilling in Prydz Bay, the Ross Sea, and the Antarctic Peninsula also shows evidence for repeated advances and retreats of the Antarctic ice sheet during the late Miocene and early Pliocene. For example, the silicoflagellate assemblages at Site 1165 in Prydz Bay pinpoint three intervals within the Pliocene (3.7, 4.3–4.4, and 4.6–4.8 Ma) with sea surface temperatures in the Southern Ocean roughly 5°C warmer than today (Whitehead and Bohaty, 2003). In the Antarctic Peninsula a strong decrease in sea ice coverage starting at 5.3 Ma and maintained during the early Pliocene is indicated by opal deposition (Grützner et al., 2005; Hillenbrand and Ehrmann, 2005). Diatom stratigraphic analyses in these sediments show three warming events between 3.5 and 3.7 Ma, which can be also recognized in cores from the Antarctic Peninsula, implying that these events were of continent-wide significance (Escutia et al., 2007)

Indirect evidence (i.e., sea level changes and ocean floor sediments) also suggests that ice volume during the Pliocene was subject to cyclical variability. Because 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 that the relatively unstable WAIS, grounded below sea level and thus thought to be less stable, was responsible for these changes. The role of the much larger and presumed 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 eastern Wilkes Land margin receives sediment delivered through the Wilkes subglacial basin, where the EAIS is partly grounded below sea level and thus may have been more sensitive to climate changes in the late Neogene. The record of ice sheet fluctuations during the times that the EAIS is thought to be more stable (after 15 Ma–Holocene) is critical for developing reliable models of ice sheet behavior, which may be the basis for future predictions of Antarctic ice sheet stability as a response to global climate change.

4. Ultrahigh resolution Holocene record.

Addressing questions of the circum-Antarctic and global response to climate forcing will advance our understanding of the relative roles of the Pacific, Atlantic, and Indian Oceans in influencing decadal- to millennial-scale climate variation during the Holocene. In addition, these data will help the assessment of the forcing factors (solar, ocean-atmosphere interaction, and volcanic) responsible for climate change over the past 10,000 y. A continuous Holocene section from the Indian Ocean sector of the East Antarctic margin is desirable as it will provide a comparison to existing Pacific Ocean records, such as those from the Palmer Deep (Antarctic Peninsula-ODP Leg 178) and Ross Sea. In particular we note that modern East Antarctic margin climate is not strongly influenced by ENSO, as is the case for the Arctic Peninsula (Domack et al., 1993, 2001, 2003, 2005; Shevenell et al., 1996; Shevenell and Kennet, 2002; Leventer et al., 1996, 2002; Domack and Mayewski, 1999) and Ross Sea (Leventer and Dunbar, 1988; Leventer et al., 1993; Cunningham et al., 1999; Domack et al., 1999). Rather, this region responds to variability in the SAM, drainage from the East Antarctic ice sheet, and the relative strength of the polar easterlies. Variability in these signals over interannual to millennial timescales needs to be established if we are to understand how forcing factors, such as solar variability, ocean-atmosphere interactions, orbital parameters, and volcanic activity, influence climate and oceanographic processes in the Southern Ocean. Development of high-quality, high-resolution Holocene climate records from the East Antarctic margin is a necessary step toward understanding the circum-Antarctic response to climate forcing and addressing similarities, differences, and possible links to the global record (as in Domack and Mayewski, 1999, for example). These data will help us evaluate the response of the EAIS and margin to global warming. Scientific questions specific to the Adélie Drift proposed drilling are:

1. What was the response of a cold-based glacial system to global and regional Holocene climate fluctuations? Was the response similar to and/or synchronous with marine records obtained from the Antarctic Peninsula and Ross Sea?
2. Was East Antarctica substantially influenced in a substantial way by solar cycle variability? Are the 90, 200, and 400 y cycles of paleoproductivity, as seen in the Palmer Deep, recorded here as well?
3. Are global climatic events such as the Little Ice Age, Holocene climatic optimum, and Younger Dryas preserved in the Eastern Antarctic margin record? More broadly, does the paleoclimatic record from the Eastern Antarctic margin demonstrate synchroneity with Northern Hemisphere records (see Domack and Mayewski, 1999) and, during deglaciation, with other parts of the Antarctic Margin (Siegert et al., 2008)?
4. With annual resolution through at least part of the Holocene, can we observe clear interannual variability in the sea ice extent and/or wind regime (an established fact at the Palmer long-term ecological research location in the Antarctic Peninsula, where this is linked to ENSO)?

Additional objectives

Drilling in the Wilkes Land margin will help assess the main controls on sediment transport and deposition on ice-dominated continental shelves and rises in order to test present architectural models of glacial processes and facies for high-latitude margins. Efforts should be made to understand the controlling factors in continental rise deposition in order to make more informed interpretation about their significance in terms of Antarctica's glacial evolution. For example, what is the influence of ice sheet development and evolution that result on the development of the large mounded deposits (i.e., up to 700 m relief) and large upper-fan channel-levee complexes (i.e., 900 m relief) on the continental rise? What caused the shift in depocenters landward causing a decrease in sediment supply to the continental rise deposits? In these mixed turbidite and contourite systems, how we can differentiate the glacial advances and retreats signal of the gravity flow deposits from the paleoceanographic signal represented in the bottom-contour current deposits?

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