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doi:10.2204/iodp.proc.318.101.2011

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 paleoceanographic changes along the inshore–offshore transect (Fig. F6). Of particular interest are testing the sensitivity of the EAIS to episodes of global warming and the detailed analysis of critical periods in Earth’s climate history (i.e., the Eocene–Oligocene and Oligocene–Miocene glaciations, middle Miocene, Pliocene, and the last deglaciation) during which the Antarctic cryosphere evolved in a step-wise fashion to ultimately assume its present-day configuration, characterized by a relatively stable EAIS.

To attain these objectives we obtained sedimentary records along the inshore–offshore gradient to constrain the age, nature, and environments of deposition inferred from seismic surveys of the Wilkes Land continental shelf, rise, and abyssal plain (Fig. F12). The expected improved chronostratigraphy and integrated multidisciplinary climatic proxy reconstructions are essential to provide accurate constraints 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 inferred to have occurred during the earliest Oligocene (Fig. F12). The late middle Eocene to early Oligocene is thought to represent a long-term episode of global cooling, the culmination of which led to Antarctic ice sheet development. Ice sheet development is presumed to have been a response to decreasing atmospheric greenhouse gas concentrations (Figs. F1, F2) 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 deepening of the Tasmanian Gateway is significantly 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 δ¹⁸O Oi-1 isotope event (Miller et al., 1985; Zachos et al., 1997; Coxall et al., 2005; Pälike et al., 2006) is widely regarded as marking the strongest step of rapid continental ice growth on Antarctica (Fig. F1) with concomitant strong sea level response. However, more recent studies (e.g., Coxall et al., 2005) indicate that this onset was in fact a two-step phased event. In any case, following the initiation of a significant Antarctic cryosphere, there are many 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). 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. Cores from the Ross Sea (Cape Roberts Project) suggest that the onset of glaciation at that margin occurred at 34–33 Ma (e.g., Florindo et al., 2005). Results from ODP drilling in Prydz Bay are inconclusive regarding the timing of the onset of glaciation along the Antarctic margin in that sector (e.g., Macphail and Truswell, 2004; Cooper et al., 2004). Ice sheet development models (Huybrechts, 1993; DeConto and Pollard, 2003a, 2003b; DeConto et al., 2007) suggest that the arrival of the first ice sheet to the Wilkes Land margin may 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 (U1358 and U1360) and continental rise Sites U1355 and U1356 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 foraminifer δ¹⁸O (Mi-1 glaciation) (e.g., Miller et al., 1985; Zachos et al., 2001, 2008) (Fig. F1). In the early Miocene, a general trend toward moderately larger ice sheets, tracking global cooling, was interrupted by the middle Miocene climatic optimum from ~17 to 14 Ma (Zachos et al., 2001; Lewis et al., 2007) (Fig. F1). At the mid-Miocene transition (~14 Ma) and shortly afterward, again tracking apparent renewed global cooling, the benthic foraminifer oxygen isotopic records imply that the EAIS evolved from a wet-based 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; Naish et al., 2009; Escutia et al., 2009). The glaciomarine continental shelf deposits recovered from Site U1358 and continental rise deposits from Site U1356 are expected to provide the required chronostratigraphic and paleoenvironmental records to help solve this controversy.

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. F12), were sampled at the continental rise Sites U1359 and U1361. Additionally, at these sites we planned to explore the stability of the ice sheet during the late Miocene and, together with the record obtained at Site U1358, the extremely 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, 2008; Lisiecki and Raymo, 2005). 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 cover 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 also can be recognized in cores from the Antarctic Peninsula and Prydz Bay, implying that these events were of continent-wide significance (Escutia et al., 2009).

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 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 (Figs. F4, F5) 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 in response to global climate change.

4. Ultrahigh resolution Holocene record

The nature, range, and rates of Holocene climate change remain critical research targets, as this information defines the background variability against which we assess anthropogenic change. Because atmospheric pCO₂ levels vary by <8% during most of the Holocene, this interval also provides a means of assessing the influence of other climate change forcing factors (solar, ocean-atmosphere interaction, and volcanic). Holocene records are rare from the Antarctic, yet the phenomenon of polar amplification combined with evidence for significant Holocene variability in the mid-latitudes of the Southern Hemisphere (Gilli et al., 2005; Lamy et al., 2001; Theissen et al., 2008; Baker et al., 2001; and many others) suggests the signal may be large. The absence of ice advance over the Antarctic continental shelf during the Holocene means that rapidly accumulating sediments are preserved, whereas the sediment records of previous interglacials have mostly been removed by glacial erosion. Site U1357 contains nearly 200 m of laminated, presumably varved, Holocene diatomaceous sediments. If continuously (or nearly so) varved, it will be the first of its kind from the Antarctic margin. This will allow analysis of changes in wind regime, water temperature, and sea ice conditions at annual to decadal timescales and permit correlation to rapidly accumulating ice-core records from Antarctica’s coastal ice domes.

A continuous Holocene section from the Australian sector of the East Antarctic margin will also 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 Antarctic Peninsula (Domack et al., 1993, 2001, 2003, 2005; Shevenell et al., 1996; Shevenell and Kennett, 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 EAIS, 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 (i.e., as in Domack and Mayewski, 1999). These data will help us evaluate the response of the EAIS and margin to global warming. Scientific questions specific to the Adélie Drift drilling are

1. What was the response of the East Antarctic margin 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 by solar cycle variability? Are the 90, 200, and 400 y cycles of paleoproductivity and sea ice extent, as seen in the Palmer Deep, recorded here as well?

3. How do global climatic events such as the Little Ice Age, Holocene climatic optimum, and Younger Dryas affect the East Antarctic margin? More broadly, does the paleoclimatic record from the East Antarctic margin demonstrate synchronicity 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) and what does this tell us about the recent trend toward great sea ice extent in East Antarctica (Zhang, 2007; Turner et al., 2009)?

5. How is the SAM modulated through time?

If sufficient benthic signal carriers are present, we can address questions about Holocene variability in Antarctic Bottom Water (AABW) production along the Wilkes Land margin (current estimates are that as much as 25% of all AABW may be generated in the nearby Mertz polynya) (Rintoul, 1998; Marsland et al., 2004; Williams et al., 2008).

Additional objectives

Wilkes Land margin cores 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 will be made to understand the controlling factors in continental rise deposition in order to make more informed interpretations about their significance in terms of Antarctica’s glacial evolution. For example, how does ice sheet development and evolution influence or control the development of the large mounded deposits (i.e., as much as 700 m relief) and large upper-fan channel-levee complexes (i.e., 900 m relief) on the continental rise? What factors produced 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 gravity flow depositional signal of glacial advances and retreats from the paleoceanographic signal represented in the bottom contour-current deposits? Finally, tying biostratigraphic datums to the geomagnetic polarity timescale (GPTS) has been notoriously difficult at high southerly latitudes. Cores obtained from the Wilkes Land expedition will provide the opportunity to improve the magnetostratigraphic calibration for the biostratigraphic zonations.

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