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doi:10.2204/iodp.sp.318.2008

Abstract

Understanding the evolution and dynamics of the Antarctic cryosphere, from its inception during the Eocene–Oligocene transition (~33 Ma) through the significant periods of climate change during the Cenozoic, is not only of major scientific interest but also is of great importance for society. The transition from Greenhouse to Icehouse Earth conceivably was the most significant step in large-scale planetary change, impacting global sea level, albedo, and oceanographic and biotic evolution, among other changes. State-of-the-art climate models combined with paleoclimatic proxy data suggest that the main triggering mechanism for initial inception and development of the Antarctic glaciation was the decreasing levels of CO₂ concentration in the atmosphere. With current rising atmospheric greenhouse gases resulting in rapidly rising global temperatures, studies of polar climates, and the Antarctic cryosphere behavior in particular, are prominent on the research agenda.

Drilling the Antarctic Wilkes Land margin is designed to provide a long-term record, obtained from sedimentary archives along an inshore to offshore transect, of Antarctic glaciation and its intimate relationships with global climatic and oceanographic change. Stratigraphic interpretations indicate that the Wilkes Land record will include the critical periods in Cenozoic Earth climate evolution when the cryosphere formed, likely in step-wise fashion, and subsequently evolved to assume its present-day configuration. The principal goals are

1. To obtain the timing and nature of the first arrival of ice at the Wilkes Land margin (referred to as the "onset of glaciation") inferred to have occurred during the earliest Oligocene (Oligocene isotope event-1),
2. To obtain the nature and age of the changes in the geometry of the progradational wedge interpreted to correspond with large fluctuations in the extent of the East Antarctic Ice Sheet (EAIS) and possibly coinciding with the transition from a wet-based to a cold-based glacial regime (late Miocene–Pliocene?),
3. To obtain a high-resolution record of Antarctic climate variability during the late Neogene and Quaternary, and
4. To obtain an unprecedented, ultrahigh resolution (i.e., annual to decadal) Holocene record of climate variability.

The Wilkes Land drilling program is designed to constrain the age, nature, and paleoenvironment of deposition of the previously only seismically inferred glacial sequences. Determining the chronostratigraphy of the Wilkes Land sediments, which is at present nonexistent, is critical to ground-truth the existing glacial-stratigraphic and ice sheet volume models for this margin. Ice sheet models suggest that the Wilkes Land margin became glaciated in the later stages of East Antarctic glaciation, after Prydz Bay and the Weddell Sea; therefore, it is presumed to be more sensitive to future temperature changes. Drilling the Wilkes Land margin has a unique advantage in that Unconformity WL-U3, inferred to separate preglacial strata below from glacial strata above in the continental shelf, can be traced to the continental rise deposits, allowing sequences to be linked from shelf to rise. Because strata below and above the "glacial onset" unconformity can be sampled at relatively shallow penetration depths, the record of the onset of glaciation can be obtained during a single drilling expedition from two depositional environments, the shelf foreset (proposed Sites WLSHE-07, WLSHE-09, WLSHE-10, and WLSHE-11) and the abyssal plain hemipelagic (proposed Site WLRIS-02A) strata. The shelf foreset section provides a direct record of first occurrence of grounded ice but one that is less continuous and harder to date. The rise hemipelagic section provides an indirect record of glaciation but one that is more continuous and easier to date. Constraints on the age and nature of the Wilkes Land glacial sequences is essential to provide age constraints for models of Antarctic ice sheet development.

The EAIS in the Wilkes subglacial basin is grounded below sea level and therefore may have been more sensitive to climate changes in the late Neogene. The sedimentary sections on the Wilkes Land margin may therefore not only hold the record of the time when the EAIS first reached this margin, but also the record of ice sheet fluctuations during times when the EAIS is thought to be more stable (15 Ma–recent). This information is critical for developing reliable models of future Antarctic ice sheet behavior.

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