STADIAL-SCALE DRIFT STRATIGRAPHY FOR THE WEST ANTARCTIC ICE-SHEET

Late Pleistocene drift stratigraphy for the marine-based West Antarctic Ice Sheet (WAIS) in the Ross Sea embayment is dominated by late Wisconsin deglacial landsystems of the Ross Sea 1 (RS-1) drift. Older RS drift sheets are considered to date to Marine Isotope Stage 6 or older, consistent with a small number of exposure-age and U/Th dates. Assuming significant sea-level control of WAIS extent, we reexamined selected Wisconsin-age RS drift complexes with ground-penetrating radar (GPR) and cosmogenic dating techniques in search of stadial events. We focus on a RS lateral moraine sequence at 300 m elevation on Hjorth Hill, a coastal headland adjacent to Taylor Valley, where moraine boulders have 3He exposure ages between 40 and 106 Ka. These exposure ages are regarded as problematic because they conflict with extensive radiocarbon dating of the moraines to the late Wisconsin.

We collected GPR profiles at both 50 and 100 MHz along seven 500-m transects layed out in a grid over the Hjorth moraine complex. Using an interpreted dielectric permittivity of 4.8, persistent horizons appear as deep as 34 m. The GPR results show that at least 5 different laterally persistent till units compose the Hjorth Hill moraine complex along with a number of separate glaciolacustrine and glaciofluvial units. The strength and lateral continuity of the first-order reflectors as well as their upslope convergence imply that most of the sequence was deposited through separate WAIS advances. Distinct moraines upslope from and draped on the lateral drift complex, whose stratigraphic relationships are ambiguous in plan-view, can be traced into the drift body with GPR, revealing vertical stratigraphy and relative age relations.

The heterogeneity of the radiometric dates associated with the Hjorth Hill lateral moraines is consistent with the GPR-based interpretation for multiple, stacked WAIS drift-sheets. Hence, we propose that the existing dates are accurate and that further dating will reveal a series of stadial-scale WAIS drift units. Support for this interpretation comes from erratic boulders on Mt. Waesche, Marie Byrd Land, West Antarctica, with exposure ages that imply WAIS thickening within MIS 5. We suggest that the WAIS is sensitive to stadial-scale sea-level fluctuations into and out of the major lowstands of MIS 3-5.