A 3-D Sedimentary Model of Frozen Terrain in Taylor Valley, Antarctica from Ground Penetrating Radar: Implications for the West Antarctic Ice Sheet During the Last Deglaciation and the Physical Properties of Sediment in the Ross Sea Region

Abstract:

We used GPR at 100, 400, and 900 MHz to resolve whether frozen geomorphic features in Taylor Valley, Antarctica, were deposited in large, deep lakes fronting an expanded West Antarctic Ice Sheet in the last glacial cycle or were in direct contact with this ice. Answering this question is important for resolving how the WAIS responds to climate change, past and future, as well as for predicting change in the frozen Antarctic landscape. Here, we present a GPR-based, 3-D model for one terrace on the valley side (7 x10³ m²) that has been interpreted as a delta built at the margin of an extensive proglacial lake adjacent to the WAIS during the last deglaciation.

We interpreted several GPR reflections based on their geometry and polarity sequence. One high-amplitude reflection with negative polarity is nearly horizontal over much of the terrace. We suggest it represents ice-rich layers that formed on the floor of ephemeral ponds. A sloping, high-amplitude reflection of positive polarity at the base of the terrace is an ice-poor, compacted, deformed erosion surface. We subdivided the terrace into GPR units or facies. One unit has geometry and internal architecture consistent with a block of ice-contact glaciofluvial sediment. Another is devoid of reflections. Yet another drapes the terrace flanks that face both the open valley and a tributary stream valley. The latter unit also contains reflection architecture that is known to indicate deposition by fluvial processes.

The GPR evidence is incompatible with the deposition of this terrace as a delta. Rather, the terrace is likely a wedge of ice-contact glaciofluvial and glaciolacustrine sediment mixed with diamicton deposited at the margin of an active Taylor Valley lobe of the WAIS, in effect a kame terrace. The GPR of the terrace signifies valley glacier advance upslope, stillstand, and retreat to emplace the wedge of sediment with lee-side facing the open valley on an eroded sloping surface.

If this deposit is representative, our interpretation implies that the grounded WAIS was more extensive during the last glacial cycle than is currently believed. Further, it implies that the ice-free valleys of the Transantarctic Mountains that open to the Ross Sea contain an extensive terrestrial retreat sequence of glacial ice-rich sediments, not fine-grained, ice-poor proglacial lake sediment.