Cosmogenic ³He concentrations in ancient flood deposits from the Coombs Hills, northern Dry Valleys, East Antarctica: interpreting exposure ages and erosion rates

In situ produced cosmogenic He-3 analyses provide independent support for the model of a stable, hyper-arid polar climate persisting in East Antarctica since the mid-Miocene and provide quantitative constraints on long-term rates of erosion within the Dry Valleys. In the Coombs Hills area, a series of cobble- size boulders form mega-ripples with wavelengths of approximately 50 m. Their topographic position and association with features characteristic of scabland, such as stripped, corrugated bedrock surfaces, indicate the boulders were deposited by subglacial floodwaters. Such outburst flooding could only have occurred during overriding of the northern Dry Valleys by a greatly expanded East Antarctic ice sheet. Timing of the overriding episode has been previously assigned to 14.8 to 13.6 Ma by correlation with volcanic ash deposits dated by Ar-40/Ar-39 in the Asgard Range of the Dry Valleys. Cosmogenic He-3 concentrations in clinopyroxene from Ferrar dolerite boulders are consistent with 8.6 to 10.4 Ma exposure, calculated using scaling factors appropriate for Antarctica and assuming zero erosion. These are an tong the oldest surface exposure dates yet measured on Earth, but are not however consistent with the Ar-40/Ar-39 chronology used to define the age of the landscape due to unconstrained levels of erosion. Erosion rates of 0.03-0.06 m Ma(-1) are necessary to have produced the measured boulder exposure age if they were deposited at 14.8 Ma. These are less than half the steady-state erosion rate derived from cosmogenic He-3 in the nearby bedrock surfaces (0.17 m Ma(-1)) and testify to the extreme stability of the landscape