AGU Fall Meeting 2009

Sublimation of ground ice and erosion of glacial deposits quantified with cosmogenic nuclides, McMurdo Dry Valleys, Antarctica
D. J. Morgan¹; J. Putkonen²; G. Balco³; J. Stone⁴
1. Earth and Environmental Sciences, Vanderbilt University, Nashville, TN, USA.
2. Geology and Geological Engineering, University of North Dakota, Grand Forks, ND, USA.
3. Berkeley Geochronology Center, Berkeley, CA, USA.
4. Earth and Space Sciences, University of Washington, Seattle, WA, USA.

Glacial deposits in the Quartermain Mountains, Antarctica present two apparent contradictions regarding the degradation of unconsolidated deposits. First, these deposits are up to millions of years old, yet they have maintained their meter-scale morphology despite the fact that bedrock and regolith erosion rates in the Quartermain Mountains have been measured at 0.1-4.0 m/Ma. Secondly, ground ice persists in Miocene-aged soils in the Quartermain Mountains even though modeled and measured sublimation rates of ice in Antarctic soils suggest that without any recharge mechanisms ground ice should sublimate in the upper few meters of soil on the order of 1,000-100,000 years. Using the concentration of cosmogenic nuclides Be-10 and Al-26 in bulk sediment samples from depth profiles, we examined how three glacial deposits in the Quartermain Mountains degrade through time. The units sampled include an undated moraine that consists of the Taylor Drift in Beacon Valley, and the Arena (>11.3 Ma) and Quartermain I (>3.5 Ma) tills in Arena Valley. At each sample site, we found that the measured nuclide concentrations are inconsistent with the ages of the deposits, that burial of the sites cannot explain this discrepancy, and that erosion alone does not always explain these concentrations. The exposure model that best explains the measured nuclide concentrations couples deflation of the tills by the sublimation of ice and erosion of the subsequently overlying till. The degradation rates that best match the data range from 0.7-12 m/Ma for the sublimation of ice with initial debris concentrations ranging from 7.9-33%, and erosion of the overlying till at rates of 0.5-1.2 m/Ma. Interestingly, though the results suggest that these units used to contain significant amounts of ground ice, vertical mixing of the tills by cryoturbation or soil creep is not indicated by the cosmogenic nuclide profiles, and erosion appears to be limited to within a few centimeters of the surface. Degradation of these tills without vertical mixing may partially explain how glacial deposits in the MDV maintain their morphology and contain ground ice close to the surface for millions of years.

Contact Information
Daniel J. Morgan, Nashville, Tennessee, USA, 37235, click here to send an email