A Method for Constraining Glacial Boulder Exposure Ages with Bedrock Erosion Rates Utilizing Cosmogenic Ne-21 from the Central Transantarctic Mountains

Monday, 14 December 2015
Poster Hall (Moscone South)
Daniel J Morgan1, Sarah Sams1, Julia Liu1, Katherine Leigh Edwards1, Carson Paige Hedberg1, Kyle Ringger1, Mitchell Stocky1, Ashley Ball2, Michael S Diamond1, Josh Cox3, Eli Orland4, Frank Lyles5, Theodore Bibby6, Collin Giusti6, Erin Hoeft6, Jaakko Putkonen7 and Greg Balco8, (1)Vanderbilt University, Earth & Environmental Sciences, Nashville, TN, United States, (2)Johns Hopkins University, Baltimore, MD, United States, (3)Vanderbilt University, Nashville, TN, United States, (4)Middlebury College, Middlebury, VT, United States, (5)Pomona College, Claremont, CA, United States, (6)University of North Dakota, Grand Forks, ND, United States, (7)University of North Dakota, Harold Hamm School of Geology and Geological Engineering, Grand Forks, ND, United States, (8)Berkeley Geochronology Center, Berkeley, CA, United States
Exposure dating with cosmogenic nuclides can be an underdetermined problem because the measured concentration depends on three unknowns: the nuclides accumulated during previous exposure, the erosion rate, and the time period of exposure. Frequently, assumptions have to be made about inherited nuclide concentrations and erosion rates, which can limit the interpretation of results. For example, if erosion is assumed to be zero, then the measured concentration must be interpreted as a minimum exposure age because an erosion rate greater than zero would require more time to pass to accumulate the measured nuclide concentration. Rock erosion rates are rarely measured, so erosion rates have to be assumed from other sources of data. Here we present a method for determining the rock erosion rate from bedrock of the same lithology as boulders that we want to exposure date. We use cosmogenic Ne-21 concentrations in quartz to determine the bedrock erosion rate, and then apply the measured bedrock erosion rate to constrain the cosmogenic Ne-21 exposure age of glacially transported boulders. Because the glacially transported boulders are the same lithology as the bedrock, and they are from the same general locale and have experienced the same climate conditions during exposure, the boulder erosion rates should be consistent with the measured bedrock erosion rates. We collected samples from two sites within the Central Transantarctic Mountains: Ong Valley (157.5°E, 83.25°S), where the bedrock consists of Hope Granite and the Argo Gneiss, and Moraine Canyon (157.55°W, 86.1°S), where the bedrock is a silicic porphyry of the Wyatt Formation. At both sites, we collected bedrock samples above the glacial limit and boulders from the glacial drifts on the valley floor. Preliminary results are that the bedrock is eroding at rates of 17 - 41 cm/Myrs, averaging 23 cm/yr. The range of erosion rates is used to constrain the age of glacial drifts in these valleys, which vary from 10 kyrs to 4.5 Myrs.