The probable importance of snow and sediment shielding on cosmogenic ages of north-central Colorado Pinedale and pre-Pinedale moraines

Authors

Larry Benson, University of Colorado at BoulderFollow
Richard Madole, US Geological Survey
William Phillips, University of Edinburgh
Gary Landis, US Geological Survey
Terry Thomas, New Mexico Bureau of Geology
Peter Kubik, Honggerberg

Date of this Version

2004

Citation

Quaternary Science Reviews 23 (2004) 193–206

Abstract

Eight uncorrected ³⁶Cl ages for Pinedale boulders in north-central Colorado fall in the range 16.5 to 20.9 kyr. ¹⁰Be age determinations on four of five boulders are in close agreement (<6% difference) with ³⁶Cl determinations. Hypothetical corrections for snow shielding increased the ³⁶Cl ages of Pinedale boulder surfaces by an average of ~12%. Most ages for pre-Pinedale (Bull Lake) boulders fall within marine-isotope stage (MIS) 5, a time when continental and Sierran ice accumulations were small or nonexistent. Under the assumption that these boulders were deposited on moraines that formed before the end of MIS 6 (~140 kyr BP), calculations indicated that rock-surface erosion rates would have had to range from5.9 to 10.7mmkyr^‒1 to produce the observed ³⁶Cl values. When compared to rates that have been documented for the past 20 kyr, these erosion rates are extremely high. Snow shielding accounts for 0–48% of the additional years needed to shift pre-Pinedale dates to MIS 6. This suggests that some combination of snow shielding, sediment shielding, or ³⁶Cl leakage has greatly decreased the apparent ages of most pre-Pinedale boulders. Inability to account for the effects of these processes seriously hinders the use of cosmogenic ages of pre-Pinedale boulders as estimators of the timing of alpine glaciation.