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

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 from 5.9 to 10.7 mm kyr −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.