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

Abstract

Peter, "The probable importance of snow and sediment shielding on cosmogenic ages of north-central Colorado Pinedale and pre-Pinedale moraines" (2004 36 Cl ages of Pinedale boulder surfaces by an average of B12%. 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 (B140 kyr BP), calculations indicated that rock-surface erosion rates would have had to range from 5.9 to 10.7 mm kyr À1 to produce the observed 36 Cl 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 36 Cl 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. Published by Elsevier Ltd.

New data for Late Pleistocene Pinedale alpine glaciation from southwestern Colorado

“…In this paper, we present new surface-exposure ages of Pinedale terminal moraines in southwestern Colorado and discuss them in context with previously published surface-exposure ages for Pinedale terminal moraines in north-central Colorado (Benson et al, 2004) and with recalculated surface-exposure ages of Pinedale moraines in west-central Wyoming (Gosse et al, 1995(Gosse et al, , 2003. With knowledge of the sources of error inherent in surface-exposure ages, we compare and contrast methods used by various authors for determining moraine ages and discuss the difficulties in extracting objective information on the initiation, duration, and termination of moraine formation.…”

“…36 Cl and 10 Be determinations Laboratory and numerical procedures for 36 Cl and 10 Be age determinations are the same as those in Benson et al (2004) and are discussed in Appendix B. The chemical data and isotopic ratios of Cl used in the calculation of surface-exposure ages for southwestern Colorado moraine-crest boulders also are given in Appendix B.…”

“…The chemical data and isotopic ratios of Cl used in the calculation of surface-exposure ages for southwestern Colorado moraine-crest boulders also are given in Appendix B. Chemical data used in the calculation of north-central Colorado moraine-crest boulders are tabulated in Benson et al (2004). The isotopic ratios of Cl used in the calculation of surface-exposure ages for north-central Colorado boulders are given in Appendix B and the data necessary for calculation of 10 Be ages of west-central Wyoming (Pinedale) moraine-crest boulders are given in Gosse et al (1995Gosse et al ( , 2003.…”

“…One way to approximate changes in snow cover is to determine changes in effective wetness for a lake at the terminus of a surface-water system containing a Pinedale or Tioga glacier. Benson et al (2004) determined the average effective wetness ðW eff ¼ 4:3Þ of the Sierra Nevada for the past 25 ka, using changes in lake size documented in the Lahontan Basin. The Sierran values of W eff are not strictly applicable to many of the sites mentioned in this paper.…”

Deglaciation and postglacial treeline fluctuation in the northern San Juan Mountains, Colorado

“…(about 35,000 cal yr) and were near their maximum extent about 22,000 yr B.P. (about 26,000 cal yr) (Benson et al, 2004).…”