New data for Late Pleistocene Pinedale alpine glaciation from southwestern Colorado

Abstract

New cosmogenic surface-exposure ages of moraine-crest boulders from southwestern Colorado are compared with published surface-exposure ages of boulders from moraine complexes in north-central Colorado and in west-central (Fremont Lake basin) Wyoming.10 Be data sets from the three areas were scaled to a single 10 Be production rate of 5.4 at/g/yr at sea level and high latitude (SLHL), which represents the average 10 Be production rate for two high-altitude, mid-latitude sites in the western United States (US) and Austria. Multiple nuclide ages on single boulders indicate that this 10 Be production rate yields ages comparable to those calculated with a commonly used 36 Cl production scheme. The average age and age range of moraine-crest boulders on terminal moraines at the southwestern Colorado and Wyoming sites are similar, indicating a retreat from their positions $16.8 36 Cl ka (Cosmogenic ages in this paper are labeled 10 Be or 36 Cl ka or just ka when both 10 Be or 36 Cl ages are being discussed; radiocarbon ages are labeled 14 C ka, calibrated radiocarbon are labeled cal ka, and calendar ages are labeled calendar ka. Errors (71s) associated with ages are shown in tables. Radiocarbon ages were calibrated using the data of Hughen et al. (Science 303 (2004) 202). This suggests a near-synchronous retreat of Pinedale glaciers across a 470-km latitudinal range in the Middle and Southern Rocky Mountains. Hypothetical corrections for snow shielding and rock-surface erosion shifts the time of retreat to between 17.2 and 17.5 10 Be ka at Pinedale, Wyoming, and between 16.3 and 17.3 36 Cl ka at Hogback Mountain, Colorado.

Deglaciation of the Colorado Rocky Mountains following the Last Glacial Maximum

“…36 Cl ages from LLGM moraines at Hogback Mountains on the eastern side of the San Juan Mountains (Fig. 3, site C) suggest that post-LLGM recession may have begun there as late as 17.5 ka (Benson et al, 2005). Those ages are not, however, recalculated with more recent production rates, and are likely too young compared to the recalculated 10 Be ages.…”

“…The two uncertainties are summed in quadrature to produce an estimate of uncertainty of the reported mean age. To insure consistency across studies, for studies where authors have presented multiple interpretations of CRN ages depending on post-depositional erosion and shielding assumptions (e.g., Benson et al, 2004Benson et al, , 2005Benson et al, , 2007Brugger, 2007), we utilize the values reported assuming zero-erosion/zero-shielding histories.…”

“…Ward et al (2009) used this chronology to constrain a 2-D numerical mass-balance and flow model (Kessler et al, 2006) of the glacier through the deglaciation interval to provide insights to ice dynamics and ELA change. Benson et al (2005) reported fifteen 36 Cl moraine-boulder surface-exposure ages from the LLGM outlet glaciers at Continental Reservoir and Hogback Mountain on the east side of the range (Fig. 2, sites B and C).…”

“…2, site M) four LLGM moraine-boulder surface-exposure ages are available including one 10 Be (Ward et al, 2009), one 26 Al (Schildgen, 2000), and two 36 Cl ages (Benson et al, 2005). Ward et al (2009) reported eighteen 10 Be ages from upvalley polished bedrock in the drainage.…”

Surface-exposure ages of Front Range moraines that may have formed during the Younger Dryas, 8.2calka, and Little Ice Age events

“…Laboratory and procedures for 10 Be age determinations are the same as those in Benson et al (2005). The chemical and isotopic ratios used in the calculation of surfaceexposure ages are given in Table 3.…”