Cosmogenic ¹⁰Be exposure dating of Bull Lake and Pinedale moraine sequences in the upper Arkansas River valley, Colorado Rocky Mountains, USA

Abstract

Many formerly glaciated valleys in the western United States preserve detailed glacial features that span the penultimate glaciation through the last deglaciation; however, numerical age control is limited in many of these systems. We report 35 new cosmogenic 10Be surface exposure ages of moraine boulders in the Sawatch Range, Colorado. Eight ages suggest Bull Lake moraines in Lake Creek (range: 132–120 ka, n = 4) and Clear Creek (range: 187–133 ka, n = 4) valleys may correlate with Marine Isotope Stage 6. In Lake Creek valley, 22 10Be ages from Pinedale end moraines average 20.6 ± 0.6 ka, and 5 10Be ages from a recessional moraine average 15.6 ± 0.7 ka, indicating that glaciers occupied two extended positions at ~21–20 and ~16 ka. The glacial extent dated to ~16 ka was nearly as great as that of the earlier glacial phase, suggesting that climate conditions in the Colorado Rocky Mountains at this time were similar to those of the last glacial maximum. Combining these moraine ages with seven previously published 10Be ages from cirque and valley-bottom bedrock reveals that the Lake Creek paleoglacier lost 82% of its full glacial length in ~1.5 ka and was completely deglaciated by ~14 ka.

Delayed and rapid deglaciation of alpine valleys in the Sawatch Range, southern Rocky Mountains, USA

“…However, once retreat was underway, all sites observed thus far reveal that glaciers completely retreated at least up to their cirques prior to the Younger Dryas been thoroughly surveyed and dated (Fig. 2;Briner, 2009;Young et al, 2011;Schweinsberg et al, 2020). The moraine chronologies reveal that following the LGM, which culminated between ~22 and 19 ka, glaciers remained at (100%) or near (82 -83%) their LGM lengths until ~16 -15 ka, after which the moraine record stops; in all https://doi.org/10.5194/gchron-2020-13 Preprint.…”

“…Over a decade of work has resulted in detailed moraine chronologies in three adjacent alpine valleys in the Sawatch Range of central Colorado (Fig. 2;Briner, 2009;Young et al, 2011;Shroba et al, 2014;Leonard et al, 2017b;Schweinsberg et al, 2020). While these studies primarily focused on mapping and dating the range-front 65 moraines and associated outwash terraces, a transect of ages from bedrock samples in Lake Creek valley (Fig.…”

“…While these studies primarily focused on mapping and dating the range-front 65 moraines and associated outwash terraces, a transect of ages from bedrock samples in Lake Creek valley (Fig. 2) documented rapid retreat between 15.6 ± 0.7 ka and 13.7 ± 0.2 ka (Leonard et al, 2017b;Schweinsberg et al, 2020). Schweinsberg et al (2020) suggested a possible link between North Atlantic climate forcing and the rapid https://doi.org/10.5194/gchron-2020-13 Preprint.…”

“…2) documented rapid retreat between 15.6 ± 0.7 ka and 13.7 ± 0.2 ka (Leonard et al, 2017b;Schweinsberg et al, 2020). Schweinsberg et al (2020) suggested a possible link between North Atlantic climate forcing and the rapid https://doi.org/10.5194/gchron-2020-13 Preprint. Discussion started: May 2020 c Author(s) 2020.…”

“…https://hess.ess.washington.edu/math/index_dev.html; Balco et al, 2008). We calculate ages using the Promontory Point production rate (Lifton et al, 2015) and the LSDn scaling model (Lifton et al, 2014)a combination used extensively throughout the western US (e.g., Licciardi and Pierce, 2018;Quirk et al, 2018;Brugger et al, 2019; 165 Schweinsberg et al, 2020). Below, we discuss in more detail how different production rate calibrations and scaling schemes impact our results.…”