Climate during the Last Glacial Maximum in the Northern Sawatch Range, Colorado, USA

Abstract

Temperature-index modeling is used to determine the magnitude of temperature depression in the northern Sawatch Range required to maintain steady-state mass balances of six reconstructed glaciers at their extent during the local Last Glacial Maximum (LLGM), dated at ~21 ka. Assuming no significant differences in precipitation compared to modern values, mean annual temperatures in the region were on average 8.8 + 0.5/– 0.8 °C cooler than they are today. Allowing for modest (± 10 cm) increases or decreases in precipitation, required temperature depressions only differ by ±0.2 °C. Temperature depression in the northern Sawatch Range is consistent, although slightly greater, with those determined in other ranges in Colorado using similar approaches. The estimates presented here are, however, substantially less than those suggested by several downscaled simulations of global Last Glacial Maximum (LGM) climate, that might be due to the need for improved calibration of such downscaled simulations, or the models from which they are derived. Our estimates of LGM temperature depression are considerably greater than that previously determined in the study area and those in two other ranges in Colorado derived using different methodologies, the latter being most likely responsible for the discrepancies.

Late Pleistocene glacial chronologies and paleoclimate in the northern Rocky Mountains

“…Nowhere in the U.S. northern Rocky Mountains have more recent paleoglaciological methods, particularly distributed energy/mass-balance models or degree-day mass-balance models, been applied to reconstructed late Pleistocene glaciers, as they have been successfully applied in the Middle Rocky Mountains (Laabs et al, 2006;Refsnider et al, 2008;Birkel et al, 2012;Quirk et al, 2018Quirk et al, , 2020 and Southern Rocky Mountains (Ward et al, 2009;Brugger, 2010;Brugger et al 2018Brugger et al , 2019Dühnforth and Anderson, 2011;Leonard et al, 2014Leonard et al, , 2017aSchweinsberg et al, 2016;). In this study we apply a modified version of the Plummer and .…”

“…Elsewhere in the Rocky Mountains, moraines with age limits of ca. 18-17 ka are found in the Sawtooth Range in Idaho (Thackray et al, 2004), the Wasatch and Uinta Mountains in northern Utah, and numerous glacial valleys in the Southern Rocky Mountains in Colorado (Leonard et al, 2017a;Brugger et al, 2018Brugger et al, , 2019Schweinsberg et al, 2020). Where sequences of moraines are exposure-dated in the Rocky Mountains, the outermost moraines of the last glaciation generally have ages that fall within the early Pinedale interval of 22-18 ka and inner moraines (representing near-maximum glacier lengths) that fall within the middle Pinedale interval (Quirk et al, 2020;Laabs et al, 2020).…”

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

“…During the Bull Lake and Pinedale glaciations, Sawatch Range glaciers extended beyond the range front, forming large lateral and end-moraine complexes tied to extensive outwash sequences (Nelson and Shroba, 1998;Shroba et al, 2014). The preservation of these surficial deposits and their associated landforms has attracted many geological investigations (e.g., Hayden, 1874;Capps and Leffingwell, 1904;Davis, 1905;Westgate, 1905;Capps, 1909;Ray, 1940;Richmond and Tweto, 1965;Tweto and Case, 1972;Nelson and Shroba, 1998;Schildgen, 2000;Briner, 2009;Young et al, 2011;Ruleman et al, 2013;Shroba et al, 2014;Schweinsberg et al, 2016;Kellogg et al, 2017;Brugger et al, 2019b). Although the relative ages of these moraines can be distinguished by morphostratigraphic features, such as the sharpness of moraine crests as well as boulder abundance and weathering (Nelson and Shroba, 1998), numerical age constraints were recently established with cosmogenic 10 Be exposure dating.…”

“…These chronologies highlight differences in glacial and deglacial patterns. Numerical modeling studies have also been conducted for several paleoglacier systems, providing quantitative limits on temperature and precipitation conditions that drove Pinedale glaciation and deglaciation (Brugger, 2006, 2010; Brugger et al, 2009, 2019b; Ward et al, 2009; Dühnforth and Anderson, 2011; Schweinsberg et al, 2016; Leonard et al, 2017b) as well as Bull Lake glaciation (Leonard et al, 2014).

Figure 1.(Top) Last glacial maximum glacial extents in the western United States (gray shading; adopted from Porter et al [1983] and Young et al [2011]).

…”

“…These chronologies highlight differences in glacial and deglacial patterns. Numerical modeling studies have also been conducted for several paleoglacier systems, providing quantitative limits on temperature and precipitation conditions that drove Pinedale glaciation and deglaciation (Brugger, 2006(Brugger, , 2010Brugger et al, 2009Brugger et al, , 2019bWard et al, 2009;Dühnforth and Anderson, 2011;Schweinsberg et al, 2016;Leonard et al, 2017b) as well as Bull Lake glaciation (Leonard et al, 2014). Established glacial chronologies suggest that glaciers throughout Colorado reached their maximum Pinedale extents before ∼23.5-19.5 ka, with near-complete deglaciation in most valleys by ∼15-13 ka (all previously published 10 Be ages are recalculated using the Promontory Point production rate [Lifton et al, 2015] and LSDn scaling scheme [Lifton et al, 2014], unless noted otherwise).…”