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

We quantify retreat rates for three alpine glaciers in the Sawatch Range of the southern Rocky Mountains following the Last Glacial Maximum using 10Be ages from ice-sculpted, valley-floor bedrock transects and statistical analysis via the BACON program in R. Glacier retreat in the Sawatch Range from at (100 %) or near (∼83 %) Last Glacial Maximum extents initiated between 16.0 and 15.6 ka and was complete by 14.2–13.7 ka at rates ranging between 35.6 and 6.8 m a−1. Deglaciation in the Sawatch Range commenced ∼2–3 kyr later than the onset of rising global CO2 and prior to rising temperatures observed in the North Atlantic region at the Heinrich Stadial 1–Bølling transition. However, deglaciation in the Sawatch Range approximately aligns with the timing of Great Basin pluvial lake lowering. Recent data–modeling comparison efforts highlight the influence of the large North American ice sheets on climate in the western United States, and we hypothesize that recession of the North American ice sheets may have influenced the timing and rate of deglaciation in the Sawatch Range. While we cannot definitively argue for exclusively North Atlantic forcing or North American ice sheet forcing, our data demonstrate the importance of regional forcing mechanisms for past climate records.

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

We quantify retreat rates for three alpine glaciers in the Sawatch Range of the southern Rocky Mountains following the Last Glacial Maximum using 10Be ages from ice-sculpted, valley-floor bedrock transects and statistical analysis via the BACON program in R. Glacier retreat in the Sawatch Range from at (100 %) or near (~ 83 %) Last Glacial Maximum extents initiated between 16.3 and 15.6 ka and was complete by 14.2–13.7 ka at rates ranging between 9.9 and 19.8 m a−1. Deglaciation in the Sawatch Range commenced ~ 2–3 kyr later than the onset of rising global CO2, but approximately in-step with rising temperatures observed in the North Atlantic region at the Heinrich Stadial 1/Bølling transition. Our results highlight a possible teleconnection between the North Atlantic sector and the southern Rocky Mountains. However, deglaciation in the Sawatch Range also approximately aligns with the timing of Great Basin pluvial lake lowering. Recent data-modeling comparison efforts highlight the influence of the large North American ice sheets on climate in the western United States, and we hypothesize that recession of the North American ice sheets may have influenced the timing and rate of deglaciation in the Sawatch Range. While we cannot definitively argue for exclusively North Atlantic forcing or North American ice sheet forcing, our data demonstrate the importance of regional forcing mechanisms on past climate records.

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

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.

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

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.