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

Keith A Brugger,Chester A Ruleman,Marc W Caffee,Cody C Mason

doi:10.3390/quat2040036

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.

Highlights

The development of glacial chronologies in the Rocky Mountains have constrained the timing of the Last Glacial Maximum (LGM, senso lato) in many of the individual ranges and provided valuable insights regarding late Pleistocene climate change [1,2,3,4,5,6,7]
In this paper we present estimates of temperature depression in the northern Sawatch Range during the LGM, the local timing of which is constrained by new 10 Be surface-exposure ages of terminal moraines
Our results suggest that the last glaciation in the northern Sawatch Range culminated ~21–20 ka

Summary

Introduction

The development of glacial chronologies in the Rocky Mountains have constrained the timing of the Last Glacial Maximum (LGM, senso lato) in many of the individual ranges and provided valuable insights regarding late Pleistocene climate change [1,2,3,4,5,6,7]. Rocky Mountains suggest no apparent coherent geographic pattern of glacial behavior. Assuming these ages represent the onset of moraine abandonment [5], initial ice retreat began as early as ca. Similar asynchronous glacier behavior is implied by both the ages and extents of the oldest (farthest downvalley) recessional moraines. Exposure ages on these moraines mark the earliest stillstands or readvances during retreat and range from 20.4 to 14.1 ka [9]. Glaciers in several valleys remained near their maximum extents well after abandoning the terminal moraines while at the same time in others, glaciers had retreated significantly [10]

Methods

Results

Discussion

Conclusion