Glacier History of the Western United States

Glacier History of the Western United States – Wind River Range, Wyoming & the Sierra Nevada, California

Aaron Putnam, Sean Birkel, George Denton, Kathryn Ladig

Collaborators: Joerg M. Schaefer (Columbia University), Edward B. Evenson (Lehigh University), David E. Putnam (University of Maine – Presque Isle), Peter Quesada, Peter Huybers (Harvard University), Richard B. Alley (Penn State), Patrick Applegate (Universitet Stockholm), David Vacco (Penn State), Robert Finkel (CEREGE)

Wind River Range, Wyoming & Sierra Nevada, California, June & July 2009, June & July 2008

The ice ages remain an outstanding mystery of Earth Sciences.  Determining what causes ice ages is key to developing a comprehensive understanding of how Earth’s climate system operates, and is of utmost importance for establishing a benchmark against which sophisticated predictive climate models can be calibrated.  Intrinsic to deriving the causes of ice ages is understanding also why they tend to end so abruptly in what have been deemed ‘glacial terminations.’  Our group has been tackling this problem by studying the history of Earth’s mountain glaciers, which are very sensitive recorders of atmospheric temperature.  We are interested in whether glacier advances during the last glacial maximum (LGM), as well retreat at the onset of the last deglaciation, occurred synchronously across the planet, or whether glaciers behaved out of phase between the hemispheres.  The answer to this question could narrow the field of potential drivers of interhemispheric climatic change.  For comparison with results from our southern mid-latitude field programs in New Zealand and Patagonia, we chose to develop glacial chronologies from the exceptionally well-preserved moraine systems flanking the Wind River Range, Wyoming, and the Sierra Nevada, California.

The Wind River Range and Sierra Nevada are well suited to develop a detailed glacial history of the Western United States for the following reasons.  First, these mountain ranges intersect the modern snowline and sustain glaciers at present, thus providing a complete glacial geomorphological record since the last ice age.  Second, former glacier lobes that flowed from these mountain ranges during the last ice age extended into the mountain forelands as piedmont lobes.  Oscillations of these piedmont glaciers produced well-spaced belts of moraine systems that are ideal for establishing a detailed record of glacier activity during and since the last ice age.  Third, the Wind River Range and Sierra Nevada are located west, and hence upwind, of where the former Laurentide Ice Sheet sat, thus minimizing the regional climatic influence of that giant ice sheet on the mountain glaciers.  Fourth, these western U.S. mountain ranges are located away from the North Atlantic region, and are thus well-suited to test whether climate changes registered in the North Atlantic were spread throughout the hemisphere.  Finally, the Wind River Range and Sierra Nevada are located east and west of where the great former ice age lakes existed in the Great Basin.  Thus, side-by-side reconstructions of atmospheric temperature (from glacier chronologies) and precipitation (from lake-level chronologies) are possible.

To develop glacier chronologies, we use the 10Be surface-exposure dating method to determine the ages of glacial landforms.  We work in close collaboration with the Lamont-Doherty Earth Observatory Cosmogenic Isotope Laboratory, where all laboratory procedures are conducted.  Extracted cosmogenic beryllium is analyzed by accelerator mass spectrometry at the Lawrence-Livermore National Laboratory.  To derive the climatic significance of these moraine systems, we are using a state-of-the-art glaciological model tailored to the modern climatology of the Western U.S. This model simulates development of former glaciers of our study regions, and recent model realizations effectively reproduce LGM ice extent in the Wind River Range inferred from mapped LGM moraine ridges (locally attributed to the ‘Pinedale’ type glaciation).

We have embarked on two field expeditions during the summers of 2008 and 2009 in the Wind River Range, and one season in the Sierra Nevada during the summer of 2009.  During these expeditions we have conducted glacial geomorphological mapping and collected samples for surface-exposure dating.  Most of our work has focused on moraine ridges attributed to the LGM, however we have also mapped and sampled moraine sets situated high in the cirques of the Wind River Range, just outboard of modern glacier termini. Together, our glacial geomorphologic and geochronological approach affords the opportunity to derive a detailed and robust temperature history of the Western U.S. since the last glacial period.  With such a tool we can evaluate potential drivers of global climate and approach resolutions to the problems of ice ages and abrupt climate change.