Date of Award

2011

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

George H. Denton

Second Committee Member

Brenda L. Hall

Third Committee Member

Karl J. Kreutz

Abstract

First described by Blackwelder in 1915 in the Wind River Range of western Wyoming, the Bull Lake (outboard) and Pinedale (inboard) moraine sets, or their equivalents, have since been recognized in most formerly glaciated basins of the Rocky Mountains in the western United States. I present a detailed 10Be surface-exposure chronology for these classic moraine sets alongside Fremont Lake that were deposited by an ice lobe that drained a high plateau ice cap of the Wind River Range. The Bull Lake complex consists largely of hummocky ground moraine interspersed with a few distinct moraine ridges. The Bull Lake moraine ridges are 5-10 m high and ~200 m wide. Four samples collected from boulders rooted in the most prominent Bull Lake moraine ridges afford a mean age of 148,500 ± 4,650 years (n = 4). The Pinedale moraine complex abuts the southern, eastern, and western margins of Fremont Lake. Pinedale-type moraines consist of hummocky ground moraine, along with several well-preserved moraine ridges. Pinedale ridges are sharp, with well-defined crests. The moraines are as much as 50 m high and 150 m wide. I divided these ridges into four distinct groups, dubbed Pinedale 1 through Pinedale 4 from outermost to innermost. Pinedale 1, which marks the outer margin of the Pinedale complex, yielded an age of 20,360 ± 500 years (n = 12). The Pinedale 2 moraine, which lies a kilometer inboard of Pinedale 1, gave an age of 17,580 ± 460 years (n = 8). Pinedale 3 laterals, located about 2 km north of the Pinedale 2 moraine, date to 16,870±430 years (n = 3). Pinedale 4 laterals, about 3 km north of the Pinedale 3 laterals, afford an age of 16,070 ± 400 years (n = 6). The outer Pinedale moraines correspond in age with the outboard moraines deposited by the Laurentide Ice Sheet in the Great Lakes region and on Long Island. This common signal implies that the Fremont Lake moraine chronology in not an outlier for the middle latitudes of North America. The Pinedale moraines formed during a period of low atmospheric CO2 and shortly after a low in summer insolation at 45° N latitude, in accordance with Milankovitch's version of the orbital theory. However, two problems with Milankovitch's theory arise when the Pinedale chronology is compared with similar glacier records from the middle latitudes of the Southern Hemisphere. First, the maxima for the Southern Alps in New Zealand and for the Andes in the Chilean Lake District are both coincident with the Pinedale maximum, even though summer insolation signals are out of phase. This indicates the importance of atmospheric CO2 in producing global temperature depression during the LGM. Second, recession of Pinedale ice was slow early in the termination despite rising summer insolation. In contrast, early recession in the south was massive despite falling summer insolation. This differing glacier behavior might point to a simultaneous southward shift of the westerly wind belts in both polar hemispheres as an important factor in initiating the last termination.

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