Date of Award

Summer 8-22-2020

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Brenda Hall

Second Committee Member

George Denton

Third Committee Member

Aaron Putnam

Additional Committee Members

Katherine Allen

Abstract

Accurate reconstructions of the Antarctic Ice Sheet (AIS) are important for evaluating past, present, and future sea-level change. Insight into future changes of the AIS and its tolerances to various climate variables can come from investigation of its past behavior. During the last glacial maximum (LGM), ice grounded in the Ross Sea, reaching close to the continental shelf edge. One hypothesis is that this event was caused largely by changing sea level that led to widespread grounding of floating portions of the ice sheet. This grounding buttressed the inflowing East Antarctic outlet glaciers and caused thickening on the lower reaches of these glaciers; interior ice remained the same or even thinned because of reduced accumulation. The Holocene was characterized by rapid recession of marine portions and possible thickening of interior ice and growth of local glaciers in response to accumulation increase. In contrast, an alternate hypothesis is that expansion of grounded Ross Sea ice was due to growth of local glaciers and East Antarctic outlets. These glaciers are thought to have receded to their present positions in the Holocene despite relatively high accumulation. These hypotheses have very different implications for the future of the ice sheet under global warming. Koettlitz Glacier, a large local glacier, flows from the Royal Society Range into McMurdo Sound (78°S, 163°E) and is ideal for testing these two hypotheses. Competing hypotheses as to how this glacier behaved during the LGM range from minor recession to significant expansion. Today, Koettlitz Glacier blocks the mouth of ice-free Pyramid Trough. However, based on surficial mapping, I infer that grounded Ross Sea ice blocked the valley mouth at the LGM. Radiocarbon dates of subfossil lacustrine algae from a lake dammed in Pyramid Trough by the Ross Sea ice date to 11-23 ka, suggesting the ice dam existed throughout that time period. The stratigraphic position and geometry of moraines indicates that Koettlitz Glacier was smaller at the LGM than it is at present. A single radiocarbon age suggests Koettlitz Glacier has advanced within the last 3 ka. Altogether, existing data suggest that Koettlitz Glacier, and by inference other local glaciers in the region, retreated during the LGM and advanced in the Holocene, probably because of fluctuations in accumulation. My work favors the first hypothesis of growth of local glaciers and at least terrestrial portions of the ice sheet during times of high accumulation, which correspond to warm periods in the Antarctic. In contrast, marine-based areas of the ice sheet, such as in the Ross Sea, appear to have advanced during the LGM and retreated in the Holocene, likely in response to changing sea level. This bimodal response of the ice sheet to climate change has implications for future ice-sheet behavior and implies that the future of the ice sheet will depend on the interaction between accumulation-caused thickening and retreat due to marine instabilities.

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