Date of Award

Summer 8-23-2019

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Quaternary and Climate Studies

Advisor

Karl Kreutz

Second Committee Member

Dominic Winski

Third Committee Member

Kristin Schild

Additional Committee Members

Bradfield Lyon

Abstract

I use instrumental and ice core records to examine drivers of observed isotope variability in the Upper Kaskawulsh-Donjek (UKD) region of the St. Elias Mountains, Yukon, Canada over the time frame of instrument-proxy overlap (mid-1900s to present). One of the drivers of post-depositional isotope signal alteration is the vertical percolation of meltwater from the glacier surface through shallow layers of snow, which causes a reduction in the amplitude of the isotope signal recorded in ice cores. I examine isotope signal preservation in two sites in the St. Elias Mountains: Eclipse Icefield and Icefield Divide. These sites are relatively close (~30 km apart and 414 m elevation difference), yet display marked differences in melt amounts and isotope signal preservation related a ~1.8 °C increase in temperature along the downward altitudinal transect from Eclipse Icefield to Icefield Divide. The increase in melt and loss of isotope signal preservation in response to this relatively small temperature rise suggests that the isotope signal at Eclipse Icefield will begin to degrade by the mid-21st century if rapid Arctic warming continues as projected. However, temperatures in northwestern Canada have already exceeded those of the Holocene Thermal Maximum. This indicates that, given Eclipse Icefield’s lack of melt-related signal alteration at present, its ice may contain a complete and unaltered record of past regional climate variability through the Holocene— regardless of its ability to record climate variability in the near future. Extending this analysis to other ice core sites in the Arctic, I identify a meteorological threshold for melt-related signal alteration, characterized by high mean summer temperatures (approximately -1.5 °C and above) and low accumulation rates (less than ~1.2 m water-equivalent snowfall per year). In addition, I investigate mechanisms driving observed seasonality in the Eclipse Icefield isotope record using instrumental records and climate reanalysis products, which summarize broad-scale meteorological patterns. A local weather station record from Icefield Divide shares 15% of variance, at most, with the Eclipse stable isotope timeseries on annual and seasonal timescales. Seasonal anomaly composites of sea level pressure fields and geopotential height at the surface and mid-troposphere, which are indicators of atmospheric circulation, show the most consistent patterns during cold seasons with high isotope ratios and high deuterium excess values. These patterns are characteristic of the Pacific-North America atmospheric teleconnection pattern and its expression at sea level, the Aleutian Low Pressure System. I conclude that seasonal differences in atmospheric circulation are a probable driver of isotope variability at Eclipse, with high isotope ratio cold seasons characterized by a more zonal moisture flow regime and local moisture sources. This finding suggests that high-isotope ratio cold seasons in the Eclipse Icefield record may provide a useful proxy of past variability in the Pacific-North America pattern and Aleutian Low Pressure System.

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