Modeling Glacier Mass Balance and Glacial Dating in Alaska

In this 2-part thesis, tree ring records are used to study Alaskan climate and glacier variability on two different timescales. One study is reconstructing glacier mass balance from Southern Alaska back into the 18th century CE. The second part is dating geomorphic changes associated with glacial activity in Glacier Bay National Park and Preserve approximately 2000 BP. Tree ring-based glacier mass balance reconstructions aim to extend mass balance records back in time to more fully analyze variability and better identify potential forcings. The current 40 to 50-year observational mass balance records in western North America are dominated by the process of ablation and document a rather narrow span of a much longer glacial history. Traditionally, mass balance reconstructions based on tree-rings have been conducted using ring width or maximum latewood density parameters. However, an additional and newly developed proxy, blue light intensity (BI), reflects lignin content in latewood tree growth and demonstrates a strong, high frequency signal for temperature in the warmer months along the Gulf of Alaska. This new BI parameter, together with conventional ring width data, was leveraged for the first time to produce seasonal and annual models of mass balances at two USGS Benchmark Glaciers: the coastal Wolverine Glacier and the more interior Gulkana Glacier. These models demonstrate BI to be a strong predictor of mass balance, coinciding with published moraine ages and explaining up to 58% of the variance in the mass balance datasets. The models also exhibit unprecedented and increasingly negative balances over their full periods. A composite Gulf of Alaska chronology gives insight into how the glaciers respond to regional versus local climate. Correlations between winter mass balance and ring width data elucidate a probable response of the Bering Sea to regime shift in the Aleutian Low Pressure System in the mid-1970’s. The Late Holocene glacial history of Glacier Bay National Park and Preserve is not fully understood. A tree ring chronology generated from subfossil wood collected in Muir Inlet partially fills in a gap in the master ring width Glacier Bay 2K chronology, from 2310 to 1800 cal yr. BP. The new radiocarbon-calibrated chronology spans 490 years, beginning at 2400 cal yr. BP through 1911 cal yr. BP. Previous analyses of the master Glacier Bay timeline likely misinterpret dates as constraining ages on ice advance in Muir Inlet ~2000 cal yr. BP. However, with the addition of the new chronology, it becomes clear ages in the 2K chronology are highly coincident with the timing of Glacial Lake Muir. Kill dates on the trees in the master chronology indicate Lake Muir flooded their locations in three separate episodes at approximately 2050, 1950 and 1900 cal yr. BP