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Corbett, Lee

Natural Resources

2016

PhD

Constraining past episodes of climate change and glacial response is critical for understanding future impacts of climate change, especially in the high latitudes where warming is expected to be rapid and most of Earth's glaciers exist. Many studies of past glacier size utilize rare isotopes called cosmogenic nuclides to perform surface exposure dating. Since most areas of Earth's surface that were previously glaciated were covered by erosive ice, which stripped away pre-existing cosmogenic nuclides, surface exposure dating yields the timing of the most recent deglaciation. However, in some high latitude areas where glacial ice is cold-based and non-erosive (so-called "ghost glaciers"), the assumptions of surface exposure dating are violated. Alternate approaches are required to constrain the complex histories of such landscapes. My doctoral dissertation focuses on both developing and employing alternative approaches to studying glacial history in the high latitudes, where glacial ice is non-erosive and dating rock surfaces with a single cosmogenic nuclide does not yield exposure ages. Here, I utilize optimized laboratory methods, paired analyses of two cosmogenic isotopes (10Be and 26Al), numerical models to assess possible exposure/burial histories, and Monte Carlo simulations to constrain uncertainties. To study the exposure and burial history of long-preserved landscapes in the Arctic, I investigate landscapes in two high-latitude locations: Thule, northwestern Greenland; and Cumberland Sound, southern Baffin Island, Canada. Bedrock surfaces, sampled on Baffin Island, exhibit evidence of long-lived subaerial weathering and have simple 10Be exposure ages up to 160,000 yr, despite being glaciated until ~10,000 yr. Simple exposure ages tend to increase with elevation, suggesting more effective erosion in the fjords and longer-term preservation of the uplands. Minimum limiting total histories calculated with 26Al/10Be range up to several million years, with periods of exposure representing ~20% of the total history, describing surfaces that have been alternately preserved beneath non-erosive glacial ice and weathered subaerially over many glacial/interglacial cycles. Boulders, sampled at both sites, have simple 10Be exposure ages up to 78,000 yr in Thule and 79,000 yr on Baffin Island, and yield multi-modal age distributions. Simple exposure ages of boulders tend to under-estimate bedrock ages in the cases of paired bedrock/boulder samples. Minimum limiting total histories calculated with 26Al/10Be range up to 700,000 yr in Thule and several million years on Baffin Island, with periods of exposure representing only a small portion of the total history. Forward numerical models suggest that boulders have been repeatedly reworked, likely experiencing partial or complete shielding during interglacial periods because of rotation and/or burial by till. The landscapes I assess here preserve histories of hundreds of thousands to millions of years, and represent a complex interplay of interglacial exposure, subglacial preservation beneath cold-based ice, periglacial processes, and subaerial weathering. Although such landscapes represent methodological challenges, they contain valuable information about long-term variations in glacial extent and climate.