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Abstract
When and how quickly the Greenland Ice Sheet (GIS) will retreat and raise sea levels is one of the most pressing societal issues. Reconstructing past ice sheet configurations and extent is imperative for predicting its response to future warming. In this dissertation, three studies add new constraints to past configurations of the GIS and the Laurentide Ice Sheet (LIS) of North America. All three studies apply the geochronometer Beryllium-10 cosmogenic nuclide surface exposure dating to calculate a duration of exposure for erratic boulders deposited by the GIS and the LIS. In Chapter 1, cosmogenic exposure dating is applied to 16 boulders on the Arnott Moraine in central Wisconsin to constrain the deposition of the moraine by the LIS, approximating the age be ~35,000 years old, just before the global Last Glacial Maximum (LGM) (26,000–19,000 years ago). These results indicate that preceding the LGM the LIS grew to an extent slightly larger than during the LGM, when sea level was thought to be ~30 m higher than during the LGM. This research adds new constraints on past ice sheet size relative to sea level, which is an important connection in the climate system. This research also applies a landscape diffusion model to the modern moraine relief and predicts that permafrost conditions during the LGM contributed to the moraine’s diffuse morphology. In Chapter 2, the timing of deglaciation of the northwestern sector of GIS is investigated by applying cosmogenic surface exposure dating to erratic boulders across Washington Land in northwest Greenland. The 71 exposure dates suggest that the northwestern sector of the GIS retreated ~8,500 years ago, which is ~2,500 years later than regional warming seen in ice core records. The GIS gradually thinned, with higher elevation areas experiencing the latest retreat. Results from samples proximal to the Petermann Glacier suggest a retreat of ice to within its fjord 8,500 years ago. The work presented in Chapter 3 uses a subset of the exposure dates from Chapter 2, i.e. those that contained cosmogenic isotopes inherited from periods of prior exposure, and applies an additional cosmogenic isotope, 26Al. The paired isotope analysis suggests that erratic boulders in Washington Land record burial and exposure durations spanning 0.2–1.8 million years. This research suggests that the GIS experienced greatly reduced configurations relative to modern during the late Pleistocene.
