Effective coastal adaptation to sea-level rise requires an understanding of how much and how fast glaciers and ice sheets will melt in the coming decades, together with an understanding of the provenance of that ice melt. When land ice is lost to the oceans, sea-levels do not rise uniformly across the globe, but exhibit a “sea-level fingerprint” specific to the source of ice melt, posing an important question motivating this thesis: Which ice mass(es) will contribute the first 1m/3 feet of sea-level rise? The glacial-geologic record archives the vulnerability of ice sheets and their sub-sectors to past warming. To analyze this record of past glacial change, I develop and apply cosmogenic-nuclide techniques for investigating the climate sensitivity of four key ice sheets. The novel geochemical techniques described here also allow me to investigate processes of landscape evolution, including subglacial and subaerial erosion. Subglacial erosion dictates landscape development in glaciated and formerly glaciated settings, which in turn influences ice-flow dynamics and the climate sensitivity of ice masses, making it an important input in ice-sheet models. In Chapter 1, I use 10Be measurements in surficial bedrock and a 4-m-long bedrock near Jakobshavn Isbræ, to constrain the erosion rate beneath the Greenland Ice Sheet (GrIS) on historical and orbital timescales. 10Be concentrations measured below ~2 m depth in a 4-m-long bedrock core are greater than what is predicted by an idealized production-rate depth profile and I develop a model to utilize this excess 10Be at depth to constrain orbital-scale erosion rates. I find that erosion rates beneath GrIS were 0.4–0.8 mm yr-1 during historical times and 0.1–0.3 mm yr-1 on Pleistocene timescales. The broad similarity between centennial- and orbital-scale erosion rates suggests that subglacial erosion rates adjacent to Jakobshavn Isbræ have remained relatively uniform throughout the Pleistocene.
In Chapter 2, I present cosmogenic 10Be and 3He data from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock core from the McMurdo Dry Valleys, Antarctica, opening new opportunities for exposure dating in mafic rocks. I describe scalable laboratory methods for isolating beryllium from pyroxene, estimate a spallation production rate for 10Be in this mineral phase, referenced to 3He, of 3.6 ± 0.2 atoms g-1 yr-1, and present initial estimates for parameters associated with 10Be and 3He production by negative muon capture. I also demonstrate that the 10Be-3He pair in pyroxene can be used to simultaneously resolve exposure ages and subaerial erosion rates, and that the precision of my 10Be measurements in pyroxene enable exposure dating on Last Glacial Maximum to Late Holocene surfaces, including moraines, on a global scale.
In Chapter 3, I apply exposure dating locally to investigate the Last Glacial Maximum (LGM) and initial deglaciation of the Laurentide Ice Sheet (LIS), the most dynamic continental ice sheet, in southern New England and New York City. I synthesize new and existing exposure age chronologies from moraines and other glacial deposits that span ~26 to 20.5 ka, and quantify retreat rates for the southeastern LIS margin. Initial retreat at <5 to 30 m yr-1 started within the canonical LGM period, representing the slowest LIS retreat rates of the entire New England deglacial record, which I relate to a slow rise in modeled local summer temperatures through the LGM.
Employing similar exposure dating techniques in Chapter 4, I describe the first 10Be ages from nunataks of the Juneau Icefield (JIF), Alaska, that I collected through the Juneau Icefield Research Program (JIRP) in order to evaluate icefield thinning during the Late Glacial and Holocene. I find that the JIF was smaller-than-present under warm climate conditions during the early-to-mid Holocene, elucidating the sensitivity of the icefield to warming.
Tackling the climate crisis more broadly and in turn, addressing pressing Earth science questions like those posed in this dissertation, requires diverse perspectives. Yet, the Earth sciences have historically been among the least diverse of the STEM disciplines. As one contribution to a comprehensive effort through JIRP to increase diversity in the geosciences pipeline, Chapter 5 details the curriculum for a two-week course titled ‘A Virtual Expedition to the Juneau Icefield’ that I co-designed and co-taught in 2021 to bring accessible polar science experiences to high school students.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/nt03-9t74 |
| Date | January 2023 |
| Creators | Balter-Kennedy, Alexandra |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
Page generated in 0.0017 seconds