Date of Award
Doctor of Philosophy (PhD)
Antarctica has been glaciated for the past 35-40 million years (Denton et al., 1991) and evidence of periodic fluctuations of the Antarctic ice sheet (AIS) during the Cenozoic are recorded in the ice sheet itself, deep sea sediments, and glacial deposits on the continent (Ingólfsson, 2004). Quaternary continental records of AIS extent is limited to few locations along the Transantarctic Mountains (TAM) and coastal continental boundaries (Denton et al., 1984; Denton et al., 1989). Records of atmospheric variation over time, glacial extents, and ice sheet responses to environmental changes are required to understand modern day forces on climate and the environment and provide a context in which to relate modern observations to the past. In this framework, this paper evaluates the geomorphic stability of Ong Valley within the Central Transantarctic Mountains (CTM) and the preservation of Pleistocene aged ice underneath an insulating lag deposit.
Ong Valley in the Central Transantarctic Mountains (CTM) contains ancient buried glacier ice derived from past flow of the adjacent Argosy Glacier. The valley floor is covered with patterned ground and has three distinct glacial tills. Geomorphic and stratigraphic evidence shows that these deposits originate from sublimation of debris-laden glacier ice. Buried glacier ice is still present beneath the youngest two drifts, one of which is older than one million years. The tills above the ice record the repeated advances and stagnations of the Argosy Glacier. Cosmogenic exposure age dating of these tills provides ages, ice sublimation rates and regolith erosion rates that support the antiquity of the ice below.
The oldest ice on Earth has an undisputed age of 800 Kya and is at the bottom of large ice sheets (Fischer et al., 2013). Access to it requires extensive drilling through kilometers of ice. Conversely, the ice in Ong valley is preserved beneath only 1 m of till and is over 1 million years old. Geomorphically similar ice was found in Beacon Valley, but its age (~8.1 Mya) was inferred from dating of volcanic ash above the ice (Sugden et al., 1995). Subsequent analysis from other investigators suggest that the age of the ash may not be a good indicator for the age of the ice below it (Hindmarsh et al., 1998; Ng et al., 2005; Stone et al., 2000).
Concentrations of cosmogenic 10Be, 26Al, and 21Ne were measured in regolith samples collected every ~10 cm in 1 m deep vertical transects through three tills in Ong Valley. Transects reach the buried ice surface in the two younger tills. Cosmogenic-nuclide concentrations in these transects are functions of: i) the age of the till and ice below it; ii) the rate of formation of the till by sublimation of underlying ice; and iii) the rate of surface erosion of the till. In general, a young till unit will have 26Al and 10Be concentrations that are primarily a function of till age; however, over time, 26Al and 10Be concentrations reach equilibrium with erosion, sublimation rates, and radioactive decay; thus, 26Al and 10Be concentrations in older tills primarily provide information about those rates. Conversely, stable nuclide 21Ne only accumulates over time which makes it useful in determining the age of older tills where as 26Al and 10Be provide minimum limiting ages.
26Al, 10Be, and 21Ne measurements in Ong Valley are consistent with a scenario in which tills are derived from progressive sublimation of glacial ice containing 10% by volume englacial debris. 26Al and 10Be concentrations in the youngest till constrain its emplacement age at 18.4 Kya. 21Ne nuclide concentrations in the two oldest tills are best explained by ice sublimation rates on the order of tens of m/Mya and surface erosion rates of the till on the order of m/Mya for at least 0.9 Mya to 1.5 Mya. Concentrations of nuclides in the bottom of the second drift suggest that local sublimation rates have increased slightly in the past 40-150 Kya. These observations imply that the ice below the middle drift is the oldest undisturbed glacier ice currently known on Earth and should provide ancient atmospheric records within one meter of Earth’s surface.
Bibby, Theodore C., "Landscape evolution and preservation of ice over one million years old quantified with cosmogenic nuclides 26Al, 10Be, and 21Ne, Ong Valley, Antarctica" (2014). Theses and Dissertations. 20.