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Title: The application of cosmogenic 3He surface exposure dating within the dry valleys of East Antarctica
Author: Margerison, H. R.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2005
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Abstract:
In situ produced cosmogenic 3He provides independent support for the existence of a stable hyper-arid polar climate within East Antarctica since the mid-Miocene. Cosmogenic 3He concentrations have been measured in over sixty boulders from glacial deposits in the Dry Valleys providing both age constraints for glacial events and quantitative erosion rate estimates for the region. Through these analyses, the processes that control cosmogenic 3He concentrations in moraine boulders are assessed. The interpretation of cosmogenic 3He exposure ages has been improved through, (a) the incorporation of erosion rate estimates, and (b) quantitative correction for non-cosmogenic 3He in Ferrar dolerite pyroxenes. Sub-glacial flood deposits in the northern Dry Valleys were exposed as the overriding ice retreated over the Transantarctic Mountains and out of the Dry Valleys, at approximately 15 Ma. The cosmogenic 3He concentrations suggest exposure ages of between 8.6 and 10.4 Ma approximately 25% lower than 40Ar/39Ar age estimates that have been used to constrain the retreat of the ice sheet. Applying an erosion rate correction of between 0.03 and 0.04 m Ma-1 implies these samples were deposited 15 Ma as the ice retreated from the Dry Valleys. Nearby Ferrar dolerite bedrock surfaces confirm the occurrence of very low rates of erosion in the region, with surfaces eroding at an average rate of 0.17 m Ma-1. Non-cosmogenic 3He, both derived from mantle and nucleogenic sources, can cause cosmogenic 3He analyses to overestimate the surface exposure age of magnetic rocks > 1 Ma. Ferrar dolerite from a mining drill core at a depth of over 600 m has never been exposed to cosmic rays. The 8.46 x 106 at g-1 measured in pyroxene from this sample is close to the theoretical production of nucleogenic 3He in the mineral based on measured U, Th and Li concentrations. This makes less than 1% difference to the majority of samples analysed within this work but becomes more important in Quaternary surfaces. Constraining erosion rates and estimating levels of non-cosmogenic 3He increases the accuracy with which cosmogenic 3He dating techniques can be applied to the glacial history the Dry Valleys. The existence of mid-Miocene aged flood deposits from the northern Dry Valleys confirm the antiquity of the landscape and support the existence of a stable hyper-arid polar climate and consequent limited geomorphic landscape evolution since this time. There must however be some flexibility within the stability model.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.657309  DOI: Not available
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