<i>In situ </i>produced cosmogenic <sup>3</sup>He provides independent support for the existence of a stable hyper-arid polar climate within East Antarctica since the mid-Miocene. Cosmogenic <sup>3</sup>He 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 <sup>3</sup>He concentrations in moraine boulders are assessed. The interpretation of cosmogenic <sup>3</sup>He exposure ages has been improved through, (a) the incorporation of erosion rate estimates, and (b) quantitative correction for non-cosmogenic <sup>3</sup>He 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 <sup>3</sup>He concentrations suggest exposure ages of between 8.6 and 10.4 Ma approximately 25% lower than <sup>40</sup>Ar/<sup>39</sup>Ar 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<sup>-1</sup> 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<sup>-1</sup>. Non-cosmogenic <sup>3</sup>He, both derived from mantle and nucleogenic sources, can cause cosmogenic <sup>3</sup>He 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 10<sup>6</sup> at g<sup>-1</sup> measured in pyroxene from this sample is close to the theoretical production of nucleogenic <sup>3</sup>He 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 <sup>3</sup>He increases the accuracy with which cosmogenic <sup>3</sup>He 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657309 |
| Date | January 2005 |
| Creators | Margerison, H. R. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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