Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Development of in situ cosmogenic 21Ne exposure dating, and dating of Australian arid landforms by combined stable and radioactive in situ cosmogenic nuclides

Auteur : Fujioka, Toshiyuki

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2006

Résumé partiel
The chronology and history of formation of Australian arid landforms including stony deserts and dune fields were investigated using cosmogenic exposure dating. Cosmogenic exposure dating offers a method to date arid landforms, in which the amount of a nuclide, produced in surface rocks by interaction of secondary cosmic rays with rock elements, can be used to estimate surface exposure ages and erosion rates. Combined use of two or more cosmogenic nuclide measurements in a sample also enables the estimation of the burial history of a sample. Among several cosmogenic nuclides of interest for exposure dating, 21N e is stable, and thus has the advantage of being useful for measuring long exposure (>5 Ma), where radionuclides such as 10Be and 26 Al cannot reach, owing to their radioactive decay. More than half of the research time of this thesis was devoted to development of a method for accurately determining the amounts of cosmogenic 21Ne, together with 21Ne produced nucleogenically as well as trapped crustal 21Ne, in a sample. In summary, evaluating crustal 21Ne by crushing data, and estimating in situ nucleogenic 21Ne from U-Th contents and sample rock ages were regarded as the best methods at this stage. Cosmogenic 21Ne is evaluated by subtracting these components from the total non-air 21Ne in a sample. Cosmogenic 21Ne in surface rocks ("gibbers") from stony deserts in central Australia was determined by following the method established in this study, together with cosmogenic 10Be, to evaluate formation history of the stony deserts. Results of cosmogenic 21Ne and 10Be measured in stony gibbers showed discordant exposure ages with excess 21Ne, implying accumulation of cosmogenic nuclides at a shallow depth, presumably under a soil mantle, prior to exhumation of parent silcrete. Two-nuclide calculations of nuclide production prior to silcrete exhumation gave corrected exposure ages that suggest that Australian stony gibbers began to form as early as 4 Ma, and that dissection of silcrete tableland and fluvial distribution of gibbers continued 2-3 Ma, followed by soil stripping and break down of exposed silcrete occurred during last 1 Ma

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