Doctor of Philosophy (PhD)
Geology and Geophysics
This dissertation consists of two independent topics; both are related to non-mass dependent isotope fractionation. 1) Atacama Gravel Sulfate: The onset of hyperaridity of Atacama Desert is of geological importance and has been controversial. I put constraints this timing by looking into a new record, a deep sulfate profile extending from the surface down to the oxide zone of a copper ore in the Atacama Gravels, whose deposition covers a majority of the Central Depression and temporarily spans from semi-arid to hyper-arid climate. The gravel sulfates show a strong depth-dependent pattern through ~50 meters (15 to 65 meter depth): narrow ranges of Δ17O (+0.22‰ to +0.38‰), δ18O (+8.1 to +10.3‰) and δ34S (~+5.0‰) for the upper 15-30 meter and gradual changes from 30 to 65 meters with decreasing Δ17O (+0.22‰ to -0.25‰), increasing δ18O (+4.5‰ to +11‰) and increasing δ34S (+2.2 ‰ to +3.5 ‰). Considering the sedimentological features of gravel deposits we propose that the pattern was mainly climate controlled with a desiccation trajectory from semi-arid to hyper-arid conditions, hyper-aridity was achieved at the present level since at least 9 million years ago. 2) Thermal gradient induced NMD effect: I report that a surprising non-mass-dependent 17O anomaly can be generated simply by subjecting O2 gas in an enclosure to a thermal gradient. The authenticity of this discovery is substantiated by a series of blank tests and isotope mass-balance calculations. Contamination effects have been exhaustedly tested and are proven to not be able to generate this phenomenon. To further explore the underlying mechanism for the anomalies, I tested the effects of gas pressure, duration of experiment, and geometry of the apparatus on the 17O anomalies for O2 as well as on the 33S or 36S anomalies for SF6 gas. The results are consistent with our proposal that a previously ignored nuclear spin effect on the gas diffusion coefficient may be largely responsible for generating the observed anomalies. This discovery provides clues to some of the puzzling non-mass-dependent isotope signatures encountered in experiments and in nature, including the triple oxygen or quadruple sulfur isotope heterogeneity in the Solar system.
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Sun, Tao, "Non-Mass Dependent Isotope Fractionation: An Application in the Hyperarid Environment and a Set of Experiments on Thermal Diffusion" (2011). LSU Doctoral Dissertations. 1832.