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Development and application of luminescence dating approaches to constrain the timing of tephra deposition and the formation of fluvial landforms
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In recent decades, luminescence dating, a family of dating techniques used to determine the time elapsed since mineral grains were exposed to light or heat, has emerged as a promising tool for constraining the rates and timing of earth surface processes. In addition to a non-specialist review of the physical processes involved in luminescence geochronology (Chapter 1), the work described in this PhD dissertation contributes to the improvement of luminescence dating techniques from the perspective of both technique development and application. In Chapter 2, TL (thermoluminescence) dating techniques were developed and refined for application to the volcanic glass constituents of tephra deposits. The utility of both blue (320 nm to 450 nm) and red (587 nm to 651 nm) TL emissions for dating volcanic glasses using single aliquot regenerative dose (SAR) techniques were assessed by comparing age results against independent age control for three tephras deposited in the Great Basin of the USA. Both blue and red TL emissions from the volcanic glass shards were dim but reproducible and displayed no evidence for significant sensitivity changes occurring between the natural TL and the first test dose during the SAR protocol. Anomalous fading rates (g-values) for the blue TL ranged from 1.2 ± 1.1 to 3.1 ± 1.3 %/decade and were statistically consistent with zero at 2σ for the red TL. Bleaching experiments showed that both signals were sensitive to light exposure, with sensitivity corrected signals declining by ~60% and ~40% over a 2-hour period for blue and red TL signals, respectively. With the exception of one sample, the Turupah Flat tephra, which yielded both blue and red TL ages ~600 years older than independent age control, both the fading-corrected blue and red SAR-TL ages were fully consistent with age expectation. These successful results demonstrate the effectiveness of TL techniques for determining the eruption ages of tephra deposits in primary position between ~1.5 and at least ~30 ka. In Chapter 3, single grain quartz OSL dating techniques were applied to alluvium overlying strath terraces along the Buffalo National River (BNR) where incision has occurred across lithologies of variable resistance during the Quaternary. OSL-derived incision rates ranged from 0.3 mm/yr in the more resistant sandstone-dominated reaches relative to 0.03 mm/yr in the less resistant limestone-dominated reaches. Comparisons of these incision rates with estimates of lateral migration based on in-situ produced 10Be concentrations support the hypothesis that lateral processes outpace vertical ones in the less resistant reaches of the BNR. In combination with previous observations of valley morphology, these geochronological results suggest that lithologic resistance imparts a strong influence on both the rates and styles of incision and consequently on landscape evolution. Importantly, the OSL results also demonstrate that overbank deposition can occur over 100,000 years after strath terrace abandonment even 5+ m above the modern channel, and that interpretations of strath planation age developed from sampling alluvial drapes can be subject to erroneous age interpretation if care is not taken to sample close to the strath surface.