Optically stimulated luminescence dating of tephra-bearing deposits from eastern Beringia

• Main Author: Demuro, Martina
• Format: Thesis
• Language: unknown
• Published: School of earth and Environmental Sciences, Faculty of Science 2009
• Subjects: Ash Bend; Yukon; Ice Sheet; Old Crow; Alaska; Beringia
• Online Access: https://ro.uow.edu.au/theses/3196; https://ro.uow.edu.au/cgi/viewcontent.cgi?article=4196&context=theses

Eastern Beringia (Alaska and the Yukon Territory) represents a unique high-latitude setting in that it has remained largely ice-free throughout the major global glaciations of the Quaternary. As a result, it contains extensive terrestrial palaeoenvironmental records in the form of perennially-frozen loess and loess-derived deposits, which span the last 3 million years. Numerous tephra horizons across eastern Beringia also provide invaluable chronostratigraphic markers that are capable of correlating regional stratigraphic sequences over large areas. These preserved records offer a unique opportunity to reconstruct detailed palaeoenvironmental, palaeoclimatic, palaeobiological and archaeological histories for this region. However, there remains a need to improve and expand the chronological approaches used to interpret these Quaternary records, particularly over timescales that lie beyond the upper age range of radiocarbon (14C) dating. The main aim of this thesis is to provide an improved, numerical-age, chronostratigraphic framework for Quaternary environmental reconstructions in eastern Beringia. For this purpose, optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) dating techniques are used to provide chronological constraints on key stratigraphic units containing correlative tephra horizons within Pleistocene loess and glaciofluvial gravels from eastern Alaska and the Yukon Territory. Importantly, the suitability of OSL dating of tephra-bearing deposits in this region remains to be fully investigated, and this forms an integral part of the research undertaken in this thesis. To do this, single-grain and multi-grain OSL dating techniques were applied to 140,000 to 30,000 year-old deposits associated with geochemically-characterised late Pleistocene tephra layers from the Yukon Territory (Klondike district, Ash Bend, Ch’ijee’s Bluff) and eastern Alaska (Chester Bluff), which have been constrained by independent age control (14C and fission-track ages). Multi-grain feldspar IRSL dating was also conducted on these known-age tephra-bearing deposits using a range of emission bands (ultraviolet, blue, yellow and orange-red emissions). An additional objective of the research was to investigate the OSL behavioural characteristics and grain-to-grain variability of sedimentary quartz grains from this I region, as a means of developing more reliable single-grain and multi-grain OSL chronologies. The single-aliquot regenerative-dose protocol was found to be well suited for dating both quartz and feldspar fractions of these tephra-bearing deposits. Sedimentary quartz grains from this region were, however, generally characterised by dim OSL signals. A key finding of this study was that multi-grain OSL dating of known-age deposits typically resulted in severe age underestimation. Several possible reasons for this were explored through signal-component investigations and the construction of ‘synthetic’ aliquots from single-grain measurements. Single-grain OSL dating produced widely spread (overdispersed) dose distributions, but ages that are in broad agreement with independent age control when using either of two well-established models to estimate the burial dose (the central and minimum age models). The findings of this thesis indicate that single-grain quartz OSL techniques are better suited to dating loess deposits in this region than are multi-grain quartz OSL techniques, because the former allow for the exclusion of aberrant grains that are particularly prominent in these samples. A major source of uncertainty in the OSL dating of these perennially-frozen deposits was the long-term water content during their burial periods. In some instances, the OSL ages obtained using the ‘as measured’ water content greatly underestimated the expected ages of the deposits. Investigations into the multi-grain feldspar IRSL characteristics showed that anomalous fading was ubiquitous in these samples, regardless of the emission band. In most cases, however, it was possible to derive IRSL ages that were in agreement with the corresponding single-grain quartz OSL ages by applying suitable fading-correction procedures. This thesis represents the first comprehensive study to apply single-grain OSL dating techniques to eastern Beringian deposits and provides some of the first OSL-based numerical-age constraints for both existing tephrochronologies and newly-identified tephras across Alaska and the Yukon Territory. The OSL chronologies established in this study include (i) maximum and minimum age constraints on the Reid glacial deposits (which represent the penultimate advance of the Cordilleran ice sheet) in central Yukon Territory; (ii) a depositional age for numerous, but previously undated, late Pleistocene tephras in this region; (iii) the first numerical ages for the interglacial deposits and Old Crow tephra at Ch’ijee’s Bluff; and (iv) numerical age constraints on extensive Pleistocene loess and organic beds at Chester Bluff.