OSL dating of loess deposits bracketing Sheep Creek tephra beds,northwest Canada: Dim and problematic single-grain OSL characteristics and their effect on multi-grain age estimates |
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| Institution: | 1. Centro Nacional de Investigación sobre la Evolución Humana, CENIEH, Paseo de Atapuerca s/n, 09002 Burgos, Spain;2. School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia;3. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada;1. Soil Geography and Landscape Group & Netherlands Centre for Luminescence Dating, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands;2. Nordic Laboratory for Luminescence Dating, Department of Geoscience, Aarhus University, Risø Campus, DK-4000 Roskilde, Denmark;3. Centre for Nuclear Technologies, Technical University of Denmark, Risø Campus, DK-4000 Roskilde, Denmark;4. School of Geographic and Oceanographic Sciences, Nanjing University, China;1. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China;2. Laboratory for Earth Surface Processes, Department of Geography, Peking University, Beijing 100871, China;1. Department of Geography, University of Sheffield, Winter Street, Sheffield S10 2TN, United Kingdom;2. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA;1. Department of Geography and Planning, University of Liverpool, Liverpool, UK;2. Center for Nuclear Technologies, Technical University of Denmark, DTU Risø Campus, Denmark;3. Nordic Laboratory for Luminescence Dating, Department of Geoscience, Aarhus University, Risø Campus, DK-4000 Roskilde, Denmark;4. Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, H-1112 Budapest, Hungary;5. Department of Lithospheric Research, University of Vienna, A-1090 Vienna, Austria |
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| Abstract: | Geochemically-fingerprinted tephra beds provide unique chronostratigraphic markers for comparing Quaternary sedimentary records across eastern Beringia (Alaska and Yukon Territory). Establishing reliable numerical age control on these tephra horizons enables them to be placed within firm temporal frameworks and increases their potential as correlative tools for regional palaeoenvironmental reconstructions. To this end we present new single-grain and multi-grain quartz optically stimulated luminescence (OSL) chronologies for loess deposits bracketing three well-documented and regionally significant variants of the Sheep Creek tephra (SCt) at two sites in west-central Yukon Territory (Ash Bend and Quartz Creek). Single-grain OSL ages bracketing the SCt-A and SCt-K reveal that these tephras were deposited during late Marine Isotope Stage (MIS) 5 or early MIS 4. The SCt-C variant and associated organic-rich bed at Ash Bend were likely deposited during late MIS 5, based on a single-grain OSL age of ～81 ka for the overlying sediments. The single-grain OSL ages obtained for these deposits are in stratigraphic order and in broad agreement with a fission track age estimate of ～77 ka for the SCt-K. In contrast, comparative chronologies obtained using multi-grain aliquots are stratigraphically inconsistent and unexpectedly young when compared with the independent SCt-K age. Detailed examination of the single-grain OSL datasets reveal a range of unfavourable luminescent properties that could have contributed to the multi-grain aliquot age discrepancies; including, very low yields of luminescent grains, weak OSL signal sensitivities and large populations of aberrant grains (particularly 0 Gy grains and ‘dim’ grains with a tendency to sensitise during the equivalent dose (De) measurement sequence) that have similarly sized OSL signals as grains used for De analysis. Synthetic aliquot De datasets constructed from single-grain OSL measurements reveal that the large proportional light sum contributions of 0 Gy and dim grains could possibly account for multi-grain age underestimations in some of the Ash Bend samples. In light of these potentially problematic averaging effects, we do not consider the multi-grain OSL ages to be reliable and suggest that single-grain approaches may be preferable for dating sediments with similar quartz luminescence behaviours across this region. |
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