ABSTRACT: Optically stimulated luminescence chronologies of late Pleistocene permafrost deposits and tephra sequences from eastern Beringia.

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Abstract

We report on the ongoing results of our optical dating program of Beringian permafrost deposits containing fossils and sediments from which the ancient DNA of extinct and extant species has been recovered. The particular region of study, Beringia, which includes the glacial land bridge connecting the two great landmasses of Eurasia and the Americas, presently submerged by the Bering Strait, has dictated the flow of plants and animals between the two continents over the last several million years. A number of well characterised tephra beds have been documented across Alaska and Yukon Territory, collectively known as eastern Beringia. The reconstruction of Quaternary stratigraphic sequences and palaeoenvironments in this region has benefited greatly from the study of these important stratigraphic markers. In order to test the applicability of optically stimulated luminescence (OSL) dating on perennial permafrost of East Beringia, the technique was applied to Pleistocene ice-rich loess-derived deposits associated with known-age tephra beds of the Klondike district, south of Dawson City, Yukon Territory, Canada, and neighbouring regions. Our analysis focus on a number of these important tephras, namely (i) Dawson tephra - originating from the Aleutian Arc-Alaska Peninsula - at Quartz Creek, Klondike; this particular tephra has recently been associated with high-resolution palaeoenvironmental data producing evidence for the nature of local-scale vegetation composition during the onset of the last Glacial Maximum in Eastern Beringia, and (ii) Sheep Creek tephra - derived from the Wrangell volcanic field eruptions – found at Ash Bend, Stewart River, central Yukon. We report our findings of OSL measurements made on quartz extracts and the resultant equivalent dose (De) values obtained using the single-aliquot regenerative-dose (SAR) protocol at the single-aliquot and single-grain scale of analysis. This study represents the first single-grain OSL study of deposits associated with tephra beds in East Beringia. In this paper we focus on (i) the luminescence characteristics and behavioural trends of the permafrost-tephra samples collected, (ii) the applicability of the SAR method at the single-aliquot and single-grain scale of analysis, and (iii) interpretation of resultant De distributions and comparisons with established thermoluminescence(TL) and 14C ages.

Bibliography of Canadian Geomorphology