SINGLE-GRAIN OSL DATING OF GLACIOFLUVIAL QUARTZ CONSTRAINS PENULTIMATE REID GLACIATION, YUKON, TO MIS 6

The extensive non-glaciated portions of the northern Cordillera of Yukon Territory, Canada, and Alaska are fringed by deposits associated with advances of the Cordilleran Ice Sheet (CIS) and local mountain glaciers. These earlier advances of the CIS reflect changes in the forcing conditions affecting the northern Cordillera, and most likely the greater advection of moisture into the region. Relatively few of these earlier advances, however, have a robust chronology, precluding a fuller understanding of their regional climatic significance and whether they truly represent coherent, synchronous advances. Here we present new constraints on the timing of the Reid glaciation at its reference locale in the Stewart River valley, central Yukon, using single-grain optically stimulated luminescence (OSL) dating of quartz. The single-aliquot regenerative-dose (SAR) protocol was used to obtain equivalent dose (D_e) values for individual quartz grains from glaciofluvial gravels bracketing the Reid till. Single-grain D_e distributions were assessed according to the weighted skewness test and overdispersion, and were analysed using the central age model. It was found that the OSL signal intensities of the quartz were generally low, with the majority of grains (70%) producing no OSL; only 1% of the measured grains were accepted for final age estimation (n≈20). The samples overlying and underlying the till produced ages of 158 ± 18 and 132 ± 18 ka, respectively, indicating that the penultimate Reid advance, at its reference locale, occurred during MIS 6. This glacial advance may be synchronous with deposits associated with the Delta glaciation of central Alaska, and may be correlative with the Mirror Creek glacial deposits in southern Yukon Territory.