Thermoluminescence dating of the Pleistocene Old Crow tephra and adjacent loess, near Fairbanks, Alaska

Abstract

Two different thermoluminescence (TL) techniques have produced an average age of 109 ± 14 ka for the widespread Pleistocene Old Crown tephra, previously of uncertain age. A deposit of the tephra from the Halfway House site at Fairbanks, Alaska, was dated directly at 110 ± 12 ka by analyses of 4–11 μm glass, using the additive-dose TL technique. A lower age limit of 108 ± 16 ka for this tephra was obtained indirectly by analysis of 4–11 μm grains of loess just above it, using the partial-bleach TL technique.For this partial-bleach technique, the assumptions of "zeroing" of TL and of the saturating exponential form of the TL growth curves were checked by analyses of a surface loess deposited within the last century. Also, most or all of an unstable (anomalous fading) TL component was eliminated from the older polymineralic loess in only a few days by storing irradiated subsamples at elevated temperatures. In contrast, several months storage was required to achieve the same effect at room temperature for this sample.