Optical dating studies of mud-dominated alluvium and buried hearth-like features from Red River Valley, southern Manitoba, Canada

Optical dating studies of samples of late-Holocene mud-dominated alluvium, and heated alluvium in features interpreted to be hearths, collected from the banks of the Red River, southern Manitoba, Canada, were undertaken to assess the viability of these deposits as chronometers for floodplain evolution. Optical dating was done by utilizing the violet luminescence emitted during infrared excitation of the potassium feldspar in the fine-silt polymineral fraction. The alluvial samples, dated by radiocarbon to be about 1 ka old, yielded maximum limiting optical ages between about 1 and 5 ka, indicating that optical dating of such samples may provide useful chronological information in cases where material suitable for radiocarbon dating is lacking, or in cases where a radiocarbon chronology needs support. Furthermore, the surprisingly low residual optical ages from sediments of this type indicate that there is a high proability that sediments representing similar mud-dominated alluvial environments in the Pleistocene record will yield ages with adequate precision and accuracy. Bulk sediment extracted from the centre of a buried hearth yielded an optical age (1.4 ± 0.2 ka) that was broadly consistent with its associated calibrated radiocarbon age (0.23-1.05 ka), but good agreement (0.79 ± 0.09 ka) was achieved when small fragments of hard-baked mud (ñ0.5 g) extracted from within the feature were optically dated separately; hard-baked fragments from two other hearths yielded similar results. Optical dating of sediments from hearths formed in mud-dominated alluvial sediments thus represents a viable method for obtaining a useful chronology in circumstances where organic material suitable for radiocarbon dating is absent.