Luminescence Dating of Sediments Rrom the Western Hertiage Excavation of Archaeological Sites at Thunder Bay (Ontario, Canada)

This report present optically stimulated luminescence (OSL) dates for six sediment samples collected from archaeological sites on the shoreline of Lake Superior (Ontario, Canada). The Ontario Ministry of Transport (MTO) commissioned Western Heritage to undertake rescue archaeological work ahead of the four-lane expansion of Highway 11/17 in 2010. Previous archaeological investigations in this region have uncovered evidence of early human occupations, potentially associated with early occupation in the region following the retreat of the Laurentide Ice Sheet. 18 samples were submitted to the Scottish Universities Environmental Research Centre (SUERC) for OSL dating, from four archaeological sites in the vicinity of Thunder Bay: Mackenzie, Woodpecker 1 and 2, and Hodder. In February 2012, approval was given for quartz SAR OSL analysis on 3 samples each from Mackenzie and Woodpecker 1. Section drawings were provided with luminescence profiles obtained using portable OSL instrumentation developed at SUERC. All samples were subjected to laboratory preparation of sand-sized quartz, and purity checked by scanning electron microscopy. Dose rates for the bulk sediment were evaluated using analyses of the uranium, thorium and potassium concentrations obtained by high resolution gamma spectrometry coupled with beta dose rate measurement using thick source beta counting, in combination with the in-situ gamma dose measurements. Equivalent doses were determined by OSL from 16 aliquots of quartz per sample using the quartz single-aliquot-regenerative (SAR) procedure. The material exhibited good OSL sensitivity and produced acceptable SAR internal quality control performance. Dose distributions from the aliquots were examined using radial plotting methods. All samples revealed some heterogeneity in their equivalent dose distribution, reflecting the main sediment source in the region i.e. glacial sediments. The profile at the Mackenzie site encompasses the lower and upper fills of a cultural pit feature: two samples were taken from the lower fill, in its basal and upper layers, and a single sample was taken from the upper fill. The lower fill (unit 4) is dated to between 6.09 ± 0.41 ka (4080 ± 410 BC) and 5.77 ± 0.44 ka (3760 ± 440 BC); and the upper fill (unit 6) from 5.50 ± 0.32 ka (3490 ± 320 BC). The profile at the Woodpecker 1 site is taken through a series of alluvial units and palaeosols that occur adjacent to a pit-like feature; the upper units in this succession cap/seal the sediments in the pit-like structure in the profile, thereby providing a terminus ante quem for the age of the pit structure. The sample at c. 50 cm DBS in the sequence of alluvial units, at the top of unit 6 is dated to 7.51 ± 0.47 ka (5500 ± 470 BC).The samples taken from the basal and upper layers of unit 3, between 15 and 25 cm DBS in the succession, are dated to 6.19 ± 0.45 ka (4180 ± 450 BC) and 2.91 ± 0.25 ka (900 ± 250 BC), respectively. The OSL investigations at SUERC have verified that there is sufficient quartz in the sediment for dating purposes and that this quartz has promising behaviour for luminescence dating. Having established the suitability of the quartz SAR OSL method for dating the sediment in profiled sections at Mackenzie and Woodpecker 1, it would now be possible to undertake a further programme of work to date additional units at both sites, and the sequences at Hodder and Woodpecker 2. Further work on additional material would have the potential for examining the relationships between the landscape de-glaciation history and its occupation by early human settlers