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Thermal activation characteristics and thermoluminescence of chert from the Red Wing, Ontario region, and its putative heat treatment in prehistory

Godfrey-Smith D. I., Bouchet-Bert L., Von Bitter P. H., Storck P. L.

Geochronometria

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2005

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Vol. 24

13-20

EN

One of the still-outstanding questions in New World archaeology is whether prehistoric toolmakers here heat-treated chert raw material prior to the manufacture of stone tools, as had been demonstrated for a number of cultural loci in the Old World. This question is of particular relevance to our understanding of the technological behaviour of the Early Paleoindian people of the Parkhill complex, which has been dated between 10,400 and 11,300 14C years ago. To address it, we studied chert samples from in situ geological contexts, from reworked contexts such as glacier-plucked surface scatters or creek gravels, and from a nearby archaeological context. The samples from archaeological contexts have been flaked in antiquity, subsequently buried, and recovered during an archaeological excavation. We compared the thermoluminescence (TL) properties of these unknowns with those of experimentally annealed cherts. The control samples, collected and flaked from the lowest of four chert layers at the Fossil Hill Formation outcrop, were annealed for four hours at 300 C degrees, 400 C degrees, and 600 C degrees. Our investigation focused on the TL sensitivity of the 100°C TL peak, which is not present in natural TL but is easily observed by prompt TL following beta or gamma dose irradiation. This peak undergoes a greater sensitivity change than the high-temperature TL following heat treatment, therefore it may be considered a far more sensitive paleothermometer. The thermal activation characteristic (TAC) of the cherts was also examined. In addition, we studied the natural TL and dose response of the high-temperature TL of the cherts, and their response to illumination by natural light. On the basis of these investigations we conclude that prior heat treatment is readily detectable in Red Wing chert, however that it has not taken place in the archaeological material we examined. This conclusion is confirmed by the excessive apparent ages of the archaeological cherts, which are an order of magnitude higher than any reasonable archaeological estimates for the presence of humans in the New World. Chert self dose rates were based on ICPMS-determined U and Th radioisotope chain concentrations, and XRF-determined K concentrations. In addition, we found that the high-temperature TL signal in chert is sensitive to reduction by exposure to natural light. This may possibly yield a spuriously lower TL signal in surfacecollected archaeological material than in self-same geological samples collected recently, and may thus lead to an erroneous conclusion of past heat treatment. A correct procedure for the accurate detection of chert heat treatment in antiquity is proposed.

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Beta dosimetry of potassium feldspars in sediment extracts using imaging microprobe analysis and beta counting

Godfrey-Smith D. I., Scallion P., Clarke M. L.

Geochronometria

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2005

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Vol. 24

7-12

EN

We provide a comparison of two independent methods used to determine the K concentration of potassium feldspar extracts from sand-sized lacustrine, colluvial, and aeolian sediments. The two methods being compared are gas-flow beta counting and imaging microprobe X-ray fluorescence analysis. Imaging analysis demonstrated that the proportion of potassium feldspar grains in a K-feldspar extract varied from a low of 7% to a high of 84%. All extracts included a significant proportion of quartz, and some also contained a few plagioclase feldspar grains. However, the K2O concentration in individual potassium feldspar grains of all eight extracts examined was within the range of 15.5 š 0.7 % K2O by weight. The K2O concentrations in four of five extracts, measured using gas-flow beta counting, were significantly lower than this value. This reflects the dilution effect of significant amounts of non-K bearing grains in these extracts. This difference can result in up to an 80% underestimate in the internal beta dose rate of a potassium feldspar grain. A 14% underestimate in the total dose rate to potassium feldspar grains, and therefore a 14% overestimate in sediment ages determined by luminescence dating of nominally pure potassium feldspar extracts is thus possible. We suggest that gas-flow beta counting is suitable for the determination of K concentrations of bulk sediments and therefore the external beta dose rate, but that internal beta dosimetry is best performed by imaging microprobe X-ray analysis. An alternative, inexpensive approach would be to assume a 15.5 š 0.7 % K2O concentration for all nominal K-feldspar extracts separated using a heavy liquid with specific gravity of 2.58 gźcm-3, and use this value to determine the internal beta dose rate. Internal beta dose rate errors incurred with this approach are expected to be significantly less than those incurred by the small sample gas-flow beta counting method.

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Determination of usage and absolute chronology of a pit feature at the Early Bronze I Ashkelon Marina, Israel, archaeological site

Godfrey-Smith D. I., Shalev S.

Geochronometria

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2002

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Vol. 21

163-166

EN

A study of the luminescence properties of one of several pit features removed from the Ashkelon Marina EB1 (Early Bronze I) archaeological site during a 1998 excavation unequivocally determines their function in antiquity. The features are shallow (Ł 50 cm) cup-shaped pits preserved in the ground. A hardened and reddened layer of earth Ł 3cm thick forms the shape of each pit, and a thin layer of white calcite is observed to lie upon the hardened earth. The pit is filled with soft buff coloured, quartz-dominated sediments, indistinguishable from the sediments which underlie it. This feature, and several others at the site, are suspected to be putative fire pit installations, over which crucibles for the melting of copper had been placed. However, the lack of any direct association of copper residues or artifacts found at the site with any of the pit features leaves this hypothesis unproven. Previous investigations, which included X-ray diffraction, differential thermal analysis, optical mineralogy studies, and FT-IR spectral measurements, have been unable to confirm the association of the pit features with fire. Calibrated radiocarbon dates place the use of the site at 5500-5300 BP. We applied optical dating and thermoluminescence (TL) dating to the hardened red layer and the overlying fill sediments, in order to determine the last time of firing and/or exposure to sunlight of the two components of the feature. The premise driving our investigations is the fact that heated sediments will give the correct TL age. In contrast, unheated sediments will give an incorrect TL age far in excess of the depositional age, but will give a correct optical dating age. The TL analyses yielded ages of 5160 š380 years for the hardened rim and 24,600 š1600 years for the fill. Optical dating of the fill yielded an age of 5260 š380 years, which is in excellent agreement with the TL age on the rim. These results fulfil the hypothesized results precisely. On this basis, we conclude that the pit features at the Ashkelon Marina archaeological site were fire pits used in early copper smelting technology.