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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

2005

Vol. 24

13-20

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.