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Annexe 1 - Rapport sur l’analyse isotopique de résidus carbonisés sur la céramique
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243 ANNEXE 1 Rapport sur l’analyse isotopique de résidus carbonisés sur la céramique Stable isotopic analyses of charred food residues on potsherds, Saguenay Area, Québec Henry P. Schwarcz School of Geography and Geology McMaster University Hamilton, ON, L8S 4M1 March 29, 2003 We were provided with a total of 20 samples,all of which had clearly developed encrustations of charred material on their interior surfaces.Some also had significant encrustations on the exterior surface which we also analysed.The charred crusts (henceforth“crusts”) resembled those which we have encountered on other ceramics which we have analysed including those reported by David Black in his thesis. Analytical Procedures We removed parts of each crust to obtain samples weighing 2-10 milligrams,and subjected them to chemical and stable isotopic analysis.We did not attempt to pre-treat the samples to remove humic matter as the crusts were rather delicate (friable) and we were concerned that they would have disintegrated in such treatment.The chemical analyses of the crusts confirm that the proportion of humics must be small. A portion of each crust was analysed in a Carlo Erba Element Analyser (EA) to determine the content of carbon and nitrogen, and the C/N ratio, which is a good indication of the type of material in the residue. Using the EA,we generated carbon dioxide gas which we then analysed on an Optima stable isotope mass spectrometer to determine the carbon isotope ratio.This is reported as 13 C values defined as follows 13 C = ({[13 C/12 C]sample / [13 C/12 C]standard } - 1) H 1000 in per mil (l) The standard is PDB, which is a marine carbonate.All organic materials have lower 13 C/12 C ratios than the standard, with the result that their 13 C values are all negative. 244 ANNEXE 1 Using the C/N ratio obtained in the first step, we weighed out additional aliquots for determination of the 15 N/14 N ratio, assuring that the N content would be adequate for analysis.These samples were placed in sealed tubes with cupric oxide and heated to 900 EC to convert all nitrogen to N2 gas.This gas was then analysed on the Optima mass spectrometer to obtain 15 N values (defined similarly to 13 C; the standard is atmospheric N2 ). The precision of the analyses is 0.1 l for both isotope ratios. Results All of the sherds had sufficient residues attached for isotopic analysis.For two sherds (mp–1 and mp–20) multiple fragments were present which differed in appearance; each of these was analysed, and the external residue on one of these was also analysed. Multiple analyses of three other samples were also made to test for internal variability. The data are presented in Table 1 (attached as an Excel file).They are also shown in Figure 1 (attached as a PowerPoint file).The 13 C values range from –28 to –20 l while the 15 N data range from 5 to 15 l.The C/N ratios vary from 8.6 to 22.1,which is comparable to the range observed in other food residues (Morton, 1989; Morton et al. 1991). Interpretation It has been found that the isotopic composition of charred residues attached to potsherds conserve the isotopic composition of the foods that were prepared in the pot (Hastorf and DeNiro, 1985; Morton, 1989; Morton and Schwarcz, in prep.).This is true both for the C/N ratio and the 13 C and 15 N values of the food.In most cases the foods in such residues appear to have been a mixture of more than one component (a “meal”).The mixture may have been some kind of a stew (ragoût) for which we can assume that the isotopic composition represents the weighted average of the isotopic compositions of the components in the stew.Thus, if the stew had consisted of equal parts of some terrestrial plant food and fish, then the composition of the mixture would be equidistant between points representing these two foods, on a graph of 13 C vs 15 N. The isotopic data are shown on such a plot in Figure 1,and compared with the estimated ranges for typical food sources, as follows : T = flesh of terrestrial herbivores (deer, moose, elk, rabbit, etc.). Note that the 13 C values also largely overlap the composition of wild plant foods.These would, however have generally lower 15 N values (ranging...