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Recent studies comparing dates from the carbon content of mortars with dendrochronologic dates for the same material have shown considerable inconsistencies related to mortar type (Van Strydonck et al, 1985). Even after the best possible removal of “dead” calcite, some mortars are unsuitable for dating. We describe here our experimental study of carbon isotope fractionation during the manufacture and hardening of mortars. Preliminary experiments established overall uptake of CO2 from the air. We then measured isotopic ratios in identical mortars at different hardening times.
The influence of the aggregate in mortar dating is examined. Sample activity as well as isotopic fractionation approach the expected values at lower yields of the preparation reaction of the counting gas. Good results are obtained at low fossil carbonate concentration. δ13C cannot give information about this concentration but preliminary visual and chemical analysis of the mortar makes prediction of sample validity possible.
This list includes most of the measurements made since the beginning of the compilation of our previous list (R, 1983, v 25, p 59–126), in 1982 and 1983, as well as some earlier measurements of extended geologic or archaeologic studies.
This list includes most of the measurements made in 1979 through 1981 and some values obtained during preceding years. The reporting of results, their calculation (half-life: 5570 ± 0, standard 13C correction only for bones), and the dilution ratios are as previously described in Lyon VIII (R, 1979, v 21, p 402–452).
Mollusks living only on ground surface can be expected to give the most reliable results in 14C dating from carbonates of continental origin. One may assume they have a homogeneous biotope and are not affected by any hard-water effect. In order to verify these assumptions and to test shells as routine dating material, results from terrestrial gastropods are compared with other 14C dates from classic biologic material, such as peat, charcoal, or bone, collected in the same archaeologic or geologic levels in miscellaneous places. Two sites were selected for which other chronologic data, such as prehistoric industries or malacologic diagrams were available.
All results indicate older values for 14C shell dates. The discrepancy between “normal” and snail dates amounts to 300 to 1200 14C years and remains the same whatever the absolute age of the sample. All 13C values of perfectly cleaned shells are between —5 to —10%, versus PDB. The initial 14C content of shells that is too low may be different according to species, as suggested by 13C variations.
Although fairly constant, this deviation of 14C ages generally makes such samples unreliable for most archaeologic studies, which often need more precise results. However, some measurements were performed on microfauna shells from several Würmian loess to show that dating of shells may be useful in fairly ancient geologic sediments for lack of better carbonaceous samples. Good agreement of some snail dates with expected sediment ages point to the importance of proper sample selection and pretreatment that might be checked by 13C measurements.
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