Conditions Involved in Dating Terrestrial Shells

Abstract

Mollusks living only on ground surface can be expected to give the most reliable results in 14 C 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 14 C 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 14 C shell dates. The discrepancy between “normal” and snail dates amounts to 300 to 1200 14 C years and remains the same whatever the absolute age of the sample. All 13 C values of perfectly cleaned shells are between —5 to —10%, versus PDB. The initial 14 C content of shells that is too low may be different according to species, as suggested by 13 C variations. Although fairly constant, this deviation of 14 C 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 13 C measurements.