RADIOCARBON DATING TERRESTRIAL GASTROPODS: ASSESSING OPEN-SYSTEM BAHAVIOR

Material capable of yielding reliable radiocarbon ages has two main attributes: 1) the material was in equilibrium with the atmosphere when it was alive, and 2) the material behaved as a closed system with respect to carbon after burial. Terrestrial gastropod shells, which contain ~12% carbon, are common in geological and archaeological deposits worldwide, but are seldom used for radiocarbon dating due to concerns over the limestone effect (i.e. lack of equilibrium with the atmosphere) and concern regarding open-system behavior. Recent work by our research group has addressed the limestone problem in gastropod shells. Here we assess whether gastropod shells are capable of acting as a closed system with respect to carbon, and if open system behavior can be detected in gastropod shells prior to radiocarbon dating. We radiocarbon dated 10 terrestrial gastropod shells from geologic strata in the northern Midwest that are >130 ka. Eight gastropod shells yielded radiocarbon ages between 40 to 45 ka, indicating about 0.0025 to 0.005 fraction modern carbon (Figure 4). Two gastropod shells, however, yielded ages of 32.66±0.76 and 35.1±1.0 kyrs, indicating significant open-system behavior. Scanning electron microscopy identified that these shells display clear evidence of secondary mineral precipitation. Trace element analysis of these shells indicated that they contain higher concentrations of Mg and Al, and lower concentrations of Ba and Sr, than the other infinitely-aged gastropod shells. These data suggest that both scanning electron microscopy and trace element chemistry can be used as screening techniques to descriminte between terrestrial gastropod shells that have experienced open-system behavior and those that have not.