INTERPRETATION OF OXYGEN ISOTOPIC VALUES OF NORTH AMERICAN LAND SNAILS

Land snails are highly abundant and diverse in both the modern world and the Quaternary fossil record, occupying a wide range of climatic regimes and environments. Their aragonitic shells incorporate oxygen isotopes as they grow, principally from environmental water. Previous studies on modern systems have found a positive relationship between the δ18O values of the precipitation and the shell. However, this relationship seems to break down in colder environments and high latitude areas, where snails generally exhibit higher δ18O values than expected. Moreover, preliminary δ18O values reported here on Last Glacial Maximum (LGM) snails from North America, as well as published values on LGM snails from Europe, are higher than expected if indeed the δ18O values of the precipitation and the air temperature were the main controls on the isotopic composition of the shell. These findings suggest that other environmental parameters, such as the δ18O values of the water vapor and the relative humidity, may, at least in part, affect the δ18O values of the snail shell. This study investigates the oxygen isotopic composition of terrestrial gastropod shells from several modern taxa along a north-south gradient in North America (from 24° to 58°N) to further understand the environmental controls on shell δ18O values. Measured shell δ18O values are compared to observed δ18O values of precipitation, and to shell δ18O values predicted by a published flux balance model that incorporates four environmental variables (i.e., δ18O values of rain and water vapor, temperature and relative humidity). Finally, the shell δ18O values of late Pleistocene fossil sequences from areas where past climate and environmental changes are well known are presented to discuss their potential for recording past environmental changes in the continental fossil record of North America.