CALIBRATION AND DEVELOPMENT OF AN OXYGEN ISOTOPE PALEOCLIMATE PROXY FROM TERRESTRIAL GASTROPODS, SAN PEDRO VALLEY, ARIZONA

Over the last several years our research group has been working to calibrate the environmental controls on the oxygen isotope composition of terrestrial gastropod shells in North America, as well as understanding potential vital effects between species and/or influences of micro-habitat and snail ecology on shell δ¹⁸O values. Initial results have identified significant differences of up to 3‰ between taxa, and that the dominant control on shell oxygen isotopic values is the δ¹⁸O value of precipitation. Here we present a paleoclimatic proxy record derived from δ¹⁸O analyses of 456 individual shells collected from desert wetland deposits in the San Pedro Valley, Arizona, USA. The deposits span approximately 20,000 years from ~29.4 to 9.9 ka, with the age of deposits constrained by ¹⁴C ages on Succinea spp. gastropod shells. Systematic offsets of ~2‰ were identified among the four taxa analyzed (Succinea spp., Pupilla hebes, Gastrocopta tappaniana, Vallonia gracilicosta), with Succinea spp. displaying the highest values and Vallonia gracilicosta having the lowest values. These trends are similar to those identified in our modern calibration study in Minnesota, USA. We then constructed one composite gastropod shell δ¹⁸O record incorporating the systematic offsets among taxa that shows an increase in δ¹⁸O values of ~4‰ from the Last Glacial Maximum (LGM) to the early Holocene, with an average value of -5‰ VPDB for the LGM and a value of -1‰ VPDB for the early Holocene. A comparison between the gastropod δ¹⁸O record from the San Pedro Valley with speleothem records in the American Southwest shows a remarkable similarity, especially between the San Pedro gastropod record and the Ft. Stanton, NM speleothem record. These results indicate that terrestrial gastropod shells, one of the most ubiquitous fossils in the Quaternary geologic record of North America, can be used as a robust δ¹⁸O proxy once the modern system has been calibrated.