USING THE OXYGEN ISOTOPIC COMPOSITION OF TERRESTRIAL GASTROPOD SHELLS AS A PALEOCLIMATIC PROXY: A CASE STUDY FROM THE SAN PEDRO VALLEY, SOUTHEASTERN ARIZONA

Although land snail shells are one of the most common and abundant fossils in Quaternary deposits of North America, they have been minimally investigated compared to other continental proxies. As these shells are composed of calcium carbonate, the oxygen isotopic composition (δ¹⁸O) of shell material has the potential to record paleoclimatic change. However, shell δ¹⁸O values are influenced by many variables, including temperature, δ¹⁸O of meteoric water, and relative humidity. To test the robustness of shell δ¹⁸O values as a proxy for past δ¹⁸O of precipitation, we analyzed fossil gastropod shells from the late Pleistocene Coro Marl (~28-19 ka) in Arizona and compared them to the adjacent Cave of the Bells (COB) δ¹⁸O speleothem record. Bulk samples were collected in 10cm increments over a 110cm unit of this wetland deposit at the Murray Springs locality (1274 m a.s.l). Fossil gastropods (n=359) include Succinea spp. (avg. δ¹⁸O = -4.6‰; n=108), Pupilla hebes (avg. δ¹⁸O = -5.6‰; n=73), Vallonia gracilicosta (avg. δ¹⁸O = -6.5‰; n=72), and Gastrocopta tappaniana (avg. δ¹⁸O = -6.6‰; n=73). The difference in δ¹⁸O values between species is related to variations in ecology and distribution. Species displaying lower values are from upland habitats that were washed into the wetland whereas those with higher values prefer wetter, more lowland habitats. Modern samples of Succinea spp. (avg. δ¹⁸O = -2.2 ‰; n=10) from the San Pedro Riparian area (1234m a.s.l.) were ~2.5‰ higher than the Pleistocene samples. We interpret that the significantly lower δ¹⁸O values observed in fossil shells is primarily explained by lower δ¹⁸O values of precipitation during the Pleistocene, associated with colder temperatures than today. The land snail δ¹⁸O proxy presented here follow the same general pattern as the COB speleothem δ¹⁸O profile, emphasizing that the δ¹⁸O of snail shells here recorded paleoclimatic shifts that are similar in magnitude and direction to speleothems in the same area.