ISOTOPIC SYSTEMATICS OF MODERN FRESHWATER SNAIL SHELLS: IMPLICATION FOR PALEOCLIMATE AND PALEOELEVATION RECONSTRUCTION

Oxygen isotope analysis of biogenic carbonates has long been established as a valuable tool to reconstruct the past environmental conditions as marine molluscan carbonate precipitates in oxygen isotopic equilibrium with the ambient water. In this study, we analyze the isotopic systematics in modern freshwater molluscs Radix sp. from lakes along an elevation/climate transect spanning on a range of elevations above seal level (SL). Radix sp. were chosen for two main reasons: (1) they are abundant in Late Cenozoic sedimentary deposits and are potential archives of paleo-temperature and paleo-elevation; and (2) modern Radix snails occur worldwide and can be used for isotope study of ancient continental climates. Our objective in this study is to determine how the isotopic compositions of modern freshwater shells reflect ambient climate/elevation conditions and whether fossil freshwater snails can be used as proxy to improve the accuracy of interpretation of paleoclimate. We have measured d¹⁸O of modern Radix snail shells and water samples from three water bodies at different elevations in Tibet and South China: Lake Manasarovar (4590 m above SL), Fuxian Lake (1722 m above SL) and Liuxihe Reservoir (168 m above SL). The d¹⁸O of bulk shells vary from −2.8‰ at Lake Fuxian (n=17) to −7.3‰ (n=10) at Liuxihe reservoir. Using traditional aragonite oxygen isotope thermometer (Kim et al., 2007), water temperature was calculated from measured d¹⁸O values of aragonite shells and average water d¹⁸O values. The calculated temperature ranges within the annual temperature variations and the average temperatures are similar or very close to the mean annual temperature for all three locations. Both calculated and mean annual temperatures are highest for Liuxihe reservoir (~24⁰ C) and the lowest for Lake Manasarovar (~4⁰ C). Since Radix sp. tend to grow at slower rates in the winter, temperature record may be biased towards warmer temperatures in upper elevations. The negative correlation between calculated temperatures and elevation and the similarity between calculated and mean annual temperatures suggest bulk Radix shells may be a proxy to reconstruct paleoclimate and paleoelevation.