SEASONAL VARIABILITY OF THE OXYGEN ISOTOPE COMPOSITION OF THE LAND SNAIL SPHINCTEROCHILA CANDIDISSIMA, AN ABUNDANT PALEOCLIMATIC PROXY IN THE MEDITERRANEAN

Land snail shells may be used to infer paleoclimatic conditions because their aragonitic shells are usually well preserved, they often can be age-dated, and they record valuable climatic information in the form of oxygen isotope codes. However, the climatic interpretation extracted from these shells remains difficult, in part due to the small number of present-day calibration studies available in the published literature. This study examines the environmental significance of the seasonal variations in the oxygen isotopic composition (δ¹⁸O) of living specimens of Sphincterochila candidissima, a common fossil and subfossil gastropod present in paleontological and archeological sites across the Mediterranean area. Forty-four individuals were live-collected throughout one year (from September 2013 to October 2014) in Tarragona, NE Spain, and the oxygen isotope composition was measured at the shell margin, which depicts the last growth episode closest to organisms’ collection date. Shell margin δ¹⁸O values varied from +3.1‰ in September to -0.8‰ in April, varying near ~4‰ between seasons. As hypothesized, monthly-averaged shell margin δ¹⁸O values positively correlated with monthly-averaged rainwater δ¹⁸O values (R²=0.58; p<0.05; n=9); and negatively correlated with monthly-averaged maximum relative humidity values (R²=0.42; p<0.05; n=9). This work illustrates that the oxygen isotope values of land snail shell margin primarily record local rainwater δ¹⁸O values and maximum relative humidity conditions, reinforcing that Sphincterochila can indeed be used as a valid paleoprecipitaiton/paleohumidity proxy in the region. The results also suggest that land snail shell margin values may be used to infer the approximate season of snail collection during prehistoric times.