Growth pattern and oxygen isotopic systematics of modern freshwater mollusks along an elevation transect: Implications for paleoclimate reconstruction

Abstract Fossil mollusk shells are widely distributed in the geologic records and their oxygen isotopic compositions have been used to reconstruct seasonality, local climatic and elevation conditions. However, interpretation of isotope data from fossil shells to estimate paleoclimate and elevation is often ambiguous. This study examines the oxygen isotopic systematics of modern freshwater gastropods ( Bellamya and Radix ) shells along an elevation/climate transect in the Asian monsoon region of China to improve the accuracy of paleoclimate and paleoelevation reconstruction using stable isotopes in fossil shells. The results from sclerochronological analyses of the shells show that the intra-shell oxygen isotopic pattern is determined by seasonal variations in lake water temperature and water isotopic composition as well as the life cycle of the organism. Both Bellamya and Radix appear to grow throughout the year in lakes where water temperature does not fall below 13 °C. Radix in the high elevation cold habitat, although prefers to grow in the warmer months, can survive through the freezing temperature. The δ 18 O patterns in the shells are similar across the elevation/climate transect, showing high δ 18 O values in the winter months and low δ 18 O values in the summer months, consistent with the expected pattern in Asian monsoon region. The average growth rates of the shells were highest at the lowest elevation site (warm and humid climate) but were similar at the mid to high elevation sites. For large shells (>2.2 cm for Bellamya and >1.4 cm for Radix ), average growth temperatures, calculated using the aragonite oxygen isotope thermometer, closely approximate the annual mean water temperature while their intra-shell variability is a good proxy for the amplitude of seasonal variations in monthly air temperature. This suggests that large shells of both Bellamya and Radix are excellent archives of lake environmental conditions and most suitable for paleoenvironmental studies.