Geochemistry of modern shells of the gastropod Radix in the Tibetan Plateau and its implications for palaeoenvironmental reconstruction

Abstract

The widely distributed shells of the gastropod Radix are a potential high resolution archive of environmental information. However, previous studies of Radix have used the shells of unidentified species, and the implications of inter- and intra-specific differences in the geochemical attributes of the shells for paleoenvironmental reconstruction are unclear. Here we report the results of a modern process study and species level classification of Radix based on measurements of the trace element and stable isotope composition of the shells, together with an investigation of the occurrence of living Radix in lakes, wetlands and rivers in the Tibetan Plateau. The results indicate that vital effects on the trace element and stable isotope composition of the modern aragonite shells of the four most common species (R. auricularia, R. cucunorica, R. lagotis and R. acuminata Lamark) are not big, especially for the two dominant species (R. auricularia and R. cucunorica). Intraspecies differences in both the major elemental ratios and the stable isotopic values can be ignored in the larger shells of R. auricularia (length >14 mm). For different species of Radix of similar sizes, the values of Sr/Cashell (＜0.0003) and Mg/Cashell (＜0.0002) are similar in all four common species. By evaluating the importance of vital effects on quantitative paleohydrochemical constructions, we demonstrate that Sr/Cashell of Radix is a valuable tool for reconstructing the Sr/Ca ratio and conductivity of the paleo-lake water. However, the Mg/Cashell ratios of Radix have no clear environmental implications, possibly because of significant vital effects. Intraspecies differences of <0.4‰ and interspecies difference of <0.8‰ for δ18Oshell are observed amongst the four species; in addition, the intraspecies (<0.6‰) and interspecies variations of δ13Cshell of Radix (<1.7‰) are larger than those of δ18Oshell. δ13Cshell of the identified Radix mainly reflects the δ13CDIC of the ambient water and is probably limited by DIC, which is of inorganic origin and is seldom limited by DIC derived from the decomposition of organic matter. By evaluating the effects of different transfer functions of living Radix on paleohydrochemical reconstructions (Sr/Capalaeo-water, ECpalaeo-water, δ18Opalaeo-water and δ13Cpalaeo-DIC), we show that the results obtained from transfer functions using R. auricularia and R. cucunorica are very similar, and we recommend the use of transfer functions based on living Radix which are of the same species as the fossil shells. Finally, we show that δ18Oshell of both identified and unidentified Radix can be used to determine lake hydrological status (open or closed).