Examining the relationship between temperature and δ18O of freshwater molluscan carbonate from modern and Pleistocene fluvial sediments from the British Isles

The δ 18 O of freshwater molluscan carbonate (δ 18 O c ) from fluvial sequences has the potential to be a key palaeoclimate archive, given the close association between δ 18 O c values and temperature in temperate lowland freshwater systems. However, the scarcity of systematic modern and Pleistocene studies from these deposits has limited the use of molluscan carbonate δ 18 O c as a proxy for climate change. Here we present a study of fluvial molluscan carbonate δ 18 O c from modern river systems and Pleistocene fluvial sequences located in central and southeast Britain. We compare molluscan carbonate δ 18 O c from four species of gastropod to independently-derived estimates of temperature from the same contexts. We demonstrate that the δ 18 O c of modern shells from three sites in southeast Britain are consistent with formation under equilibrium conditions with modern water temperature and δ 18 O w values, which are in turn controlled by prevailing air temperature. The relationship between the δ 18 O c value and temperature is also observable in the data obtained from fossil shells from ten Pleistocene sequences. Despite uncertainties with the interpretation of both the δ 18 O c and palaeotemperature data at several localities, and the paucity of mollusc shells from cold climate contexts, this study highlights the potential of using δ 18 O c analysis to obtain palaeoclimatic records from long Pleistocene fluvial sequences in temperate regions. • Analysed δ¹⁸O in British modern and Pleistocene river molluscs, comparing with temperature and water isotope data. • Modern and Pleistocene δ¹⁸O values seem primarily driven by air temperature during carbonate mineralisation. • Interspecific δ¹⁸O variability may relate to growth differences; however, this requires further study. • Highlights the potential for δ¹⁸O of molluscan carbonate to provide palaeoclimatic records from long fluvial sequences.