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

The δ¹⁸O of freshwater molluscan carbonate (δ¹⁸O_c) from fluvial sequences has the potential to be a key palaeoclimate archive, given the close association between δ¹⁸O_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 δ¹⁸O_c as a proxy for climate change. Here we present a study of fluvial molluscan carbonate δ¹⁸O_c from modern river systems and Pleistocene fluvial sequences located in central and southeast Britain. We compare molluscan carbonate δ¹⁸O_c from four species of gastropod to independently-derived estimates of temperature from the same contexts. We demonstrate that the δ¹⁸O_c of modern shells from three sites in southeast Britain are consistent with formation under equilibrium conditions with modern water temperature and δ¹⁸O_w values, which are in turn controlled by prevailing air temperature. The relationship between the δ¹⁸O_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 δ¹⁸O_c and palaeotemperature data at several localities, and the paucity of mollusc shells from cold climate contexts, this study highlights the potential of using δ¹⁸O_c analysis to obtain palaeoclimatic records from long Pleistocene fluvial sequences in temperate regions.