Recent and historical discharge of a large European river system – oxygen isotopic composition of river water and skeletal aragonite of Unionidae in the Rhine

Seasonal variations in the oxygen isotopic composition of Rhine River water were analyzed in detail and compared with the oxygen isotopic record from recent and historical specimens of freshwater bivalves (Unionidae). The purpose of this study was to investigate the potential use of these aragonitic bivalves as proxy recorders for varying amounts and sources of discharge, and thereby infer climate change. Seasonal variations in the river water δ18O are on the order of 1–2‰. During summer, Alpine melt-water contributes significantly to the total discharge, resulting in average values of −10 to −10.5‰, whereas the non-Alpine contribution is higher during winter, as indicated by mean δ18O values of −8.5 to −9‰. The basic pattern of the modern seasonal variation of δ18O of river water can be described by a numerical mass balance approximation of the various contributions from the Alpine and non-Alpine catchments with their average δ18O composition. The δ18O of growth increments in the prismatic shell layer of Anodonta corresponds perfectly to what is predicted by known fractionation of 18O between water and aragonite. Shell growth is restricted to water temperatures above 8–10°C, so variations in δ18O and the river water temperature are faithfully recorded by relatively large growth increments during summer. The distinctive isotopic signatures of individual flood events during summer and autumn are also recorded in the shells.