The temporal variability of sediment composition in modern rivers: provenance or grain size signal?

The first step in any provenance study is sampling the sediment of interest, which is subject to many potential sources of error. It is nevertheless commonly assumed that one sample of sediment is enough to provide a somewhat “basin-averaged” compositional signal. Spatial or temporal variability of this signal is often not considered. In this study, we test the temporal variability of sediment composition in modern fluvial deposits in four German rivers: the Gersprenz, Modau and Mümling in the Odenwald, and the Neckar close to the city of Tübingen. We revisited the same locations 12 times in the course of one year to take a sample. The samples were analyzed for grain size distribution (GSD) and geochemistry via XRF measurement. The results show that (1) the four river sediments have overall different GSD, which could be related to their different source lithologies; (2) there are variabilities in the GSD of samples taken in different months, and these can be correlated with flood events; (3) the bulk geochemistry changes significantly between months; (4) within narrow grain size windows (1Φ-steps), the chemical composition varies less than the bulk geochemistry. We conclude that bulk geochemistry of fluvial sediment varies mostly as a result of varying GSD, and not due to actual provenance changes throughout the year. On the one hand, this is promising for studies that assume fluvial sediment to faithfully reflect a “basin-averaged” provenance signal. On the other hand, it shows that geochemical data should always be interpreted in tandem with GSD.