Reconstruction of Late Holocene Floods and Droughts of the Rivers Rhine and Meuse by Means of &delta18O Analyses on Freshwater Mussels (Unionidae)

The Meuse and Rhine are large rivers in the Netherlands. They both have distinct seasonal δ¹⁸O patterns caused by relative inputs of groundwater, surface runoff and Alpine glacier melting. Damaging floods are known to occur in the Dutch river area and are becoming more frequent as discharges have increased over the last century due to increased precipitation. In order to improve our understanding of river dynamics and to predict impacts of future climate change, insight in past river conditions is essential.

In unionid freshwater bivalves, δ¹⁸O has previously been applied as a proxy for water source or discharge. As shell aragonite is precipitated in isotopic equilibrium with the ambient water, analyzing growth increments can reveal seasonal patterns in δ¹⁸O.

The project (BSIK) aims at the reconstruction of late Holocene discharge patterns of the Rhine and Meuse rivers through geochemical analyses on growth increments of freshwater mussels.

We studied freshwater bivalves of the genus Unio. These are found naturally in the rivers Rhine and Meuse. The results of a monitoring experiment in which we examine the relationship between water and shell chemistry at high resolution are presented. The Unionids were kept in cages while temperature and pH were measured and water samples were taken every 14 days. After 18 months the shells were taken out and sampled at high resolution. We demonstrate that these species precipitate their shell in equilibrium with the ambient water. As such, unionid shell δ¹⁸O can serve as a proxy for past river composition and wet and dry periods.

Subsequently an extrapolation is made to the past. We show records of shells from archaeological finds and we present reconstructions of river water composition, floods and droughts for time slices in the Neolithic, Roman times, Middle Ages and Little Ice Age.