Multi-method geochronological approach to reconstruct post-1800 floodplain sedimentation in the upper Rhine plain, France

The precise dating of sedimentary archives covering the last 200 years in floodplains massively impacted by human activities is a major challenge. A combination of geochronological approaches is necessary to accurately date post-1800 sedimentary deposits. Here, we use a combination of a wide range of methods to unravel floodplain sedimentary dynamics, through the example of the Upper Rhine which is a highly regulated river. This comprises short--lived radionuclides (¹³⁷Cs, ²¹⁰Pb_xs) and Infrared Stimulated Luminescence (IRSL) single-grain dating. Luminescence profiling methods (IRSL screening, portable luminescence reader) were also used to further characterise sedimentation dynamics. These were combined with a hydrogeomorphological approach based on historical planimetric and hydrological data, the knowledge of engineering works as well as the morpho-sedimentary adjustments they induced. Our study demonstrates the value of historic maps as well as historical hydrological data, which provide precise time markers for dating the sedimentary archive under study. We illustrate different assumptions, validity domains and limitations inherent to each method, especially the complexity of ¹³⁷Cs to date floodplain sediments and the potential of luminescence methods for dating and estimating sedimentation continuity. We finally show the advantage of combining geochronological approaches in the construction of robust age models for young floodplain sedimentary archives in highly anthropized fluvial environments.