A carbon storage perspective on alluvial sediment storage in the Rhine catchment

Floodplains are important sinks in terms of the sediment and carbon flux in fluvial systems. Despite the large amount of detailed geomorphological studies, which include measurements on total organic carbon, little is known of the coupling of long-term carbon and sediment flux, and on the amount of carbon storage in floodplains. Therefore, the influence of the sedimentary facies in fluvial systems was investigated based on a statistical analysis of 1948 organic carbon measurements in different parts of the Rhine catchment. The analysis allows to develop a conceptual carbon budget model of fluvial systems, which was coupled with a Holocene alluvial sediment budget, to estimate the Holocene sequestration rates of carbon storage in floodplains. Averaged over the Rhine catchment the sedimentary carbon sequestration ranges between 3.4 to 25.4 g m^{- 2} yr^{- 1} with more reasonable values between 5.3 to 17.7 g m^{- 2} yr^{- 1}. Compared to the recent particulate carbon export, these values are in the same order of magnitude but somewhat smaller indicating that approximately the same amount of the exported carbon may be stored in floodplains. However, compared to sedimentary carbon sequestration rates obtained elsewhere, the presented values are at the lower limit, corresponding to the lower mean Holocene soil erosion and floodplain accumulation rates.