Sedimentation rate decrease in the Skagerrak and its implication for human and natural impacts in the North Sea

Since industrial times, human and natural processes have affected the sediment system of the North Sea. As a substantial proportion of the suspended sediment in the North Sea is ultimately deposited in the Skagerrak, it offers a representative archive for reconstructing the temporal variability of the North Sea sediment system. However, little is known about how sedimentation rates in the Skagerrak may have changed over time. In this study, we present high-resolution age-depth models based on the natural radionuclide 210Pb and the anthropogenic time markers 137Cs, 14C and mercury to determine average sedimentation rates before and after the year 1963 at six stations in the Skagerrak. This year was selected because its age-depth relationship was clearly reflected by peak activities or concentrations in the sedimentary data of the time markers. The main result of this study is a consistent decrease in sedimentation rates at all stations. On average, sedimentation rates decreased from 0.36 to 0.15 cm yr-1, suggesting a substantial alteration of the North Sea sediment system. We tentatively discuss possible driving factors including a shift in the North Sea circulation pattern, increased sediment deposition in the Wadden Sea, and reduced sediment inputs into the North Sea due to coastal protection and river damming. In terms of the overall North Sea sediment cycle, these processes may outweigh the effects of sediment resuspension by human activities and storm events, as well as temperature, humidity and sea level rise caused by climate change.