Mass accumulation rates decreased in the Skagerrak basin over the last 100 years

Since the 19th century, the North Sea sediment system has been subject to a dynamic hydrographic regime and intense human alteration. The Skagerrak serves as the largest depocenter for suspended sediment originating from the North Sea. Thus, deposits in the Skagerrak provide a historical record of potential shifts in the sediment cycle of the North Sea. Despite the availability of mass accumulation rate (MAR) data in the Skagerrak, previous studies focused on steady-state reconstructions and little is known about how these rates may have changed over time. To address this knowledge gap, we present age-depth models based on the natural radionuclide ²¹⁰Pb and the anthropogenic time markers ¹³⁷Cs, fraction modern ¹⁴C (F¹⁴C) and mercury (Hg) to determine the MAR before and after the year 1963 at six stations in the deep Skagerrak basin between 434 and 677 m water depth. We applied 1963 as the boundary since this year is constrained by ¹³⁷Cs and F¹⁴C peaks in the sediment cores due to atomic weapons testing and changes in sedimentary Hg contents. Our primary result reveals that the MAR in the deep Skagerrak basin decreased from 0.17 to 0.14 g cm⁻² yr⁻¹ averaged across the stations. We further simulate the effect of bioturbation on the solid phase profiles by applying a reaction transport model to the data, revealing that the decline in MAR is more pronounced when bioturbation is considered (from 0.17 to 0.09 g cm⁻² yr⁻¹). Decreasing MARs in the Skagerrak basin indicate that the sediment system of the North Sea substantially changed over time. Possible reasons include a shift in the North Sea circulation pattern, enhanced sediment trapping in the Wadden Sea and reduced sediment inputs due to river damming, deepening of harbor channels and coastal protection. However, we stress that our data do not allow for a quantitative analysis of the major driving factors behind the temporal variability of sediment cycling. Hence, we recommend combining our results with information on the provenance of the Skagerrak deposits and integrating the Skagerrak data into larger-scale physical models that consider non-steady state particle transport in the North Sea.