Patterns and dynamics of sedimentation and biogeochemical processes in the Skagerrak
The Skagerrak serves as the largest depocenter for sediments and POC in the North Sea. Hence, Skagerrak deposits provide a historical record of how the North Sea ecosystem has changed over time. This thesis revisits sedimentation and biogeochemical processes in the Skagerrak based on a combination of solid phase and porewater geochemistry, in-situ measurements and modeling. Highly resolved temporal and spatial data are used to characterize the benthic sediment, POC and biogenic silica (bSi) cycles in the Skagerrak and to track the impact of human and natural processes in the North Sea over the past 100 years. Major findings reveal that average sedimentation rates and POC burial fluxes decreased from before to after the year 1963. Possible explanations involve a shift in the North Sea circulation pattern and increasing POC settling in other depocenters, such as the Wadden Sea. The spatial distributions reveal areas of elevated 210Pb rain rates and MAR at intermediate water depths following the shape of the general water circulation pattern. A North Sea sediment budget highlights the Skagerrak as the primary sink for material in the North Sea. The benthic cycles of POC and bSi show that 50 % of the POC and bSi are recycled and reintroduced to bottom waters. The rest is buried in the sediment. In a global context, comparably high POC and bSi burial efficiencies as well as low sedimentary bSi:POC ratios reflect the influence of the large lateral sediment input from the North Sea. The 210Pb and bSi rain rates are further used to estimate the contributions of locally produced and laterally transported POC to the Skagerrak. They show that 19±6 % of the deposited POC are derived from local primary production, whereas 81±6 % are transported laterally into the Skagerrak. The results of this thesis extend our knowledge of sedimentation and biogeochemical processes in the Skagerrak under changing environmental conditions.
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