Sub-recent nitrogen-isotope trends in sediments from skagerrak (North sea) and kattegat:: Changes in N-budgets and N-sources?

We determined N-15/N-14 ratios of total nitrogen in surface sediments and dated sediment cores to reconstruct the history of N-loading of the North Sea. The isotopic N composition in modern Surface sediments is equivalent to and reflects the isotopic mixture of oceanic nitrate on the one hand (delta N-15=5 parts per thousand) and the imprint of river-borne nitrogen input into the SE North Sea (delta N-15 up to 12 parts per thousand in estuaries of the SE North Sea) on the other hand. We compare the results with 615 N records from pre-industrial sediment intervals in cores from the Skagerrak and Kattegat areas, which both constitute significant depositional centres for N in the North Sea and the Baltic Sea/North Sea transition. As expected, isotopically enriched anthropogenic nitrogen was found in the two records from the Kattegat area, which is close to eutrophication sources on land. Enrichment of delta N-15 in cores from the Skagerrak - the largest sediment sink for nitrogen in the entire North Sea - was not significant and values were similar to those found in sediment layers representing pre-industrial conditions. We interpret this isotopic uniformity as an indication that most riverine reactive nitrogen with its characteristic isotopic signature is removed by denitrification in shallow shallow-water sediments before reaching the main sedimentary basin of the North Sea. (C) 2008 Elsevier B.V. All rights reserved.