Wim Boer

Recent sedimentation and organic carbon burial in a shelf sea: the North Sea

To obtain a carbon budget of the North Sea, recent sedimentation rates and organic carbon and nitrogen concentrations were determined on 27 box cores from the shallow North Sea (with a water depth < 200 m and a surface area of 500 × 103 km2). The sedimentation rates in these box cores range from 5 to 35 cm · 100 year−1, Corg concentrations are < 0.05 − 1%. Based on these numbers and published data the total amount of Corg buried in the shallow North Sea is calculated as ~ 1 × 105 tons · year−1. In large parts of the shallow North Sea recent sedimentation is (nearly) absent. Reworking of relict deposits and partial mixing of these deposits with small amounts of recent fine grained sediments by biological and physical processes and human activities (beam trawl fishery) result in apparent sedimentation rates which are likely to exceed the real net sedimentation rates considerably. The results of this study suggest that the majority of the organic matter is exported from the shelf over the shelf edge into the Norwegian Sea and into inner shelf deeps (Skagerrak and Norwegian Channel). Coarse grained shelf sediments are only of minor significance for the preservation of organic matter.

Historical trends in Hg, Pb and Zn sedimentation in the central shelf area of Portugal

Temporal records of excess 210Pb, and the determination of major (Al and Ca) and trace elements (Zn, Cr, Ni, Pb, Cu and Hg) in two sediment box-cores, collected in the central area of the Portuguese shelf of North of the Nazare canyon (offshore from the Lis River), allow evaluation of the deposition of various chemical elements normally associated with anthropogenic activities. In order to compensate for the natural sediment variability, heavy metal contents were normalised to Al. Temporal variations of Hg, Pb and Zn (Al-normalised) show an increasing trend since the beginning of the 1920’s, recording the development of industrial activities. Enrichment factors (EF) were calculated to estimate the level of contamination in these sediments. Mercury is the element with the highest average EF values (EF = 3), followed by Pb (EF = 1.5) and Zn (EF = 1.2). The results indicate that since 1991 64% of total Hg, 44% of total Pb and 24% of total Zn are derived from anthropogenic sources. The average anthropogenic fluxes of Hg, Pb and Zn (0.008, 3, 6 μg cm-2yr-1, respectively) for the last 40 years in a ca. 400 km2 deposition area represent a total accumulation of approximately 30, 12000 and 24400 kg per year of Hg, Pb and Zn, respectively. These results indicate that despite the high-energy conditions and the generally sandy nature of the Portuguese shelf sediments, it is possible to identify significant anthropogenic enrichments in some areas of sediment accumulation. These contaminants are not necessarily related to immediate sources but may instead indicate atmospheric and or marine transport from more distant sources.