ALBERT

Description: Coastal areas are dynamic environments due to both natural changes and human impact. This is especially the case for estuaries, which evolve in response to sea level change and which are sensitive to human activities affecting the river outflow and geochernistry. To be able to restrict the human impact, knowledge about its effects and mechanisms is needed, which can be hard to obtain on the background of natural variability. Therefore, in this study, the sediments from the outer, less anthropogenically disturbed part of the Göta älv estuary, a rnicro-tidal estuary in SW Sweden, are used to reconstruct the Holocene sedimentation history as a tool to evaluate natural and anthropogenic influences in the area. Due to the isostatic rebound the sea level has been falling at a decreasing rate since 7000 BP, changing morphology and sedimentation conditions. Human impact is connected to the regulation of the Göta älv River and the development of industries along the river and the city and harbour of Göteborg in the inner estuary. A seisrnic survey of the estuary conducted by Dr. P. Maurenbrecher, Delft University, and Dr. B.T.A.J. Degen, Geocom, yielded high-resolution (10 cm) profiles penetrating to about 20 m. Using this as an orientation, sediments where sampled on transects with a 3 m piston corer and on a grid using a box corer in the outer estuary and at selected sites of the inner estuary. The piston cores were used to establish sediment facies, which were dated by 14 C and pollen dating, and for 5 detailed geochernical depth profiles. Diatom analysis was conducted to aid environmental interpretations, and recent sediments were dated by 210Pb- and 137 Cs-dating. The box cores were sub-sampled at four levels and used to assess the spatial distribution of density, grain size, organic-matter and heavy-metal concentrations. Selected box cores were taken for the investigation of methylmercury and organotin compounds. The sediments in the estuary are mainly silty clays with different degrees of consolidation and organic-matter content. The setting changed from a distal glaciomarine environment to an estuarine setting between 10,200 BP and 4000 BP. Deposition has been unstable in response to sea level change, as indicated by two major hiatuses. Recent, organic matter rich sediments started to accumulate ca. 100 years ago, at rates between 0.4 and 2.5 cm/yr. The onset of sedimentation may be connected to river regulation and other anthropogenic influences. The recent clays are contaminated by heavy metals, especially mercury, originating mainly from alkali chloride and paper industry and sewage, which had highest discharges from the 1930's to the 1980's. Mass calculations suggest that the estuary has a relatively high trapping efficiency for heavy metals. High methylmercury-concentrations in the porewater of sediments from the inner estuary indicate higher bioavailability there than in the outer estuary, while the overall bioavailability is in a common range and does not seem to increase markedly by sediment dredging. The whole estuary is aff ected by contarnination with organotin compounds. lt can be concluded that the Göta älv estuary has mainly been affected by river regulation and pollution in the last 100 years, and that the pollution from these "old sins" is stored in the sediments. The recent sediments of the whole estuary contain ca. 20 t Hg and 4000 t Zn, including the geogenic portion. These can act as a contarnination source, especially when they are disturbed.