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Diagenetic trace metal mobilisation in sediments of the Scheldt estuary
Zwolsman, J.J.G.; de Lange, G.J.; van Eck, G.Th.M. (1999). Diagenetic trace metal mobilisation in sediments of the Scheldt estuary, in: Zwolsman, J.J.G. Geochemisch gedrag van zware metalen in het Schelde-estuarium = Geochemistry of trace metals in the Scheldt estuary. Geologica Ultraiectina, 171: pp. 113-141
In: Zwolsman, J.J.G. (1999). Geochemisch gedrag van zware metalen in het Schelde-estuarium = Geochemistry of trace metals in the Scheldt estuary. Geologica Ultraiectina, 171. PhD Thesis. Universiteit Utrecht: Utrecht. ISBN 90-5744-030-X. 183 pp., more
In: Geologica Ultraiectina. Universiteit Utrecht: Utrecht, more

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Keywords
    Diagenesis; Mobility; Sediments; Trace metals; ANE, Netherlands, Westerschelde [Marine Regions]; Marine; Brackish water

Authors  Top 
  • Zwolsman, J.J.G., more
  • de Lange, G.J.
  • van Eck, G.Th.M., more

Abstract
    Sediment cores were collected along a wide salinity gradient in the Scheldt estuary in March/April and June 1989. Cores were taken at two coastal marine stations, one brackish water station, and one station in the low-salinity, high-turbidity zone. The cores were cut into thin sections (1-2 cm), which were pressure-filtered in order to collect the pore waters (under nitrogen atmosphere). Pore waters were., analysed for major elements, nutrients, Mn, Fe, sulphate, Ha, and trace metals (Cd, Cu, Zn). The trace metal profiles recorded in the pore waters are related to the position of the redox boundaries in the sediments. Oxidation of labile organic matter and sulphides leads to concentration maxima of dissolved Cd, Cu, and Zn near the sediment-water interface (SWI). The intensity of the trace metal peaks at the SWI depends on the redox condition of the surface sediments, but not on sediment trace metal content or pore water salinity. In general, high concentrations of Cd, Cu, and Zn are observed in the surficial pore waters of sandy sediments, which are mildly oxidising at the SWI. Silty sediments, in which reducing conditions are established close to the SWI, show low concentrations of Cd, Cu, and Zn in the surficial pore waters. This observation is related to trace metal scavenging by Fe oxyhydroxides near the SWI (suggested for one marine sediment core), or to precipitation of metal sulphides just below the SWI (in the low-salinity sediment cores). In the suboxic zone of the sediments, mobilisation of Cu and Zn is also observed, which is ascribed to release from Mn and Fe oxyhydroxides. Very low trace metal concentrations were found in the sulphate reduction zone of the sediments (if present), reflecting metal sulphide precipitation. The extent of post-depositional mobility of sediment- bound trace metals is also related to the position of the redox boundaries, in agreement with the pore water profiles. Thus, post-depositional mobility is minor in the sulphidic low-salinity sediments, because of efficient scavenging of trace metals released during organic matter decomposition. Diagenetic processes have a significant impact on the solid-phase distribution of trace metal$ in the brackish sediments, which is related to the well-resolved spacing of the redox zones in these sediments. The impact of diagenesis on the trace metal content of the marine sediments is limited, which may reflect their recent time of deposition. Due to the presence of dissolved metal peaks near the SWI, the sediments of the lower estuary may act as a source of dissolved Cd, Cu, and Zn to the water column. In the future, when oxic conditions are restored in the Scheldt river, this may also become true for the sediments of the upper estuary. Restoration of dissolved oxygen in the upper estuary is expected to cause a temporary increase in the outflow of dissolved Cd, Cu, and Zn from the sediments, because metal sulphide precipitation at the SWI would become less important than it is today.

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