Varied pathways of river-borne clay minerals in a near-shore marine region: A case study of sediments from the Elbe- and Weser rivers, and the SE North Sea

Abstract

The <2 μm fraction of 45 sediment samples from the Elbe and Weser rivers and from the southeastern North Sea (German Bight) was analyzed for its contents of clay minerals and selected chemical elements. This was done to provide new information to the controversial question about the origin and distribution of clays in this near-shore marine region. The proportions of smectite, chlorite, illite and kaolinite were determined from intensities of the (001) basal reflection of the XRD pattern using weighted integrated factors and IR-mineral spectra, which were fitted to the values of the chemical analyses. Illite – the major clay mineral – was characterized by its K/Rb ratios and K-Ar ages. Anthropogenic metal contamination was demonstrated by the Cu, Pb and Zn contents. The results outline significant systematic changes in the proportions of clay minerals, in the K/Rb-ratios, the K-Ar data, and in the heavy-metal contents at the fluvial-estuarine transition and towards the marine environment. These changes result from a progressive mixing of suspended fluvial material with reworked Pleistocene sediments at the bottom of the North Sea. Near shore, the proportion of reworked Pleistocene sediment amounts only 20–30%. The mixture is distributed within a 100 km wide zone, mainly by currents parallel to the coast. It is also transported during flood tide into the estuaries, where it is mixed further with suspended fluvial material. The K/Rb ratios and K-Ar dates obtained from illite prove to be particularly useful for modeling the mixing of the clay minerals as well as their distribution patterns and origin.