Ultra-low density sampling of overbank sediment in Fennoscandia

Abstract

Abstract Ultra-low density sampling of overbank sediment, till, river water and humus from 49 large river catchments in Fennoscandia was conducted at the Abo Akademi University in Finland as a pilot study for IGCP Project 259, International Geochemical Mapping. The −62 μm fraction of overbank sediment was analysed by ICP for 30 elements after an Aqua-Regia leach, and for 35 elements after “total” digestion in HF-HClO 4 -HCl. Till was treated and analysed in the same way and in the same laboratory as the sediment. The purpose of the project was to test the usefulness of extremely low sampling density (1 sample/23 000 km 2 ) and the usefulness of overbank sediment compared with other sampling media. Even at this sampling density overbank sediment depicts regional content patterns for many elements, some of which can be related to known geological features and some of which cross geological boundaries. The geochemistry of overbank sediment also indicates some ore provinces and mineralized areas, the most distinct ones being two Mo-provinces, one in southern Norway and another in northern Sweden. The total leach gives more distinct patterns, which are easier to interpret, that those of the partial leach. The content-variation patterns are similar to those found in other surveys in overbank sediment, stream sediment and till. The geochemistry of overbank sediment and till are almost identical. This is seen in a good correlation and similar medians and ranges for over 20 elements. The only major difference is higher contents of Mn, Fe, Pb and Zn in the sediment, probably due to Mn-Fe-precipitates. Thus one sample of overbank sediment represents large catchment basins and substitutes for composites of 6–20 till samples. The surface layers of the sediment are anthropogenically contaminated at some sampling sites situated downstream from mines or major industries. In basins, where mines have been in operation for several hundred years, even the bottom layers are apparently polluted. The southern parts of Fennoscandia are clearly polluted with Pb through atmospheric deposition. Outside areas of mining activities or major industries, there is no other obvious pollution. The results of this survey show that overbank sediment is a suitable sampling medium for Global Geochemical Mapping.