Geochemistry of till in Fennoscandia from ultra-low density sampling

Abstract

As a pilot survey for IGCP Project 259, International Geochemical Mapping, ultra-low density sampling of till, overbank (flood-plain) sediment, humus and river water was done in 49 large catchment basins (500–7000 km 2) in Fennoscandia. The main objectives of the survey were to test the usefulness of extremely low sampling densities (1 station per 23 000 km 2) and of different sampling media for global geochemical mapping. Till was sampled at 6–20 sites from each basin. The − 62 μm fraction of all samples was analysed by ICP-ES for 30 elements after an Aqua-Regia leach. Composite samples for each basin were analysed for 35 elements by ICP-ES after “total” digestion in HF-HClO 4-HNO 3-HCl. The results show that the element-content variation in till even at this sampling density form distinct regional patterns, most of which do not coincide with the major geological domains. The content levels for most elements are, however, significantly different in the different domains. The contents are significantly different for groups classified according to underlying bedrock type. The results of this survey are in good agreement with those of other regional surveys. Total contents give more reliable and more easily interpretable results than Aqua-Regia soluble contents. The regional content patterns in till are for many elements less distinct than those in overbank sediment from the same basins. The C-horizon of till is a suitable medium for global mapping in glaciated terrain. For most elements it depicts broad regional content-variation patterns of natural origin, which are unaffected by anthropogenic factors.