GEOCHEMISTRY OF TILL IN FENNOSCANDIA FROM ULTRA-LOW DENSITY SAMPLING

As a pilot survey for IGCP Project 259, International Geochemical Mapp ing, ultra-low density sampling of till, overbank (flood-plain) sedime nt, humus and river water was done in 49 large catchment basins (500-7 000 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 mu m fraction of all samples was analysed by ICP-ES for 30 elements after an Aqua-Regia leach. Composite samples for each basin were analysed fo r 35 elements by ICP-ES after ''total'' digestion in HF-HClO4-HNO3-HCl . The results show that the element-content variation in till even at this sampling density form distinct regional patterns, most of which d o not coincide with the major geological domains. The content levels f or most elements are, however, significantly different in the differen t domains. The contents are significantly different for groups classif ied according to underlying bedrock type. The results of this survey a re in good agreement with those of other regional surveys. Total conte nts 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 map ping in glaciated terrain. For most elements it depicts broad regional content-variation patterns of natural origin, which are unaffected by anthropogenic factors.