WIDE-SPACED SAMPLING OF HUMUS IN FENNOSCANDIA

In support of IGCP Project 259 (International Geochemical Mapping), a pilot survey designed to test the effectiveness of analysing dried hum us collected at an extremely low sampling density (one composite sampl e per 23 000 km(2)) for geochemical mapping was carried out in 49 larg e catchment basins in Fennoscandia. The samples were analysed for thei r aqua-regia leachable contents of 30 elements by ICP-ES. The results were compared with those of till sampled at the same sites and with mo ss from previous surveys. The correlation with underlying bedrock was also studied. Geological features exert only a weak influence on the r egional distribution of elements in humus. Exceptions are high levels of Ca and Sr in areas of carbonate bedrock and low contents of K in ar eas of Archean rocks. The distribution patterns of Co, Cr, Cu, Mg, Ni and Zn in humus coincide partly, and the levels correlate positively w ith those in till. The contents of Ni, Cr and Cu are lower in humus ov erlying acid magmatic rocks and arkose than other rock types. Pollutio n is the obvious source of high concentrations of Pb, Cd and Zn in sou thern Sweden and southern Norway. Acid fallout seems to cause leaching of Al, P and La from the humus horizon. The contents of these element s are low in the south where deposition of S and N is high and pH of r ainwater is below 4.5. Deposition of sea salts gives high levels of Na , Mg, K and B along the coast of Norway and southern Sweden. The conte nts of several elements in humus correlate positively with the amount of organic matter (LOI). Comparison between four types of forest shows that the contents of Ca, Mg and Na in birch forest and of Cd, Mn, Pb and Zn in spruce forest are significantly higher than in other forest types. These features, however, are considered false and are due to so urces such as atmospheric deposition of anthropogenic metals and sea s alt. The results of the relatively strong chemical attack used for ana lyses of humus samples are less informative than those from till and o verbank sediments. A weaker attack may have given more useful informat ion. The humus horizon could be suited for environmental monitoring if used in global geochemical mapping.