ENRICHMENT OF METALS IN THE ORGANIC-SURFACE LAYER OF NATURAL SOIL - IDENTIFICATION OF CONTRIBUTIONS FROM DIFFERENT SOURCES

The humus layer of natural soils is frequently used as a sampling medi um in geochemical exploration surveys, This procedure has been questio ned because the concentration level in the humus layer does not necess arily reflect the geochemistry in the underlying mineral material, Fou r sources of metals-to the organic surface layer are identified: (1) b ioturbation; (2) the 'vascular pump'; (3) atmospheric input of marine origin; and (4) airborne pollution, Two methods to define and discuss the contributions from these sources are presented for a survey of Nor wegian soils, The first method is based on calculation of ratios of co ncentrations between humus and undisturbed subsoil within regions of s imilar topography, climate and air pollution, Concentration ratios con sistently below 0.1 and low inter-region variation, such as for Al and Fe, characterize elements supplied to the humus by mixing-in of miner al soil, while concentration ratios appreciably higher than 0.1 identi fy some additional mechanism, The latter is evident for elements such as Pb, Zn and Cd (air pollution), Sr and Mg (atmospheric input of sea- salt), and Ca (the 'vascular pump'), The second method involves subtra ction of the contributions from mineral matter, assuming that the ash content reflects the mass, and C-horizon soil the composition of the m ineral matter in the humus layer, As mineral grains mixed into the hum us layer are often fairly strongly weathered compared with the undistu rbed mineral soil, this method will result in a certain over-compensat ion; nevertheless it leads to some interesting conclusions as is shown for Ba (the 'vascular pump'), and Zn and V (Long-range transport and local air pollution).