Enrichment of metals in the organic surface layer of natural soil: identification of contributions from different sources

Abstract

The humus layer of natural soils is frequently used as a sampling medium in geochemical exploration surveys. This procedure has been questioned because the concentration level in the humus layer does not necessarily reflect the geochemistry in the underlying mineral material. Four sources of metals to the organic surface layer are identified: (1) bioturbation; (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 Norwegian soils. The first method is based on calculation of ratios of concentrations between humus and undisturbed subsoil within regions of similar topography, climate and air pollution. Concentration ratios consistently below 0.1 and low inter-region variation, such as for Al and Fe, characterize elements supplied to the humus by mixing-in of mineral soil, while concentration ratios appreciably higher than 0.1 identify 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 subtraction of the contributions from mineral matter, assuming that the ash content reflects the mass, and C-horizon soil the composition of the mineral matter in the humus layer. As mineral grains mixed into the humus layer are often fairly strongly weathered compared with the undisturbed mineral soil, this method will result in a certain over-compensation; 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).