residue, but with a low base saturation) but no Luvisols, as in the claystone series. The
high proportion of illite in the limestone dissolution residue was the reason for the
high K signal generated by the Alisols. What is remarkable is that the ratio between
the Alisol and limestone signals was different for each element. Though variability in
the limestone dissolution residues might have contributed to this, it is a hint that
leaching might have contributed to the loss, especially of U. This appears logical
assuming U is predominantly present as carbonate in the limestone. In solution above
pH 5.5, the preferred ligand of U is carbonate (Unsworth et al. 2002), so that both
components are leached together. As soon as a decalcified solum is established, U
tends to residually accumulate, while potassium-bearing minerals are decomposed in
the acid environment and K is leached out of the profile. In line with these processes,
the K signal should decrease (loss of silicates) and U and Th signals should further
increase in Ferralsols, as the final well-aerated weathering product. However, the
latter was not found to occur for Th, due to the fact that Ferralsols developed in this
environment only in the vicinity of hydrothermal pipes (Herrmann et al. 2007), which
contain a high proportion of aluminium-hydroxides. Consequently, they developed
from a mixture of parent materials. Umbrisols in the limestone domain represented an
intermediate soil with respect to the intensity of gamma-ray emissions. These soils
tended to appear in sinkholes, and thus represented a mixture of colluvial material
from the surrounding slopes.
Dominant soils from claystone (Luvisols, Alisols) showed the expected decrease
in potassium due to silicate weathering, but increasing U and Th concentrations due
to residual accumulation. The stronger accumulation of Th was again a hint of the
higher level of mobility of U in this environment. The high K and U radiation levels
measured in the Umbrisols on claystone might have been due to either local
inhomogeneities related to topography or grain-size selective erosion and transport,
and this requires further research. Soils from the latite showed the expected trends
in the weathering sequence from Cambisol to Luvisol, but unexpected ratios in
relation to the supposed parent material, especially for K. The latite here is only a
small magmatic intrusion and was only sampled once. Thus, the unexpected
differences found might be attributed to parent material in-homogeneity.
In conclusion, the gamma-ray signal of soils is first of all inherited from the
parent material, and with continuing soil formation, the signal changes. Secondary
carbonate accumulation dilutes the signal, whereas silicate weathering leads to
decreasing K concentrations due to leaching, whereas the U and Th signal increases
to a different degree depending upon the residual accumulation. U appears to be
more mobile than Th, especially at a neutral to alkaline pH (carbonate buffer range).
2.2.5 Gamma-Ray Signals at the Soil Profile Scale
Here, we exclusively deal with Reference Soil Groups, as characterised by clay
illuviation, since they dominate with respect to surface coverage. Of interest is the
question as to whether clay illuviation and chemical weathering change the signal at
42
K. Stahr et al.
high proportion of illite in the limestone dissolution residue was the reason for the
high K signal generated by the Alisols. What is remarkable is that the ratio between
the Alisol and limestone signals was different for each element. Though variability in
the limestone dissolution residues might have contributed to this, it is a hint that
leaching might have contributed to the loss, especially of U. This appears logical
assuming U is predominantly present as carbonate in the limestone. In solution above
pH 5.5, the preferred ligand of U is carbonate (Unsworth et al. 2002), so that both
components are leached together. As soon as a decalcified solum is established, U
tends to residually accumulate, while potassium-bearing minerals are decomposed in
the acid environment and K is leached out of the profile. In line with these processes,
the K signal should decrease (loss of silicates) and U and Th signals should further
increase in Ferralsols, as the final well-aerated weathering product. However, the
latter was not found to occur for Th, due to the fact that Ferralsols developed in this
environment only in the vicinity of hydrothermal pipes (Herrmann et al. 2007), which
contain a high proportion of aluminium-hydroxides. Consequently, they developed
from a mixture of parent materials. Umbrisols in the limestone domain represented an
intermediate soil with respect to the intensity of gamma-ray emissions. These soils
tended to appear in sinkholes, and thus represented a mixture of colluvial material
from the surrounding slopes.
Dominant soils from claystone (Luvisols, Alisols) showed the expected decrease
in potassium due to silicate weathering, but increasing U and Th concentrations due
to residual accumulation. The stronger accumulation of Th was again a hint of the
higher level of mobility of U in this environment. The high K and U radiation levels
measured in the Umbrisols on claystone might have been due to either local
inhomogeneities related to topography or grain-size selective erosion and transport,
and this requires further research. Soils from the latite showed the expected trends
in the weathering sequence from Cambisol to Luvisol, but unexpected ratios in
relation to the supposed parent material, especially for K. The latite here is only a
small magmatic intrusion and was only sampled once. Thus, the unexpected
differences found might be attributed to parent material in-homogeneity.
In conclusion, the gamma-ray signal of soils is first of all inherited from the
parent material, and with continuing soil formation, the signal changes. Secondary
carbonate accumulation dilutes the signal, whereas silicate weathering leads to
decreasing K concentrations due to leaching, whereas the U and Th signal increases
to a different degree depending upon the residual accumulation. U appears to be
more mobile than Th, especially at a neutral to alkaline pH (carbonate buffer range).
2.2.5 Gamma-Ray Signals at the Soil Profile Scale
Here, we exclusively deal with Reference Soil Groups, as characterised by clay
illuviation, since they dominate with respect to surface coverage. Of interest is the
question as to whether clay illuviation and chemical weathering change the signal at
42
K. Stahr et al.