56 Peter Stille and Graham Shields
granite are around 1.212. The Pb isotopic compositions of the insoluble residue
give values varying between 1.21 and 1.215 and are therefore identical to the
unweathered granite (Table 4.1). The soluble labile component (LR) shows not
only a trend in Pb isotope composition but also in Pb concentration with
increasing depth in the soil profile. The Pb concentration decreases with
increasing depth while the Z~176
ratios increase. On the basis of isotope
geochemistry, two distinct regions can be recognized in the soil profile:
In the uppermost centimeters of the soil profile ~-~
ratios between 1.19
and 1.217 can be determined for the LR. These values are close to those of the
atmosphere in this region (average ~pb/2~
since 1870: 1.17; present-day:
1.22). It can therefore be assumed that the uppermost centimeters of the soil
profile contain a significant component of industrial lead. Deeper in the profile,
the ratios vary around 1.3, which is far higher than that characteristic for industrial
lead and is also higher than that of fresh, unweathered granite or that of the
insoluble residue (NLR; 1.22). Erel et al. consider that this part is not influenced
by industrial lead but by the weathering of accessory minerals such as apatite,
allanite and monazite with high U/Pb ratios and therefore high Pb isotopic
compositions. This free, radiogenic lead was released into the labile reservoir
(LR) of the rock system.
The soil profile was also measured for its organic carbon content (Table 4.2).
The concentrations of lead and organic carbon allow us to recognize an
anthropogenic as well as a geogenic trend (Fig. 4.7).
6~176
500
lOO
0
rat trend
20
40
60
8o
~oo
Organic carbon (%)
Fig. 4.7. Relationship between Pb concentration and organic matter in soils, (Erel et al.
1990)
granite are around 1.212. The Pb isotopic compositions of the insoluble residue
give values varying between 1.21 and 1.215 and are therefore identical to the
unweathered granite (Table 4.1). The soluble labile component (LR) shows not
only a trend in Pb isotope composition but also in Pb concentration with
increasing depth in the soil profile. The Pb concentration decreases with
increasing depth while the Z~176
ratios increase. On the basis of isotope
geochemistry, two distinct regions can be recognized in the soil profile:
In the uppermost centimeters of the soil profile ~-~
ratios between 1.19
and 1.217 can be determined for the LR. These values are close to those of the
atmosphere in this region (average ~pb/2~
since 1870: 1.17; present-day:
1.22). It can therefore be assumed that the uppermost centimeters of the soil
profile contain a significant component of industrial lead. Deeper in the profile,
the ratios vary around 1.3, which is far higher than that characteristic for industrial
lead and is also higher than that of fresh, unweathered granite or that of the
insoluble residue (NLR; 1.22). Erel et al. consider that this part is not influenced
by industrial lead but by the weathering of accessory minerals such as apatite,
allanite and monazite with high U/Pb ratios and therefore high Pb isotopic
compositions. This free, radiogenic lead was released into the labile reservoir
(LR) of the rock system.
The soil profile was also measured for its organic carbon content (Table 4.2).
The concentrations of lead and organic carbon allow us to recognize an
anthropogenic as well as a geogenic trend (Fig. 4.7).
6~176
500
lOO
0
rat trend
20
40
60
8o
~oo
Organic carbon (%)
Fig. 4.7. Relationship between Pb concentration and organic matter in soils, (Erel et al.
1990)