『Abstract
Rare earth element (REE) distributions and Pb isotope compositions
were explored in soils varying in age from ca. 0.4 to ≧300 ka,
developed on moraines in the Wind River Mountains, Wyoming. Soil
extracts (0.6 M HCl) were used to examine the soil labile pool
while the major element distribution in soil profiles was used
to determine the extent of weathering at different soil depths.
The results show that the chondrite-normalized REE patterns of
the deepest bulk soil within each profile reflects the composition
of the moraine till, except for the oldest soil. Up to ca. 12
ka, the soil extract fraction is enriched in light REE, indicating
early release of light REE to the soil labile pool while that
of the two oldest soils are relatively enriched in heavy REE.
In the soil extracts the La/Sm ratio normalized to the deepest
soil (LaD/SmD) decreases
systematically with soil age. Similarly, the Eu-anomaly in the
deepest soil from each profile (EuD/EuD*) decreases slightly with soil age
in the three young soils; however, EuD/EuD* increases with soil age in the older
soils. This systematic trends of these two ratios indicate the
depletion of light REE in young soils and the enrichment of Eu
and heavy REE in the older soils. Based on the Pb isotope ratios,
the relative contribution of Pb to the soil labile pool via mineral
weathering of U- or Th-rich phases was assessed for the different
stages of weathering. The whole-soil profile 208Pb/204Pb
ratio was found to decrease with soil age and with LaD/SmD, whereas it increased with the EuD/EuD* ratio. In each horizon, Pb isotope
ratios (206Pb/204Pb, 207Pb/204Pb,
and 208Pb/204Pb) ratio generally decrease
with soil age. In order to overcome possible effects from parent
material heterogeneity, the amount of radiogenic Pb as compared
to the whole-soil composition was calculated and this was found
to decrease systematically with soil age.』
1. Introduction
2. materials and methods
2.1. Study area and sample collection
2.2. Methodology and analytical procedures
3. Results
3.1. Major and trace elements in soil extract and bulk soil
3.2. REE in soil extracts and bulk soil
3.3. Soil horizons
3.4. Lead isotopic composition in soil extracts and bulk soil
4. Discussion
4.1. The significance of the dilute acid-extract analyses
4.2. The soil REE pool
Bulk soil
Soil extracts
4.3. The soil Pb isotopic composition
5. Conclusions
Acknowledgments
References