『Abstract
The sustainability of soil is a major issue for society. In principle,
the evolution of soil resources can be constrained by comparing
the rates of soil erosion and production. Cosmogenic isotopes
provide one measurement of soil erosion rates. They can also be
used to estimate soil production rates but only if erosion is
assumed to be balanced by production. This implies that the evolution
of soil resources (thinning, thickening or constant) since soil
thickness is assumed to be constant with time. Here we utilise
an independent method to estimate soil production rates, using
uranium series (U-series) isotopes. The study of a site in temperate
Australia undisturbed by human activity shows that soil production
rates inferred from U-series isotopes are similar to erosion rates
derived from beryllium-10 (10Be) measurements, implying
that at this site there is no net accumulation or loss of soil.
Saprolite production rates (the migration rate of the weathering
front into the bedrock) are also similar to erosion rates so the
thickness of the entire weathering profile is effectively in steady-state.
This study demonstrates that the combination of U-series and cosmogenic
isotopes can be used to quantitatively assess soil evolution and
the development of weathering profiles. Preliminary observations
suggest that the rate of bedrock weathering (i.e. saprolite production)
in temperate Australia is of the same order of magnitude as that
inferred for laterites in tropical climates. This may suggest
that, for thick weathering profiles, although the extent of weathering
strongly differs between temperate and tropical climates, the
migration of the weathering front into the bedrock occurs at a
relatively uniform rate regardless of present-day climatic conditions.
Keywords: soil production; radioactive disequilibrium; uranium-series
isotopes; chemical weathering; landscape evolution』
1. Introduction
2. Study area and analytical techniques
3. Methods
3.1. Chemical depletion fraction
3.2. Model for determining the residence time of a sample in
the saprolite
3.3. Model for determining the residence time of a sample in
the soil
4. Results
5. Discussion
5.1. Geochemical characteristic of the saprolite
5.2. Geochemical characteristics of the soil
5.3. Saprolite residence time and production rate
5.4. Soil residence time and production rate
6. Conclusions
Acknowledgments
Appendix A
A1. Saprolite residence time
A2. Soil residence time and production rate
A2.1. Lateral << vertical soil transport
A2.2. Lateral transport considered
Appendix B. Supplementary data
References