『Abstract
Hillslopes have been intensively studied by both geomorphologists
and soil scientists. Whereas geomorphologists have focused on
the physical soil production and transport on hillslopes, soil
scientists have been concerned with the topographic variation
of soil geochemical properties. We combined these differing approaches
and quantified soil chemical weathering rates along a grass covered
hillslope in Coastal California. The hillslope is comprised of
both erosional and depositional sections. In the upper eroding
section, soil production is balanced by physical erosion and chemical
weathering. The hillslope then transitions to a depositional slope
where soil accumulates due to a historical reduction of chemical
incision at the hillslope's base. Measurements of hillslope morphology
and soil thickness were combined with the elemental composition
of the soil and saprolite, and interpreted through a process-based
model that accounts for both chemical weathering and sediment
transport. Chemical weathering of the minerals as they moved downslope
via sediment transport imparted spatial variation in the geochemical
properties of the soil. Inverse modeling of the field and laboratory
data revealed that the long-term soil chemical weathering rates
peak at 5 g m-2 yr-1 at the downslope end
of the eroding section and decrease to 1.5g m-2 yr-1
within the depositional section. In the eroding section, soil
chemical weathering rates appear to be primarily controlled by
the rate of mineral supply via colluvial input from upslope. In
the depositional slope, geochemical equilibrium between soil water
and minerals appeared to limit the chemical weathering rate. Soil
chemical weathering was responsible for removing 6% of the soil
production in the eroding section and 5% of colluvial influx in
the depositional slope. These were among the lowest weathering
rates reported for actively eroding watersheds, which was attributed
to the parent material with low amount of weatherable minerals
and intense coating of the primary minerals by secondary clay
and iron oxides. We showed that both the morphologic disequilibrium
of the hillslope and the spatial heterogeneity of soil properties
are due to spatial variations in the physical and chemical processes
that removed mass from the soil.
Keywords: weathering; erosion; soil geochemistry; hillslope processes;
sediment transport; channel incision』
1. Introduction
2. Conceptual framework
3. Study site and laboratory analysis
4. Results and discussion
4.1. Geomorphic context
4.2. Mineralogical context
4.3. Elemental concentrations of soils and saprolites
4.4. Inverse modeling calculation of colluvial flux
4.5. Inverse modeling calculation of chemical weathering rates
in the colluvial soil
4.6. Contribution of soil chemical weathering to landscape denudation
4.7. Topographic pattern of total chemical weathering rates within
the eroding hillslope
4.8. Reduction in chemical weathering rate as the colluvial soil
moves from the eroding to depositional slope
5. Conclusions
Acknowledgements
Appendix A. Supplementary data
References