『Abstract
Located in the uplands of the Valley and Ridge physiographic
province of Pennsylvania, the Susquehanna/Shale Hills Critical
Zone Observatory (SSHO) is a tectonically quiescent, first-order
catchment developed on shales of the Silurian Rose Hill Formation.
We used soil cores augered at the highest point of the watershed
and along a subsurface water flowline on a planar hillslope to
investigate mineral transformations and physical/chemical weathering
fluxes. About 25 m of bedrock was also drilled to estimate parent
composition. Depletion of carbonate at tens of meters of depth
in bedrock may delineate a deep carbonate-weathering front. Overlying
this, extending from 〜6 m below the bedrock-soil interface up
into the soil, is the feldspar dissolution front. In the soils,
depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock
define the illite and chlorite reaction fronts. When combined
with a cosmogenic nuclide-derived erosion rate on watershed sediments,
these depletion profiles are consistent with dissolution rates
that are several orders of magnitudes slower for chlorite (1-5×10-17
mol m-2 s-1) and illite (2-9×10-17
mol m-2 s-1) than observed in the laboratory.
Mineral reactions result in formation of vermiculite, hydroxy-interlayered
vermiculite, and minor kaolinite. During weathering, exchangeable
divalent cations are replaced by Al as soil pH decreases.
The losses of Mg and K in the soils occur largely as solute fluxes;
in contrast, losses of Al and Fe are mostly as downslope transport
of fine particles. Physical erosion of bulk soils also occurs:
results from a steady-state model demonstrate that physical erosion
accounts for about half of the total denudation at the ridgetop
and midslope positions. Chemical weathering losses of Mg, Na,
and K are higher in the upslope positions likely because of the
higher degree of chemical undersaturation in porewaters. Chemical
weathering slows down in the valley floor and Al and Si even show
net accumulation. The simplest model for the hillslope that is
consistent with all observations is a steady-state, clay weathering-limited
system where soil production rates decrease with increasing soil
thickness.』
1. Introduction
2. Methods
2.1. Site description
2.2. Drill core
2.3. Augered soil cores
2.4. Sample preparation and elemental analysis
2.5. Separation of clay fraction
2.6. XRD and SEM analysis
2.7. Cation exchange characterization
2.8. Stream water samples
2.9. Sampling and characterization of stream sediment for meteoric
10Be
3. Results
3.1. Parent shale
3.2. Soils
3.3. Exchangeable cations
3.4. Stream water chemistry
3.5. Basin-scale erosion rate
4. Discussion
4.1. Bedrock transformation and weathering fronts
4.2. Strain and element immobility
4.3. Elemental changes along the planar transect
4.4. Mineral transformations
4.5. Stoichiometric reactions
4.6. Hillslope evolution along a planar transect
4.7. Mineral weathering rates
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References