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| Titel |
Quantifying regolith production rates with Uranium-series isotopes at Shale Hills Critical Zone Observatory: implications for chemical weathering and landscape evolution |
| VerfasserIn |
Lin Ma, François Chabaux, Eric Pelt, Estelle Blaes, Lixin Jin, Susan Brantley |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033590
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| Zusammenfassung |
| Quantifying regolith production rates is essential in understanding many important Earth’s
surface processes such as nutrient cycling, carbon sequestration, erosion, and acid rain
mitigation. Over the long term, the rates of weathering and erosion also combine to
control the evolution of surface landscapes. Uranium-series isotopes offer a powerful
tool to investigate regolith production rates and weathering timescales within a
weathering system because of their well-documented fractionation behavior during
chemical weathering and transport by water. To quantify regolith formation rates on
shale lithology, we measured U-series isotopes (238U, 234U, and 230Th) in three
weathering soil profiles along a planar north-facing hillslope at the Susquehanna
Shale Hills Observatory (SSHO) in central Pennsylvania. All regolith samples show
significant U-series disequilibrium: (234U/238U) and (230Th/238U) activity ratios
range from 0.934 to 1.072 and from 0.903 to 1.096, respectively. These values
display depth trends that are consistent with fractionation of U-series isotopes during
chemical weathering, i.e., the relative mobility decreases in the order 234U > 238U
>> 230Th. The activity ratios observed in the soils are explained by i) loss of U-series
isotopes during water-rock interactions and ii) re-precipitation of 234U and 238U
downslope.
Regolith production rates calculated with U-series isotopes for these soil profiles decrease
systematically with increasing distance from the ridge: from ~45 m/Myr at the ridge top, the
highest point along the hillslope, to ~26 m/Myr at the middle slope site, and to ~15 m/Myr
at the valley floor. Soil weathering timescales within these profiles range from 7 kyr to 45 kyr,
increasing from the ridge to the valley floor. Given that the SSHO experienced peri-glacial
climate ~15 ky ago, we conclude that the hillslope retains regolith formed before
that glacial period and that the hillslope is not at geomorphological steady state.
The regolith production rates at Shale Hills vary as an exponential function of soil
thickness. With the local regolith production function at Shale Hills, a hillslope
soil transport model is used to predict the landscape evolution and change of soil
thickness along the planar transect. The simulation suggests that both the landscape and
soil thickness along the planar hillscope at Shale Hills are currently at a transient
state.
This research documents that U-series isotopes are powerful tools to constrain the time
scales of chemical weathering and to quantify regolith production rates. Regolith production
rates at the SSHO should be useful as a reference value for future work at other shale
weathering localities. Furthermore, this study also enhances our understanding of the
response of regolith production to climate perturbations, and the landscape evolution for this
first-order catchment. |
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