Geology Reference
In-Depth Information
Steady-State
Erosion
Landslide-Dominated
Erosion
Time
Time
Incipient
landslide
A
contours of
CRN concentration
15
Modeled Erosion Rates
in Landslide-Dominated Area
5 mm/yr =
l on g-term r at e
75%
mean
10
25%
V o lumet r ic
CRNs
5
0
B
12
3
45
1
2
3
4
5
Basin Order
Basin Order
Fig. 7.17 Cosmogenic erosion rates in landslide-dominated catchments.
A. Contrasting models for cosmogenic radionuclides (CRNs) in landscapes dominated by grain-by-grain erosion
versus ones where landsliding is prevalent. Whereas in the grain-by-grain case, erosion removes those rocks and
soils with the highest CRN concentrations, landslides slice downward to include rocks with low CRN
concentrations that will lower the overall detrital CRN concentration. B. Results of a numerical model for a
landslide-dominated landscape where the overall long-term erosion rate is 5 mm/yr and where grain-by-grain
weathering is 0.1 mm/yr. In the model, landslides are drawn from a magnitude-frequency distribution (Fig. 7.15)
and randomly dropped on the landscape to yield a long-term rate of 4.9 mm/yr. The model keeps track of both
the volumetric erosion rate (left) and the cosmogenic erosion rate (right). At small catchment sizes (first-
and second-order), both volumetric and cosmogenic concentrations will greatly underestimate the long-term
erosion rate. The largest catchments come closest to approximating the actual erosion rate. The model results
indicate that, in rapidly eroding regions, cosmogenic field studies should target high-order catchments in order
to obtain the best estimate of the long-term erosion rate. Modified after Niemi et al.  (2005).
upstream catchment; known extent of glaciers
and annual variations in snow cover; and
nuclide concentrations that are independent of
grain size or the erosion process. Although
some of these assumptions are impossible to
test, when they are violated, they can warp or
invalidate interpretations. If a large fraction of
erosion occurs by bedrock landsliding, for
example, cosmogenic concentrations are likely
to significantly underestimate the actual ero-
sion rate, especially in low-order catchments
(Fig. 7.17). When erosion rates are slow and
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