Geology Reference
In-Depth Information
lTributary tracer contribution
A
Tracer
abundance
Tracer
value
Denudation
rate
Lithological
variation
Yield of tracer
per unit area per unit time
Area
B
Isotopic Mixing
relative
denudation
rate
total
load
isotopic
contribution
area
percentage
15%
60%
25%
20%
20%
0.15 / 0.2 = 0.75
0.6 / 0.2 = 3.0
0.25 / 0.6 = 0.42
60%
100%
100%
mean denudation
rate for each area
total load
per unit time
isotopic
x / contributing area =
contribution
C
Detrital Age Mixing
Down-
stream
0.4 x A
0.6 x B
Trib A
Trib B
+
Age (Ma)
Age (Ma)
Age (Ma)
Fig. 7.11
Use of tracers to determine relative and absolute erosion rates.
A. The contribution of a tracer from a tributary catchment depends on lithologic variability, denudation rates, and
catchment size. B. Unmixing of the downstream signal derived from source areas with discrete isotopic signatures can
reveal relative erosion rates. If the actual erosion rate is known for any of the source areas, then a rate for each source
area can be estimated from the isotopic contribution. C. Mixing models for populations of detrital ages from two
catchments. The downstream probability distribution of detrital ages (right) can be deconvolved to determine
the relative contribution from two tributary catchments, if the detrital age distribution from each catchment is known.
Here, the downstream catchment records 40% and 60% of its ages from tributaries A and B, respectively. Modified
after Brewer
et al.
(2005) and Amidon
et al.
(2005).
reflect the nearby lithologic characteristics. Now
imagine measuring water or sediment in the
stream at a point many kilometers downstream
from the drainage divide, where the zone con-
taining the high peaks constitutes only a small
fraction of the upstream catchment. If the tracer
value in the water or sediment in this area is still
dominated by contributions from the high
peaks, it would indicate that these peaks are the
sites of the most rapid denudation.
