Geology Reference
In-Depth Information
If you know only the isotopic contributions
and the area of each isotopically distinct source
area, you can estimate the relative denudation
rates between different source areas (Fig. 7.11).
If you also know the total sediment load in
either the trunk stream or from one of the source
areas, then you can also calculate absolute ero-
sion rates for each contributing area (Fig. 7.11).
Hence, a simple mass balance based on tracer
abundances and source-area ratios can provide
a powerful tool for estimating modern erosion
rates and their basin-to-basin variation. Keep in
mind, however, that any calculated erosion rate
is relevant only for the phenomenon being
measured. For example, if isotopic compositions
of bedload are measured, then one can calculate
the rate of mechanical erosion in the source area
that produces the observed bedload. If one con-
siders a carbonate terrane versus a granitic one,
it is easy to imagine that the relative contribu-
tions of these rocks to the dissolved load and
the detrital load will vary greatly between these
areas, irrespective of the relative erosion rate. In
the northwest Himalaya, for example, calcite
accounts for less than 1% of the outcrop area,
but, because it dissolves much more readily than
do the prevalent aluminosilicates, it dominates
the Sr isotopic composition of the surface waters
(Blum
et al.
, 1998).
Whenever an attempt is made to infer long-
term conditions from short-term measurements
of conservative tracers, one must ask whether
present conditions are representative of the
long-term average. Certainly, one year's sediment
flux should not be considered to typify even dec-
adal rates. Comparisons of detrital ages between
different years or in different grain-size popula-
tions commonly display significant year-to-year
variation (Ruhl and Hodges, 2005). Typically, the
impact of unusual or catastrophic events on
tracer distributions is unknown. For example, in
several places in the Nepal Himalaya, massive
rockfalls and landslides have originated from the
slopes of high peaks within the Higher Himalaya
(Fort, 1987). The run-out of some of these land-
slides carried them more than 30 km into the
Lesser Himalaya (Yamanaka and Iwata, 1982).
Today, the Lesser Himalayan rivers have incised
into the landslide debris, leaving behind steep
risers sweeping up to the former upper surface
of the deposit. As these rivers continue to cut
into the bases of these landslide deposits, debris
collapses into the river and is washed away. Such
landslide deposits can distort the tracer signal in
at least two ways. First, because landslide debris
is banked against the adjacent slopes, it creates a
barrier or buffer that inhibits entry into the river
of locally derived bedrock material with its
Lesser Himalayan signature. Second, the poorly
consolidated Higher Himalayan debris is being
actively input into the river due to erosion of the
steep risers. In this circumstance, within the
Lesser Himalaya, Higher Himalayan contribu-
tions to the tracer composition would be
expected to be over-represented and inferences
based on these tracer studies would be biased
and perhaps misleading.
Regolith production rates
In soil-mantled landscapes and in the absence of
surface processes that remove unweathered bed-
rock, the rate of bedrock lowering or erosion is
primarily a function of the rate at which bedrock
is converted into transportable material (rego-
lith) by chemical and mechanical processes. In
the past, these rates have typically been poorly
known and difficult to measure. Nonetheless, if
the average regolith production rate across a
landscape can be determined and if the land-
scape is in an approximate steady state, then
long-term rates of bedrock lowering can be esti-
mated. With the advent of cosmogenic radionu-
clide techniques, rates of conversion of bedrock
to soil can now be assessed more reliably (see
the primer on cosmogenic nuclides in Box 3.1).
On a bare bedrock knob that is steadily erod-
ing, the concentration of cosmogenic nuclides
[CRN] is dependent primarily on the rate of bed-
rock lowering (
E
) (Fig. 7.12A):
Pz
E
0
*
(7.1)
=
[CRN]
where
P
0
is the production rate at the surface
and
z*
is the rock depth (commonly
∼
60 cm) at
which the production rate drops to 1/e of
P
0
,
and any decay of the isotopes is ignored. Results
