Geology Reference
In-Depth Information
where
r
is the erosion rate of the surface, and
t
is time,
t
= 0 being the time of sampling and
t
into the past being positive. Ignoring decay, for
the moment, and solving Eqn 3.4 with this
steady production-rate history yields
eroding. The principal problem lies in the
likelihood that the clasts being used to assess
the concentration of CRNs have experienced
prior exposure elsewhere within the geo-
morphic system before being deposited on the
surface to be dated. The nuclides accumulated
during this exposure are called the “inheritance.”
Consider first a fluvial terrace. The inheritance
derives from a combination of: (i) exhumation
through the cosmogenic nuclide-production
boundary layer as the hillslope surface is
lowered; (ii) transport within the hillslope
system; (iii) transport within the fluvial system,
which will entail occasional burial in fluvial
bars; and (iv) final deposition on the terrace to
be dated.
Tactics must be employed in the sampling of
the terrace materials that allow separation of the
inheritance signal. In several fluvial systems
studied to date, the inheritance can represent a
significant (several tens of percent) portion of
the total CRN concentration measured in clasts
sampled from the terrace surface (e.g., Anderson
et al.
, 1996; Repka
et al.
, 1997) (Fig. 3.18).
Because the inheritance differs markedly from
one surface to another, one must employ some
means of constraining this inheritance on a site-
by-site basis. One effective strategy is to collect
samples at varying depths in a terrace profile.
Each sample should consist either of sand or of
a contribution of an equal amount of rock from
numerous clasts buried at the same level, for
example, 10 g from each of 30 clasts. Assuming
that the terrace aggraded quickly so that each
sampled layer was rapidly buried to near its
present depth, then the age of each sample
represents a combination of the time since
deposition and the inheritance at the time of
deposition. When samples are drawn from deep
enough in the terrace, say greater than
∼
2 m,
their concentrations should reflect almost
exclusively the inherited nuclides, because CRN
production rates are negligible at this depth.
One other field situation bears mentioning. On
many surfaces, the topmost few decimeters have
been well stirred - turbated - by one or another
mechanism: rodents, earthworms, tree roots, frost
churning. This churning homogenizes all constitu-
ents within that layer, including the minerals
N
=
P
0
(
z
*/
r
)
(3.6)
which can be solved for the erosion rate,
r
. Here
the term
z
*/
r
represents the time it takes the
sample to travel through the boundary layer in
which the production rate is significant.
Importantly, it is this time scale over which
the measurement of erosion rate is being
averaged in this application. The faster the
parcel is exhumed, the lower the resulting
concentration.
Including decay alters the equation to
N
=
P
0
z
*/[
r
+ (
z
*/
l
)]
(3.7)
If, on the other hand, the erosion occurs in
steps - due to fire spalls, for instance (Bierman
and Gillespie, 1991) or to joint block removal
(Small
et al.
, 1997) - then the calculation of
a long-term erosion rate from employment of
Eqn 3.6, and the CRN measurement of a surface
sample, will vary depending on the time since
the last spall event. The error in the estimate
will depend on the thickness of the spall. In
situations like those involving spalls, averaging
of several sample sites is required to provide a
more robust sense of the mean rate at which
the surface is being lowered (Small
et al.
, 1997).
If one would still like to extract an exposure
age from this site, and not just an erosion rate, Lal
(1991) has shown that the use of two cosmogenic
radionuclides with differing half-lives allows
some additional constraint on the exposure age
in certain scenarios. Working in the
36
Cl system,
supplemented by
10
Be analyses, Phillips
et al.
(1997) have demonstrated the usefulness of this
technique in constraining the age of large boul-
ders whose surfaces have been slightly eroded
since emplacement on the surface to be dated.
Depositional surfaces
Depositional surfaces, such as fluvial and marine
terraces, present their own problems, even if
the surfaces can be safely assumed not to be
