Geology Reference
In-Depth Information
must be done carefully, as the magnetic field
intensity has been shown to vary significantly
over the last 140 ka (e.g., Meynadier
et al.
, 1992).
This record of field intensity variations will no
doubt be extended by future research. The
production-rate history at a sampling site should
be calculated from a knowledge of the average
production rate deduced from a sample of
known age, the magnetic field history, and the
relationship between field intensity and produc-
tion rate (e.g., Clark
et al.
, 1995).
Knowledge of the production rate and its
dependence on elevation (e.g., Stone, 2000),
latitude, and time has greatly advanced over the
last decade in part due to concerted and coordi-
nated efforts in both Europe and North America.
Among other activities, sites of independently
constrained age are being used to calibrate the
production-rate functions. One result is a calcu-
lator in which these functions are embedded
that allows one to calculate the local production
rate of a sample of given thickness and density
(see Balco
et al.
, 2008). The half-life of
10
Be has
also been reassessed (see table in Box 3.1),
placing this method on yet more firm ground.
It remains the case, however, that the quality
of the interpretation of the CRN concentration
extracted from a sample, for either the exposure
age or the erosion history at the site, is dependent
on the degree to which the geomorphic processes
active at that site over the history of the surface
can be captured in a quantitative model that
incorporates a reliable geomorphic history. Not
only must the inheritance of a particular sample
be assessed, but the post-depositional processes
that might alter the production-rate history of
the sample should be dealt with. On boulders
presently at the surface, one must worry about
whether the boulder has always been at the
surface, as well as the likelihood of erosion of
the boulder once it was emplaced on the surface.
Where range fires cause boulders to spall,
samples must be collected to avoid or account
for this process. One may also have to assess the
role of burial by snow for some portion of the
age of the surface. At present, this new set of
dating techniques is generating much interest.
Moreover, these new dating capabilities are
opening a set of problems that provide an
excellent opportunity for the marriage of new
chronological tools with new modeling tools: a
topic to which we turn later in the topic.
Conclusions
In this chapter, we have sampled the dating
methods available to provide the time scales for
tectonic-geomorphic studies. The choice of
method must be dictated by the availability
of the proper materials, the details of the
geomorphic setting, and, of course, the cost.
Whereas relative dating methods are quick and
dirty, they should not automatically be shunned.
They do produce immediate results, whereas
other techniques might take months to a year
for processing of samples. In using any
method, it is incumbent upon the researcher to
document carefully both the geomorphic and
depositional setting. Useful interpretation of the
dates, sometimes painstakingly and expen-
sively obtained, relies heavily on careful field
observations: the field context and geomorphic
history of a sample provide the critical
framework for appropriate interpretations of
any date or rate obtained from it.
