Geology Reference
In-Depth Information
Cosmogenic radionuclide dating
come in essentially uniformly from all angles.
Being charged, they are steered by the magnetic
field of the Earth, generating a stronger beam of
particles at high geomagnetic latitudes and
reducing the downward flux at lower latitudes.
These particles interact with atoms in the atmos-
phere and, thereby, create such familiar species
as
14
C. These interactions reduce the number of
energetic particles that penetrate to lower levels
in the atmosphere: the production rate of radio-
nuclides declines with a 1/e length scale of
roughly 1.5 km within the lower atmosphere.
Owing to this atmospheric attenuation, produc-
tion rates of cosmogenic nuclides at altitudes of
3 km will be e
2
(about 7) times higher than those
at sea level. These nuclides generated in the
atmosphere have been dubbed “garden-variety”
CRNs or meteoric radionuclides. Cosmic rays
that survive to impact the surface of the Earth
are capable of producing CRNs in near-surface
materials:
in situ cosmogenic radionuclides
. The
production rate is highest at the surface, and it
decays with depth in bedrock with a 1/e scale of
roughly 50-70 cm, the difference in attenuation
length scales reflecting the relative density of
rock and air. Minerals comprising atoms that are
susceptible to the nuclear reactions that gener-
ate CRNs must be present in these near-surface
materials to be useful. For instance, and quite
fortunately, the very common mineral quartz
(SiO
2
) is a target mineral for both
10
Be and
26
Al:
10
Within the last three decades, the nuclear phys-
ics community has introduced the geological
community to a new technology that allows
dating of bare bedrock surfaces and of alluvial
deposits that have been continuously exposed
to cosmic radiation since formation. These tech-
niques are evolving rapidly. For the first decade,
much of the work was performed in geomor-
phic situations in which both the geomorphic
and nuclear physics communities could benefit.
Using surfaces previously dated using other
methods, the production rate resulting from
cosmic-ray bombardment of rock was calibrated,
showing a strong dependence on both altitude
and latitude. Since then, surfaces of unknown
age have been dated in a wide variety of set-
tings. Because the materials being dated are
commonly available rocks and because the time
scale over which the technique may be applied
covers the entire Quaternary, this new technique
is often the only method available. Here we
briefly review the theory of the use of
cosmogenic radionuclides (CRNs) in a variety of
geomorphic settings. This treatment is by no
means exhaustive, but is intended to serve as an
introduction to this new and exciting field. More
detailed reviews may be found in Lal (1991),
Morris (1991), Bierman (1994, 2007), Cerling
and Craig (1994), Granger and Muzikar (2001),
and Gosse and Phillips (2001). We will see that
the major problems facing the community lie in
the interpretation of the CRN concentrations,
that considerable care needs to be exercised in
sampling appropriately, and that much effort
must be put into developing a relevant
geomorphic model of the site. Although much
has been accomplished in this new field,
the techniques continue to evolve, and the
applications of the methods continue to broaden.
Be being produced from
18
O, and
26
Al being
produced from
32
Si. See the table within Box 3.1
for a compilation of the most commonly used
species, their half-lives, and the sea-level, high-
latitude production rates.
Note that the production rates for most
cosmogenic nuclides are only a few atoms per
gram of target mineral per year (see Box 3.1).
These rates result in such low concentrations
that conventional mass spectrometry and
counting of decays are unreasonable methods
for measuring concentrations. Analyses of the
concentrations of those CRNs produced
in situ
requires separation of the target mineral from
the rock, ridding the sample of garden-variety
CRNs by a leach step, and subsequent chemical
separation of the CRNs. The minuscule sample
is then analyzed in an accelerator mass
Background
We summarize here and in the cosmogenic
primer (Box 3.1) the concepts that underpin the
essential uses of cosmogenic radionuclides in
geomorphological studies. Cosmic rays isotropi-
cally bombard the Solar System, meaning they
