Geoscience Reference
In-Depth Information
mass spectrometry (AMS) is the most commonly used procedure. AMS uses a particle
accelerator combined with a high-resolution detector system that can identify and count
single atoms. Cosmogenic nuclide dating can be used to date material ranging from less than
1000 years old to millions of years in age depending on the material that is dated, its age and
the analysis facilities that are available. Systematic errors can occur due to the exposure
history of the surface being poorly known, the surface having undergone significant erosion
during the examined time interval or due to invalid assumptions concerning the isotope
production rates. For accurate dating to be possible it is important that samples are taken
from unweathered surfaces. It is possible to take samples for dating surface exposure if
sufficient rock volume has been removed to expose previously shielded rock surfaces. This
method has been successfully applied to the dating of rock fall events, in addition to the
dating of glacial deposits, impact craters, basalt flows and other volcanic features and
measurements of erosion rates.
Optically stimulated luminescence (OSL) dating
OSL dating can be used to determine the age of aeolian, fluvial, colluvial and littoral
sediments. Luminescence is caused by stimulation of electrons trapped in metastable energy
levels at crystal defects. These electrons subsequently recombine in luminescence centres. This
process leads to the emission of photons when the electrons are returned to shallower trap
levels. The electrons can be released when the mineral grain is exposed to light or heat. Thus,
luminescence describes emissions of photons over and above black-body radiation by a
non-conducting solid to which energy is supplied. The OSL clock is reset in a mineral when it
is last exposed to sunlight long enough for all the trapped electrons to be released from the
crystal lattice. It has been found that only a few seconds of exposure can reduce the signal to
below the detection level. The necessary time for exposure varies with radiation levels.
Aeolian sediments have usually been well exposed to sunlight and the mineral grains often
have the previously stored luminescence energy released during transport. Fluvial sediments
are also often quite well bleached of previous luminescence energy but at times turbid flood
flows and those that occur specifically at night can contain mineral grains that have not been
well bleached. There are a variety of techniques used in order to test for adequate bleaching
and resolving this problem during the dating process. Samples from landslide events can be
obtained from the shear surface and at the accumulation lobe. Dating at the shear surface is
possible where the energy release of the mass movement was great enough to heat the
material at the shear surface to temperatures of 500 Cormore or where frictionites were
formed. Dating of material from the accumulation lobe can possibly be achieved in debris
flows, mudflows and rock avalanches that contain sandy or silty material if the mineral
grains were exposed to sunlight for a sufficient time period to reset the OSL clock.
Uranium-series dating
Uranium-series dating is based on radioactive disequilibrium in the decay chains of
238 U and 235 U. This disequilibrium is created by geochemical fractionation. The amount of
time since the end of the fractionation can be measured by the extent of balance
accomplished in the decay chains. Thus, the time span since an event that affected the decay
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