ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

common accessory mineral in many rocks types so that fission
track thermochronology is almost universally applicable
to large areas of the earth's crust. The link to denudation
arises in that cooling of rocks through the thermal gradient
that prevails in this upper crustal domain is mostly
dominated by denudation at the surface. In effect, the
quantitative thermal history for each rock sample gives a
measure of its movement towards the landsurface as material
is gradually removed from that surface.
The fission track dating method relies on the accumulation
of microscopic lines of radiation damage in natural uranium-bearing
minerals, such as apatite, from the spontaneous
nuclear fission of 238U. In general, the older the sample
the greater the number of tracks that can be observed.
However, over geological time-scales, fission tracks are stable
only at temperatures relatively low temperatures. As
temperature increases the fission tracks are gradually repaired
and, eventually disappear, in a processes known as
annealing. During annealing the tracks gradually shrink
from their ends so that the degree of annealing is indicated
by the lengths of individual tracks. The apparent fission
track age of the sample and the distribution of track
lengths are analysed together to reconstruct the thermal history
up to the temperature at which the tracks are lost
completely. For apatite, fission track annealing takes place
from ambient surface temperatures up to a maximum of
about 110°-120°C. Such temperatures are typical of the
upper 3-4 km of the crust.
Understanding of the long-term stability of fission tracks is
largely based on laboratory annealing studies, which can
reproduce the natural geological annealing process on
shorter time scales, but at higher temperatures. These studies
have led to numerical modelling procedures which
can quantitatively reconstruct the thermal history experienced
by any particular apatite sample. Fission track modelling
procedures are now routinely used to quantify the
thermal histories (i.e. the variation of temperature through
time) for the host rock masses containing the apatites. Such
reconstructions of thermal history make realistic predictions
which can be tested in deep drill holes which access
temperatures across the geological annealing zone for apatite.
Other environments such as sedimentary basins also
enable the fission track thermal histories to be tested
against other kinds of geological information and palaeotemperature
indicators. These studies indicate that thefission
track annealing models are well established and provide
realistic estimates of past temperature variations.
Different styles of thermal histories give rise to distinctive
fission track length distributions which show that only rarely
can a fission track age be taken as indicating the timing
of some particular event. Most fission track ages are
cooling ages or mixtures of two different age components
which can be misleading if interpreted at face-value. Where
sampling is available over a significant vertical interval,
such as from a drill hole or in areas of high surface relief,
then distinctive profiles of fission track age with sample
elevation are found which are also indicators of the style of
thermal history experienced. Because the samples in such a
profile have a fixed geometric relationship to each other,
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reconstruction of the thermal histories also enables the palaeo-thermal
gradients to be estimated directly. Knowledge
of the thermal gradient is important in calculating denudation
rates from the observed cooling rates. Even without
such vertical sampling information, however, it is still possible
to estimate denudation rates because the range of variation
of thermal gradients is reasonably well known in a
range of geological settings.
Using these fission track techniques, thermal histories of
upper crustal rocks can now be reconstructed in great detail
and have proved effective in the study of patterns of
denudation in various tectonic settings over time-scales of
millions, to hundreds of millions of years. These patterns
can also be studied on various spatial scales from regional
up to continental with a relatively high spatial resolution.
The wide applicability of fission track methods means that
this information can be collected relatively easily from large
areas of the crust. This type of information, together
with associated studies of the natural controls on denudation,
provides an extremely powerful methodology for investigating
the relationship between tectonics and the evolution
of topography. A large fission track data set covering
much of southeastern and eastern Australia illustrates
the usefulness this approach in reconstructing past variations
in palaeotemperature, denudation and landscape
evolution in an evolving rifted continental margin setting.
VALENTIN N. GOLOSOV
Redeposition Chernobyl Cs-137 in small basins
of Central Russia
Laboratory of Soil Erosion and Fluvial Processes
Department of Geography, Moscow State University, Vorob'evy Gory,
119899 Moscou, Russia
Chernobyl accident caused serious radionuclide pollution
of the vast areas of the Russian Plain. According to the latest
maps of radionuclide pollution the highest level of pollution
(more than 1 ku per sq. m.) are to be found in the
Tula, Kaluga, Orel and Bryansk regions of Russia. Detailed
large-scale maps of radionuclide inventories are available
for all areas with high level of pollution.
The dominant pathway for radionuclide redistribution is
associated with soil erosion and sediment delivery, since
the radionuclides are strongly sorbed by soil particles. Recent
field investigation undertaken by Laboratory of Soil
Erosion and Fluvial Processes has shown that significant
redistributional transport of radionuclides has occurred
within balka or dry creek systems, where substantial levels
of accumulation have been dominated. Preliminary estimates
of future redistribution and accumulation indicate that
40 years after Chernobyl accident radionuclide inventories
in the balka bottoms will be in 7 times higher than at present.
The balka bottoms thus represent important sinks for
radionuclides and other chemical pollutants, which could