Geoscience Reference
In-Depth Information
Table 1.2
Isotopic tracing methods used in soil science.
Method
Examples of application
13
C/
12
C
Dynamics of compounds in organic matter; study of neogenesis of
CaCO
3
15
N/
14
N
Origin and fate of nitrogen in soils
Deuterium/H
Conditions of clay mineral formation; quantification of evaporation
and
18
O/
16
O and transpiration
44
Ca/
40
Ca
Cycle of basic cations in soils and plants
66
Zn/
64
Zn
Dynamics of Zn; pedogenesis
87
Sr/
86
Sr
Study of the dynamics of the soil/plant/atmosphere system
By way of example, let us examine the third row of the table. Clay
minerals contain different isotopes of oxygen and of hydrogen. Various
factors are liable to alter the contents of these isotopes (Savin and Hsieh
1998). It is therefore difficult to interpret the results. In the present state
of knowledge, the
18
O and deuterium contents of a kaolinite seem to
increase with temperature, which could give indications of the climate
(cold, temperate or tropical) that reigned in the epoch during which the
mineral was formed.
1.3.4
Other Dating Methods
Tree rings enable counting the years of growth, provided the base of the
trunk is considered (for at the top of the tree, the year-old shoot, when
cut, will show only one growth ring!). The rings are thicker when the
climatic conditions are favourable. From one tree to another, it is possible
to find out characteristic growth sequences corresponding to a given
group of years, for example, 'p-p-p-V-p-V-V-V-p' where p indicates
poor growth and V, vigorous growth. In USA, using trunks preserved
in a peat bog, researchers were able to splice such 'barcodes' from one
tree to another and reconstruct little by little a complete sequence for
nearly 9000 years. Carbon dating of different rings gives the limits of
this isotopic method (see above).
Dendrochronology
Crystals in nature are irradiated by radioactive elements (
238
U,
235
U,
232
Th,
40
K, etc.) that emit alpha-, beta- and gamma-rays. To this irradiation by
natural radioactivity is added that caused by cosmic rays. Some atoms
of the crystals impacted by such rays are found to be ionized, whereby
electrons are liberated to be trapped in defects in the mineral structure.
Thermoluminescence
