Geoscience Reference
In-Depth Information
Fi rst ly, i f t hey a re u n st able a nd d i sappea r over t i me (rad ioi sotope s),
they can serve as clocks under certain conditions for dating. Let us
recall that alpha radiation corresponds to loss of two protons and
two neutrons or to the emission of one nucleus of helium
4
He
2+
;
the atomic mass is thereby reduced by 4. In beta-radioactivity,
the nucleus will lose or gain an electron without change in mass,
but the atomic number changes and therefore the properties.
In gamma-radioactivity there is emission of electromagnetic
energy corresponding to a lowering of the excitation level of
the nucleus.
Secondly, some substances comprise a fixed proportion
of different stable isotopes. Thus C3 plants and C4 plants
(photosynthesis types) absorb
13
C and
12
C in different proportions.
This constitutes a sort of signature that can be used for 'isotopic
tracing'. For example, we can determine in what proportion the
organic matter of the soil found under maize comes from the
decomposition of roots of that crop (Balesdent
et al.
1987).
1.3.2 Isotopic Dating
Dating with
14
C was invented before 1950 by the American physicist
Willard Frank Libby and his team at the University of Chicago. Libby
received the Nobel Prize in Chemistry in 1960 for this work. It is not
necessary to develop the principle of the method here because it is
easily found on the Internet. As the rate of decay of
14
C corresponds to a
half-life of 5734 years, we can get valid periods between a few centuries
and about 45,000 years. Of course, the sample should contain carbon,
which practically limits the use of the method to organic materials.
Theoretically, carbonates are affected but as the proportion is presumed
identical in the material and in the atmosphere, substances resulting
from physicochemical precipitation are excluded.
The combined use of another dating method (tree rings, see below)
has shown, from 1958 on, that the
14
C method is associated with a
systematic bias: the
14
C age is less than the actual age and the difference
varies with time. It can go as high as 17 per cent. The error is linked
to the variation in cosmic rays responsible for the transformation of
irradiated nitrogen in the upper atmosphere to
14
C. It is therefore
necessary to introduce a correction obtained from a calibration curve.
The analytical technique has long depended on the measurement of
radioactivity. But although many atoms of
14
C are present in the sample,
a few of them decay during the measurement time in the laboratory. To
