Geoscience Reference
In-Depth Information
limited lifetime, neutrons do not have to overcome the Coulomb barrier of the nucleus and
react more easily than charged particles of the same energy, such as protons. An important
source of
14
C is the reaction between neutrons and nitrogen, which produces radioactive
carbon-14 and a proton:
14
N
14
C
+
n
→
+
p
(4.17)
β
−
emission with a decay constant
The
14
C atom decays to
14
Nby
10
−
4
y
−
1
. Before disappearing, it mixes very quickly with the stable isotopes of carbon, the most
14
C
12
C
λ
14
C
of 1.2
×
14
C
12
C
e
−
λ
14
C
t
t
=
(4.18)
0
Plants exchange their carbon with the atmosphere, with which they are in isotopic equi-
librium until they die. It can be seen that if the ratio (
14
C/
12
C)
0
in the atmosphere is
constant and known, measurement of the
14
C/
12
C ratio in wood or a fossil carbonate will
date the death of the organism.
This approach is complicated by several effects. First, ever since the nineteenth century,
the burning of coal and oil has released a large quantity of “dead” carbon devoid of
14
C
into the atmosphere, thus complicating estimates of the (
14
C/
12
C)
0
ratio. To make things
worse, above-ground nuclear explosions until the mid 1970s have contaminated the atmo-
sphere with artificial
14
C, some of which has invaded the surface of the oceans. Finally,
the variation in solar activity modulates galactic cosmic radiation received by the Earth
and therefore changes the rate of production of
14
C in the atmosphere over very long peri-
ods. To overcome these difficulties, the
14
C scale has been calibrated for more recent ages
by dendrochronology - a method based on counting the growth rings of very old trees -
Californian bristle-cone pines or German oaks. For older ages, calibration is achieved by
comparison with the thorium-230 method on corals.
There are various applications for the
14
C method; it can provide dates as old as 40 000
years ago. Measurement methods using a linear accelerator have pushed this limit a little
further back, but above all they have reduced the quantities of material required for anal-
ysis to be conducted. The
14
C method has been used very successfully in archeology and
Quaternary geology. It also has applications in dating groundwater sources and, as we will
see later, deep water of the oceans.
4.1.2 Beryllium-10
The radioactive nuclide
10
Be is one of the pieces produced by spallation, the breaking of
atmospheric
14
N and
16
O nuclei by cosmic rays. Metallic Be is rapidly oxidized to BeO,
scavenged by atmospheric particles, and finally incorporated into soil and sediments by
rain water and run off. The
10
Be decay constant is 4.62
10
−
7
y
−
1
and it is customary to
normalize its abundance to that of the stable isotope
9
Be. Beryllium is an element similar
to aluminum and is found in clay and soils. Beryllium-10 dating is much used in oceanog-
raphy for measuring sedimentation rates or manganese-nodule growth rates. For samples
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