Chemistry Reference
In-Depth Information
drawn from the carbon exchange reservoir. As a consequence of the with-
drawal, the amount of radiocarbon in dead plant and animal matter does
not remain constant, as in the carbon exchange reservoir: since radiocar-
bon undergoes radioactive decay, its relative amount in the dead remains
of organisms decreases gradually and continuously. The rate of decrease is
independent of the nature of the organism of which the radiocarbon is a
component, its physical condition, and the nature and conditions of the
surroundings of the dead remains: it is determined only by the half-life of
the isotope, 5730
40 years. This means that the remains of any type of
vegetable or animal matter 5730 years old contain only half as much radio-
carbon as contemporary living matter, one-quarter after 11,460 years, only
one-eighth after 17,190 years, and so on (see Table 67). Thus, although
there are some differences in isotopic abundance between different
parts of the exchange reservoir, the
mean ratio
between the isotopes
(the proportion carbon-12 : carbon-13 : radiocarbon) in the biosphere is
kept constant throughout, at approximately 1 : 0.01 : 0.0000000000001
(10
−
12
). In other words, for every atom of carbon-12 in the biosphere there
are only 0.01 atoms of the isotope carbon-13 and as little as 10
−
12
atoms
of radiocarbon. Such a minute concentration of radiocarbon is practically
impossible to detect, let alone to assess by most chemical and physical tech-
niques. It can, however, be detected and accurately measured because of
its radioactive properties. Measuring either (1) the amount of beta radia-
tion emitted by the decaying radiocarbon or (2) the actual number of
radiocarbon atoms provides a means to determine the total amount of
radiocarbon in any sample.
±
TABLE 67
Decay of Radiocarbon in Dead Remains
Number of atoms
a
Time since death of
Number of
an organism (years)
half-lives
Carbon-14
Carbon-12
0
0
1N
1N
5,730
1
1
2
N
1N
11,460
2
1
4
N
1N
17,190
3
1
8
N
1N
22,920
4
1
16
N
1N
28,650
5
1
32
N
1N
34,380
6
1
64
N
1N
40,110
7
1
128
N
1N
45,840
8
1
256
N
1N
51,570
9
1
512
N
1N
57,300
10
1
1,026
N
1N
a
N = number of atoms at the time of death.
