39
Descriptive Oceanography
where N 0 is the initial concentration, and λ = 1.22 × 10 –4 yr –1 . The amount of 14 C being
generated over thousands of years to the surface of the earth is constant. It is disappearing by radioactive decay at the same rate as it is being produced by cosmic rays. During
the nuclear weapons tests, large amounts of 14 C were produced and added to the amount
formed naturally.
Since the half-life (t 1/2 = ln 2/λ = 5700 yr) and the lifetime (τ = 1/λ = 8200 yr) for the
decay of 14 C to 14 N are the same order as the mixing times of the oceans and its mixing
with ordinary carbon ( 12 C) is not complete, it has become an important and valuable tool
3 H
14 C
Figure 1.43
Production of carbon-14 and tritium in an atomic bomb.
0
200
400
600
800
1000
Tritium (TU)
Tritium
0
1950 '54
'58
'62
'66
Year
'70
'74
'78
'80
200
400
600
800
δ
14
C (‰)
RadioCarbon
Figure 1.44
The input of carbon-14 and tritium into the atmosphere from bomb tests.