accumulate at a predictable rate. By measuring both
the tritium concentration and the accumulated
3
He
in the fluid parcel, the time that has elapsed since the
fluid was last in contact with the surface can be determined according to the equation:
t ¼ 17:96ln 1 þ
3 He
Â
Ã
3 H
½ Š
where t is the tritium–
3
He age in years, and [
3
He]
and [
3
H] are the concentrations of
3
He and tritium in
the water, respectively. For typical surface water
concentrations of a few tritium units, elapsed times
as short as a month or two can be detected, and the
upper limit to the dating technique is of the order of
10–20 years (see discussion below). This range of
timescales is ideal for studying shallow-ocean circulation, ventilation, and biogeochemical processing.
Figure 7 The tritium–helium dating concept.
Age (y)
Above 7
6 7
−
5 6
−
4 5
−
3 4
−
2 3
−
1 2
−
Below 1
_
1 0 0
_
1 5 0
_
2 0 0
D e p th
_ 2 5
_ 2 0
_ 3 0
_ 3 5
L o n g it u d e
2 5
3 0
3 5
2 0
_ 10 0
_ 15 0
_ 20 0
L
a
ti
tu
d
e
Figure 8 The distribution of tritium–helium age on a constant density surface (26.4 kg m
À3 ) in the subtropical North Atlantic.
TRITIUM–HELIUM DATING 143