Jökull - 01.12.1962, Síða 53

A/V£fí VAfíMA
x(2>
s (re)
f/VFRAG£fíO/
R£YKJA V/fíf Afí£A
Fig. 2. Location of samples from the Reykjavik area.
heavy isotope concentration as the latitude
varies from equatorial to polar. This refiects
a continuous loss of vapour from the poleward
moving air masses. As a consequence of this the
precipitation has a latitude label.
In general, the D-concentration in fresh water
varies by about 30%, ranging from 3% higher
to 27% lower than in average ocean water. The
018-concentration varies by only 4%, ranging
from 0,6% higher to 3,4% lower than average
ocean water.
On the other hand, it must be emphasized
that the dependance on the latitude is often
quite complex and depending on the local
meteorological conditions. Dansgaard (1961) has
shown tliat there are substantial time-variations
in the 018-concentration of the local precipita-
tion.
In experimental work it is necessary to mea-
sure the D-concentration with an accuracy of
0,1% and the 018-concentration with an ac-
curacy of 0,1 to 0,2%.
The radioactive isotope tritium. The third
isotope of importance is tritium or H8. It is
radioactive with a half-life of 12,5 years and
decays to He8. It is found in a very low con-
centration in natural waters. The pattern of
distribution has not been studied in detail and
the experimental work lias been highly com-
plicated by the H-bomb tests. Libby has carried
out pioneering work in this field. The pre-
bomb ratio T/H may for average rain water
have amounted to 5xl0-18.
Natural water derives tritium from the atmos-
phere. It is believed to be produced at high
altitudes by the action of cosmic radiation.
Therefore, tritium is not produced in surface
or ground water. Its concentration in ground
water will consequently decrease as a result of
JÖKULL 51