SESSION HIB DISCUSSION
697
HAWLEY: Dr. Libby has suggested that there may be a selective
plant-uptake mechanism involved in the transpiration of tritiated water
over normal water. This, if true, may help explain such things as the
greater concentrations of 3H over continents than over oceans and
over large forested areas than over semiarid areas and also seasonal
variations. Dr. Thatcher, has your group been considering this?
THATCHER: By analogy with deuterium, one might expect that
a selective plant-uptake mechanism with respect to tritium might
exist. The plant would reject tritium as it usually does deuterium in
the metabolic process, and this would increase the evaporation of
tritiated water leading to a higher air concentration. The IAEA data
do not appear to support this mechanism, however, since the concentrations found in semiarid regions, such as at Teheran, are generally just as high as those in regions that support an abundant vegeta-
tion, such as at Vienna. The more humid regions are more greatly
influenced by oceanic air, however, which would counteract the pro-
posed effect for vegetation. The data should be carefully scanned in
an attempt to identify this effect.
BOLIN: There is, of course, fractionation when evapotranspiration takes place from vegetation. It is important to realize, however,
that,
for estimating this, the effect of water transfer through the
capillary systems in the plants must be considered. We do not know
precisely what the final effect is, but changes in the composition of
water due to fractionation insofar as tritium is concerned is small
compared to the variations that have been observed in precipitation.
REITER: Dr. Bolin, did you imply that from tritium measure-
ments at the ground, together with knowledge of the observed tritium
gradient in the atmosphere, you could make inferences on the precipitation physics in the atmosphere?
BOLIN: The vertical transfer of tritium, deuterium, ‘80, and
particulate matter takes place through the same mechanism of turbu-
lence. Since the sources and sinks of these tracers are different but
partially related, observations of the vertical distribution of all of
them would yield some information on the turbulent processes and
the intensity of the transfer. Considering the vertical distribution in
the trade-wind belt to be in an approximate steady state might yield
information on the turbulent transfer of water vapor and have a bearing on our estimates of evaporation from the ocean surface. I am not
sure how such data would be precisely interpreted, but in any case
the simultaneous use of several tracers for studies of the vertical
transfer should be advantageous.