38
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“I
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10Pbh “AND #°Po IN WOOD AND THE CIRCULATION OF LEAD IN TREES
R. B. Holtzman and F. H. Ilcewicz
Measurements of *°Pb and *°Po concentrations in tree rings
from four 100-year-old trees were made in order to estimate
the rates of radial translocation. Decrease of the concentration
of the ™Pb with a 214-year half-life showed that there is
little circulation of lead in hickory heartwood more than 20
years old. In oak there appears to be little circulation in wood
less than 5 years old, but this conclusion is less reliable because
of the low concentration of the ™°Pb and the presence of ”*Ra.
The concentrations of the ™°Po correlated fairly well with those
of the *°Pb. Preliminary measurements of the stable lead concentrations did not show increased uptake in recent times.
However, more detailed studies on the ™°Pb and stable lead
concentrations are needed to determine the value of the heartwood of trees as a temporal record of lead contamination of
the environment.
In recent years the concentration of 7!°Pb, a natur-
ally-occurring radionuclide with a 21.4-year half-life,
has been used for age determination of materials such
as snow’? and lead in artists’ paints,‘* *») and as a
tracer for stable lead in rain.) As a time-dependent
tracer it could also be useful in estimating the temporal
distribution of lead over the last century.
Despite the generally acknowledged increase in con-
tamination of man’s environment by lead overthelast
century or more, many of the quantitative aspects are
uncertain, such as the rates of change of this contami-
nation in the biosphere in general and in man in
ticular. Environmental levels of lead in the past ec:
be obtained from contemporary measurements, wl
however, are scarce, often of low sensitivity and
curacy, and not easily compared to present day m:
urements. Another possibility is the measuremen:
lead in relics of the past which are uncontamimatec.
present day lead. Thus, Chow and Patterson
Jaworowski'®) have measured lead in glacial cores
Jaworowski also determined lead in bone from anc
gravesites.{®) Similarly, Ault et al.have lookec
the lead concentrations and isotopic composition 1
set of 3 tree rings covering a 30-year period. This t
of sampling may be useful in estimating the amou
of lead contamination in trees in different environm«
and possibly the change in contamination over
years, if one assumes that the trees absorb. a measuré
amount of the additional lead either through the ro
or leaves.
This use of trees depends on the stabilization of
Jead concentration within a few years of formation
the wood. Although the heartwood does contain liv
cells, the arguments on metabolism of Stewart? ir
cate that radial transport of materials in heartwoox