Lead has four stable, naturally occurring isotopes: 204Pb
(1.4%), 206Pb (24.1%), 207Pb (22.1%) and 208Pb
(52.4%). 206Pb, 207Pb and 208Pb are all
radiogenic, and are the end products of complex decay chains that begin
at 238U, 235U and 232Th respectively.
The corresponding half-lives of these decay schemes vary markedly: 4.47
x 109, 7.04 x 108 and 1.4 x 1010 years,
respectively. Each is reported relative to 204Pb, the only non-radiogenic
stable isotope. The ranges of isotopic ratios for most natural materials
are 14.0-30.0 for 206Pb/204Pb, 15.0-17.0 for 207Pb/204Pb
and 35.0-50.0 for 208Pb/204Pb, although numerous
examples outside these ranges are reported in the literature. Because U,
Th and Pb have different geochemical behaviors, the fact that the Pb isotopic
composition of any material is the composite of the three independent decay
chains creates the potential for greater variability of isotopic values
in minerals of a single rock relative to that for the Rb-Sr system. Also,
atmospheric Pb tends to become concentrated in the uppermost, organic-rich
soil horizons, while Pb in ground water should be dominated by that derived
from rock weathering. Thus, the relative importance of the two sources
in stream water can be determined.
Potential complications to the use of Pb isotopes, such as anthropogenic
contamination, immobility of Pb in ground water, and the possibility for
highly radiogenic Pb to exist in accessory minerals, can also be used to
an advantage. Anthropogenic Pb (Veron et al., 1994) and accessory-phase
Pb in minerals have distinct compositions, and the immobility of Pb produces
concentration covariance (Nimz, 1998).
Another useful lead isotope for catchment research is 210Pb,
a radiogenic (via 238U and 222Rn) and radioactive
isotope with a half-life of 22.1 years. 210Pb can be used to
age-date materials formed in the last 100 or so years, with typical precision
of a few years under favorable conditions. The 210Pb "age"
reflects the time since the lead became incorporated in the material--
usually because of 222Rn or atmospheric fallout--or can be augmented
by additional 210Pb contributed by decay of uranium in the underlying
geologic materials, thus complicating age determinations. A very powerful
application is to use 210Pb to age-date the deposition of layered
materials and then to relate the changes in the chemical and isotopic compositions
of the layers to changes in ambient environmental conditions.
Source of text: This review was assembled by Dan Snyder,
Carol Kendall and Eric Caldwell, from the references below.
||Bernat, M. and Church, T. M. (1989). "Uranium and thorium decay
series in the modern marine environment." In: P. Fritz and J.-Ch.
Fontes (Eds.), Handbook of Environmental Isotope Geochemistry, Volume
3, Amsterdam, Elsevier Science, pp. 357-383.
||Bullen, T.D., and Kendall, C. (1998). "Tracing of Weathering Reactions
and Water Flowpaths: A Multi-isotope Approach." In: C. Kendall and
J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier,
||Kendall, C., Sklash, M. G., Bullen, T. D. (1995). "Isotope Tracers
of Water and Solute Sources in Catchments", in Solute Modelling
in Catchment Systems, John Wiley and Sons, New York, pp. 261-303.
||Nimz, G. J., (1998). "Lithogenic and Cosmogenic Tracers in Catchment
Hydrology". In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers
in Catchment Hydrology. Elsevier, Amsterdam, pp. 247-290.
||Veron, A. J., Church, T. M., Patterson, C. C. and Flegal, A. R. (1994).
"Use of stable lead isotopes to characterize the sources of anthropogenic
lead in North Atlantic surface waters." Geochim. et Cosmochim.
Acta, 58: 3199.