Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 97

Table 4.3 Pb isotopic
abundance

Isotope Abundance (%)

204 Pb

1.48

206 Pb

23.60

207 Pb

22.60

208 Pb

52.30

used to determine the source of Pb in air, aerosols, and dust (Ault et al. 1970 ;Chow
and Johnstone 1965 ;Gulsonetal. 1994 , 1996 ;Rosmanetal. 1993b ; Chiaradia and
Cupelin 2000 ; Laidlaw et al. 2014 ), snow and ice (Rosman et al. 1993a , b , 1997 ;
Veysseyre et al. 2001 ; Shotyk et al. 2005 ), soils (Gulson et al. 1981 ; Steinmann and
Stille 1997 ; Hansmann and Köppel 2000 ; Mihaljeviç et al. 2006 ; Reimann et al.
2012 ; Mackay et al. 2013 ; Kristensen et al. 2014 ), lacustrine and reservoir deposits
(Shirahata et al. 1980 ; Petit et al. 1984 ; Chiaradia et al. 1997 ; Foster et al. 2002 ),
wetlands and peat (Shotyk et al. 1998 ; Weiss et al. 1999 ; Marcantonio et al. 2000 ;
Cloy et al. 2008 ), plants, mosses, and tree rings (Bellis et al. 2004 ; Bindler et al. 2004 ;
Bi et al. 2009 ), human tissues and blood (Manton 1977 ; Keinonen 1992 ;Gulsonet
al. 1996 ) and other biota (Sangster et al. 2000 ; Miller et al. 2005 ; Soto-Jiménez et al.
2008 , 2009 ; Sondergaard et al. 2010 ; Potot et al. 2012 ). Lead isotopes have also been
extensively utilized to determine the origins of Pb and other correlated trace met-
als/metalloids in riverine environments, mostly from point-sources of contamination
(see Bird 2011 ) for a detailed review of Pb as a contaminant tracer in rivers).
Like Sr, the isotopic abundances of Pb within geological materials are reported as
ratios (e.g., 206 Pb/ 204 Pb). Unfortunately, investigators have not used a consistent set
of ratios. While one study may report the values in terms of 206 Pb/ 207 Pb, others,
particularly geologically oriented investigations, may report the same measured
abundances in terms of 207 Pb/ 206 Pb. These differences in reporting make it more
difficult than necessary to compare the results between investigations. Investigators
have also focused on different ratios. For example, geologically oriented studies
have extensively used the ratio of the radiogenic isotopes to 204 Pb (i.e., 206 Pb / 204 Pb,
207 Pb/ 204 Pb, and 208 Pb/ 204 Pb) (Bird 2011 ). In contrast, most environmental studies
only use the radiogenic isotopes of Pb as geochemical tracers for two reasons: (1) the
abundance of 204 Pb is relatively low (Table 4.3 ) making it more difficult to accurately
measure, particularly with an MC-ICP-MS, and (2) the discriminative (fingerprint-
ing) power of Pb isotopes is primarily due to 206 Pb, 207 Pb, and 208 Pb (Sangster et al.
2000 ).
The effectiveness of Pb isotopes as a geochemical tracer is related to several fac-
tors. First, the radiogenic isotopes of Pb can be measured with a high degree of
precision and accuracy. Second, effective contaminant tracers generally exhibit con-
servative behavior over a wide-range of environmental (geochemical) conditions. In
the case of sediment, conservative behavior means that the tracer will move with the
sediment without any significant loss in elemental mass (Yeager et al. 2005 ;Bird
2011 ). Moreover, fractionation of the radiogenic Pb isotopes by physical, chemi-
cal, and biological processes is limited because of the low relative atomic weight