Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 96

Fig. 4.5 a Variations in Sr concentration, 87 Sr/ 86 Sr ratios, and ʵ Nd values as a function of depth
below the ground surface and age within a peat deposit of southeast Florida. Age estimates based
on 210 Pb and calibrated radiocarbon data from the 72-76 cm depth interval. b Bivariate 87 Sr/ 86 Sr
versus

ʵ Nd plot showing mixing curve between suggested end-member sources (Saharan dust and
Ocala limestone) and the peat deposits from southeast Florida. Filled squares represent relative
mixing proportions between sources in terms of percent Saharan dust. See Kamenov et al. ( 2009 )
for parameters used to develop mixing curve. Plio-Pleistocene seawater value is based on Nd data
for northeast Atlantic from Abouchami et al. ( 1999 ) (adapted from Kamenov et al. 2009 )

contribution of dust from the Saharan. The argument is supported by the plotting of
the sampled marsh sediments on a ʵ Nd - 87 Sr/ 86 Sr mixing curve (Fig. 4.5 b) such that

Nd isotopic variations can be explained by the mixing of 70-85% sediment from the
local bedrock and 15-30% of the sediment from the Saharan.
The study by Kamenov et al. ( 2009 ) suggests that the use of Nd and Sr isotopes
to track contaminant influx to aquatic systems may be more effective than their
limited use for this purpose implies. Thus, the applications of Sr and Nd isotopes
to contaminant problems in rivers and other aquatic systems is likely to increase in
the future. They will probably be most effective when combined with other isotopes,
such as those of Pb, or when used in combination with other forms of tracers (e.g.,
selected trace metals, metalloids, or Rare Earth Elements).

4.3 Pb Isotopes

4.3.1 General Characteristics

Pb has three radiogenic isotopes, 206 Pb, 207 Pb, and 208 Pb, which are derived from
the radioactive decay of 238 U, 235 U, and 232 Th, respectively (Table 4.1 ). They are
often combined in tracer studies with 204 Pb, which has no known radiogenic parent.
Pb isotopes have not been extensively utilized to determine the source of sedimen-
tary particles per se. However, it is fair to say that they now represent the method
of choice for tracking Pb and Pb contaminated sediment. To date, they have been