Stable ‘Non-Traditional’ Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 123

66 Zn values corresponded with a change in ore processing techniques and
an increase in Zn extraction efficiencies. Thus, it appears that sediments within the
reservoir were not only able to differentiate between natural and anthropogenic Zn,
but Zn associated with different industrial processing methods.
Two inherent assumption in the use of both elemental concentration and isotopic
data for the construction of pollution histories are that (1) the concentration or iso-
topic signature within the deposits correlate with the influx of metals to the river,
and (2) spatial changes in concentrations or isotopic abundances reflect the varia-
tions within the river sediment at the time of deposition (Miller and Orbock Miller
2007 ). In other words, the post-depositional migration of trace metals within the sedi-
ment profile is limited. Several studies (e.g., Bradley andCox 1987 ; Hudson-Edwards
et al. 1998 ) have shown that migration on the order of a few centimeters can occur.
In addition, isotopic abundances of many of the non-traditional isotopes, including
Zn, may be affected by post-depositional (diagenetic) fractionation processes. Weiss
et al. ( 2007 ), for example, found that diffusion of dissolved Zn in ombrotrophic peats
altered the isotopic composition of the sediment-associated Zn, resulting in higher
ʴ

ʴ

heavier

66 Zn values at the bottom of collected cores. However, the degree of fractionation
(and mobility) depends on the physiochemical conditions at the site, and the systema-
tic variations in isotopic ratios observed by Sivry et al. ( 2008 ) (and Thapalia et al.
2010 ) suggest that in the case of Zn such post-depositional alterations may not be
so significant that they inhibit the use of Zn isotopes to decipher changes in Zn
provenance through time or space.

5.2.2 Use of Cd Isotopes as Contaminated Sediment Tracers

In comparison to Zn, exploratory investigations of the use of Cd isotopes as an
environmental tracer aremore limited. In fact, with the exception of a fewpreliminary
studies, their application to contaminated sediments in rivers is completely lacking.
Thus, while the few studies that have been conducted suggest that Cd isotopes may
serve as effective tracers to determine Cd provenance, their actual use has yet to be
effectively demonstrated.
In a detailed review of the existing Cd isotope data, Rehkämper et al. ( 2011 )
found that Cd isotopic composition of igneous rocks derived from the crust and
mantle exhibited

114 / 110 Cd values between

4 and 4. Loess and clastic sediments
from rivers and continental margins also fell within this narrow range of values
(Fig. 5.4 a). Thus, they concluded that there was no “evidence for systematic Cd iso-
tope fractionation between or within the silicate Earth.” In contrast, Cd isotope values
of extraterrestrial, impact, marine and polluted materials exhibited a wider range of
values. With regards to the latter, most attention to date has focused on the Cd isotope
fractionation associated within the processing of Pb and Zn ores with which Cd is
associated. Cloquet et al. ( 2006 ), for example, analyzed the Cd isotopic composition
of dust trapped in exhaust filters and slag produced in the furnace of a Pb-Zn refinery.
They found that dust exhibited lighter

ʵ −

114 Cd values than the slag by about

ʵ − ʵ

10 .