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by mining and smelting operations. In fact, Cd enrichment factors calculated for
samples collected along the river and its tributaries are extremely high, ranging from
11.5 to 1,100 (Gao et al.
2013
). Gao et al. (
2008
) showed that two samples collected
near a Pb-Zn smelter exhibited
114
Cd values that were distinctly different from
uncontaminated sediments (Fig.
5.4
b). The observed differences led them to suggest
that Cd isotopes may serve as an effective tracer. However, additional data provided
on samples collected along the North River and downstream of the smelters were less
convincing. The range of
ʵ
114
Cd variations (relative to SPEX Cd reference solution)
ʵ
was limited to 0
.
42
ʵ
(
−
0
.
35-0
.
07). In addition, 10 of the samples, while exhibiting
114
Cd values in comparison to effluent from the smelters, were
similar, falling within a narrow range, in spite of differences in Cd concentration
within the samples or the location at which the samples were located with respect to
the upstream smelters. Gao et al. (
2013
) argued that the differences in
markedly different
ʵ
114
Cd values
that were observed were likely to reflect the mixing of dust from the smelter, slag
from the smelter, and local background/ore materials.
ʵ
5.3 Copper Isotopes
Copper is an essential (nutrient) trace metal, but is highly toxic to aquatic, photosyn-
thetic microorganisms and algae. It can also be toxic to higher trophic level animals in
which it can cause a condition known as oxidative stress. Its release, then, to aquatic
environments is often of considerable concern. Unlike Cd and Zn, Cu is stable in
the near surface environment in two oxidation states, Cu
+
(CuI) and Cu
2
(CuII).
It therefore participates in a number of abiotic and biotic redox reactions. CuI is
the common form associated with sulfide minerals including chalcopyrite (CuFeS
2
),
chalcocite (Cu
2
S), enargite (Cu
3
AsS
4
), and covellite (CuS
2
), whereas CuII is the
common aqueous form. Cu may also occur on rare occasions as a native element.
It possesses two isotopes,
65
Cu and
63
Cu, with relative abundances of 30
.
83% and
69
17%, respectively. Like most non-CHONS, the Cu isotopic composition of phys-
ical and biological materials is reported in units of per mil (
.
ʴ
) typically (but not
always) relative to the NIST 976 Cu standard.
Analyses of the Cu isotopic composition of Earth and biological materials are
still limited, particularly for silicate rocks and sediments. Data collected to date on
dust, deep sea sediments, sandstones, shale and basalts show that
65
Cu values fall
ʴ
within a relatively narrow range (
) (Vance et al.
2008
, citing data
from Maréchal et al.
1999
; Archer and Vance
2004
; Asael et al.
2007
). These values
are similar to those often reported for natural (uncontaminated) soils, which exhibit
variations of
+
0
.
16
±
0
.
16
∼
1
with most around 0
(Bigalke et al.
2009
). In marked con-
65
Cu values measured in ores and minerals are highly variable, ranging
trast, the
ʴ
65
Cu values noted for
Cu ores and minerals appear to be rather uncommon, prompting some investigators
(e.g., Weiss et al.
2008
) to argue that
from
−
17 to
+
10
(Mathur et al.
2009
). The extremes of
ʴ
65
Cu values typically fall within a range of
ʴ
−
+
.
about
3to
5
7
for sediment, secondary ore minerals, and biological materials,
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