Stable ‘Non-Traditional’ Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 117

ducted to date to have the most potential to track sediment-associated trace metals
in rivers. It should be noted, however, that the potential application of other non-
traditional isotopes as tracers in riverine systems is currently being explored, includ-
ing Ag, Cr, Fe, Mo, Ni, Sb, Se, and U (see, for example, EnvironMetal Isotopes
Conference Program, 2013). Which, if any, of these isotope systems can served as
effective sediment tracers is yet to be determined; thus, they will not be examined
herein. In addition, we have elected not to include a discussion on the use of the
stable isotopes of C, H, O, N, S as tracers as they have been extensively discussed in
other sources.

5.2 Zn and Cd Isotopes

Cadmium and Zn sulfide minerals are commonly associated with one another in
ore deposits. As a result, Cd is often recovered along with Zn and Pb during the
processing of Zn and Pb ores (Rehkämper et al. 2011 ). Zn and Cd also exhibit a
number of other geochemical similarities (Bullen 2011 ; Rehkämper et al. 2011 ).
Both, for example, are stable at the Earth's surface in a single (

2) oxidation state,
and therefore are not significantly influenced by redox processes. They also are
isotopically fractionated by a number of similar processes, such as by evaporation,
condensation, or electroplating.
As was the case of Nd, the isotopic abundance of the non-CHONS including Cd
and Zn are expressed per 1,000 (mil,

+

ʴ ʵ

) relative to a standard, due
to extremely small variations that occur in nature. Mathematically,

) or per 10,000 (

isotopic ratio of sample
isotopic ratio of standard −

1

ij

ʴ = × 1 , 000 (5.1)

and

isotopic ratio of sample
isotopic ratio of standard −

1

ij

ʵ = × 10 , 000 (5.2)

ij and ij are the

where values of the element, respectively, for the isotopic

ratio represented by ij . In most instances, the lighter isotope is used as the denomi-
nator, in which case

ʴ ʵ ʴ and ʵ

0 lighter. In the case
of Zn, which has five stable isotopes, most studies have utilized 66 Zn/ 64 Zn ratios
(and often reported as

ʴ> 0 is heavier than the standard and ʴ<

66 Zn) due to the high relative abundances of 66 Zn and 64 Zn,
and the ability to precisely measure their contents in geological materials. Unfor-
tunately, studies to date have utilized different materials as a standard, making the
direct comparison of reported

ʴ

66 Zn values difficult (Cloquet et al. 2008 ). Cd iso-
topic abundances may be reported in terms of either

ʴ

. In terms of isotopic ratios,
most environmental studies have utilized 114 Cd/ 110 Cd ratios, although 112 Cd/ 110 Cd
has also been used (e.g., Schmitt et al. 2009 ). As is the case for Zn, previous Cd
isotope investigations have utilized differing standards (see Rehkämper et al. 2011
for a more detailed discussion on Cd isotopes and isotopic analysis).

ʴ or ʵ