Stable ‘Non-Traditional’ Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 115

The term 'non-traditional' was originally coined to differentiate the investigation
of these alternative isotopes from the much more intensively studied and applied
isotopes of C, H, O, N, and S. Bullen ( 2013 ) objected to the continued use of 'non-
traditional' to describe work on these lesser studied isotopes, arguing that recent
research has rendered the term obsolete. Moreover, he argued that the term posed
“more of a roadblock than a proper description of the field” and suggested that use
may lead to the risk of “having the science relegated to a niche market that is difficult
for other scientists to access”. He went on to propose the term of 'non-CHONS'
as a replacement for non-traditional to distinguish these isotopes from the more
extensively studied isotopes of C, H, O, N, and S. While we have elected to use the
term non-CHONS here, it seems highly unlikely that their study will be relegated
to the back corners of isotope geochemistry anytime soon. On the contrary, their
study appears to be accelerating at a nearly exponential rate as they are being used to
address a wide range of issues, such as the paleochemistry of the oceans, metal trans-
fer processes in both physical and biological systems, and the identification of the
source and cycling of metal and metalloid contaminants (Bullen and Walczyk 2009 ).
With regards to the latter, interest in the isotopes stems from the fact that a number of
metals (Cd, Cr, Cu, Hg, Ni, Se, Ag, and Zn) are considered by the USEPA and other
regulatory bodies as priority pollutants. Thus, if their isotopes can be used as tracers,
then it may be possible to directly determine their source, dispersal patterns/rates,
and/or cycling processes in near-surface systems.
At the present time, use of the non-CHONS is still in the developmental stages;
while some notably successes have demonstrated their potential use as environmental
tracers, their application is muchmore difficult than, for example, themore traditional
radiogenic isotopes of Pb, Nd, or Sr. The primary difference rests on the magnitude
of isotopic fractionation by physical and biological processes. In the case of Pb, for
example, fractionation by industrial or biological processes is negligible and the Pb
isotopic composition of an anthropogenic substance depends upon the ore deposits
fromwhich the Pbwas derived. Since the Pb isotopic signature of metallic ores gener-
ally differs frommost other (geogenic) rocks and minerals, the measured Pb isotopic
ratios within alluvial sediments results exclusively from the mixing of particles with
different isotopic abundances, and anthropogenic materials can often be isotopically
fingerprinted to determine the source of Pb in the river system. In contrast, variations
in the isotopic abundances of non-CHONS tend to be more limited within natural
geological materials (e.g., rocks, minerals, and ore deposits). However, small but
measurable variations in the isotopic composition of a material derived from the
original rocks, minerals, ores, etc., may occur as a result of physical and biologi-
cal isotopic fractionation processes (Table 5.1 ) (Wombacher et al. 2004 ; Cloquet et
al. 2006 ; Shiel et al. 2010 ; Rehkämper et al. 2011 ). These new isotopic abundances,
whichmay, for example, be linked to a specific industrial process, may then be utilized
as a geochemical tracer to determine a contaminant source (Bullen 2011 ; Rehkämper
et al. 2011 ). Alternatively, low temperature fractionation of elements which partici-
pate in redox reactions may provide insights into biogeochemical cycling processes.
In the following sections, we turn our attention to four stable metal isotopes (Cd,
Cu, Hg, and Zn) that appear on the basis of the limited studies that have been con-