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On a much smaller scale Sr and Nd isotopes have been used to assess changes
in the source of dissolved constituents within individual catchments. Voss et al.
(
2014
), for example, utilized a time series of
87
Sr/
86
Sr data from the Fraser River
of Canada to determine changes in the source of dissolved constituents within a
relatively pristine environment. They found that the Fraser River was characterized by
seasonal variations in
87
Sr/
86
Sr ratios with higher values occurring during the spring
and summer and lower values during fall and winter. The isotopic data were then
utilized in a mixing model to show that the higher
87
Sr/
86
Sr values were associated
with enhanced chemical weathering fluxes from old sedimentary rock units within
the headwaters of the catchment, in spite of the fact that the area contributed relatively
little water to the river.
Our primary interest here is in the use of Sr and Nd isotopes as a tracer of sed-
iment and contaminated sediment provenance. The use of Sr isotopes as a means
of determining sediment provenance was first demonstrated by Dasch (
1969
)who
attributed large differences in the spatial variations of
87
Sr/
86
Sr within the north
Atlantic to variations in sediment source. Since then, both Sr and Nd isotopic sys-
tems have been widely used as sediment tracers. Their utilization is driven in large
part by their highly conservative behavior in sediments that allows the isotopic sig-
nature of the source materials to be preserved within the sediment throughout its
erosion, transport, deposition, and subsequent diagensis (DePaolo
1981
; Goldstein
et al.
1984
; Jones et al.
1994
; Winter et al.
1997
; Tripathy et al.
2011
). The Sr, and
to lesser extent, Nd, isotopic composition of sediments has, however, been shown to
vary with the particle size distribution of the sediments in some (e.g., Dasch
1969
;
Grousett and Biscaye
2005
), but not all (e.g., Padoan et al.
2011
) studies. Thus, the
isotopic ratios within the sampled material may be altered from that of the source
rocks by weathering and hydraulic sorting processes. In general,
87
Sr/
86
Sr ratios
are thought to be more significantly influenced than Nd because the latter is more
uniformly distributed among the minerals in the parent rock. Thus, the preferential
dissolution of specific minerals will impact
87
Sr/
86
Sr ratios more than
143
Nd/
144
Nd
ratios. Similarly, mineralogical differences produced by particle sorting are likely to
influence
87
Sr/
86
Sr ratios more than
143
Nd/
144
Nd ratios because of the wider range
of the former among the minerals. The more conservative behavior of Nd is often
considered to make it a more robust tracer (Walter et al.
2000
; Tripathy et al.
2011
).
In addition, the observed differences in the geochemical behavior of Sr and Nd,
when combined with their known inverse correlation in source rocks, have led to
their combined use to assess sediment provenance and transport patterns (Tripathy
et al.
2011
). In other words, coherence in the temporal and spatial patterns within the
sampled material is taken as an indication that the source signatures are preserved
allowing for a determination of sediment provenance.
In light of the above, it is not uncommon for sediment sources and sampled
mixtures to be shown on bivariate
87
Sr/
86
Sr -
143
Nd/
144
Nd (
ʵ
Nd
) plots (Fig.
4.2
).
Samples with
87
Sr/
86
Sr -Nd values that systematically plot along a path between
two end-member sources can be viewed as containing a mixture of sediments from
the two sources. The isotopic Sr and Nd ratio of the mixture can be expressed in
terms of the two sources as (Allègre
2008
):
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