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is the chondritic uniform reservoir. Mathematically,
143
Nd
144
Nd
143
Nd
144
Nd
(
/
)
sample
−
(
/
)
chondrite
10
4
ʵ
Nd
=
×
(4.1)
143
Nd
144
Nd
(
/
)
chondrite
Aswas the case of
87
Sr/
86
Sr, the
143
Nd/
144
Nd ratio of rocks andminerals increases
with time as a result of the production of
143
Nd from
147
Sm; thus,
143
Nd/
144
Nd
ratio (
ʵ
Nd
) depends on both the initial Sm/Nd ratio and age of the source rocks. In
contrast to
87
Sr/
86
Sr, the highest (most radiogenic)
143
Nd/
144
Nd ratios are found
in mid-oceanic-basalts whereas the lowest ratios are associated with granites. Once
again, however, the range of
143
Nd/
144
Nd ratios are limited in basalts (about 0.5128-
0.5120) but vary widely in continental crustal rocks from around 0.5080-0.5110
(Allègre
2008
). Moreover,
87
Sr/
86
Sr and
143
Nd/
144
Nd ratios are inversely correlated
indicating that there is a high degree of coherence between the Sr and Nd isotopes
(Allègre
2008
).
The wide variations in both
87
Sr/
86
Sr and
143
Nd/
144
Nd within geological mate-
rials, their highly conservative behavior, and the limited degree to which they are
fractionated, suggests that the Sr and Nd isotopes will provide a diagnostic finger-
print of the constituents within both the dissolved and particulate load of rivers (as
well as other water bodies). With regards to the dissolved load, Sr isotopes have
been most extensively utilized, primarily to determine the source of the constituents
within the water and the rates of chemical weathering (Moon et al.
2007
; Rai and
Singh
2007
; Tripathy et al.
2010
;Vossetal.
2014
). On a global scale, the long-
residence time of Sr in ocean waters compared to oceanic mixing times suggests
that (1) the Sr isotopic composition of authigenic marine precipitates will exhibit the
same ratios as the water at the time of their formation, and (2) the Sr values within the
marine sediments will exhibit a high degree of homogeneity throughout the world's
oceans (Banner
2004
). Thus, recorded temporal variations in
87
Sr/
86
Sr ratios through
time, which now extend back to the Precambrian, describe global changes in ocean
chemistry. In contrast, the residence time of
143
Nd/
144
Nd in ocean waters is shorter
than that of
87
Sr/
86
Sr and shorter than the time required for complete oceanic mix-
ing. Thus, observed variations in the
143
Nd/
144
Nd values in marine sediments are
thought to reflect regional differences in ocean chemistry. Significant effort has been
devoted in recent years to determining the processes or factors that account for the
noted variations in both the Sr and Nd isotopic ratios in marine sediments, includ-
ing hydrothermal circulation at mid-oceanic ridges, alterations in weathering rates
and intensities associated with climate change and/or tectonic activity, shifts in the
configuration of the continents, and a number of other processes (see Banner
2004
for a review). It has been argued, for example, that the preferential release of Sr by
the chemical weathering of newly exposed rock surfaces (resulting from tectonics
or glaciation) can alter the
87
Sr/
86
Sr signal of river systems, and the input of the
Sr ratios to the oceans, in some areas for periods of about 20,000 years. Thus, it
may be possible to link glacial episodes (or other mineral exposure processes) to
documented alterations in the oceanic
87
Sr/
86
Sr record.
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