Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 92

The term K in Eq. 4.6 has the value

C A / C B

K = (4.7)

C A / C B

In the case of Pb, as we will see below, the denominator of the two plotted ratios is
the same; thus, the equation forms a straight mixing line between the two sources. A
straight-line also is produced when the denominators differ (as is true for the use of
87 Sr/ 86 Sr - 143 Nd/ 144 Nd), but K is equal to 1 indicating the Nd/Sr ratios are constant
in both systems. However, when K is equal to any other negative or positive value,
the line between the end-member sources forms a hyperbola, the shape of which
varies with the value of K (Allègre 2008 ) (Fig. 4.2 ). Regardless of form, a systematic
trend between two end-members allows for an estimation of the relative proportion
of sediment from the two sources.
To date, Sr and Nd isotopes have primarily been used to determine the provenance
of siliciclastic sedimentary rocks, particularly within sedimentary basins (McCul-
loch and Wasserburg 1978 ; Goldstein and Jacobsen 1988 ; Awwiller and Mack 1989 ;
McLennan 1989 ; McLennan et al. 1990 , 1993 ; Awwiller 1994 ). With regard to con-
temporary sediments, Sr and Nd isotopes have mainly been applied to large systems
characterized by a diversity of rock types. Both Sr and Nd have, for example, been
extensively utilized to assess the origins and transport patterns of dust (aerosols,
loess and sand) on a worldwide basis (Grousset et al. 1992 , 2005 ; Biscaye et al.
1997 ; Basile et al. 1997 , 2001 ).
With regard to contemporary riverine systems, Sr-Nd fingerprints have often been
applied to marine and coastal sediments to determine the relative contribution and
dispersal patterns of sediment from river basins underlain by differing rock types
and which therefore exhibit specific Sr-Nd isotopic signatures (Weldeab et al. 2002 ;
We i e t a l . 2012 ; Rosenbauer et al. 2013 ). Weldeab et al. ( 2002 ), for example, utilized
Sr-Nd isotopic data to determine the source and primary transport pathways of sus-
pended particulate matter within the Eastern Mediterranean Sea (Fig. 4.3 ). Similarly,
Rosenbauer et al. ( 2013 ) applied Sr and Nd isotopes, along with selected trace
elements and rare earth elements, to San Francisco Bay to determine the relative
contributions of beach-sized sands from the major inflowing rivers. They found that
the majority of the sand within three areas of the Bay (Suisun Bay, San Pablo Bay,
and north Central Bay) originated from the Sierra Nevada Batholith via the Sacra-
mento River, while input from other rivers including the Napa and San Joaquin
provided lesser contributions. The Sr-Nd isotopic data also revealed that other sed-
iment sources were locally important. Once the sand-sized particles exited the Bay,
the isotopic data indicated that the materials were transported southward along the
outer coast by long-shore currents (Rosenbauer et al. 2013 ).
The Sr-Nd signatures of sediment within individual tributaries have also been
used to assess the primary sources and dispersal mechanics within large river basins.
For instance, Padoan et al. ( 2011 ) used Sr-Nd isotopic ratios to determine the relative
contributions of sediment input from the major tributaries to the Nile and the effects