Radiogenic Isotopes - Application of Geochemical Tracers to Fluvial Sediment - page 93

Fig. 4.3 The primary transport pathways of suspended riverine sediment within the Eastern
Mediterranean Sea as determined from Sr and Nd isotopes. Open arrows indicate turbid flow
directions as provided by Reeder et al. ( 1998 ) (from Weldeab et al. 2002 )
of hydraulic sorting processes on the sediment size distribution. The Nile River Basin
is well-suited for the study because individual catchments are underlain by rocks of
different age and therefore exhibit different Sr-Nd isotopic ratios. More specifically,
they found that tertiary volcanic rocks of the Ethiopian highlands, drained by the
Blue Nile and Atbara (Fig. 4.4 a), were characterized by low 87 Sr/ 86 Sr values and
high 143 Nd/ 144 Nd ratios. Sediments from these rocks could be discriminated from
Archean to Proterozoic crystalline basement rocks and Mesozoic sedimentary rocks
exposed along the Western margin of the Red Sea and East African Rift (which
were characterized by relatively high 87 Sr/ 86 Sr values and low 143 Nd/ 144 Nd ratios).
When combined with isotopic data on samples from the main channel of the Nile,
they were able to determine that the majority of the sediment was derived from the
Blue Nile and Atbara. In spite of the fact that the White Nile supplies about 33%
of the discharge to the main Nile, and encompasses about 50% of the total basin
area, it contributes relatively little sediment to the main River (Fig. 4.4 b). Apparently,
very little sediment is transported through various East African Lakes and the vast
wetlands of the Sudd and Machar marshes of South Sudan. The Sr-Nd isotopic data
demonstrated that sediment fluxes to the lower Nile do not correspond to discharge or
basin area, but are related to high rates of erosion in the Ethiopian highlands (Padoan
et al. 2011 ).
4.2.2 Tracing Contaminated Particles
While the use of Sr and Nd isotopes to determine the source and dispersal pat-
terns of sediment within siliciclastic strata, atmospheric systems, and large river-
marine systems is widely recognized, their application to diffuse or point-sources