Identifying Sources of Aeolian Mineral Dust: Present and Past - Mineral Dust: A Key Player in the Earth System - page 65

3.4.5 Isotopic Methods of Identifying Dust Sources

One of the most powerful methods of identifying dust sources is through isotopic
“fingerprinting.” This approach has become so important that many dust provenance
studies, particularly for LRT dust, routinely incorporate isotopic analyses in their
analyses. Isotopic studies have formed the foundation for some of our current
understanding of the sources of loess, deep-sea sediments, and dust particles found
in polar ice caps.
Commonly used elements for identifying dust sources are all radiogenic isotopes,
Sr, Nd, and Pb. Taylor et al. ( 1983 ) were among the first to show the relation of
Nd and Sr isotopes in loess to possible source rocks. Grousset and Biscaye ( 2005 )
review the use of these isotope systems in dust provenance studies in general,
and Scheuvens et al. ( 2013 ) review the Sr-Nd-Pb data that have been generated
specifically for major dust sources in Africa. Sr-Nd-Pb-isotopic compositions of
dust grains are highly dependent on particle size, as we show below with examples
from China (Chen et al. 2007 ) and Africa (Meyer et al. 2011 ).
The source or sources of silt particles on the Loess Plateau of China (Fig. 3.7 a)
have been debated for decades. Here, 87 Sr/ 86 Sr vs. 143 Nd/ 144 Nd studies provide
a means to evaluate hypothesized sources, and a recent study not only applies
this approach but also provides a cautionary note in the methodology. Particle
size trends in the Loess Plateau suggest that loess could have originated from
any or all of several desert basins found to the north of the loess body. Chen
et al. ( 2007 ) analyzed sediment samples from most of these basins, which here
we collapse into three groups, the “western deserts” (Taklimakan Desert, Qaidam
Basin, Badain Jaran Desert, and Tengger Desert), the “northeastern deserts,” and
the two deserts immediately to the north of the Loess Plateau (Mu Us and Hobq
Deserts). Analytical results show that the 87 Sr/ 86 Sr and 143 Nd/ 144 Nd compositions
of sediments from these three desert-basin groups are distinct from one another
(Fig. 3.7 b). Further, Chen et al. ( 2007 ) show that the 87 Sr/ 86 Sr and 143 Nd/ 144 Nd
compositions of sediments in all three deserts are a function of particle size.
Although the 143 Nd/ 144 Nd compositions are not significantly different for the
<75 m fraction compared to the <5 m fraction, 87 Sr/ 86 Sr values are consistently
more radiogenic for the <5 m fraction. For samples from the Loess Plateau, both
size fractions show 87 Sr/ 86 Sr and 143 Nd/ 144 Nd compositions that fall closest to the
western deserts. Given the close geographic association of the Loess Plateau to the
Mu Us and Hobq Deserts, this interesting result shows the interpretive power of this
isotopic approach to identifying loess or dust sources, as long as due attention is paid
to the potential effects of particle size on isotopic composition. Similar effects of
grain size on 87 Sr/ 86 Sr composition were reported for aeolian and fluvial sediments
analyzed from cores off northwestern Africa (Meyer et al. 2011 ).
The Pb-isotopic system has also been employed to identify dust and loess
sources. Dust source sediments will have Pb-isotopic signatures that reflect the
rocks they are derived from, although care must be taken to avoid local urban
and industrial anthropogenic additions of Pb (from gasoline, mining, and smelting