On Composition, Morphology, and Size Distribution of Airborne Mineral Dust - Mineral Dust: A Key Player in the Earth System - page 20

In eastern Asia, elevated smectite contents were not reported so far. However, Shao
et al. ( 2007 ) detected extremely high contents of illite-smectite mixed layers in the
fine fraction of Asian dust samples collected in Beijing in 2002. Further XRD data
is needed to clarify if the data of Shao et al. ( 2007 ) is representative for Asian dust in
general or if it represents a single unusual event. Palygorskite, an Mg-bearing fibrous
clay mineral, has not been detected in Asian dusts. In northern Africa, it is only
rarely found in Sahelian dusts but is frequently observed as a minor phase in mineral
dusts uplifted in the region of Morocco, Algeria, and Tunisia. Hence, palygorskite
was proposed as a source tracer for dusts originating in the northwestern part of
northern Africa (e.g., Schütz and Sebert 1987 ). Far-travelled palygorskite-bearing
African dusts were found, for example, in western Europe (e.g., Avila et al. 1997 )
and on the Canary islands (Menéndez et al. 2007 ).

2.2.2 Isotope Data

Isotope data of mineral dusts are mainly used for source apportionment analyses.
Studies in the last two decades have shown that the analysis of Nd and Sr isotopes is
an especially valuable tool to assign actual and paleo-dust samples (e.g., marine
sediments, ice core samples) to a specific source region. For northern Africa,
there exists an impressive data set of calculated ￿ Nd (0) and 87 Sr/ 86 Sr values from
continental and marine surface sediments, which were mainly measured by Grousset
and coworkers (Grousset et al. 1988 , 1990 , 1992 , 1998 ; Grousset and Biscaye
2005 ; see also compilation by Scheuvens et al. 2013 ). However, isotope analyses
of mineral dust or deposited samples are much rarer (Grousset et al. 1988 , 1992 ,
1998 ; Rognon et al. 1996 ; Meyer et al. 2011 , 2013 ), owing to the high amount of
particulate mass required. Northern African dusts which were mainly sampled off
the western coast of northern Africa exhibit ￿ Nd (0) values between

10
(with the exception of data from Goldstein et al. 1984 ; Frost et al. 1986 )and
87 Sr/ 86 Sr values between 0.713 and 0.725. In conjunction with analyses from marine
surface sediments, an indistinct trend with decreasing ￿ Nd (0) values to the south can
be observed along the western coast of northern Africa. However, the extremely low

15 and

16.0) that were obtained in northern Mauretania and Western
Sahara on surface sediment samples (Grousset et al. 1998 ) were not detected in
African dust samples so far.
Isotope analyses of mineral dusts from other regions are rare. For example,
Kanayama et al. ( 2005 ) presented isotope data from aerosol samples collected with a
high-volume sampler at different sites in China and their data can be compared to the
numerous analyses of potential source sediment and loess samples in eastern Asia
(see Formenti et al. 2011b ). On average, Asian dusts and source sediments exhibit
slightly higher ￿ Nd (0) values and lower 87 Sr/ 86 Sr values than northern African dusts.
However, the interregional variability within a continent is much more pronounced
than the differences between African and Asian dusts. Hence, it is suggested that
isotope data are best suited for regional source apportionment studies and that some

Nd (0) values (<