Subaquatic Dust Deposits - Mineral Dust: A Key Player in the Earth System - page 444

the material it transports; ice-rafted sediments tend to be completely unsorted
as opposed to aeolian sediments, which are very well sorted. Based on this
assumption, a numerical model was developed (Weltje 1997 ), with which a data set
of grain-size distributions can be modelled and deconvolved into subpopulations -
without a priori information about these subpopulations - that can subsequently be
interpreted in terms of sediment transport mechanism. The end-member modelling
algorithm (EMMA) does not prescribe anything in terms of shape of the grain-size
distributions of the subpopulations and has been successfully applied to recognise
ice-rafted sediments (e.g. Prins et al. 2002 ) and wind-blown dust (Stuut et al.
2002b , 2007 , 2014 ; Weltje and Prins 2003 , 2007 ). Using the end-member approach,
the river-transported and wind-blown fractions can be distinguished and quantified
downcore provided the source-to-sink distance is small enough for the aeolian
fraction to be coarser grained than the river-derived sediment fraction, which is
typically around 4-6 m in deep-marine sediment archives (e.g. Prins and Weltje
1999 ).
Intuitively, it may seem more logical to assume that wind-blown sediments are
relatively fine grained, but numerous studies on present-day dust have demonstrated
that aeolian particles (including volcanic grains) can be up to 300 m, even
thousands of kilometres from their source (e.g. Ram and Gayley 1991 ). This
observation of large wind-blown particles is fully accepted in the loess community
(see, e.g. Chap. 16 ; Prins and Vriend 2007 ; Prins et al. 2007 ), but on land coarse-
grained wind-blown deposits could be the result of sequential short-term suspension
events (Fig. 17.2 ).
In the marine realm such short-term suspension events can be excluded, but
nonetheless there is the issue of the so-called “giant” wind-blown particles (Betzer
et al. 1988 ; Middleton et al. 2001 ), which are particles that can only be transported
through the atmosphere, like particles from a dust outbreak in China, retrieved on
the islands of Hawaii, >10,000 km from their source (Betzer et al. 1988 ), but which
are too coarse grained to be explained using currently acknowledged atmospheric
transport mechanisms. Potentially, there are turbulences in the atmosphere, which
can keep relatively large particles suspended in air over thousands of kilometres.
This observation is important as the particle size and shape are directly related to
optical properties of the mineral dust. It is well established that small particles in
the higher parts of the atmosphere reflect incoming radiation (Chap. 11 ) , but in the
lower parts of the atmosphere, larger particles may absorb solar energy that was
reflected at the Earth's surface and would have otherwise been radiated into space
(e.g. Otto et al. 2007 ; Satheesh et al. 2007 ).
The shape of the aeolian particles also plays a big role in their long-distance
transport as platy minerals such as micas aerodynamically behave like much smaller
particles (e.g. Stuut et al. 2005 ). At very large source-to-sink distances (several
tens of thousands of kilometres), wind-blown particles usually are very small (e.g.
Prospero and Bonatti 1969 ; Rea et al. 1985 , this topic: Chap. 18 ) and can be so small
that they would only settle from the atmosphere as nuclei of ice crystals (Chap. 12
and, e.g. Franzén and Hjelmroos 1988 ).