Subaquatic Dust Deposits - Mineral Dust: A Key Player in the Earth System - page 441

Fig. 17.5 Maps of the equatorial northwest Atlantic, showing dust flux and dust particle size of
sea-floor sediments (Sarnthein et al. 1981 ( left )), with each black dot indicating individual samples
that were analysed for particle size, and an 8-year average of TOMS aerosol index data (TOMS
data are available at http://toms.gsfc.nasa.gov ) showing the position of the northwest African dust
plume travelling westward (Mulitza et al. 2008 ( right ))

geologic history may have favoured river run-off. A classic example of this is the
Sahara Desert in northwest Africa, which presently is hyper-arid and basically only
producing dust, but which was characterised by the so-called African Humid Period
about 10,000 years ago (e.g. deMenocal et al. 2000b ) with increased rainfall at
such magnitudes that present-day deserts were turned into grasslands with ample
shrubs and trees (e.g. Tjallingii et al. 2008 ). Such humid periods occurred regularly
through geologic time at a pacing of about 23,000 yrs, and the lakes that were filled
with water during these wet phases now act as sources of dust (Prospero 1996 ;
Prospero et al. 2002 ; Drake and Bristow 2006 ; Stuut et al. 2009 ). Undoubtedly,
such high amounts of rainfall also caused significant river run-off, and evidence of
this is found in large palaeo-river systems (Vörösmarty et al. 2000 ; Krastel et al.
2004 ). The terrigenous fraction of deep-sea sediments that was carried to sea in
such intervals in geologic history most likely was dominated by river-transported
sediments instead of dust.

17.4 Proxies Used in Subaquatic Sediments to Reconstruct

Palaeo-environmental Conditions

17.4.1 Particle Size of Mineral Dust

The particle size of the wind-blown sediment fraction is intuitively related to
wind vigour (e.g. Janecek and Rea 1985 ), and several studies therefore used
the particle size of the wind-blown sediment fraction to reconstruct atmospheric
circulation patterns in the past (Prins and Weltje 1999 ; Stuut et al. 2002b ;