Loess Records - Mineral Dust: A Key Player in the Earth System - page 420

deserts are regarded as the main source regions, they serve as dust and silt holding
areas rather than dominant producers. The mountain processes (including glacial
grinding, frost weathering, salt weathering, tectonic processes, and some fluvial
comminution) in the Gobi Altay Mountains, Hangayn Mountains, and the Qilian
Mountains have played an important role in producing the vast amounts of loess-
sized material for forming the Loess Plateau (Sun 2002a ).
Sun et al. ( 2007 ) argued that fine silt- to clay-sized quartz in the northwestern
deserts are heterogeneous and derived mainly from the nearby high-altitude moun-
tains in East Asia, whereas the eastern deserts in China mainly have a local source.
Based on the luminescence sensitivity variations of quartz grains from Chinese
deserts, four regional groups of Chinese deserts can be distinguished including
the eastern, central, western, and northwestern deserts. The different luminescence
sensitivity signals are dominantly related to the rock types of mountains surrounding
or adjacent to the deserts (Lü and Sun 2011 ).
Temporal variations of loess provenance have also been studied. Based on tem-
poral variations in Sr, Nd, and Pb isotopes and trace element concentrations within
airborne dust on the central Loess Plateau (Sun 2005 ; Sun and Zhu 2010 ), a distinct
change in source occurred at about 2.6 Ma. This change in the source of aeolian dust
at 2.6 Ma is considered to be coincident with the initiation of Quaternary glaciation
in the Northern Hemisphere. The dramatic climatic cooling induced glacial grind-
ing, which plays an important role in modifying the source material of the dust.
Loess accumulation in Central Asia is closely linked to the geographic and
atmospheric conditions in the area. The great deserts such as Karakum, Kyzylkum,
and Sary-Ishikotrau are situated directly to the west and northwest of the loess
region, covering the windward piedmont forelands and river valleys of Tian Shan
and the Pamir Plateau (e.g., Dodonov and Baiguzina 1995 ;Finaev 1995 ; Ding et al.
2002a ). In addition, airborne dust can be also transported to Iran and accumulate
as loess (Asadi et al. 2012 ). Paleowinds in northeastern Iran were dominated by
northerly or northeasterly winds blowing from Central Asia, as suggested by the
delineation of dune fields in the Karakum Desert, in Turkmenistan (Rozychi 1991 ;
Letolle and Mainguet 1993 ), and by the spatial distribution of loess deposits along
Kopeh Dagh (Kehl 2009 ).
Loess-paleosol sequences on the Loess Plateau of north-central China
(Fig. 16.1 a) have provided abundant information about regional and global climate
changes during the Quaternary (Heller and Liu 1982 ; Kukla and An 1989 ). The
most important studies of the orbital-scale paleoclimatic record on the Loess
Plateau were completed by Ding's group (Ding et al. 1993 , 1994 , 2002b ). Based on
detailed pedostratigraphy and magnetostratigraphy of the Baoji section, thirty-seven
pedostratigraphic units are identified, representing thirty-seven major cold-to-warm
climatic cycles in the past 2.5 Ma (Ding et al. 1993 ).
For the Central Asia loess, the best long-term paleoclimatic record is the
Chashmanigar loess section (Fig. 16.8 ) of southern Tajikistan (Ding et al. 2002a ;
Ya n g a n d D i n g 2006 ;Yangetal. 2006 ). This section is about 200 m thick
and consists of an alternation of loess and paleosol layers with a basal age of
about 1.77 Ma (Ding et al. 2002a ). The high-resolution magnetic susceptibility,