Loess Records - Mineral Dust: A Key Player in the Earth System - page 419

can be dated back to 22-25 Ma (Guo et al. 2002 ; Qiang et al. 2011 ), and loess can
be hundreds of meters thick, as at Lanzhou in the western Loess Plateau (Burbank
and Li 1985 ). Loess is also found in Central Asia including Tajikistan, Kyrgyzstan,
Turkmenistan, Uzbekistan, and Kazakhstan. Regardless of where the loess is found
in Asia, all loess bodies are bordered by or in the downwind areas of gobi (stony
desert) and sand deserts.
Loess in China is widespread between latitudes 34-45 ı N and longitudes 75-
130 ı E(Fig. 16.3 b). Although the most continuous loess cover is in the central
part of northern China, forming the Loess Plateau, loess deposits also occur on
pediments in the forelands of high mountains in northeastern China. On the Loess
Plateau, the thickness of loess varies from tens to hundreds of meters, found at
elevations ranging from 1,000 to 1,500 m. In northwestern China, loess is found
mainly blanketing mountain slopes, with thickness usually less than tens of meters,
but the elevation where it is found can be up to 4,000 m.
In Central Asia, loess deposits are adjacent to mountain regions and dominantly
cover piedmonts and hills (Dodonov 1991 , 2007 ). In contrast to the Chinese Loess
Plateau, loess in Central Asia mostly accumulates on the windward slopes of the
Central Asian orogenic belt (including the Tian Shan, Kunlun, Hindu Kush, and
Pamir Mountains), where loess can be found at elevations of up to 2,500-3,000 m
(Dodonov 1991 ). Generally, loess deposits in Central Asia are several tens of meters
thick, except in certain regions, such as Tajikistan, or in the vicinity of Tashkent,
where the loess strata can be up to 100-200 m (Dodonov 1991 ; Ding et al. 2002a ).
For Chinese loess, the basal age of the Quaternary loess-paleosol successions is
near the Gauss/Matuyama (G/M) geomagnetic polarity boundary (Heller and Liu
1982 ;Liu 1985 ), with an age of about 2.58 Ma. More recently, the Neogene Red
Clay (Fig. 16.1 b), also on the Loess Plateau, has now been demonstrated to be
aeolian. The latter deposits are discontinuously overlain by the Quaternary loess-
soil successions, being usually younger than 8 Ma in most sites (Ding et al. 1998 ;
Qiang et al. 2001 ), with the exception of the Qinan section that extends to 22 Ma
(Guo et al. 2002 ). The most recent study in the Junggar basin of northwestern China
reveals aeolian deposits of 24 Ma (Sun et al. 2010 ), whereas a long core taken on the
western Loess Plateau reveals aeolian dust accumulation that can be dated back to
25 Ma (Qiang et al. 2011 ). In Central Asia, stratigraphic and chronological studies
have previously shown that loess began to accumulate about 2-2.5 Ma ago and the
loess-soil sequences at different sites can be correlated with each other (Dodonov
1991 ; Dodonov and Baiguzina 1995 ).
For the most extensive and thickest loess deposits on the Loess Plateau, a
vast provenance of northwestern China had been proposed (Liu 1985 ). However,
multiple isotopic, chemical, and mineralogical data demonstrate that the loess
deposits on the Loess Plateau are mainly derived from the gobi deserts that lie to the
north of the Loess Plateau, whereas aeolian dust entrained from the three inland arid
basins (the Tarim, Junggar, and Qaidam basins) of northwestern China contributes
very little to the loess deposits on the Loess Plateau. Dust from these basins
accumulates mainly on the piedmont forelands of high mountains surrounding the
basins (Sun et al. 2001 ;Sun 2002a , b ). Moreover, although these gobi and sand