ELSEVIER
Earth and Planetary Science Letters 165 (1999) 129–144
Rock magnetic and grain size evidence for intensified
Asian atmospheric circulation since 800,000 years B.P. related to
Tibetan uplift
Xiao-Min Fang a,b , Ji-Jun Li a , Rob Van der Voo b,*
b
a Department of Geography, Lanzhou University, Lanzhou, Gansu, China
Department of Geological Sciences, the University of Michigan, Ann Arbor, MI 48109-1063, USA
Received 18 May 1998; accepted 3 November 1998
Abstract
Paleomagnetic, rock magnetic, and grain size studies of a thick loess sequence in the West Qin Ling (mountain range)
show that loess deposition there began about 800 ka. The data reveal a progressively increasing coarse grain size fraction
upwards into the Holocene. The averages of these coarse size fractions are higher than in the central Loess Plateau, which
was apparently farther from the source area, and slightly lower than those of the western Loess Plateau and the eastern
Tibetan Plateau, which were therefore closer to the source area. The coarsening and source area location suggest (1) that
Asian air circulation may have changed and intensified at about 800 ka resulting in dust deposition in West Qin Ling;
(2) that dust-carrying winds were driven not only by the Asian winter monsoon, but included also the westerlies and
a winter monsoon caused by the Tibetan Plateau High, and (3) that intensification of all these air circulation systems
continues to the present. Increased elevation of the Tibetan Plateau so that it reached into the cryosphere by about 800
ka and a subsequent persistent uplift of the plateau may have been the mechanisms to trigger a change and intensify
the air circulation system. Moreover, this circulation shift and intensification, simultaneous with a shift in Milankovitch
periodicity, may have contributed to large global climate changes such as the 15% increase in global ice volume at ca. 800
ka.  1999 Published by Elsevier Science B.V. All rights reserved.
Keywords: Brunhes Epoch; Matuyama Epoch; loess, grain size; Asia; monsoons; circulation; Qinghai-Xizang Plateau;
uplifts
1. Introduction
The Asian monsoonal circulation system is one of
two key atmospheric systems that control much of
northern hemispheric climate change, the other being the North Atlantic air–ocean circulation system.
Ł Corresponding
author. Tel.: C1 734 764 8322; Fax: C1 734
763 4690; E-mail: [email protected]
Numerical modeling has demonstrated that initiation
and evolution of the Asian monsoons are closely
coupled to uplift of the Tibetan Plateau and that
these monsoons have great influence on global climate change [1–3]. Knowledge of the long-term
evolutionary history of the Asian circulation system, therefore, can provide important information
to improve our understanding of ocean–continent
air systems, the timing of the uplift of the Tibetan
0012-821X/99/$ – see front matter  1999 Published by Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 2 5 9 - 3
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X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
Plateau, and the mechanisms that force Asian and
global climate changes.
Large areas of Central and East Asia are covered
by loess. These deposits offer a unique opportunity
to trace past air circulation paths, because loess materials are transported and deposited by Asian winter
monsoons and other coupled circulation systems of
the westerlies and the Tibetan winter monsoon [4].
Grain size variation in loess has proven to be an excellent indicator for this purpose [4–8]. Spatial grain
size variation within the Loess Plateau has clearly
delineated three NE-oriented loess belts (the sand,
silt and clay loess belts). The sequence of these belts
indicates a principal NW wind direction, caused by a
combination of Asian winter monsoons and the westerlies, with the loess derived from multiple Asian inland dust source areas [4]. In contrast, Tibetan loess
is much coarser than that of the Loess Plateau, with
grain-size analysis showing a westward-coarsening
trend [5]. This demonstrates that the Tibetan loess
was derived chiefly from glacial and peri-glacial
source areas in the central and western parts of the
Tibetan Plateau.
The relative abundance of coarse grains (>63 µm
or >32 µm) has been demonstrated to be proportional to the strength of the wind and the intensity
of the Asian winter monsoon [4,7,8]. Here, we use
grain size evidence to explore the history of Asian
air circulation during the last 800,000 years. Our
sampling has been carried out in the Zhaodaisuogou
loess section at Wudu in the Qin Ling mountains
(Fig. 1) because of its large thickness and its location
in an area that is sensitive to circulation change.
2. Geological setting and stratigraphy
The Qin Ling mountains, striking W–E, form an
important natural boundary that divides temperate
and subtropical zones in China and make up the
southern boundary of the Loess Plateau. The study
section is located at Wudu (104º550 E, 33º250 N) in
southern West Qin Ling and is close to the northeastern margin of the Tibetan Plateau to the west
(Fig. 1). West Qin Ling has elevations ranging from
over 3.5 km in the west to ca. 1.5 km in the east, rising about 1–1.5 km above the Loess Plateau (Fig. 1).
Further east, in central Qin Ling, the altitude in-
creases again to over 3 km. Thus, the east end of
West Qin Ling is actually a wind pass as well as
a divide between Jialing Jiang (River), a tributary
of the Yangtze River (Chang Jiang), and Wei He,
a tributary of the Yellow River (Huang He) (see
Fig. 1). Modern winter air circulation patterns show
that lower level (<3 km) northerly and northwesterly
winds proceed southward only through the pass, and
then enter the Bailong Jiang valley in the eastern
Tibetan Plateau and West Qin Ling (see Fig. 1) from
a southeasterly direction. In contrast, winds above
3 km are northwesterly throughout the map area
[9]. The lower-level winds are generated mainly by
the Asian winter monsoon driven by the Siberian–
Mongolian High and by the northern branch of the
circum-Plateau westerlies. High-level winds are derived from a combination of the Tibetan Plateau
winter monsoon and higher levels of the westerlies
[9–12] (Fig. 1, inset).
The study section at Zhaodaisuogou in Wudu is
situated on the fourth terrace (T4) of the Bailong
Jiang (River), a tributary of Jialing Jiang, and is one
of the thickest loess sections in the West Qin Ling
(Figs. 1 and 2). It consists of two parts, with the
upper part consisting mainly of 55 m of loess, which
contains 18 individual paleosols that can be grouped
into 8 paleosol complexes (S0–S7), as well as a layer
of debris flows in the middle (Figs. 2 and 3). Six thin
reworked loess layers are observed in the sequence.
The lower part consists of about 25 m of alluvial silt,
alluvial gravels and debris flows.
The uppermost paleosol is a well developed,
brownish dark (10YR 6=3-2) distinctive paleosol. At
a depth of approximately 12–13 m, a second well developed brownish (7.5YR 6=4) paleosol complex is
found. Between these two well developed paleosols
is brownish yellow (10YR 7=3), coarse and loose
Malan Loess (L1) and its four embedded weakly developed yellowish brown (10YR 6=4) paleosols. By
analogy with other pedostratigraphic models [4,13–
15], these paleosols are correlated with pedo-complexes S0, Sm, and S1 in the Loess Plateau (Figs. 2
and 3). Between 14 to 23 m three well developed
soil complexes are correlated to paleosols S2 to S4
in the Loess Plateau (Fig. 3). At depths of 27 to 41
m the three best-developed paleosols of the section
occur, characterized by clear horizons of Bt–Bk–C.
The Bt horizon has distinct vivid brown colors (7.5
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
131
Fig. 1. Location map of the area near the Wudu section in Qin Ling (Mts.), with elevations and loess distribution. The inset shows the
configuration of large landforms and wind streamlines at the 1500 m level (arrow-headed solid lines) and 5000 m level (arrow-headed
dashed lines) during January (averaged for 1961–1970) [9]. The divergence of the westerlies into north and south branches at the west
end of the Tibetan Plateau and convergence at its east end and in East China occurs for winds on the lower (1500) as well as the higher
(5000 m) levels. The north branch of the westerlies passing around the plateau is largely responsible for the lower-level formation and
intensity of several anticyclones (A) in the Asian inland, such as the Siberian–Mongolian High centered at 100ºE, 50ºN.
to 5YR 6=4), medium–fine granular structure, many
clay channel coatings and infillings, abundant biologic remains, and very low contents of carbonate
(8–0.5 wt%) [16]. In the Loess Plateau, paleosol
complex S5 consists of three welded or individual
soils that are the most strongly developed in the
entire loess–paleosol sequence of the last 2.5 Ma
[4,13–15]. We correlate the three most strongly developed paleosols in our section with the S5 complex
of the Loess Plateau [16]. Consequently, the lowermost two paleosol complexes in our section are S6
and S7 (Fig. 3). Interspersed with these paleosols
are loess layers L1–L8 (Figs. 2 and 3), including
reworked loess, which is recognized by its weak
bedding structure and transported soil relics.
3. Sampling and laboratory measurements
Paleomagnetic samples were taken in the Zhaodaisuogou section (T4) at 0.5–1 m intervals within
the paleosol–loess sequence and at variable inter-
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Fig. 2. Stratigraphy of the Zhaodaisuogou loess section on the fourth terrace (T4) of Bailong Jiang (River) at Wudu in central West Qin
Ling. Broken lines indicate lower terrace profiles (T2, T3) at nearby sites.
vals in lenses within the alluvial gravels and debris
flows, wherever sampling was possible. One oriented
block sample of 10 ð 10 ð 10 cm was taken at each
site (level), from which three to four oriented cubic
sub-samples of 2 ð 2 ð 2 cm were cut, yielding a
total of 260 sub-samples. A parallel section on an
older terrace (T7) was similarly sampled at Dazhaicun in Wudu, in order to confirm the location of the
Brunhes–Matuyama boundary. Here a total of 520
sub-samples was collected. The Dazhaicun section is
located near the Zhaodaisuogou section, but is about
200 m higher in elevation; the two sections have very
comparable stratigraphies (Figs. 3 and 4).
Frequency-dependent magnetic susceptibility
(fd ) was calculated from measurements of bulk
susceptibility using a Bartington MS2 susceptibil-
ity meter at low (0.47 kHz) and high (4.7 kHz)
frequencies corrected for the weight of the sample
and is expressed as a percentage of low frequency
susceptibility [17]. The natural remanent magnetization (NRM) of the samples was measured using a
DIGICO magnetometer and was demagnetized with
stepwise alternating fields (AF) from 5 to 50 mT
in ten increments of 5 mT at Lanzhou University.
One sub-sample from each site in the lower part of
the two sections was subjected to thermal demagnetization from 100 to 700ºC in fifteen steps varying
between 10 and 50ºC, and measured with a 2G cryogenic magnetometer in a magnetically shielded room
at the University of Michigan.
Demagnetization diagrams demonstrate that AF
and thermal demagnetization on the same sam-
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
133
Fig. 3. Lithology, frequency-dependent magnetic susceptibility .fd /, declination, inclination, and NRM intensity in the Wudu loess
section at Zhaodaisuogou. The available age dates, obtained with 14 C, thermoluminescence (TL) and optical stimulated luminescence
(OSL) techniques, are shown next to the lithological column. The dashed lines indicate an anomalous intermediate magnetic direction
obtained in thermal demagnetization. For the locations marked by asterisks, representative demagnetization diagrams are shown in Fig. 5.
ple yields essentially the same direction for most
of the measured samples. A lower-coercivity or
lower-blocking temperature component is removed
by about 20 mT or 250ºC, whereupon characteristic
directions become clearly revealed in trajectories to
the origin below 50 mT or 700ºC (Fig. 5). While
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X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
Fig. 4. Lithology, magnetic susceptibility, virtual geomagnetic pole latitude, NRM intensity, and polarity in the Wudu loess section at
Dhazhaicun. For the location at 55 m marked by an asterisk, a representative demagnetization diagram is shown in Fig. 5.
thermal demagnetization shows a contribution of
hematite (or maghemite?) to the remanence in the
interval 580–700ºC, most of the magnetization is
removed in fields or temperatures that are rather
characteristic of magnetite.
Paleomagnetic directions were obtained using
principal component analysis for each sub-sample.
Mean directions for each site were obtained by averaging the three or four sub-sample directions from
each level. Site-mean declinations and inclinations
are plotted alongside the stratigraphic column of
Fig. 3.
Additional bulk samples were collected at similar intervals as for the paleomagnetic sites, but at
increased intervals in soil horizons. Samples were
divided into two portions. One portion was used to
measure coarse (sand) size fraction <4 φ (>63 µm)
by using a wet-sieve method. The other was pre-
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
135
Fig. 5. Lower-hemisphere stereographic projection and orthogonal projections of progressive demagnetization results for representative
samples of loess and paleosol from the T4 terrace, Zhaodaisuogou section (a–c), and from (d) the T7 terrace, Dazhaicun section, in
Wudu, along with their respective intensity decay curves (e–h). Solid (open) symbols (in b–d) represent projections onto the horizontal
(vertical) plane. The positions of the samples shown are marked by asterisks in Figs. 3 and 4.
treated by H2 O2 to remove organic matter and by
calgon solution (3.3% sodium hexametaphosphate
and 0.7% sodium carbonate) and subjected to 10
minutes of ultrasonic vibration to scatter fine particles (clays) for homogeneous emulsion. Then the
emulsion was moved onto a Sedigraph 500ET laser
sizer for analysis of fine grades (>4 φ). The system
of Pettijohn [18] was used for grain size classification, which bins sand, silt and clay as particles <4 φ
(>63 µm), 4–9 φ (63–2 µm) and >9 φ (<2 µm).
Common statistic parameters (mean and sorting coefficient D standard deviation (¦ )) were calculated
by matrix methods [18].
4. Chronology
4.1. Paleomagnetic and rock magnetic stratigraphy
Stepwise AF- and thermal demagnetizations of
loess and paleosols show that there are no clearly
reversed directions observed in the upper 60 m of
the Zhaodaisuogou section, although one sub-sample
from a depth of 52 m in paleosol S7 has an intermediate (westerly and shallowly inclined) direction
(Fig. 3). The oldest loess is L8 in this section and
is still of normal polarity. Below L8, between 60
and 85 m, sampling was hampered by the poorly
suited lithologies, but the few paleomagnetic directions obtained in this part (not shown) were all of
normal polarity, with only one exception that has
little credibility. In the parallel Dazhaicun section,
sampled about 200 m higher than the Zhaodaisuogou
section, on the seventh terrace (T7) of the Bailong
Jiang at Wudu, the samples are reversed in the bottom 3 m in Paleosol S8 and Loess L9 (Fig. 4 and
Fig. 5d). The Brunhes–Matuyama boundary therefore occurs in the lowermost part of L8 or the top
of S8. We conclude that the loess–paleosol sequence
(upper 60 m) on the terrace T4 at Wudu was formed
entirely during the Brunhes Chron, in agreement
with the pedostratigraphic division mentioned above
and magnetostratigraphic determinations elsewhere
for S1–S7 and L8 [4,22,23].
The frequency-dependent magnetic susceptibility
(fd ) of the sequence shows a clear magnetic en-
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hancement in the paleosols and sharp decreases in
loess (Fig. 3), as has also been found elsewhere in
China [17,19,23]. The reworked loess generally also
shows higher values of fd (Fig. 3). This may suggest
that slope processes have caused an accumulation of
some fine-grained magnetic minerals eroded from
the upper part of the slope. Except for this minor
deviation caused by the reworking of loess, the variation pattern of fd in the section can be easily
correlated with that of loess–paleosol sequences on
the Loess Plateau [4,17] and with the nineteen MIS
stages during the last 800 ka [24]. In summary, the
evidence collectively indicates that the age of the
bottom of the Zhaodaisuogou section is best placed
at about 770 ka, whereas the somewhat older bottom
part of the Dazhaicun section is reversely magnetized
and estimated to be about 830 ka.
4.2. Radiocarbon and TL=OSL dating
Three bulk samples collected from the top and
bottom of the Bw horizon of paleosol S0 and the
top of paleosol Sm-1 were used for carbon dating
on diffuse organics, yielding ages of 2 to 27 ka
(Fig. 3). Five thermoluminescence (TL) and optical
stimulated luminescence (OSL) samples collected at
depths of 8 to 14 m yielded ages ranging from 54
to 141 ka (Fig. 3). Although the TL and OSL ages
have large errors, they suffice to constrain the ages
of the upper 14 m of section. Here, the youngest
and second-youngest well developed paleosol complexes (S0 and S1), as well as the weakly developed
(Sm) paleosol complex between them, are located
with ages of about 2–10 14 C ka (S0), 91–141 ka
(S1a and S1b) and 27–54 ka (Sm-1 to Sm-3), respectively (Fig. 3). These ages confirm our pedostratigraphic correlations and are further supported
by a good correlation between the fd data in our
section (Fig. 3) and fd records from the Loess
Plateau [17,19]. Further, marine isotope-stage climatic records (MIS) confine the Holocene (MIS1),
last interglacial (MIS5) and last mega-interstadial
(MIS3) to ages of 0–12, 74–130 and 24–59 ka,
respectively [20]. This, largely, agrees with our data.
The radiocarbon and TL=OSL ages also guide
age estimates for the lower parts of the section.
Based on our ages, the inverse sedimentation rate
of the late Pleistocene Malan Loess L1 and the
Holocene loess L0 (Fig. 3) is calculated at ca. 75.8
years=cm, assuming no erosion. It has long been
known that the Malan and Holocene loesses in China
have higher rates of sediment accumulation than the
older loesses below [4,13,14]. This is also true for
deposits in the North Pacific Ocean [25]. On the
central Loess Plateau (e.g., at the Heimogou section in Luochuan), inverse sedimentation rates of
loess L1–L0 and loess between L1 and L8 at the
Brunhes–Matuyama boundary (780 ka [21]) are ca.
74.5 and 166.3 years=cm, respectively [4]. On the
western Loess Plateau (e.g., at the Jiuzhoutai section
in Lanzhou), the corresponding rates are 23.9 and
58.9 years=cm, respectively [13,14], averaging to an
increase in sedimentation rate by a factor of 2.35 for
the Loess Plateau. Taking the same factor of 2.35
for the Wudu area, and subtracting the 5 m of debris
flow in the loess section at Wudu, yields an age for
the bottom of the Zhaodaisuogou loess section of ca.
770 ka. This estimate agrees well with the paleomagnetic results. Moreover, ages of paleosols S2–S7
estimated by this model are consistent with those
on the Loess Plateau [4], suggesting a reasonable
pedostratigraphic division for the lower part of the
Wudu section.
5. Grain size results, and areal and vertical extent
of the loess
The clay-size fraction is greater in paleosols and
smaller in loess, while the sand-size fraction just
shows the opposite tendency (Fig. 6). Not surprisingly this indicates a high degree of pedogenesis in
paleosols. The fluctuation of clay-sized grains agrees
well with the fd curve. However, grain sizes of
loess and paleosols in our section reveal two other
striking characteristics. The first is that grain size
shows a progressively coarsening upward trend since
ca. 800 ka, especially if the debris flow intervals are
discounted. Moreover, this trend appears to be somewhat accelerated since the last interglacial (74–130
ka [20]) (Fig. 6). The coarsening is manifested as
an increase in the sand-size and coarse silt-size fractions (>32 µm, Fig. 6). This trend is equally well
indicated by the plot of mean φ (Fig. 6).
The second characteristic that we observe is that
the average grain size in the Wudu Brunhes-age
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
137
Fig. 6. Lithology and variation of grain size parameters (see text for explanation) in the loess–paleosol sequence of the Zhaodaisuogou
section at Wudu.
loess is larger than that in the central Loess Plateau,
which is surprising given the closer proximity of
the (lower-elevation) central Loess Plateau to Asian
inland dust sources. Although stronger northwesterly
winds would result in comparably larger grain sizes
in the Qinling, such winds would have deposited
even larger grains in the central Loess Plateau. This
indicates that additional source areas may need to
be identified. At Wudu, the average grain size is
only slightly smaller than those in the western Loess
Plateau and in East Tibet (Table 1), which suggests
that these additional source areas may be located
in the Tibetan Plateau. This last observation implies
that there must have been a shift, either gradual or
sudden, in the air circulation at about 800 ka that
allowed different winds to bring coarser dust into
West Qin Ling than to the central Loess Plateau.
A simple intensification of the previously existing
air circulation system can not have accomplished
this, for several reasons. First, as already mentioned,
the central Loess Plateau is closer to Asian inland
dust source areas and has a much lower average
elevation than West Qin Ling (Fig. 1), so it would
have received coarser loess in greater quantities than
the Qin Ling. Second, the previously existing winter
monsoon was chiefly transporting coarser dust near
the land surface (<2 km) [9,12,26], so it would have
been largely stopped by the higher-elevation West
Qin Ling. Third, there are no likely local dust sources
in West Qin Ling. The wind pass in the eastern end
of West Qin Ling (Fig. 1) could have allowed dust
from Central Asia (e.g., the Gobi Desert) into the
area, as occurs today with the dominant low-level
northerly winter winds, but this would only lead to
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X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
Fig. 7. A model proposed for the air circulation resulting from Tibetan Plateau uplift at times (a) well before, and (b) largely after the
Brunhes–Matuyama boundary at about 800 ka. Before 800 ka and after 2.5 Ma, the average height of the Tibetan Plateau is assumed to
be lower than the threshold height of 3 km but higher than 1.5 km [12,33,45], so that lower-level flow passed mainly around the plateau,
but higher-level (‘Main westerly’) flow passed directly over the plateau [44]. The north branch of the low-level westerlies was much
weaker than at present and did not yet move in a south–southeasterly direction towards Central China. The Asian and plateau winter
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
139
Table 1
Average grain sizes of loess in Qin Ling and other parts of China
Clay (>9 φ)
(wt%)
Mean
(φ)
Sorting coefficient
Xuancheng (118.8ºE, 31ºN), Anhui Province, East China [28]
ML
4.1
56.6
ULL
3.1
45.1
LLL
2.1
36.0
39.3 a
51.9 a
61.9 a
–
–
–
–
–
–
Luochuan (109.5ºE, 35.5ºN), Central Loess Plateau [4]
ML
6.8
65.8
ULL
10.0
63.7
LLL
5.3
62.6
27.4
26.0
31.2
6.00
5.92
6.34
2.98
2.68
2.97
Wudu (105ºE, 33.5ºN), western Qin Ling (this study)
ML
8.2
69.1
ULL
5.5
65.3
22.7
29.2
6.71
7.44
2.58
2.77
Lanzhou–Linxia (103.5ºE 36ºN), western Loess Plateau [14]
ML
11.6
70.0
ULL
9.1
66.6
LLL
5.3
72.8
18.5
24.3
20.7
6.35
7.08
5.9
2.71
3.02
2.56
Eastern Tibetan Plateau
ML [5]
ULL (this study)
18.0
23.4
6.64
7.12
3.01
3.34
Stratigraphy
Sand (<4 φ)
(wt%)
17.9
13.7
Silt (4–9 φ)
(wt%)
64.1
62.9
ML D Malan Loess (ca. 12–73 ka). ULL D Upper Lishi Loess (ca. 73–500 ka). LLL D Lower Lishi Loess (ca. 500–1200 ka).
a Sizes >8 φ.
the deposition of thicker and older, not coarser, loess
at the pass [27].
The areal and vertical extent of loesses of different ages provides another line of evidence for a shift
in regional atmospheric circulation at ca. 800 ka.
Loess deposited during the Brunhes normal-polarity
Chron (hereafter called Brunhes loess) has a much
wider distribution than loess deposited during the
Matuyama Chron between 0.78 and 2.5 Ma [4,22,
23]). Matuyama-aged loess is confined to the Loess
Plateau to the north of ca. 34ºN in central North
China (north of the double dotted line in Fig. 7a;
see also fig. 2.1 in [4]), but Brunhes loess extends
much farther to the southeast to the lower reaches
of the Yangtze River and the coastal area of East
China north of 30ºN [4,28–30] and even into Japan
[31] (Fig. 7b). The increase of dust influx into the
northern Pacific Ocean since ca. 800 ka [32] may
monsoons were much weaker than at present [1–3], due to the existence of prevailing overflow of the westerlies in winter and the Indian
monsoon in summer and the predominance of grass and forest on the plateau. Before about 800 ka, limited transport of dust occurred,
as shown by the fine grain size (Table 1). The southern areal distribution limit of Matuyama loess (double dotted line) was limited to
the north of 35ºN [4] in Central and East China, and no loess deposition occurred in the Tibetan–Himalayan area [33–40]. After about
800 ka, the plateau rose over the threshold height and into the cryosphere [33,45,46]. The flow of the westerlies went mainly around
the plateau and was more intense. After flowing past the plateau, the north branch began to move in a more southeastward direction
than before, and the convergence point of the two branches of the westerlies shifted southward to the lower reaches of the Yangtze River
(Chang Jiang) at about 30ºN. The plateau and Asian winter monsoons were greatly strengthened due to the replacement of grass and
forests in the plateau by a glacier- and snow-covered surface, and by diversion of the westerlies, as indicated by the 600 and 850 mbar
geopotential height (light gray continuous lines) of the January average [10]. The air circulation shift caused a stronger development of
dust storms both in the Asian inland and on the plateau, a large southeastward expansion of Brunhes loess to East China and Japan north
of 30ºN (broken double line), and the first loess deposition in the Tibetan–Himalayan area. Subsequent continued uplift of the plateau
caused further expansion of loess towards the southeast, reaching latitudes near 27ºN (combined dotted and solid line). Heavy dotted
single line marks the area influenced by the plateau monsoon [10]. L indicates low pressure centers (cyclones).
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X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
Table 2
Eastward changes in grain size of Malan Loess from the eastern Tibetan Plateau to West Qin Ling
Locality
Latitude, longitude
Sand (<4 φ)
(wt%)
Mean
(φ)
Source
Tibetan Plateau
Henan
Maqu
Luqu
Longmusi
101º300 E, 34º200 N
102ºE, 34ºN
102º400 E, 34º50 N
102º300 E, 34º350 N
34.5
24.1
18.7
17.7
5.80
6.14
6.19
7.31
This study
[5]
This study
[5]
West Qin Ling
Tewo
Wudu
Huixian
103º130 E, 34º50 N
104º550 E, 33º250 N
106º50 E, 33º450 N
10.0
8.2
7.5
6.50
6.71
6.8
This study
This study
This study
The eastward decreasing grain sizes of Malan Loess can be traced from the westernmost location on the Tibetan Plateau (top) to the
easternmost location in West Qin Ling (bottom).
also be related to this phenomenon. At the same
time, at about 800 ka, loess began to be deposited at
higher elevations than before, and high Asian loess
appeared in Tibet at about 800 ka [33–36].
In the last 90,000 years, loess deposits show yet
a further expansion, both horizontally and vertically.
Malan and Holocene loess reaches 2–3º southward
beyond the limits of the earlier Brunhes loess (to the
combined solid–dotted line in Fig. 7b), has higher
rates of deposition, is coarser in grain size, and is
found at higher elevations than older Brunhes loess
[4,13,14,28–30] (Table 1). Modern dust distributions
and historical records show dust storms reaching to
23.5ºN in China in some cases, as well as southern
Japan (e.g., at Chichijima at 27ºN [4,37,38]. Malan
and Holocene loess occurs up to 2.8 km in the western Loess Plateau and up to 2.9 km at the northern
slopes of the West Qin Ling, whereas the maximum
elevations of older Brunhes loess in these areas were
2.5 and 2.3 km, respectively. The younger loess is
found to extend over a larger area in the Himalayas
and Tibetan Plateau and adjacent southern Asia than
older Brunhes loess [5,33–36,39,40]. The latter area
is presently controlled by the Tibetan monsoon, the
westerlies, easterlies and Indian monsoon [9–12],
a ‘Tibetan’ system that is very different from the
air circulation system operating in Central and East
China (Fig. 1 and Fig. 7b). Without the ‘Tibetan’
system, High Asian loess would not be deposited.
We argue that it is only because of this ‘Tibetan’
system that coarse dust could have been transported
to West Qin Ling and other areas adjacent to the
plateau. The higher-level winds of the Tibetan winter monsoon (Fig. 1 inset) pass over Tibetan sanddunes, providing a ready source area in the western
and central plateau, and over loess deposits in the
eastern plateau [5,41]. The mineralogical and chemical similarities between Tibetan, West Qin Ling and
Lanzhou area loesses [5,42,43] and their eastward
decreasing grain sizes (Tables 1 and 2), lend further
support to this idea.
6. Discussion
The grain-sizes, and the changes in the temporal, areal, and vertical distributions of the loess in
Asia, suggest that atmospheric circulation conditions
changed at about the Brunhes–Matuyama boundary
and further continued to intensify during the Late
Pleistocene and Holocene. The change in grain size
has been observed in the Qin Ling as well as the
central and western Loess Plateau [4,14,23]. We argue that these patterns are due to changes in the
orography of the Tibetan Plateau and 100 k-year
orbital power dominating the paleoclimate. We will
describe, in the following, a conceptual model that
aims to explain them. Fig. 7 illustrates the conditions
we hypothesize as prevailing well before and largely
after the Brunhes–Matuyama boundary at about 800
ka, but we must note that we cannot be definitive
about whether the transition was sudden or gradual.
Modern dust is transported by two important air
circulation systems, as already mentioned. One is
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
the Tibetan monsoon, which, combined with the
high-level westerly winds that pass over the plateau
and the southern branch of the lower-level westerlies,
deposits loess in Tibet and adjacent southern Asia
[5,9–12] (Fig. 7b). The other system consists of
the Asian winter monsoon and the northern branch
of the westerlies, which deposit loess in Central
and East China [4,9]. The existence of the two air
circulation systems, of course, depends critically on
the elevation of the Tibetan Plateau, and the first
appearance of the Tibetan circulation system is a key
to explain the observed features of Brunhes loess.
Theoretical studies have shown that a critical
height exists that determines whether flow tends to
go over, or around the Tibetan Plateau [44]. When
the elevation of the Tibetan Plateau is less than half
the critical height, most of the flow tends to pass
directly over the plateau, whereas when elevations
are about twice the critical height (called ‘threshold’
below), much of the flow tends to go around it.
Between these two elevations, flow will go over the
plateau at the higher level and around the plateau at
lower levels. Estimates of threshold height are complicated, but center around mean elevation values of
about 3 km [44], which is also a threshold height
for the plateau to become glaciated [45]. Studies
of Tibetan glaciations have shown that the plateau
reached the cryosphere at about 800 ka [33,45,46].
These observations and theoretical considerations
are generally consistent with modeling using numerical GCMs (general circulation models), which
show that without a Tibetan Plateau above a certain
height, there would be no subtropic easterly jet and
no Siberian High, whereas only a weak high would
be found near the northern margin of the Tibetan
Plateau (causing a much weaker Asian winter monsoon); in such a situation, the Eurasian landmass
would be mostly influenced by smooth and weaker
westerlies [1,2]. Kutzbach and his co-workers have
further demonstrated that above a certain average
elevation (‘critical height’) of the plateau, a circulation system is configured that begins to resemble,
in general, that of today. It must be noted that their
‘critical height’, estimated as ca. 1.5 km, includes
averaging of elevations between the plateau and adjacent areas, and so is low for the plateau proper.
With further progressive uplift of the plateau, the
system is linearly intensified, and when full uplift
141
to ca. 2.6 km has occurred (again this is on the
low side), a modern circulation pattern is fully established [2]. Refinements in this GCM modeling
using more precise landform data [3], together with
detailed past plateau monsoon records [46], have further demonstrated that the Tibetan Plateau monsoon
develops in concert with the Asian monsoon, such
that a stronger Tibetan High (depending on plateau
height) enhances the strength of the Asian winter
monsoon, as well as the intensity of the westerly and
easterly jets. This means that the two systems are
closely coupled and directly linked with uplift of the
plateau.
Based on these models, we propose that uplift
of the Tibetan Plateau above a critical threshold of
some 3 km average height may have triggered a shift
in air circulation in Asia, at about 800 ka, and, in
turn, may have caused expansion of Brunhes loess to
more southerly locations in China and facilitated the
first appearance of the Tibetan loess.
Fig. 7a illustrates the situation as we envision
it well before the Tibetan Plateau reached a 3 km
threshold height. At that time, the Indian monsoon
could move directly into and over much of the
plateau during the summer, bringing moisture to the
surface area of the plateau and the present-day dry
areas to the north and east [47–50]. The plateau was
then covered by forests and grass, with relatively low
albedo. Few possible Tibetan source areas for dust
existed, therefore, at that time. The main westerly
flow ran directly over the plateau, resulting in weak
Tibetan and Siberian Highs (compare their symbolized sizes in Fig. 7a,b), although at lower levels a
weak diversion occurred, around the plateau to the
north or south. This air circulation system could generate only small dust storms, and transport of dust
would be limited in distance, as observed for the
Matuyama loess (2.5–0.8 Ma), which is confined to
the Loess Plateau north of about 34ºN (Fig. 7a).
In contrast, after the plateau reached the threshold
height, it began to force the main westerly flow to
go principally around it [44] (Fig. 7b). This diversion of the westerlies causes (1) intensification of
the Siberian and Tibetan Highs, (2) a strengthened
westerly flow, with inherently increased instability,
and (3) a much more south-directed orientation of
the northern branch of the westerlies after its passage
around the plateau [1–3,9–12]. This last charac-
142
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
teristic causes a displaced convergence of the two
branches of the westerlies towards lower latitudes,
e.g., towards the Yangtze River (Chang Jiang) north
of 27ºN (Fig. 7b), and changes the direction of the
lower level flow of the Asian and plateau winter
monsoons from eastward to southeastward. The increased instability of the westerlies facilitates the
formation of cyclones and troughs in the middle and
upper troposphere, which in turn create cold fronts
and trigger a release of accumulated Siberian cold
air masses that generate the dust storms of the Asian
winter monsoon [9–11].
That the plateau reached the cryosphere at the
same time at about 800 ka, may have further enhanced the processes described above. The disappearance of vegetation and increase in snow cover of
the glacial and periglacial plateau environment during the Brunhes Chron must have increased albedo,
produced cooler near-surface air, enhanced the Tibetan High during winter, and very importantly, created a large and new source area of dust. As a
corollary, feedback mechanisms associated with the
loss of heat and a strengthened Tibetan High will
enhance global cooling, and enlarge the temperature
gradient between high-latitude areas and the equator,
thereby further strengthening the westerlies and the
Asian winter monsoon.
In Tibet, stronger currents generated sand dunes
and dust [41] and transported dust to eastern and
southern parts of the plateau, as well as nearby
areas such as West Qin Ling and the western Loess
Plateau [5]. Continued Late Pleistocene uplift of the
Plateau caused further intensification of the Asian
air circulation system and aridity in enlarged Asian
inland dust-source areas. We believe that this caused
the grain size coarsening in the loess at Wudu and the
further expansion of loess in the Late Pleistocene–
Holocene (Fig. 7b).
If further research confirms the likelihood of this
plateau–air circulation coupling model, the global
cooling, enhanced seasonality, and magnified summer monsoons (well known to occur during interglacials [4,7,8]) or winter monsoons (during glacial
intervals) [51] may have made important contributions to the increased global ice volume in the
Mid-Pleistocene [52], approximately contemporaneous with the Milankovitch climatic periodicity shift
at ca. 800 ka [53].
Acknowledgements
We cordially thank Prof. David K. Rea of the
University of Michigan, and Profs. Tang Maocang,
and Luo Siwei at the Lanzhou Institute of (Tibetan)
Plateau Atmospheric Physics of CAS for their helpful discussions on air circulation systems, Prof. Shi
Yafeng for his heuristic suggestions, Mr. Shi Zhengtao for his help, and Dr. M.L. Clarke and Mr. Cao
Jixiu for their assistance in the laboratory work.
Helpful reviews were provided by R. Rutter, G.
Rambstein and an anonymous reviewer. This work
is co-supported by grants from the NECC (grant for
Trans-Century Excellent Researchers and the Key
Program), NSFC and the National and CAS Tibet
Research Project. [AC]
References
[1] S. Manabe, T.B. Terpstra, The effects of mountains on
the general circulation of the atmosphere as identified by
numerical experiments, J. Atmos. Sci. 31 (1974) 3–42.
[2] J.E. Kutzbach, P.J. Guetter, W.F. Ruddiman, W.L. Prell,
Sensitivity of climate to late Cenozoic uplift in Southern
Asia and the American West: numerical experiments, J.
Geophys. Res. 94 (1989) 18393–18407.
[3] X.D. Liu, Z.G. Wei, Numerical simulation of the change
of Tibetan Plateau Monsoon since the last glacial maximum, in: X.D. Liu (Ed.), Studies in the Regional Climate
Variation of the Western China and Its Relevant Problems,
Lanzhou Univ. Press, Lanzhou, 1995, pp. 1–10.
[4] T.S. Liu, Loess and the Environment, China Ocean Press,
Beijing, 1985, 251 pp.
[5] X.M. Fang, The origin and provenance of Malan loess
along eastern margin of Qinghai–Xizang (Tibetan) Plateau
and its adjacent area, Sci. China B 38 (1995) 876–887.
[6] J.A. Mason, E.A. Nater, H.C. Hobbs, Transport direction
of Wisconsinan loess in southeastern Minnesota, Quat. Res.
41 (1994) 44–51.
[7] S.C. Porter, Z. An, Correlation between climate events in
the North Atlantic and China during the last glaciation,
Nature 375 (1995) 305–308.
[8] J. Xiao, S. Porter, Z.S. An, H. Kumai, S. Yoshikawa, Grain
size of quartz as an indicator of winter monsoon strength on
the Loess Plateau of Central China during the last 130,000
yr, Quat. Res. 43 (1995) 22–29.
[9] D.Z. Ye, Y.X. Gao, Meteorology of the Tibetan Plateau,
Sci. Press, Beijing, 1988, 420 pp., (in Chinese).
[10] M.C. Tang, Z.B. Sheng, Y.Y. Chen, General climatic characteristics of (Tibetan) Plateau Monsoon, Acta Geogr. Sin.
34 (1979) 33–42.
[11] M.C. Tang, E.R. Reiter, Plateau monsoons of the northern
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
Hemisphere: A comparison between North America and
Tibet, Mon. Weather Rev. 112 (1984) 617–637.
M.C. Tang, X.D. Liu, A new mark for delimitation of
Quaternary: stable appearance of Plateau Monsoon and
its environmental effects, Quat. Sci. 1 (1995) 82–88 (in
Chinese with English abstract).
D.W. Burbank, J.J. Li, Age and palaeoclimatic significance
of the loess of Lanzhou, north China, Nature 316 (1985)
429–431.
F.H. Chen, W. Zhang et al., Loess Stratigraphy and Quaternary Glacier Problems in Gansu–Qinghai Region, Sci.
Press, Beijing, 1993, 201 pp., (in Chinese).
Z.L. Ding, N.W. Rutter, T.S. Liu, Pedostratigraphy of Chinese loess deposits and climatic cycles in the last 2.5 Ma,
Catena 20 (1993) 73–91.
X.M. Fang, 730,000 year debris flow strata records and environmental change in West Qin Ling Mountains and their
implications on the uplift of the (Tibetan) Plateau, in: Proc.
4th Nat. Symp. Debris Flow, Gansu Culture Press, Lanzhou,
1994, pp. 10–20, (in Chinese with English abstract).
F. Heller, X.M. Liu, T.S. Liu, T.C. Xu, Magnetic susceptibility of loess in China, Earth Planet. Sci. Lett. 103 (1991)
301–310.
F. Pettijohn, Sedimentary Rocks, 3rd ed., Harper Row, New
York, 1975, 580 pp.
X.M. Liu, T. S Liu, F. Heller, T.C. Xu, Frequency-dependent susceptibility of loess and Quaternary paleoclimate,
Quat. Sci. 1 (1990) 42–50 (in Chinese with English abstract).
D.G. Martinson, N.G. Pisias, J.D. Hays, J. Imbrie, T.C.
Moore Jr., N.J. Shackleton, Age dating and the orbital
theory of the ice ages: development of a high-resolution 0
to 300,000-year chronostratigraphy, Quat. Res. 27 (1987)
1–29.
S.C. Cande, D.V. Kent, Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and
Cenozoic, J. Geophys. Res. 100 (1995) 6093–6095.
F. Heller, T. Liu, Magnetism of Chinese loess deposits,
Geophys. J. R. Astron. Soc. 77 (1984) 125–141.
G. Kukla, Loess stratigraphy in Central China, Quat. Sci.
Rev. 6 (1987) 191–219.
N.J. Shackleton, A. Berger, W.R. Peltier, An alternative
astronomical calibration of the lower Pleistocene timescale
based on ODP Site 667, Trans. R. Soc. Edinburgh, Earth
Sci. 81 (1990) 251–261.
D.K. Rea, The paleoclimatic record provided by eolian
deposition in the deep sea: The geologic history of wind,
Rev. Geophys. 32 (1994) 159–195.
R.A. Bagnold, The Physics of Blown Sand and Desert
Dunes, Methuen, London, 1941, 280 pp.
X.Y. Lei, L.P. Yue, J.Q. Wang, L. Zhang, Magnetic characteristic of loess at Fengzhou in Qin Ling and its paleoclimatic implication, Chin. Sci. Bull. 43 (1998) 1537–1540
(in Chinese).
D.Y. Yang, The Quaternary dust-fall accumulation and the
monsoon variability in eastern China, Quat. Sci. 4 (1991)
354–360 (in Chinese with English abstract).
143
[29] F. Jiang, X. Wu, H. Xiao, S. Wang, B. Xue, Age of
the vermiculated red soil in Jiujiang area, Central China,
J. Geomech. 3 (1997) 27–32 (in Chinese with English
abstract).
[30] J.J. Li, L.Y. Zhang, Y.X. Dong, X.Z. Zhou, Problems of
Quaternary environmental evolution and geomorphological
development in Lu Shan, Sci. Sin. Ser. B 8 (1983) 734–
742.
[31] T. Suzuki, Discharge rates of fallout tephra and frequency
of plinian eruptions during the last 400,000 years in the
southern northeast Japan ARC, Quat. Int. 34 (3536) (1996)
79–87.
[32] D.K. Rea, H. Snoeckx, L.H. Joseph, Late Cenozoic eolian
deposition in the North Pacific: Asian drying, Tibetan uplift,
and cooling of the northern hemisphere, Paleoceanography
13 (1998) 215–224.
[33] F.B. Chen, S.H. Gao, J.L. Chen, Preliminary paleomagnetic
study of Ganzi loess section, Sci. Bull. Chin. 35 (1990)
1600 (in Chinese).
[34] X.M. Fang, F.B. Chen, Y.F. Shi, J.J. Li, Ganzi loess and
its suggested glacial evolution on the Qinghai–Xizang (Tibetan) Plateau, Chin. Sci. Bull. 41 (1996) 1865–1867 (in
Chinese).
[35] H.L. Jin, G.R. Dong, S. Li, Y.Z. Liu, J.Y. Shen, J.X.
Cao, The climate and southwest monsoon change in the
middle ‘One River Two Tributaries’ Basin, Tibet since 0.80
Ma B.P., J. Desert Res. 16 (1996) 9–12 (in Chinese with
English abstract).
[36] A. Bronger, R.K. Pant, A.K. Singhvi, Pleistocene climatic
changes and landscape evolution in the Kashmir Basin,
India: paleopedologic and chronostratigraphic studies, Quat.
Res. 27 (1987) 167–181.
[37] D.E. Zhang, Synoptic climate studies of dust fall in China
since historic time, Sci. Sin. Ser. B 27 (1984) 825–836.
[38] T. Suzuki, S. Tsunogai, Origin of calcium in aerosols over
the western North Pacific, J. Atmos. Chem. 6 (1988) 363–
374.
[39] R. Gardner, Late Quaternary loess and paleosols, Kashmir
Valley, India, Z. Geom. Suppl. 76 (1989) 225–245.
[40] H.M. Rendell, R.A.M. Gardner, D.P. Agrawal, N. Juyal,
Chronology and stratigraphy of Kashmir loess, Z. Geom.
Suppl. 76 (1989) 213–223.
[41] X.M. Fang, J.J. Li, S.Z. Zhou, S.C. Kang, Aeolian sand
deposition in the source area of Huang He (Yellow River)
and its significance, J. Sediment. 16 (1998) 124–131 (in
Chinese with English abstract).
[42] G.Y. Chen, S.R. Sun, X.M. Fang, Heavy minerals and
provenance of the Malan loess on the Qinghai–Xizang
(Tibet) Plateau and its adjacent area, J. Sediment. 15 (1997)
134–142 (in Chinese with English abstract).
[43] M.L. Clarke, Sedimentological characteristics and rare earth
element fingerprinting of Tibetan silts and their relationship
with the sediments of the western Chinese Loess Plateau,
Quat. Proc. 4 (1995) 41–51.
[44] K.E. Trenberth, S.-C. Chen, Planetary waves kinematically
forced by Himalayan Orography, J. Atmos. Sci. 45 (1989)
2934–2948.
144
X.-M. Fang et al. / Earth and Planetary Science Letters 165 (1999) 129–144
[45] Y.F. Shi, B.X. Zheng, S.J. Li, B.S. Ye, Studies on altitude
and climatic environment in the middle and east parts of
Tibetan Plateau during Quaternary maximum glaciation,
J. Glaciol. Geocryol. 17 (1995) 97–112 (in Chinese with
English abstract).
[46] L.G. Thompson, T.D. Yao, M.E. Davis, K.A. Henderson, E.
Mosley-Thompson, P.-N. Lin, J. Beer, H.-A. Synal, J. ColeDai, J.F. Bolzan, Tropical climate cycle from a Qinghai–
Tibetan ice core, Science 276 (1997) 1821–1825.
[47] Z.J. Cui, Y.Q. Wu, G.N. Liu, D.K. Ge, Q.Q. Pang, Q.H.
Xu, The ‘Kunlun-Huang He movement’, Sci. Chin. D 28
(1) (1988) 53–59.
[48] L.S. Zhang, Major characteristics of environmental change
of China since the Quaternary, J. Beijing Normal Univ.
(Nat. Sci. Ser.) 4 (1984) 81–86 (in Chinese with English
abstract).
[49] S.M. Wang, L. Ji, B. Xue, Study of lake deposition and reconstruction of paleoenvironment in the southeastern mon-
[50]
[51]
[52]
[53]
soon region and the eastern Tibetan Plateau, Quat. Res. 3
(1995) 243–248 (in Chinese with English abstract).
B.Y. Li, Evolution of the lakes in the North Qiangtang
Plateau, in: Tibet Project Expert Comm. (Eds.), Studies on
Formation and Evolution of the Tibetan Plateau, Environmental Changes and Ecological System, Paper Collections,
Sci. Press, Beijing, 1995, pp. 1261–1266, (in Chinese with
English abstract).
J.J. Li, Uplift of Qinghai–Xizang (Tibet) Plateau and
Global Change, Lanzhou Univ. Press, Lanzhou, 1995, 207
pp.
W.H. Berger, The central mystery of the Quaternary ice
age, Oceanus 36 (1993) 53–56.
W.F. Ruddiman, M.E. Raymo, D.G. Martinson, B.M.
Clement, J. Backman, Pleistocene evolution: northern hemisphere ice sheets and north Atlantic Ocean, Paleoceanography 4 (1989) 353–412.