Earth and Planetary Science Letters 237 (2005) 45 – 55
www.elsevier.com/locate/epsl
Stepwise expansion of desert environment across northern
China in the past 3.5 Ma and implications for
monsoon evolution
Z.L. Ding a,*, E. Derbyshire b, S.L. Yang a, J.M. Sun a, T.S. Liu a
a
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Department of Geography, Royal Holloway (University of London), Egham, Surrey TW20 0EX, UK
b
Received 20 January 2005; received in revised form 26 May 2005; accepted 8 June 2005
Available online 19 July 2005
Editor: E. Boyle
Abstract
A systematic study of the last glacial cycle along three transects across the Chinese Loess Plateau shows that sand-sized particle
content within loess decreases rapidly from north to south, and that markedly high sand particle contents in loess horizons occur
only in the northern part of the Plateau. This suggests that variation in the sand-sized particle fraction within loess near the desert
margin is closely linked to migration of the southern desert border in northern China where sand grains move mainly in saltation or
modified saltation mode near the ground surface. As desert margin shift is essentially controlled by the amount of monsoon
precipitation, the sand-sized particle content within loess near desert margin is regarded as a new and readily applied proxy for
variations in the strength of the East-Asian summer monsoon. A continuous record of sand content in loess along the loess–desert
transitional zone shows that the Mu Us Desert migrated southward at 2.6, 1.2, 0.7 and 0.2 Ma, suggesting a stepwise weakening of
the East-Asian summer monsoon during the past 3.5 Ma. This evolutionary pattern is significantly different from that previously
inferred from loess magnetic susceptibility records, a widely used monsoon proxy. Our results further suggest that changes in
global ice volume may have been an essential factor in controlling Plio–Pleistocene monsoon evolution, and that the anticipated
future melting of polar ice cover may lead to a northward migration of the monsoon rainfall belt in northern China.
D 2005 Elsevier B.V. All rights reserved.
Keywords: East-Asian summer monsoon; loess–red clay sequence; Mu Us desert; sand content of loess; Plio–Pleistocene
1. Introduction
* Corresponding author. Tel.: +86 10 62008111; fax: +86 10
62010846.
E-mail address: [email protected] (Z.L. Ding).
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.06.036
Formation and evolution of deserts can be regarded
as a result of interactions between the atmosphere,
hydrosphere, biosphere and lithosphere [1–4]. Changes
in desert environment can, in turn, exert a significant
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Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
influence on the climate system by altering surface
albedo feedback and by supplying to the atmosphere
and oceans variable quantities of mineral aerosols that
have the potential to affect the radiative balance of the
atmosphere and the global carbon cycle [5–7]. The
deserts of northern China, covering an area of about
1.5 106 km2, together make up the worldTs largest
mid-latitude, temperate, continental interior desert.
The location of this desert zone is thought to be closely
associated with the uplift of the Tibetan Plateau during
the late Cenozoic, a process that progressively hindered northward penetration of moisture-laden air
from the Indian Ocean [8]. Another forcing mechanism leading to dryland environmental change is the
variation in the strength of the East-Asian summer
monsoon. A compilation of paleo-data from within
the North China deserts has demonstrated that the
southern desert margin migrated several hundred kilometers north of its last glacial maximum (LGM, ~20 ka
BP) limit in response to increased monsoon rainfall
during the Holocene Optimum (~8–4 ka BP) [9].
The long-term evolution of the deserts of northern
China remains poorly understood because of the
sparseness of directly extractable geological evidence
of suitable type and quality to be found within them.
Given that the loess deposits making up the Loess
Plateau of China lie immediately south and southeast
of these deserts, and so are largely a product of winds
from these dryland sources [10–12], it is considered
that well-dated, semi-continuous loess records may
provide valuable insights into the recent history of
the deserts and their margins. In this study, three
loess transects covering the last glacial cycle, from
Hongde (HD) to Yangling (YAL), Zichang (ZC) to
Lantian (LAT), and Yulin (YL) to Weinan (WN),
were sampled and analyzed, together with a thick
eolian loess–red clay sequence at Jingbian near the
Mu Us desert (Fig. 1). The aim was to use spatial
changes in loess particle size to establish a semi-quantitative relation between particle size and desert margin
location, and to reconstruct shifts in desert margins
since the late Pliocene. The linkage between desert
Fig. 1. The sampling localities and annual precipitation isopleths (mm) in the Loess Plateau. The southern border of the Mu Us Desert during the
LGM lay in a location similar to that of the present [9]. The three loess transects studied here stretch from YL to WN, ZC to LAT and HD to
YAL. Within the loess–desert transitional zone, few loess sections with complete last glacial loess deposits have been found.
Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
changes in northern China and East-Asian monsoon
evolution is also briefly addressed.
2. Spatial variation in grain size of late Pleistocene
loess
Fig. 2 shows the down-section changes in median
grain size and content of sand particles (N 63 Am%)
47
for the three transects. The HD–YAL (upper panel),
YL–WN (middle panel) and ZC–LAT (lower panel)
transects consist of 12, 8, and 7 sections, respectively.
All sections include the S1–S0 stratigraphic units. The
S1 soil formed in the last interglacial period, and has a
brownish or reddish color. In the northernmost part of
the transects, this soil consists of three discrete pedogenic units and two intercalated loess horizons. Loess
unit L1, which accumulated in the last glacial, can be
Fig. 2. Changes in median grain size and sand-grade particle (N63 Am%) content for the HD–YAL (upper panel), YL–WN (middle panel) [15]
and ZC–LAT (lower panel) transects. The left curve in each pair is median grain size and the right curve is sand-grade particle content. The
shaded zones indicate interglacials. The loess sections were sampled at 5–10 cm intervals.
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Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
subdivided into 5 loess layers, namely L1-1, L1-2, L13, L1-4 and L1-5. Chronological studies have shown
that the L1-2, L1-3 and L1-4 units together accumulated in marine isotope stage 3 (MIS 3), and that L1-1
and L1-5 formed in MIS 2 and MIS 4, respectively
[13–15]. In the YL section, the L1-1, L1-3 and L1-5
horizons are composed of medium sand particles,
indicating southward advance of paleo-desert during
these times [14]. The S0 soil, developed in the Holocene, has a darkish color because of relatively high
organic matter. In some sections of the northern Loess
Plateau, e.g. at ZC, the S0 soil has been partly or
totally eroded (Fig. 2). Across the Loess Plateau, there
is a consistent northward decrease both in present-day
mean annual temperatures (from ~13 to ~8 8C) and
mean annual precipitation (from ~600 to below 400
mm) (Fig. 1). The northward increase in aridity is
essentially controlled by the systematic decrease in
summer rainfall derived from the East-Asian summer
monsoon over the Loess Plateau.
The stratigraphy of the sections is correlative in the
field, suggesting quasi continuous dust accumulation
during the last glacial cycle. Particle size was determined for all samples taken at 5–10 cm intervals downsection, using the method described in Ding et al. [16].
The following features are clearly evident in Fig. 2. (1)
Thickness, median size and sand particle percentage all
decrease consistently from north to south, indicating
strong spatial differentiation of dust during subaerial
transport. (2) Loess units L1-1 and L1-5 are characterized by much coarser median grain sizes and higher
sand contents than soils S0 and S1 and the middle part
of L1 in each section. (3) Markedly high sand particle
contents in the glacial loess (L1-1 and L1-5) occur only
in the northern part of the Plateau, a time when the
desert margin was close by [9].
In order to demonstrate further the spatial differentiation of eolian sediments, changes in sand particle
content (N 63 Am%) with increasing distance southward of the present Mu Us Desert margin (Fig. 1) are
shown for L1-1, L1-4, L1-5 and S1 (Fig. 3). All
horizons exhibit a consistent southward decrease in
the sand particle content, the rate of decrease for L1-1
and L1-5 being more rapid than that for L1-4 and S1.
Both the L1-1 and L1-5 records show an abrupt
decrease near the desert margin and a gradual decrease
beyond. According to the reconstruction of Sun et al.
[9], the southern border of the Mu Us Desert during
Fig. 3. Changes in the content of sand-sized particles southward
from the southern border of the Mu Us Desert (Fig. 1) for L1-1, L14, L1-5 and S1. The values for each horizon in each section were
averaged from between 4 and 20 samples (depending on estimated
sedimentation rates) from the portions with coarsest (L1-1 and L15) or finest (L1-4 and S1) median grain size. Changes in these
values are assumed to represent an approximately synchronous
differentiation of sand-sized particles on a millennial-scale average.
the cold-dry LGM (L1-1) was broadly similar to that
of today, whereas it retreated several hundred kilometers to the north during the warm-humid Holocene
Optimum. This implies that this desert margin experienced wide-ranging advance–retreat cycles in response to climatic oscillations at orbital time scales.
It is clear that significant increase in sand percentages
in L1-1 and L1-5 relative to S1 and L1-4 were con-
Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
trolled at the first order by the desert advance during
their accumulation. Using the sand content–distance
relation of L1-1 (Fig. 3), it is therefore inferred that
sand particle contents of ~30% and ~15% within the
loess indicate a distance from the desert margin of
~100 and ~200 km, respectively.
In general, loess particles are transported by winds
from the source area to the depositional area in two
modes, namely saltation and suspension. Theoretical
and experimental studies by Pye [17] and Tsoar and
Pye [18] concluded that, during low-level atmospheric
dust storms, sand-sized particles are usually transported by saltation or modified saltation near the
desert surface, and that any sand particles transported
in suspension quickly settle back on the ground surface. Our observations show that significant volumes
of sand particles are present within the wind-blown
loess only near the desert margin during glacial periods, which is consistent with the Tsoar and Pye
model. Moreover, our data suggest a statistical relationship from which may be derived a semi-quantitative estimation of the distance between dust sources
and depositional areas. Of course, as previously suggested [11], other factors such as wind velocity play a
part in sand particle transport. It is widely accepted
that the Chinese loess was transported mainly by
winter monsoon winds driven by the Siberian–Mongolian high pressure system [11,19]. The mean and
maximum velocities of winter monsoon winds during
the LGM may well have been higher than during other
intervals of the last glacial cycle, because of the likely
intensification of the Siberian High associated with
expanded polar ice sheets and a more extensive sea ice
cover [20]. It follows from this that source-to-sink
estimates for loess horizons other than L1-1, based
on the N 63 Am% to distance relationship provided by
L1-1 (Fig. 3), should be regarded as upper limits.
3. Desert migrations during the past 3.5 Ma
The Jingbian section (37840V54W N, 108831V15W E),
at 1370 m above sea level, lies on the summit of the
Baiyu Mountains. It is located only ~12 km south of
the present margin of the Mu Us Desert. No local
sources of sand, such as river channels, are present in
this area. The section is composed of a ~252 m thick
Pleistocene loess-soil sequence resting on a ~30 m
49
Pliocene red clay deposit, making a total thickness of
~282 m. A previous magnetic polarity study showed
that this sequence has a basal age of 3.5 Ma [21].
Field observations have demonstrated that the development of the paleosols within both the Pleistocene
and Pliocene eolian deposits is much weaker at Jingbian than in the main body of the Loess Plateau series,
and that the Pleistocene loess-soil stratigraphy correlates well with the classic loess sections. To the best of
our knowledge, the Jingbian section is the only desert
margin eolian sequence known to cover the whole
Pleistocene and the late Pliocene. Its proximity to
the dust source region makes it ideal for the study
of long-term desert changes.
In the field, samples were taken at 5–10 cm intervals, making up a total of 3440 samples. The magnetic
susceptibility and median grain-size records at Jingbian
are shown in Fig. 4. As seen in other loess sections, the
magnetic susceptibility values are 2–4 times higher in
paleosols than in loess horizons, and grain sizes are
much coarser in loess than in soils. Both the susceptibility and grain-size records further support our field
observation that this eolian sequence is a well preserved and almost complete record, making it possible
to develop a time scale for this sequence by correlating
its loess-soil unit succession to the Chinese Loess
Particle Time Scale (Chiloparts) [22]. The Chiloparts
record was developed by stacking five individual loess
grain-size records that were tuned to the obliquity and
precession records of the EarthTs orbits.
The Jingbian magnetic susceptibility and grain-size
records, plotted on the Chiloparts time scale, are
shown in Fig. 5, together with a composite marine
oxygen isotope record [23–25]. The N 63 and N125
Am% records (Fig. 5C and D) at Jingbian show four
stepped increases in sand-sized particle content. The
late Pliocene red clay (below L33) contains few sand
particles, indicating that the dust was transported in
suspension, mainly from a remote source. From ~2.6
to ~1.2 Ma, sand content in interglacial soils remains
low, whereas it varies generally between 18% and
25% in glacial loess except for the case of L15 and
L16. This suggests that, during glacial periods, the
desert environment advanced to a location no more
than 200 km from the present northern margin of the
Loess Plateau. In the part of the section deposited
between ~1.2 and ~0.7 Ma, sand content increases
to ~12% in soils and to ~43% in loess with a sub-
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Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
Fig. 4. The magnetic susceptibility and median grain-size records of the Jingbian section, located in the northernmost part of the Loess Plateau
(Fig. 1). The Pleistocene loess (above L33) at Jingbian is ~252 m thick, and the Pliocene red clay ~30 m thick. The red clay accumulated during
the time interval ~3.5–2.6 Ma, as suggested by magnetic polarity studies [21]. The Jingbian section was sampled at 5–10 cm intervals. The
major loess-paleosol units are labeled for the section.
stantial increase in N125 Am particles, implying a
large-scale advance of the desert margin during both
glacial and interglacial times. Throughout material
deposited in the interval ~0.7–0.2 Ma, N 63 Am particles range from ~30% in soils and ~55% in loess
units, with the N 125 Am particles exceeding 8%.
This suggests that the distance between the Loess
Plateau and the present desert margin was less than
100 km. During the last two glacial periods, eolian
sand was directly deposited at Jingbian, indicating a
further southward desert shift.
4. Discussion and conclusions
In previous studies, two factors have been proposed as controls on grain-size variations within the
Chinese loess, namely strength of the transporting
winds, specifically those associated with the winter
monsoon [11,19], and the location of the desert margin [15,22]. If the source-to-sink distance remains
unchanged, an increase in the winter monsoon intensity will result in the deposition of coarser dust particles. On the other hand, if the desert margin advances
southwards, the loess deposited at a specific site on
the Plateau will also be coarser. In this study, we show
that loess particle size is much coarser during glacial
periods than during interglacials. This may have
resulted from either or both desert margin advance
and wind intensity increase due to cold-dry climatic
conditions. However, a significant sand content within
glacial loess horizons is present only in the northern
part of the Loess Plateau, i.e. near desert margins
where saltation or modified saltation may be the
Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
51
Fig. 5. Changes in magnetic susceptibility (A) and grain-size data (B, C, D) at Jingbian, and correlation with a stacked marine d 18O record (E)
[23–25]. The time scale of the Jingbian section was developed by correlating the loess-soil units with the Chiloparts record [22].
dominant mode of transport. It is conceivable that,
regardless of the wind intensity, loess deposited close
to desert margins will contain a high sand-sized particle fraction, and that this fraction will rapidly decrease with distance from the margins, as shown in
Fig. 3. It is thus concluded that changes in the sandsized particle fraction within loess can be used as an
indicator of advance–retreat cycles along desert–loess
transition zones.
Humidity is the most important factor affecting
desert margin shift, since desert margin advance can
occur only when the vegetation cover there is
destroyed with a critical reduction of humidity. In
the loess–desert transitional region of northern
China, about 80% of rainfall is attributable to the
East-Asian summer monsoon. When this summer
frontal rainfall belt penetrates farther to the north,
the desert margins show a corresponding northward
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shift, as in the case of the Holocene Optimum [9]. In
this context, the sand-sized fraction in the loess near
desert margin can be regarded as a proxy of summer
monsoon intensity.
In most previous studies, loess magnetic susceptibility [26,27] and various chemical element ratios [28]
of the Chinese loess-paleosol series have been used as
summer monsoon proxies, a practice based, respectively, on the recognition that the susceptibility values
of interglacial paleosols are 2–4 times greater than
those found in the loess units laid down in glacials,
and that paleosols show much more advanced chemical weathering than is seen in adjacent loess. However, the processes and factors affecting variations in
loess-paleosol susceptibility remain controversial [29–
32]. Recent studies have shown that increased magnetic susceptibility in paleosols may not result solely
from intensified pedogenic processes arising from
increased monsoonal precipitation [31,32]. Here, we
propose that changes in elemental concentrations
within loess-paleosol sequences may be determined
largely by three factors: chemical weathering in
source regions, grain-size distribution, and post-depositional weathering. It is only the signature resulting
from post-depositional weathering that can be linked
to summer monsoon intensity. Unfortunately, the detailed studies required to distinguish the major controlling factors in loess element ratios are still
pending.
The East-Asian summer monsoon affects vast areas
of the continent. Essentially, monsoon precipitation is
a product of the interaction between warm-moist
southerly airmasses and cold northerly airflows from
middle and high latitudes. Generally, the more northerly the penetration of the frontal rainfall belt the
greater is the intensity of the summer monsoon [33].
It follows that desert margin retreat in northern China
is consistent with a stronger summer monsoon. Thus,
the linkage described above between the sand-sized
particle fraction within loess, desert margin location
and summer monsoon intensity forms the basis of a
novel, and readily applied monsoon proxy. As desert
margin shift is a geographical phenomenon, the sandsized particle content in loess near desert margin
should be regarded as a more direct and sensitive
proxy for monsoon strength than either loess magnetic
susceptibility or elemental ratios derived from physical and chemical characteristics of loess, respectively.
The Jingbian sand-sized particle record clearly
demonstrates that, superimposed on the glacial–interglacial oscillations, the deserts in northern China
experienced significant expansion at ~2.6, ~1.2,
~0.7 and ~0.2 Ma, directly implying a stepwise
southward retreat of the monsoon rainfall belt, associated with a complementary reduction in summer
monsoon strength, in the past 3.5 Ma. This evolutionary pattern conflicts with current understanding
based on loess magnetic susceptibility records, which
show a significant increase in susceptibility values in
sediments from early to late Pleistocene in age
[26,34], thus implying a progressively enhanced
monsoon precipitation. This suggests that factors
affecting variations in loess magnetic susceptibility
require further investigation.
Two forcing factors may have driven the long-term
evolution of the East-Asian monsoon, namely changes
in the elevation of the Tibet Plateau [3,35] and changing global ice volumes [20,34]. Both meteorological
observations [33,36] and numerical model experiments [35,37,38] indicate that uplift of the Tibet
Plateau plays a critical role in initiation and maintenance of the Asian monsoon system. Geological studies show that the main body of the Plateau had
reached its highest elevation by ~14 Ma [39] or ~8
Ma [40], although a late phase (~3.6 Ma) of uplift of
its northern parts may have occurred [41]. This
implies that Tibetan tectonic changes were not a significant influence on Plio–Pleistocene monsoon evolution. In addition, any substantial uplift of the
Plateau, if such occurred during the past 3.5 Ma,
would be expected to have caused an increase in
monsoon strength. The Jingbian record shows that
this was not the case.
The main causal factor in the stepwise weakening
of the summer monsoon, therefore, was probably the
increase in global ice volume. The marine oxygen
isotope record [23–25] shows a gradual, oscillatory
increase in global ice volume from the late Pliocene
until replaced by a dstationaryT fluctuation at around
0.8 Ma (Fig. 5E); this is broadly consistent with the
trend in East-Asian monsoon evolution (Fig. 5C).
Numerical modeling results also suggest that Pleistocene glaciation should lead to a substantial weakening
of the East-Asian summer monsoon [35]. Here, we
propose that the global ice volume signatures were
transferred into the monsoon system by means of
Z.L. Ding et al. / Earth and Planetary Science Letters 237 (2005) 45–55
three well-known mechanisms. (1) Development and
expansion of polar ice sheets and sea ice cover would
lead to increased temperature and atmospheric pressure gradients between the polar regions and the
middle-low latitudes, thereby impeding northward
movement of moisture-laden monsoon airflows. (2)
Ice accumulation on continents would cause sea-level
lowering and exposure of vast continental shelves in
the Pacific marginal seas [42], thus resulting in enhanced continentality (increased distances to ocean
moisture sources, affecting monsoon rainfall belt migration towards the inland desert margins). Given the
steep monsoon precipitation gradients, particularly
over northern China (Fig. 1), such changes would
have the effect of weakening the summer monsoon.
(3) The expansion of polar ice sheets and sea ice cover
would result in the intensification of the northerly
winter monsoon winds, imposing a downstream cooling on the low-latitude oceans. This would thereby
lead to weakened oceanic water evaporation and a
decreased summer monsoon season via delayed
onset. The abruptness of desert margin shift (Fig.
5C and D) also suggests that threshold mechanisms
should be recognized as fundamental factors in assessment of long-term monsoon evolution.
By generating a multiple-proxy data set, Clemens
et al. [43] found that the growth of ice sheets in the
Northern Hemisphere over the past 3.5 Ma has weakened the Indian summer monsoon and increased the
aridity of subtropical Asia and eastern Africa. With
respect to long-term evolution, the phase of summer
monsoon maxima relative to global ice volume maxima has changed significantly over both the precession and obliquity bands, with major changes
occurring at about 2.6, 1.2 and 0.6 Ma. This would
lead us to expect a similar evolutionary history for
closely coupled though discrete East-Asian and Indian
monsoons, although such abruptness of events shown
here for China is not evident in the Indian monsoon
records. The Indian subsystem has a north–south
land–ocean configuration with a latent heat source
in the southern hemisphere part of the Indian Ocean.
In contrast, the East-Asian subsystem has a significant
east–west land–ocean configuration with a major Pacific Ocean latent heat source (e.g. [44]). The close
coupling between the Indian and East-Asian monsoon
subsystems indicates that the common mechanism
driving the long-term evolution of these subsystems
53
lies within the Asian continent, rather than within the
oceans, providing further support for a link between
northern hemisphere ice volume and monsoon
strength, as argued here.
The close linkage between the East-Asian monsoon and global ice volume, on both long and glacial
cycle time scales, may have important implications
for assessing the consequences of human-induced
global warming for ecosystems in the semi-arid
regions in northern China, where devastating desertification now threatens. If the anticipated future
melting of polar ice sheets and sea ice cover [45]
leads to a large-scale reorganization of atmospheric
circulation patterns, a northward shift of the summer
monsoon rainfall belt is highly likely, thus raising the
possibility of reversing the current trend towards
ecological deterioration.
Acknowledgments
This research is financially supported by the CAS
(grant KZCX2-SW-133) and NNSFC (grants
90202020 and 40021202). We thank Dr. Steven Clemens for valuable comments on an early version of the
manuscript.
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