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Earth and Planetary Science L
Mineral magnetic variation of the Jingbian loess/paleosol sequence in
the northern Loess Plateau of China: Implications for Quaternary
development of Asian aridification and cooling
Chenglong Deng a,b,*, John Shaw b, Qingsong Liu a,c, Yongxin Pan a, Rixiang Zhu a
a Paleomagnetism and Geochronology Laboratory (SKL-LE), Institute of Geology and Geophysics,
Chinese Academy of Sciences, Beijing 100029, Chinab Geomagnetism Laboratory, Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 7ZE, UK
c Earth Sciences Department, University of California, Santa Cruz, CA 95064, USA
Received 29 June 2005; received in revised form 8 October 2005; accepted 14 October 2005
Available online 22 November 2005
Editor: V. Courtillot
Abstract
A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern
extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain
and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM100mT/SIRM,
SIRM100mT/SIRM30mT and SIRM100mT/SIRM60mT (SIRM is the saturation isothermal remanent magnetization; SIRMnmT repre-
sents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change
in the Asian interior by correlating the mineral magnetic record with the composite d18O record in deep-sea sediments. The long-
term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the
relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively,
which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and
deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-
timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and
interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary
aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the
Himalayan–Tibetan complex during this period.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Chinese Loess Plateau; loess; paleosol; mineral magnetism; magnetoclimatology; Asian aridification and cooling
0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2005.10.020
* Corresponding author. Paleomagnetism and Geochronology Lab-
oratory (SKL-LE), Institute of Geology and Geophysics, Chinese
Academy of Sciences, Beijing 100029, China. Tel.: +86 10 6200
7913; fax: +86 10 6201 0846.
E-mail addresses: [email protected] (C. Deng),
[email protected] (J. Shaw), [email protected] (Q. Liu),
[email protected] (Y. Pan), [email protected] (R. Zhu).
1. Introduction
The wind-blown deposits derived from the arid Gobi
and associated deserts of northwestern China cover
much of the arid and semi-arid regions in northern
China, and in particular the Chinese Loess Plateau [1]
etters 241 (2006) 248–259
Fig. 1. Schematic map showing the Loess Plateau, Tibetan Plateau
Gobi and other deserts in northwestern China and location of the
Jingbian section studied (modified from Guo et al. [24] and Sun [23])
The Yellow River and Yangtze River are the major river systems in
north and south China, respectively. The east–west trending Qinling
Mountains are the traditional dividing line between temperate north
ern China and subtropical southern China. The dotted and solid
arrows respectively show summer and winter monsoon directions.
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259 249
where loess/paleosol sequences contain essentially con-
tinuous both geomagnetic and paleoclimate records from
Late Pliocene throughout the entire Quaternary (see
reviews by Heller and Evans [2], Liu and Ding [3],
Evans and Heller [4] and Porter [5]). Over the past two
decades, studies on these sequences have provided a
wealth of information on both global and regional paleo-
climate change, as revealed by a number of proxy indi-
cators (see Ding et al. [6] and references therein). These
proxies consistently demonstrate that the rhythm of Chi-
nese loess/paleosol alternations was directly influenced
by alternating strengthening and weakening of the East-
ern Asian paleomonsoon, with loess deposited during
glacial periods and soils developed during interglacial
periods. In particular, mineral magnetic proxies have
been successfully used to retrieve the paleoclimatic sig-
nals recorded in Chinese loess/paleosol sequences [7–
11]. Since the pioneering work of Heller and Liu [12–14]
on Chinese loess in the early 1980s, environmental
magnetism has played an indispensable role in recon-
structing the variability of the Eastern Asian monsoons
over the Quaternary period (see reviews by Heller and
Evans [2], Verosub and Roberts [15] and Maher [16]).
Heller and Liu [12,13] conducted hallmark work on
the magnetoclimatological records in Chinese loess/
paleosol sequences. They firstly suggested a possible
link between magnetic properties and paleoclimate
recorded in the sequences [12], and further proposed
that low-field magnetic susceptibility variations can be
correlated with marine oxygen isotope records [13,14].
Subsequently, this correlation was further examined by
a series of mineral-magnetic and non-magnetic proxies,
especially by low-field magnetic susceptibility [17–20]
and bulk grain size [6,20]. There is currently a consen-
sus that there are genetic relationships between the
Chinese eolian deposits, the volume of land-based ice
and global climate [17], making the Chinese loess/
paleosol sequence one of the best terrestrial archives
of paleoclimate.
The Chinese loess/paleosol sequences can also be
used to monitor the evolution of Asian deserts and
atmospheric circulation [1,21–24]. These sequences
originate mainly from the Gobi and desert lands in
northwestern China carried by northwesterly winter
monsoon [1,22–25]; they are therefore expected to be
sensitive to the aridification and cooling history of the
Asian interior. However, mineral magnetic investiga-
tions of the long-term evolution of Quaternary Asian
aridification and cooling remain very scarce [10]. Re-
cent developments in magnetic bunmixingQ techniques[26,27] now allow us to quantitatively determine the
eolian inputs.
This study aims to explore the long-term develop-
ment of Asian aridification and cooling over the Qua-
ternary period in terms of mineral magnetic data from
the Jingbian loess/paleosol sequence at the northern
extremity of the Chinese Loess Plateau. This site
appears to be an ideal natural recorder of climate
change because of its high dust sedimentation rates
and because it is in the transition zone between the
dust source region and the deposition area, which is
sensitive to Quaternary climate change [28].
2. Geological setting, stratigraphy and sampling
The Jingbian loess section (108.88E, 37.48N) is
located in the transition zone between the Gobi and
associated deserts in northwestern China and the north-
ern part of the Chinese Loess Plateau (Fig. 1). The
region, dominated by an arid/semi-arid continental cli-
mate [29], has a mean annual precipitation of 395.4 mm
and a mean annual temperature of 7.8 8C. The mean
July temperature is 22.5 8C and the mean January
temperature �8.8 8C [30].
The 252-m-thick Jingbian loess/paleosol sequence
consists of Holocene soil (S0), the Malan Formation
(L1), the Lishi Formation (S1/L2 to S14/L15) and the
Wucheng Formation (S15/L16 to S32/L33) [28]
(according to the nomenclature of Chinese loess stra-
tigraphy by Liu [1]). The Pliocene Red Clay underlying
the loess/paleosol sequence is about 28 m thick [28]. In
,
.
-
ig. 2. Temperature-dependence of magnetic susceptibility of selected
amples of loess and paleosols.
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259250
this area, the paleosols are weakly developed and visu-
ally indistinct in the field [31]. However, some pedos-
tratigraphic marker layers and magnetostratigraphy give
a reliable stratigraphic constraint on this sequence. The
prominent paleosol unit S5, two thick and coarse-tex-
tured loess layers L9 and L15, and some thick loess
units, such as L24, L27 and L32 [1,6], were initially
identified and then used as stratigraphic markers in the
field. Previous magnetostratigraphic investigations
have provided robust age control for the Jingbian se-
quence [31,32]. The Matuyama–Brunhes boundary was
identified in the middle to lower part of loess unit L8
and the upper and lower boundaries of the Jaramillo
normal subchron in loess L10 (close to the stratigraphic
boundary L10/S9). The Jaramillo normal subchron is
found in the lower part of loess layer L12, the upper
and lower boundaries of the Olduvai normal subchron
in the middle part of loess unit L25 and in the lower
part of paleosol S26, and the Gauss–Matuyama bound-
ary in the middle part of loess unit L33. The positions
of geomagnetic reversals and subchrons at Jingbian are
essentially similar to those of other loess/paleosol
sequences over the Loess Plateau (e.g., Heller and
Evans [2], Evans and Heller [4] and references therein).
Samples were collected from the Holocene soil (S0)
to the bottom of the Pliocene Red Clay at 5�10 cm
intervals. A total of 4400 samples were collected in this
section. 1584 samples at 10�20 cm intervals were used
in this study, 1564 from the Quaternary loess and
paleosols (S0/L1 to S32/L33), and 20 from the upper
part of the Pliocene Red Clay.
3. Methodology
Saturation isothermal remanent magnetizations
(SIRM) of all samples were produced in a steady
constant field of 1 T and demagnetized at peak alter-
nating fields (AFs) of 10, 20, 30, 40, 60, 80 and 100
mT. These magnetic measurements were made using a
2G Enterprises Model 760-R cryogenic magnetometer
situated in a magnetically shielded room.
High-temperature magnetic susceptibilities (v�T) of
selected loess and paleosol samples were measured
using a KLY-3 Kappabridge with a CS-3 high-temper-
ature furnace (Agico Ltd., Brno) in an argon atmo-
sphere. The sample holder and thermocouple
contributions to magnetic susceptibility were sub-
tracted.
Hysteresis parameters of selected samples were mea-
sured using a MicroMag 2900 Alternating Gradient
Magnetometer (AGM) (Princeton Measurements Corp.,
USA). Themagnetic fieldwas cycledbetweenF1.5T for
each sample. Saturation magnetization (Ms), saturation
remanence (Mrs) and coercivity (Bc) were determined
after correction for the paramagnetic contribution iden-
tified from the slope at high fields. Samples were then
demagnetized in alternating fields up to 150 mT, and
an isothermal remanent magnetization (IRM) was
imparted from 0 to 1.5 T also using the MicroMag
2900 AGM. Subsequently, the 1.5 T IRM was demag-
netized in a stepwise backfield to obtain the coercivity of
remanence (Bcr).
One loess sample at a depth of 4.0 m was extracted
at 1.5 T field. X-ray diffraction (XRD) analysis of the
magnetic extracts was carried out using a DMAX2400
X-ray diffractometer with the following parameters:
Cu-Ka/40kV/80mA, scattering slit of 18, receiving
slit of 0.15 mm, continuous scan mode, scanning
speed of 28/min and a scanning step of 0.028.
4. Results
4.1. High-temperature magnetic susceptibility (v�T)
and XRD
All selected samples display a clear susceptibility
drop near 585 8C (Fig. 2), suggesting the existence of
nearly stoichiometric magnetite in both loess and paleo-
F
s
Fig. 3. XRD spectrum of magnetic extracts from a loess sample (depth
of 4.0 m), showing the presence of magnetite (M) and hematite (H) in
the sediment. The major impurity is quartz (Q).
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259 251
sol samples. A steady (but nearly reversible) increase of
susceptibility below ~200 8C is observed for the paleo-
sol samples, which can be ascribed to gradual unblock-
ing of fine-grained (near the superparamagnetic/single-
domain (SP/SD) boundary) ferrimagnetic particles
[33,34]. The further drop of magnetic susceptibility
between ~300 8C and ~450 8C is generally interpreted
as the conversion of ferrimagnetic maghemite to weak-
ly magnetic hematite [35–38]. Liu et al. [34] further
show that this susceptibility decrease results from the
inversion of fine-grained maghemite to hematite. Such
a susceptibility drop is more prominent for paleosol
than loess samples, suggesting that paleosols contain
more fine-grained pedogenic maghemite grains.
Unlike the paleosol samples, however, the heating
curves for the loess samples is almost flat up to ~550
8C. The weakness or absence of the 250–300 8C hump
in the heating curves is indicative of absence of fine-
grained pedogenic maghemite grains [38]. This almost
temperature-independent nature of low-field suscepti-
bility below 585 8C, the Curie point of magnetite,
indicates that detrital coarse-grained MD-like magnetite
is the major contributor to the magnetic susceptibility of
the loess samples [10,39].
Paleosol samples show a sharp increase in magnetic
susceptibility when cooled below 580 8C. The increasein susceptibility is mainly due to the transformation
from iron-containing silicates/clays to magnetite during
thermal treatment [33,38]. In the northern and north-
western Loess Plateau, where pedogenesis is much
weaker than in the central and southern plateau, the
weathering has not depleted the supply of weatherable
Fe-minerals in loess and paleosols and has left most of
the iron-bearing precursor phases nearly untouched,
leading to a sufficiency of iron that can serve as the
Fig. 4. Hysteresis loops for representative paleosol and loess samples after slope correction for paramagnetic contribution. The hysteresis loops were
measured in fields up to F1.5 T.
source for the formation of new magnetite during lab-
oratory heating [33].
The XRD spectrum reveals that a mixture of
magnetite and hematite is present in the loess
(Fig. 3). Maghemite, another important ferrimagnetic
phase responsible for the enhanced magnetic suscepti-
bility and remanences of Chinese loess sediments
[34,37,38,40,41], was not clearly indicated by the
XRD spectrum, possibly because its grain size is too
fine to be extracted.
4.2. Hysteresis properties
Fig. 4 shows the hysteresis loops for representative
loess and paleosol units. All the samples display a
significant paramagnetic contribution (not shown).
Loess samples generally have wider hysteresis loops
than paleosols. The loops do not close even at 500 mT,
especially for the very weakly weathered loess units L6,
L9, L15 and L33, which may arise from high contribu-
tions of high-coercivity phases, such as hematite, goe-
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259252
thite and partially oxidized eolian magnetite [33,42].
Some hysteresis loops are slightly wasp-waisted [43],
such as S5, L15, S18 and S32, which have been inter-
preted as resulting from the competing contributions of
the composition, concentration and grain-size of mag-
netic minerals [33,44]. In addition, the values of Mrs /
Ms versus Bcr /Bc when plotted on a Day diagram [45]
indicate that the magnetic mineralogy is dominated by
large PSD or MD-like magnetic grains (Fig. 5).
4.3. SIRM and AF demagnetization of SIRM
SIRM values of the entire Jingbian sequence show
variations that correspond to loess/paleosol alternations,
with high values in paleosols and low values in loess
horizons (Fig. 6a). The highest SIRM is observed in the
most prominent paleosol unit S5.
AF demagnetization of SIRM reflects the magnetic
hardness of a material [10,27,39,46]. Loess and paleo-
sols display different behavior during AF demagneti-
zation of SIRM (Figs. 6 and 7). Generally, the SIRM
of paleosols is more easily AF demagnetized than that
of loess. Paleosol SIRM shows a steep decrease
in intensity between 0 and 30 mT. Specifically, the
spectra of the median destructive field of SIRM
(MDFSIRM) for some representative loess and paleosol
samples are shown in Fig. 7a. The MDFSIRM values of
the selected paleosols range from 16.7 mT to 26.2 mT
with a mean value of (19.6F2.1) mT (N =30), and of
the selected less weathered loess samples, from 19.7
mT to 36.8 mT with a mean value of (31.0F4.2) mT
(N =31). MDFSIRM decreases non-linearly with in-
Fig. 5. Hysteresis ratios plotted on a Day diagram [45] of the loess
and paleosol samples of the Jingbian section. SD=single domain;
PSD=pseudo-single domain and MD=multidomain.
creasing pedogenesis (represented by the increased
SIRM, Fig. 7b).
Fig. 6b shows the variations of the residual SIRM
after progressive AF demagnetization normalized by
the initial SIRM. Similar to SIRM behavior, the
normalized residual SIRM of the entire Jingbian se-
quence exhibits variations that correspond to loess/
paleosol alternations, with low values in paleosols and
high values in loess units. In this paper, we use
SIRMnmT to represent the residual SIRM after an n
mT AF demagnetization. In particular, there is a long-
term up-section decreasing trend in the SIRM10–100mT/
SIRM ratios. This feature becomes more and more
pronounced with increasing alternating fields (Fig.
6b). In addition, the ratios of SIRM100mT/SIRM30mT
and SIRM100mT/SIRM60mT show a long-term up-sec-
tion decreasing trend, with the former almost
paralleling that of the SIRM100mT/SIRM ratio and
the latter showing no obvious changes in loess/paleo-
sol rhythms (Fig. 8d,e). Stratigraphic variations of
MDFSIRM exhibit distinct loess/paleosol alternations;
however, this parameter does not show a clear long-
term increasing or decreasing trend (Fig. 8b). The
paleoclimatic significance of these long-term varia-
tions will be addressed in detail in the Discussion
section.
5. Discussion
5.1. Magnetic mineralogy of the Jingbian loess/paleo-
sol sequence
High-temperature magnetic susceptibility measure-
ments (Fig. 2) and XRD spectrum (Fig. 3) reveal the
presence of a mixture of magnetite, maghemite and
hematite in the Jingbian loess and paleosols. In this
section, maghemite makes significant contributions to
the magnetic susceptibility of paleosols but only a
minor contribution to that of loess units (Fig. 2).
Maghemite was not positively detected by the XRD
analysis (Fig. 3), supporting the concept that maghe-
mite particles are fine-grained grain and generally not
extractable [42,47]. In addition, the magnetite grains
in the loess samples display a coarse-grained MD-like
behavior, as suggested by the nearly temperature-in-
dependent nature of low-field susceptibility below
585 8C (Fig. 2). Hematite, another important carrier
of the natural remanence [12,37,47], is not clearly
present in the v�T curves because of its inherently
weak magnetism. Its presence, however, is unambig-
uously indicated by the XRD spectrum (Fig. 3).
Furthermore, the open nature of hysteresis loops
Fig. 6. (a) Intensity of SIRM after AF demagnetization at peak fields of 0, 10, 20, 30, 40, 60, 80 and 100 mT for the Jingbian loess/paleosol
sequence. (b) Stratigraphic variations of the residual SIRM after AF demagnetization normalized by the initial SIRM.
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259 253
above 300 mT (Fig. 4) strongly indicates the presence
of high-coercivity phases in the weakly weathered
loess layers.
Loess samples generally exhibit higher coer-
civties than paleosols, clearly suggested by their higher
MDFSIRM values and higher ratios of SIRM100mT/SIRM,
SIRM100mT/SIRM30mT and SIRM100mT/SIRM60mT in
our study (Fig. 8). Three magnetic components may
be responsible for this behavior: partially oxidized
coarse-grained eolian magnetite, eolian hematite and
goethite, and pedogenic hematite and goethite. Their
contributions to the high-coercivity fraction of SIRM
are addressed below.
Firstly, the coarse-grained (large PSD or MD) mag-
netites of eolian origin have usually been altered by
low-temperature oxidation, which, at the initial stage of
pedogenesis, can significantly increase the coercivity
by increasing the internal stress caused by the coupling
between the maghemite rim and the magnetite core of
the particles [42,48,49]. However, with further pedo-
genic enhancement, pedogenesis can further oxidize the
coarse-grained lithogenic magnetites, thus decreasing
Fig. 7. (a) SIRM demagnetization curves of selected loess (thin lines) and paleosol samples (thick lines). (b) SIRM versus MDFSIRM for all samples
of the entire Jingbian loess/paleosol sequence.
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259254
the coercivity of these magnetites by decreasing the
oxidation gradient between the magnetite core and the
maghemite rim [50]. A 30 mT AF demagnetization can
significantly attenuate the pedogenic ferrimagnetic par-
ticles contributions to SIRM (Figs. 6 and 7a). In addi-
tion, Liu et al. [11] show that the partially oxidized
eolian coarse-grained magnetite makes significant con-
tributions to SIRM60mT. However, the much elevated
peak field of AFs to 100 mT can efficiently remove the
contributions of these particles and simultaneously en-
hance the relative contributions of the magnetically
harder phases, e.g., hematite or goethite.
The presence of goethite in Chinese loess/paleosols
has been suggested by heavy mineral analysis [1], color
indices [51] and diffuse reflectance spectrophotometry
[52]. However, the goethite in terrestrial environments
is usually Al-substituted. Al-substitution may suffi-
ciently suppress its Neel temperature (TN) below
room temperature (300 K). Around TN, Al-substituted
goethite has maximum magnetic susceptibility but loses
its ability to carry a remanence [53]. In addition, low-
temperature magnetic susceptibility measurements in-
dicate that in Chinese loess/paleosols the dominant
antiferromagnetic phase is hematite rather than goethite
[54]. Liu et al. [47] has further examined the thermal
behavior of the high-coercivity fraction of SIRM. Their
findings show a dominant unblocking temperature of
~670 8C, unambiguously suggesting that hematite rath-
er than goethite dominates the SIRM. Thus, the contri-
bution of goethite to the laboratory SIRM of the
Jingbian loess/paleosols is possibly negligible. We
therefore believe that SIRM100mT is carried dominantly
by hematite.
In less weathered loess units, it has been suggested
that eolian hematite grains dominate high-coercivity
fraction [46]. In paleosols, however, much pedogenic
hematite was formed due to strong weathering during
interglacial periods [33,47,52]. Considering all the
evidence together, it is reasonable to hypothesize
that in Chinese loess/paleosols the high-coercivity
fraction SIRM100mT is dominated by hematite. Thus,
SIRM100mT can serve as a measure of hematite contri-
butions in the Chinese loess/paleosol sequences, and
the SIRM100mT/SIRM ratio can be used as a proxy for
variations in the relative contributions of hematite in the
sequences, which mainly originates from the dust
source region in less weathered loess layers and is
mainly produced due to in situ pedogenesis in weath-
ered paleosol units.
The most striking feature in the mineral magnetic
records of the Jingbian loess/paleosol sequence is the
up-section gradual decrease of the SIRM100mT/SIRM
ratio for both loess and paleosol units over the entire
sequence (Fig. 8c). This behavior is more pronounced
for the glacial loess than for the interglacial paleosols.
To remove the effects of pedogenic ferrimagnetic com-
ponents, and then better isolate the magnetic responses
of the long-term paleoclimate evolution separately in
glacial and interglacial stages, we further employ
SIRM30mT and SIRM60mT as normalizing parameters.
The SIRM30mT is dominated by hematite and low-
temperature oxidized coarse-grained lithogenic magne-
tite, and at least partly by pedogenic magnetite/maghe-
mite while the SIRM60mT is dominated by hematite
and low-temperature oxidized coarse-grained lithogenic
magnetite.
5.2. Interpretation of the long-term mineral magnetic
variations
The dust source region in the Gobi and sand deserts
of northwestern China is dominated by an arid conti-
Fig. 8. Correlation of magnetic hardness of the Jingbian loess/paleosol sequence with the marine oxygen isotope records. (a) SIRM, (b) MDFSIRM,
(c) SIRM100mT/SIRM, (d) SIRM100mT/SIRM30mT, (e) SIRM100mT/SIRM60mT, (f) composite d18O records. The d18O curve is a composite record of
V19-30 (0 to 0.34 Ma) [56], ODP 677 (0.34 to 1.811 Ma) [57] and ODP 846 (1.811 to 2.6 Ma) [58,59].
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259 255
nental climate [29] where there are moderately high
temperatures and short periods of limited rainfall in
summer. It is expected that the formation of hematite
due to in situ chemical weathering in the source region
is favored under such a climate regime [52]. In glacial
periods, the formation of hematite in the source region
may be reduced due to increased aridification and cool-
ing. However, the hematite in interglacial paleosols is
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259256
dominantly formed by in situ chemical weathering in
the depositional area. In the Jingbian section, where
loess layers are very weakly weathered and paleosols
moderately to well developed [28], hematite in loess
layers is dominantly of eolian origin and in paleosol
units is mainly of pedogenic origin. Therefore, the
content of hematite in loess and paleosols mainly
reveals the degree of chemical weathering intensity
respectively in the glacial-stage source region (the
Gobi and deserts in northwestern China) and in the
interglacial-stage depositional area (the Loess Plateau
region), further reflecting the degree of aridification and
cooling respectively in glacial-stage source region and
in interglacial-stage depositional area.
Therefore, the long-term up-section decreasing
trend in the ratios of SIRM100mT/SIRM, SIRM100mT/
SIRM30mT and SIRM100mT/SIRM60mT suggests a long-
term decreasing trend in the relative contributions of
both eolian hematite in the glacial loess units and
pedogenic hematite in the interglacial paleosols. This
long-timescale variation possibly further signals a long-
term increasing aridification and cooling during both
glacial extrema in the source region and interglacial
extrema in the depositional area over the entire Quater-
nary period. In addition, we note that the long-term
variation in the glacial loess layers is clearly shown by
all the three ratios. However, in the interglacial paleosol
units, this feature is weakly revealed by the SIRM100mT/
SIRM ratio and much more distinctly defined by the
other two ratios because SIRM is more affected by fine-
grained pedogenic ferrimagnetic grains.
Following the long-term ongoing deterioration of
Cenozoic climate [55], our observed trend coincides
well with the long-term increase in d18O values seen
in the marine oxygen isotopic record [56–59] (Fig. 8f),
which reflects a long-term global cooling trend
and expansion of the Arctic ice-sheet. Each of the
glacial–interglacial climatic oscillations show a close
match between the ratios of SIRM100mT/SIRM and
SIRM100mT/SIRM30mT and the composite marine oxy-
gen record, with a discrepancy present only in the upper
sandy loess unit L9, which will be separately addressed
in the last paragraph of this section. However, all the
three ratios mentioned above and the composite marine
oxygen records show a good correlation over the whole
2.6 Ma time period (Fig. 8).
The onset of desertification in the Asian interior has
been believed to be at least 22 Myr ago in terms of
magnetostratigraphic results of the Miocene loess/
paleosol sequences in the western Loess Plateau [24].
The ultimate cause for the onset and development of
aridification and cooling in the Asian interior remains
unsettled although several factors have been invoked as
driving forces for this scenario such as ongoing global
cooling, uplift of the Himalayan–Tibetan Plateau and
changes in land–sea distribution [21,22,24,60,61], with
the former two being the main forces responsible for the
Quaternary development of Asian aridification and
cooling, and the last one playing a background role in
the long-term Cenozoic climate deterioration.
Firstly, ongoing global cooling and expansion of the
Arctic ice-sheet during the Quaternary contributed sig-
nificantly to the development of Asian aridification and
cooling. A cooler high-latitude ocean provides less
moisture to the continental interior, thus promoting
aridification in the Asian mainland [24,60]. Secondly,
climate simulations suggest that the Himalayan–Tibetan
uplift could serve as a key contributor to the develop-
ment of Asian aridification [22,60]. Plateau uplift
enhances summer monsoon and brings wetter climates
to India and Southeast Asia, but this moisture cannot
reach the Asian interior because the uplifted Himala-
yan–Tibetan topography blocks airflow from the south
[24]. Meanwhile, the plateau uplift strengthens the
winter monsoon, causing additional mid-latitude to
high-latitude Asian interior cooling by greatly increas-
ing the atmospheric dust loading [21,22,24]. As a re-
sult, the interior of Asia becomes increasingly dry and
cold as a climatic response to the progressive uplift,
leading to an effective increase in aridification and
cooling during the Quaternary and ultimately resulting
in a long-term decreasing trend in chemical weathering
intensity in both the dust source region and the dust
depositional area over that period. This is reflected in
our observed long-term trend in the relative content of
hematite in both glacial loess and interglacial paleosols
of the Jingbian section (Fig. 8).
Several lines of geological evidence from the loess/
paleosol sequences at the studied Jingbian section, in
the central and southern Loess Plateau support this
argument. For example, the N63 and N25 Am% records,
which mainly reflect proximal signals of the sand-sized
particle content, at Jingbian show four stepped
increases. These four increases indicate that the Mu
Us Desert migrated southward at 2.6, 1.2, 0.7 and 0.2
Ma, further suggesting a stepwise weakening of the
East Asian summer monsoon during the past 3.5 Ma
[28]. A general up-section increase in the ratios of Na/
Al and Fe2+/Fe3+ from the Lingtai (35.18N, 107.78E)sequence indicates a long-term increase in the rate of
mechanical weathering and erosion in the source
regions over the last 2.6 Myr [62]. A general coarsening
trend in bulk median grain size from the Luochuan
(35.758N, 109.428E) sequence suggests a long-term
C. Deng et al. / Earth and Planetary Science Letters 241 (2006) 248–259 257
intensification of the winter monsoon over the last 2.6
Myr [63]; multiple mineral magnetic parameters re-
trieved from the Jiaodao (35.88N, 109.48E) sequence
suggest a long-term decrease in summer monsoon in-
tensity and a long-term increase in winter monsoon
intensity over the last 2.6 Myr [10]. A general decreas-
ing trend in the 87Sr/86Sr ratio of the Luochuan se-
quence indicates a long-term decrease in source area
weathering intensity in the Asian interior over the last
2.6 Myr [64]. Sporopollen records obtained from the
Chaona (107812VE, 3587VN) sequence indicate shifts of
vegetations from forest-steppe (1.5�0.95 Ma) to open
forest and steppe (0.95�0.5 Ma) and then to steppe (0.5
Ma to present), implying a stepwise drying in the south-
central plateau over the last 1.5 Myr [65]. High-tem-
perature magnetic susceptibility measurements of the
Jiaodao section also indicate a general decrease in
weathering intensity over the last 1.2 Myr [33]. And
an up-section increasing trend in illite content in the
Baoji sequence may indicate an increase in aridity over
the last 1.2 Myr [66].
Specifically, there is an exception in the upper sandy
loess layer L9 of the Jingbian section, which was given
an age of 0.865–0.943 Ma by the new astronomical
timescale for the Chinese loess/paleosol sequences
developed by Ding et al. [6]. This loess layer has
prominent low coercivities, namely, low values of
MDFSIRM, SIRM100mT/SIRM, SIRM100mT/SIRM30mT
and SIRM100mT/SIRM60mT (Fig. 8). This behavior is
inconsistent with the general changes of these para-
meters. This abnormal behavior may be related to an
episode of accelerated uplift of the Tibetan Plateau (at
least the northern and northeastern Tibetan Plateau)
about 0.9 Myr ago [10,63]. The accelerated tectonic
uplift enhanced rock weathering and mountain erosion
in the Tibetan Plateau and its adjacent mountainous
regions, leading to the dominant input of weakly weath-
ered materials rich in coarse-grained magnetite grains
into the source sedimentary basins near the plateau in
northwestern China [10]. Subsequently, these coarse-
grained magnetite grains with low coercivities were
transported through the East Asian monsoonal circula-
tion and deposited in the Loess Plateau region.
6. Conclusions
We have analyzed in detail a high-resolution
mineral magnetic record from the Jingbian loess/
paleosol sequence at the northern extremity of
the Chinese Loess Plateau. Results show that the
ratios of SIRM100mT/SIRM, SIRM100mT/SIRM30mT
and SIRM100mT/SIRM60mT all display a long-term up-
section oscillatory decreasing trend in both glacial and
interglacial periods over the Quaternary period. This
long-term trend is attributed to a long-term decrease
in the relative contributions of eolian hematite during
glacial extrema and of pedogenic hematite during
interglacial extrema. We then interpret this long-term
variation pattern to reveal a long-term decreasing
trend in chemical weathering intensity in both gla-
cial-stage source region (the Gobi and other deserts in
northwestern China) and interglacial-stage deposition-
al area (the Loess Plateau region) over the Quaternary
period. This long-timescale variation also indicates a
long-term increasing aridification and cooling during
both glacial extrema in the source region and inter-
glacial extrema in the depositional area over this
period. In addition, the three ratios and the composite
marine oxygen records show good correlation be-
tween glacial–interglacial climatic oscillations and/or
long-term trends. In summary, the up-section gradual
decrease of those ratios over the entire Jingbian loess/
paleosol sequence possibly reflects an increasing ari-
dification and cooling of the climate system in the
Asia interior over the last 2.6 Myr, which is most
likely controlled by ongoing global cooling, expan-
sion of the Arctic ice-sheet and progressive uplift of
the Himalayan–Tibetan Plateau complex within this
interval.
Acknowledgements
This research used samples provided by Professors
Zhongli Ding and Jimin Sun. We are grateful to the two
anonymous reviewers for their insightful comments and
suggestions to improve the manuscript. C. Deng thanks
Dr. Shiling Yang for useful discussion. C. Deng is
grateful for the hospitality of his Liverpool friends at
the Geomagnetism Laboratory of the University of
Liverpool, where the paper was written. C. Deng fur-
ther acknowledges support from the Royal Society in
the form of a BP Amoco Fellowship. This work was
supported by the National Natural Science Foundation
of China and Chinese Academy of Sciences through
grants 40221402, KZCX2-SW-133, KZCX3-SW-150
and 40325011.
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