Mineral magnetic variation of the Jingbian loess/paleosol sequence

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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).


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-


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.


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.


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].


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


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


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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Mineral magnetic variation of the Jingbian loess/paleosol sequence