Process C
Resume:
SCIENCE CHINA
Earth Sciences
RESEARCH PAPER March 2010 Vol.53 No.3: 419 431
doi: 10.1007/s11430-010-0011-5
Loess magnetic properties in the Ili Basin and their correlation
with the Chinese Loess Plateau
SONG YouGui1*, SHI ZhengTao2, FANG XiaoMin3, NIE JunSheng4, NAOTO Ishikawa5,
QIANG XiaoKe1 & WANG XuLong1
1
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi an 710075, China;
2
School of Tourism and Geography, Yunnan Normal University, Kunming 650092, China;
3
Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China;
4
Jackson School of Geosciences, University of Texas at Austin, Austin TX 78712, USA;
5
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
Received June 11, 2009; accepted November 30, 2009; published online January 14, 2010
Over the past two decades, magnetoclimatological studies of loess-paleosol sequences in the Chinese Loess Plateau (CLP)
have made outstanding achievements, which greatly promote the understanding of East Asian paleomonsoon evolution, inland
aridification of Asia, and past global climate changes. Loess magnetic properties of the CLP have been well studied. In con-
trast, loess magnetic properties from outside the CLP in China have not been fully understood. We have little knowledge about
the magnetic properties of loess in the Ili Basin, an intermontane depression of the Tianshan (or Tien Shan) Mountains. Here,
we present the results of rock magnetic measurements of the Ili loess including mass magnetic susceptibility and anhyster-
etic remanent magnetization (ARM), high/low temperature dependence of susceptibility (TDS) and hysteresis, as well as X-ray
diffraction (XRD) for mineral analysis. Based on the comparison with loess-paleosol sequences in the CLP (hereafter referred
to as the Chinese loess), we discuss the possible magnetic susceptibility enhancement mechanism of the Ili loess. The results
show that 1) the total magnetic mineral concentration of the Ili loess is far lower than that of the Chinese loess, though they
have similar magnetic mineral compositions. The ferrimagnetic minerals in the Ili loess are magnetite and maghemite, and the
antiferromagnetic mineral is hematite; XRD analysis also identifies the presence of ilmenite. The ratio of maghemite is lower
in the Ili loess than in the Chinese loess, but the ratios of magnetite and hematite are higher in the Ili loess than in the Chinese
loess. 2) The granularity of magnetic minerals in the Ili loess, dominated by pseudo-single domain (PSD) and multi-domain
(MD) grains, is generally much coarser than that of the Chinese loess. Ultrafine pedogenically-produced magnetic grains have
a very limited contribution to the susceptibility enhancement. Rather, PSD and MD particles of magnetite and maghemite are
the main contributors to the enhancement of susceptibility in the Ili loess. 3) The susceptibility enhancement mechanism for
the Ili loess is complicated and superimposes both a wind velocity/vigor model (Alaskan or Siberian model) and the in situ ul-
trafine grain pedogenic model; the former might play an important role in the Ili loess. 4) Magnetic susceptibility enhance-
ments of the Ili loess are related not only to the eolian input of the source area, but also to the local climate, landform, and
geological background. Therefore, great care should be taken when reconstructing paleoclimate using magnetic susceptibility
data from the Ili loess.
loess, magnetic property, magnetic susceptibility, the Ili Basin, the Chinese Loess Plateau
Citation: Song Y G, Shi Z T, Fang X M, et al. Loess magnetic properties in the Ili Basin and their correlation with the Chinese Loess Plateau. Sci China Earth
Sci, 2010, 53: 419 431, doi: 10.1007/s11430-010-0011-5
*Corresponding author (email: abpo3h@r.postjobfree.com)
Science China Press and Springer-Verlag Berlin Heidelberg 2010 earth.scichina.com www.springerlink.com
420 SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
Magnetic properties of Quaternary loess-paleosol sequences bility than those of their adjacent paleosol layers [32 34].
in the Chinese Loess Plateau (CLP) have been extensively Magnetic susceptibility measurements show that different
studied during the past two decades, and have been widely sections at different magnetic altitudes and landform loca-
used for paleoclimate reconstruction, land-ocean compari- tions in the Ili Basin have different behaviors. An important
son and past global changes. These studies represent re- question then is what the reasons are for causing the mag-
markable achievements [1 5]. For example, magnetic sus- netic differences. Can magnetic susceptibility be taken as a
ceptibility as an important paleoclimatic proxy (summer paleoclimatic proxy in the Ili Basin? The paucity of system-
monsoon intensity) has been widely applied to reconstruct atic rock magnetic studies greatly hampers our understand-
East Asian paleomonsoon evolution [6 8], land and ocean ing of the magnetic susceptibility enhancement mechanisms
comparison [9 12], quantitative reconstruction of paleocli- in the Ili loess sediments and thus prevents us from obtain-
mate (such as paleorainfall and paleotemperature) [13 15], ing paleoenvironmental information encoded by magnetic
the uplifting of the Qinghai-Tibet Plateau [3, 8, 16], Asian susceptibility variations. This paper selected two loess sec-
inland aridification studies [5] and so on. Central Asia is tions, located at the southern margin of the Ili Basin at the
one of the most significant loess regions on Earth, located northern foot of the Tianshan Mountains, as systemic rock
between the well-studied European loess sequences to the magnetic and mineralogical studies, and then compared the
west and the extensive CLP region to the east. This enables magnetic properties of the Ili loess with Chinese loess and
researchers to carry out interregional paleoclimatic investi- discussed enhancement mechanisms of the Ili loess mag-
gations along a west-east transect across the entire Eurasian netic susceptibility.
loess belt of the Northern Hemisphere [17]. The loess in
Central Asia has important implications for the studies on
1 Geographic background and sampling
Asian inland aridification, dust sources of the Northern
Hemisphere, past atmospheric circulation, and past global
The Ili Basin is a Mesozoic-Cenozoic faulted depression
climate change.
surrounded by the Tianshan orogenic belt; the topography
In recent years, international Quaternary geologists have
of the Ili Basin is shaped like a trumpet, with the mouth in
paid much attention to the loess sediments in Tajikistan and
the west; i.e., the altitudes are higher in the eastern part and
southern Kazakhstan, which are situated in the southern part
lower in the western part, and the Kazakhstan Gobi Desert
of Central Asia [17 24]. Previous studies show that pa-
is to the west (Figure 1). The Ili Basin is located in the
leoenvironment information recorded in south Central Asia
inland of the Eurasian continent and thus is far away from
loess sediments is very comparable with that recorded in
the ocean. The basin has a temperate, continental, arid cli-
Chinese loess. However, few similar studies have been done
mate. Westerly airflow from the Atlantic Ocean or the
on the loess sediments in the Tianshan area of northeastern
Mediterranean or the Black Seas easily reaches the valley
Central Asia. Previous studies indicate that the formation
and form precipitation because of the unique landforms and
and evolution of the Tianshan loess are closely related to the
the mountain strike. The climate is controlled mostly by
development of Central Asia deserts [23, 25 27] and Asian
Mongolian high-pressure and is influenced mainly by the
inland aridification [25, 28], but little is known about the
north branch of westerly airstream during the winter. The
paleoclimate implications of the Tianshan loess magnetic
summer climate is affected mainly by the Indian hot low
properties.
pressure, and the south branch of westerly airstream shifts
Correlation between magnetic susceptibility of modern
to this area and controls the climate. Westerly winds prevail
soil samples in the CLP and mean annual temperature
at high-altitudes throughout the year [30]. The MAT of the
(MAT) or mean annual precipitation (MAP) shows mag-
netic susceptibility increases with the increase of MAT or Ili Basin ranges from 2.6 to 10.4 . The distribution of the
MAP. This correlation in the Xinjiang area, though, is very MAP influenced landform is also uneven, with the general
complicated, and there are no obvious relationships among trend of the spatial distribution in the eastern part being
susceptibility and MAT/MAP [13]. However, recent rock higher than that in the western part and higher in the moun-
magnetic studies of Central Asian topsoil samples [29] tains than in the plains. The MAP of the plains ranges from
suggest that magnetic mineral contents are generally lower 200 to 500 mm, but the MAP in the mountains is more than
than those in the CLP, and are proportional to spatial pre- 600 mm.
cipitation. Loess investigations and paleoenvironment stud- Loess sediments are widely distributed in the Ili Basin
ies [30 32] have been carried out in the Ili Basin, an inter- (Figure 1(b)); they lie mainly at different terraces of the Ili,
montane basin in the Tianshan Mountains of Central Asia, Kunse, Turks, and Kashgar Rivers, hilly areas, piedmonts
during the last several years. These studies show that mag- and at the edge of the deserts. The thickness of loess sedi-
netic susceptibility of loess-paleosol sequences is far more ments ranges from several meters to nearly 100 m, and the
complex in the Ili Basin than in the CLP. Magnetic suscep- eastern basin is thicker than the western basin. The age of
tibility of paleosol layers is not always higher than that of most of natural loess outcrops is dated to the last glaciation,
loess layers; some loess layers even have a higher suscepti- and reaches Middle Pleistocene in the Kunes valley in the
421
SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
eastern basin [30]. The maximum distribution altitudes typical eolian sediments. Since the late 1990s, we have car-
(MDAs) of loess sediments vary with landforms. For exam- ried out investigations on the Ili loess sediments, and, in the
ple, the MDA in the Zhaosu Basin is 1900 2100 m, whereas summer of 2001, we obtained a 96 m long drilling core
the MDA of loess in the northern Ili Basin and southern from a Kunes terrace near Talede town in Xiyuan County.
piedmonts of the North branch of Tianshan tend to increase Magnetic measurements reveal that there are different
from the west (1200 1600 m) to the east (1800 2000 m). magnetic susceptibility behaviors for different loess-paleo-
The MDAs of loess rang from 700 to 1800 mm. The MDAs sol sequences at different altitudes [32, 34], magnetic sus-
of loess range from 1700 to 1800 m in the southern Ili Basin, ceptibility values in some of paleosols are higher, while
with the loess mainly distributed on hills, higher terraces, some are lower, and there is no clear relationship between
and piedmonts in the Kunes river valley with an altitude of magnetic susceptibility and paleosols. Therefore, it is criti-
at 900 1500 m [30]. The composition [30], grain size [35, cal to study the enhancement mechanism of magnetic sus-
36], quartz surface morphology [30, 37] and geochemistry ceptibility.
of the Ili loess [30, 31] analysis show that the Ili loess are Two sections and some powder samples from the drilling
Figure 1 Distribution of loess sediments in Asia [4] (a) and the Ili Basin in the Tianshan Mountains (b) and the locations of the studied sections.
422 SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
core were selected in order to study loess magnetic proper- analyses were carried out on a PANalytical X pert Pro
X-ray diffractometer (PW3071) with Cu-K radiation in the
ties. One section (ZSP) (80.25 E, 42.69 N, 1875 m) is lo-
cated in western Zhaosu County near the boundary between Environmental Mineralogy Laboratory of IEE, CAS. The
China and Kazakhstan, and another section (TLD) (83.01 E, XRD was measured using a step size of 0.02 2 and a scan
43.42 N, 850 m) is located at Talede Town in Xinyuan speed of 2 2 /min from 5 2 to 60 2 on air-dried sam-
County, Xinjiang. Both of the sections are situated on river ples.
terraces on the southern margin of the Ili Basin (Figure
1(b)), and the direct distance between them is about 240 km.
3 Results
Though the thicknesses of the loess sections are different,
the loess sediments show typical features of eolian deposi-
3.1 Magnetic susceptibility and frequency-dependent
tion [31]. The Zhaosu loess section (ZSP) is 6.9 m thick
susceptibility
from the top to the upper part the first paleosol (S1), and the
bottom consists of fluvial gravel sediments. According to Figure 2 shows mass magnetic susceptibility ( lf) and fre-
IRSL (infrared stimulated luminescence) dating of this sec- quency-dependent susceptibility ( fd and fd%) variations
tion, the bottom is 80 ka old (Figure 2). The Talede outcrop versus depth for the three sections. It is very easy to see that
(TLD) is 40 m thick, and the thickness above S1 is about 34
both lf and fd% values of the Ili loess (Figure 2(a) and (b))
m [31]. In order to compare rock magnetic properties of the
are lower than those of Chinese loess (Figure 2(c)). Fluctua-
Ili loess with Chinese Loess, we have also collected a loess
tion amplitudes of the Ili loess are also smaller than those of
section above S1, named Chaona section (CN) (107.2 E,
Chinese loess. lf in paleosol layers is not always higher
35.1 N, 1495 m), (Figure 1(a)) in the CLP. The sampling
than in loess layers. For example, lf of some samples is
interval in the ZSP and CN is 5 cm whereas in the TLD
lower in S1, and even lower than L1 in the TLD section.
section, it is 10 cm.
Likewise, lf values in the Talede drilling core have no clear
relationship to paleosol layers. To a certain extent, magnetic
2 Methods susceptibility values reflect the total content of magnetic
minerals in the samples. The average lf values of the ZSP
and TLD sections are 5.43 10 7 m3/kg and 7.69 10 7 m3/kg,
We measured the mass magnetic susceptibility and an-
respectively, which is far less than that of the CN section
hysteretic remanent magnetization (ARM) at the Paleo-
(11.68 10 7 m3/kg). This fact shows that the total amount of
magnetism Laboratory of the Institute of Earth Environment
magnetic minerals in the Ili loess sediments is far less than
(IEE), Chinese Academy of Sciences (CAS). Magnetic sus-
that in Chinese Loess. The average lf value of the ZSP sec-
ceptibility was measured with a Bartington MS2 meter at
tion is lower than that of the TLD section, which means that
frequencies of 470 Hz (i.e., lf) and 4700 Hz (i.e., hf). From
the total amount of magnetic minerals in the loess in the
these measurements, the absolute frequency dependence of
western Ili Basin is lower than that of the loess in the east-
susceptibility fd (defined as lf hf) and frequency-
ern basin. fd% of the Ili loess is characterized by lower
dependent susceptibility fd% (defined as ( lf hf)/
values and high frequency fluctuation in a relatively narrow
lf 100%) were calculated. fd and fd% represent the ab-
range. fd% varies from 1.33% to 6.07% with an average of
solute and relative behavior of frequency-dependent suscep-
2.9%, which reflects a weak pedogenesis process. fd% in
tibility, respectively. ARM was imparted using a 100 mT
the paleosol is higher than in the loess within the same sec-
peak alternating field with a superimposed 0.05 mT direct
tion (Figure 2(a) and (b)), and it seems that fd% is a more
bias field. ARM was then normalized by the bias filed to
sensitive indicator in the paleosol and loess in the Ili Basin.
obtain ARM susceptibility ( ARM). We measured hysteresis
fd% of the CN section from the CLP shows an approxi-
parameters (including saturation magnetization (Ms), rema-
mately linear relationship with lf, with fd% of the CN
nent saturation magnetization (Mr), coercivity (Bc), and
section ranging from 7.5% to 12.15%, which is far higher
Remanent coercivity (Bcr)) by using a Princeton Measure-
than that of the Ili loess. The weakly-developed paleosol
ments MicroMag 2900-02C alternating gradient force mag-
(Sm) (equivalent to Marine Isotope Stage 3) intercalated in
netometer at the Paleomagnetism and Rock Magnetism
L1 of the Chinese loess shows obvious peaks in both lf and
Laboratory of Kyoto University. Temperature-dependent
fd%, but there is no clear peak in the Ili loess (Figure 2).
susceptibility (TDS) was measured continuously from room
fd% is usually regarded as a proxy to determine the contri-
temperature up to 700 C and back down to room tempera-
ture using a KLY-3 Kappabridge with CS3 high tempera- bution of superparamagnetic particles to magnetic suscepti-
ture device in the Fort Hoofddijk Paleomagnetism Labora- bility [1, 38, 39], which indicates that the content of super-
tory of Utrecht University of the Netherlands. The pow- paramagnetic particles is very low, and is a very limited
dered whole-rock samples were heated and cooled in an contributor to the susceptibility enhancements. The rela-
argon atmosphere to prevent possible oxidation. XRD tionships between lf and fd% of the three sections (Figure
423
SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
3) indicated that the correlation coefficient in the CN sec- of any of these parameters for the Ili loess are significantly
tion is 0.76, while in the ZSP section (Figure 3(a)), it is only lower than those of Chinese loess. However, the coefficients
0.4, and there are no obvious relationships in the TLD sec- between the Ili loess and Chinese loess are similar to some
tion (Figure 3(b)). extent, which implies that superparamagnetic grains pro-
Previous work [40, 41] has shown that the parameter fd duced by pedogenic process have a limited contribution to
the magnetic susceptibility enhancements of the Ili loess.
is especially sensitive to grain sizes of ~20 ~25 nm, i.e.,
Meanwhile, the lower coefficients indicate that the pe-
larger or viscous superparamagnetic maghemite. Likewise,
dogenic model is not completely suitable to the Ili loess,
the parameter ARM is sensitive to grain sizes of ~25 ~100
especially to the TLD loess (Figures 3(b), 4(b)).
nm, i.e., larger stable-single-domain (SSD) and smaller
pseudo-single-domain (PSD) maghemite [42 44]. Both fd
and ARM values of the Ili loess are lower than those of Chi- 3.2 Temperature dependence of magnetic susceptibil-
nese loess in the CLP. fd differences, in particular, can ity
reach almost an order of magnitude lower (Figure 4(a) and
Temperature dependence of magnetic susceptibility (TDS)
(b)), and the highest ARM value of the Ili loess is equivalent
is sensitive to subtle changes in magnetic minerals during
to the lower values of Chinese loess (Figure 4(c)). This in-
thermal treatments and has been widely used as a routine
dicates that the contents of super-paramagnetic particles
rock magnetic tool in order to identify the magnetic miner-
with either ~20 ~25 nm or ~25 100 nm grain sizes are lower
alogy and grain size distribution [45 49]. All the TDS
in the Ili loess than in Chinese loess. It is easy to distinguish
heating curves (Figure 5) of both the Ili loess and Chinese
the Ili loess from Chinese loess by the relationship plots
among fd, ARM and lf. The absolute values or coefficients loess are characterized by a major decrease in susceptibility
Figure 2 Mass magnetic susceptibility ( lf) and frequency-dependent susceptibility ( fd%) variations versus depth of the Zhaosu and Talede section in the
Ili Basin and Chaona section in the Chinese Loess Plateau. Solid circles indicate the rock magnetic samples, and triangles represent the IRSL dating data.
Figure 3 Correlations of lf and fd% in the Zhaosu section(a), the Talede section(b) in the Ili Basin and in the Chaona section in the Chinese Loess Plateau.
424 SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
The relationship between fd versus ARM (a), fd versus lf (b), lf versus ARM (c). Triangle, ZSP; crisscross, TLD; open circle, CN.
Figure 4
Figure 5 Temperature dependence of magnetic susceptibility curves of the Ili loess and Chinese loess. Bold lines represent heating; thin lines represent
cooling.
at about 580 C, the Curie point of magnetite. This behavior ing curves below the heating curves might suggest that
indicates that magnetite is the major contributor to the sus- maghemite is transformed to hematite during the heating
ceptibility enhancements. The cooling curves of most of the process, which causes the susceptibility decrease. These
samples lie above the heating curves, indicating that new phenomena are also seen in Siberian loess [50] and Alaskan
magnetic minerals form during the cooling treatment. The loess [51, 52].
TDS shows a sharp increase in magnetic susceptibility when In order to study magnetic susceptibility changes during
cooled below magnetite s Curie point at ~580 C. This phe- the heating process and to facilitate the comparison, we will
nomenon indicates that newly formed ferrimagnetic miner- normalize susceptibility curves by the initial susceptibility
als during heating are dominated by magnetite. The in- values at room temperature conditions (Figure 6). It is easy
creases of susceptibility during the cooling process of near to see the significant differences of the heating curves be-
surface samples are higher than those of the lower samples, tween the Ili loess and the Chinese loess. There are two ob-
which indicate that more minerals in the near surface sam- vious peaks occurring around 260 C and 520 C for the
ples can be transformed into ferrimagnetic minerals. Cool- heating curves of both L1 and S0 samples in the ZSP sec-
425
SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
Figure 6 Correlation of normalizing heating curves between the Ili loess and Chinese loess. (a) Zhaosu section; (b) Talede section; (c) Chaona section.
tion (Figure 6(a)). In the cooling curves, the gradual in- the Chinese loess. The decrease in amplitude from 580 to
crease of susceptibility from room temperature to 260 C 600 C of the Ili loess (~50%) is about 10% higher than that
might be related to a gradual unblocking of fine-grained of the Chinese loess (~38%), which indicates that the pro-
single domain (SD) particles [49], or, likely, it is due to the portion of magnetite to magnetic minerals in the Ili loess is
conversion of maghemite ( Fe2O3) from the dehydration of higher than the Chinese loess. The decrease from 600 to
700 C of the Ili loess during heating is slightly higher than
some lesser magnetic Fe-hydroxides (e.g., lepidocrocite,
that seen in the Chinese loess, which means there is more
FeOOH) during heating [53]. The decrease in susceptibil-
hematite preset in the Ili loess.
ity between 300 and 440 C probably results from the ther-
Low-temperature magnetic susceptibility can effectively
mally-induced conversion of metastable ferrimagnetic
determine the grain size of magnetite and hematite. The
maghemite [15, 46], or fine-grained pedogenic particles [49]
curves of the Ili loess show an obvious Verway transition
or goethite [53] to weakly magnetic hematite ( Fe2O3).
between 197 C and 150 C (Figure 7), suggesting that
Both of the heating curves of the S0 and L1 samples display
coarser grains of MD magnetite/maghemite are present in
a pronounced magnetic susceptibility peak at around 520 C.
the Ili loess samples. However, there is no significant Ver-
The susceptibility peak mainly results from the production
way transition in the Chinese loess, and paleosol samples
of magnetite grains [50, 52], which are decomposed from
show the characteristics of ultra-paramagnetic magnetite. A
ferrous silicates/clay mineral or transformed from iron-
Verway transition is also identified at the Kurtak loess in
hydrate (e.g., limonite) under high temperature. The peaks
Siberia [54], and it seems to suggest that the magnetic
around 520 C in the heating curves of the samples from S1
properties of the Ili loess are more similar to the Siberian
in the ZSP section, TLD section, and CN section are not
loess, although the decreases in the amplitude of the Ili
clear (Figure 6(b) and (c)). The heating curves are mainly
loess are much lower than those of the Siberian loess.
divided into three stages around 260 and 580 C, which first
show a trend of increasing magnetic susceptibility, then
gradually decreasing, and finally suddenly dropping. It is
noted that the heating curves of the L3 samples from the
TLD section (Figures 5(g), 6(b)) and the Sm sample from
the CN section (Figures 5(k), 6(c)) are significantly differ-
ent. The former increases very slightly from 260 to 580 C,
and drops suddenly after 580 C, representing the presence
of multi-domain (MD) magnetite. The latter increases con-
tinually from room temperature to 380 C, then drops rap-
idly, and reaches a value of nearly 0 until 600 C, which
indicates that magnetic minerals in this samples are domi-
nated by magnetite, perhaps with ferrous silicates/clay min-
erals. Further detailed observation reveals that the magnetic
susceptibility intensity decrease of the Ili loess from 260 C
to 580 C is around 30%, whereas the decrease of the Chi-
nese loess reaches around 50%. This indicates that the con- Figure 7 Correlations of low-temperature susceptibility curves for se-
lected samples from the Ili loess, Chinese loess, and Siberian loess [54].
centrations of maghemite are lower in the Ili loess than in
426 SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
3.3 Hysteresis parameters netic minerals during heating, we also performed X-ray
diffraction analysis on the same samples (Figure 9). The
The shape of the hysteresis loop can be an indicator of main peaks of these spectra are very similar; however, there
magnetic mineral types [55]. The Ili loess hysteresis loops are intensity differences. This suggests that the mineral
are like an S in shape, and are, in general, similar to the compositions of the Ili loess are the same, but the concen-
Chinese loess. Most of the loops tend to close at 200 300 trations of minerals in different samples are different. Sam-
mT, and close completely at 400 500 mT; the loops in the ples consist mainly of quartz, feldspar, mica, and carbonate
Ili loess also have their own characteristics (Figure 8). The (mainly calcite and dolomite), as well as iron-rich chlorite
hysteresis loops of the Chinese loess, except from Sm (Fig- clay minerals; ilmenite is also identified in some samples
ure 8(j)), have a steep and narrow slim shape and tend to (Figure 9(j) and (h)). Unfortunately, typical peaks of mag-
close before 200 mT (Figure 8(i), (k) and (l)), whereas the netite, maghemite, and hematite are not observed, which
Ili loess hysteresis loops are relatively wide and short and suggests that the concentrations of these minerals are very
tend to close at 300 mT (Figure 8(a) (h)). These coarser low ( the heavy minerals are less than 5% in the Ili loess
loops suggest the presence of a mix of considerably [37]) and beyond the limited of XRD. Magnetic mineral
low-coercivity ferrimagnetic minerals and paramagnetic extracts contain a large number of iron-rich silicates (mainly
minerals, and the different shapes are mainly due to content iron magnesium hornblende) and clay minerals (mainly
differences. Previous studies [56] indicate that hysteresis iron-rich chlorite).
loops in loess layers are relatively wide and short, because
there are much higher coercivity antiferromagnetic minerals
4 Discussions
(e.g., hematite, goethite). The results suggest that the Ili loess
has a relatively higher proportion of antiferromagnetic miner-
4.1 Concentrations and types of magnetic mineral of
als (e.g., hematite) than the Chinese loess does. This result
the Ili loess
also supports the temperature-dependent interpretation.
To some extent, magnetic susceptibility reflects the total
3.4 X-ray diffraction spectra
concentration in magnetic minerals of sample. The average
lf values of the Ili loess are far smaller than those of Chi-
To further explain the transformation mechanisms of mag-
Figure 8 Typical magnetic hysteresis loops of the loess and paleosol samples from the ZSP, TLD and CN section.
427
SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
Figure 9 X-ray diffraction spectrum of the Ili loess. Q, Quartz; CL, iron-rich chlorite; Mu, muscovite; Ab, abite; Cc, calcite; Do, dolomite; IL, ilmenite.
nese loess, indicating that the Ili loess samples with low more high-coercivity antiferromagnetic minerals such as
magnetic mineral contents are far from the Loess Plateau. hematite and goethite than the Chinese loess. Previous re-
This also suggests that the total magnetic mineral concen- search [43] also shows that samples located in Day-plot [57]
tration of the Ili loess is far lower than that of the Chinese will shift towards the increase direction with the increase in
loess. The average lf values of the ZSP section, located in antiferromagnetic minerals (Figure 10). This also shows that
the Ili loess has more antiferromagnetic minerals such as
the western Ili Basin, are lower than those of the TLD sec-
hematite than the Chinese loess. The XRD analysis and
tion in the eastern basin, which also indicates that higher
heavy mineral analysis [37] showed that the Ili loess con-
magnetic mineral concentrations are present in the eastern
tains iron-rich silicates (e.g., amphibole) and clay minerals
basin. This may be caused by the geographical location dif-
(e.g., iron-rich chlorite); during the heating, iron-rich min-
ferences such as altitude, or the distances to dust sources
erals may be decomposed into magnetite, resulting in in-
region. The ZSP section is much closer to the desert, with a
creased magnetic susceptibility.
high altitude (1875 m a.s.l.) and 512 mm annual precipita-
Magnetite, maghemite, hematite, and goethite are the
tion, whereas the TLD section that lies on the interior of the
main magnetic minerals in loess-paleosol sediments in the
Tianshan has a relatively low altitude (850 m a.s.l.) and
CLP. TDS studies have shown that magnetic mineral com-
only 350 mm of annual precipitation [34]. It is difficult to
ponents of the Ili loess are similar to the Chinese loess. The
use the pedogenic model to explain the higher magnetic
contributions of hematite and goethite to magnetic suscepti-
susceptibility in the TLD section. The possible mechanism
bility in the CLP can be ignored due to the contributions of
is that the eolian sediments from nearby riverbeds are plen-
magnetite and maghemite, because the mass magnetic sus-
tiful in the TLD section because of its low altitude, and
ceptibility of pure SD magnetite is 450 10 6 m3/kg, which is
more magnetic minerals have been transported to the terrace,
75 times that of hematite (6 10 6 m3/kg). In other words, 75
which results in the increase of magnetic susceptibility. This
suggests that there is no linear relationship between mag- units of hematite contribution to the magnetic susceptibility
netic susceptibility and precipitation. The studies of TDS is equivalent to 1 unit magnetite contribution. Therefore,
and hysteresis loops have shown that the Ili loess contains loess magnetic susceptibility mainly depends on the content
428 SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
of magnetite. However, the TDS curves of the Ili loess show
the presence of hematite distinctively (Figure 7(a) and (b)),
which means not only that hematite significantly contributes
to the Ili loess magnetic susceptibility, but also that the
hematite concentrations are much higher. Unfortunately,
further X-ray diffraction analysis only suggests the presence
of ilmenite, and there are no characteristic spectra of con-
firming of hematite. However, the Ili loess heavy mineral
analysis [37] proved the presence of magnetite, limonite (a
mixture of goethite and lepidocrocite), and ilmenite. Mag-
netite is sharply angular and subangular with fine grains
under a single microscope observation (much coarser rela-
tive to SP grain), whereas limonite is subrounded with
coarser grain.
4.2 Granularity of the Ili loess magnetic minerals and
pedogenesis
Figure 10 DAY plot of the Ili loess. Open circle, ZSP; diamond, TLD;
The DAY plot between the magnetization intensity ratio of solid circle, CN; triangle, topsoil of Xinjiang.
magnetic minerals (Mrs/Ms) and coercivity ratio (Bcr/Bc) is
one ideal approach to identify the granularity of magnetic
just half that of the Chinese loess (Figure 6), indicating that
minerals [57]. The critical value of granularity in this paper
the overall pedogenic degree of the Ili loess is far weaker
follows Thompson et al. [55]: SD magnetic particles are
than the Chinese loess. Frequency-dependent susceptibility
characterized by Mrs/Ms>0.5 and Bcr/Bc
has been used as a sensitive indicator of SP concentrations
magnetic grains are characterized by Mrs/Ms 4; PSD grains lie between MD and SD grains. The
than the Chinese loess, but slightly higher than the Siberian
Chinese loess contains a large number of SSD and SP
loess (close to 0) [54], which illustrates that the pedogenic
fine-grained mineral particles. From Figure 10, we can ob-
process is weaker in the Ili loess than in the Chinese loess,
serve that the Chinese loess samples scatter in the PSD re-
but stronger than in the Siberian loess.
gion, whereas nearly half of the Ili samples lie in the PSD
area (more offset to MD) and other samples fall in the MD
region. The distribution of Xinjiang topsoil samples from
4.3 Enhancement mechanisms of magnetic susceptibi-
the Tarim Basin, Ili Basin, Junggar Basin, and the Tianshan
lity in the Ili loess sediments
and Altay Mountains is similar to that of the Ili loess, al-
though the distributions are wider. These features show that The mechanism of loess magnetic susceptibility enhance-
the grain size of ferrimagnetic minerals in the Ili loess is ment has been a controversial issue. The magnetic suscepti-
much coarser than in the Chinese loess, which is also con- bility values are affected by sources, sedimentary processes,
sistent with the results of low-temperature magnetic suscep- post-depositional pedogenesis, weathering processes, and
tibility measurements. These results confirm that PSD and biological effects. Therefore, magnetic susceptibility en-
MD grains dominate the Ili loess. A careful comparison of hancement mechanism of loess-paleosol sequences in dif-
the DAY plot (Figure 10) reveals that the magnetic grains of ferent regions may be different [50, 52, 58]. Several possi-
the ZSP samples are slightly coarser than that of the TLD ble mechanisms for the magnetic enhancement have been
section and close to MD zone significantly. Average values suggested in the CLP. These include eolian dust dilution
of frequency-dependent susceptibility fd% (a sensitive pa- [59], sediment compression and carbonate leaching [60],
rameter for detecting the presence of SP grains in soils) are decomposition of plant residues [61], and the pedogenic
lower than those of the Chinese loess, indicating a relatively model [1, 41]. Currently, the most widely accepted inter-
low concentration of ~25 100 nm SP grains. DAY plot pretation is in situ formation of ultra-fine magnets during
consistency between the Xinjiang loess topsoil samples and pedogenesis (i.e., pedogenic model), which suggests that the
the Ili samples may suggest that the Ili loess is transported magnetic enhancement is the result of the formation of ul-
from nearby sources, originally from the adjacent Gobi de- trafine (SP and SSD), magnetic minerals during the pe-
sert by wind. dogenic process. Warm humid interglaciation is favorable to
The attenuation values between 300 and 400 C in TDS forming new fine-grained magnetic minerals, and strong
curves are regarded as a pedogenic intensity indicator, that pedogenesis produces new magnetic minerals. In general,
is, the concentration of SP grains changes during pedogenic the stronger the pedogenesis occurs, the higher the magnetic
processes [49]. The decreasing values of the Ili loess are susceptibility produces. During dry climate conditions, the
429
SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
pedogenic process in the Ili loess is weak, which is unfa- the heavy mineral compositions of the Ili loess are domi-
vorable to producing much more SP magnetic grains, so the nated by amphibole, which is the common component of
SP grains have a very limited contribution to magnetic en- bedrock in the Tianshan Mountains, the Ili River Valley,
hancement. The main contributors to magnetic susceptibil- and the western desert. Amphibole also contains plenty of
ity are PSD and MD magnetite. There are two kinds of iron magnesium hornblende, which perhaps has a greater
sources of loess magnetic minerals: protogenesis and epi- contribution to magnetic susceptibility. We found that the
genesis. The protogenesis comes from eolian dust, and epi- magnetic susceptibility value of an individual sample is
genesis means that the SP magnetic minerals form during several times higher than the adjacent samples during
pedogenesis when loess is deposited. DAY plot and TDS measurements, indicating the presence of near-source strong
results show the presence of MD magnet grains, which are magnetic minerals. Therefore, the Ili loess originated
difficult to produce during the pedogenesis process, though mainly from Gobi desert in Central Asia area; another
they may originate from detritus magnetic minerals from the source may be the Ili River valley sediments. The Ili loess
source region. Therefore, the pedogenic model cannot com- preserves more near-source material, which contains coarse
pletely explain the magnetic susceptibility enhancements. ferrimagnetic minerals. Rock magnetic studies confirm the
When climate varies toward higher temperature and higher presence of MD ferrimagnetic minerals in the Ili loess. With
moisture, weathering and pedogenesis also increases, which the changes of wind strength, the concentrations of mag-
favors the development of an oxidizing pedogenic environ- netic minerals carried by wind also changes, resulting in
ment. It is easy to form ultrafine magnet grains such as magnetic susceptibility fluctuations.
maghemite and magnetite in this oxidizing environment, Wind velocity theory can explain the phenomenon that
resulting in an increase of magnetic susceptibility. There- the magnetic mineral grains in the ZSP section are larger
fore, magnetic susceptibility of the Chinese loess shows a than those of TLD (Figure 10), but it cannot explain the
general positive proportional relationship to temperature lower average magnetic susceptibility in the ZSP section. It
and moisture of the paleoclimate, which is used as a proxy is believed that magnetic susceptibility is related to local
for paleoprecipitation [14, 15, 62]. The MAP (~512 mm) at precipitation because of the difference of topography (e.g.,
the ZSP section is equivalent to Luochuan (~550 mm) and altitude, slope). Liu et al. [52] performed a good comparison
Xifeng (~560 mm), but the magnetic properties of loess of pedogenic model between the Chinese loess and the Si-
show distinctive differences. This indicates that the rela- berian [54] or Alaskan [52] loess. They conclude that the
tionship between the MAP and pedogenic intensity is much influence of pedogenic processes on magnetic susceptibility
more complex. Different hydrothermal combinations (pre- should not be ignored under either the pedogenic model or
cipitation, temperature, and evaporation) also have a sig- wind velocity model, and they further argue that wind plays
nificant influence on pedogenic process, resulted in differ- an important role in the initial loess deposition, and that the
ent magnetic properties. The East Asian monsoon climate in post-depositional environment becomes a main factor con-
the CLP, with synchronous high temperature and rainy trolling the transformation of iron minerals. Magnetic sus-
weather is conducive to chemical weathering and biological ceptibility enhancements of the Ili loess provide a good test
processes and pedogenesis; it is easy to form new SP mag- of their hypothesis. The magnetic enhancement mechanism
net grains. Since the westerly climate of in the ZSP section in the Ili loess is a mixture of the wind velocity model and
has lower MAP (~2.9 C) and higher annual evaporation the pedogenic model; however, the former is prevailing.
(1261 mm), chemical weathering and biological processes The Ili loess has a similar mechanism to the Xining loess in
are weak, and thus it is an unfavorable location for pe- the Northeastern Tibetan Plateau [58]. However, because
dogenic processes, so magnetic susceptibility is lower. In the relationships between magnetic susceptibility and loess
addition, the granularity of soil-forming parents can also or paleosol are uncertain, explaining the susceptibility en-
affect the intensity of pedogenesis. hancement mechanism is more complicated. Special topog-
Another well-known theory of magnetic enhancements is raphical conditions, deposition environment, and climate
the wind velocity/vigor hypothesis (also named the Alaskan determine the complexity of magnetic susceptibility en-
or Siberian model), where wind strength affects magnetic hancement in the Ili loess. Therefore, more attention should
susceptibility values of loess through physical sorting of be paid to topography, depositional environment, and cli-
magnetic grains [63]. The loess sediments have relatively matic factors, when loess magnetic properties are used for
high magnetic susceptibility and coarser magnetic grains paleoclimate reconstruction.
under strong winds during interglacial periods or near the
dust sources. Mineralogical studies indicate that minerals in
5 Conclusions
the Ili loess are dominated by sharply angular and subangu-
lar grains with clean surfaces [37]. These mineral surface
(1) Rock magnetic studies of the Ili loess sediments show
morphological features suggest that the Ili loess is near
that the total magnetic mineral concentration of the Ili loess
source sediments and experiences a weak biological and
is far lower than that of the Chinese loess, though they have
chemical weathering process. In comparison with the CLP,
430 SONG YouGui, et al. Sci China Earth Sci March (2010) Vol.53 No.3
similar magnetic mineral compositions. The ferrimagnetic 305 308
11 An Z S, Liu T S, Lu Y C, et al. The long-term paleomonsoon varia-
minerals in the Ili loess are magnetite and maghemite, and
tion recorded by the loess-paleosol sequence in Central China. Quat
the antiferromagnetic mineral is hematite; XRD analysis Int, 1990, 7 8: 91 95
also identifies the presence of ilmenite. The ratio of 12 Li J J, Zhu J J, Kang J C, et al. The comparison of Lanzhou loess
profile with Vostok ice core in Antarctica over the last glaciation cy-
maghemite in the Ili loess is lower than that of the Chinese
cle. Sci China Ser B, 1992, 35: 476 487
loess, but the ratios of magnetite and hematite are higher in
13 Han J, L H, Wu N, et al. The magnetic susceptibility of modern
the Ili loess than in the Chinese loess. soils in China and its use for paleoclimate reconstruction. Stud Geo-
(2) The granularity of magnetic minerals in the Ili loess, phys Geod, 1996, 40: 262 275
14 Maher B A, Thompson R. Paleorainfall reconstructions from pe-
which are dominated by pseudo single domain (PSD) and
dogenic magnetic susceptibility variations in the Chinese loess and
multi-domain (MD) grains, is generally much coarser than
paleosols. Quat Res, 1995, 44: 383 391
th
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