1. health and fitness
2. disease
3. cholesterol

51
Chapter 5
An integrated study of the grain-size dependent magnetic mineralogy of the
Chinese loess/paleosol and its environmental significance
(Published in Journal of Geophysical Research)
(108, B9, 2437, doi: 10.1029/2002JB002264, 2003)
Abstract
To investigate the grain-size dependent properties of the magnetic minerals in Chinese
loess/paleosol samples (Touxiangdao, Xining, Gansu province, China), magnetic extracts
were divided into two size fractions by gravitational settling. Based on hysteresis
measurements, thermal demagnetization of low temperature saturation isothermal remanent
magnetization, and non-magnetic studies (SEM and XRD), we identified magnetic phases
both in the grain size fractions of the magnetic extracts and in the less magnetic residues, to
provide more accurate and complete descriptions of all the magnetic components in the bulk
natural samples. The results show that the oxidation degree (non-stoichiometry) of magnetic
minerals is strongly affected by both grain size and the paleoclimatic environment in which
they were deposited and altered. In ascending order of the oxidation degree of our samples,
we find: (1) loess-coarse particles (LC) are multi domain (MD) magnetite with slight
oxidation; (2) paleosol-coarse (PC) particles are also MD magnetite but with a higher
oxidation degree compared to LC; (3) loess-fine (LF) particles are PSD magnetite with a high
oxidation degree; and (4) paleosol-fine (PF) particles are pseudo-single domain (PSD)
maghemite. Single domain (SD) and superparamagnetic (SP) maghemite mainly stay in the
residues. Further thermomagnetic analysis of PF (PSD maghemite) revealed that this natural
maghemite has a Curie temperature identical to that of magnetite, and that the conversion
efficiency of transformation from maghemite to hematite is only about 50% after a 700oC
heating/cooling cycle. These new results identify the sources of multi-component NRM in
Chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls
on the remanence components.
52
1. Introduction
Extremely thick Chinese loess/paleosol sequences of eolian origin have been shown to be
excellent recorders of the variations in both paleomagnetic field (Heller & Evans, 1995; Guo
et al., 1999; Zhu et al., 1999) and paleoclimate (Kukla 1988; Maher and Thompson, 1992;
Banerjee and Hunt, 1993; An and Porter, 1997; Liu et al., 1999). The sequence consists of
periodically alternating less-altered loess horizons and highly weathered paleosol formations.
The two kinds of units correspond to cold/arid and warm/humid climate, respectively, and are
characterized by variations in both non-magnetic (e.g., CaCO3 content, clay composition,
color, etc.) and magnetic properties (Chen et al., 1997).
It is well accepted that the magnetic enhancement (increased susceptibility and remanent
magnetization) in such paleosols is due to the formation of new SP and SD magnetic minerals
by in situ pedogenesis (Zhou et al., 1990; Maher and Thompson; 1991; Heller et al., 1993).
Apparently, the climate has had a significant impact on the magnetic mineralogy of the loess
during the process of soil development.
The mineralogical difference (Heller and Liu, 1986; Han et al., 1991; Maher and
Thompson 1992; Verosub et al., 1993; Eyre and Shaw 1994) and grain-size difference (Zhou
et al., 1990; Maher and Thompson, 1991, 1992) between loess and paleosol have been
extensively studied. The results show that magnetite, maghemite, and hematite are the
dominant magnetic minerals in the Chinese loess/paleosol, and the size distribution of
strongly magnetic grains ranges from below 30 nm (SP) to above 30 µm (MD). Thus the bulk
magnetic properties of the loess/paleosol arise from a mixture of magnetite, maghemite and
hematite with a wide range of grain sizes.
However, to quantitatively reconstruct variations both in paleoclimate and in
paleomagnetic field behavior, such bulk magnetic properties are not sufficient because of
inherent ambiguities due to mixture of different magnetic minerals and grain sizes. We need
to investigate the grain-size dependent mineralogy and rock magnetic properties to provide
sound constraints for interpreting the bulk signals. An accurate separation will allow us to
clearly distinguish the original signal of inherited lithogenic magnetic minerals from the
secondary overprinting of the pedogenically formed minerals by combining information about
grain-size, mineral phases, and remanent magnetization.
53
Generally, two kinds of techniques (chemical extraction and mathematical separation of
bulk signals) serve this purpose. The citrate-bicarbonate-dithionite (CBD) method has been
suggested to separate the finer-grained magnetic particles (generally of pedogenic origin)
from the coarser-grained detrital magnetic particles (Verosub et al., 1993; Hunt et al., 1995;
Sun et al., 1995; van Oorschot and Dekkers, 1999). Recently, van Oorschot et al. (2002)
reported on the acid-ammonium-oxalate/ferrous-iron (AAO-Fe2+) extraction technique as a
new approach to remove very fine-grained magnetite and maghemite from coarse-grained
magnetite. However, these chemical extraction techniques potentially alter the magnetic
properties. Mathematically, Banerjee et al. (1993) retrieved information of extremely fine (<
20 nm) superparamagnetic (SP) magnetite from that of multi-domain (MD) magnetite by
taking advantage of its distinct low-temperature properties. However, there still remain
ambiguities for this method because SP magnetite/maghemite is not the only material that
gradually decreases its low-temperature remanent intensity during warming. Other various
mathematical techniques involve separating coercivity fractions from IRM acquisition curves
or alternating field (AF) demagnetization curves (e.g., Robertson and France, 1994;
Stockhausen, 1998; Kruiver et al., 2001; Heslop et al., 2002). However, these methods are
also indirect means to determine the mineralogy and grainsize distribution, inferred from
coercivity spectra. Low-temperature oxidation can significantly enhance magnetite’s
coercivity (van Velzen and Zijderveld, 1992, 1995; Cui et al., 1994; Kosterov, 2002) resulting
in uncertain interpretation.
To accurately determine the grainsize-dependent magnetic properties of the Chinese loess
sequences, physical separation of the magnetic particles in the natural samples is still
necessary even though aggregation of very fine-grained magnetic material during sample
separation may change its magnetic properties (van Oorschot and Dekkers, 1999). Our
approach in this study uses two methods of physical separation. First, we extract the highly
magnetic material from a couple of the Chinese loess/paleosol from Xining, western China.
Then, based on gravitational setting, we physically divide the magnetic extracts into two
groups with grain size generally less than and greater than ~4-5 µm. The purpose of this size
threshold is to investigate specifically whether finer pseudo-single domain (PSD) maghemite
particles could be formed during and from pedogenesis. We then systematically measured
both magnetic and non-magnetic properties of each group.
54
2. Sampling
Xining Touxiangdao profile (36.6o N, 101.7o E), an 80m thick loess-paleosol sequence, is
located on the 4th terrace of the Huanghe river. The paleosol unit S1 is located between 30.138.0m depth, and consists of three paleosol sub-units (S1S1, S1S2, and S1S3) interbedded
with two loess units (S1L1 and S1L2) (Chen et al. 1999). The upper two weakly developed
paleosol sub-units S1S1 and S1S2 are yellow-brown and the lower mature one (S1S3) is dark
brown to red-brown.
Oriented block samples were collected from a cleaned natural outcrop. Cubic specimens
of 1.5×1.5×1.5cm3 were then cut from the block samples spanning from L2 to S1S3 (Fig. 1) in
the University of Lanzhou, and transported to the University of Minnesota.
∆JTv (10-3Am2kg-1)
S1L2
0.6 0.8
1
1.2
0.10
36.6
0.08
-dJ/dT (10-3Am2kg-1T-1)
Depth (m)
37.0
37.2
37.4
37.6
37.8
0.06
37.54m
38.11 m
0
(a)
0.02
37.87 m
38.2
Stratigraphy
0.04
37.13 m
38.0
L2
Altered L2
S1S3
36.8
5 10 15
50
100
150
200
250
0.00
300
Temperature (K)
χ (m 3/kg 10-7 )
(c)
(b)
Figure 1. (a) Stratigraphy of the TXD section. (b) susceptibility (rectangle) and remanence
drop at Verwey transition (∆JTV) as a function of depth. Two arrows point to the samples used
for magnetic separation. (c) first derivative of LT-SIRM, the magnetic background (dashed
lines) were fitted by third-order polynomials by using data between 50-70 K and 150-300 K.
3. Methods
3.1 Preliminary analysis and sample selection
Low-field susceptibility of bulk samples, χ, was first measured with a Bartington
susceptibility bridge to outline the general stratigraphy of the profile (Fig. 1). A more accurate
55
stratigraphy was later determined based on the new method put forward by Liu et al. (2003a).
Because the upper part of the L2 glacial sediments were overprinted to a depth of about 1-1.5
m below the surface during deposition of the overlying interglacial deposits, the true climatic
boundary between L2 and S1S3 has been obscured by pedogenesis. The method of Liu et al.
(2003a) is based on the observation that the concentration-proxy (intensity drop at the Verwey
transition, ∆JTV, explained in Fig. 1) for coarse (PSD/MD) magnetite clearly distinguishes the
highly altered upper L2 samples from the overlying S1S3 samples. The method involves two
steps. First, we calculate the derivative of low temperature thermal demagnetization of
saturation isothermal magnetization (LT-SIRM) (Fig 1c). Second, polynomial curves fitted to
the data between 50-70 K and 150 –300 K are subtracted from the total derivatives (Fig 1c).
∆JTV is the area under the background-corrected derivative between 70 and 150 K. A general
interpretation of ∆JTV is that it quantifies the concentration of the relatively coarse-grained
stoichiometric magnetite, which serves as a proxy for eolian inputs. Material deposited during
cold/dry periods (e.g. L2 and S1L2) and warm/humid periods (S1S3) are observed to have
∆JTV values greater than and less than 0.9 ×10-3Am2/kg, respectively. Detailed discussion
about the parameter ∆JTV and its geological indications are given in Liu et al. (2003a). Based
on the pedostratigraphy (Fig 1a), two representative samples (a typical weakly-altered loess in
L2 at ~38.1 m and a mature paleosol in S1S3 at ~ 37.2 m) were selected for separation and
analysis.
3.2 Sequential separation and magnetic characterization
Magnetic extracts were obtained in a continuous-loop flow driven by a pump, using a
high-gradient magnet. Each separation ran for about two weeks to sufficiently extract the
magnetic minerals from samples.
Magnetic extracts (not the bulk sample) were then physically divided into two size
fractions by gravitational settling based on Stokes’ Law, which states that particles settle in a
viscous fluid at a rate proportional to the square of their effective diameter and to the density
contrast between particles and fluid. In this study, first, about 0.1 ml of the magnetic extract
slurry was dispersed in ~ 10 ml water, and then dropped into distilled water contained in a
50cm-length-tube. After thirty minutes, the upper 15 cm of supernatants were siphoned off,
and transferred into another container. To avoid further aggregation of these finer particles,
56
the supernatants with water were directly mixed with non-magnetic CaF2 matrix, and dried at
room temperatures.
Based on Stokes’ law, 4-5 µm magnetite/maghemite particles will drop about 15 cm
within 30 minutes. In our study, the coarser-grained fraction undoubtedly may contain some
clumped finer-grained particles. However, this does not affect the overall comparison of finergrained and coarser-grained fraction of magnetic extracts. The fraction that remains in
suspension must consist entirely of fine grains, and the more rapidly settled fraction, though
consisting of a mixture of fine-particle aggregates and coarse individual grains, is enriched in
the coarser grains. The 4-5 µm grainsize threshold estimation is further constrained by
scanning electron microscopy (SEM) (see discussion 5.1) and thermomagnetic analysis (see
discussion 5.2).
For convenience, in the following discussion, the four magnetic extract fractions are
referred to as paleosol coarse (PC), paleosol fine (PF), loess coarse (LC), and loess fine (LF),
respectively. Residues after magnetic extraction were obtained by evaporating the associated
water (about 2 L) in an air-drying oven for three days at a temperature of 80-90oC. The “nonmagnetic” residue (R) and bulk (B) raw sediments of the loess/paleosol are named as LR/PR,
LB/PB, respectively. Hysteresis loops were measured for all of the material with an automated
Princeton vibrating sample magnetometer (VSM). ARM was imparted in an alternating field
of 100 mT with a bias field of 0.05 mT.
Thermal
demagnetization
of
low
temperature
saturation
isothermal
remanent
magnetization (LT-SIRM) acquired at 20K with a 2T applied field was performed by a
Quantum Design susceptometer. The temperature was continuously swept at a constant rate of
2 K/min, and measurements were made when temperature increased in order to reduce the
experimental time. Even though the uncertainties in measuring temperature are on the order of
10-3 K, the temperature change during individual measurement is relatively larger, but
generally less than +0.5 K. Therefore, we refer to the latter as the temperature error for each
measurements. The first derivative of the absolute or relative intensity curve was calculated to
enhance the signature of the Verwey transition produced by magnetite at ~120 K.
To check the existence of hematite in the residues, PR and LR were first imparted an IRM
with a field of 1.2 T (IRM1.2T), then stepwise AF demagnetized with a maximum field up to
200 mT, to destroy the remanence carried by the strongly ferrimagnetic minerals
57
(magnetite/maghemite). The AF demagnetization was followed by a thermal demagnetization
from room temperatures to 700oC. The 1.2 T applied field may not fully saturate the hematite
remanence. However, it is sufficient for determining its Curie temperature.
The contributions of “soft” and “hard” components to the remanence, carried by PR and
LR, were quantified by a simple decomposition of the AF demagnetization spectra of the
IRM1.2T suggested by Dekkers (1990). The observed linear trend of the high-coercivity
fraction (fitted to the data between 70 and 200 mT) were subtracted. The “soft” remanence
components were normalized again and referred as to modified IRM1.2T. An anhysteretic
remanent magnetization (ARM) for the paleosol residue was imparted in a 100 mT alternating
field with a superimposed 50 µT direct bias field using a Dtech D2000 Alternating Field
Demagnetizer. This ARM was further AF demagnetized with a procedure identical to that
used for IRM1.2T.
Thermomagnetic analysis of magnetization in an applied field of 1 T was measured using
a VSM for the paleosol magnetic extracts PF and PC. The samples were heated in air at a rate
of 10oC min-1 and cooled at the same rate.
Mineral phases in the magnetic extracts were identified by X-ray diffraction (XRD) using
a Siemens D5005 X-ray diffractometer with a sealed tube, monochromatic Cu-Kα radiation.
The scan speed was 0.005o 2θ s-1. Because the “zero-position” of different diffractograms
could be slightly different, peaks of quartz (as an internal standard) were selected to determine
the “zero-position”. The raw diffractograms were repositioned before estimating the unit cell
of magnetite/maghemite peaks.
Scanning electron microscopy (SEM) with a JEOL 8900 Electron Probe Microanalyzer
was used to examine the physical size and composition of the two size groups of magnetic
extract. Before taking digital photos, Energy Dispersive Spectra (EDS) were obtained to
check the composition of the particles to make sure they were Fe-bearing oxides.
4. Results
4.1 Efficiencies of magnetic extraction and separation properties
Concentration-dependent properties (Table 1) for the bulk sediments show that the
paleosol (Ms ~ 70×10-3 Am2kg-1) contains twice as much ferrimagnetic material as the loess
(Ms ~ 35×10-3 Am2kg-1). The magnetic enrichment is somewhat larger for the carrier of
58
saturation remanence (Mrs,
paleosol
/Mrs,
loess
~ 2.5), and much higher for susceptibility
(χpaleosol/χloess ~ 5.8).
Table 1. Bulk properties and extract efficiency for the loess and paleosol samples.
Parameter
χ (10 m kg )
ARM(10-4Am2kg-1)
Ms (10-3Am2kg-1)
Mrs (10-3Am2kg-1)
-7
3
-1
Loess
Bulk
2.60
0.53
35.0
4.40
Residue
0.68
0.27
2.50
0.64
Efficiency (%)
74.0
48.0
93.0
86.0
Paleosol
Bulk
15.1
4.7
70.71
11.79
Residue
6.89
4.5
20.0
2.73
Efficiency(%)
55.0
5.0
72.0
77.0
Table 2. Hysteresis properties of the different grain-size extracts and residues from loess and
paleosol
Sample
Bc (mT)
Bcr (mT)
Bcr/Bc
Mrs/Ms
χ/Ms (10-6mA-1)
PF
10.3
22.4
2.18
0.21
PC
11.6
27.4
2.36
0.18
PB
8.49
22.5
2.65
0.17
21.4
PR
4.3
15.8
3.67
0.133
34.5
LF
20.6
55.0
2.67
0.25
LC
14.0
48.8
3.48
0.12
LB
14.8
52.6
3.54
0.14
7.4
LR
24.3
117
4.81
0.20
27.2
Because of the possible loss of magnetic material during extraction and subsequent size
separation, the extraction efficiencies are quantified by the following equation: efficiency (%)
= 100(1-λresidue/λbulk), where λresidue and λbulk are the parameters (susceptibility, ARM, and
SIRM) of the residue and bulk material, respectively. The extraction efficiencies for
susceptibility, ARM, and SIRM are summarized in Table 1.
For the loess sample, the extraction efficiency is 73.9% for susceptibility, 48.1% for
ARM, 92.8% for Ms, and 85.5% for Mrs. The variation results from the different dependencies
of these properties on mineralogy and grain size. The high Ms efficiency (92.8%) for the loess
sample indicates that almost all of the strong ferrimagnetic material has been extracted. The
reduced efficiencies for ARM and χ indicates that SD and SP grains are less effectively
extracted. For the paleosol sample, the efficiencies as measured by susceptibility, ARM, Ms,
and Mrs are 54.4%, 5.0%, 71.7%, and 76.8%, respectively. Compared to the loess, the lower
efficiencies for the paleosol samples reveal that there are more magnetic components left in
the “non-magnetic” residues. Especially, the extremely low ARM efficiency (5%) of the
59
paleosol sample indicates that single domain (SD) particles remain almost entirely in the
paleosol residue (PR).
Using our separation procedure, it is difficult to quantify precisely the mass ratios of the
magnetic extract size fractions. Qualitatively, however, we can say that the fraction remaining
in suspension is much smaller than the fraction that settled down, for both the magnetic
extracts from the loess and those from the paleosol.
4.2 Magnetic properties of separated fractions
Hysteresis parameters of different grain-size extracts and the corresponding raw and
residual material are shown in both Fig. 2 and table 2. Clearly, the hysteretic properties of LC
are nearly identical to those of LB (Fig. 2). In contrast, the finer fraction (LF) has a much
higher Bc (20.6 mT) and Mrs/Ms (0.25), and the “non-magnetic” residue (LR) has even higher
Bc and Bcr. On the whole, the hysteresis parameters of all paleosol samples (PB, PC, and PF)
are comparable with one another except that the paleosol residue (PR) has a very low Bc due
to a large SP content, as explained below.
0.3
0.25
LF
Mrs/Ms
LR
PF
0.2
PC
PB
0.15
LB
PR
0.1
LC
PSD
0.05
MD
1.5
2
2.5
3
3.5
4
4.5
5
Bcr/Bc
Figure 2. Ratio plots of Mrs/Mr versus Bcr/Bc. Large solid dot, small solid dot, and star and
triangle stand for coarse magnetic extract, fine magnetic extract, and raw material, and residue
respectively. The light crosses are the data from Day et al. (1977) for magnetite samples with
different mean grain sizes.
LT-SIRM warming curves for paleosol magnetic extracts and residue are shown in Fig. 3.
Above 100 K, both PB and PC show asymmetric peaks in their derivative curves (Fig 3b) with
60
a maximum peak at 120 K, corresponding to the Verwey transition temperature for
stoichiometric magnetite. The derivative for paleosol residue PR simply increases with
decrease of temperature, forming the SP background contributed by the finest grains (solid
line in Fig. 3b).
PF
PB
PR
-d(J/Jo)/dT
J/Jo
0.8
PC
0.6
0.4
Paleosol
0.005
1
0.004
0.003
0.002
0.2
(a)
(b)
0
0
50
100 150 200 250 300
0.001
80
90 100 110 120 130 140 150
T (K)
T (K)
Figure 3. (a) Normalized LT-SIRM. (b) the first derivative of the data from (a). Samples are
from mature paleosol at the depth of 24.4 m.
LC
LF
LB
LR
0.012
0.01
-d(J/Jo)/dT
J/Jo
0.8
0.6
0.4
0.2
0.006
0.004
(b)
0
50
0.008
0.002
(a)
0
Loess
1
100 150 200 250 300
0
80
90 100 110 120 130 140 150
Temperature (K)
Temperature (K)
Figure 4. (a) Normalized LT-SIRM. (b) the first derivative of the data from (a). Samples are
from typical loess at the depth of 23.5 m.
For LB and LC, sharp decreases in SIRM are observed around 110K (Fig.4 a). In the firstorder derivative curves (Fig. 4b), both LB and LC have two peaks, around 110 and 120 K. LF
and the loess residue (LR) also show a drop in intensity around 110 K, but the decreases are
much lower in amplitude than those of LB and LC (Fig 4).
61
The temperature dependence (the maximum temperature is 700oC) of induced
magnetization M1T for the paleosol extracts PC and PF is shown in Fig. 5. The heating and
cooling curves of PF show essentially reversible patterns with maximum temperatures up to
455oC (Fig. 5a). After heating to 700oC, the heating and cooling curves presented irreversible
patterns. About 15% and 50% of the initial magnetization of PC (Fig. 5b) and PF (Fig. 5c)
were removed after the higher thermal cycle, indicating some inversion (e.g. maghemite
hematite) took place during heating.
200
300
400
1
0.9
0.8
455 oC run
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
o
250 C
350 oC
0.4
(a)
0.9
PC
0.2
(b)
0.1
455 oC
0
0.2
100
200
300
400
0
500
0
Temperature (oC)
SP
PF
se
co
nd
0.5
0.4
rst
0.7
run
0.6
0.5
0.4
run
0.3
0.3
(c)
0.1
0
0
0.1
0
100 200 300 400 500 600 700
o
Temperature ( C)
100
1000
0.3
0.25
PF
0.2
de Boer and Dekkers
(1996)
0.15
??
40
30
0.1
20
0.2
0.2
10
PSD
SD
50
PI (%)
PF
fi
0.6
1
60
0.8
0.7
0.1
70
0.9
PF
0.8
J/J0
0.001 0.01
100 200 300 400 500 600 700
1
0.9
0.1
0
100 200 300 400 500 600 700
Temperature (oC)
1
0.8
0.2
0.4
0
100 200 300 400 500 600 700
1
350 oC run
0.8
J/J0
1
250 oC run
0.8
0.6
0
500
1.2
Mrs/Ms
100
J/J0
0
1.2
PI
10
(d)
0
0.001 0.01
0.05
Mrs/Ms
0.1
1
10
100
0
1000
Grain size (µm)
Figure 5. (a) Thermomagnetic curves for PF heating to 250, 350, 455oC, respectively.
Reversible patterns show PF is stable up to at least 455oC. (b) and (c) thermomagnetic curves
for paleosol magnetic extracts PC and PF (dashed curve for the first run and solid curve for
the second run), respectively. (d) Grain size dependent Mrs/Mr (open circle) and percentage of
maghemite (solid circle) inverted to hematite (PI) during a thermomagnetic run to 650oC (after
de Boer and Dekkers, 1996). The light shade areas indicates the size range of SD particles and
estimated size range of PF. The solid triangle is the results from ? zdemir, 1990.
For the second run of PF, the cooling curve is higher than the heating curve, indicating
that magnetite particles were produced during the second heating run. This magnetic
enhancement is probably caused in part by reduction of hematite and maghemite from the
burning of organic matters trapped in magnetic extracts (Kletetschka and Banerjee, 1995) and
62
in part from the reducing environment caused by the high-temperature cement. During the
first run, the slight intensity enhancement due to reduction was totally masked by the
significant loss of remanence due to transformation from maghemite to hematite. Further
enhancement is shown in the second run because the major inversion was completed after the
first run.
25 125 225 325 425 525 625
Intensity (A/m),
Normlaize intensity
Paleosol residue
(4.974 g)
1.4
1.2
0.8
IRM1.2T
Normalized IRM1.2T
1.0
0.6
Modified IRM1.2T
0.8
ARM
0.4
0.6
0.4
0.2
Normalized Intensity
1.0
1.6
0.2
0.0
0.0
0
50
100
150
200
T (degree)
AF field (mT)
25 125 225 325 425 525 625
1.0
Loess residue
(3.430 g)
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
Normalized intensity
Intensity (A/m),
Normlaize intensity
1.0
0.0
0
50
100
150
200
T (degree)
AF field (mT)
Figure 6. Combining studies of AF and thermal demagnetization of IRM1.2T for loess/paleosol
residue after magnetic extraction. Open circles stand for the IRM1.2T normalized by initial
value. Dashed lines were fitted by the values at fields higher than 100 mT and are projected to
the virgin state. The dashed lines are used to stand for the behavior of high coercivity material
(hematite). After extracting the dashed line from the AF curve, the residual curves were
normalized by the value at zero field again. The new curve was named as modified IRM1.2T, in
which effects of hematite had been removed. The shadows in the right part of figures show
possible inversion temperature and Curie points of maghemite and magnetite. Clearly, the
main Curie point is at about 670o. This demonstrates that hematite is the main phase that
controls the high coercivity behavior.
SEM studies (Fig. 7) showed that both LC and PC contain abundant grains with diameters
of several tens of µm (MD). There are also finer grains that may have settled rapidly due to
clumping. The average size of LC is larger than that of PC. Consistent with the theoretical
63
calculation by Stokes’ law, the average grain size of LF and PF is generally less than 5 µm
(Fig. 7). We note that Fig. 7 represents the upper limits of the grainsize of each separated
fraction.
Figure 7. SEM studies for different grain size of magnetic extract from the loess/paleosol. The
scale and grain-size are marked on each of the sub figure. Each row of the sub figures stands
for the same material.
Sequential demagnetization of IRM1.2T for both the paleosol and the loess residues are
shown in Fig. 6. Both PR and LR exhibit a bimodal AF demagnetization spectrum. The soft
remanent fraction (low coercivity) can be easily removed by a 50 mT field. In contrast, the
64
hard components (high coercivity) decay slowly, with approximately linear trends between 70
and 200 mT. Based on these trends, the extrapolated hard components before AF
demagnetization for PR and LR are ~30% and ~80% of the initial IRM, respectively. For PR,
the AF demagnetization spectrum of modified SIRM1.2T (after subtracting the hard
components) almost replicates that of ARM. The thermal demagnetization spectra of the hard
components after 200 mT AF demagnetization for both PR and LR show a consistent
unblocking temperature of ~ 670oC, indicating that hematite is the main carrier of the hard
component for both PR and LR.
4.3 XRD
MH
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30
2-Theta (degree)
Figure 8. XRD spectrum for different grain size of magnetic extract from loess/paleosol. The
label of M, MH, MV, H, and Q stand for magnetite, maghemite, muscovite, hematite, quartz,
respectively. The peak around 30.8o probably relates to titanium iron oxide.
65
XRD analysis was used to characterize the mineralogy of samples. The mineralogy
identification was performed based on the data between 15 and 90 degrees (2θ), and the
results are presented in Fig 8. The 33.15o peak corresponding to hematite exists in all the
material. At 35.45o (hkl, 311) and 43.10o (hkl, 400), two peaks are caused by magnetite. Just
to the right of the magnetite peaks, two characteristic peaks for maghemite appeared at 35.63o
(hkl, 311) and 43.28o (hkl, 400). In addition, peaks for muscovite and quartz show that they
are also present in the extracts. For LC, there is a magnetite peak at 35.45o, with a right
shoulder suggesting that the magnetite particles are slightly oxidized. LF has a peak at 43.28o
corresponding to maghemite. For PF, between 35o and 36o, the maghemite peak (311) at
35.63o is much stronger than that the magnetite peak (311) at 35.45o. We believe that this
indicates that maghemite instead of magnetite is the main magnetic mineral phase in PF. PC
had two peaks with nearly equal amplitudes at 35.45o and 35.63o.
The unit cells (D) for LC, LF, PC, PF calculated from the XRD spectrum were
8.399±0.010 (hkl, 311, 400, 531, 533), 8.360±0.008 (hkl, 311, 400, 440, 533), 8.393±0.010
(hkl, 220, 311, 222, 511, 440, 531), and 8.339±0.010 (hkl, 311, 400, 440, 642), respectively.
The unit cell of (titano-) magnetite is mainly determined by its ulv?spinel content and
oxidation degree (Readman and O’Reilly, 1972; Nishitani and Kono, 1983; Zhou et al., 1999).
Moskowitz et al. (1998) showed that the remanence transitions are suppressed to temperatures
below 80 K even for low-Ti substitution (e.g., TM05). The 110-120 K Verwey transition of
the Chinese loess/paleosol samples therefore indicates that titanium substitution is
insignificant. Therefore, in this study, we used Readman and O'Reilly (1972) reference curve
to correlate the unit cell to the corresponding oxidation parameter z because the subsequent
studies (e.g., Nishitani and Kono, 1983; Zhou et al., 1999) focused more on titanomagnetite.
The resolution of the nominal z values calibrated from the D-z curves is highly dependent
on the curve’s slope. The error ∆z due to errors in the determined value of D (∆D) is
∆z=∆D/(dD/dz). Especially for the lower z values corresponding to nearly stoichiometric
magnetite, where dD/dz is almost zero, slight errors in D could be translated into very large
errors in estimated value of z. With these caveats in mind, we use these z estimates as nominal
values for purpose of discussion: LC (0-0.05), PC (0.27), LF (0.87), and PF (1.0). More subtle
transitions from magnetite to maghemite and coexistence of these two minerals for different
size fractions are indicated in Fig. 8.
66
5. Discussion
5.1 Magnetic mineralogy and grain-size
Under our experimental procedure, it is extremely difficult to obtain an accurate
estimation of the mass of each size fraction of the magnetic extracts. Therefore, we
qualitatively evaluate the relative abundance of each in bulk sediments by comparing its rock
magnetic properties with the bulk properties.
In this section, based on XRD and magnetic measurements, we first discuss the magnetic
mineralogy of the two size fractions of the magnetic extracts and residues. Second,
constrained by SEM, Day-plot, characteristic magnetic properties, and thermomagnetic
analysis (refer to section 5.2), we link the magnetic phases to the corresponding grainsizes to
provide a clearer picture of the magnetic components in the Chinese loess/paleosol. Finally,
the geological implications for the loess/paleosol natural remanent magnetization (NRM)
carrier and model of low-temperature oxidation are discussed in some detail.
XRD revealed that the oxidation degree in an ascending order is LC, PC, LF and PF. The
fine fraction of paleosol (PF) is characterized by a characteristic maghemite peak at 35.63o
(hkl 311) in the XRD pattern. The highly depressed Verwey transition at ~ 110K confirms that
PC is more oxidized than LC (Kosterov, 2002) (Fig. 3). The highest oxidation degree of PF is
further supported by the following facts: 1) brown color, which is typical for maghemite; and
2) no Verwey transition (Fig. 3).
The AF demagnetization to 200 mT followed by thermal demagnetization of the loess
residue IRM1.2T shows unblocking up to ~670oC, the Curie temperature (Tc) of hematite (Fig.
6). LT-SIRM curves for the loess residue LR show a weak Verwey transition. Meanwhile, LR
has a Bcr value larger than 100 mT, which would also seem to require a significant
contribution from hematite or other antiferromagnetic minerals. After subtracting the linear
trend fitted to the data between 100 and 200 mT for the AF demagnetization curves, for the
paleosol residue, the AF demagnetization spectrum of the normalized IRM1.2T is almost
identical to that of ARM, indicating that the soft-fraction of the remanence (lower coercivity)
is carried by SD particles. Further, the AF demagnetization spectrum of the normalized
IRM1.2T for the loess residue almost replicates that of the paleosol residue. Therefore, we
67
conclude that the main mineral phase in LR is hematite, but it also contains a small percentage
of SD/PSD magnetite/maghemite.
Therefore, LC and PC are dominated by less-oxidized and more-oxidized magnetite,
respectively. PF is an extremely oxidized magnetic phase (maghemite). LF has an oxidation
degree between PC and PF. Hematite has minor effects on the magnetic properties of the
extracts, but has stronger effects on the residue properties.
The estimation of the grainsizes was constrained by multiple parameters. First, SEM
observations (Fig. 7) provide an upper size limit of each size fraction. In ascending order, the
largest observed grainsizes of these fractions are LC (42.0 µm), PC (16.0 µm), LF (9.0 µm),
and PF (2.6 µm). This order matches exactly the order of the corresponding oxidation degree,
indicating that the oxidation degree is grainsize dependent. A smaller grainsize corresponds to
higher oxidation degree.
Second, the existence and contribution of SP grains are estimated by hysteretic
parameters. SP grains can result in a lower value of Bc, but have no effects on Bcr, which
depends only on remanence. The high values of Bcr/Bc for the paleosol residue (Fig. 2 and
Table 2) probably reflect the existence of both SP and stable SD, which also account for much
of the bulk paleosol susceptibility and ARM.
The indicator of superparamagnetic fraction χ/Ms for LB (~ 7.4 × 10-6 m/A) is typical for
coarser-grained (>SP) ferrimagnetic minerals (magnetite or maghemite). In contrast, LR, PB
and PR all have ratios at least three times larger, indicating significant contribution of SP
particles.
Unlike the loess residue, the paleosol residue PR has a quite low Bc (4 mT) suggesting a
large SP fraction. These SP magnetic grains formed during the pedogenic processes (Zhou et
al., 1990).
The Day plot (Day et al., 1977) is another tool for estimating the average grain-size of
magnetic minerals in a whole sample. However, the presence of multiple mineral phases and
grain size populations in the sample as well as their non-stoichiometry can lead to serious
ambiguities (Dunlop 2002a, b). Discrete size fractions of magnetite particles plot along the
trend shown in Fig. 2 by light crosses (Day et al., 1977). Grain size mixtures (Tauxe et al.,
1996; Carter-Stiglitz et al., 2001; Dunlop 2002a, b) and mineral mixtures (Jackson et al.,
1990; Roberts et al., 1995) generally plot above and to the right of the trend. Determining the
68
components of a mixture from a Day plot can thus be quite difficult and indirect (Fukuma and
Torii, 1998). To allow a more unique interpretation, our present study physically separates the
raw loess/paleosol material into different parts (Table 2).
Fig. 2 shows that bulk samples of loess (LB) and paleosol (PB) are located in the PSD
region, and the overall grain-size of the PB is finer than that of LB. The bulk loess properties
are dominated by the material recovered in the coarse extract, because both plot near the
magnetic grain-size boundary between PSD and MD regions (Fig. 2). Thus the coarse extracts
represent the characteristic magnetic material in bulk loess. For the paleosol, all fractions have
similar characteristics except for the residue PR, which plots in an apparently coarser grainsize region than the others, near LB and LC. It is well known that abundant SP grains can
form during pedogenesis, and contribute to the susceptibility enhancements of paleosols.
These extremely fine SP particles can not be extracted by our method and they stay in the
residue, resulting in a lower extraction efficiency based on susceptibility (54.5%) compared to
that of loess. Further, this low susceptibility efficiency for paleosol confirms that these SP
particles are the main contributor of enhancement of susceptibility.
In summary, the oxidation degree in an ascending order for the magnetic extracts is: 1)
LC, consisting mainly of slightly oxidized MD magnetite; 2) PC, which is also mainly MD
magnetite, but with a higher oxidation degree, and a smaller average grain size than LC; 3)
LF, which consists of heavily oxidized magnetites with a small nearly-stoichiometric
magnetite core and a maghemite shell; 4) PF, which is dominated by PSD maghemite. LR is
composed mainly of hematite and partially of small PSD maghemite. PR is a little more
complicated than LR. It mainly consists of SP and SD maghemite grains formed during the
pedogenic processes.
For paleosol material, the average grain size from the Day plot of the bulk sample PB is
consistent with both fine and coarse fraction from the SEM study of the corresponding
magnetic extraction. This indicates that the PSD behavior of PB is not caused by a mixture of
SD and MD magnetic particles because the SD particles mainly stay in the residue PR. In
contrast, the SP particles had significantly shifted the plots of PB toward the MD region by
decreasing Bc but without effecting Bcr. Therefore, the true average grain size of the PB ought
to be smaller than that suggested by its position on the Day plot. For loess material, the coarse
69
magnetic fraction (LC) dominates the behavior of LB. So the PSD behavior of the loess bulk
sample (LB) indicated by the Day plot is also reasonable.
One more point about the magnetic minerals in the loess samples is that they are not
stoichiometric. It has been well documented that the increase of non-stoichiometry in MD
(especially just above PSD size) magnetic particles can result in a PSD behavior. So the grain
size suggested by the Day plot is in terms of magnetically effective size instead of the true
physical size.
This study confirmed that the magnetite particles in both loess and paleosol samples are
non-stoichiometric. Meanwhile, the partially oxidized coarse-grained magnetite particles (PC
and LC) have higher remanence coercivity than the fine-grained single-phase maghemite
(PF). This indicates that the partially oxidized magnetite has a high potential for recording
stable detrital remanent magnetization (DRM). The maghemite (SD and small PSD) particles
are strongly linked to pedogenesis, and these particles probably carry a chemical remanence
magnetization (CRM) during its formation by pedogenesis. Further detailed studies on the
remanence character of these different magnetic particles will bear on our understanding of
the loess NRM acquisition history. However, more sensitive parameters are needed to
separate the pedogenically-produced maghemite particles from the inherited maghemite of
eolian origin, including the maghemite shell covering the coarse-grained magnetite caused by
the low temperature oxidation.
This study also has important implications for the low-temperature oxidation model for
the magnetite particles in natural samples. The bulk and the coarse extract fraction of the
paleosol samples contain a sharper Verwey transition, i.e. more surface area centered on 120
K, than the loess sample, which has a broad peak (or a double peak) from 105-120 K. Based
on a comparison with Tv data for uniformly oxidized synthetic samples (?zdemi r et al., 1993;
Honig, 1995; Moskowitz et al., 1998), the loess Verwey transition curves seem to contain
signal more from 'altered magnetite' than the paleosol. This appears to conflict with the XRD
analysis, which indicated that PC has a higher oxidation degree than LC.
We tentatively interpret this paradox in the following way. The synthetic samples used in
these previous studies were homogeneously oxidized and uniform. However, the lowtemperature oxidized products of initially stoichiometric magnetite in natural samples may
consist of an inner magnetite core, covered by a maghemitized rim generally occurring mainly
70
at the crystal surface or in fissures (van Velzen and Zijderveld, 1992, 1995; Cui et al., 1994;
Kosterov, 2002).
One dramatic effect of the maghemite rim on the whole magnetic particle is to increase its
coercivity due to the enhanced stresses induced by a large gradient in the oxidation degree
from the maghemitized rim to the magnetite core (van Velzen and Zijderveld, 1995). The
stress is related to the difference in lattice constants of the maghemite rim and magnetite core.
van Velzen and Zijderveld (1995) further proposed that the coercivity of the oxidized
magnetite is proportional to this oxidation gradient. We suggest that as the oxidation front
penetrates more deeply into the grain, the oxidation gradient is reduced, and the sample’s
coercivity will also decrease.
LC is only slightly oxidized, as indicated by the stoichiometric-like unit cell. This slightly
oxidized shell will shift Tv to a lower temperature, and the magnetite core contributes to the
stoichiometric-like Tv peak. Compared to LC, the lower coercivity of PC confirms that it has
a lower oxidation gradient caused by a higher oxidation. The nearly stoichiometric Tv
(~120K) peak of PC (Fig. 3b) is most probably caused by the reduced in size magnetite core,
but the shell has been oxidized to a higher degree. Its total contribution to Tv is small, and the
SP and maghemite background are simultaneously enhanced. This would further suggest that
the reduced (in size) magnetite core in PC may behave independently from the maghemite rim
for the low-temperature thermal demagnetization of SIRM curves.
Therefore, for the natural samples, the highly-suppressed but near stoichiometric Verwey
transition (~120K), masked by a strong maghemite and SP background, appear to indicate a
high degree of low-temperature oxidation. This weak but stoichiometric-like Verwey
transition is caused by the highly reduced (in size) magnetite-core through low-temperature
oxidation, which behaves independently from the maghemite-rim. In contrast, the broad
Verwey transition peak and relatively large intensity drop for the loess indicates a lower lowtemperature oxidation degree. This can be further demonstrated by its coercivity. The paleosol
has a lower coercivity than the loess. Based on this model, the absence of Verwey transition
for PF strongly suggests that it is a homogeneously maghemite phase without a magnetitecore.
5.2 High temperature behavior of natural maghemite in Chinese loess
71
Magnetite and maghemite are two common strongly ferrimagnetic minerals. They are the
main natural remanent magnetization (NRM) carriers and are also the main contributors to
magnetic susceptibility in the Chinese loess/paleosols (Evans and Heller, 1994; Eyre and
Shaw, 1994; Sun et al., 1995). Better understanding of the origin of these two magnetic
minerals is essential for precise retrieval of the paleoclimatic information from Chinese loess.
However, compared to magnetite, there remains a lack of detailed research on the magnetic
properties of the natural maghemite in Chinese loess.
Based on the thermomagnetic behavior of synthetic maghemite samples, Liu et al. (1999)
concluded that maghemite in the loess/paleosol sequences originated mainly from eolian
sources, with some additional maghemite generated during pedogenesis. This conclusion
rested on the observation that the synthetic maghemite used in their studies fully inverted to
hematite at temperatures above 500oC. However, the magnetic properties of maghemite are
highly dependent on the presence of impurities, method of preparation and morphology of the
particles. In contrast to Liu et al. (1999)’s reports, synthetic samples used by ? zdemir (1990)
showed that only 30% of the initial Ms was destroyed by a thermal cycle from room
temperature to 700oC indicating only some inversion from maghemite to hematite occurred
during heating. The PSD natural maghemite particles from the Robe River mining district
(Australia) showed similar thermal behavior (de Boer and Dekkers, 1996). These earlier
studies indicate that the exact correspondence between the synthetic and natural maghemite in
Chinese loess/paleosol proposed by Liu et al. (1999) needs further investigation.
Constrained by low temperature measurements and XRD analysis, we believe that our
finer fraction of paleosol (PF) is mainly composed of PSD maghemite. Four aspects
associated with natural maghemite’s high temperature properties, including its Curie
temperature, thermal stability, grain size and additional inflection of thermomagnetic curves at
350oC, will be addressed now.
Fig. 5 shows that PF has a Curie temperature of 575oC, which is about 10oC lower than
that of partially oxidized magnetite (PC), and much lower than the Tc (645oC) for a synthetic
maghemite (a cubic spinel with tetragonal superstructure) measured by ? zdemir (1990). In
addition, the Tc of PF is also lower than that (610oC) of natural maghemite reported by de
Boer and Dekkers (1996).
72
The commonly present larger strain and higher amount of imperfections for natural
maghemite, especially in the pedogenic environment, can significantly decrease its Tc (de
Boer and Dekkers, 1996). In addition, substitution of Fe by Al can also lower the Tc of
maghemite by reducing the exchange interactions (da Costa et al., 1995). A recent study by de
Boer and Dekkers (2001) showed that maghemite particles inverted from hematite during
heating have a Curie temperature 470-475oC. They suggested that the lower Tc of this unusual
type of maghemite is related to vacancy ordering over the magnetic sublattices. Thus, the
relatively lower Curie temperature (575oC) of PF suggests that this maghemite may have been
significantly affected by one or more of those factors. Clearly, the difference of Tc between
the maghemite (PF) and the partially oxidized magnetite (PC) is not sensitive enough to be a
routine tool suggested by the early studies (Zhu et al., 1995) for distinguishing these two
ferrimagnetic minerals in the Chinese loess/paleosol sequences.
It has been well documented that maghemite can lose its initial magnetic intensity during
thermal treatments due to partial transformation from maghemite to hematite. In this study, we
are interested in both the corresponding inversion temperature and inversion efficiency
because these two parameters are necessary for quantifying the concentration of maghemite in
natural samples. The exact transformation temperature from maghemite to hematite is not
fixed, and it can vary between 250-900oC (Kachi et al., 1963; Sato et al., 1967). ? zdemir
(1990) reported that the transformation of synthetic SD maghemite to hematite take places at
725oC. Based on the stepwise thermal demagnetization of IRM1.2T and low-field bulk
susceptibility measurements, de Boer and Dekkers (1996) found the majority of inversion of
PSD and MD maghemite to hematite starts after the 600oC run, and is not fully complete even
after the 800oC run. Our measurements show that the thermomagnetic curves are reversible at
least up to 455oC (Fig. 5a), indicating the natural maghemite in our samples is moderately
thermally stable. Our recent study (Liu et al., 2003b) also revealed that the inversion occurs
just above 550oC.
de Boer and Dekkers (1996) defined the inversion efficiency PI as the fractional decrease
of Mrs after heating to 600oC. and found that PI increased from 0.13 for large MD (150-250
µm) to 0.16 for relatively small grains (< 5 µm). The ratio Mrs/Ms has a similar grain size
dependence (Fig. 5d). The SD maghemite of ? zdemir (1990) does not follow this trend due to
its acicular shape and tetragonal superstructure, which is not common in the natural samples.
73
Therefore, the natural samples used by de Boer and Dekkers (1996) are more analogous to PF
even though PF may have a higher content of imperfections or impurities than the former.
In this study, the corresponding Mrs/Ms and the inversion efficiency (PI) of PF are 0.21 and
0.5, respectively. These values are higher than those of the smallest size grain (2.5 µm) used
by de Boer and Dekkers (1996). Even though the exact relationships between maghemite’s
grain size and its PI and the other magnetic parameters are not available due to the scarcity of
systematic measurements especially for grains smaller than 2 µm, we believe that the general
trend of PI and Mrs/Ms with decrease of maghemite’s grain size is valid. Assuming that de
Boer and Dekkers’s data can be extended to SD maghemite particles, then the average grain
size of PF is about 200 nm, which is just above the threshold between SD and PSD particles.
Grain size can also be directly estimated by SEM observation, but is highly dependent on the
selection of particles and spatial resolution of the technique. Fig. 7 shows that the grain sizes
of four typical PF particles are between 0.6 to 2.6 µm. The latter can only be considered as the
maximum size of PF. In contrast, the smaller maghemite particles (e.g., just larger than 100
nm) can not be clearly characterized by direct SEM observation. To be safe, we conclude that
the size range of PF is between 100 nm and 3 µm. The lower size limit rests on the fact that
SD particles mainly stay in the paleosol residue (PR).
Another interesting characteristic of the thermomagnetic curves from the earlier studies is
the additional inflection occurring at 350oC (de Boer and Dekkers, 1996; Liu et al., 1999).
This probably indicates the lowest initial inversion temperature from maghemite to hematite.
However, Fig. 5A shows that the inflection for PF occurs at 250oC, whereas, PC has a 350oC
inflection. A reasonable interpretation is that the maghemite particles contained in PC were
formed on the surface of coarser magnetite (larger PSD/MD) by low temperature oxidation.
The grain size of these maghemite particles is generally SP/SD (Dunlop and ?z demir, 1997).
Therefore, the 350oC inflection relates more to PC rather than the PSD maghemite contained
in PF.
6. Conclusion
The gravitational separation of the magnetic extracts is an effective method to investigate
the grain-size dependent magnetic properties for loess/paleosol. This method can clarify
74
inherent ambiguities of bulk magnetic parameters. This, then, makes it feasible to assign
appropriate grain-size to the corresponding magnetic composition. In turn, we can provide a
more accurate and detailed description of the magnetic minerals in the loess deposits. Based
on the discussion above, the main conclusions are:
1) The magnetic extracts are not pure magnetic minerals. All of them still contain nonmagnetic quartz and muscovite. Weakly magnetic hematite is another common phase found in
extracts, but its concentration is masked by the signal of magnetite and maghemite. It is most
evident in the loess residue after most of the ferrimagnetic material was removed by
extraction.
2) In the following conclusion, we refer the name of each group to its main magnetic
mineral. Quartz and muscovite are not considered. Among the extracts, the coarser fraction of
loess (LC) and the finer fraction of paleosol (PF) have the lowest and highest oxidation
degree, respectively. LC is slightly oxidized MD magnetite and PF is single-phase PSD
maghemite. The coarser fraction of paleosol (PC) is similar to LC, but has a higher degree of
oxidation. Different from the PF, the finer fraction of loess (LF) is a highly oxidized
magnetite with a small magnetite core in the center surrounding by a maghemite cell. The
pedogenesis-related SP/SD grains mainly stay in the paleosol residue (PR).
3) The magnetic properties of the bulk samples are dominated by genuine PSD particles
instead of a mixture of SD and MD grains. The PSD maghemite (PF) in paleosol has a Curie
temperature similar to that of magnetite. Further, only 30% of maghemite was transformed to
hematite after a heating cycle up to 700 oC. We believe that the PSD maghemite in paleosol
was produced from pedogenesis because it is abundant only in paleosols.
The TXD section is ideal for constructing the high-resolution paleoclimatic variations at
the western part of the loess plateau because of the high sedimentation rate and relatively low
pedogenesis. The exact oxidation degree of magnetite in the loess/paleosols may vary in
different profiles because of the different pedogenic degree. Nevertheless, we believe that the
order of the oxidation degree for the different separated magnetic fractions revealed by this
study is general and can be extended to the other profiles.
75
Chapter 6
Determination of magnetic carriers of the characteristic remanent
magnetization of Chinese loess by low-temperature demagnetization
(Published in Earth and Planetary Science Letters)
(216, 1-2, 175-186, 2003)
Abstract
The Chinese loess/paleosol sequences can provide excellent paleomagnetic records for
detailed studies on both magnetostratigraphy and secular variations of the Earth’s magnetic
field. However, the nature of the loess/paleosol Characteristic Remanent Magnetization
(ChRM), isolated by thermal demagnetization from the Natural Remanent Magnetization
(NRM), still remains in debate. In this study, we directly measure the thermal, alternating
field (AF), and low temperature demagnetization (LTD, cooling/warming cycle of remanence
in zero field between 300 and 50 K) spectra of ChRM of the loess samples at Touxiangdao,
Xining, China. The results show that coarse-grained, pseudo-single domain (PSD)/ multidomain (MD) magnetite and PSD maghemite particles are the main magnetic carriers of the
loess and paleosol ChRMs, respectively. By comparing the ChRM characteristics across a
loess/paleosol transition zone, we have found that during pedogenesis, the thermally-separable
ChRM (original detrital remanent magnetization, DRM), carried by the PSD/MD magnetite in
loess is gradually overprinted by the chemical remanent magnetization (CRM) carried by PSD
maghemite. Even moderate pedogenesis (e.g., for the pedogenically-altered loess) can
strongly affect the loess ChRM. Thus more attention has to be focused on the paleosol ChRM
separated solely by thermal demagnetization especially when constructing a continuous
paleomagnetic record covering both loess and paleosol units.
Key words: Chinese loess, Characteristic remanent magnetization, Magnetic carrier, Low
temperature demagnetization, Paleomagnetism
1. Introduction
76
Over the last 20 years,
paleomagnetic records have been successfully used to construct the
chronological framework of the Chinese loess sequences (Heller and Liu, 1982, 1984; Heslop
et al., 2000), and feature detailed variations in both paleomagnetic directions and intensities
during geomagnetic polarity reversals as well as excursions (Zhu et al., 1994a, 1994b, 1998a;
1998b; Fang et al., 1997; Guo et al., 1999; Pan et al., 2001). Systematic patterns of the major
geomagnetic chrons and subchrons unambiguously suggest that Chinese loess sequences
record at least these major geomagnetic events even though they may have been locally
affected by post-depositional alterations (Zhou and Shackleton, 1999).
The initial purpose of the loess natural remanent magnetization (NRM) studies was to
determine the stratigraphic location of the main geomagnetic reversals (e.g., MatuyamaBrunhes boundary as well as Jaramillo subchron), and the first-order chronological framework
(Li et al., 1974) of the susceptibility profiles for a further global correlation with the marine
isotope (Heller & Liu, 1984; Kukla et al., 1988) and ice core records (Chen et al., 1997). In
these early studies, alternating field (AF) demagnetization was first used to isolate the
Characteristic Remanent Magnetization (ChRM) from NRM. Afterwards, Heller and Liu
(1984) showed that a better component separation was achieved by progressive thermal
demagnetization, which has become the standard approach for the loess NRM cleaning.
It was Heller and Liu (1984) who first systematically investigated the remanence carriers
and the corresponding remanence origin of the Chinese loess. Their results showed that
viscous remanent magnetization (VRM) and ChRM can be successfully separated after a 250300oC thermal treatment, and this was further verified by subsequent work (Zhu et al., 1994a;
Fang et al, 1997). Heller and Liu (1984) argued that magnetite is so “soft” (low coercivity)
that it can record only VRM, which can be effectively removed by thermal demagnetization.
Therefore, they suggested that the most important ChRM carrier could be hematite. A
contrary view proposed by Zhu et al.(1998b), based on a comprehensive investigation of the
rock magnetic properties, is that pseudo-single domain magnetite accounts for the main
portion of the ChRM.
Despite these attempts, identification of the magnetic carrier of the loess ChRM is still
unresolved because of the difficulty in linking bulk rock magnetic parameters to specific
remanence carriers. Hysteresis parameters and in-field thermomagnetic analysis reflect
contributions of all magnetic minerals in the bulk samples. In contrast, the loess ChRM,
77
generally less than 20% of the initial NRM intensity, is carried only by a fraction of the
magnetic minerals (Heller and Liu, 1984).
To avoid such ambiguities, we directly measure the ChRM properties of the Chinese
loess/paleosols by combining multiple demagnetization methods (thermal, AF and low
temperature demagnetization). A similar strategy has been used by van Velzen and
Zijderveld[15,16] to investigate the remanence carrier of marine marls (van Velzen and
Zijderveld, 1992, 1995). However, in their studies, they combined only AF and thermal
demagnetization methods.
Samples were collected from the Xining Touxiangdao (TXD) loess profile, western China,
which is characterized by a high deposition rate, and relatively weak pedogenesis of the loess
(Hunt et al., 1995). In this study, we focus on the variations in remanence carriers across the
boundary between the loess unit L2 (Marine isotope stage, MIS 6) and the overlying paleosol
sub-unit S1S3 (MIS 5e). We also focus on the effects of pedogenesis on the NRM recording
history. Even though the goal of this study is to interpret the carriers of the ChRM in the
Chinese loess, we believe that our method can be extended to other materials (e.g.,
marine/lake sediments, and igneous rocks).
2. Sampling and experiments
The Chinese loess pedostratigraphy has been well studied by using lithologic and
pedogenic criteria and magnetic susceptibility. The standard stratigraphic nomenclatures L
and S are respectively assigned for the loess and paleosol units, (Kukla and An, 1989). An age
model for the last glacial (L1) /interglacial (S1) cycle is well determined by multiple methods,
including 14C (Head et al., 1989), thermoluminescence (Lu et al., 1988), 10Be dating (Beer et
al., 1993), and correlation of grainsize and susceptibility with marine oxygen-isotope records
(Kukla et al., 1988). Generally, the paleosol units have reddish color and higher susceptibility
than the yellowish lower-susceptibility loess units. The contrasts between the loess and
paleosol units reflect the effects of in-situ pedogenesis (Verosub et al., 1993; Fine et al.,
1995), which is linked to variations in the Asian summer monsoons.
The TXD profile (36.6o N, 101.7o E) is located on the 4th terrace of the Huanghe River
(for details see Chen et al. 1999). Stratigraphically, the paleosol unit S1 (Marine isotope stage,
78
MIS 5) consists of three paleosol subunits (S1S1/MIS5a, S1S2/MIS5c, and S1S3/MIS5e)
interbedded with two loess units (S1L1/MIS5b and S1L2/MIS5d). Oriented block samples
were collected from a cleaned natural outcrop. Cubic specimens of 1.5×1.5×1.5cm3 were cut
from these block samples in the University of Lanzhou, and transported to the University of
Minnesota.
0.6 0.8
S1L2
S1L2
∆JTv (10-3Am2kg-1)
1
1.2
0.10
36.6
37.2
37.4
37.6
∆JTv
38.0
Stratigraphy
a)
b)
0.04
37.13 m
37.54m
0.02
38.11 m
38.2
5
0.06
37.87 m
χ
0
0.08
-dJ/dT (10-3Am2kg-1T-1)
Depth (m)
S1S3
37.0
37.8
L2
L2
Altered L2
S1S3
36.8
50
10 15
100
150
200
250
0.00
300
Temperature (K)
χ (m3/kg 10 -7)
c)
d)
Figure 1. Plots of susceptibility, ∆JTV, and stratigraphy of the Xining profile. Arrows indicate
representative samples for low temperature experiments. a) observed stratigraphy by color and
susceptibility; b) interpreted stratigraphy by the new parameter ∆JTV; c) depth plots of
susceptibility and ∆JTV; d) the first-derivatives of LTD-SIRM for four selected samples.
Dashed lines in d) are fitted polynomials for backgrounds.
Mass normalized low-field susceptibility (χ) of bulk samples was measured with a
Bartington susceptibility bridge. Detailed stratigraphic boundaries (seen in Fig. 1a, 1b) were
determined by χ, visual field observation, and a new and more precise parameter (named
∆JTV, in Fig. 1 proposed by Liu et al., 2003) to quantify the abundance of stoichiometric
coarse-grained magnetites.
During the development of soil, pedogenesis gradually progresses down to the underlying
glacial loess deposits, and the upper part of loess unit was altered to soil, therefore obscuring
the original climatic boundary between L2 and S1S3 (Fig. 1c). The parameter ∆JTV was
79
designed to distinguish the pedogenically-altered glacial material from the overlying
authentically interglacial deposits. To obtain ∆JTV, first, we calculated the first order
derivative of low temperature thermal demagnetization of saturation isothermal magnetization
(LT-SIRM) (Fig 1d). Second, polynomial curves, fitted to the data between 50-70 K and 150300 K, were subtracted from the derivatives (dashed lines in Fig 1d). ∆JTV is the area under
the background-corrected derivative between 70 and 150 K, and it equals the absolute drop of
intensity directly associated with the Verwey transition. The differentiation, detrending and
re-integration separate out remanence loss due to unblocking at the superparamagnetic (SP)
/single-domain (SD) boundary, and isolate the loss due to the monoclinic-cubic phase
transition. ∆JTV is a highly sensitive indicator of the abundance of stoichiometric coarsegrained detrital magnetite, which marks the paleoclimatic boundary between the glacial
periods and interglacial periods (Fig. 1c) (Liu et al., 2003). Even though pedogenesis may
have altered the coarser-grained aeolian magnetites, the sharp drops in ∆JTV (e.g., at 37.3 m)
are dominantly a depositional signal rather than a pedogenic (Liu et al., 2003).
Hysteresis loops were measured on an automated vibrating sample magnetometer (VSM),
in a maximum field of 1 T. Hysteretic parameters were calculated after subtracting the
paramagnetic contribution. A Day plot (Day et al., 1977; Dunlop et al., 2002a, 2002b)[26-28]
was constructed to estimate the overall grain-size of samples.
For thermal demagnetization, samples were heated progressively from room temperature
up to 650oC with steps of 10-25oC, using a Schonstedt furnace with an internal field less than
100 nT. It was noted that ChRMs of samples were isolated after 300oC thermal treatment
(held for 25 minutes). ChRMs of parallel samples were then further demagnetized by AF and
low-temperature demagnetization (LTD) methods. AF demagnetization was performed with a
DTech-2000 AF demagnetizer, in steps of 2.5~5 mT up to a maximum applied field of 50 mT.
Low-temperature demagnetization (LTD) of ChRMs was performed after a 300oC thermal
demagnetization by putting specimens into liquid nitrogen in a zero-field container for one
hour. After warming up to room temperature, the remanences of these samples were measured
with a 2G cryogenic magnetometer. The fractional ChRM loss after LTD is referred to as
∆J%.
To monitor the continuous LTD curves of the NRM and ChRM, first, the remanence
directions of cubic bulk samples were determined using a 2G cryogenic magnetometer. Then,
80
cylinder sub-samples (~0.5 cm in diameter and ~1.5 cm in length) were cut from the cubic
samples along the direction of ChRM (NRM) for measurement in a single-axes SQUID
magnetometer (MPMS) for measuring. Before running the samples, the ambient field in the
measurement chamber of the susceptometer was carefully adjusted to 0.0 ±0.5 µT. Finally,
these sub-samples were cycled in zero-field from room temperature to 50 K, with a
temperature error less than ±1K. Low temperature cycles of three typical empty plastic straw
holders for the low temperature experiments were also measured for any contribution to the
natural remanence.
0.25
Mrs/Ms
0.2
0.15
0.1
PSD
0.05
MD
0
1.5
2
2.5
3
3.5
4
4.5
Bcr/Bc
Figure 2. Biplot of Mrs/Mr and Bcr/Bc. Small dots are the samples from this study. The arrow
represents the trend of increasing susceptibility. Background trend (cross bars) shows data
from Day et al. (1977).
To quantify the observed linear trend on the LTD warming/cooling curves, we define the
extrapolated fractional change δex for this linear trend as J50/J300-1, where J50 and J300 are
fitted intensities at 50 and 300 K, respectively. To obtain this rate, first, we fitted a line to the
data between 200 and 300K to avoid the effects of the Verwey transition. Then, J300 and J50
were calculated based on this fitted line. This procedure can avoid large errors caused by
using only single points.
81
3. Results
3.1 Susceptibility and hysteresis parameters
The susceptibility profile from the loess unit L2 to the sub-paleosol unit S1S3 is shown in
Fig. 1. Susceptibility values are between ~2 × 10-7 m3/kg for loess to ~15 × 10-7 m3/kg for
mature paleosol. There is a susceptibility transition zone from L2 to S1S3 between 38.0 and
37.35 m. Based on the parameter ∆JTV (Fig. 1c), this transition zone is recognized as the
pedogenically-altered loess (PAL) L2 material rather than the interglacial deposits (S1S3)
suggested by the stratigraphy based on color and susceptibility (Fig. 1a). This repeats the
pattern found for the same climatic boundary (MIS 5e/MIS6) at the Yuanbao section to the
east (Liu et al., 2003).
Plots of the hysteresis ratios Mrs/Ms and Bcr/Bc have been traditionally used to estimate the
overall grainsize of the magnetic minerals in bulk samples. Results show that the grainsize of
magnetic minerals becomes finer with increasing pedogenic degree, as indicated by enhanced
susceptibility (Fig. 2). The overall magnetic grainsizes are located in the pseudo-single
domain (PSD) region.
3.2 Thermal and AF demagnetization of ChRM
Detailed progressive thermal demagnetization studies (Fig. 3) show that about 80-90% of
the initial NRM intensity was removed after 300oC treatment. The thermally stable remanence
component removed between 300 and 500oC (orthogonal projections of NRM in Fig. 3) has
generally been used to define the loess ChRM vectors (Zhu et al., 1994a; Fang et al., 1997;
Guo et al., 1999; Pan et al., 2001).
Both thermal and AF demagnetization spectra of ChRM are shown in Figure 4. The
paleosol ChRM is higher by a factor of 3~5 than that of loess (Fig. 4a). With increase of
temperatures, ChRM intensities gradually decreased. Slight intensity drops around 570-580oC
indicate that minor parts of the ChRM of both loess and paleosol are carried by thermally
stable magnetite. The residual remanences at higher temperatures (>600oC) could be carried
by maghemite, hematite or both. Unfortunately, the relatively large errors of the
82
measurements at temperatures higher than 600oC do not allow us to confidently distinguish
them based on the thermal and AF demagnetization spectra (Fig. 4a, 4b).
E/Down
1
Z
0.8
J/Jo
38.1 m
0.6
0.4
0.2
N
Jo=0.009 A/m
0
0
200
400
600
Temperature (oC)
E/Down
1
Z
J/Jo
0.8
37.75 m
0.6
0.4
N
0.2
Jo=0.025 A/m
0
0
200
400
600
Temperature ( oC)
E/Down
1
Z
J/Jo
0.8
37.19 m
0.6
0.4
0.2
N
Jo=0.063 A/m
0
0
200
400
600
Temperature (oC)
Figure 3. Orthogonal projections of NRM vectors and NRM intensity behavior of three typical
samples at 38.1, 37.75, and 37.19 m during thermal demagnetization. Solid and open circles
represent vertical and horizontal projections, respectively.
83
The normalized thermal demagnetization spectra of ChRM for both loess and paleosol
samples exhibit similar patterns (Fig. 4b). The same is true for the AF demagnetization
spectra after thermal cleaning at 300oC (Fig. 4c). For AF demagnetization, about 20% of
remanence remains after 50 mT.
1.0
1.0
0.8
0.8
0.6
0.6
J/J0
1.0
J/J300
J (10-3Am-1)
1.5
0.4
0.4
0.2
0.2
0.0
300 350 400 450 500 550 600 650
0.0
0.5
Loess
Paleosol
Loess
0.0
300 350 400 450 500 550 600 650
a)
T (degree)
b)
T (degree)
0
10
c)
AF field (mT)
20
30
40
50
Figure 4. Thermal and AF demagnetization of ChRM. a) Thermal demagnetization spectrum;
b) Normalized intensity of the data from a); c) AF demagnetization spectrum. The open and
closed circles stand for the paleosol and the loess, respectively. The shaded rectangle in a)
outlines the temperature resistant component, which is probably carried by hematite,
maghemite, or both.
3.3 LTD of NRM and ChRM
The LTD results for the empty holders (one typical curve is shown in Fig. 5) show that the
background caused by the holders is generally less than 2×10-10 Am2. In contrast, the weak
remanence carried by the small cylinder samples are at least five times (1×10-9 Am2) higher
than the background. Further, the LTD curves of holder have no systematic patterns at the
characteristic points, e.g., Morin transition (~250K) (de Boer et al., 2001), and Verwey
transition (~120K). Therefore, we believe that the systematic changes in the LTD curves of
the loess samples actually reflect the behavior of the remanence carriers.
Fig. 6a shows that the percentage of ChRM to the initial NRM intensity (JChRM/JNRM) is a
function of susceptibility. For the loess samples (χ<3×10-7m3kg-1), the ratio may be as high as
20-30%. In contrast, for the paleosol sample (χ>5×10-7m3kg-1), it is only 5-10%. This pattern
is repeated in the correlation between Bcr and χ (Fig. 6b). Fig. 6c illustrates that the ChRM
loss after LTD is also negatively correlated to χ. The loess ChRMs lose by 8-16% of their
intensities, whereas, the paleosol ChRMs remain almost unchanged with ∆J% less than 4%.
84
The normalized (J/J300K) LTD curves of NRM and ChRM for both loess and paleosol
samples are shown in Fig. 7. For the loess sample (38.12 m), NRM intensity (Fig. 7a)
gradually decreases from room temperature to just above the crystallographic Verwey
transition (Tv, ~120 K). On cooling through Tv, the intensity abruptly increases, and the
normalized jump magnitude is nearly 60% (Fig. 7a). On the warming curve, the intensity is
initially reversible up to about 135 K, then departs from the cooling curve and stays nearly
Intensity of Holder (10-10 Am2)
constant up to room temperature.
3
2
1
0
-1
-2
-3
50
100
150
200
250
300
Temperature (K)
Figure 5. Low temperature demagnetization of typical empty holders.
35
60
a)
20
b)
c)
50
16
40
12
20
15
∆J %
25
Bcr (mT)
JChRM/JNRM (%)
30
30
8
20
4
10
5
0
10
0
0
5
10
χ(10 m kg )
-7
3
-1
15
0
5
10
χ(10 m kg )
-7
3
-1
15
0
5
10
χ(10-7m3 kg -1)
15
Fig. 6. Plots of ratio of ChRM/NRM (a), Bcr (b), and remanence loss after a low-temperature
cycle (c) against susceptibility.
85
For the thermally-cleaned (300oC) loess ChRM (Fig. 7b), a different pattern is displayed.
The loess ChRM does not sharply increase when cooling through Tv. Similar to the warming
curve of the loess NRM, a reversible pattern is shown by the loess ChRM up to about 130 K.
1
1
0.8
0.8
0.6
0.6
0.4
0.4
~50%
0.2
0.2
b)
a)
0
0
50
50
100 150 200 250 300
1.5
1.15
~15%
1.1
1.4
~90%
1.3
1.05
1.2
1
~22%
1.1
0.95
1
c)
PAL
J/J300K
100 150 200 250 300
Temperature (K)
temperature (K)
d)
0.9
0.9
50
100 150 200 250 300
50
100 150 200 250 300
1.2
1.2
1.16
~17%
J/J300K
Loess
ChRM
~32%
1.12
1.1
1.08
~13%
Paleosol
J/J300K
NRM
1.04
1
1
e)
f)
0.96
0.9
50
100 150 200 250 300
50
100 150 200 250 300
Figure 7. Normalized low temperature demagnetization spectra of NRM and ChRM for the
loess (38.12 m) (a and b), pedogenically-altered loess (37.6 m) (c and d), and paleosol
samples (37.2 m) (e and f). The solid and open circles represent the cooling and warming
curve, respectively. The left column (a, c, e) and the right column (b, d, f) are NRM and
ChRM measurements. PAL corresponds to the pedogenically-altered loess. One striking
feature for most of these curves is the linear trend above 160 K (inclined dashed lines) and in
some cases below 100 K. The corresponding numbers in each figure are δex values for these
linear trends.
86
Unlike the loess sample, the paleosol NRM intensity gradually increases on cooling, and
shows a weak drop at Tv (Fig. 7e). The warming curve is relatively reversible except that it is
shifted down slightly, especially for temperatures higher than 150 K. Compared to the loess
NRM curve, the paleosol NRM apparently has a much more linear trend (δex =~17%). The
LTD curves (Fig. 7f) of the paleosol ChRM can be fitted by two linear trends at the
temperature intervals between 50 and 100 K (~32%), and 150 and 300 K (~13%),
respectively. The LTD curve is completely reversible.
The NRM LTD curves (Fig. 7c) for the pedogenically altered loess, PAL, also have a twosegment behavior. A linear (δex =~15%) trend is similar to that of the paleosol NRM when
temperatures are higher than 150 K. But the intensity drop at the Verwey transition is much
higher than that of the paleosol NRM. Meanwhile, the intensity slightly increases when
passing through Tv. This pattern is more like that of the loess NRM. The LTD pattern of
ChRM (Fig. 4d) is consistent with that of the mature paleosol. However, δex values for two
linear trends are ~90% for the lower temperatures (50-100K) and ~22% for the higher
temperature (150-300K).
4. Discussion
4.1 NRM and ChRM carriers
It is well known that remanences carried by multi-domain (MD) magnetite can be partially
demagnetized by zero-field cooling from room temperatures to liquid nitrogen temperatures or
to even lower temperatures (10K), leaving a relatively stable remanence (Heider et al., 1992;
McClelland and Shcherbakov, 1995; ?zdemir and Dunl
op, 1998, 1999; Muxworthy and
McClelland, 2000; ?zdemir et al., 2002). In contrast, remanences carried by single domain
(SD) magnetite are less affected by such low temperature demagnetization (LTD) (Ozima et
al., 1964).
Several mechanisms have been proposed to explain the partial demagnetization of
remanence during LTD: 1) unpinning of the domain walls at the iostropic point Ti (Dunlop
and Argyle, 1991); 2) decrease in the magnetostrictive anisotropy energy on cooling to Tv
(Hodych et al., 1998); and 3) domain reorganization on cooling through magnetite’s
crystallographic transition (Hodych, 1991). Even though the mechanism of partial
87
demagnetization of MD magnetite is not precisely known in all cases (?zdemir and Dunlop,
1998; Muxworthy and McClelland, 2000; Hodych et al, 1998), the relatively larger partial
demagnetization after LTD, as well as the existence of Verwey transition at ~120 K, can be
confidently used as indicators for coarse-grained (large PSD/MD) magnetite particles as the
weak remanence carriers.
Chinese loess was deposited by the accumulation of dust from the north and northwest in
response to major uplift of the Tibetan plateau, and the associated change in Asian wind
patterns. Before deposition, each individual magnetite particle transported by the wind carries
a TRM. The loess acquired a (p)DRM during and after deposition on the Loess Plateau, and
then was altered by post-depositional processes. The original DRM could be overprinted not
only by VRM but also by secondary CRM carried by the newly-formed SD magnetic particles
produced during pedogenesis. Therefore, a working hypothesis is that loess/paleosols may
preserve their initial DRM only if their ChRM carrier is partially oxidized MD or PSD
magnetite. Otherwise the primary remanence may have been overprinted or totally replaced
by the secondary CRM carried by pedogenic SD/PSD maghemite.
Based on this model, first, we examine the LTD behavior for two characteristic loess
(38.12 m) and mature paleosol samples (37.2 m). One common feature of the LTD remanence
curves (NRM and ChRM) is the linear trend (Fig. 7), which is believed to be caused by
maghemite (including maghemite rims on magnetite grains and individual maghemite
particles) (Dekkers, 2002, personal communication). There are also other magnetic minerals
that can show low-temperature behavior similar to that of maghemite, e.g., greigite (iron
sulfite) and goethite (FeOOH). However, greigite is not a common phase in the Chinese
paleosol. Goethite has a N?el temperature of about 150
o
C, and moreover it dehydrates at
temperature below 300oC, thus it can not contribute to the ChRM. Therefore, we identify
maghemite as the main source for the linear LTD pattern of the paleosol ChRM.
Fig. 7 shows that the remanence (NRM and ChRM) of the loess sample changes sharply at
Tv indicating that PSD/MD magnetite particles are the main magnetic carriers. However, the
paleosol NRM is dominated by a linear trend with a slight intensity anomaly at Tv, indicating
that maghemite is the main NRM carrier.
By comparing the evolution pattern from the loess to the paleosol (Fig. 7), we can trace
the effects of pedogenesis on acquisition of NRM during the development of the soil. Clearly,
88
the DRM carried by the MD magnetite particles in the loess has been gradually overprinted or
replaced by the CRM carried by the newly-formed maghemite particles. Thus we conclude
that only the loess ChRM (after 300oC cleaning) may display reliable paleomagnetic field
variations.
Figs 2, 4 and 6 show that the magnetic properties of the bulk sample and the
corresponding ChRMs continuously change with susceptibility. Especially, Fig. 6c displays a
clear trend of ∆J% decreasing with increasing susceptibility. This pattern is further shown by
the continuous LTD curves of three representative samples (Fig. 7). The measurement of
continuous LTD curves with an instrument like MPMS is not always feasible. Nevertheless,
the parameter ∆J% (Fig. 6c) appears to be an excellent index for ChRM carriers when the
assemblage of the magnetic minerals is previously determined. For example, in this study, the
low ∆J% (<4%) for the paleosol ChRM strongly indicates that the dominant paleosol ChRM
carriers are maghemite.
4.2 Comparison of natural DRM with lab-induced weak remanence TRM and ARM
Most previous LTD studies have focused on saturation isothermal remanence (SIRM)
(
demir and Dunlop, 1998;
demir et al., 2002; Hodych 1991). Fewer studies have been
conducted on laboratory thermal remanence (TRM) (Ozima et al., 1964; Hartstra, 1983),
natural TRM (Hodych et al., 1998; Creer and Like 1967), and anhysteretic remanent
magnetization (ARM) (Muxworthy et al., 2003), and no reports have been published on
detrital remanent magnetization (DRM) because of difficulty in simulating DRM acquisition
in laboratory. Clearly, TRM and DRM are acquired though different physical processes.
When magnetic particles cool in a weak field from above their Curie temperature, they
acquire a TRM. In contrast, the DRM is acquired by rotating the magnetic moments of
magnetic particles parallel to the ambient field.
However, the remanence of individual detrital grains in sediments is most likely a TRM
inherited from the eroded source rocks (Dunlop and
demir, 1997). During the DRM
acquisition process, the weak ambient field generally does not change properties of the TRM
carried by each particle, but simply rotates the grain and its magnetic moment. Thus, we
believe that the LTD theory for the TRM can be extended to DRM. Likewise, ARM is often
89
used as an analog of TRM. Therefore, we will compare the loess NRM with both TRM and
ARM induced in the laboratory.
Muxworthy and McClelland observed that the TRM (weak-field) LTD curves of nearly
stoichiometric magnetite are grainsize dependent. For example, finer-grained particles have a
larger increase of intensity when cooling through Tv (∆VJ) than coarser-grained particles do. A
tentative explanation for this phenomenon is that screening by closure domains is eliminated
at low temperatures (below Tv) resulting in a large increase of the measured magnetization
(Muxworthy and McClelland, 2000). They further pointed out that coarse-grained (large
PSD/MD) magnetite has domain structures less controlled by the closure domain. Therefore,
no intensity increase is apparent in the TRM LTD curves just below Tv for these grains.
Based on this model, the observation that the loess NRM has a much higher ∆VJ (Fig. 7a)
than the loess ChRM, appears to indicate that the loess NRM carriers are finer-grained than
those of the loess ChRM. This conclusion seems inconsistent with the AF demagnetization
spectra of ChRM (after a 300oC thermal demagnetization), which are more SD-like than
PSD/MD-like. The stoichiometric-like magnetite behavior of the loess remanences across the
120 K Verwey transition, which strongly supports the idea that the loess remanence carriers
are coarser than the paleosol remanence carriers.
However, the positive intensity jump associated with Tv for magnetite particles depends
not only on grainsize, but is also coercivity-dependent. Further LTD studies on ARM revealed
that the higher coercivity fraction of remanence exhibits a higher ∆VJ. Muxworthy et al.
(2003) proposed that the ARM results could be extended to identify NRM carriers, with high
memory ratios and large ∆VJ values indicating a high-coercivity MD remanence.
Abundant evidence indicates that high-coercivity of the loess magnetites is enhanced by
low-temperature oxidation. Fig. 2 shows that the measured hysteresis ratios gradually depart
from the background trend of Day et al. (1977) with decreasing pedogenesis. This pattern is
probably caused by an increase of Mrs through low-temperature oxidation (van Velzen and
Zijderveld, 1992, 1995). Based on theoretical mixing calculations, Dunlop (2002b) tentatively
interpreted the Chinese loess ratios in terms of SD + 10-nm SP mixtures. However, this
interpretation apparently conflicts with the accepted view that the effects of SD+SP particles
are less important for these less-altered loess samples. Therefore, we currently accept the first
90
mechanism, that the loess bulk remanence is controlled by partially oxidized magnetite of
high-coercivity.
The similar LTD behavior of the loess NRM (Fig. 7a) and ARM carried by fine-grained,
high-coercivity magnetite (Muxworthy et al., 2003) suggests that the loess NRM is carried
dominantly by fine partially-oxidized magnetite. After thermal treatments, the stress caused
by the large compositional gradient between the maghemite shell and the magnetite core is
greatly reduced due to increase of the oxidation degree (van Velzen and Zijderveld, 1992,
1995). This is consistent with the LTD behavior of the loess ChRM, characterized by the
absence of a remanence jump around Tv.
However, direct comparison of the loess NRM and lab-induced remanences (TRM and
ARM) is still problematic because of the possibilities of very different microcoercivity
distributions and even mineral assemblages in the synthetic samples (Muxworthy et al., 2003),
and in the natural samples. Nevertheless, it is reasonable to deduce that the intensity jump in
the LTD of the loess NRM curve is caused by high-coercivity, partially-oxidized PSD/MD
magnetites rather than the relatively finer SD-sized magnetite grains, because the former are
more abundant in the loess.
4.3 Unsolved problems and future works
The main unsolved problem in this study is the origin of the linear trend of LTD curves
and the relationship between the slope and the grainsize and occurrence (rims or individual) of
maghemite. First, we find that δex varied from ~13% (Fig. 7f) up to ~90% (Fig. 7d). Second,
for a single sample (Fig. 7d and 7f), two linear trends can be perfectly fitted at the lower and
higher temperatures, respectively.
Temperature dependent variations in Js can not explain the large variations (much larger
than 10%) in intensity during the low temperature cycle. We tentatively attribute the linear
changes larger than 10% in intensity to progressive changes in domain configuration and
moment orientation, related to changing magnetocrystalline and/or magnetoelastic anisotropy.
However, we have to design more subtle experiments with synthetic samples to convincingly
demonstrate this.
The second question concerns the two kinds of maghemite particles controlling the
partially altered loess (PAL) and paleosol ChRM (Fig. 7d and 7f). We associate the former
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with maghemite rims on magnetites and the latter with individual maghemite particles. The
maghemite rim is a part of the partially oxidized magnetite and results in a higher coercivity
compared to the stoichiometric magnetite or maghemite. In contrast, the individual
maghemites can either be formed by pedogenesis or of aeolian detrital origin (Liu et al.,
1999). The saturation remanent magnetization for these two kinds of particles has been
recently investigated by Liu et al. (2003). However, their weak remanent properties have not
yet been fully understood. Low-temperature behavior of lab-induced remanence in
stoichiometric magnetites with a wide size range (from SD to MD) have been well
investigated. However, careful comparison of the lab-induced weak-field remanences (e.g.,
TRM, ARM, or DRM) for stoichiometric and partially oxidized magnetite samples with a
wide range size (from SD to MD) is still essential to clarify this problem.
5. Conclusions
The magnetic carriers of the loess /paleosol ChRM isolated by a 300oC thermal treatment
are more complex than previously thought (e.g. nearly stoichiometric PSD magnetite). Based
on the discussion above, the main conclusions are:
1) The loess and paleosol ChRMs are dominated by PSD/MD magnetite and PSD
maghemite, respectively. Therefore, we believe that the loess ChRM is a (p)DRM. However,
the contribution of a small maghemite component, added by pedogenesis to the loess ChRM,
may affect the paleomagnetic results when scrutinizing the detailed variations of the magnetic
field within a geomagnetic reversal. Due to a similar thermal coercivity spectrum for
magnetite and maghemite particles, it is problematic to construct continuous paleomagnetic
records spanning both loess and paleosol layers because the ChRM carried by the latter is
totally controlled by maghemite particles of pedogenic origin.
2) A combined demagnetization (AF, thermal, LTD) approach is a more direct way to
detect the magnetic carriers of the weak-field remanences (NRM and ChRM, or the other labinduced remanences, e.g., ARM, TRM, and DRM) than the traditional approaches, e.g.,
hysteresis ratios, and thermomagnetic analysis.
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