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The magnetic mechanism of paleosol S5 in the Baoji section of the southern Chinese Loess Plateau
Quaternary International 306 (2013) 129e136
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The magnetic mechanism of paleosol S5 in the Baoji section of the southern Chinese Loess Plateau Xuelian Guo a, *, Xiuming Liu b, c, d, Pingyuan Li d, Bin Lü b, Hui Guo d, Qu Chen e, Zhi Liu d, Mingming Ma d a
School of Earth Sciences & Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University, South Road Tianshui, Lanzhou 730000, China Research Centre of Global Change, School of Geographical Science, Fujian Normal University, Fuzhou 350007, China c Department of Environment and Geography, Macquarie University, NSW 2109, Australia d Key Laboratory of Western China’s Environmental Systems, Ministry of Education, Lanzhou University, Lanzhou 730000, China e College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua, Zhejiang 321004, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 4 March 2013
In the central Chinese Loess Plateau (CLP), the magnetic susceptibility has the highest value in the S5 paleosol unit. However, at the southern of the CLP, such as in Baoji, the magnetic susceptibility value in the S5 paleosol unit is on the lower range of the top 6 paleosols (S0, S1.S5). The studies of rock magnetism and geochemistry showed that the Baoji S5 experienced a stronger pedogenesis and chemical weathering than the S3. The paleosol S3 was formed under an oxidizing environment where the maghemite and hematite are formed simultaneously. Maghemite is the main contributor to magnetic enhancement. However, the concentrations of maghemite in the S5 are lower than in the S3. With the development of pedogenesis, the proportion of the maghemite component decreased in the S5, while the concentrations of anti-ferromagnetic minerals (mainly goethite) increased. In the field, measurable amounts of dark brown iron-manganese cutans can be observed on the cranny surface of S5. These indicate that the ironemanganese elements in the S5 were assembled, and pedogenesis occurred intermittently between wet and dry, and strong leaching soil conditions. This may have led to finegrained magnetite, maghemite and hematite, which were gradually converted into weak magnetic goethite, therefore resulting in the lower magnetic susceptibility of S5. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction As a valuable climatic proxy for Chinese loess deposits, magnetic suscepstibility has been used for quantitative reconstruction of precipitation (Heller et al., 1993; Maher and Thompson, 1994; Liu et al., 1995; Han et al., 1996; Hao and Guo, 2005; Balsam et al., 2011). Magnetic susceptibility shows a generally positive correlation with pedogenesis in the central Chinese Loess Plateau (CLP), such as Xifeng (Liu et al., 2001) and Lingtai (Ding et al.,1998) sections. The magnetic enhancement of paleosols is thought to predominantly be due to the formation of hematite and maghemite under elevated temperatures and rainfall conditions (Maher,1998; Chen et al., 2005). However, the relationship between magnetic susceptibility and pedogenesis was not clearly observed around the southern edge of CLP (Ding et al., 1991). Furthermore, magnetic susceptibility of the deposits of loess in Siberia and Alaska profiles demonstrate a * Corresponding author. E-mail address: [email protected] (X. Guo). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.02.033
negative relationship with pedogenesis. This phenomenon is explained by the strong activity of winds during the glacial periods (Begét et al., 1990; Chlachula, 2003; Kravchinsky et al., 2008), or the reducing conditions during pedogenesis which led to the transformation of wind-blown ferrimagnetism minerals and a decrease in magnetic susceptibility (Liu et al., 2008; Bábek et al., 2011). The S5 is the most prominent and well-developed paleosol in the CLP, and can be easily recognized by its thickness and dark red color. The S5 period represents a climatic optimum for the past 2.5 Ma (An et al., 1987). In general, the paleosol unit S5 has the highest or near highest susceptibility value in the central CLP. However, in the Baoji section of the southern CLP, the precipitation is more abundant than in the central CLP, yet the magnetic susceptibility value of S5 is not the highest. Some scholars have studied the Baoji section (Ding et al., 1991; Kalm et al., 1996; Han et al., 1998), but the reason for the relatively lower magnetic susceptibility of the S5 is still not completely resolved. This paper compares paleosols S5 and S3 in the Baoji section to explore the reasons for the relatively lower magnetic
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susceptibility of the Baoji S5. Using geochemical methods based on rock magnetism, the variation in the types of magnetic mineral, concentration and grain sizes during pedogenesis was studied, with further analysis of the transformation of magnetic minerals in intermittently oxidizing and reducing conditions. The conclusions may shed light on the weak relationship between magnetic susceptibility and pedogenesis around the edge of the CLP.
F300 ¼ IRM300 mT/SIRM) is expressed on a mass-specific basis. Hysteresis loop and temperature-dependent magnetization (JeT) curves were measured using a Magnetic Measurements Variable Field Transition Balance (MMVFTB). The hysteresis parameters, including saturation magnetization (Ms), remanence saturation magnetization (Mrs), coercivity (Bc) and remanent coercivity (Bcr) of paleosols were determined on hysteresis loops (Guo et al., 2011).
2. Materials and methods
2.3. Geochemical analysis
2.1. Sampling
Rb, Sr concentrations were measured using the XRF spectrography, For each XRF measurement, a 4 g whole-rock sample was powdered in agate mortar, sieved to a size-fraction of 200 mesh, and then compacted into a round disc and measured with the VP320 XRF spectrometer (made in Japan) (Chen et al., 1999). The relative standard deviation is about 1%, and the relative errors estimated from the measured values and the recommended values for the standards are less than 2%. The diffuse reflectance spectra were obtained over the range from 400 to 700 nm at 2 nm intervals using a Perkin Elmer Lambda 950 spectrophotometer. The method of Torrent et al. (2007) was used to derive the second-order derivative of the KubelkaeMunk remission spectrum. The intensities of the bands at w425 nm (I425) and w535 nm (I535) are proportional to the concentration of goethite and hematite, respectively (Scheinost et al., 1998), and can be used as proxies for relative changes in the mass concentration of goethite and hematite (Torrent et al., 2007), allowing calculation of the hematite and goethite concentrations in the studied loess samples.
Baoji is located at the southern edge of the CLP, at the north foot of the Qinling Mountains. The present climate is mildly humid with a mean annual precipitation (MAP) of more than 700 mm (up to 1100 mm in Qinling), 60% of which is distributed from July to September. The mean annual temperature (MAT) is 12 C (25.4 C in July). Precipitation is carried by the southeast summer monsoon, which decreases from southeast to northwest. The samples were collected from the Baoji loess-paleosol sequence (34 250 N, 107 070 E) (Fig. 1) from S3 to S5 in 5 cm intervals; 430 samples were collected in total. 2.2. Magnetic measurements The samples were air-dried, then weighed and packed in nonmagnetic plastic boxes. Magnetic susceptibility (c, mass-specific) was measured using Bartington MS-2 meter (low frequency cLF with 470 Hz, high frequency cHF with 4700 Hz). The absolute frequency dependent susceptibility was calculated cfd ¼ (cLF cHF). Lowtemperature susceptibility was measured using a Kappabridge KLY3S low temperature system (from 200 w 0 C), using about 0.4 g of each sample. Anhysteretic remanent magnetization (ARM) was measured in the minispin magnetometer after magnetization with a DTECH AF demagnetizer, the peak AF field used was 100 mT and the DC bias was 50 mT. ARM was then normalized by the bias field to obtain ARM susceptibility (cARM). Isothermal remanent magnetization (IRM) was acquired in progressively increasing magnetic fields up to 1 T with a MMPM10 pulse magnetizer and the induced remanence after each field was measured in a Minispin Magnetometer. The IRM acquired in the maximum field of 1 T was defined as the saturation isothermal remanent magnetization (SIRM ¼ IRM1T), S300 (defined as S300 ¼ (1(IRM300 mT/SIRM))/2) and F300 (defined as
Fig. 1. Map showing the study localities. Also shown are the deserts (reticulated) and mountains (grey) around and within the CLP.
3. Results 3.1. Magnetic properties at room temperature 3.1.1. Magnetic parameters and their ratios Magnetic parameters in the Baoji section are shown in Fig. 2. The parameters of c (Fig. 2a), cfd (Fig. 2b), SIRM (Fig. 2c), cARM/ SIRM (Fig. 2d), cARM/c (Fig. 2e), and S300 (Fig. 2f) show generally consistent trends, where the higher values correspond with paleosols and the lower values correspond with loess. The c values are highest in the most developed S5 horizon from the Xifeng (Liu et al., 2001) section in the central CLP. However, in the Baoji, the c values of the S5 horizon are on the lower range (max. 18.73 107 m3 kg1) within the upper 6 paleosols (max. 36.75 107 m3 kg1). Moreover, c values in the S3 horizon are two times greater than the S5 horizon (Fig. 2a). cfd indicates the concentration of the superparamagnetic (SP) ferrimagnetic minerals produced during pedogenesis. The maximum cfd value is 4.16 107 m3 kg1 in the S3 samples; and 2.17 107 m3 kg1 in the S5 samples (Fig. 2b), indicating that the concentration of SP ferrimagnetic minerals in the S5 samples are lower than in the S3 samples. As the Ms of anti-ferromagnetic minerals (hematite and goethite) is about two orders of magnitude lower than that of ferrimagnetic minerals, the Ms value provides an estimation for the absolute mass concentration of the ferrimagnetic minerals, and is independent of the grain size distribution (Liu et al., 2010a). The maximum Ms value for the S3 samples is 18.3 Am2 kg1, with a mean value of 6.849 Am2 kg1; the maximum Ms value for the S5 samples is 5.62 Am2 kg1, with a mean value of 3.336 Am2 kg1, showed that ferrimagnetic minerals (magnetite and/or maghemite) concentrations of the S3 samples are about two times higher than the S5 samples. As cARM is extremely sensitive to stable single domain (SSD) ferrimagnetic particles, the cARM/c ratio is usually used as an indicator of ferrimagnetic particles (King and Channell, 1991). The mean values of cARM/c for S3 and S5 are 4.24 and 5.18, with maximum values of 5.33 and 6.48, respectively (Fig. 2e).
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Fig. 2. The variations for magnetic parameters, hematite, goethite concentrations and Rb/Sr ratio of paleosol S3 and S5 in Baoji section.
These indicate that the SSD particles fractions are higher, but the SP magnetic particles fractions are relatively lower for the S5 samples. F300 is the ratio of the IRM acquisition at 300 mT over SIRM, which reflects the relative abundances of ferrimagnetic minerals (magnetite and/or maghemite) and anti-ferromagnetic minerals (hematite and/or goethite) in the samples (Evans and Heller, 2003). For the S3 samples, F300 > 0.92 (Fig. 3a), indicating that the IRM is approximately saturated at 300 mT. F300 is positively correlated with c (r ¼ 0.719), suggesting that the soft magnetic mineral components increase with pedogenic development as soft magnetic mineral components are the main contributor to remanence. For the S5 samples, 0.75 < F300 < 0.85 (Fig. 3a), indicating that the IRM reaches about 75% w 85% of saturation at 300 mT. Obviously, the content of hard magnetic mineral components of the S5 samples is higher than the S3 samples. A weakly correlation (r ¼ 0.087) between F300 and c in the S5 samples indicate that the percentage of soft magnetic mineral components slightly decreased with pedogenesis progression. The hematite are positively correlated with c and cfd for the S3 and S5 samples (Fig. 3d and f), showed that the concentrations of hematite and pedogenic ferrimagnetic minerals increased with increasing pedogenic degree in the S3 and S5 layers, ferrimagnetic magnetite/maghemite and anti-ferromagnetic hematite are produced by pedogenic processes and they co-exist in paleosols. Pedogenic ferrimagnetic mineral concentrations were also positively correlated with hematite in Lingtai and Luochuan loess-paleosol sequences (Balsam et al., 2004). The goethite and goethite þ hematite concentrations are also positively correlated with c and cfd for the S3 samples, while negatively correlated with c and cfd for the S5 samples (Fig. 3b,c and e), and the goethite concentrations are relatively higher in the S5 layer than the S3 layer (Fig.2h), suggesting that the goethite concentrations increased with decreasing concentrations of pedogenic ferrimagnetic minerals in the S5 layer, which means that parts of ferrimagnetic minerals were converted into weakly magnetic goethite during pedogenesis in the S5 layer.
3.1.2. Hysteresis loops The shape of hysteresis loops can indicate magnetic mineral types (Evans and Heller, 2003). Using 300 mT magnetic fields, soft magnetic minerals, hard magnetic minerals, and their relative proportions were analyzed. The two loops (Fig. 4) show different degrees of hysteresis below 300 mT, indicating that they contained soft magnetic mineral (magnetite and/or maghemite). The soft magnetic mineral components of the S3 samples were higher than the S5 samples. However, the two loops remained unsaturated above 300 mT, suggesting that the high-coercivity of hard magnetic minerals, such as goethite and/or hematite, also existed in these samples (Guo et al., 2011). The two curves were compared, and it was found that the S5 samples contained more high-coercivity hard magnetic minerals (hematite and/or goethite) than the S3 samples. The Bcr of the S5 and S3 samples were 46.48 mT and 19.97 mT, the Ms values were 3.98 105 Am2 kg1 and 18 105 Am2 kg1, respectively (Fig. 4). This further suggested that hard magnetic mineral concentrations in the S5 samples were higher than S3 samples, and soft magnetic mineral concentrations were relatively lower. These results are consistent with results from the F300. 3.2. Low temperature susceptibility The low-temperature dependence of magnetic susceptibility can determine the presence of SP, single domain (SD) and multidomain (MD) magnetite (Özdemir et al., 1993). The results of the L5, S3 and S5 samples from the Baoji section are shown in Fig. 5. The L5 sample showed low and relatively uniform magnetic susceptibility values with increasing temperature, indicating that the size of the magnetic particles of the loess sample are mainly SD grain. Magnetic susceptibility values increased linearly with increasing temperature for the two paleosol samples, which showed the characteristics of SP magnetic grains (regardless of magnetite or maghemite). When the temperature was decreased below 196 C, the SP magnetite or maghemite could not carry remanence, and showed SD characteristics. However, with increasing temperature,
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Fig. 3. c correlations (a) F300, (c) goethite and (d) hematite concentration, cfd correlations (b) goethite þ hematite, (e) goethite, and (f) hematite concentration from S3 and S5 samples.
the particles were restored to the SD state. Magnetic susceptibility of the SP fraction was higher than the SD. Therefore, with increasing temperature, these particles were transformed from SD to SP, resulting in an increase in magnetic susceptibility (Liu et al., 2010b). The low temperature curve of the S5 sample showed a less steep slope than the S3 sample, indicating that the S5 sample contained a smaller SP fraction and greater SD fraction than the S3 sample.
3.3. Magnetic properties at high temperature Detecting the Curie point temperature of magnetic minerals using a thermomagnetic curve is often used for determining the composition of magnetic minerals in a sample. Magnetization versus temperature curves of the S3 and S5 samples are shown in Fig. 6. For all samples, the magnetizations of heating curves significantly decreased at about 120 C, indicating the existence of
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during pedogenesis are maghemite rather than magnetite (Van Velzen and Dekkers, 1999; Hao et al., 2012). 3.4. Rb/Sr ratio Rb is a stable element formed during pedogenesis, showing characteristics for residual enrichment, and Sr is an active element with content reduced with increasing pedogenic degree. The Rb/Sr ratio indicates the weathering degree of loess and the intensity of the summer monsoon circulation (Chen et al., 1999). The S5 samples had a Rb/Sr > 0.9, with a mean value of 0.951; and the S3 samples had a Rb/Sr < 0.8, with mean value of 0.715. The higher Rb/ Sr ratio of the S5 samples compared to the S3 samples indicate that the S5 experienced a stronger chemical weathering than the S3. The climatic condition during the S5 period was confirmed to be the warmest and wettest over the last 0.5 Ma. 3.5. Ironemanganese cutans
Fig. 4. Magnetic hysteresis loops of the S3 and S5 samples.
goethite. The decreased magnetization between 300 and 410 C probably results from a thermally-induced conversion of metastable ferrimagnetic maghemite to weakly magnetic hematite (gFe2O3 / aFe2O3) (Dunlop and Özdemir, 1997). As the magnetic susceptibility gradually increased, the Curie temperature of the S3 (Fig. 6a and c) and S5 samples (Fig. 6d and f) rise from w580 C to w600 C, shown by their thermomagnetic curves. This indicates that with increasing pedogenic degree, magnetic minerals were gradually transformed from magnetite into maghemite, which may be a result of the warm-wet and oxidizing conditions. These conditions result in the surface oxidation of magnetite grains and the formation of small thermally stable maghemite grains at the outermost rim of the grain’s surface, which could lead to a higher Curie temperature (w600 C) and higher magnetic susceptibility. This maybe confirmed that main ferrimagnetic minerals formed
The field observations revealed that the paleosol S3 color was redder than the S5, and had dark brown ironemanganese cutans in the S5 horizon, but not in the S3 horizon (Fig. 7). Ironemanganese cutans were formed intermittently between wet and dry, and weathering and leaching soil conditions. Weathering made iron and manganese silicate minerals or clay minerals which were destroyed to release Fe, Mn, etc. under moisture over saturated soil conditions. Iron and manganese oxides were destroyed, releasing Fe2þ and Mn2þ which migrated with water into the soil surface, which then oxidized in dried climatic conditions and formed irone manganese oxide cutans (Huang et al., 2008). 4. Discussion As stated above, c is a useful palaeoclimatic indicator in the central CLP. In both the Xifeng (Liu et al., 2001) and Lingtai (Ding et al., 1998) sections, the S5 layer has the highest or near highest c value in the whole section. In combination with pedogenic study (An et al., 1987), this indicates that S5 is the most developed paleosol. However, in the Baoji section, the pattern is different. The c value in the S3 horizon is about two times greater than in the S5 horizon (Fig. 2a). Baoji is located south of Xifeng and Lingtai, with higher MAP (Fig. 1) and MAT, indicating it has better pedogenic conditions, but the paleosol unit S5 has a lower c value (Fig. 2a). Is the lower magnetic susceptibility of the S5 horizon caused by weak pedogenesis, or by over-saturated soil? Paleosol S5 and S3 in the Baoji section are compared to analyze the possible reasons. 4.1. Weathering conditions of the paleosols S3 and S5 The Rb/Sr ratio indicated the degree of weathering of the loess and the intensity of the summer monsoon circulation (Chen et al., 1999). The lower Rb/Sr ratio of the S3 samples (Fig. 2h) reflected the relatively weak weathering degree, and dry oxidizing soil conditions. The high Rb/Sr ratio of the S5 samples in the Baoji section (Fig. 2h) reflected a strong weathering degree during the S5 period, which indicated that the climate condition during the S5 period was the warmest and wettest of the last 0.5 million years. 4.2. Magnetic enhancement mechanics of paleosols S3 and S5
Fig. 5. Low temperature susceptibility behaviour of the paleosol S3, S5 and loess L5 samples.
The previous studies have proposed that the mechanism of magnetic mineral transformation to generate magnetic enhancement was the conversion of non-ferromagnetic mineral to crystallites of magnetite and/or maghemite (Zhou et al., 1990; Barron and Torrent, 2002; Torrent et al., 2007). However, this mechanism has
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Fig. 6. Magnetization as a function of temperature for palaeosol S3(aec) and S5(def) samples. Thicker lines represent heating and thinner lines represent cooling.
not been fully applied to some paleosol horizons in the Baoji section at the southern edge of the CLP, because the maximum magnetic enhancement (S3 horizon) in this section is not associated with the maximum weathering intensity (S5 horizon) as indicated by the Rb/ Sr proxy. The magnetic minerals transformed in the Baoji section may be related with the strong leaching soil conditions during pedogenesis. 4.2.1. Magnetic enhancement mechanics of Paleosol S3 For the S3 layer, c was significantly positive linearly correlated with cfd (r ¼ 0.97), Ms (r ¼ 0.94), SIRM (r ¼ 0.87), hematite (r ¼ 0.91) and goethite (r ¼ 0.57). F300 > 0.92, which showed that with increasing pedogenetic degree, soft magnetic minerals and hard magnetic minerals components increased simultaneously. The relationship between iron oxide and magnetic susceptibility of the Luochuan and Lingtai loess-paleosol in CLP also showed that pedogenic ferrimagnetic minerals and hematite had a positive correlation (Balsam et al., 2004). Torrent et al. (2010) and Liu et al.
(2007) confirmed that maghemite and hematite were produced simultaneously during pedogenesis, but maghemite and goethite were not. Under relatively weak pedogenesis, and dry, oxidizing soil conditions, more SP ferrimagnetic minerals (mainly maghemite) and stable hematite are formed with pedogenesis enhancement. The high-temperature magnetic characteristics further confirmed that with increasing pedogenic development, magnetite was gradually converted into stable maghemite (Fig. 6). It also confirmed that pedogenic ferrimagnetic minerals were mainly composed of maghemite instead of magnetite because pedogenic magnetite can be easily oxidized to maghemite under oxidizing conditions (Van Velzen and Dekkers, 1999). Therefore, pedogenic SP maghemite is a main contributor to magnetic enhancement in the paleosol S3. 4.2.2. Magnetic enhancement mechanics of paleosol S5 For the S5 layer, c and cfd, (r ¼ 0.92), cARM (r ¼ 0.93), Ms (r ¼ 0.79), SIRM (r ¼ 0.83), and hematite (r ¼ 0.55) showed a
Fig. 7. Ironemanganese cutans of the paleosol S3 and S5 surface in the Baoji section.
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significant positive relationship, indicating that the magnetic enhancement of paleosol S5 was mainly affected by SP and SSD ferrimagnetic minerals (mainly maghemite), even if concentrations were much lower than anti-ferromagnetic minerals. Recent studies showed that the magnetic susceptibility enhancement was not only caused by SP magnetic particles, but that the contents of pedogenic SSD and PSD particles also played an important role (Deng et al., 2005; Wang et al., 2006). In addition, the higher goethite (Fig. 2h), goethite þ hematite (Fig. 3b), remanence coercivity (Fig. 4), and lower F300 values (Fig. 3a), suggests that the S5 layer contains higher goethite concentrations compared to the S3 layer. The goethite increased with decreasing c and cfd (Fig. 3c and e), which means that the fine ferrimagnetic mineral (mainly maghemite), which was affected by environmental conditions, became unstable, and converted to stable goethite under the moist environment, thus showing a negative correlation between goethite and c. Baoji, located at the southern edge of the CLP, is the warmest and wettest region of the CLP. The modern MAP is more than 700 mm, mainly distributed from July to September (Fig. 1). The stronger weathering, and warm-wet soil conditions during the S5 period showed that the strong water leaching during pedogenic processes caused fine-grained ferrimagnetic minerals (mainly maghemite) to dissolve. Fe3þ was reduced to Fe2þ which migrated with water into cracks or surface of soil, which then oxidized under an aerobic soil environment, deposited, and formed iron oxide cutans. Goethite, lepidocrocite, and amorphous oxide can form cutans on clay surfaces (Roth et al., 1969), but it is mainly stable goethite under moist soil conditions (Huang et al., 2008). Low temperature susceptibility curves (Fig. 5) and cARM/c ratios (Fig. 2e) also showed lower SP and higher SSD fractions in the S5 samples. As a result of the conversion of parts of fine-grained maghemite into weakly magnetic goethite, there was a decline in the SP ferrimagnetic minerals produced through pedogenesis, and c decreased in the paleosol S5. This phenomenon is common in reticulate red clay (RRC) in southern China. Pedogenic maghemite was dissolved under longterm water logging conditions, which caused the significant decrease of clf in the RRC (Hu et al., 2009a). Many studies (Maher, 1998; Cornell and Schwertmann, 2003; Liu et al., 2008; Hu et al., 2009b; Bábek et al., 2011) reported that pedogenic fine maghemite and/or magnetite are reductively dissolved in anaerobic soil conditions, causing the depletion of soil magnetism. Modern soil studies also showed that magnetic susceptibility increased with increasing pedogenic degree in aerobic soil with low and moderate rainfall. When the moisture is high, the magnetic susceptibility decreased (Han et al., 1996; Balsam et al., 2011; Long et al., 2011). 5. Conclusions The climate during the S5 period was warmer and wetter, and paleosol S5 experienced a stronger chemical weathering than the paleosol S3 in the Baoji section. The concentrations of maghemite and hematite increased simultaneously with increasing pedogenic degree in S3 and S5 palaeosol horizons. SP maghemite was the main contributor to magnetic susceptibility enhancement. Pedogenic ferrimagnetic mineral (mainly maghemite) concentrations were lower, and the anti-ferromagnetic mineral (mainly goethite) concentrations were relatively higher in the S5 horizon. SP components were lower, and SD components were relatively higher. Overall, the higher goethite concentrations and the SD magnetic particles mainly contributed to the magnetic susceptibility. There were measurable amounts of dark brown ironemanganese cutans on the surface of the palaeosol S5. Parts of fine-grained strongly magnetic minerals (magnetite and/or maghemite) were
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converted into weakly magnetic minerals (mainly goethite), which resulted in a decline in SP ferrimagnetic minerals and lower magnetic susceptibility values in the Baoji S5. Acknowledgements We thank Prof. Weiguo Zhang, Ph.D. Yan Dong, and Chenying Dong, State Key Laboratory of Estuarine and Coastal Research, East China Normal University, for their help in the diffuse reflectance spectra laboratory work. The authors would like to express their gratitude to two anonymous reviewers. This research was supported by the National Natural Science Foundation of China (grants no. 40830105, 41072124, 41202129, 41210002). References An, Z.S., Liu, T.S., Zhu, Y.Z., Sun, F., 1987. The paleosol complex S5 in the China Loess Plateau-A record of climatic optimum during the last 1.2 Ma. Geojournal 15, 141e143. Bábek, O., Chlachula, J., Matys Grygar, T.M., 2011. Non-magnetic indicators of pedogenesis related to loess magnetic enhancement and depletion: examples from the Czech Republic and southern Siberia. Quaternary Science Reviews 30, 967e979. Balsam, W., Ji, J.F., Chen, J., 2004. Climatic interpretation of the Luochuan and Lingtai loess sections, China, based on changing iron oxide mineralogy and magnetic susceptibility. Earth and Planetary Science Letters 223, 335e348. Balsam, W.L., Ellwood, B.B., Ji, J.F., Williams, E.R., Long, X.Y., Hassani, A.E., 2011. Magnetic susceptibility as a proxy for rainfall: worldwide data from tropical and temperate climate. Quaternary Science Reviews 30, 2732e2744. Barron, V., Torrent, J., 2002. Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochimica et Cosmochimica Acta 66, 2801e2806. Begét, J., Stone, D., Hawkins, D., 1990. Paleoclimate forcing of magnetic susceptibility variations in Alaskan loess. Geology 18, 40e43. Chen, J., An, Z.S., Head, J., 1999. Variation of Rb/Sr ratios in the loess-paleosol sequences of central China during the last 130,000 years and their implications for monsoon paleoclimatology. Quaternary Research 51, 215e219. Chen, T.H., Xu, H.F., Xie, Q.Q., Chen, J., Ji, J.F., Lu, H.Y., 2005. Characteristics and genesis of maghemite in Chinese loess and paleosols: mechanism for magnetic susceptibility enhancement in paleosols. Earth and Planetary Science Letters 240, 790e802. Chlachula, J., 2003. The Siberian loess record and its significance for reconstruction of Pleistocene climate change in north-central Asia. Quaternary Science Reviews 22, 1879e1906. Cornell, R.M., Schwertmann, U., 2003. The Iron Oxides, second ed. John Wiley, New York, 664 pp. Deng, C.L., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q.S., Shaw, J., Zhu, R.X., 2005. Mineral magnetic of the Jiaodao Chinese loess/paleosol sequence and its bearing on long term climatic variability. Journal of Geophysical Research 110, B03103. http://dx.doi.org/10.1029/2004JB00345. Ding, Z.L., Sun, J.M., Yang, S.L., Liu, T.S., 1998. Preliminary magnetostratigraphy of a thick eolian red clay-loess sequence at Lingtai, the Chinese Loess Plateau. Geophysical Research Letters 25, 1225e1228. Ding, Z.L., Yu, Z.W., Liu, T.S., 1991. Progress in loess research (part 3): time scale. Quaternary Sciences 4, 336e348 (in Chinese). Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge University Press, New York, 573 pp. Evans, M.E., Heller, F., 2003. Environmental Magnetism: Principles and Applications of Environ-Magnetics. Academic Press, New York, 299 pp. Guo, X.L., Liu, X.M., Lü, B., Tang, D.P., Mao, X.G., Chen, J.S., Chen, X.Y., 2011. Comparison of topsoil magnetic properties between the loess region in Tianshan Mountains and Loess Plateau, China, and its environmental significance. Chinese Journal of Geophysics 54, 485e495. Han, J.M., Lü, H.Y., Wu, N.Q., 1996. Magnetic susceptibility of modern soils in China and climate conditions. Study Geophysics Geodetica 40, 262e275. Han, J.T., William, S.F., Fred, J.L., 1998. Climatic implications of the S5 paleosol complex on the southernmost Chinese Loess Plateau. Quaternary Research 50, 21e33. Hao, Q.Z., Guo, Z.T., 2005. Spatial variations of magnetic susceptibility of Chinese loess for the last 600 kyr: implications for monsoon evolution. Journal Geophysical Research 110, B12101. Hao, Q.Z., Oldfield, F., Bloemendal, J., Guo, Z.T., 2012. Hysteresis and thermomagnetic properties of particle-sized fractions from loess and palaeosol samples spanning 22 Myr of accumulation on the Chinese Loess Plateau. Geophysical Journal International 191, 64e77. Heller, F., Shen, C.D., Beer, J., Liu, X.M., Liu, T.S., Bronger, A., Suter, M., Bonani, G., 1993. Quantitative estimates of pedogenic ferromagnetic mineral formation in Chinese loess and palaeoclimatic implications. Earth and Planetary Science Letters 114, 385e390. Hu, X.F., Wei, J., Xu, L.F., Zhang, G.L., Zhang, W.G., 2009a. Magnetic susceptibility of the Quaternary Red Clay in subtropical China and its
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paleoenvironmental implications. Palaeogeography, Palaeoclimatology, Palaeoecology 279, 216e232. Hu, X.F., Xu, L.F., Pan, Y., Shen, M.N., 2009b. Influence of the aging of Fe oxides on the decline of magnetic susceptibility of the Tertiary red clay in the Chinese Loess Plateau. Quaternary International 209, 22e30. Huang, L., Hong, J., Tan, W.F., Hu, H.Q., Liu, F., Wang, M.K., 2008. Characteristics of micromorphology and element distribution of ironemanganese cutans in typical soils of subtropical China. Geoderma 146, 40e47. Kalm, V.E., Rutter, N.W., Rokosh, C.D., 1996. Clay minerals and their paleoenvironmental interpretation in the Baoji loess section, Southern Loess Plateau, China. Catena 27, 49e61. King, J., Channell, J., 1991. Sedimentary magnetism, environmental magnetism and magneto- stratigraphy, 1987e1990. Review of Geophysics 39, 358e 370. Kravchinsky, V.A., Zykina, V.S., Zykin, V.S., 2008. Magnetic indicator of global paleoclimate cycles in Siberian loess-paleosol sequences. Earth and Planetary Science Letters 265, 498e514. Liu, Q.S., Hu, P.X., Torrent, J., Barrón, V., Zhao, X.Y., Jiang, Z.X., Su, Y.L., 2010a. Environmental magnetic study of a Xeralf chronosequence in northwestern Spain: indications for pedogenesis. Palaeogeography, Palaeoclimatology, Palaeoecology, 144e156. Liu, X.M., Shaw, J., Jiang, J.Z., Bloemendal, J., Hesse, P., Rolph, T., Mao, X.G., 2010b. Analysis on variety and characteristics of maghemite. Science China Earth Science (Series D) 53, 1153e1162. Liu, Q.S., Deng, C.L., Torrent, J., Zhu, R.X., 2007. Review of recent developments in mineral magnetism of the Chinese loess. Quaternary Science Reviews 26, 368e385. Liu, X.M., An, Z.S., Rolph, T., Q, X.K., Hesse, P., Lu, H.Y., Zhou, J., 2001. Magnetic properties of the Tertiary red clay from Gansu Province, China and its paleoclimatic significance. Science China Earth Sciences (Series D) 44, 635e 651. Liu, X.M., Liu, T.S., Xia, D.S., et al., 2008. The analysis of two different pedogenesis models in reductive and oxidative conditions record by Chinese Siberia loess. Science China Earth Sciences (Series D) 37, 1382e1391.
Liu, X.M., Rolph, T., Bloemendal, J., Shaw, J., Liu, T.S., 1995. Quantitative estimates of paleoprecipitation at Xifeng in the loess plateau of China. Palaeogeography, Palaeoclimatology, Palaeoecology 113, 243e248. Long, X., Ji, J., Balsam, W., 2011. Rainfall-dependent transformations of iron oxides in a tropical saprolite transect Hainan Island, South China: spectral and magnetic measurements. Journal of Geophysical Research. http://dx.doi.org/10.1029/ 2010JF0 01712. Maher, B.A., 1998. Magnetic properties of modern soils and Quaternary loessic paleosols: paleo-climatic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 137, 25e54. Maher, B.A., Thompson, R., 1994. Comments on: pedogenesis and paleoclimateinterpretation of the magnetic susceptibility record of Chinese loess-paleosol sequences. Geology 23, 857e858. Özdemir, Ö., Dunlop, S.J., Moskowitz, B.M., 1993. The effect of oxidation on the verwey transition in magnetite. Geophysical Research Letters 20, 1671e1674. Roth, C.B., Jackson, M.L., Syers, J.K., 1969. Clays Clay Minerals 17, 253e264. Scheinost, A.C., Chavernas, A., Barrón, V., Torrent, J., 1998. Use and limitations of the second-derivative diffuse reflectance spectroscopy in the visible to nearinfrared range to identify and quantify Fe oxide minerals in soils. Clays Clay Minerals 46, 528e536. Torrent, J., Liu, Q.S., Barrón, V., 2010. Magnetic minerals in Calcic Luvisols (Chromic) developed in a warm Mediterranean region of Spain: origin and paleoenvironmental significance. Geoderma 154, 465e472. Torrent, J., Liu, Q.S., Bloemendal, J., Barrón, V., 2007. Magnetic enhancement and iron oxides in the upper Luochuan loess-paleosol sequence, Chinese Loess Plateau. Soil Science Society of America Journal 71, 1570e1578. Van Velzen, A.J., Dekkers, M.J., 1999. Low-temperature oxidation of magnetite in loess-paleosol sequences: a correction of rock magnetic parameters. Studia Geophysica Geodaetica 43, 357e375. Wang, X.S., Yang, Z.Y., Løvie, R., Sun, Z.M., Pei, J.L., 2006. Magnetostratigraphic reassessment of correlation between Chinese loess and marine oxygen isotope records over the last 1.1 Ma. Physics of the Earth and Planetary Interiors 159, 109e117. Zhou, L.P., Oldfield, F., Wintle, A.G., Robinson, S.G.., Wang, J.T., 1990. Partly pedogenic origin of magnetic variations in Chinese loess. Nature 346, 737e739.
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The magnetic mechanism of paleosol S5 in the Baoji section of the southern Chinese Loess Plateau Xuelian Guo a, *, Xiuming Liu b, c, d, Pingyuan Li d, Bin Lü b, Hui Guo d, Qu Chen e, Zhi Liu d, Mingming Ma d a
School of Earth Sciences & Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University, South Road Tianshui, Lanzhou 730000, China Research Centre of Global Change, School of Geographical Science, Fujian Normal University, Fuzhou 350007, China c Department of Environment and Geography, Macquarie University, NSW 2109, Australia d Key Laboratory of Western China’s Environmental Systems, Ministry of Education, Lanzhou University, Lanzhou 730000, China e College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua, Zhejiang 321004, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 4 March 2013
In the central Chinese Loess Plateau (CLP), the magnetic susceptibility has the highest value in the S5 paleosol unit. However, at the southern of the CLP, such as in Baoji, the magnetic susceptibility value in the S5 paleosol unit is on the lower range of the top 6 paleosols (S0, S1.S5). The studies of rock magnetism and geochemistry showed that the Baoji S5 experienced a stronger pedogenesis and chemical weathering than the S3. The paleosol S3 was formed under an oxidizing environment where the maghemite and hematite are formed simultaneously. Maghemite is the main contributor to magnetic enhancement. However, the concentrations of maghemite in the S5 are lower than in the S3. With the development of pedogenesis, the proportion of the maghemite component decreased in the S5, while the concentrations of anti-ferromagnetic minerals (mainly goethite) increased. In the field, measurable amounts of dark brown iron-manganese cutans can be observed on the cranny surface of S5. These indicate that the ironemanganese elements in the S5 were assembled, and pedogenesis occurred intermittently between wet and dry, and strong leaching soil conditions. This may have led to finegrained magnetite, maghemite and hematite, which were gradually converted into weak magnetic goethite, therefore resulting in the lower magnetic susceptibility of S5. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction As a valuable climatic proxy for Chinese loess deposits, magnetic suscepstibility has been used for quantitative reconstruction of precipitation (Heller et al., 1993; Maher and Thompson, 1994; Liu et al., 1995; Han et al., 1996; Hao and Guo, 2005; Balsam et al., 2011). Magnetic susceptibility shows a generally positive correlation with pedogenesis in the central Chinese Loess Plateau (CLP), such as Xifeng (Liu et al., 2001) and Lingtai (Ding et al.,1998) sections. The magnetic enhancement of paleosols is thought to predominantly be due to the formation of hematite and maghemite under elevated temperatures and rainfall conditions (Maher,1998; Chen et al., 2005). However, the relationship between magnetic susceptibility and pedogenesis was not clearly observed around the southern edge of CLP (Ding et al., 1991). Furthermore, magnetic susceptibility of the deposits of loess in Siberia and Alaska profiles demonstrate a * Corresponding author. E-mail address: [email protected] (X. Guo). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.02.033
negative relationship with pedogenesis. This phenomenon is explained by the strong activity of winds during the glacial periods (Begét et al., 1990; Chlachula, 2003; Kravchinsky et al., 2008), or the reducing conditions during pedogenesis which led to the transformation of wind-blown ferrimagnetism minerals and a decrease in magnetic susceptibility (Liu et al., 2008; Bábek et al., 2011). The S5 is the most prominent and well-developed paleosol in the CLP, and can be easily recognized by its thickness and dark red color. The S5 period represents a climatic optimum for the past 2.5 Ma (An et al., 1987). In general, the paleosol unit S5 has the highest or near highest susceptibility value in the central CLP. However, in the Baoji section of the southern CLP, the precipitation is more abundant than in the central CLP, yet the magnetic susceptibility value of S5 is not the highest. Some scholars have studied the Baoji section (Ding et al., 1991; Kalm et al., 1996; Han et al., 1998), but the reason for the relatively lower magnetic susceptibility of the S5 is still not completely resolved. This paper compares paleosols S5 and S3 in the Baoji section to explore the reasons for the relatively lower magnetic
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susceptibility of the Baoji S5. Using geochemical methods based on rock magnetism, the variation in the types of magnetic mineral, concentration and grain sizes during pedogenesis was studied, with further analysis of the transformation of magnetic minerals in intermittently oxidizing and reducing conditions. The conclusions may shed light on the weak relationship between magnetic susceptibility and pedogenesis around the edge of the CLP.
F300 ¼ IRM300 mT/SIRM) is expressed on a mass-specific basis. Hysteresis loop and temperature-dependent magnetization (JeT) curves were measured using a Magnetic Measurements Variable Field Transition Balance (MMVFTB). The hysteresis parameters, including saturation magnetization (Ms), remanence saturation magnetization (Mrs), coercivity (Bc) and remanent coercivity (Bcr) of paleosols were determined on hysteresis loops (Guo et al., 2011).
2. Materials and methods
2.3. Geochemical analysis
2.1. Sampling
Rb, Sr concentrations were measured using the XRF spectrography, For each XRF measurement, a 4 g whole-rock sample was powdered in agate mortar, sieved to a size-fraction of 200 mesh, and then compacted into a round disc and measured with the VP320 XRF spectrometer (made in Japan) (Chen et al., 1999). The relative standard deviation is about 1%, and the relative errors estimated from the measured values and the recommended values for the standards are less than 2%. The diffuse reflectance spectra were obtained over the range from 400 to 700 nm at 2 nm intervals using a Perkin Elmer Lambda 950 spectrophotometer. The method of Torrent et al. (2007) was used to derive the second-order derivative of the KubelkaeMunk remission spectrum. The intensities of the bands at w425 nm (I425) and w535 nm (I535) are proportional to the concentration of goethite and hematite, respectively (Scheinost et al., 1998), and can be used as proxies for relative changes in the mass concentration of goethite and hematite (Torrent et al., 2007), allowing calculation of the hematite and goethite concentrations in the studied loess samples.
Baoji is located at the southern edge of the CLP, at the north foot of the Qinling Mountains. The present climate is mildly humid with a mean annual precipitation (MAP) of more than 700 mm (up to 1100 mm in Qinling), 60% of which is distributed from July to September. The mean annual temperature (MAT) is 12 C (25.4 C in July). Precipitation is carried by the southeast summer monsoon, which decreases from southeast to northwest. The samples were collected from the Baoji loess-paleosol sequence (34 250 N, 107 070 E) (Fig. 1) from S3 to S5 in 5 cm intervals; 430 samples were collected in total. 2.2. Magnetic measurements The samples were air-dried, then weighed and packed in nonmagnetic plastic boxes. Magnetic susceptibility (c, mass-specific) was measured using Bartington MS-2 meter (low frequency cLF with 470 Hz, high frequency cHF with 4700 Hz). The absolute frequency dependent susceptibility was calculated cfd ¼ (cLF cHF). Lowtemperature susceptibility was measured using a Kappabridge KLY3S low temperature system (from 200 w 0 C), using about 0.4 g of each sample. Anhysteretic remanent magnetization (ARM) was measured in the minispin magnetometer after magnetization with a DTECH AF demagnetizer, the peak AF field used was 100 mT and the DC bias was 50 mT. ARM was then normalized by the bias field to obtain ARM susceptibility (cARM). Isothermal remanent magnetization (IRM) was acquired in progressively increasing magnetic fields up to 1 T with a MMPM10 pulse magnetizer and the induced remanence after each field was measured in a Minispin Magnetometer. The IRM acquired in the maximum field of 1 T was defined as the saturation isothermal remanent magnetization (SIRM ¼ IRM1T), S300 (defined as S300 ¼ (1(IRM300 mT/SIRM))/2) and F300 (defined as
Fig. 1. Map showing the study localities. Also shown are the deserts (reticulated) and mountains (grey) around and within the CLP.
3. Results 3.1. Magnetic properties at room temperature 3.1.1. Magnetic parameters and their ratios Magnetic parameters in the Baoji section are shown in Fig. 2. The parameters of c (Fig. 2a), cfd (Fig. 2b), SIRM (Fig. 2c), cARM/ SIRM (Fig. 2d), cARM/c (Fig. 2e), and S300 (Fig. 2f) show generally consistent trends, where the higher values correspond with paleosols and the lower values correspond with loess. The c values are highest in the most developed S5 horizon from the Xifeng (Liu et al., 2001) section in the central CLP. However, in the Baoji, the c values of the S5 horizon are on the lower range (max. 18.73 107 m3 kg1) within the upper 6 paleosols (max. 36.75 107 m3 kg1). Moreover, c values in the S3 horizon are two times greater than the S5 horizon (Fig. 2a). cfd indicates the concentration of the superparamagnetic (SP) ferrimagnetic minerals produced during pedogenesis. The maximum cfd value is 4.16 107 m3 kg1 in the S3 samples; and 2.17 107 m3 kg1 in the S5 samples (Fig. 2b), indicating that the concentration of SP ferrimagnetic minerals in the S5 samples are lower than in the S3 samples. As the Ms of anti-ferromagnetic minerals (hematite and goethite) is about two orders of magnitude lower than that of ferrimagnetic minerals, the Ms value provides an estimation for the absolute mass concentration of the ferrimagnetic minerals, and is independent of the grain size distribution (Liu et al., 2010a). The maximum Ms value for the S3 samples is 18.3 Am2 kg1, with a mean value of 6.849 Am2 kg1; the maximum Ms value for the S5 samples is 5.62 Am2 kg1, with a mean value of 3.336 Am2 kg1, showed that ferrimagnetic minerals (magnetite and/or maghemite) concentrations of the S3 samples are about two times higher than the S5 samples. As cARM is extremely sensitive to stable single domain (SSD) ferrimagnetic particles, the cARM/c ratio is usually used as an indicator of ferrimagnetic particles (King and Channell, 1991). The mean values of cARM/c for S3 and S5 are 4.24 and 5.18, with maximum values of 5.33 and 6.48, respectively (Fig. 2e).
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Fig. 2. The variations for magnetic parameters, hematite, goethite concentrations and Rb/Sr ratio of paleosol S3 and S5 in Baoji section.
These indicate that the SSD particles fractions are higher, but the SP magnetic particles fractions are relatively lower for the S5 samples. F300 is the ratio of the IRM acquisition at 300 mT over SIRM, which reflects the relative abundances of ferrimagnetic minerals (magnetite and/or maghemite) and anti-ferromagnetic minerals (hematite and/or goethite) in the samples (Evans and Heller, 2003). For the S3 samples, F300 > 0.92 (Fig. 3a), indicating that the IRM is approximately saturated at 300 mT. F300 is positively correlated with c (r ¼ 0.719), suggesting that the soft magnetic mineral components increase with pedogenic development as soft magnetic mineral components are the main contributor to remanence. For the S5 samples, 0.75 < F300 < 0.85 (Fig. 3a), indicating that the IRM reaches about 75% w 85% of saturation at 300 mT. Obviously, the content of hard magnetic mineral components of the S5 samples is higher than the S3 samples. A weakly correlation (r ¼ 0.087) between F300 and c in the S5 samples indicate that the percentage of soft magnetic mineral components slightly decreased with pedogenesis progression. The hematite are positively correlated with c and cfd for the S3 and S5 samples (Fig. 3d and f), showed that the concentrations of hematite and pedogenic ferrimagnetic minerals increased with increasing pedogenic degree in the S3 and S5 layers, ferrimagnetic magnetite/maghemite and anti-ferromagnetic hematite are produced by pedogenic processes and they co-exist in paleosols. Pedogenic ferrimagnetic mineral concentrations were also positively correlated with hematite in Lingtai and Luochuan loess-paleosol sequences (Balsam et al., 2004). The goethite and goethite þ hematite concentrations are also positively correlated with c and cfd for the S3 samples, while negatively correlated with c and cfd for the S5 samples (Fig. 3b,c and e), and the goethite concentrations are relatively higher in the S5 layer than the S3 layer (Fig.2h), suggesting that the goethite concentrations increased with decreasing concentrations of pedogenic ferrimagnetic minerals in the S5 layer, which means that parts of ferrimagnetic minerals were converted into weakly magnetic goethite during pedogenesis in the S5 layer.
3.1.2. Hysteresis loops The shape of hysteresis loops can indicate magnetic mineral types (Evans and Heller, 2003). Using 300 mT magnetic fields, soft magnetic minerals, hard magnetic minerals, and their relative proportions were analyzed. The two loops (Fig. 4) show different degrees of hysteresis below 300 mT, indicating that they contained soft magnetic mineral (magnetite and/or maghemite). The soft magnetic mineral components of the S3 samples were higher than the S5 samples. However, the two loops remained unsaturated above 300 mT, suggesting that the high-coercivity of hard magnetic minerals, such as goethite and/or hematite, also existed in these samples (Guo et al., 2011). The two curves were compared, and it was found that the S5 samples contained more high-coercivity hard magnetic minerals (hematite and/or goethite) than the S3 samples. The Bcr of the S5 and S3 samples were 46.48 mT and 19.97 mT, the Ms values were 3.98 105 Am2 kg1 and 18 105 Am2 kg1, respectively (Fig. 4). This further suggested that hard magnetic mineral concentrations in the S5 samples were higher than S3 samples, and soft magnetic mineral concentrations were relatively lower. These results are consistent with results from the F300. 3.2. Low temperature susceptibility The low-temperature dependence of magnetic susceptibility can determine the presence of SP, single domain (SD) and multidomain (MD) magnetite (Özdemir et al., 1993). The results of the L5, S3 and S5 samples from the Baoji section are shown in Fig. 5. The L5 sample showed low and relatively uniform magnetic susceptibility values with increasing temperature, indicating that the size of the magnetic particles of the loess sample are mainly SD grain. Magnetic susceptibility values increased linearly with increasing temperature for the two paleosol samples, which showed the characteristics of SP magnetic grains (regardless of magnetite or maghemite). When the temperature was decreased below 196 C, the SP magnetite or maghemite could not carry remanence, and showed SD characteristics. However, with increasing temperature,
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Fig. 3. c correlations (a) F300, (c) goethite and (d) hematite concentration, cfd correlations (b) goethite þ hematite, (e) goethite, and (f) hematite concentration from S3 and S5 samples.
the particles were restored to the SD state. Magnetic susceptibility of the SP fraction was higher than the SD. Therefore, with increasing temperature, these particles were transformed from SD to SP, resulting in an increase in magnetic susceptibility (Liu et al., 2010b). The low temperature curve of the S5 sample showed a less steep slope than the S3 sample, indicating that the S5 sample contained a smaller SP fraction and greater SD fraction than the S3 sample.
3.3. Magnetic properties at high temperature Detecting the Curie point temperature of magnetic minerals using a thermomagnetic curve is often used for determining the composition of magnetic minerals in a sample. Magnetization versus temperature curves of the S3 and S5 samples are shown in Fig. 6. For all samples, the magnetizations of heating curves significantly decreased at about 120 C, indicating the existence of
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during pedogenesis are maghemite rather than magnetite (Van Velzen and Dekkers, 1999; Hao et al., 2012). 3.4. Rb/Sr ratio Rb is a stable element formed during pedogenesis, showing characteristics for residual enrichment, and Sr is an active element with content reduced with increasing pedogenic degree. The Rb/Sr ratio indicates the weathering degree of loess and the intensity of the summer monsoon circulation (Chen et al., 1999). The S5 samples had a Rb/Sr > 0.9, with a mean value of 0.951; and the S3 samples had a Rb/Sr < 0.8, with mean value of 0.715. The higher Rb/ Sr ratio of the S5 samples compared to the S3 samples indicate that the S5 experienced a stronger chemical weathering than the S3. The climatic condition during the S5 period was confirmed to be the warmest and wettest over the last 0.5 Ma. 3.5. Ironemanganese cutans
Fig. 4. Magnetic hysteresis loops of the S3 and S5 samples.
goethite. The decreased magnetization between 300 and 410 C probably results from a thermally-induced conversion of metastable ferrimagnetic maghemite to weakly magnetic hematite (gFe2O3 / aFe2O3) (Dunlop and Özdemir, 1997). As the magnetic susceptibility gradually increased, the Curie temperature of the S3 (Fig. 6a and c) and S5 samples (Fig. 6d and f) rise from w580 C to w600 C, shown by their thermomagnetic curves. This indicates that with increasing pedogenic degree, magnetic minerals were gradually transformed from magnetite into maghemite, which may be a result of the warm-wet and oxidizing conditions. These conditions result in the surface oxidation of magnetite grains and the formation of small thermally stable maghemite grains at the outermost rim of the grain’s surface, which could lead to a higher Curie temperature (w600 C) and higher magnetic susceptibility. This maybe confirmed that main ferrimagnetic minerals formed
The field observations revealed that the paleosol S3 color was redder than the S5, and had dark brown ironemanganese cutans in the S5 horizon, but not in the S3 horizon (Fig. 7). Ironemanganese cutans were formed intermittently between wet and dry, and weathering and leaching soil conditions. Weathering made iron and manganese silicate minerals or clay minerals which were destroyed to release Fe, Mn, etc. under moisture over saturated soil conditions. Iron and manganese oxides were destroyed, releasing Fe2þ and Mn2þ which migrated with water into the soil surface, which then oxidized in dried climatic conditions and formed irone manganese oxide cutans (Huang et al., 2008). 4. Discussion As stated above, c is a useful palaeoclimatic indicator in the central CLP. In both the Xifeng (Liu et al., 2001) and Lingtai (Ding et al., 1998) sections, the S5 layer has the highest or near highest c value in the whole section. In combination with pedogenic study (An et al., 1987), this indicates that S5 is the most developed paleosol. However, in the Baoji section, the pattern is different. The c value in the S3 horizon is about two times greater than in the S5 horizon (Fig. 2a). Baoji is located south of Xifeng and Lingtai, with higher MAP (Fig. 1) and MAT, indicating it has better pedogenic conditions, but the paleosol unit S5 has a lower c value (Fig. 2a). Is the lower magnetic susceptibility of the S5 horizon caused by weak pedogenesis, or by over-saturated soil? Paleosol S5 and S3 in the Baoji section are compared to analyze the possible reasons. 4.1. Weathering conditions of the paleosols S3 and S5 The Rb/Sr ratio indicated the degree of weathering of the loess and the intensity of the summer monsoon circulation (Chen et al., 1999). The lower Rb/Sr ratio of the S3 samples (Fig. 2h) reflected the relatively weak weathering degree, and dry oxidizing soil conditions. The high Rb/Sr ratio of the S5 samples in the Baoji section (Fig. 2h) reflected a strong weathering degree during the S5 period, which indicated that the climate condition during the S5 period was the warmest and wettest of the last 0.5 million years. 4.2. Magnetic enhancement mechanics of paleosols S3 and S5
Fig. 5. Low temperature susceptibility behaviour of the paleosol S3, S5 and loess L5 samples.
The previous studies have proposed that the mechanism of magnetic mineral transformation to generate magnetic enhancement was the conversion of non-ferromagnetic mineral to crystallites of magnetite and/or maghemite (Zhou et al., 1990; Barron and Torrent, 2002; Torrent et al., 2007). However, this mechanism has
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Fig. 6. Magnetization as a function of temperature for palaeosol S3(aec) and S5(def) samples. Thicker lines represent heating and thinner lines represent cooling.
not been fully applied to some paleosol horizons in the Baoji section at the southern edge of the CLP, because the maximum magnetic enhancement (S3 horizon) in this section is not associated with the maximum weathering intensity (S5 horizon) as indicated by the Rb/ Sr proxy. The magnetic minerals transformed in the Baoji section may be related with the strong leaching soil conditions during pedogenesis. 4.2.1. Magnetic enhancement mechanics of Paleosol S3 For the S3 layer, c was significantly positive linearly correlated with cfd (r ¼ 0.97), Ms (r ¼ 0.94), SIRM (r ¼ 0.87), hematite (r ¼ 0.91) and goethite (r ¼ 0.57). F300 > 0.92, which showed that with increasing pedogenetic degree, soft magnetic minerals and hard magnetic minerals components increased simultaneously. The relationship between iron oxide and magnetic susceptibility of the Luochuan and Lingtai loess-paleosol in CLP also showed that pedogenic ferrimagnetic minerals and hematite had a positive correlation (Balsam et al., 2004). Torrent et al. (2010) and Liu et al.
(2007) confirmed that maghemite and hematite were produced simultaneously during pedogenesis, but maghemite and goethite were not. Under relatively weak pedogenesis, and dry, oxidizing soil conditions, more SP ferrimagnetic minerals (mainly maghemite) and stable hematite are formed with pedogenesis enhancement. The high-temperature magnetic characteristics further confirmed that with increasing pedogenic development, magnetite was gradually converted into stable maghemite (Fig. 6). It also confirmed that pedogenic ferrimagnetic minerals were mainly composed of maghemite instead of magnetite because pedogenic magnetite can be easily oxidized to maghemite under oxidizing conditions (Van Velzen and Dekkers, 1999). Therefore, pedogenic SP maghemite is a main contributor to magnetic enhancement in the paleosol S3. 4.2.2. Magnetic enhancement mechanics of paleosol S5 For the S5 layer, c and cfd, (r ¼ 0.92), cARM (r ¼ 0.93), Ms (r ¼ 0.79), SIRM (r ¼ 0.83), and hematite (r ¼ 0.55) showed a
Fig. 7. Ironemanganese cutans of the paleosol S3 and S5 surface in the Baoji section.
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significant positive relationship, indicating that the magnetic enhancement of paleosol S5 was mainly affected by SP and SSD ferrimagnetic minerals (mainly maghemite), even if concentrations were much lower than anti-ferromagnetic minerals. Recent studies showed that the magnetic susceptibility enhancement was not only caused by SP magnetic particles, but that the contents of pedogenic SSD and PSD particles also played an important role (Deng et al., 2005; Wang et al., 2006). In addition, the higher goethite (Fig. 2h), goethite þ hematite (Fig. 3b), remanence coercivity (Fig. 4), and lower F300 values (Fig. 3a), suggests that the S5 layer contains higher goethite concentrations compared to the S3 layer. The goethite increased with decreasing c and cfd (Fig. 3c and e), which means that the fine ferrimagnetic mineral (mainly maghemite), which was affected by environmental conditions, became unstable, and converted to stable goethite under the moist environment, thus showing a negative correlation between goethite and c. Baoji, located at the southern edge of the CLP, is the warmest and wettest region of the CLP. The modern MAP is more than 700 mm, mainly distributed from July to September (Fig. 1). The stronger weathering, and warm-wet soil conditions during the S5 period showed that the strong water leaching during pedogenic processes caused fine-grained ferrimagnetic minerals (mainly maghemite) to dissolve. Fe3þ was reduced to Fe2þ which migrated with water into cracks or surface of soil, which then oxidized under an aerobic soil environment, deposited, and formed iron oxide cutans. Goethite, lepidocrocite, and amorphous oxide can form cutans on clay surfaces (Roth et al., 1969), but it is mainly stable goethite under moist soil conditions (Huang et al., 2008). Low temperature susceptibility curves (Fig. 5) and cARM/c ratios (Fig. 2e) also showed lower SP and higher SSD fractions in the S5 samples. As a result of the conversion of parts of fine-grained maghemite into weakly magnetic goethite, there was a decline in the SP ferrimagnetic minerals produced through pedogenesis, and c decreased in the paleosol S5. This phenomenon is common in reticulate red clay (RRC) in southern China. Pedogenic maghemite was dissolved under longterm water logging conditions, which caused the significant decrease of clf in the RRC (Hu et al., 2009a). Many studies (Maher, 1998; Cornell and Schwertmann, 2003; Liu et al., 2008; Hu et al., 2009b; Bábek et al., 2011) reported that pedogenic fine maghemite and/or magnetite are reductively dissolved in anaerobic soil conditions, causing the depletion of soil magnetism. Modern soil studies also showed that magnetic susceptibility increased with increasing pedogenic degree in aerobic soil with low and moderate rainfall. When the moisture is high, the magnetic susceptibility decreased (Han et al., 1996; Balsam et al., 2011; Long et al., 2011). 5. Conclusions The climate during the S5 period was warmer and wetter, and paleosol S5 experienced a stronger chemical weathering than the paleosol S3 in the Baoji section. The concentrations of maghemite and hematite increased simultaneously with increasing pedogenic degree in S3 and S5 palaeosol horizons. SP maghemite was the main contributor to magnetic susceptibility enhancement. Pedogenic ferrimagnetic mineral (mainly maghemite) concentrations were lower, and the anti-ferromagnetic mineral (mainly goethite) concentrations were relatively higher in the S5 horizon. SP components were lower, and SD components were relatively higher. Overall, the higher goethite concentrations and the SD magnetic particles mainly contributed to the magnetic susceptibility. There were measurable amounts of dark brown ironemanganese cutans on the surface of the palaeosol S5. Parts of fine-grained strongly magnetic minerals (magnetite and/or maghemite) were
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