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Rock magnetism, iron oxide mineralogy and geochemistry of Quaternary red earth in central China and their paleopedogenic implication
Palaeogeography, Palaeoclimatology, Palaeoecology 379–380 (2013) 95–103
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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo
Rock magnetism, iron oxide mineralogy and geochemistry of Quaternary red earth in central China and their paleopedogenic implication Siyuan Wang, Shi Lin, Shenggao Lu ⁎ College of Environmental and Resource Sciences, Zhejiang University, Zhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrient, Hangzhou 310058, China
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 26 March 2013 Accepted 15 April 2013 Available online 20 April 2013 Keywords: Quaternary red earth (QRE) Rock magnetism Iron oxide Weathering index Paleopedogenesis
a b s t r a c t This paper presents a detailed investigation on the rock magnetism, iron oxide mineralogy and geochemistry of a Quaternary red earth (QRE) in central China. The magnetic profile of QRE showed a characteristic vertical variation pattern: the uniform red clay (URC) section (0.2–2 m) of the profile had a higher magnetic susceptibility (χ) while vermiculated red clay (VRC) section (2–6 m) had a very low χ (b 30 × 10−8 m3 kg−1). Rock magnetism and differential XRD analysis revealed that magnetite/maghemite and hematite were the main magnetic carriers in the URC section. The major elements Fe2O3 and Al2O3 of QRE increased with depth while the SiO2 content decreased with depth, indicating a leaching of the mobile elements and an enrichment of the immobile elements. In comparison with the URC, the VRC has a greater depletion in the content of Na, Mg, K and Mg, indicating a stronger weathering intensity. The dithionite–citrate–bicarbonate (DCB) extracted iron (Fed), the ratio of Fed to total iron oxide (Fe/Fet), and the weathering indices of QRE confirmed that the VRC had experienced a stronger pedogenesis than the URC. Strong pedogenesis in the VRC resulted in a transformation of the highly magnetic maghemite to the weakly magnetic hematite, which was responsible for the weak magnetism and hard magnetic behavior of the VRC. Our studies suggest that the VRC had experienced a hot climate with a dry season following one that was warm and moister. Magnetic reduction of the VRC section resulting from the transformation of highly magnetic maghemite to weakly magnetic hematite appeared to be an important paleopedogenic process in the QRE. We concluded that the magnetic parameters, hard isothermal remanent magnetization (HIRM) and SIRM/χ of QRE, can be used for the analysis of paleopedogenic processes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Quaternary red earth (QRE) is widely distributed in southern and southeastern China and believed to be an important terrestrial record archive of Quaternary paleoclimatic and paleoenvironment changes in subtropical–tropical areas (Huang et al., 1996; Lu et al., 1999; Yin and Guo, 2006; Lu, 2007; Hu et al., 2008, 2010; Hong et al., 2010a,b, 2012). Paleomagnetic, optically-stimulated luminescence (OSL) and electron spin resonance (ESR) dating so far suggest that the QRE in China had been deposited since the mid-Pleistocene age (Zhao and Yang, 1995; Huang et al., 1996; Yang et al., 1996). This QRE sequence was usually thought to be produced by strong pedogenesis with intense oxidation and leaching, which resulted from enhanced East Asian summer monsoon activity (Huang et al., 1996; Yin and Guo, 2006; Yuan et al., 2008). Yin and Guo (2006) pointed out that the QRE in southern China was developed during the periods of greatest warmth and humidity, which correlates with the paleosol units S4 and S5 of midPleistocene (approx. 0.8 Ma) in the Loess Plateau of northern China. The QRE in southeastern China often consists of three different morphological sections from top to bottom: yellow-brown earth ⁎ Corresponding author. E-mail address: [email protected] (S. Lu). 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.04.010
(YBE), uniform red clay (URC) and vermiculated red clay (VRC). The VRC layer constitutes the main Quaternary stratigraphic unit and is much more weathered than the sections above and below (Huang et al., 1996; Yuan et al., 2008; Hu et al., 2009; Hong et al., 2010a). The underlying bottom section is the fluvial gravel deposits. The QRE is characterized by the red color, strong acidity, clayey structure and high degree of weathering, with heavy leaching of mobile elements (Ca, Mg, K, Na) and relative enrichment of immobile elements (Fe and Al), and kaolinite as the dominant clay mineral (Hu et al., 2009; Hong et al., 2012). Therefore, QRE is commonly used to probe into the Quaternary paleoclimatic and paleoenvironmental processes (Huang et al., 1999; Qiao et al., 2011; C.C. Liu et al., 2012). Soil magnetism studies have indicated that the magnetic enhancement effects of pedogenesis could be used as a proxy to estimate the pedogenic chemical weathering intensity and a tool to reconstruct pedoenvironmental processes (Maher, 1998; Maher et al., 2003; Torrent et al., 2007; Q.S. Liu et al., 2012). In classic loess–paleosol studies of China, the soil magnetic measurements have been widely used to indicate the climate fluctuations recorded in loess–paleosol sequences (Maher, 1998; Balsam et al., 2004; Q.S. Liu et al., 2012). The magnetic susceptibility of QRE also has been used to investigate paleoenvironmental change as in loess−paleosol sequences (Yang et al., 1995). However, Hu et al. (2009) thought that the magnetic
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susceptibility of QRE could not be used as a paleoclimatic proxy after they measured the magnetic susceptibility of 11 typical QRE profiles in subtropical China. Recent studies indicated that strong pedogenesis occurring in QRE could enhance the transformation of highly magnetic maghemite to weakly magnetic hematite. The relative concentration of pedogenic maghemite versus hematite is correlated with the pedogenic intensity (C.C. Liu et al., 2012). Therefore, parameters such as magnetic susceptibility (χ), anhysteretic remanent magnetization (χARM) and saturation isothermal remanent magnetization (SIRM), which denote the absolute magnetization of the soils, cannot accurately reflect the pedogenic intensity in QRE. The S-ratio, which can be used to quantify the relative concentration of antiferromagnetic minerals, is a good climate index in the vermiculated red soils (C.C. Liu et al., 2012). The hard isothermal remanent magnetization (HIRM) can also be used to indicate pedogenic intensity. Current research indicates that there have been at least three environmental shifts in southeast China since the early Pleistocene. A very warm and dry climate followed by significant rainfalls in 1.0–1.2 Ma ago resulted in the occurrence of VRC. During the period of 1.0–1.2 to 0.85–0.9 Ma, pedogenesis was strengthened gradually possibly by a strong East Asian summer monsoon (Xiong et al., 2002; Yin and Guo, 2006; Liu et al., 2008). After 0.85–0.9 Ma, pedogenesis in southeast China gradually became weak because the strengthened winter monsoon had significantly affected the paleoclimate in southeast China since 0.85 Ma, which caused a weakening of pedogenesis after that time. Apparently, further study is needed to understand the role of pedoenvironmental changes in determining the soil magnetic properties and to reveal whether the magnetism of the QRE has any specific implications on paleoclimate. Previous studies on QRE were mainly focused on the south of middle to lower reaches of the Yangtze River in China (Fig. 1) and detailed magnetic investigation on QRE in the subtropical–tropical regions as a whole remained very scarce. These studies were only focused on the QRE in several limited areas in the middle to lower reaches of the
Yangtze River. The mechanisms and paleoenvironmental implication of magnetic variation in the QRE are still in debate. Especially, the mechanism of the magnetization reduction in the vermiculated section has not been resolved. Few studies using integrated proxies such as magnetic susceptibility, iron oxide minerals and geochemistry have been used for examining these climate changes. The objective of this study was to investigate if the magnetic mineralogy and geochemistry of QRE can provide more insights into the paleoenvironmental change in central China, with particular reference to the pedogenic processes involved in the magnetic mineral transformation and the relationships between these and iron oxide phases. 2. Materials and methods 2.1. Soil profile and sampling The studied Quaternary red earth (QRE) profile is situated in Qiyang County (111°52′E, 26°45′), Hunan Province of central China (Fig. 1). The present climate is a typical middle subtropical monsoon climate with a mean annual precipitation of 1255 mm, mean annual evaporation of 1431 mm, and mean annual temperature of 18 °C. This profile was excavated for building materials with a depth of 650 cm, which provided a suitable profile to collect soil samples. The profile was subdivided into four pedostratigraphical units from top to bottom according to the color, texture, and occurrence of worm- or net-like white veins. The upper first section (0 to 20 cm) was a cultivated soil horizon with loose texture and abundant organic matter and plant roots. The second section (20 to 200 cm) was a uniform red clay (URC), consisting of reddish to red-brown clays with a nuclear blocky structure and abundant dark films and Fe and Mn nodules. The third section (200 to 600 cm) was the vermiculated red clay (VRC) consisting of red-brown to darkbrown clays with white worm- or net-like veins. The bottom section (b 600 cm) was the underlying fluvial gravel deposits. Soil samples were collected at an interval of 10 cm. The samples were air-dried,
Fig. 1. Schematic map showing the distribution of Quaternary red earth (QRE) in southern and southeastern China. The studied QRE profile is located in Qiyang County of Hunan Province of central China. The shaded area indicates loess deposits in northern China and brown-red area is Quaternary red earth region of southern China. The Qiliting (1), Xuancheng (2) and Jiujiang (3) QRE sequences were typical QRE profile in the middle to lower reaches of the Yangtze River (Liu et al., 2008; Hu et al., 2009; Hong et al., 2010a). Map is modified after Huang et al. (1999) and C.C. Liu et al. (2012).
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ground and passed through a 2 mm sieve. For geochemical analyses, the soil samples were ground to pass 0.15 mm nylon sieve. 2.2. Soil chemical analysis Soil pH was measured in H2O and in 1 M KCl (soil:solution, 1:2.5) using a pH meter. Organic carbon was determined according to the wet oxidation procedure. Particle size distribution was determined by a combination of the wet-sieving and pipette method. Soil color was determined using the Munsell color charts (X-Rite, Grand Rapids, MI, USA). Total iron content (Fet) of soil samples was determined on dissolution extracts obtained by acid attack with a mixture of HClO4: HNO3:HF. The free iron in soils was determined by extraction with dithionite–citrate–bicarbonate (DCB) (Mehra and Jackson, 1960) and amorphous iron was extracted using a 0.2 M ammonium oxalate (pH 3.0) (Schwertmann, 1973). Iron content of the extracted solution was determined by an ultraviolet spectrophotometry. The concentration of major elements was determined by X-ray fluorescence spectrometry (XRF) (Philips PW2440 XRD). A national standard material (GBW07407) was measured in the same manner to check the accuracy of the analysis. Analytical uncertainties were controlled within 5% for all major elements. Elemental content was converted to the weight percentage of oxide. The weight percentage of oxide was normalized to molecular weight and used to calculate the following molecular ratio as the indicators of soil-forming environment, for example, Sa = SiO2/Al2O3 and Saf = SiO2/Al2O3 + Fe2O3. The chemical index of alteration (CIA) was calculated following Nesbitt and Young (1982), i.e., CIA = [Al2O3 / (Al2O3 + Na2O + K2O + CaO*)] × 100, where CaO* represents the amount of CaO in the silicate fraction of the sample. A larger CIA value corresponds to a greater degree of leaching and removal of cations (Ca 2+, Na + and K+) with a CIA of 100 being referred to kaolinite and/or gibbsite. The molar ratios of free Fe-oxide (Fed) and amorphous Fe oxide (Feo) to total elemental Fe-oxide (Fed/Fet and Feo/Fet) were calculated as the additional weathering indices (Schwertmann, 1985).
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using the DCB procedure. XRD analyses were carried out on a Rigaku X-ray diffractometer (D/Max 2550 PC, Rigaku Corporation, Japan) using Cu Kα radiation (45 kV, 300 mA) at the XRD Laboratory, Zhejiang University. The XRD patterns of the DCB treated and untreated samples were collected from 5° to 75° 2θ at a scan rate of 0.2° 2θ/min. 3. Results 3.1. Soil properties of QRE The QRE has a hue of 5.5 YR or 7.5 YR (Fig. 2b). This hue indicates the presence of hematite in the soil (Schwertmann, 1985). The soil pHH2O and pHKCl values of QRE are generally low, especially in the URC (Fig. 2a), reflecting the predominance of acidic cations (Al3+ and H+) at exchange sites. The organic matter concentration in the QRE shows the highest values in the surface soil and decreases sharply with depth (Fig. 2c). In the 450–550 cm layer, the pH and organic matter of QRE profile fluctuate due to the fluctuation in pedogenic environment and weathering intensity. Clay is the dominant size fraction in all soil horizons (Fig. 2d), with a relatively greater change in the amounts of silt and sand particles. The sand/clay ratio increases with profile depth (Fig. 2e). The VRC unit has the highest sand/clay ratio. 3.2. Magnetic properties of the QRE
2.4. Differential X-ray diffraction (DXRD)
3.2.1. Magnetic parameter profiles The magnetic parameter profiles of QRE are shown in Fig. 3. The χlf of soil decreases with depth and reaches the lowest level in the VRC section. Magnetic susceptibility variation pattern of the QRE, being consistent with previous studies in the middle to lower reaches of the Yangtze River, had the lowest value in the VRC section and higher values in the URC section (Lu et al., 1999; Liu et al., 2008; Hu et al., 2009; Liu et al., 2010; C.C. Liu et al., 2012). The IRM20 mT and SIRM values of soil showed similar variation patterns, and were much lower in the vermiculated section than those of the other sections at the upper portion of the profile. However, SIRM showed a sudden enhancement in 370–460 cm layer. The correlation between χ and IRM20 mT was highly significant (R 2 = 0.772, p b 0.01), suggesting that the magnetism of the QRE was mainly due to ferrimagnetic minerals. Much lower χ and SIRM values in the VRC section suggested small concentration of ferrimagnetic minerals. However, other parameters, such as HIRM and SIRM/χ, showed variation patterns significantly different from those of the magnetic susceptibility. The VRC section had much higher HIRM and SIRM/χ values than the URC section. The χfd profile of QRE indicated that the absolute χfd of URC ranged from 5 × 10 −8 to 18 × 10 −8 m 3 kg −1, and that χfd of VRC was b5 × 10 −8 and SIRM/χ increased with depth. The SIRM/χ can be used to detect the variation of the magnetic grain size (Zhang et al., 2007; Q.S. Liu et al., 2012). The lower SIRM/χ values can be ascribed to the higher concentration of small particles, e.g., viscous SP grains. In contrast, the HIRM curves of the QRE profile were different from the χ curves. HIRM values were lowest in the URC and significantly enhanced in the VRC. HIRM reflects the absolute and relative concentrations of antiferromagnetic minerals (such as hematite and goethite) in the mineral mixtures. Results indicated that the weakly magnetic and high-coercivity antiferromagnetic minerals contributed to magnetic signals of VRC section. This suggested that the VRC with weak magnetism contained less ferrimagnetic minerals but more hematite and/or goethite.
Iron oxide mineralogy was determined by differential X-ray diffraction (DXRD) (Schulze, 1981). The clay fraction (b2 μm) of the soil samples was separated using a sedimentation method according to Stokes' law and treated for selective dissolution of iron oxides
3.2.2. Magnetic mineralogy of QRE High-temperature magnetization (Ms–T curve) measurements provide useful information about magnetic mineralogy. Fig. 4 shows the Ms–T curves of selected samples of the QRE sequence. The Ms–T
2.3. Rock magnetic measurements Soil magnetic susceptibility was measured using a Bartington MS2 meter at low (0.47 kHz, χlf) and high (4.7 kHz, χhf) frequency, respectively. Isothermal remanent magnetization (IRM) was produced in progressively increasing magnetic fields up to 1000 mT with a Molspin pulse magnetizer, and the induced remanence after each field was measured using a Molspin spinner magnetometer. The IRM acquired in the maximum field of 1000 mT was defined as the saturation isothermal remanent magnetization (SIRM) of the sample. Massspecific frequency-dependent susceptibility (χfd) and hard isothermal remanent magnetization (HIRM) were calculated by the following formulae: χfd = (χlf − χhf), and HIRM = (SIRM − IRM300 mT). χfd reflects the presence of fine viscous ferrimagnetic grains close to the superparamagnetic/stable single domain (SP/SD) boundary (~30 nm) (Maher, 1998; Zhang et al., 2007; Q.S. Liu et al., 2012). HIRM provides a rough estimate on the concentration of antiferromagnetic minerals (Zhang et al., 2007; Q.S. Liu et al., 2012). High-temperature magnetization (Ms–T curves) and hysteresis parameter measurements on the selected samples were performed using a Magnetic Measurements Variable Field Translation Balance (VFTB) at the State Key Laboratory of Estuarine and Coastal Research (SKLEC), East China Normal University.
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Fig. 2. Vertical distribution of soil properties in Quaternary red earth (QRE) profile. a. pH; b. soil color; c. organic carbon; d. particle size composition; and e. sand/clay ratio.
curve of URC samples showed a rapid decrease in magnetization with the temperature rise and gave the Curie temperature of a magnetite. The rapid and significant loss in Ms after heating might indicate a full oxidation and even a destruction of maghemite/magnetite to hematite, which indicated the presence of pedogenic maghemite/ magnetite in the URC section. The Ms–T curves of VRC section displayed a steady magnetization increase below ~300 °C and a steady decrease above 300 °C. The characteristic Ms–T curves of VRC sample may arise from the new formation of minor amount of ferrimagnetic phases, e.g. maghemite (γ-Fe2O3) and the conversion of fine-grained maghemite to hematite (α-Fe2O3) (Deng et al., 2005; C.C. Liu et al., 2012; Q.S. Liu et al., 2012). The Ms decrease between 300 and 450 °C for VRC sample is much smaller than that for the URC sample, which is ascribed to lower concentration of maghemite in the URC sample. Hematite was usually not obvious in the Ms–T curves due to its weak magnetism, however, Ms–T curves of VRC samples displayed a marked Ms decrease in the temperature range between 580 °C and ~680 °C (the Néel temperature of hematite). The significant Ms decrease between 580 °C and ~680 °C
indicates the presence of hematite in the VRC sample. Therefore, the Ms–T curves in the URC and VRC sections display significant difference in the Ms decrease between 580 °C and ~680 °C. The contribution of hematite to magnetization of the VRC samples was greater than that of the URC samples. The cooling curves of the different layers in the QRE profile showed a similar trend, except for a sharp increase around 680 °C of VRC samples. Hysteresis loops provide information about the coercivity spectrum (Dunlop and Özdemir, 1997). All selected samples of QRE profile displayed wasp-waisted hysteresis loops (Fig. 5), which usually are due to the coexistence of two magnetic components with strongly contrasting coercivities. The relative concentrations of two magnetic components varied with the different layers. Hysteresis loops of the URC samples were closed in lower fields than those of the VRC samples. In contrast, hysteresis loops of the VRC samples do not close above 800 mT. This evidence suggested that the VRC samples had a higher content of high-coercivity phases than the URC samples, which was consistent with the Ms–T curves.
Fig. 3. Vertical distribution of magnetic parameters in Quaternary red earth (QRE) profile. a. χ; b. IRM20 mT and SIRM; c. χfd; d. HIRM; and e. SIRM/χ.
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Fig. 4. High-temperature magnetization (Ms–T curves) of selected samples from Quaternary red earth (QRE). a and b represent heating and cooling runs, respectively. The applied field is 300 mT.
Clay minerals in soils are frequently used as the indicator of pedogenesis because the clay mineralogy is related to the pedogenic processes occurring in the soil. The XRD spectra indicated that the different layers of QRE profile had similar XRD patterns, suggesting the presence of a mixture of quartz, kaolinite, and small amounts of iron oxide (maghemite/hematite) (Fig. 6). The maghemite and hematite were clearly detected by the peak of 0.252 nm in the XRD spectra. Compared with the URC, the VRC had obviously stronger hematite absorption intensity. Comparing the original samples with the DCB-treated samples (Fig. 6), the maghemite/hematite almost disappeared, and the other clay minerals did not change. These results further indicated the presence of maghemite/hematite in QRE. The intensity of kaolinite (0.71 nm) was obviously strong in the VRC. This suggested that the VRC had been more intensively weathered.
3.3. Iron oxide of QRE The DCB-extracted iron oxides (Fed) and the Fed/Fet ratio effectively indicate the degree of pedogenesis in tropical or subtropical region (Schwertmann, 1985). The Fed content of QRE increased with depth, which suggested that the degree of pedogenesis increased downwards (Fig. 7). The middle and lower portions of the QRE contained more free iron oxide (Fed), whereas the upper portion contained more amorphous iron oxide (Feo). QRE had a low value of the Feo/Fed ratio, which was the characteristic of mature, well drained soils, and indicated the dominance of crystalline iron oxides (hematite and goethite). Fig. 7 shows that the Feo and Feo/Fed values in VRC section were much less than those in the upper section. The
Fig. 5. Hysteresis loops of selected samples from QRE. For each sample, the hysteresis parameters were measured up to ±1.0 T.
Feo/Fed ratio in VRC section was lower than 0.10, indicating that only a small fraction of Fe oxides was in the poorly crystalline form. The χlf profiles of DCB- and acid ammonium oxalate (AAO)treated soils are shown in Fig. 8. After DCB treatment, the χlf of URC section in the QRE profile was reduced to about 20 × 10−8 m 3 kg−1, indicating the presence of small concentrations of magnetite and maghemite. The XRD could not detect these minerals due to their low contents. The χlf of VRC section in the QRE profile was reduced to less than 10 × 10 −8 m 3 kg −1. The χlf profiles of AAO-treated soils showed a similar vertical trend as the original soil (Fig. 8), which indicated that the AAO treatment did not affect the magnetic susceptibility of VRC. The χlf/Fed ratio reflects a proportion of the ferrimagnetic minerals to total secondary Fe oxides. The χlf/Fed curve of QRE profile showed a sharply decreasing trend with depth (Fig. 7). χlf/Fed ratio on VRC was much less than those in URC section. Combined with χlf profiles before and after the DCB treatment, we can conclude that the difference in values before and after extraction was due to the contribution of pedogenic ferrimagnetic minerals.
3.4. Geochemical properties of QRE The data of major elements indicated that the QRE was dominated by SiO2 (average 59.64%), Al2O3 (17.45%), and Fe2O3 (8.34%), with a relatively strong negative correlation of SiO2 with Al2O3 (R2 = 0.628, p b 0.01) and Fe2O3 (R2 = 0.569, p b 0.01). The vertical distribution of major elements showed an enrichment of Al2O3 and Fe2O3 and a leaching of SiO2 in the VRC section. These results indicated that the VRC had a higher weathering effect (Fig. 9). The significant reduction of magnetic susceptibility in the VRC section corresponded to the enrichment of Fe2O3 and Al2O. The Fe2O3 concentration from QRE ranged from 6.28 to 10.82 g kg −1 in comparison with a 6.53 g kg −1 in the URC and a 9.31 g kg−1 in the VRE (Fig. 9). Compared with the loess–paleosol sequence of northern China (Yang et al., 2004; Hao et al., 2010; Buggle et al., 2011; Qiao et al., 2011), the QRE in southern China had much lower contents of CaO, Na2O, MgO and K2O. The Mn content showed a great variation in the QRE profile, which might be attributed to the presence of Fe–Mn nodules and films in the soil. The molecular ratios of SiO2/Al2O3 (Sa), SiO2/(Al2O3 + Fe2O3) (Saf) and (Al2O3/Al2O3 + Na2O + K2O + CaO) (CIA) were used to evaluate the weathering degree of the soil (Nesbitt and Young, 1982). The Sa and Saf values of the QRE profile decreased with soil depth, which indicated that the degree of weathering and leaching gradually increased with depth. The Sa and Saf ratios of QRE were low, being in the range of 2.40–4.18 and 1.55–2.97, respectively. These results indicated a dramatic loss in the easily weatherable minerals and enrichment in the immobile elements. The Sa and Saf values in the VRC section were lower than those of the URC section, implying that the VRC with weak magnetism was more intensively weathered than the URC.
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Fig. 6. XRD spectra of selected samples in QRE profile before and after DCB treatment. A is XRD pattern details with enlarged scale before and after DCB treatment. H, hematite; K, kaolinite; and Q, quartz.
In tropical and subtropical climate conditions, intense chemical weathering resulted in a depletion of mobile elements and an accumulation of immobile elements, as compared with the original unweathered materials. The chemical index of alteration (CIA) value is often a good measure of the weathering degree of rock or soil. Obviously, the changes in CIA value along with the soil profile exhibited a similar trend as the Sa and Saf values. The CIA values of QRE profile were ranged from 69 to 77 with an average value of 75. However, the CIA values of the VRC section were relatively higher than those of the URC section. 4. Discussion 4.1. Rock magnetism of QRE sequence Magnetic variation of QRE profile in central China showed a similar vertical trend as that of QRE profiles in the middle–lower reaches
of the Yangtze River reported previously (Liu et al., 2008; Hu et al., 2009; Hong et al., 2010a). Previous studies reported that the vermiculation process in the QRE resulted in a significant reduction of magnetization (Hu et al., 2009; C.C. Liu et al., 2012). This result was confirmed by our rock magnetic results. Rock magnetic and iron oxide mineral analyses revealed that maghemite, hematite, magnetite and possibly goethite co-existed in the QRE sequence. The rock magnetic results demonstrated that the magnetic mineral composition of the VRC section was similar to that of the URC section except for a small difference in the relative concentration of these magnetic minerals. The fine-grained maghemite was shown to be preferentially depleted during vermiculation process, as suggested by the marked low χfd values and a steady decrease of magnetization below ~ 300 °C in the Ms–T curves of the VRC samples. The presence of hematite was unambiguously verified by the high-temperature magnetic susceptibility measurements (Fig. 4) and the differential XRD spectra (Fig. 6).
Fig. 7. Vertical distribution of DCB-extractable iron oxide (Fed) and amorphous iron oxide (Feo) concentrations, the ratio of DCB-extractable Fed to total iron (Fed/Fet) and the ratio of oxalate-extractable iron to free oxide iron (Feo/Fed), and χ/Fed of QRE.
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Fig. 8. Vertical distribution of magnetic susceptibility of QRE after DCB and ammonium oxalate (AO) treatments.
4.2. Mechanism of weak magnetism in the VRC section The magnetization reduction of VRC section was likely due to either the depletion of ferrimagnetic maghemite or the conversion of highly magnetic maghemite into weakly magnetic hematite (C.C. Liu et al., 2012). The content of pedogenic ferrimagnetic maghemite in the VRC section was lower and the concentration of high-coercivity hematite was higher than that of the non-vermiculated section (Fig. 3). There are several hypotheses for the weak magnetism in the VRC section. Yin and Guo (2006) reported that the weak magnetism in the VRC section resulted from the iron leaching in the white veins caused by abundant rainfall. Hu et al. (2009) suggested that the dissolution of fine-grained pedogenic maghemite during post-depositional groundwater fluctuations was responsible for the weak magnetism of VRC. They further pointed out that maghemite was more sensitive to waterlogging and more easily to be dissolved than hematite. The hypothesis of magnetic mineral dissolution, however, cannot explain the high absolute Fed concentration and the high-coercivity hematite as indicated by the peak HIRM values in the VRC section (Fig. 3). Maghemite is a common pedogenic iron oxide, which is usually formed in the initial stage of pedogenesis under aerobic conditions due to the oxidation of pre-existing
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magnetite or as an intermediate product in the transformation from ferrihydrite to hematite (Maher, 1998; Barrón and Torrent, 2002; Barrón et al., 2003). The formation of hematite is favored by the high temperatures and dry climate following a short period of limited rainfall (Schwertmann, 1985; Balsam et al., 2004). Torrent et al. (2006) showed that the relative concentration of maghemite versus hematite was influenced by the particular environment (such as seasonal drying and high temperature) and the degree of soil development. Soil studies on different location in the world indicated that the soil factors favorable to hematite formation are: high soil temperature, free drainage, pH values of 7–8, and low organic matter (Schwertmann, 1985). The abundant occurrence of pedogenic hematite in the VRC section indicates stronger weathering intensity under a warm and dry soil condition. Results of soil geochemical analyses suggested that the VRC experienced a much stronger pedogenesis than the URC. Therefore, the relatively low concentration of maghemite in the VRC section could be ascribed to an enhanced transformation from maghemite to hematite under a strong pedogenesis with a complicated geochemistry processes in the vermiculation. In addition, the small size of the pedogenic maghemite may also favor the transformation of maghemite to hematite in the vermiculation process. This mechanism is applicable to the weak magnetism and hard magnetic behavior of the vermiculated section. The pedogenic vermiculation process caused a very complicated chemical weathering, which might result in the transformation from maghemite to hematite. The transformation process not only decreased the total magnetization but also altered the relative concentration of magnetic minerals, which can be responsible for the much lower χ and χfd values. As discussed above, the pedogenic fine magnetic minerals were a predominant magnetic carrier in the QRE, however, they were largely transformed in the vermiculation process. As a result, χ of the VRC was inconsistent with the strong degree of pedogenesis. This inconsistency was also reported previously (Zhang et al., 2007; Hu et al., 2009; C.C. Liu et al., 2012). Therefore, in most cases, χ of the QRC in subtropical China cannot directly be used as a paleoclimatic proxy. Previous studies on Xuancheng soil profile showed that the SiO2/Al2O3 and Al2O3/Fe2O3 ratios and CIA values were fluctuated with the soil profile as a result of the intense climatic oscillations between warm and cool climate conditions during the soil-forming periods (Hong et al., 2010a). The leaching of mobile elements (K, Na, Ca, and Mg) in the QRE was also characterized by the CIA value, which indicated a strong chemical weathering effect, typically in a warm and humid region. The VRC showed an obvious depletion in Na2O, MgO, K2O and MnO, which indicated a strong weathering intensity or a
Fig. 9. Vertical distribution of major elements (SiO2, Al2O3, and Fe2O3) and chemical weathering indices (Sa, Saf, and CIA) of QRE.
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Table 1 Correlation coefficients between magnetic parameters and the geochemical index of weathering in QRE.
HIRM SIRM/χ
Sa
Saf
CIA
Fed
Fed/Fet
Feo
Feo/Fed
−0.718⁎⁎ −0.575⁎⁎
−0.699⁎⁎ −0.584⁎⁎
−0.778⁎⁎ −0.828⁎⁎
0.691⁎⁎ 0.567⁎⁎
0.429⁎ 0.281
−0.462⁎ −0.565⁎⁎
−0.635⁎⁎ −0.635⁎⁎
⁎⁎ p b 0.01. ⁎ p b 0.05.
longer period of weathering. Strong pedogenesis in the VRC resulted in the magnetic mineral transformation significantly different from that of the upper URC section. Therefore, the concentration-related magnetic parameters, such as χ, χfd, and SIRM, cannot accurately reflect the pedogenic intensity. 4.3. Magnetic parameters reflecting pedogenic intensity of QRE As the strong intensity of pedogenic weathering resulted in the significant reduction of magnetic susceptibility in the QRC profile, magnetic susceptibility of the QRC in subtropical China could not be used as a proxy to indicate the paleopedogenic environment during the Quaternary period. In the present study, combined with previous research, we showed that the magnetic depletion in the vermiculated section was related to the transformation from highly magnetic maghemite to weakly magnetic hematite. This process correlated with pedogenesis and provided evidence about the development of paleoclimate. Therefore, we have a chance to use the magnetic index reflecting the magnetic mineral transformation to reconstruct the paleoclimatic evolution in the vermiculation period. HIRM and SIRM/χ are two magnetic parameters most consistently correlated with other geochemical weathering indicators. As indicated in Table 1, the HIRM and SIRM/χ had a significant negative relationship with Sa, Saf and CIA, and a strong positive relationship with Fed and a negative relationship with Feo and Feo/Fed. The HIRM and SIRM/χ values increase with increasing degree of pedogenesis, which was probably caused by the transformation of remanence carrying ferrimagnetic grains to hematite. Based on the analyses of rock magnetism and iron oxide mineralogy, the pedogenic processes and paleoenvironmental implications of the VRC section could be speculated. In the initial period of the VRC formation, the pedogenic environment was moist and warm, and suited for significant formation of maghemite. After this, a period with a high temperature and seasonal drying climate prevailed, which favored the transformation from maghemite to hematite. Therefore, we speculated that south China had a much hotter and dry climate alternating with a short period of highly seasonal rainfall in the vermiculation processes. The pedogenic evolution essentially depended on the soil-forming environmental conditions; therefore, the alteration of magnetic parameters caused by pedogenic evolution can be used as the indicator of paleoclimatic evolution. Strong pedogenesis occurring in VRC section can enhance the transformation from maghemite to hematite. Low relative χlf/Fed and high HIRM correspond to an enhanced transformation of maghemite to hematite. Therefore, it may be concluded that HIRM and SIRM/χ of QRE are potentially useful for the study of paleopedogenic environments. 5. Conclusions A detailed rock magnetic investigation, coupled with iron oxides and geochemical analyses, was conducted on a Quaternary red earth of southern China. Results on the rock magnetic study indicated that magnetic minerals, including maghemite, hematite, magnetite and possibly goethite, co-existed in the QRE sequence, all of which contributed to the magnetic susceptibility. The differential XRD results showed that the VRC section had a relatively richer hematite than the URC section. The VRC section had a significantly lower magnetic susceptibility than other sections of QRE profile. A transformation from maghemite to
hematite caused by a strong weathering led to a lower magnetism of VRC section. The values of DCB-extracted iron (Fed), DCB-extracted iron/total iron ratio (Fed/Fet), and the weathering indices of the QRE sequence have confirmed that the VRC section experienced a stronger pedogenesis than the other sections. A decrease in the weathering indices of Sa and Saf suggested that the degree of weathering and leaching gradually was increased with depth. The CIA values revealed that the QRE experienced a strong chemical weathering, typically in a warm and humid region. The VRC section suffered from a more prominent depletion of NaO2, MgO, K2O and MnO as compared with the URC section, indicating a strong weathering intensity. Ferrimagnetic minerals were first formed in the wet episodes of monsoon climate, and then converted into hematite in a long drier and warmer period. The pedogenic ferrimagnetic minerals transformed into hematite under a warmer climate. Therefore, the parameters, such as χ, χfd and SIRM, commonly used as the indicator of climate changes, were not consistently reliable for the proxy of pedogenesis intensity. However, a combined use of the geochemical weathering indexes (Sa, Saf, and CIA), and magnetic parameters (HIRM and SIRM/χ) revealed a clear weathering intensity variation of Quaternary period, and therefore it might be used to detect the extent of pedogenesis. Acknowledgments This project was supported by the National Natural Science Foundation of China (No. 41171182). References Balsam, W., Ji, J., 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, 335–348. Barrón, V., Torrent, J., 2002. Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochimica et Cosmochimica Acta 66, 2801–2806. Barrón, V., Torrent, J., de Grave, E., 2003. Hydromaghemite, an intermediate in the hydrothermal transformation of 2-line ferrihydrite into hematite. American Mineralogist 88, 1679–1688. Buggle, B., Glaser, B., Hambach, U., Gerasimenko, N., Markovic, S., 2011. An evaluation of geochemical weathering indices in loess–paleosol studies. Quaternary International 240, 12–21. Deng, C.L., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q.S., Shaw, J., Zhu, R.X., 2005. Mineral magnetic variation 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/10.1029/2004JB003451. Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge University Press, Cambridge, UK 573. Hao, Q.Z., Guo, Z.T., Qiao, Y.S., Xu, B., Oldfield, F., 2010. Geochemical evidence for the provenance of Middle Pleistocene loess deposits in southern China. Quaternary Science Reviews 29, 3317–3326. Hong, H.L., Gu, Y.S., Li, R.B., Zhang, K.X., Li, Z., 2010a. Clay mineralogy and geochemistry and their palaeoclimatic interpretation of the Pleistocene deposits in the Xuancheng section, southern China. Journal of Quaternary Science 25, 662–674. Hong, H.L., Gu, Y.S., Yin, K., Zhang, K.X., Li, Z., 2010b. Red soils with white net-like veins and their climate significance in south China. Geoderma 160, 197–207. Hong, H.L., Churchman, G.J., Gu, Y.S., Yin, K., Wang, C.W., 2012. Kaolinite–smectite mixed-layer clays in the Jiujiang red soils and their climate significance. Geoderma 173 (174), 75–83. Hu, X.F., Jiang, W., Ye, W., Shen, M.N., Zhang, W.G., Wang, H.B., Lu, C.W., Zhu, L.D., 2008. Yellow-brown earth on Quaternary red clay in Langxi County, Anhui Province in subtropical China: evidence for paleoclimatic change in late Quaternary period. Journal of Plant Nutrition and Soil Science 171, 542–551. Hu, X.F., Wei, J., Xu, L.F., Zhang, G.L., Zhang, W.G., 2009. Magnetic susceptibility of the Quaternary Red Clay in subtropical China and its paleoenvironmental implications. Palaeogeography, Palaeoclimatology, Palaeoecology 279, 216–232.
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and weathering intensity. Palaeogeography, Palaeoclimatology, Palaeoecology 308, 513–523. Schulze, D.G., 1981. Identification of soil iron oxide minerals by differential X-ray diffraction. Soil Science Society of America Journal 45, 437–440. Schwertmann, U., 1973. Use of oxalate for Fe extraction from soils. Canadian Journal of Soil Science 53, 244–246. Schwertmann, U., 1985. The effect of pedogenic environments on iron oxide minerals. Advances in Soil Science 1, 171–200. Torrent, J., Barrón, V., Liu, Q.S., 2006. Magnetic enhancement is linked to and precedes hematite formation in aerobic soil. Geophysical Research Letters 33, L02401. http://dx.doi.org/10.1029/2005GL024818. Torrent, J., Liu, Q., 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, 1570–1578. Xiong, S.F., Sun, D.H., Ding, Z.L., 2002. Aeolian origin of the red earth in southeast China. Journal of Quaternary Science 17, 181–191. Yang, H., Xia, Y.F., Zhao, Q.G., Li, X.P., 1995. The character of magnetic susceptibility of red earth profile in south China and palaeo-climate changes. Acta Pedologica Sinica 32, 195–200 (in Chinese with English abstract). Yang, H., Zhao, Q.G., Li, X.P., Xia, Y.F., 1996. ESR dating of eolian sediment and red earth series from Xuancheng profile in Anhui province. Acta Pedologica Sinica 33, 293–300 (in Chinese with English abstract). Yang, S.Y., Li, C.X., Yang, D.Y., Li, X.S., 2004. Chemical weathering of the loess deposits in the lower Changjiang Valley, China, and paleoclimatic implications. Quaternary International 117, 27–34. Yin, Q.Z., Guo, Z.T., 2006. Mid-Pleistocene vermiculated red soils in southern China as an indication of unusually strengthened East Asian monsoon. Chinese Science Bulletin 51, 213–220. Yuan, B.Y., Xia, Z.K., Li, B.S., Qiao, Y.S., Gu, Z.Y., Zhang, J.F., Xu, B., Huang, W.W., Zeng, R.S., 2008. Chronostratigraphy and stratigraphic division of red soil in southern China. Quaternary Science 28, 1–13 (in Chinese with English abstract). Zhang, W.G., Yu, L.Z., Lu, M., Zheng, X.M., Shi, Y.X., 2007. Magnetic properties and geochemistry of the Xiashu Loess in the present subtropical area of China, and their implications for pedogenic intensity. Earth and Planetary Science Letters 260, 86–97. Zhao, Q.G., Yang, H., 1995. A preliminary study on red earth and changes of Quaternary environment in south China. Quaternary Science 15, 107–115 (in Chinese with English abstract).
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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo
Rock magnetism, iron oxide mineralogy and geochemistry of Quaternary red earth in central China and their paleopedogenic implication Siyuan Wang, Shi Lin, Shenggao Lu ⁎ College of Environmental and Resource Sciences, Zhejiang University, Zhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrient, Hangzhou 310058, China
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 26 March 2013 Accepted 15 April 2013 Available online 20 April 2013 Keywords: Quaternary red earth (QRE) Rock magnetism Iron oxide Weathering index Paleopedogenesis
a b s t r a c t This paper presents a detailed investigation on the rock magnetism, iron oxide mineralogy and geochemistry of a Quaternary red earth (QRE) in central China. The magnetic profile of QRE showed a characteristic vertical variation pattern: the uniform red clay (URC) section (0.2–2 m) of the profile had a higher magnetic susceptibility (χ) while vermiculated red clay (VRC) section (2–6 m) had a very low χ (b 30 × 10−8 m3 kg−1). Rock magnetism and differential XRD analysis revealed that magnetite/maghemite and hematite were the main magnetic carriers in the URC section. The major elements Fe2O3 and Al2O3 of QRE increased with depth while the SiO2 content decreased with depth, indicating a leaching of the mobile elements and an enrichment of the immobile elements. In comparison with the URC, the VRC has a greater depletion in the content of Na, Mg, K and Mg, indicating a stronger weathering intensity. The dithionite–citrate–bicarbonate (DCB) extracted iron (Fed), the ratio of Fed to total iron oxide (Fe/Fet), and the weathering indices of QRE confirmed that the VRC had experienced a stronger pedogenesis than the URC. Strong pedogenesis in the VRC resulted in a transformation of the highly magnetic maghemite to the weakly magnetic hematite, which was responsible for the weak magnetism and hard magnetic behavior of the VRC. Our studies suggest that the VRC had experienced a hot climate with a dry season following one that was warm and moister. Magnetic reduction of the VRC section resulting from the transformation of highly magnetic maghemite to weakly magnetic hematite appeared to be an important paleopedogenic process in the QRE. We concluded that the magnetic parameters, hard isothermal remanent magnetization (HIRM) and SIRM/χ of QRE, can be used for the analysis of paleopedogenic processes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Quaternary red earth (QRE) is widely distributed in southern and southeastern China and believed to be an important terrestrial record archive of Quaternary paleoclimatic and paleoenvironment changes in subtropical–tropical areas (Huang et al., 1996; Lu et al., 1999; Yin and Guo, 2006; Lu, 2007; Hu et al., 2008, 2010; Hong et al., 2010a,b, 2012). Paleomagnetic, optically-stimulated luminescence (OSL) and electron spin resonance (ESR) dating so far suggest that the QRE in China had been deposited since the mid-Pleistocene age (Zhao and Yang, 1995; Huang et al., 1996; Yang et al., 1996). This QRE sequence was usually thought to be produced by strong pedogenesis with intense oxidation and leaching, which resulted from enhanced East Asian summer monsoon activity (Huang et al., 1996; Yin and Guo, 2006; Yuan et al., 2008). Yin and Guo (2006) pointed out that the QRE in southern China was developed during the periods of greatest warmth and humidity, which correlates with the paleosol units S4 and S5 of midPleistocene (approx. 0.8 Ma) in the Loess Plateau of northern China. The QRE in southeastern China often consists of three different morphological sections from top to bottom: yellow-brown earth ⁎ Corresponding author. E-mail address: [email protected] (S. Lu). 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.04.010
(YBE), uniform red clay (URC) and vermiculated red clay (VRC). The VRC layer constitutes the main Quaternary stratigraphic unit and is much more weathered than the sections above and below (Huang et al., 1996; Yuan et al., 2008; Hu et al., 2009; Hong et al., 2010a). The underlying bottom section is the fluvial gravel deposits. The QRE is characterized by the red color, strong acidity, clayey structure and high degree of weathering, with heavy leaching of mobile elements (Ca, Mg, K, Na) and relative enrichment of immobile elements (Fe and Al), and kaolinite as the dominant clay mineral (Hu et al., 2009; Hong et al., 2012). Therefore, QRE is commonly used to probe into the Quaternary paleoclimatic and paleoenvironmental processes (Huang et al., 1999; Qiao et al., 2011; C.C. Liu et al., 2012). Soil magnetism studies have indicated that the magnetic enhancement effects of pedogenesis could be used as a proxy to estimate the pedogenic chemical weathering intensity and a tool to reconstruct pedoenvironmental processes (Maher, 1998; Maher et al., 2003; Torrent et al., 2007; Q.S. Liu et al., 2012). In classic loess–paleosol studies of China, the soil magnetic measurements have been widely used to indicate the climate fluctuations recorded in loess–paleosol sequences (Maher, 1998; Balsam et al., 2004; Q.S. Liu et al., 2012). The magnetic susceptibility of QRE also has been used to investigate paleoenvironmental change as in loess−paleosol sequences (Yang et al., 1995). However, Hu et al. (2009) thought that the magnetic
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susceptibility of QRE could not be used as a paleoclimatic proxy after they measured the magnetic susceptibility of 11 typical QRE profiles in subtropical China. Recent studies indicated that strong pedogenesis occurring in QRE could enhance the transformation of highly magnetic maghemite to weakly magnetic hematite. The relative concentration of pedogenic maghemite versus hematite is correlated with the pedogenic intensity (C.C. Liu et al., 2012). Therefore, parameters such as magnetic susceptibility (χ), anhysteretic remanent magnetization (χARM) and saturation isothermal remanent magnetization (SIRM), which denote the absolute magnetization of the soils, cannot accurately reflect the pedogenic intensity in QRE. The S-ratio, which can be used to quantify the relative concentration of antiferromagnetic minerals, is a good climate index in the vermiculated red soils (C.C. Liu et al., 2012). The hard isothermal remanent magnetization (HIRM) can also be used to indicate pedogenic intensity. Current research indicates that there have been at least three environmental shifts in southeast China since the early Pleistocene. A very warm and dry climate followed by significant rainfalls in 1.0–1.2 Ma ago resulted in the occurrence of VRC. During the period of 1.0–1.2 to 0.85–0.9 Ma, pedogenesis was strengthened gradually possibly by a strong East Asian summer monsoon (Xiong et al., 2002; Yin and Guo, 2006; Liu et al., 2008). After 0.85–0.9 Ma, pedogenesis in southeast China gradually became weak because the strengthened winter monsoon had significantly affected the paleoclimate in southeast China since 0.85 Ma, which caused a weakening of pedogenesis after that time. Apparently, further study is needed to understand the role of pedoenvironmental changes in determining the soil magnetic properties and to reveal whether the magnetism of the QRE has any specific implications on paleoclimate. Previous studies on QRE were mainly focused on the south of middle to lower reaches of the Yangtze River in China (Fig. 1) and detailed magnetic investigation on QRE in the subtropical–tropical regions as a whole remained very scarce. These studies were only focused on the QRE in several limited areas in the middle to lower reaches of the
Yangtze River. The mechanisms and paleoenvironmental implication of magnetic variation in the QRE are still in debate. Especially, the mechanism of the magnetization reduction in the vermiculated section has not been resolved. Few studies using integrated proxies such as magnetic susceptibility, iron oxide minerals and geochemistry have been used for examining these climate changes. The objective of this study was to investigate if the magnetic mineralogy and geochemistry of QRE can provide more insights into the paleoenvironmental change in central China, with particular reference to the pedogenic processes involved in the magnetic mineral transformation and the relationships between these and iron oxide phases. 2. Materials and methods 2.1. Soil profile and sampling The studied Quaternary red earth (QRE) profile is situated in Qiyang County (111°52′E, 26°45′), Hunan Province of central China (Fig. 1). The present climate is a typical middle subtropical monsoon climate with a mean annual precipitation of 1255 mm, mean annual evaporation of 1431 mm, and mean annual temperature of 18 °C. This profile was excavated for building materials with a depth of 650 cm, which provided a suitable profile to collect soil samples. The profile was subdivided into four pedostratigraphical units from top to bottom according to the color, texture, and occurrence of worm- or net-like white veins. The upper first section (0 to 20 cm) was a cultivated soil horizon with loose texture and abundant organic matter and plant roots. The second section (20 to 200 cm) was a uniform red clay (URC), consisting of reddish to red-brown clays with a nuclear blocky structure and abundant dark films and Fe and Mn nodules. The third section (200 to 600 cm) was the vermiculated red clay (VRC) consisting of red-brown to darkbrown clays with white worm- or net-like veins. The bottom section (b 600 cm) was the underlying fluvial gravel deposits. Soil samples were collected at an interval of 10 cm. The samples were air-dried,
Fig. 1. Schematic map showing the distribution of Quaternary red earth (QRE) in southern and southeastern China. The studied QRE profile is located in Qiyang County of Hunan Province of central China. The shaded area indicates loess deposits in northern China and brown-red area is Quaternary red earth region of southern China. The Qiliting (1), Xuancheng (2) and Jiujiang (3) QRE sequences were typical QRE profile in the middle to lower reaches of the Yangtze River (Liu et al., 2008; Hu et al., 2009; Hong et al., 2010a). Map is modified after Huang et al. (1999) and C.C. Liu et al. (2012).
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ground and passed through a 2 mm sieve. For geochemical analyses, the soil samples were ground to pass 0.15 mm nylon sieve. 2.2. Soil chemical analysis Soil pH was measured in H2O and in 1 M KCl (soil:solution, 1:2.5) using a pH meter. Organic carbon was determined according to the wet oxidation procedure. Particle size distribution was determined by a combination of the wet-sieving and pipette method. Soil color was determined using the Munsell color charts (X-Rite, Grand Rapids, MI, USA). Total iron content (Fet) of soil samples was determined on dissolution extracts obtained by acid attack with a mixture of HClO4: HNO3:HF. The free iron in soils was determined by extraction with dithionite–citrate–bicarbonate (DCB) (Mehra and Jackson, 1960) and amorphous iron was extracted using a 0.2 M ammonium oxalate (pH 3.0) (Schwertmann, 1973). Iron content of the extracted solution was determined by an ultraviolet spectrophotometry. The concentration of major elements was determined by X-ray fluorescence spectrometry (XRF) (Philips PW2440 XRD). A national standard material (GBW07407) was measured in the same manner to check the accuracy of the analysis. Analytical uncertainties were controlled within 5% for all major elements. Elemental content was converted to the weight percentage of oxide. The weight percentage of oxide was normalized to molecular weight and used to calculate the following molecular ratio as the indicators of soil-forming environment, for example, Sa = SiO2/Al2O3 and Saf = SiO2/Al2O3 + Fe2O3. The chemical index of alteration (CIA) was calculated following Nesbitt and Young (1982), i.e., CIA = [Al2O3 / (Al2O3 + Na2O + K2O + CaO*)] × 100, where CaO* represents the amount of CaO in the silicate fraction of the sample. A larger CIA value corresponds to a greater degree of leaching and removal of cations (Ca 2+, Na + and K+) with a CIA of 100 being referred to kaolinite and/or gibbsite. The molar ratios of free Fe-oxide (Fed) and amorphous Fe oxide (Feo) to total elemental Fe-oxide (Fed/Fet and Feo/Fet) were calculated as the additional weathering indices (Schwertmann, 1985).
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using the DCB procedure. XRD analyses were carried out on a Rigaku X-ray diffractometer (D/Max 2550 PC, Rigaku Corporation, Japan) using Cu Kα radiation (45 kV, 300 mA) at the XRD Laboratory, Zhejiang University. The XRD patterns of the DCB treated and untreated samples were collected from 5° to 75° 2θ at a scan rate of 0.2° 2θ/min. 3. Results 3.1. Soil properties of QRE The QRE has a hue of 5.5 YR or 7.5 YR (Fig. 2b). This hue indicates the presence of hematite in the soil (Schwertmann, 1985). The soil pHH2O and pHKCl values of QRE are generally low, especially in the URC (Fig. 2a), reflecting the predominance of acidic cations (Al3+ and H+) at exchange sites. The organic matter concentration in the QRE shows the highest values in the surface soil and decreases sharply with depth (Fig. 2c). In the 450–550 cm layer, the pH and organic matter of QRE profile fluctuate due to the fluctuation in pedogenic environment and weathering intensity. Clay is the dominant size fraction in all soil horizons (Fig. 2d), with a relatively greater change in the amounts of silt and sand particles. The sand/clay ratio increases with profile depth (Fig. 2e). The VRC unit has the highest sand/clay ratio. 3.2. Magnetic properties of the QRE
2.4. Differential X-ray diffraction (DXRD)
3.2.1. Magnetic parameter profiles The magnetic parameter profiles of QRE are shown in Fig. 3. The χlf of soil decreases with depth and reaches the lowest level in the VRC section. Magnetic susceptibility variation pattern of the QRE, being consistent with previous studies in the middle to lower reaches of the Yangtze River, had the lowest value in the VRC section and higher values in the URC section (Lu et al., 1999; Liu et al., 2008; Hu et al., 2009; Liu et al., 2010; C.C. Liu et al., 2012). The IRM20 mT and SIRM values of soil showed similar variation patterns, and were much lower in the vermiculated section than those of the other sections at the upper portion of the profile. However, SIRM showed a sudden enhancement in 370–460 cm layer. The correlation between χ and IRM20 mT was highly significant (R 2 = 0.772, p b 0.01), suggesting that the magnetism of the QRE was mainly due to ferrimagnetic minerals. Much lower χ and SIRM values in the VRC section suggested small concentration of ferrimagnetic minerals. However, other parameters, such as HIRM and SIRM/χ, showed variation patterns significantly different from those of the magnetic susceptibility. The VRC section had much higher HIRM and SIRM/χ values than the URC section. The χfd profile of QRE indicated that the absolute χfd of URC ranged from 5 × 10 −8 to 18 × 10 −8 m 3 kg −1, and that χfd of VRC was b5 × 10 −8 and SIRM/χ increased with depth. The SIRM/χ can be used to detect the variation of the magnetic grain size (Zhang et al., 2007; Q.S. Liu et al., 2012). The lower SIRM/χ values can be ascribed to the higher concentration of small particles, e.g., viscous SP grains. In contrast, the HIRM curves of the QRE profile were different from the χ curves. HIRM values were lowest in the URC and significantly enhanced in the VRC. HIRM reflects the absolute and relative concentrations of antiferromagnetic minerals (such as hematite and goethite) in the mineral mixtures. Results indicated that the weakly magnetic and high-coercivity antiferromagnetic minerals contributed to magnetic signals of VRC section. This suggested that the VRC with weak magnetism contained less ferrimagnetic minerals but more hematite and/or goethite.
Iron oxide mineralogy was determined by differential X-ray diffraction (DXRD) (Schulze, 1981). The clay fraction (b2 μm) of the soil samples was separated using a sedimentation method according to Stokes' law and treated for selective dissolution of iron oxides
3.2.2. Magnetic mineralogy of QRE High-temperature magnetization (Ms–T curve) measurements provide useful information about magnetic mineralogy. Fig. 4 shows the Ms–T curves of selected samples of the QRE sequence. The Ms–T
2.3. Rock magnetic measurements Soil magnetic susceptibility was measured using a Bartington MS2 meter at low (0.47 kHz, χlf) and high (4.7 kHz, χhf) frequency, respectively. Isothermal remanent magnetization (IRM) was produced in progressively increasing magnetic fields up to 1000 mT with a Molspin pulse magnetizer, and the induced remanence after each field was measured using a Molspin spinner magnetometer. The IRM acquired in the maximum field of 1000 mT was defined as the saturation isothermal remanent magnetization (SIRM) of the sample. Massspecific frequency-dependent susceptibility (χfd) and hard isothermal remanent magnetization (HIRM) were calculated by the following formulae: χfd = (χlf − χhf), and HIRM = (SIRM − IRM300 mT). χfd reflects the presence of fine viscous ferrimagnetic grains close to the superparamagnetic/stable single domain (SP/SD) boundary (~30 nm) (Maher, 1998; Zhang et al., 2007; Q.S. Liu et al., 2012). HIRM provides a rough estimate on the concentration of antiferromagnetic minerals (Zhang et al., 2007; Q.S. Liu et al., 2012). High-temperature magnetization (Ms–T curves) and hysteresis parameter measurements on the selected samples were performed using a Magnetic Measurements Variable Field Translation Balance (VFTB) at the State Key Laboratory of Estuarine and Coastal Research (SKLEC), East China Normal University.
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Fig. 2. Vertical distribution of soil properties in Quaternary red earth (QRE) profile. a. pH; b. soil color; c. organic carbon; d. particle size composition; and e. sand/clay ratio.
curve of URC samples showed a rapid decrease in magnetization with the temperature rise and gave the Curie temperature of a magnetite. The rapid and significant loss in Ms after heating might indicate a full oxidation and even a destruction of maghemite/magnetite to hematite, which indicated the presence of pedogenic maghemite/ magnetite in the URC section. The Ms–T curves of VRC section displayed a steady magnetization increase below ~300 °C and a steady decrease above 300 °C. The characteristic Ms–T curves of VRC sample may arise from the new formation of minor amount of ferrimagnetic phases, e.g. maghemite (γ-Fe2O3) and the conversion of fine-grained maghemite to hematite (α-Fe2O3) (Deng et al., 2005; C.C. Liu et al., 2012; Q.S. Liu et al., 2012). The Ms decrease between 300 and 450 °C for VRC sample is much smaller than that for the URC sample, which is ascribed to lower concentration of maghemite in the URC sample. Hematite was usually not obvious in the Ms–T curves due to its weak magnetism, however, Ms–T curves of VRC samples displayed a marked Ms decrease in the temperature range between 580 °C and ~680 °C (the Néel temperature of hematite). The significant Ms decrease between 580 °C and ~680 °C
indicates the presence of hematite in the VRC sample. Therefore, the Ms–T curves in the URC and VRC sections display significant difference in the Ms decrease between 580 °C and ~680 °C. The contribution of hematite to magnetization of the VRC samples was greater than that of the URC samples. The cooling curves of the different layers in the QRE profile showed a similar trend, except for a sharp increase around 680 °C of VRC samples. Hysteresis loops provide information about the coercivity spectrum (Dunlop and Özdemir, 1997). All selected samples of QRE profile displayed wasp-waisted hysteresis loops (Fig. 5), which usually are due to the coexistence of two magnetic components with strongly contrasting coercivities. The relative concentrations of two magnetic components varied with the different layers. Hysteresis loops of the URC samples were closed in lower fields than those of the VRC samples. In contrast, hysteresis loops of the VRC samples do not close above 800 mT. This evidence suggested that the VRC samples had a higher content of high-coercivity phases than the URC samples, which was consistent with the Ms–T curves.
Fig. 3. Vertical distribution of magnetic parameters in Quaternary red earth (QRE) profile. a. χ; b. IRM20 mT and SIRM; c. χfd; d. HIRM; and e. SIRM/χ.
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Fig. 4. High-temperature magnetization (Ms–T curves) of selected samples from Quaternary red earth (QRE). a and b represent heating and cooling runs, respectively. The applied field is 300 mT.
Clay minerals in soils are frequently used as the indicator of pedogenesis because the clay mineralogy is related to the pedogenic processes occurring in the soil. The XRD spectra indicated that the different layers of QRE profile had similar XRD patterns, suggesting the presence of a mixture of quartz, kaolinite, and small amounts of iron oxide (maghemite/hematite) (Fig. 6). The maghemite and hematite were clearly detected by the peak of 0.252 nm in the XRD spectra. Compared with the URC, the VRC had obviously stronger hematite absorption intensity. Comparing the original samples with the DCB-treated samples (Fig. 6), the maghemite/hematite almost disappeared, and the other clay minerals did not change. These results further indicated the presence of maghemite/hematite in QRE. The intensity of kaolinite (0.71 nm) was obviously strong in the VRC. This suggested that the VRC had been more intensively weathered.
3.3. Iron oxide of QRE The DCB-extracted iron oxides (Fed) and the Fed/Fet ratio effectively indicate the degree of pedogenesis in tropical or subtropical region (Schwertmann, 1985). The Fed content of QRE increased with depth, which suggested that the degree of pedogenesis increased downwards (Fig. 7). The middle and lower portions of the QRE contained more free iron oxide (Fed), whereas the upper portion contained more amorphous iron oxide (Feo). QRE had a low value of the Feo/Fed ratio, which was the characteristic of mature, well drained soils, and indicated the dominance of crystalline iron oxides (hematite and goethite). Fig. 7 shows that the Feo and Feo/Fed values in VRC section were much less than those in the upper section. The
Fig. 5. Hysteresis loops of selected samples from QRE. For each sample, the hysteresis parameters were measured up to ±1.0 T.
Feo/Fed ratio in VRC section was lower than 0.10, indicating that only a small fraction of Fe oxides was in the poorly crystalline form. The χlf profiles of DCB- and acid ammonium oxalate (AAO)treated soils are shown in Fig. 8. After DCB treatment, the χlf of URC section in the QRE profile was reduced to about 20 × 10−8 m 3 kg−1, indicating the presence of small concentrations of magnetite and maghemite. The XRD could not detect these minerals due to their low contents. The χlf of VRC section in the QRE profile was reduced to less than 10 × 10 −8 m 3 kg −1. The χlf profiles of AAO-treated soils showed a similar vertical trend as the original soil (Fig. 8), which indicated that the AAO treatment did not affect the magnetic susceptibility of VRC. The χlf/Fed ratio reflects a proportion of the ferrimagnetic minerals to total secondary Fe oxides. The χlf/Fed curve of QRE profile showed a sharply decreasing trend with depth (Fig. 7). χlf/Fed ratio on VRC was much less than those in URC section. Combined with χlf profiles before and after the DCB treatment, we can conclude that the difference in values before and after extraction was due to the contribution of pedogenic ferrimagnetic minerals.
3.4. Geochemical properties of QRE The data of major elements indicated that the QRE was dominated by SiO2 (average 59.64%), Al2O3 (17.45%), and Fe2O3 (8.34%), with a relatively strong negative correlation of SiO2 with Al2O3 (R2 = 0.628, p b 0.01) and Fe2O3 (R2 = 0.569, p b 0.01). The vertical distribution of major elements showed an enrichment of Al2O3 and Fe2O3 and a leaching of SiO2 in the VRC section. These results indicated that the VRC had a higher weathering effect (Fig. 9). The significant reduction of magnetic susceptibility in the VRC section corresponded to the enrichment of Fe2O3 and Al2O. The Fe2O3 concentration from QRE ranged from 6.28 to 10.82 g kg −1 in comparison with a 6.53 g kg −1 in the URC and a 9.31 g kg−1 in the VRE (Fig. 9). Compared with the loess–paleosol sequence of northern China (Yang et al., 2004; Hao et al., 2010; Buggle et al., 2011; Qiao et al., 2011), the QRE in southern China had much lower contents of CaO, Na2O, MgO and K2O. The Mn content showed a great variation in the QRE profile, which might be attributed to the presence of Fe–Mn nodules and films in the soil. The molecular ratios of SiO2/Al2O3 (Sa), SiO2/(Al2O3 + Fe2O3) (Saf) and (Al2O3/Al2O3 + Na2O + K2O + CaO) (CIA) were used to evaluate the weathering degree of the soil (Nesbitt and Young, 1982). The Sa and Saf values of the QRE profile decreased with soil depth, which indicated that the degree of weathering and leaching gradually increased with depth. The Sa and Saf ratios of QRE were low, being in the range of 2.40–4.18 and 1.55–2.97, respectively. These results indicated a dramatic loss in the easily weatherable minerals and enrichment in the immobile elements. The Sa and Saf values in the VRC section were lower than those of the URC section, implying that the VRC with weak magnetism was more intensively weathered than the URC.
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Fig. 6. XRD spectra of selected samples in QRE profile before and after DCB treatment. A is XRD pattern details with enlarged scale before and after DCB treatment. H, hematite; K, kaolinite; and Q, quartz.
In tropical and subtropical climate conditions, intense chemical weathering resulted in a depletion of mobile elements and an accumulation of immobile elements, as compared with the original unweathered materials. The chemical index of alteration (CIA) value is often a good measure of the weathering degree of rock or soil. Obviously, the changes in CIA value along with the soil profile exhibited a similar trend as the Sa and Saf values. The CIA values of QRE profile were ranged from 69 to 77 with an average value of 75. However, the CIA values of the VRC section were relatively higher than those of the URC section. 4. Discussion 4.1. Rock magnetism of QRE sequence Magnetic variation of QRE profile in central China showed a similar vertical trend as that of QRE profiles in the middle–lower reaches
of the Yangtze River reported previously (Liu et al., 2008; Hu et al., 2009; Hong et al., 2010a). Previous studies reported that the vermiculation process in the QRE resulted in a significant reduction of magnetization (Hu et al., 2009; C.C. Liu et al., 2012). This result was confirmed by our rock magnetic results. Rock magnetic and iron oxide mineral analyses revealed that maghemite, hematite, magnetite and possibly goethite co-existed in the QRE sequence. The rock magnetic results demonstrated that the magnetic mineral composition of the VRC section was similar to that of the URC section except for a small difference in the relative concentration of these magnetic minerals. The fine-grained maghemite was shown to be preferentially depleted during vermiculation process, as suggested by the marked low χfd values and a steady decrease of magnetization below ~ 300 °C in the Ms–T curves of the VRC samples. The presence of hematite was unambiguously verified by the high-temperature magnetic susceptibility measurements (Fig. 4) and the differential XRD spectra (Fig. 6).
Fig. 7. Vertical distribution of DCB-extractable iron oxide (Fed) and amorphous iron oxide (Feo) concentrations, the ratio of DCB-extractable Fed to total iron (Fed/Fet) and the ratio of oxalate-extractable iron to free oxide iron (Feo/Fed), and χ/Fed of QRE.
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Fig. 8. Vertical distribution of magnetic susceptibility of QRE after DCB and ammonium oxalate (AO) treatments.
4.2. Mechanism of weak magnetism in the VRC section The magnetization reduction of VRC section was likely due to either the depletion of ferrimagnetic maghemite or the conversion of highly magnetic maghemite into weakly magnetic hematite (C.C. Liu et al., 2012). The content of pedogenic ferrimagnetic maghemite in the VRC section was lower and the concentration of high-coercivity hematite was higher than that of the non-vermiculated section (Fig. 3). There are several hypotheses for the weak magnetism in the VRC section. Yin and Guo (2006) reported that the weak magnetism in the VRC section resulted from the iron leaching in the white veins caused by abundant rainfall. Hu et al. (2009) suggested that the dissolution of fine-grained pedogenic maghemite during post-depositional groundwater fluctuations was responsible for the weak magnetism of VRC. They further pointed out that maghemite was more sensitive to waterlogging and more easily to be dissolved than hematite. The hypothesis of magnetic mineral dissolution, however, cannot explain the high absolute Fed concentration and the high-coercivity hematite as indicated by the peak HIRM values in the VRC section (Fig. 3). Maghemite is a common pedogenic iron oxide, which is usually formed in the initial stage of pedogenesis under aerobic conditions due to the oxidation of pre-existing
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magnetite or as an intermediate product in the transformation from ferrihydrite to hematite (Maher, 1998; Barrón and Torrent, 2002; Barrón et al., 2003). The formation of hematite is favored by the high temperatures and dry climate following a short period of limited rainfall (Schwertmann, 1985; Balsam et al., 2004). Torrent et al. (2006) showed that the relative concentration of maghemite versus hematite was influenced by the particular environment (such as seasonal drying and high temperature) and the degree of soil development. Soil studies on different location in the world indicated that the soil factors favorable to hematite formation are: high soil temperature, free drainage, pH values of 7–8, and low organic matter (Schwertmann, 1985). The abundant occurrence of pedogenic hematite in the VRC section indicates stronger weathering intensity under a warm and dry soil condition. Results of soil geochemical analyses suggested that the VRC experienced a much stronger pedogenesis than the URC. Therefore, the relatively low concentration of maghemite in the VRC section could be ascribed to an enhanced transformation from maghemite to hematite under a strong pedogenesis with a complicated geochemistry processes in the vermiculation. In addition, the small size of the pedogenic maghemite may also favor the transformation of maghemite to hematite in the vermiculation process. This mechanism is applicable to the weak magnetism and hard magnetic behavior of the vermiculated section. The pedogenic vermiculation process caused a very complicated chemical weathering, which might result in the transformation from maghemite to hematite. The transformation process not only decreased the total magnetization but also altered the relative concentration of magnetic minerals, which can be responsible for the much lower χ and χfd values. As discussed above, the pedogenic fine magnetic minerals were a predominant magnetic carrier in the QRE, however, they were largely transformed in the vermiculation process. As a result, χ of the VRC was inconsistent with the strong degree of pedogenesis. This inconsistency was also reported previously (Zhang et al., 2007; Hu et al., 2009; C.C. Liu et al., 2012). Therefore, in most cases, χ of the QRC in subtropical China cannot directly be used as a paleoclimatic proxy. Previous studies on Xuancheng soil profile showed that the SiO2/Al2O3 and Al2O3/Fe2O3 ratios and CIA values were fluctuated with the soil profile as a result of the intense climatic oscillations between warm and cool climate conditions during the soil-forming periods (Hong et al., 2010a). The leaching of mobile elements (K, Na, Ca, and Mg) in the QRE was also characterized by the CIA value, which indicated a strong chemical weathering effect, typically in a warm and humid region. The VRC showed an obvious depletion in Na2O, MgO, K2O and MnO, which indicated a strong weathering intensity or a
Fig. 9. Vertical distribution of major elements (SiO2, Al2O3, and Fe2O3) and chemical weathering indices (Sa, Saf, and CIA) of QRE.
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Table 1 Correlation coefficients between magnetic parameters and the geochemical index of weathering in QRE.
HIRM SIRM/χ
Sa
Saf
CIA
Fed
Fed/Fet
Feo
Feo/Fed
−0.718⁎⁎ −0.575⁎⁎
−0.699⁎⁎ −0.584⁎⁎
−0.778⁎⁎ −0.828⁎⁎
0.691⁎⁎ 0.567⁎⁎
0.429⁎ 0.281
−0.462⁎ −0.565⁎⁎
−0.635⁎⁎ −0.635⁎⁎
⁎⁎ p b 0.01. ⁎ p b 0.05.
longer period of weathering. Strong pedogenesis in the VRC resulted in the magnetic mineral transformation significantly different from that of the upper URC section. Therefore, the concentration-related magnetic parameters, such as χ, χfd, and SIRM, cannot accurately reflect the pedogenic intensity. 4.3. Magnetic parameters reflecting pedogenic intensity of QRE As the strong intensity of pedogenic weathering resulted in the significant reduction of magnetic susceptibility in the QRC profile, magnetic susceptibility of the QRC in subtropical China could not be used as a proxy to indicate the paleopedogenic environment during the Quaternary period. In the present study, combined with previous research, we showed that the magnetic depletion in the vermiculated section was related to the transformation from highly magnetic maghemite to weakly magnetic hematite. This process correlated with pedogenesis and provided evidence about the development of paleoclimate. Therefore, we have a chance to use the magnetic index reflecting the magnetic mineral transformation to reconstruct the paleoclimatic evolution in the vermiculation period. HIRM and SIRM/χ are two magnetic parameters most consistently correlated with other geochemical weathering indicators. As indicated in Table 1, the HIRM and SIRM/χ had a significant negative relationship with Sa, Saf and CIA, and a strong positive relationship with Fed and a negative relationship with Feo and Feo/Fed. The HIRM and SIRM/χ values increase with increasing degree of pedogenesis, which was probably caused by the transformation of remanence carrying ferrimagnetic grains to hematite. Based on the analyses of rock magnetism and iron oxide mineralogy, the pedogenic processes and paleoenvironmental implications of the VRC section could be speculated. In the initial period of the VRC formation, the pedogenic environment was moist and warm, and suited for significant formation of maghemite. After this, a period with a high temperature and seasonal drying climate prevailed, which favored the transformation from maghemite to hematite. Therefore, we speculated that south China had a much hotter and dry climate alternating with a short period of highly seasonal rainfall in the vermiculation processes. The pedogenic evolution essentially depended on the soil-forming environmental conditions; therefore, the alteration of magnetic parameters caused by pedogenic evolution can be used as the indicator of paleoclimatic evolution. Strong pedogenesis occurring in VRC section can enhance the transformation from maghemite to hematite. Low relative χlf/Fed and high HIRM correspond to an enhanced transformation of maghemite to hematite. Therefore, it may be concluded that HIRM and SIRM/χ of QRE are potentially useful for the study of paleopedogenic environments. 5. Conclusions A detailed rock magnetic investigation, coupled with iron oxides and geochemical analyses, was conducted on a Quaternary red earth of southern China. Results on the rock magnetic study indicated that magnetic minerals, including maghemite, hematite, magnetite and possibly goethite, co-existed in the QRE sequence, all of which contributed to the magnetic susceptibility. The differential XRD results showed that the VRC section had a relatively richer hematite than the URC section. The VRC section had a significantly lower magnetic susceptibility than other sections of QRE profile. A transformation from maghemite to
hematite caused by a strong weathering led to a lower magnetism of VRC section. The values of DCB-extracted iron (Fed), DCB-extracted iron/total iron ratio (Fed/Fet), and the weathering indices of the QRE sequence have confirmed that the VRC section experienced a stronger pedogenesis than the other sections. A decrease in the weathering indices of Sa and Saf suggested that the degree of weathering and leaching gradually was increased with depth. The CIA values revealed that the QRE experienced a strong chemical weathering, typically in a warm and humid region. The VRC section suffered from a more prominent depletion of NaO2, MgO, K2O and MnO as compared with the URC section, indicating a strong weathering intensity. Ferrimagnetic minerals were first formed in the wet episodes of monsoon climate, and then converted into hematite in a long drier and warmer period. The pedogenic ferrimagnetic minerals transformed into hematite under a warmer climate. Therefore, the parameters, such as χ, χfd and SIRM, commonly used as the indicator of climate changes, were not consistently reliable for the proxy of pedogenesis intensity. However, a combined use of the geochemical weathering indexes (Sa, Saf, and CIA), and magnetic parameters (HIRM and SIRM/χ) revealed a clear weathering intensity variation of Quaternary period, and therefore it might be used to detect the extent of pedogenesis. Acknowledgments This project was supported by the National Natural Science Foundation of China (No. 41171182). References Balsam, W., Ji, J., 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, 335–348. Barrón, V., Torrent, J., 2002. Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochimica et Cosmochimica Acta 66, 2801–2806. Barrón, V., Torrent, J., de Grave, E., 2003. Hydromaghemite, an intermediate in the hydrothermal transformation of 2-line ferrihydrite into hematite. American Mineralogist 88, 1679–1688. Buggle, B., Glaser, B., Hambach, U., Gerasimenko, N., Markovic, S., 2011. An evaluation of geochemical weathering indices in loess–paleosol studies. Quaternary International 240, 12–21. Deng, C.L., Vidic, N.J., Verosub, K.L., Singer, M.J., Liu, Q.S., Shaw, J., Zhu, R.X., 2005. Mineral magnetic variation 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/10.1029/2004JB003451. Dunlop, D.J., Özdemir, Ö., 1997. Rock Magnetism: Fundamentals and Frontiers. Cambridge University Press, Cambridge, UK 573. Hao, Q.Z., Guo, Z.T., Qiao, Y.S., Xu, B., Oldfield, F., 2010. Geochemical evidence for the provenance of Middle Pleistocene loess deposits in southern China. Quaternary Science Reviews 29, 3317–3326. Hong, H.L., Gu, Y.S., Li, R.B., Zhang, K.X., Li, Z., 2010a. Clay mineralogy and geochemistry and their palaeoclimatic interpretation of the Pleistocene deposits in the Xuancheng section, southern China. Journal of Quaternary Science 25, 662–674. Hong, H.L., Gu, Y.S., Yin, K., Zhang, K.X., Li, Z., 2010b. Red soils with white net-like veins and their climate significance in south China. Geoderma 160, 197–207. Hong, H.L., Churchman, G.J., Gu, Y.S., Yin, K., Wang, C.W., 2012. Kaolinite–smectite mixed-layer clays in the Jiujiang red soils and their climate significance. Geoderma 173 (174), 75–83. Hu, X.F., Jiang, W., Ye, W., Shen, M.N., Zhang, W.G., Wang, H.B., Lu, C.W., Zhu, L.D., 2008. Yellow-brown earth on Quaternary red clay in Langxi County, Anhui Province in subtropical China: evidence for paleoclimatic change in late Quaternary period. Journal of Plant Nutrition and Soil Science 171, 542–551. Hu, X.F., Wei, J., Xu, L.F., Zhang, G.L., Zhang, W.G., 2009. Magnetic susceptibility of the Quaternary Red Clay in subtropical China and its paleoenvironmental implications. Palaeogeography, Palaeoclimatology, Palaeoecology 279, 216–232.
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