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Roles of sorting and chemical weathering in the geochemistry and magnetic susceptibility of Xiashu loess, East China
Journal of Asian Earth Sciences 29 (2007) 813–822 www.elsevier.com/locate/jaes
Roles of sorting and chemical weathering in the geochemistry and magnetic susceptibility of Xiashu loess, East China Fuchun Li
a,*
, Zhangdong Jin b,c, Changren Xie a, Jiayi Feng d, Libo Wang e, Yongzhao Yang a
a College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China Key Laboratory of Lake Sedimentation and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710054, China d Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China e Nanjing Institute of Geology and Mineral Resources, The Ministry of Land and Resources, Nanjing 210016, China b
c
Received 11 October 2004; received in revised form 13 May 2005; accepted 26 May 2005
Abstract Uncertainties in paleoenvironmental interpretations for traditional chemical analysis of bulk samples result from different grain-size sub-populations of sediments containing variable distributions of elements and minerals. Therefore, it is important to understand the elemental and mineral distribution in different grain sizes in determining the quantitative relationship between chemical weathering and climatic change. We sieved a series of Xiashu loess samples into three sub-populations of different grain sizes (<2, 2–45 and >45 lm, respectively), and then analyzed each population for rubidium (Rb), strontium (Sr), rare earth elements and magnetic susceptibility. In comparison with elemental concentrations of bulk samples, clay mineralogy and illitic crystallinity, our results show that distinct elemental distributions and magnetic susceptibilities for different grain-size sub-populations are controlled by sorting and/or chemical weathering, although we also suggest that the Xiashu loess may have the same provenance as the Central Chinese Loess. Maximum concentrations of Rb and fine-grained magnetic minerals in the less than 2 lm sub-population, coupled with our finding of maximum Sr in the larger than 45 lm fraction, indicate that Sr was lost during chemical weathering. Grain-size sub-population analysis is, therefore, an effective method for extracting paleoenvironmental information, because individual sub-populations show minimal variations in initial Rb/Sr ratios compared to bulk analysis of all sizes together. Furthermore, a negative correlation between Rb/Sr ratios and Sr concentrations for the <2 lm fraction (R2 = 0.97) may indicate that clay is a sensitive indicator of intensity of chemical weathering and is an ideal sub-population for determining Rb/Sr ratios, but not for magnetic susceptibility. 2006 Elsevier Ltd. All rights reserved. Keywords: Grain-size sub-population; Rb/Sr ratio; Magnetic susceptibility; Clay minerals; Loess–paleosol
1. Introduction Chinese loess deposits provide good climatic signatures when compared with proxy signals obtained from d18O values of foraminifera shells preserved in deep-sea sediments (Liu, 1985; Chen et al., 1999). Loess and/or red clay in East
*
Corresponding author. Tel.: +86 25 84395014; fax: +86 25 84396326. E-mail address: [email protected] (F. Li).
1367-9120/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2005.05.011
China are also of considerable interest in reconstructing monsoonal climate, especially the Xiashu loess located in the middle and lower reaches of the Yangtze River (Fig. 1). The spatial distribution of Xiashu loess as a marginal eolian deposit has recorded variations in the eastern Asian winter monsoon for the recent past (Yang and Fang, 1991). Moreover, the weathering intensity of Xiashu loess after deposition should reflect changes in the summer monsoon that controls variations in precipitation for the middle and lower reaches of the Yangtze River (Liu and Li, 2000).
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F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
Fig. 1. Location map of the Xiashu loess showing the site examined for our work.
Although the late Pleistocene monsoonal history was reconstructed using oxygen isotopic compositions of speleothem calcite near Nanjing (Wang et al., 2001a,b), more evidence is required to verify the correlation with cyclic climatic change. The Xiashu alternating loess and paleosol horizons constitute the most complete sediment sequence in East China since the late Pleistocene and can be used as an ideal reference for the monsoonal record via d18O compositions of speleothem and also as an indicator of weathering controlled by monsoon intensity. Regional environmental change is one of the important factors in controlling weathering intensity and the chemical composition of weathering products (White and Blum, 1995; Gallet et al., 1996; Blum et al., 1998). Therefore studies of chemical weathering have served as effective tracers for environmental reconstruction (Chen et al., 1999; Jin et al., 2001). More attention has focused on understanding the weathering history via various proxies and their quantitative relationship with climatic change over the last decade. Among a series of proxies for tracing weathering intensity, Rb/Sr ratios and magnetic susceptibility are two sensitive tracers of chemical weathering and hence paleoclimatic changes (Guo et al., 1996; Chen et al., 1999; Jin et al., 2005). The mineralogical and bulk chemical compositions of Chinese loess are used to determine provenance and also to evaluate weathering related to paleomonsoons. On a short-term time scale, the provenance of eolian deposits is relatively stable, resulting in similar elemental ratios and magnetic susceptibility in bulk sediments. However, the grain-size distribution may change during weathering
after deposition, because weathering tends to convert certain minerals to clays. Furthermore, if the various grainsize sub-populations have different elemental distributions and mineral assemblages, then weathering must lead to compositional fractionation in the reworked sediments. This makes traditional bulk chemical analysis of sediments questionable for paleoenvironmental reconstruction. Consequently, to better trace weathering intensity using Rb/ Sr ratios and/or magnetic susceptibility, it is necessary to understand the distribution of Rb, Sr and magnetic minerals in different size sub-populations. In order to distinguish provenance from pedogenetic and weathering effects after deposition under late Pleistocene monsoonal conditions, we analyzed for Rb, Sr and rare earth elements (REE) contents along with magnetic susceptibility of bulk samples and three different size sub-populations of the Xiashu loess. We then evaluated the effects of sorting and weathering on the geochemical and magnetic proxies of eolian deposits. 2. Sampling and methods More than one hundred samples of Xiashu loess were taken from the Laohushan profile (3206 0 N, 11846 0 E), which is located in Shangyuan, Nanjing, Jiangsu Province (Fig. 1). The 10.5 m thick Laohushan profile is preserved artificially by an engineering project. Soil throughout the entire profile is fresh with columnar joints. The profile consists of three alternating loess and paleosol units, represented by variations of reflectivity (Fig. 2). On the basis of IRSL chronology by Lai et al. (2001) and horizon comparisons, the ages at the bottom of S1, L1 and S2 are 10.1, 71.5
Lithofacies
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
) 10
20
30 10 20 30 40 50
Ka 5
)
) 20
35
50
20
40
60
815
Illite crystallinity (IC) 80 0
1
2
3
4
0 S0
1 2
S1
3 L1
Depth (m)
4 5 S2
6 7 L2
8
S3
9 10
L3
11
Fig. 2. Profiles of stratigraphic zones, brightness, contents of illite, vermiculite, kaolinite and illite crystallinity of the Laohushan loess and paleosol versus profile depth.
and 77.1 ka, respectively. Thus the 10.5 m thick Laohushan profile was deposited after the late Pleistocene (about 120 ka B.P.) (Lai et al., 2001; Li et al., 2002). The profile was sampled at 10 cm intervals continuously from the base to the surface. The samples were dried at room temperature. The magnetic susceptibility of each sample was measured using a Bartington MS2 Meter at the Key Laboratory of Lake Sedimentation and Environment, Chinese Academy of Sciences, with an analytical error of ±0.1%. After samples were ground to a fine powder (<38 lm in a mortar), bulk samples were analyzed for Rb and Sr contents by means of a VP-320 X-ray fluorescence (XRF) spectrometer at the Modern Analysis Center, Nanjing University. The relative standard deviation was less than 1 ppm. The brightness of each sample was determined quantitatively by means of a Perkin-Elmer Lambda spectrophotometer in the Department of Earth Sciences, Nanjing University. In addition, 20 g of each sample was weighed and then shaken for 6 h at 25 C after addition of 100 mL of 1.0 M acetic acid and, on the basis of the Stokes settling rule, each sample was separated into three sub-populations; less than 2 lm, 2–45 lm and larger than 45 lm, respectively. The Rb, Sr and REE contents for the three sub-populations were analyzed by means of a Finnigan Mat Element ICP-MS at the Institute of Geochemistry, Chinese Academy of Sciences. The errors for Rb and Sr analysis were less than 3%. We selected 22 samples for clay determination by X-ray diffraction (XRD) at Nanjing Agricultural University. After the <2 lm fraction was separated from each sample and saturated by KCl and MgCl2, respectively, the suspension was transferred to a glass slide in order to obtain oriented clay specimens and then dried in air (air-dried, AD). The AD oriented specimens were
examined by XRD. KCl-treated oriented specimens were heated for 2 h at 550 C and examined by XRD again. The MgCl2-treated oriented specimens were saturated for 10 h with glycerol (GL) using an atomizer and then examined immediately by XRD to determine vermiculite and smectite. Each sample was scanned by a D/max-B XRD at 82h/min with Cu-Ka radiation, 40 kV, 20 mA, 0.012h steps with a scanning range of 3–622h. The errors were less than 5%. 3. Results and discussion 3.1. REE and eolian provenance It is generally accepted that the Xiashu loess is a windblown silt deposit, but different opinions still exist regarding its provenance. Although most researchers agree that the Xiashu loess has the same source as the Chinese Loess Plateau (Wang et al., 2001b), some suggest that material comprising the loess was mainly from a proximal source rather than from northwestern China (e.g., Liu et al., 2000). Since REEs in detritus experience little fractionation during weathering, detailed work by Cullers et al. (1988) suggested that the REE composition in clay was nearly the same as in the source materials and that the REE of clay fractions in sediments can be used to trace source provenance. Therefore, in order to determine provenance, it is important to understand the size distribution and mineral composition, especially REE patterns in clay. We analyzed the REE composition of the Laohushan profile which was listed in Table 1. REE results show that there are (i) the same REE distribution patterns for loess and paleosol, and (ii) the same REE patterns in all three size sub-populations for both loess and paleosol (Fig. 3a and b). The
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F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
Table 1 Feature parameters of rare earth element (REE) of the Xiashu loess and Central China loess P Sample Grain size REE LREE HREE LR/HR dEu
dCe
Eu/Sm
Sm/Nd
Nd/La
Paleosol
<2 lm 2–45 lm >45 lm Bulk sample Luochuan*
252.22 204.46 209.80 212.10 143.20
226.10 183.40 188.70 190.20 128.40
26.12 21.06 21.10 21.84 14.81
8.66 8.71 8.95 8.71 8.67
0.65 0.65 0.58 0.64 0.60
0.92 0.90 0.80 0.90 1.01
0.21 0.21 0.18 0.21 0.19
0.19 0.18 0.18 0.18 0.18
0.89 0.83 0.72 0.84 0.86
Loess
<2 lm 2–45 lm >45 lm Bulk sample Luochuan*
247.77 209.31 213.85 215.60 139.70
220.90 187.50 194.10 193.10 125.30
26.87 21.81 19.75 22.47 14.42
8.22 8.60 9.83 8.59 8.69
0.68 0.65 0.66 0.65 0.63
0.97 0.89 0.85 0.90 0.91
0.22 0.21 0.21 0.21 0.20
0.20 0.18 0.17 0.18 0.17
0.90 0.84 0.77 0.85 0.82
*
Average values after Chen et al. (1999); dEu = 2(Eu)N/[(Nd)N + (Gd)N], dCe = 2(Ce)N/[(La)N + (Pr)N], where N is ratio of sample to chondrite.
a
b
1000
Sample/chondrite
Loess
1000
Paleosol
< 2 um
< 2 um
2-45 um
2-45 um
> 45 um
> 45 um
Luochuan
Luochuan
100
100
10
10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 3. Chondrite-normalized REE patterns for Laohushan samples. (a) Loess, (b) paleosol. REE data of Luochuan is from Chen et al. (1996).
REEs show a moderately steep, light rare earth element enriched (LREE) pattern, with a flatter heavy rare earth elements (HREE) distribution. Similar values for dEu, dCe, Eu/Sm, Sm/Nd and Nd/La ratios of the loess, paleosol and three sub-populations indicate that all have the same source provenance. The dEu and dCe values in different size fractions vary little, with values less than 1 for both. Ratios of Eu/Sm, Sm/Nd and Nd/La vary from 0.18 to 0.21, 0.17 to 0.20 and 0.72 to 0.90, respectively, indicating that the same provenance provided both fine and coarse size deposits in the Xiashu loess. Among the three size sub-populations P of both loess and paleosol, the total REE composition ( REE) in the <2 lm sub-population is the highest. The LREE/HREE ratio increase from the fine- to coarse-grained sub-populations, indicating that the HREE have an affinity P for clays. Bulk loess samples have a slightly higher REE and lower LREE/HREE ratio than that of the paleosol (Table 1). The differences for REE between loess and paleosol and various size subpopulations indicate that compositional fractionation may arise from eolian sorting during transportation and deposition. Compared with P the Luochuan profile in the Chinese Loess Plateau, the REE in the Laohushan is obviously higher. The same REE patterns and similar values of LREE/HREE, dEu, dCe, and elemental ratios (Eu/Sm,
Sm/Nd and Nd/La) for the Laohushan and the Luochuan profiles (Fig. 3a and b) suggest that they have the same provenance. The REE retained by the clay minerals results in higher REE concentrations in clay-rich sediments (Yang and Li, 1999). However, there is no REE data for the clay fractions of northern Chinese loess at present, even though many REE studies on the loess have been done. Average clay content in the northern loess was reported to be 17% (Liu, 1985); lower than in the Laohushan loess with 29.7% clay (Li et al., 2004). Therefore, the REE of paleosol and loess in the Laohushan are 1.48 and 1.54 times higher than those in Luochuan, respectively, which might be attributed to wind sorting during distal transportation to East China. 3.2. Clay minerals and illite crystallinity (IC) Different types and assemblages of secondary minerals in loess would reflect chemical weathering intensity before and after deposition under various climatic conditions (Liu, 1985; Kisch, 1991). For example, the content and crystallinity of different clay minerals are important records of environmental conditions and serve as significant proxies of past environment (Ji et al., 1999b; Kostic and Protic, 2000). Unstable mixed-layer clay minerals are especially sensitive to environmental change. The degree of crystallinity
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
changes significantly with increased weathering intensity and therefore is an effective proxy for loess research (Liu, 1985; Kisch, 1991; Ji et al., 1999b). Research on clay minerals in loess will help us to understand chemical weathering intensities under various climatic conditions and their implication for paleoenvironmental change. However, little work has been done on clay minerals of the Xiashu loess in determining the extent and intensity of chemical weathering. Based on XRD charts of the samples after a series of treatments, including (1) saturation with KCl, (2) heating, (3) saturation with MgCl2, and (4) solvate with glycerine, clay minerals in the Laohushan sequence are found to be dominantly illite, vermiculite and kaolinite (Fig. 4). Estimates of each clay mineral abundance were based on integrated peak areas determined by basal (001) diffraction lines (1.47, 1.01 and 0.71 nm) of samples saturated with MgCl2 and GL using computer programs. The quantities of each mineral were multiplied by the power of 3 for illite, 2 for kaolinite and 1.5 for vermiculite (Gorbunov, 1978). The relative content of each clay mineral is listed in Table 2. Similar types and contents of clay minerals in the Xiashu loess and in the northern Chinese loess (Ji et al., 1999b) further support the hypothesis that both were derived from the same provenance. The average illite content is 52.8% in paleosol and 47.0% in loess. The higher kaolinite content in loess (18.1%) relative to paleosol (16.0%) for the Laohushan sequence is opposite to that of the Luochuan profile investigated by Liu (1985), indicating strong weathering and/or modern weak-acid rainfall in East China, because kaolinite is a clay mineral formed by strong chemical weathering within a slightly acidic environment. The observation is also supported by the gradual increase in kaolinite
817
Table 2 Relative contents of dominate clay minerals and illitic crystallinity (IC) Samples no.
Depth (m)
Vermiculite
Kaolinite
Illite
IC
LHS-1 LHS-5 LHS-10 LHS-15 LHS-20 LHS-25 LHS-30 LHS-35 LHS-40 LHS-45 LHS-50 LHS-55 LHS-60 LHS-65 LHS-70 LHS-75 LHS-80 LHS-85 LHS-90 LHS-95 LHS-100 LHS-105
10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
31.69 46.33 26.52 34.55 52.73 47.96 33.94 26.95 39.47 16.08 40.56 16.27 27.03 25.95 38.05 42.20 36.23 31.70 36.81 18.02 20.29 26.13
11.61 16.82 22.42 21.67 5.47 8.92 19.32 11.12 13.28 6.06 24.92 15.10 17.33 17.10 5.70 30.47 18.42 6.57 9.94 14.01 22.14 51.43
56.70 36.85 51.06 43.78 41.79 43.12 46.74 61.93 47.25 77.86 34.52 68.63 55.64 56.95 56.25 27.33 45.35 61.73 53.25 67.96 57.57 22.44
2.91 2.00 1.91 2.00 2.25 1.50 1.91 1.91 2.78 3.82 3.00 2.50 2.38 2.10 2.95 0.91 2.10 3.71 2.00 2.84 2.64 1.33
contents from a depth of 2 m to the surface in the Laohushan profile. Unlike some easily weathered clay minerals such as kaolinite or interstratified illite–smectite (I–S), as long as there is no intense late-stage metamorphism, illite is stable even in an epigenetic environment (Velde and Vasseur, 1992; Ji et al., 1999b). Consequently, illite crystallinity (IC) is used to understand formation conditions of source material and serves as an indicator of provenance. For
Fig. 4. X-ray diffractograms of the Xiashu paleosol and loess showing representative diffractograms for KCl-treated, KCl-treated and heated under 550 C (KCl + 550), MgCl2-treated and MgCl2-treated and glycolated (MgCl2 + GL) samples. Semi-quantitative abundance is given in Table 2.
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F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822 100
Y = 12.83 X + 20.65 R2 = 0.454
Content of illite (%)
80
60
40
20
0 0
1
2
3
4
5
Illite crystallinity (IC)
Fig. 5. Plot showing a positive correlation between illite content and illitic crystallinity.
example, Liu (1985) found that the loess and paleosol layers in northern Chinese loess could be distinguished by the higher IC in loess relative to paleosol. There are different methods for calculating IC. Here we used IC = H/W suggested by Liu (1985), where H and W is the height and the half-peak-width of the 1.01 nm illite peak, respectively. The IC values correlate well with variations in illite content with depth (Fig. 2), especially in the upper part of the profile (Fig. 5), indicating that the source area for Xiashu loess should be located in an arid and/or cold environment, and that eolian deposits experienced only slight weathering after deposition. 3.3. Rb, Sr distribution and Rb/Sr ratio in different size subpopulations Chemical weathering affects both the elemental distribution and magnetic susceptibility of rocks/sediments (Gallet
et al., 1998). For example, Rb and Sr are easily fractionated during weathering, resulting in variations in Rb/Sr ratios for loess–paleosol sequences and lake sediments in response to weathering intensity under different climatic conditions (Chen et al., 1999; Jin et al., 2001, 2005). Table 3 shows that Rb concentrations in clay populations of the Xiashu loess are the highest, decreasing from <2 lm to 2– 45 lm and to >45 lm sub-populations for both loess and paleosol. Variation in Sr contents with size is opposite to that exhibited by Rb. It is thought to be attributed to sorting and weathering related to monsoonal conditions. On the one hand, during eolian transportation, changes in the strength of the winter monsoon would result in different mineral compositions and initial Rb and Sr contents of eolian deposits. During glacial periods, a stronger winter monsoon should transport coarse grains dominated by quartz, feldspar and carbonates with lower Rb/Sr ratios. During interglacial periods, a weak winter monsoon can carry clay particles with higher Rb/Sr ratios (Gallet et al., 1998). Therefore part of the variations in Rb/Sr ratios for both loess and paleosol should depend on wind sorting during eolian transportation. On the other hand, Rb/Sr ratios in either loess or paleosol would vary due to leaching and weathering after deposition. Variations in Rb/Sr ratios for bulk samples from the Luochuan profile have been used to elucidate weathering history after deposition (Chen et al., 1999). Based on the acid-leaching experiment of northern Chinese loess, 20–40% Sr is easy to remove into solution, while Rb is retained by clay minerals (Chen et al., 1996). Clays are the main products of rock weathering, whose type and component budget contain important information of weathering intensity, if the provenance of the Xiashu loess remained unchanged throughout the late Pleistocene. Here, an important issue is how to distinguish the amount of sorting and weathering, and the degree of weathering before and after deposition, on
Table 3 Statistical results of concentrations of Rb, Sr and magnetic susceptibility (MS) of bulk samples and three sub-populations of different sizes Grain size
Element
S1*(11)
S1(17)
L1(12)
S2(31)
L2(5)
S3(12)
L3(17)
S(71)
L(34)
Average
<2 lm
Rb Sr Rb/Sr MS
172.94 67.13 2.58 352.65
172.52 80.03 2.17 376.27
169.18 79.93 2.12 338.33
166.86 116.05 1.45 309.85
165.31 106.33 1.56 164.17
164.85 105.68 1.56 312.75
163.57 120.19 1.37 350.16
168.82 98.09 1.82 332.88
165.80 103.94 1.66 318.63
167.84 99.99 1.77 328.26
2–45 lm
Rb Sr Rb/Sr MS
90.99 113.94 0.80 107.30
101.90 110.51 0.92 127.87
95.55 110.96 0.87 116.28
100.51 138.44 0.73 105.81
85.55 133.68 0.64 48.74
101.29 129.86 0.78 105.76
102.62 136.79 0.75 117.67
99.50 126.51 0.80 111.31
97.61 127.22 0.78 107.04
98.89 126.74 0.78 109.93
>45 lm
Rb Sr Rb/Sr MS
73.19 118.04 0.62 82.01
75.66 122.67 0.63 96.45
71.74 114.37 0.63 76.02
82.58 141.89 0.60 103.76
72.65 133.94 0.55 49.61
71.16 126.87 0.56 87.61
71.27 133.29 0.54 102.22
77.54 131.06 0.61 95.91
71.64 126.71 0.57 85.24
75.63 129.65 0.60 92.45
Bulk sample
Rb Sr Rb/Sr MS
105.55 153.45 0.69 156.42
108.59 139.41 0.78 156.28
103.67 120.08 0.87 140.97
109.84 143.10 0.77 142.80
100.00 136.80 0.73 68.60
109.25 138.33 0.79 147.55
106.00 149.18 0.71 167.70
108.66 130.76 0.84 148.94
104.29 137.09 0.77 143.69
107.25 132.81 0.82 147.24
L, loess; S, paleosol; S1*, upper part of S1, listed separated due to possible effect of human activity. (11) is number of samples.
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
the basis of Rb/Sr proxy variations in bulk samples for both loess and paleosol. One way is Rb/Sr measurements for different size sub-populations, because clays are closely related to weathering intensity whereas the coarse fractions are related to provenance. On the Earth’s surface, minerals with higher Sr would be weathered first in the order: calcite fi olivine fi feldspar fi pyroxene fi hornblende fi biotite fi K-feldspar fi muscovite fi quartz (Kukla et al., 1988). In the Laohushan profile, Rb/Sr ratios in the residue products would increase due to greater Sr loss as the intensity of chemical weathering increased, especially at the early stage of rock weathering. From Table 3 and Fig. 7, comparison of Xiashu loess and paleosol shows that variations in Rb and Sr contents for given size sub-populations are different. Rb contents vary little within <2 lm sub-population, between 164 and 169 ppm in loess (averaging 166 ppm), and between 165 and 173 ppm in paleosol (averaging 169 ppm). On the other hand, Sr contents vary between 80 and 120 ppm in loess (with an average of 104 ppm), and between 67 and 116 ppm in paleosol (averaging 98 ppm). This indicates that Rb in the Xiashu loess is relatively stable, while variable Sr contents is due to active mobility of Sr during weathering, resulting in a negative correlation between Rb/Sr ratios and Sr contents, whose correlation coefficient (R2) value can be used to evaluate the sensitivity of Rb/Sr for measuring weathering intensity. Among R2 values of bulk samples and three size sub-popa 1.1
819
ulations, the <2 lm sub-population has the highest value (R2 = 0.97), while R2 values for the >45 lm and 2–45 lm sub-populations are 0.40 and 0.49, respectively (Fig. 6b, c and d). The R2 value of the <2 lm sub-population is higher than that of the bulk samples from both the Laohushan (R2 = 0.93, Fig. 6a) and the Luochuan (R2 = 0.94, Chen et al., 1996) profiles. This suggests that Sr lost during weathering mainly affects the composition of the <2 lm sub-populations. Curves of Rb/Sr ratios versus depth (Fig. 7) show similar variations between bulk samples and the <2 lm sub-population, while Rb/Sr ratios for both the >45 lm and the 2–45 lm sub-populations do not vary. We suggest that variations in Rb/Sr ratios for bulk samples due to weathering depend on the composition of the <2 lm sub-population, while Rb/Sr ratios for both the >45 lm and the 2–45 lm sub-populations preserve information of provenance. Higher Sr contents for the loess and paleosol indicates that the Xiashu loess has experienced only slight weathering, similar to the northern Chinese loess. Increasing Rb/Sr ratios for both bulk samples and the <2 lm subpopulations indicates an increase in weathering intensity for the lower Yangtze River region since the late Pleistocene (Li et al., 2002). 3.4. Relationship between magnetic susceptibility and size Magnetic susceptibility in bulk loess is also used to determine the degree of weathering (Guo et al., 2000), b 3.0
1.0 2.5
2
Rb/Sr ratio
Rb/Sr ratio
y = 79.9 x -0.94 0.9
R = 0.93
0.8 0.7
y = 215 x -1.05 R 2 = 0.97
2.0
1.5 0.6 0.5 100
1.0 120
140
160
180
60
80
100
120
140
d 1.0
1.1
y = 81.8 x -0.96
1.0
R 2 = 0.49
0.9 0.8 0.7
y = 30.4 x
0.8
Rb/Sr ratio
Rb/Sr ratio
c 1.2
40
-0.81
R2 = 0.40
0.6
0.4
0.6 0.5
0.2 90 100 110 120 130 140 150 160
Sr (ppm)
80
115
150
185
220
Sr (ppm)
Fig. 6. Relationship between Rb/Sr ratios and Sr concentrations in Xiashu loess samples for different size sub-populations. (a) Bulk samples, (b) <2 lm, (c) 2–45 lm and (d) >45 lm.
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F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
SUS/10 -8 m 5 kg -1
Rb/Sr ratio 0.6
0.7
0.8
0.9
1.0
0
1
2
3
0
0.0
50
1.5
3.0
250
300
350
400
2-45 um
Bulk
Bulk < 2um 2-45 um > 45 um
Depth (m)
Depth (m)
200
> 45 um
3.0
6.0
150
< 2 um
1.5
4.5
100
0.0
4.5
6.0
7.5
9.0
10.5
Fig. 7. Curves of Rb/Sr ratios for bulk samples and three size subpopulations versus depth from the Laohushan profile.
7.5
9.0
10.5
Fig. 8. Curves of magnetic susceptibility of bulk samples and three size sub-populations with depth.
where increasing magnetic susceptibility reflects enhanced weathering. Higher values of magnetic susceptibility in paleosol relative to loess are thought to be caused by formation of fine superparamagnetic grains and stable single domain grains during pedogenesis (Heller and Liu, 1982; Kukla et al., 1988; Zhou et al., 1990; Maher and Thompson, 1991; Guo et al., 1996). This indicates that there is a relationship between weathering and magnetic susceptibility. In loess profiles, weathering of biotite and chlorite is thought to be a dominant process for evolution of ironbearing minerals via iron supply for magnetic minerals (Ji et al., 1999a). Further work has shown that there is a good linear correlation between mica to chlorite ratios and magnetic susceptibility (Ji et al., 1999b). Magnetic susceptibility for bulk samples and three size subpopulations of the Laohushan profile show distinctly different values. The average magnetic susceptibilities (·108 m3 kg1) of the bulk samples, the <2 lm, 2– 45 lm and >45 lm sub-populations are 147.24, 328.26, 109.93 and 92.45, respectively (Table 3). Magnetic susceptibility for both the >45 lm and the 2–45 lm sub-populations is lower than in bulk samples. Values of magnetic susceptibility in the <2 lm sub-population are three times higher than those of bulk samples, but exhibit a similar curve with depth (Fig. 8). The results indicate that weathering mainly affects magnetic susceptibility of the clay sub-population via increase in concentration of a claygrained (predominantly superparamagnetic) ferromagnetic fraction. Higher magnetic susceptibility in the clay subpopulation is also attributable to grain size, where smaller grains weather more easily (Zhou et al., 1990). Therefore clay is sensitive to weathering rates after deposition under different climatic conditions. Variations in magnetic susceptibility for different size grains help elucidate why the magnetic susceptibility increases during pedogenesis and its relationship with chemical weathering.
4. Conclusions The effects of sorting and chemical weathering are sufficient in explaining the compositional differences between bulk sediments and the three size fractions (>45 lm, 2– 45 lm and <2 lm) from the Xiashu loess in East China. Our work on the Xiashu loess not only highlights Rb/Sr ratios and magnetic susceptibility as effective proxies for weathering intensity, but also provides further evidence that variations in Rb/Sr ratios for bulk loess are controlled by the chemical and mineralogical compositions of the clay fraction. We suggest that analysis of different grain-size sub-populations is useful for identifying environmental changes, because a given sub-population may minimize the effects of sorting and weathering and thus reduce varying initial Rb/Sr ratios. The differences in Rb/Sr ratios and magnetic susceptibilities for bulk sediments and the three size subpopulations are therefore explicable if chemical weathering of the Xiashu loess after deposition played a controlling role in the Rb/Sr ratios and magnetic susceptibility of the clay sub-population. Chemical weathering results in a negative correlation between Rb/Sr ratios and Sr contents for the <2 lm fraction with R2 = 0.97. Rb/Sr ratios and magnetic susceptibility of the <2 lm sub-population stand out from the other groups (Fig. 9). Therefore variations in Rb/Sr ratios in the clay fraction could serve as an effective tracer for weathering intensity. On the other hand, the Rb/Sr ratios and magnetic susceptibility of the coarse size (here 2–45 lm and >45 lm) subpopulations provide information of source provenance controlled by weathering before transportation and sorting. To evaluate the effects of chemical weathering and sorting on the composition of loess and paleosol deposited
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822 3.0 <2μm 2-45μm
2.5
>45μm bulk
Rb/Sr ratio
2.0
1.5
1.0
0.5
0.0
Magnetic susceptibility (×10-8 m3 ·kg-1 ) Fig. 9. Plot of Rb/Sr ratios versus magnetic susceptibility in bulk samples and three size sub-populations of the Xiashu loess.
at different locations under the same climate conditions, further work should focus on comparing the Rb/Sr ratios and magnetic susceptibility in different size fractions of the Xiashu loess with those in the northern Chinese loess. Acknowledgements This work was supported by the project sponsored by National Natural Science Foundation of China through Grants 40573057 and 40373004, by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (G200406), by the Post-Doctoral Foundation of China (Batch No. 30), and by the Key Laboratory of Environment Geochemistry in Institute of Geochemistry, Chinese Academy of Sciences (HDH010903). We thank Prof. Ji Junfeng in Department of Earth Sciences, Nanjing University, Prof. Liu Chongqiang and Prof. Wang Shijie in Institute of Geochemistry, Chinese Academy of Sciences for their help during the experiments. We are gratitude to two anonymous reviewers for their constructive suggestions. References Blum, J.D., Gazis, C.A., Jacobson, A.D., Chamberiain, C.P., 1998. Carbonate versus silicate weathering in the Raikhot watershed within the High Himalayan Crystalline Series. Geology 26, 411– 414. Chen, J., An, Z., Head, J., 1999. Variation of Rb/Sr ratios in the loess– paleosols sequences of Central China during the last 130,000 years and their implications for monsoon paleoclimatology. Quaternary Research 51, 215–219. Chen, J., An, Z., Wang, Y., Ji, J., Chen, Y., Lu, H., 1996. Distribution of Rb and Sr in the Luochuan loess–paleosol sequence of China during the last 800 ka – implication for paleomonsoon variations. Science in China. Series D 42, 225–232. Cullers, R.L., Basu, A., Suttner, L.J., 1988. Geochemical signature of provenance in sand size mineral in soil and stream near the tabacco root batholith, Montana, USA. Chemical Geology 70, 335–348.
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Gallet, S., Jahn, B.M., Toril, M., 1996. Geochemical characterization of the Luochuan loess–paleosol sequence, China, and paleoclimate implication. Chemical Geology 133, 67–75. Gallet, S., Jahn, B.M., Lanoe, B.V.V., Dia, A., Rossello, E., 1998. Loess geochemistry and implications for particle origin and composition of the upper continental crust. Earth Planetary Science Letters 156, 157– 172. Gorbunov, N.I., 1978. Mineralogy and Physical Chemistry of Soils. Science Press, Moscow, pp. 77–94 (in Russian). Guo, Z.T., Biscaye, P., Wei, L.Y., Chen, X., Peng, S., Liu, T., 2000. Summer monsoon variations over the last 1.2 Ma from the weathering of loess-soil sequences in China. Geophysical Research Letter 27, 1751–1754. Guo, Z., Liu, T., Guiot, J., Wu, N., Lu, H., 1996. High frequency pulses of East Asian monsoon climate in the last two glaciations: link with the North Atlantic. Climate Dynamics 12, 701–709. Heller, F., Liu, T.S., 1982. Magnetostratigraphical dating of loess deposits in China. Nature 300, 431–433. Ji, J., Chen, J., Liu, L., Lu, H., 1999a. Enhancement of magnetic susceptibility and chemical weathering of chlorite in Luochuan loess section. Advance in Natural Science 9, 619–623 (in Chinese with English abstract). Ji, J., Chen, J., Lu, H., 1999b. Origin of illite in the loess from the Luochuan area, Loess Plateau, Central China. Clay Minerals 34, 525– 532. Jin, Z.D., Wu, Y., Zhang, X., Wang, S., 2005. Role of late glacial to midHolocene climate in catchment weathering in the Central Tibetan Plateau. Quaternary Research 63, 161–170. Jin, Z.D., Wang, S., Shen, J., Zhang, E., Li, F., Ji, J., Lu, X., 2001. Chemical weathering since the Little Ice Age recorded in lake sediments: a high-resolution proxy of past climate. Earth Surface Processes and Landforms 26, 775–782. Kisch, H.J., 1991. Illite crystallinity: recommendations on sample preparation, X-ray diffraction settings, and interlaboratory samples. Journal of Metamorphic Geology 9, 665–670. Kostic, N., Protic, N., 2000. Pedology and mineralogy of loess profiles at Kapela-Batajnica and Stalac, Serbia. Catena 41, 217–227. Kukla, G., Heller, F., Liu, X.M., Xu, T.C., Liu, T.S., An, Z., 1988. Pleistocene climate in China dated by magnetic susceptibility. Geology 16, 811–814. Lai, Z., Zhou, J., Xia, Y., Zhang, J., Chen, J., 2001. Thermoluminescence dating of Xiashu Loess in Nanjing. Advance in Natural Science 11, 203–207. Li, F., Xie, C., Pan, G., 2002. Paleoclimatic implication of distribution of Rb, Rb/Sr and magnetic susceptibility in loess and paleosols from Laohushan profile, Nanjing. Marine Geology and Quaternary Geology 22, 47–52 (in Chinese with English abstract). Li, F., Xie, C., Pan, G., Feng, J., 2004. Rare earth elements distribution of different size sub-population of the Xiashu Loess, Nanjing. Quaternary Sciences 24, 477–478 (in Chinese with English abstract). Liu, T.S., 1985. Loess and the Environment. Science Press, Beijing, pp. 219–238 (in Chinese). Liu, X., Han, D., Cui, G., Shan, Q., Lin, T., 2000. New exploration of the formation of the Xiashu loess in the Western Yellow Sea. Marine Sciences 24, 52–53 (in Chinese with English abstract). Liu, X.D., Li, L., 2000. Preliminary exploration of forcing mechanisms of summer monsoon climate change over East Asia. Marine Geology and Quaternary Geology 20, 83–90 (in Chinese with English abstract). Maher, B.A., Thompson, R., 1991. Mineral magnetic record of the Chinese loess and paleosols. Geology 19, 3–6. Velde, B., Vasseur, G., 1992. Estimation of the diagenetic smectite to illite transformation in time-temperature space. American Mineralogist 77, 967–976. Wang, Y.J., Cheng, H., Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.C., Dorale, J.A., 2001a. A high-resolution absolute-dated late Pleistocene monsoon record from Hulu cave, China. Science 294, 2345–2348.
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Wang, A., Yang, S., Li, C., 2001b. Elemental geochemistry of the Nanjing Xiashu loess and the provenance study. Journal of Tongji University 29, 657–661 (in Chinese with English abstract). White, A.F., Blum, A.E., 1995. Effects of climate on chemical weathering in watersheds. Geochimica et Cosmochimica Acta 59, 1729–1747. Yang, D.Y., Fang, Y.S., 1991. Middle-late Pleistocene eolian deposits and environmental change in the Xuancheng area, Anhui. Marine Geology
and Quaternary Geology 11, 97–104 (in Chinese with English abstract). Yang, S., Li, C., 1999. Research progress in REE tracer for sediment source. Advance in Earth Sciences 14, 164–167 (in Chinese with English abstract). Zhou, L.P., Oldfield, F., Wintle, A.G., Robinson, S.G., Wang, J.T., 1990. Partly pedogenic origin of magnetic variation in Chinese loess. Nature 346, 737–739.
Roles of sorting and chemical weathering in the geochemistry and magnetic susceptibility of Xiashu loess, East China Fuchun Li
a,*
, Zhangdong Jin b,c, Changren Xie a, Jiayi Feng d, Libo Wang e, Yongzhao Yang a
a College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China Key Laboratory of Lake Sedimentation and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710054, China d Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China e Nanjing Institute of Geology and Mineral Resources, The Ministry of Land and Resources, Nanjing 210016, China b
c
Received 11 October 2004; received in revised form 13 May 2005; accepted 26 May 2005
Abstract Uncertainties in paleoenvironmental interpretations for traditional chemical analysis of bulk samples result from different grain-size sub-populations of sediments containing variable distributions of elements and minerals. Therefore, it is important to understand the elemental and mineral distribution in different grain sizes in determining the quantitative relationship between chemical weathering and climatic change. We sieved a series of Xiashu loess samples into three sub-populations of different grain sizes (<2, 2–45 and >45 lm, respectively), and then analyzed each population for rubidium (Rb), strontium (Sr), rare earth elements and magnetic susceptibility. In comparison with elemental concentrations of bulk samples, clay mineralogy and illitic crystallinity, our results show that distinct elemental distributions and magnetic susceptibilities for different grain-size sub-populations are controlled by sorting and/or chemical weathering, although we also suggest that the Xiashu loess may have the same provenance as the Central Chinese Loess. Maximum concentrations of Rb and fine-grained magnetic minerals in the less than 2 lm sub-population, coupled with our finding of maximum Sr in the larger than 45 lm fraction, indicate that Sr was lost during chemical weathering. Grain-size sub-population analysis is, therefore, an effective method for extracting paleoenvironmental information, because individual sub-populations show minimal variations in initial Rb/Sr ratios compared to bulk analysis of all sizes together. Furthermore, a negative correlation between Rb/Sr ratios and Sr concentrations for the <2 lm fraction (R2 = 0.97) may indicate that clay is a sensitive indicator of intensity of chemical weathering and is an ideal sub-population for determining Rb/Sr ratios, but not for magnetic susceptibility. 2006 Elsevier Ltd. All rights reserved. Keywords: Grain-size sub-population; Rb/Sr ratio; Magnetic susceptibility; Clay minerals; Loess–paleosol
1. Introduction Chinese loess deposits provide good climatic signatures when compared with proxy signals obtained from d18O values of foraminifera shells preserved in deep-sea sediments (Liu, 1985; Chen et al., 1999). Loess and/or red clay in East
*
Corresponding author. Tel.: +86 25 84395014; fax: +86 25 84396326. E-mail address: [email protected] (F. Li).
1367-9120/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2005.05.011
China are also of considerable interest in reconstructing monsoonal climate, especially the Xiashu loess located in the middle and lower reaches of the Yangtze River (Fig. 1). The spatial distribution of Xiashu loess as a marginal eolian deposit has recorded variations in the eastern Asian winter monsoon for the recent past (Yang and Fang, 1991). Moreover, the weathering intensity of Xiashu loess after deposition should reflect changes in the summer monsoon that controls variations in precipitation for the middle and lower reaches of the Yangtze River (Liu and Li, 2000).
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Fig. 1. Location map of the Xiashu loess showing the site examined for our work.
Although the late Pleistocene monsoonal history was reconstructed using oxygen isotopic compositions of speleothem calcite near Nanjing (Wang et al., 2001a,b), more evidence is required to verify the correlation with cyclic climatic change. The Xiashu alternating loess and paleosol horizons constitute the most complete sediment sequence in East China since the late Pleistocene and can be used as an ideal reference for the monsoonal record via d18O compositions of speleothem and also as an indicator of weathering controlled by monsoon intensity. Regional environmental change is one of the important factors in controlling weathering intensity and the chemical composition of weathering products (White and Blum, 1995; Gallet et al., 1996; Blum et al., 1998). Therefore studies of chemical weathering have served as effective tracers for environmental reconstruction (Chen et al., 1999; Jin et al., 2001). More attention has focused on understanding the weathering history via various proxies and their quantitative relationship with climatic change over the last decade. Among a series of proxies for tracing weathering intensity, Rb/Sr ratios and magnetic susceptibility are two sensitive tracers of chemical weathering and hence paleoclimatic changes (Guo et al., 1996; Chen et al., 1999; Jin et al., 2005). The mineralogical and bulk chemical compositions of Chinese loess are used to determine provenance and also to evaluate weathering related to paleomonsoons. On a short-term time scale, the provenance of eolian deposits is relatively stable, resulting in similar elemental ratios and magnetic susceptibility in bulk sediments. However, the grain-size distribution may change during weathering
after deposition, because weathering tends to convert certain minerals to clays. Furthermore, if the various grainsize sub-populations have different elemental distributions and mineral assemblages, then weathering must lead to compositional fractionation in the reworked sediments. This makes traditional bulk chemical analysis of sediments questionable for paleoenvironmental reconstruction. Consequently, to better trace weathering intensity using Rb/ Sr ratios and/or magnetic susceptibility, it is necessary to understand the distribution of Rb, Sr and magnetic minerals in different size sub-populations. In order to distinguish provenance from pedogenetic and weathering effects after deposition under late Pleistocene monsoonal conditions, we analyzed for Rb, Sr and rare earth elements (REE) contents along with magnetic susceptibility of bulk samples and three different size sub-populations of the Xiashu loess. We then evaluated the effects of sorting and weathering on the geochemical and magnetic proxies of eolian deposits. 2. Sampling and methods More than one hundred samples of Xiashu loess were taken from the Laohushan profile (3206 0 N, 11846 0 E), which is located in Shangyuan, Nanjing, Jiangsu Province (Fig. 1). The 10.5 m thick Laohushan profile is preserved artificially by an engineering project. Soil throughout the entire profile is fresh with columnar joints. The profile consists of three alternating loess and paleosol units, represented by variations of reflectivity (Fig. 2). On the basis of IRSL chronology by Lai et al. (2001) and horizon comparisons, the ages at the bottom of S1, L1 and S2 are 10.1, 71.5
Lithofacies
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
) 10
20
30 10 20 30 40 50
Ka 5
)
) 20
35
50
20
40
60
815
Illite crystallinity (IC) 80 0
1
2
3
4
0 S0
1 2
S1
3 L1
Depth (m)
4 5 S2
6 7 L2
8
S3
9 10
L3
11
Fig. 2. Profiles of stratigraphic zones, brightness, contents of illite, vermiculite, kaolinite and illite crystallinity of the Laohushan loess and paleosol versus profile depth.
and 77.1 ka, respectively. Thus the 10.5 m thick Laohushan profile was deposited after the late Pleistocene (about 120 ka B.P.) (Lai et al., 2001; Li et al., 2002). The profile was sampled at 10 cm intervals continuously from the base to the surface. The samples were dried at room temperature. The magnetic susceptibility of each sample was measured using a Bartington MS2 Meter at the Key Laboratory of Lake Sedimentation and Environment, Chinese Academy of Sciences, with an analytical error of ±0.1%. After samples were ground to a fine powder (<38 lm in a mortar), bulk samples were analyzed for Rb and Sr contents by means of a VP-320 X-ray fluorescence (XRF) spectrometer at the Modern Analysis Center, Nanjing University. The relative standard deviation was less than 1 ppm. The brightness of each sample was determined quantitatively by means of a Perkin-Elmer Lambda spectrophotometer in the Department of Earth Sciences, Nanjing University. In addition, 20 g of each sample was weighed and then shaken for 6 h at 25 C after addition of 100 mL of 1.0 M acetic acid and, on the basis of the Stokes settling rule, each sample was separated into three sub-populations; less than 2 lm, 2–45 lm and larger than 45 lm, respectively. The Rb, Sr and REE contents for the three sub-populations were analyzed by means of a Finnigan Mat Element ICP-MS at the Institute of Geochemistry, Chinese Academy of Sciences. The errors for Rb and Sr analysis were less than 3%. We selected 22 samples for clay determination by X-ray diffraction (XRD) at Nanjing Agricultural University. After the <2 lm fraction was separated from each sample and saturated by KCl and MgCl2, respectively, the suspension was transferred to a glass slide in order to obtain oriented clay specimens and then dried in air (air-dried, AD). The AD oriented specimens were
examined by XRD. KCl-treated oriented specimens were heated for 2 h at 550 C and examined by XRD again. The MgCl2-treated oriented specimens were saturated for 10 h with glycerol (GL) using an atomizer and then examined immediately by XRD to determine vermiculite and smectite. Each sample was scanned by a D/max-B XRD at 82h/min with Cu-Ka radiation, 40 kV, 20 mA, 0.012h steps with a scanning range of 3–622h. The errors were less than 5%. 3. Results and discussion 3.1. REE and eolian provenance It is generally accepted that the Xiashu loess is a windblown silt deposit, but different opinions still exist regarding its provenance. Although most researchers agree that the Xiashu loess has the same source as the Chinese Loess Plateau (Wang et al., 2001b), some suggest that material comprising the loess was mainly from a proximal source rather than from northwestern China (e.g., Liu et al., 2000). Since REEs in detritus experience little fractionation during weathering, detailed work by Cullers et al. (1988) suggested that the REE composition in clay was nearly the same as in the source materials and that the REE of clay fractions in sediments can be used to trace source provenance. Therefore, in order to determine provenance, it is important to understand the size distribution and mineral composition, especially REE patterns in clay. We analyzed the REE composition of the Laohushan profile which was listed in Table 1. REE results show that there are (i) the same REE distribution patterns for loess and paleosol, and (ii) the same REE patterns in all three size sub-populations for both loess and paleosol (Fig. 3a and b). The
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Table 1 Feature parameters of rare earth element (REE) of the Xiashu loess and Central China loess P Sample Grain size REE LREE HREE LR/HR dEu
dCe
Eu/Sm
Sm/Nd
Nd/La
Paleosol
<2 lm 2–45 lm >45 lm Bulk sample Luochuan*
252.22 204.46 209.80 212.10 143.20
226.10 183.40 188.70 190.20 128.40
26.12 21.06 21.10 21.84 14.81
8.66 8.71 8.95 8.71 8.67
0.65 0.65 0.58 0.64 0.60
0.92 0.90 0.80 0.90 1.01
0.21 0.21 0.18 0.21 0.19
0.19 0.18 0.18 0.18 0.18
0.89 0.83 0.72 0.84 0.86
Loess
<2 lm 2–45 lm >45 lm Bulk sample Luochuan*
247.77 209.31 213.85 215.60 139.70
220.90 187.50 194.10 193.10 125.30
26.87 21.81 19.75 22.47 14.42
8.22 8.60 9.83 8.59 8.69
0.68 0.65 0.66 0.65 0.63
0.97 0.89 0.85 0.90 0.91
0.22 0.21 0.21 0.21 0.20
0.20 0.18 0.17 0.18 0.17
0.90 0.84 0.77 0.85 0.82
*
Average values after Chen et al. (1999); dEu = 2(Eu)N/[(Nd)N + (Gd)N], dCe = 2(Ce)N/[(La)N + (Pr)N], where N is ratio of sample to chondrite.
a
b
1000
Sample/chondrite
Loess
1000
Paleosol
< 2 um
< 2 um
2-45 um
2-45 um
> 45 um
> 45 um
Luochuan
Luochuan
100
100
10
10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Fig. 3. Chondrite-normalized REE patterns for Laohushan samples. (a) Loess, (b) paleosol. REE data of Luochuan is from Chen et al. (1996).
REEs show a moderately steep, light rare earth element enriched (LREE) pattern, with a flatter heavy rare earth elements (HREE) distribution. Similar values for dEu, dCe, Eu/Sm, Sm/Nd and Nd/La ratios of the loess, paleosol and three sub-populations indicate that all have the same source provenance. The dEu and dCe values in different size fractions vary little, with values less than 1 for both. Ratios of Eu/Sm, Sm/Nd and Nd/La vary from 0.18 to 0.21, 0.17 to 0.20 and 0.72 to 0.90, respectively, indicating that the same provenance provided both fine and coarse size deposits in the Xiashu loess. Among the three size sub-populations P of both loess and paleosol, the total REE composition ( REE) in the <2 lm sub-population is the highest. The LREE/HREE ratio increase from the fine- to coarse-grained sub-populations, indicating that the HREE have an affinity P for clays. Bulk loess samples have a slightly higher REE and lower LREE/HREE ratio than that of the paleosol (Table 1). The differences for REE between loess and paleosol and various size subpopulations indicate that compositional fractionation may arise from eolian sorting during transportation and deposition. Compared with P the Luochuan profile in the Chinese Loess Plateau, the REE in the Laohushan is obviously higher. The same REE patterns and similar values of LREE/HREE, dEu, dCe, and elemental ratios (Eu/Sm,
Sm/Nd and Nd/La) for the Laohushan and the Luochuan profiles (Fig. 3a and b) suggest that they have the same provenance. The REE retained by the clay minerals results in higher REE concentrations in clay-rich sediments (Yang and Li, 1999). However, there is no REE data for the clay fractions of northern Chinese loess at present, even though many REE studies on the loess have been done. Average clay content in the northern loess was reported to be 17% (Liu, 1985); lower than in the Laohushan loess with 29.7% clay (Li et al., 2004). Therefore, the REE of paleosol and loess in the Laohushan are 1.48 and 1.54 times higher than those in Luochuan, respectively, which might be attributed to wind sorting during distal transportation to East China. 3.2. Clay minerals and illite crystallinity (IC) Different types and assemblages of secondary minerals in loess would reflect chemical weathering intensity before and after deposition under various climatic conditions (Liu, 1985; Kisch, 1991). For example, the content and crystallinity of different clay minerals are important records of environmental conditions and serve as significant proxies of past environment (Ji et al., 1999b; Kostic and Protic, 2000). Unstable mixed-layer clay minerals are especially sensitive to environmental change. The degree of crystallinity
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
changes significantly with increased weathering intensity and therefore is an effective proxy for loess research (Liu, 1985; Kisch, 1991; Ji et al., 1999b). Research on clay minerals in loess will help us to understand chemical weathering intensities under various climatic conditions and their implication for paleoenvironmental change. However, little work has been done on clay minerals of the Xiashu loess in determining the extent and intensity of chemical weathering. Based on XRD charts of the samples after a series of treatments, including (1) saturation with KCl, (2) heating, (3) saturation with MgCl2, and (4) solvate with glycerine, clay minerals in the Laohushan sequence are found to be dominantly illite, vermiculite and kaolinite (Fig. 4). Estimates of each clay mineral abundance were based on integrated peak areas determined by basal (001) diffraction lines (1.47, 1.01 and 0.71 nm) of samples saturated with MgCl2 and GL using computer programs. The quantities of each mineral were multiplied by the power of 3 for illite, 2 for kaolinite and 1.5 for vermiculite (Gorbunov, 1978). The relative content of each clay mineral is listed in Table 2. Similar types and contents of clay minerals in the Xiashu loess and in the northern Chinese loess (Ji et al., 1999b) further support the hypothesis that both were derived from the same provenance. The average illite content is 52.8% in paleosol and 47.0% in loess. The higher kaolinite content in loess (18.1%) relative to paleosol (16.0%) for the Laohushan sequence is opposite to that of the Luochuan profile investigated by Liu (1985), indicating strong weathering and/or modern weak-acid rainfall in East China, because kaolinite is a clay mineral formed by strong chemical weathering within a slightly acidic environment. The observation is also supported by the gradual increase in kaolinite
817
Table 2 Relative contents of dominate clay minerals and illitic crystallinity (IC) Samples no.
Depth (m)
Vermiculite
Kaolinite
Illite
IC
LHS-1 LHS-5 LHS-10 LHS-15 LHS-20 LHS-25 LHS-30 LHS-35 LHS-40 LHS-45 LHS-50 LHS-55 LHS-60 LHS-65 LHS-70 LHS-75 LHS-80 LHS-85 LHS-90 LHS-95 LHS-100 LHS-105
10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
31.69 46.33 26.52 34.55 52.73 47.96 33.94 26.95 39.47 16.08 40.56 16.27 27.03 25.95 38.05 42.20 36.23 31.70 36.81 18.02 20.29 26.13
11.61 16.82 22.42 21.67 5.47 8.92 19.32 11.12 13.28 6.06 24.92 15.10 17.33 17.10 5.70 30.47 18.42 6.57 9.94 14.01 22.14 51.43
56.70 36.85 51.06 43.78 41.79 43.12 46.74 61.93 47.25 77.86 34.52 68.63 55.64 56.95 56.25 27.33 45.35 61.73 53.25 67.96 57.57 22.44
2.91 2.00 1.91 2.00 2.25 1.50 1.91 1.91 2.78 3.82 3.00 2.50 2.38 2.10 2.95 0.91 2.10 3.71 2.00 2.84 2.64 1.33
contents from a depth of 2 m to the surface in the Laohushan profile. Unlike some easily weathered clay minerals such as kaolinite or interstratified illite–smectite (I–S), as long as there is no intense late-stage metamorphism, illite is stable even in an epigenetic environment (Velde and Vasseur, 1992; Ji et al., 1999b). Consequently, illite crystallinity (IC) is used to understand formation conditions of source material and serves as an indicator of provenance. For
Fig. 4. X-ray diffractograms of the Xiashu paleosol and loess showing representative diffractograms for KCl-treated, KCl-treated and heated under 550 C (KCl + 550), MgCl2-treated and MgCl2-treated and glycolated (MgCl2 + GL) samples. Semi-quantitative abundance is given in Table 2.
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F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822 100
Y = 12.83 X + 20.65 R2 = 0.454
Content of illite (%)
80
60
40
20
0 0
1
2
3
4
5
Illite crystallinity (IC)
Fig. 5. Plot showing a positive correlation between illite content and illitic crystallinity.
example, Liu (1985) found that the loess and paleosol layers in northern Chinese loess could be distinguished by the higher IC in loess relative to paleosol. There are different methods for calculating IC. Here we used IC = H/W suggested by Liu (1985), where H and W is the height and the half-peak-width of the 1.01 nm illite peak, respectively. The IC values correlate well with variations in illite content with depth (Fig. 2), especially in the upper part of the profile (Fig. 5), indicating that the source area for Xiashu loess should be located in an arid and/or cold environment, and that eolian deposits experienced only slight weathering after deposition. 3.3. Rb, Sr distribution and Rb/Sr ratio in different size subpopulations Chemical weathering affects both the elemental distribution and magnetic susceptibility of rocks/sediments (Gallet
et al., 1998). For example, Rb and Sr are easily fractionated during weathering, resulting in variations in Rb/Sr ratios for loess–paleosol sequences and lake sediments in response to weathering intensity under different climatic conditions (Chen et al., 1999; Jin et al., 2001, 2005). Table 3 shows that Rb concentrations in clay populations of the Xiashu loess are the highest, decreasing from <2 lm to 2– 45 lm and to >45 lm sub-populations for both loess and paleosol. Variation in Sr contents with size is opposite to that exhibited by Rb. It is thought to be attributed to sorting and weathering related to monsoonal conditions. On the one hand, during eolian transportation, changes in the strength of the winter monsoon would result in different mineral compositions and initial Rb and Sr contents of eolian deposits. During glacial periods, a stronger winter monsoon should transport coarse grains dominated by quartz, feldspar and carbonates with lower Rb/Sr ratios. During interglacial periods, a weak winter monsoon can carry clay particles with higher Rb/Sr ratios (Gallet et al., 1998). Therefore part of the variations in Rb/Sr ratios for both loess and paleosol should depend on wind sorting during eolian transportation. On the other hand, Rb/Sr ratios in either loess or paleosol would vary due to leaching and weathering after deposition. Variations in Rb/Sr ratios for bulk samples from the Luochuan profile have been used to elucidate weathering history after deposition (Chen et al., 1999). Based on the acid-leaching experiment of northern Chinese loess, 20–40% Sr is easy to remove into solution, while Rb is retained by clay minerals (Chen et al., 1996). Clays are the main products of rock weathering, whose type and component budget contain important information of weathering intensity, if the provenance of the Xiashu loess remained unchanged throughout the late Pleistocene. Here, an important issue is how to distinguish the amount of sorting and weathering, and the degree of weathering before and after deposition, on
Table 3 Statistical results of concentrations of Rb, Sr and magnetic susceptibility (MS) of bulk samples and three sub-populations of different sizes Grain size
Element
S1*(11)
S1(17)
L1(12)
S2(31)
L2(5)
S3(12)
L3(17)
S(71)
L(34)
Average
<2 lm
Rb Sr Rb/Sr MS
172.94 67.13 2.58 352.65
172.52 80.03 2.17 376.27
169.18 79.93 2.12 338.33
166.86 116.05 1.45 309.85
165.31 106.33 1.56 164.17
164.85 105.68 1.56 312.75
163.57 120.19 1.37 350.16
168.82 98.09 1.82 332.88
165.80 103.94 1.66 318.63
167.84 99.99 1.77 328.26
2–45 lm
Rb Sr Rb/Sr MS
90.99 113.94 0.80 107.30
101.90 110.51 0.92 127.87
95.55 110.96 0.87 116.28
100.51 138.44 0.73 105.81
85.55 133.68 0.64 48.74
101.29 129.86 0.78 105.76
102.62 136.79 0.75 117.67
99.50 126.51 0.80 111.31
97.61 127.22 0.78 107.04
98.89 126.74 0.78 109.93
>45 lm
Rb Sr Rb/Sr MS
73.19 118.04 0.62 82.01
75.66 122.67 0.63 96.45
71.74 114.37 0.63 76.02
82.58 141.89 0.60 103.76
72.65 133.94 0.55 49.61
71.16 126.87 0.56 87.61
71.27 133.29 0.54 102.22
77.54 131.06 0.61 95.91
71.64 126.71 0.57 85.24
75.63 129.65 0.60 92.45
Bulk sample
Rb Sr Rb/Sr MS
105.55 153.45 0.69 156.42
108.59 139.41 0.78 156.28
103.67 120.08 0.87 140.97
109.84 143.10 0.77 142.80
100.00 136.80 0.73 68.60
109.25 138.33 0.79 147.55
106.00 149.18 0.71 167.70
108.66 130.76 0.84 148.94
104.29 137.09 0.77 143.69
107.25 132.81 0.82 147.24
L, loess; S, paleosol; S1*, upper part of S1, listed separated due to possible effect of human activity. (11) is number of samples.
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
the basis of Rb/Sr proxy variations in bulk samples for both loess and paleosol. One way is Rb/Sr measurements for different size sub-populations, because clays are closely related to weathering intensity whereas the coarse fractions are related to provenance. On the Earth’s surface, minerals with higher Sr would be weathered first in the order: calcite fi olivine fi feldspar fi pyroxene fi hornblende fi biotite fi K-feldspar fi muscovite fi quartz (Kukla et al., 1988). In the Laohushan profile, Rb/Sr ratios in the residue products would increase due to greater Sr loss as the intensity of chemical weathering increased, especially at the early stage of rock weathering. From Table 3 and Fig. 7, comparison of Xiashu loess and paleosol shows that variations in Rb and Sr contents for given size sub-populations are different. Rb contents vary little within <2 lm sub-population, between 164 and 169 ppm in loess (averaging 166 ppm), and between 165 and 173 ppm in paleosol (averaging 169 ppm). On the other hand, Sr contents vary between 80 and 120 ppm in loess (with an average of 104 ppm), and between 67 and 116 ppm in paleosol (averaging 98 ppm). This indicates that Rb in the Xiashu loess is relatively stable, while variable Sr contents is due to active mobility of Sr during weathering, resulting in a negative correlation between Rb/Sr ratios and Sr contents, whose correlation coefficient (R2) value can be used to evaluate the sensitivity of Rb/Sr for measuring weathering intensity. Among R2 values of bulk samples and three size sub-popa 1.1
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ulations, the <2 lm sub-population has the highest value (R2 = 0.97), while R2 values for the >45 lm and 2–45 lm sub-populations are 0.40 and 0.49, respectively (Fig. 6b, c and d). The R2 value of the <2 lm sub-population is higher than that of the bulk samples from both the Laohushan (R2 = 0.93, Fig. 6a) and the Luochuan (R2 = 0.94, Chen et al., 1996) profiles. This suggests that Sr lost during weathering mainly affects the composition of the <2 lm sub-populations. Curves of Rb/Sr ratios versus depth (Fig. 7) show similar variations between bulk samples and the <2 lm sub-population, while Rb/Sr ratios for both the >45 lm and the 2–45 lm sub-populations do not vary. We suggest that variations in Rb/Sr ratios for bulk samples due to weathering depend on the composition of the <2 lm sub-population, while Rb/Sr ratios for both the >45 lm and the 2–45 lm sub-populations preserve information of provenance. Higher Sr contents for the loess and paleosol indicates that the Xiashu loess has experienced only slight weathering, similar to the northern Chinese loess. Increasing Rb/Sr ratios for both bulk samples and the <2 lm subpopulations indicates an increase in weathering intensity for the lower Yangtze River region since the late Pleistocene (Li et al., 2002). 3.4. Relationship between magnetic susceptibility and size Magnetic susceptibility in bulk loess is also used to determine the degree of weathering (Guo et al., 2000), b 3.0
1.0 2.5
2
Rb/Sr ratio
Rb/Sr ratio
y = 79.9 x -0.94 0.9
R = 0.93
0.8 0.7
y = 215 x -1.05 R 2 = 0.97
2.0
1.5 0.6 0.5 100
1.0 120
140
160
180
60
80
100
120
140
d 1.0
1.1
y = 81.8 x -0.96
1.0
R 2 = 0.49
0.9 0.8 0.7
y = 30.4 x
0.8
Rb/Sr ratio
Rb/Sr ratio
c 1.2
40
-0.81
R2 = 0.40
0.6
0.4
0.6 0.5
0.2 90 100 110 120 130 140 150 160
Sr (ppm)
80
115
150
185
220
Sr (ppm)
Fig. 6. Relationship between Rb/Sr ratios and Sr concentrations in Xiashu loess samples for different size sub-populations. (a) Bulk samples, (b) <2 lm, (c) 2–45 lm and (d) >45 lm.
820
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822
SUS/10 -8 m 5 kg -1
Rb/Sr ratio 0.6
0.7
0.8
0.9
1.0
0
1
2
3
0
0.0
50
1.5
3.0
250
300
350
400
2-45 um
Bulk
Bulk < 2um 2-45 um > 45 um
Depth (m)
Depth (m)
200
> 45 um
3.0
6.0
150
< 2 um
1.5
4.5
100
0.0
4.5
6.0
7.5
9.0
10.5
Fig. 7. Curves of Rb/Sr ratios for bulk samples and three size subpopulations versus depth from the Laohushan profile.
7.5
9.0
10.5
Fig. 8. Curves of magnetic susceptibility of bulk samples and three size sub-populations with depth.
where increasing magnetic susceptibility reflects enhanced weathering. Higher values of magnetic susceptibility in paleosol relative to loess are thought to be caused by formation of fine superparamagnetic grains and stable single domain grains during pedogenesis (Heller and Liu, 1982; Kukla et al., 1988; Zhou et al., 1990; Maher and Thompson, 1991; Guo et al., 1996). This indicates that there is a relationship between weathering and magnetic susceptibility. In loess profiles, weathering of biotite and chlorite is thought to be a dominant process for evolution of ironbearing minerals via iron supply for magnetic minerals (Ji et al., 1999a). Further work has shown that there is a good linear correlation between mica to chlorite ratios and magnetic susceptibility (Ji et al., 1999b). Magnetic susceptibility for bulk samples and three size subpopulations of the Laohushan profile show distinctly different values. The average magnetic susceptibilities (·108 m3 kg1) of the bulk samples, the <2 lm, 2– 45 lm and >45 lm sub-populations are 147.24, 328.26, 109.93 and 92.45, respectively (Table 3). Magnetic susceptibility for both the >45 lm and the 2–45 lm sub-populations is lower than in bulk samples. Values of magnetic susceptibility in the <2 lm sub-population are three times higher than those of bulk samples, but exhibit a similar curve with depth (Fig. 8). The results indicate that weathering mainly affects magnetic susceptibility of the clay sub-population via increase in concentration of a claygrained (predominantly superparamagnetic) ferromagnetic fraction. Higher magnetic susceptibility in the clay subpopulation is also attributable to grain size, where smaller grains weather more easily (Zhou et al., 1990). Therefore clay is sensitive to weathering rates after deposition under different climatic conditions. Variations in magnetic susceptibility for different size grains help elucidate why the magnetic susceptibility increases during pedogenesis and its relationship with chemical weathering.
4. Conclusions The effects of sorting and chemical weathering are sufficient in explaining the compositional differences between bulk sediments and the three size fractions (>45 lm, 2– 45 lm and <2 lm) from the Xiashu loess in East China. Our work on the Xiashu loess not only highlights Rb/Sr ratios and magnetic susceptibility as effective proxies for weathering intensity, but also provides further evidence that variations in Rb/Sr ratios for bulk loess are controlled by the chemical and mineralogical compositions of the clay fraction. We suggest that analysis of different grain-size sub-populations is useful for identifying environmental changes, because a given sub-population may minimize the effects of sorting and weathering and thus reduce varying initial Rb/Sr ratios. The differences in Rb/Sr ratios and magnetic susceptibilities for bulk sediments and the three size subpopulations are therefore explicable if chemical weathering of the Xiashu loess after deposition played a controlling role in the Rb/Sr ratios and magnetic susceptibility of the clay sub-population. Chemical weathering results in a negative correlation between Rb/Sr ratios and Sr contents for the <2 lm fraction with R2 = 0.97. Rb/Sr ratios and magnetic susceptibility of the <2 lm sub-population stand out from the other groups (Fig. 9). Therefore variations in Rb/Sr ratios in the clay fraction could serve as an effective tracer for weathering intensity. On the other hand, the Rb/Sr ratios and magnetic susceptibility of the coarse size (here 2–45 lm and >45 lm) subpopulations provide information of source provenance controlled by weathering before transportation and sorting. To evaluate the effects of chemical weathering and sorting on the composition of loess and paleosol deposited
F. Li et al. / Journal of Asian Earth Sciences 29 (2007) 813–822 3.0 <2μm 2-45μm
2.5
>45μm bulk
Rb/Sr ratio
2.0
1.5
1.0
0.5
0.0
Magnetic susceptibility (×10-8 m3 ·kg-1 ) Fig. 9. Plot of Rb/Sr ratios versus magnetic susceptibility in bulk samples and three size sub-populations of the Xiashu loess.
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