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Magnetic susceptibility of surface soils in the Mu Us Desert and its environmental significance
Aeolian Research 25 (2017) 127–134
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Magnetic susceptibility of surface soils in the Mu Us Desert and its environmental significance Xiaokang Liu a,b,c, Ruijie Lu a,b,c,⇑,1, Zhiqiang Lü a,b,c, Jing Du a,b,c, Feifei Jia d, Tengfei Li a,b,c, Lu Chen a,b,c, Yongqiu Wu a,b,c a
State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China School of Geography, Beijing Normal University, Beijing 100875, China Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China d School of Urban and Environmental Sciences, Liaoning Normal University, Dalian 116029, China b c
a r t i c l e
i n f o
Article history: Received 30 October 2016 Accepted 5 April 2017
Keywords: Modern surface soils Frequency-dependent magnetic susceptibility Regional MAP Transfer function Paleo-rainfall reconstruction
a b s t r a c t Magnetic susceptibility has been widely used as a climatic proxy in paleoclimatic research. In arid and semi-arid regions, the magnetic properties of modern surface soil are significantly influenced by precipitation. This is demonstrated by observed positive correlations between percentage frequency-dependent magnetic susceptibility (vfd%), which reflects the presence of fine-grained (superparamagnetic, SP) grains produced during weathering and pedogenesis, and regional mean annual precipitation (MAP). To further investigate this relationship, we measured the magnetic properties of 104 surface soil samples collected along two transects (AA and BB) spanning a rainfall gradient across the Mu Us Desert in northern China. There were no systematic trends in magnetic properties along transect BB; the vfd% values remained relatively low and stable, probably reflecting weak pedogenesis and the domination of the magnetic properties by lithology. In contrast, along transect AA there was a significant positive correlation (p < 0.01) between vfd% and regional MAP. From this relationship, we developed a transfer function (P = 274.1 + 1424.4 vfd%) and used it to produce quantitative estimates of paleo-precipitation within three Holocene aeolian sections located in the southern the modern Mu Us Desert. The results show that the variations of reconstructed precipitation are consistent with those of lithological properties, and they also confirm previous conclusions that paleosol development in the study area is dominated by precipitation. Overall the results further demonstrate the feasibility of using frequency-dependent magnetic susceptibility to quantitatively reconstruct regional paleo-precipitation, including within a geographically diverse desert area. In addition, they provide an improved understanding of the main sand provenance in Mu Us Desert. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Magnetic susceptibility (MS) measurements, which reflect how easily a material is magnetized in a weak magnetic field, have many applications in environmental research (e.g., Evans and Heller, 2003; Liu et al., 2012). In addition, the availability of relatively inexpensive and sensitive MS meters, which enable the rapid acquisition of large volumes of data, have resulted in the widespread use of MS measurements as a paleoclimatic indicator e.g.,
⇑ Corresponding author at: State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, No. 19, Xinjiekouwai Street, Haidian District, Beijing, China. E-mail address: [email protected] (R. Lu). 1 This author contributed equally with the first author to this work. http://dx.doi.org/10.1016/j.aeolia.2017.04.003 1875-9637/Ó 2017 Elsevier B.V. All rights reserved.
(Herb et al., 2013; Kukla et al., 1988; Liu and Deng, 2009; Verosub et al., 1993). Modern soils are formed under different climatic regimes and in some environments the magnetic properties of soils are considered to directly reflect climate, especially precipitation (Liu et al., 1995; Maher et al., 1994b). For regions of uniform parent material, numerous studies have used MS measurements of surface soils for the development of climofunctions (Balsam et al., 2011; Guo et al., 2011; Porter et al., 2001; Song et al., 2014; Xia et al., 2012) which can be used to reconstruct past climatic conditions, especially paleo-precipitation (Lü et al., 1994; Maher et al., 1994b). However, other studies have shown that arid or semi-arid regions, where soil parent material and topography are complex and varied, are unsuitable for using magnetic susceptibility to estimate variations in rainfall amount (Wei et al., 2008; Xia et al., 2007).
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Frequency-dependent magnetic susceptibility (vfd%) reflects the content of superparamagnetic (SP) grains produced during weathering and pedogenesis (Liu et al., 1990; Zhou et al., 1990). In addition, vfd% is very sensitive to low amplitude climatic fluctuations (Heller et al., 1991; Liu et al., 1990) and in arid zones is well correlated with regional MAP (Xia et al., 2007). The Mu Us Desert in northern China is located in a climatically sensitive zone and is therefore well suited for studies of climatic change (Ding et al., 2005; Sun et al., 1999). Significant progress has been made in investigating late Quaternary aeolian activity (He et al., 2010; Jia et al., 2015; Lu et al., 2005; Miao et al., 2016; Xu et al., 2015) and climate change (Du et al., 2012; Liu et al., 2014; Lu et al., 2011) in the Mu Us Desert; however, there have been few attempts to reconstruct the paleo-rainfall during the Holocene (Chen et al., 2015). Here, we present the results of an investigation of the relationship between measurements of the vfd% of surface soils and regional MAP along a precipitation gradient in the Mu Us Desert. We then use the resulting climofunction to reconstruct past precipitation variations within three Holocene aeolian sections from the same region. The findings provide insights into the relationship between environmental factors and the production of secondary ferrimagnetic minerals, enable a preliminary quantitative reconstruction of paleo-rainfall in the study region, and provide information on sediment provenance. 2. Material and methods 2.1. Study area The modern Mu Us Desert (Fig. 1), with an area of about 39,000 km2, is located on the Ordos Plateau with the Loess Plateau immediately to the south. It lies on the margin of the area impacted by the East Asian monsoon. The mean annual rainfall is 250–440 mm and the mean annual temperature is 6.0–8.5 °C. Warm and humid air brought by the East Asian summer monsoon delivers more than 60% of the annual precipitation, which falls mainly from June to August. The present vegetation consists mainly of Artemisia ordosica Krasch., Tamarix chinensis Lour., and Hippophae rhamnoides Linn. Regional landscape types are varied and include stabilized, semi-stabilized and active sand dunes, grassland, and interlinked sequences of lakes and swamps (‘bead s-on-a-string’) (Zhou et al., 1996). It is noteworthy that in the northwestern Mu Us Desert there is a bedrock-dominated zone consisting of weakly-cemented Cretaceous purple sandstone with occasional Jurassic celadon mudstone strata (Department of Geography of Peking University et al., 1983; Kapp et al., 2015; Yue et al., 2007). 2.2. Sampling and laboratory measurements In April 2015, we collected a total of 104 surface soil (including sand) samples along two transects (AA and BB) spanning a modern rainfall gradient in the Mu Us Desert (Fig. 1). The samples were collected from sites as far as possible from cultivated land and human habitation and with a natural vegetation cover to avoid possible magnetic contributions from anthropogenic sources. The depth interval from 2–5 cm was sampled from typical regional soil units including sand dunes and grassland, and observations were made of landscape type and soil and vegetation conditions around the sampling site (see Supplementary information). The main soil types sampled were aeolian sandy soil in the case of sand dunes and meadow soil in the case of grassland. In addition, three natural exposures of aeolian strata, sections ZBT (38°190 4700 N, 109°430 5600 E, altitude 1108 m.a.sl.), CC
(37°400 2700 N, 108°180 5300 E, altitude 1350 m.a.sl.), and LZ (37°360 0200 N, 108°210 2400 E, altitude 1363 m.a.sl.) were chosen for study. The ZBT section is a sand-paleosol sedimentary sequence and detailed lithological description and dating results can be found in Jia et al. (2015). The CC and LZ sections are paleo-dunes, with two sandy paleosol layers and one sandy paleosol layer, respectively. Lithological descriptions of the LZ and CC sections are given in Table 1. Five samples of organic sediments were collected from the CC and LZ sections for AMS 14C dating, and the ages were converted to calendar ages using the INTACAL 13 calibration (Reimer et al., 2013). In the laboratory the samples were air-dried, disaggregated and packed in 8 cm3 plastic boxes. Measurements of low (0.47 kHz) and high frequency (4.7 kHz) magnetic susceptibility (vlf and vhf ) were made using a Bartington Instruments MS2 meter and MS2B sensor in the MOE Key Laboratory of Environmental Change and Natural Disasters, Beijing Normal University. Percentage frequency-dependent magnetic susceptibility was calculated as vfd% =ðvlf vhf Þ=vlf 100% to estimate the relative content of superparamagnetic (SP) particles. Rainfall along the transect was estimated using data from 1981–2010 from 16 meteorological stations (http://data.cma.cn/). The rainfall at each sampling site was estimated using a kriginginterpolation model (based on the Nearby Principle method) implemented in GIS 10.2 and Surfer 8.
3. Results 3.1. Variation of magnetic susceptibility and its frequency dependence and construction of the climofunction The vlf values range from 8.17–174.15 108 m3/kg, with an average of 45.69 108 m3/kg. The vfd% values range from 0– 4.62%, with an average of 1.41%, and exhibit a roughly increasing trend from northwest to southeast. In terms of the various landscape types illustrated in Fig. 2, vlf is relative high in semistabilized or semi-active dunes and lower in the stabilized sand dunes. This may be the result of contrasts in dune source material source in this geographically diverse desert area. vfd% values are higher in stabilized sand dunes and grassland and low in active sand dunes; this clearly reflects the production of fine-grained secondary ferrimagnetic minerals weathering and pedogenesis. The vegetation of the sand dunes is dominated by Artemisia ordosica Krasch., Caragana korshinskii Kom. and that of the grassland is dominated by Gramineae (see Supplementary information). The variation of vfd% and precipitation along transect BB is illustrated in Fig. 3. In the bedrock-dominated zone in the northern part of the transect (blue triangles) it is clear that there is no systematic trend with the vfd% values remaining relatively low (less than 2%) and stable. The bedrock in this zone consists of Cretaceous purple sandstone (Fig. 1A) and Jurassic celadon mudstone (Fig. 1B) and it is possible that the distinctive lithology is responsible for the low and invariant vfd% values. In the southern part of the transect, in the sand dune zone (blue dotted arrows) where the rainfall exceeds 340 mm/yr, there is a positive correlation between vfd% and regional MAP. Due to the relatively constant vfd% values along much of the transect (the reasons for which are discussed in Section 4.2, below) we chose another transect AA (Fig. 1) which also follows the steepest modern precipitation gradient; however, there are no significant variations in lithology and the temperature is almost constant. Prior to the analyses, we removed 5 samples (Fig. 1 – orange dots and see Supplementary information) whose vfd% values were more than two times larger than the neighboring points (Song et al., 2014). The occurrence of these values may be
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Fig. 1. Left: location of the study area in China, and major atmospheric circulation systems. Upper right: map of regional precipitation based on data from local meteorological stations from 1981–2010 (http://data.cma.cn/) and the distribution of bedrock derived from the Digital Library of NGAC (http://en.ngac.org.cn/Map/List). Also shown are the locations of surface soil sampling transects AA and BB. Precipitation along the transects ranges from 260 mm/yr in the near-desert conditions in the northwest to over 380 mm/yr in the southeast sandland environment, a distance of about 150 km. Lower right: examples of landscapes, landforms and lithologies in the Mu Us Desert, including A – purple sandstone; B – celadon mudstone; C – barchan; D – fixed dune; E – sand dunes and marshy grassland; F – grassland.
Table 1 Lithological descriptions of the LZ and CC Sections. Section
Depth (cm)
Lithology
Munsell color
LZ
0–40
Sod horizon with abundant modern plant roots Loose fine sand layer
Very pale brown (10YR7/4)
40–370
370–476
476–620 CC
0–30
30–85 85–112 112–268 268–370
Sandy paleosol layer with white mycelia, compacted Loose coarse sand layer Sod horizon with abundant modern plant roots Weakly-developed sandy paleosol layer Loose fine sand layer Sandy paleosol layer, compacted Loose fine sand layer
Very pale brown (10YR7/4), light yellowish brown (10YR6/4) Dark yellowish brown (10YR4/4) Very pale brown (10YR7/4) Light yellowish brown (10YR6/4) Yellowish brown (10YR5/4) Light yellowish brown (10YR6/4) Brown (10YR4/3) Light yellowish brown (10YR6/4)
explained by heteroscedasticity in which a population of relatively higher vfd% values is mixed with a population of lower values ones. The different populations may reflect different sand sources in the area. A scatter plot of vfd% and regional MAP for the 23 remaining
Fig. 2. Average values of vlf and vfd% for different landscape types. G: grassland; SS-SA: semi-stabilized or semi-active dunes; S: stabilized sand dunes; A: active sand dunes.
samples is illustrated in Fig. 4. Application of linear regression analysis produced the following line of best fit which was statistically significant which can be approved by the t-testing and F-testing at 0.01 level by SPSS:
P ¼ 274:1 þ 1424:4 vfd %; R2 ¼ 0:77 ðp < 0:01Þ where P is mean annual precipitation.
ð1Þ
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Fig. 3. Topographic profile along transect BB (along the red arrowed profile in Fig. 1) and the relationship between precipitation and ᵡfd%.
developed soil, while CC-P1 exhibits the weakest pedogenic development. The reconstructed precipitation records based on the climofunction (Eq. (1)) were smoothed by moving averaging. The results (Fig. 6) are in accordance with lithological evidence for intensified pedogenesis, hence precipitation. From the early Holocene to the late Holocene, the reconstructed precipitation curves for the three sections exhibit a similar trend of variation with an initial increase and subsequent decrease. Maximum reconstructed precipitation occurred in the mid-Holocene. The paleo-precipitation varied within a relatively broad range (more than 100 mm) during the Holocene. It is worth noting that the range of application of our transfer function may only be limited in the Mu Us Desert and adjacent regions in consideration of the rainfall inflection point between the magnetic susceptibility and regional MAP (Long et al., 2011). Fig. 4. Scatter plot of fd% versus regional MAP for transect AA with line of best fit shown in green. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Discussion 4.1. Environmental controls on the soil magnetic properties
3.2. Aeolian sections in the southern Mu Us Desert: Sedimentary characteristics of the paleosol layers and application of the transfer function We applied the transfer function to the magnetic data from Holocene aeolian sections in the same region to reconstruct paleo-rainfall. The studied sections, ZBT, CC and LZ, are located on the southeastern or southern boundary of the modern Mu Us Desert (Fig. 1). The dating results for the CC and LZ sections are listed in Table 2. The ages are in chronological order with no inversions. The sections are of Holocene age. Photos of the sections are shown in Fig. 5. All three sections contain a paleosol layer(s) reflecting the early/mid-Holocene Asian summer monsoon precipitation maximum. In section CC, the upper paleosol layer (CC-P1) is loose yellowish brown (10YR5/4) weaklydeveloped with small pseudomycelia, root tissues and bio-pores. The lower paleosol layer (CC-P2) is compacted and brown in color (10YR4/3) with abundant pseudomycelia. In section LZ, there is a sandy paleosol layer (LZ-P) which is dark yellowish brown (10YR4/4), compacted and with occasional pseudomycelia. In the lithological description of section ZBT (Jia et al., 2015), there are three paleosol layers: the upper is dark yellowish brown (ZBTP1), compacted, with abundant bio-pores and white mycelia; the middle paleosol layer (ZBT-P2) is thin, dark yellowish brown with abundant root tissues and white mycelia; and the lower paleosol layer (ZBT-P3) is light yellowish brown, sandy and weaklydeveloped. Overall, we consider that paleosol ZBT-P1 is the best
The magnetic properties of soils are affected by a wide range of factors, including parent material, duration of soil formation, vegetation, topography, and fire history (Blundell et al., 2009; Gedye et al., 2000; Maher, 1998). However, the nature of the parent material is the primary factor (Wei et al., 2008), with pedogenesis as a secondary factor (An et al., 1991) which is dominantly affected by climate (Maher et al., 1994a). Precipitation is one of the five soil forming factors and has a major effect on the magnetic characteristics of soils (Tite and Linington, 1975). With abundant precipitation, flourishing vegetation and well-drained soils, strong weathering and pedogenesis result in the production of significant amounts of SP grains. Pedogenic iron oxides mainly consist of hematite, goethite and maghemite and the content of ferromagnetic maghemite dominates the magnetic susceptibility of soils (Long et al., 2016). We divide the area along transect BB in the Mu Us Desert into two zones, an erosion zone and an accumulation zone. In the bedrock-dominated zone where the bedrock is exposed, in the northern part of the transect, erosion is dominant under the influence of strong winds, relatively low precipitation and sparse vegetation coverage. Consequently, we designate it the ‘erosion zone’. In the downwind area along transect BB where the sand is transported from upwind, there is greater precipitation which results in vegetated sand dunes and sediment accumulation is dominant. Hence, we designate it the ‘accumulation zone’. A conceptual model of the factors influencing soil magnetic properties in the study area is illustrated in Fig. 7. In the erosion
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X. Liu et al. / Aeolian Research 25 (2017) 127–134 Table 2 AMS14C dates for the CC and LZ sections. Beta Lab No.
Sample No.
Depth (cm)
Material
387098 387099 387102 387100 387101
LAA4C01 LAA4C05 CC14C13 CC14C02 CC14C05
386–388 468–470 60–65 126–128 260–262
Organic Organic Organic Organic Organic
sediment sediment sediment sediment sediment
d13C
Radiocarbon age (yr BP)
Cal age (2r, cal yr BP)
21.7 22.6 22.2 22.1 22.1
3190 ± 30 5210 ± 30 1280 ± 30 4560 ± 30 6350 ± 30
3410 ± 45 5935 ± 20 1230 ± 55 5195 ± 125 7285 ± 35
Fig. 5. Photograph of the CC, LZ and ZBT sections.
Fig. 6. Quantitative precipitation reconstructions for the LZ (a), CC (b), ZBT (c) sections. The reconstruction for the ZBT section is from Chen et al. (2015) (d). The reconstructions are based on the climofunction shown in Eq. (1).
zone, climatic factors, including precipitation, temperature and strong winds, lead to rock weathering and weak pedogenesis, and in addition the soil magnetic properties may be diluted by the influx of coarse particles. In the accumulation zone, fine dusts are blown from upwind areas and precipitation is the major factor influencing pedogenesis. In addition, to the effect of intrinsic site and soil factors, the accumulation of magnetic minerals derived from subaerial dust falls and magnetic bacteria may also contribute to the soil magnetic properties.
4.2. The characteristics of the different zones and regional comparisons In the northwest part of the study area erosion is dominant because of the high ground surface, aridity (MAP of 260– 340 mm) and strong winds (Kapp et al., 2015) and the underlying bedrock is widely exposed. The relatively weakly-developed soils with a low content of SP grains in this zone are analogous to the topsoils which have a coarse grain-size, weak pedogenic development and local sediment provenance at the northern foot of the
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Fig. 7. Conceptual model of the factors influencing soil magnetic properties in the study area. The dominant factors influencing soil magnetic properties are ① – precipitation, ② – temperature, ③ – intrinsic site and soil factors (including altitude, pH, and clay organic matter content), ④ – accumulation of magnetic minerals derived from subaerial dust falls, and ⑤ – living magnetic bacteria. (I) Rock weathering, (II) pedogenesis, (III) dilution effect, (IV) vfd% depends on the content of superparamagnetic (SP) grains.
Tien Shan Mountains (Guo et al., 2011; Jia et al., 2013). Similarly, in arid or hyper-arid Egypt there is virtually no relationship between magnetic susceptibility and rainfall (Balsam et al., 2011). Another possible interpretation for these observations is that the pedogenic magnetic signal is diluted by the influx of coarse dust from the sediment source area (An et al., 1991). All of the foregoing factors may be responsible for the limited pedogenic development and weak soil magnetic properties in this part of the study area. However, our field observations indicate that pockets of aeolian accumulation close to the bedrock-dominated erosion zone due occur, but due to the unfavorable environmental conditions the process of soil formation is extremely slow and magnetic enhancement is very weak. In addition, it is possible that any fine magnetic grains of pedogenic origin produced in such locations are removed by the strong wind activity. In the ‘accumulation-dominated’ zone pedogenesis is the major control of soil magnetic properties. If the effect of variations in the characteristics of the protolith in the accumulation zone can be eliminated, a strong positive relationship between vfd% and regional MAP may be observed, as is the case of the surficial magnetic enhancement of soil profiles in the relatively warm and moist Northern Californian coastal region (Singer et al., 1996). Precipitation amount is the factor most likely to determine the degree of pedogenesis (Liu et al., 1995), and vfd% as a measure of the contribution of SP grains to the magnetic susceptibility of samples (Liu et al., 1991, 1992; Zhou et al., 1990) is a proxy for the intensity of pedogenesis (Liu et al., 1990; Maher et al., 1994b; Maher and Thompson, 1995; Torrent et al., 2007). In summary, in the ‘accumu lation-dominated’ zone in the Mu Us Desert, the variation of vfd% is likely to be dominated by the intensity of the production of pedogenic magnetic minerals, which depends on the gradient of the regional MAP. Temperature has a secondary and much less significant effect on vfd% (Song et al., 2014). Along transect AA, temperature is relatively constant (Fig. 1) and therefore the effect of this factor on pedogenesis can be eliminated. Other soil-forming factors (including altitude, pH and clay and organic matter content) (Long et al., 2011), the accumulation of magnetic minerals derived from subaerial dust falls from other sources, such as volcanic ash, fine desert dust, and dust raised from forest fires during soil forming periods
(Kukla et al., 1988) and living magnetic bacteria (Fassbinder et al., 1990), may also have a secondary influence on vfd%.
4.3. Implications for material sources of Mu Us Desert Magnetic measurements are useful for detecting signals associated with various environmental processes (Liu et al., 2012). Our results highlight some of the complexities involved in using magnetic properties to reconstruct regional-scale paleo-rainfall in a complex desert environment. One factor is provenance-related differences in source material, and other factors include climate, and specifically different combinations of rainfall. With regard to the provenance of the soil parent materials of the Mu Us Desert, a previous hypothesis (Department of Geography of Peking University et al., 1983) proposed that they were sourced from the underlying Cretaceous aeolianite and Jurassic mudstone. However, recent research concluded that although the underlying Cretaceous and Quaternary formations are important sources (Sun, 2000), material supplied from distant sources is also significant (Stevens et al., 2013), especially eroded material from Northeastern Tibet (Nie et al., 2015). In fact, the materials of the modern Mu Us Desert may be derived from multiple sources, with the western part heavily influenced by sediment transported from Tibet via the Yellow River, and the eastern part controlled more by recycling from local sources and the underlying bedrock (Stevens et al., 2013). The absence of a systematic relationship between vfd% and regional MAP along transect BB has implications for understanding the original sand source of the Mu Us Desert. In the northern part along transect BB, where the rainfall amount does not exceed 340 mm/yr, the widespread exposed bedrock and ‘erosion-domina ted’ environment may play an important role in the soil formation process. We speculate that the bedrock-exposed eroding uplands in the northwestern Mu Us Desert are one of main material sources for sandy sediments based on the erosion-accumulation and ‘from source to sink’ relationship. Compared with the ‘accumulation-do minated’ zone where pedogenesis is relatively strong under much higher rainfall, in the northern ‘erosion-dominated’ zone pedogenesis is extremely limited which leads to the relatively low and constant vfd% values.
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4.4. Reliability of the reconstructed of paleo-rainfall record The changes in reconstructed precipitation from the three aeolian sections in the Mu Us Desert are consistent with changes in lithological properties and they confirm the view of previous studies that paleosol development in North China was dominated by variations in monsoonal precipitation (An et al., 1990). During the Holocene climatic optimum, the prevailing paleosol and sandy paleosol development in the Mu Us region indicates a warm-moist climate with flourishing vegetation growth and that the majority of the dunes were stabilized (Xu et al., 2015). Our reconstructed paleo-precipitation curves (Fig. 6) are generally consistent with the textural and other characteristics of the paleosol layers. The increased precipitation promoted pedogenesis of the dune sands, increased the frequency of paleosol and sandy paleosol development (Jin et al., 2001), and resulted in an increase in the pedogenic production of SP grains and the increased vfd% values. Comparison of our results with a stable carbon isotope record from organic matter and a paleo-rainfall record from the ZBT section of Chen et al. (2015) reveals considerable consistency (Fig. 6d). The two quantitative precipitation reconstructions for the southeastern margin of the Mu Us Desert both exhibit a trend of increasing precipitation in the early Holocene, high and uniform precipitation in the middle Holocene and decreasing precipitation in the late Holocene. Both the overall trends and relative amplitudes of the features in the two curves are consistent. In addition, two intervals of possible increased aeolian activity during the middle Holocene, indicated by the presence of sand layers and thus decreased precipitation, are quite consistent. However, unlike the ZBT section, in the early Holocene part of the LZ and CC sections (Fig. 5) there is an obvious aeolian sand layer. This difference may reflect differences in altitude, temperature and moisture conditions (Lu et al., 2011; Qiang et al., 2016, 2013). In the middle Holocene, there are some differences between the sections; however, they all exhibit either a paleosol or sandy paleosol and the reconstructed precipitation also exhibits peak values. This may indicate that during the Holocene climatic optimum most of the sand dunes were stabilized, and pedogenesis occurred under a relatively warm and humid climate. In the late Holocene, the LZ section exhibits evidence of sand dune reactivation after 3.4 ka BP; while in the CC section a sandy paleosol developed at around 1.2 ka BP, which corresponds to the Medieval Warm Period. It is likely that a variety of factors influenced the development of the study sections, including differences in landscape, sediment supply and regional environmental conditions (Telfer and Hesse, 2013) which make detailed correlations difficult. The regional comparisons demonstrate the utility of our climofunction for reconstructing paleo-precipitation in the study region (Lü et al., 1994). However, because of the numerous influences on soil development, in the region it is unlikely that a transfer function based on a single variable can be used to express the full variability of this complex soil system (cf. Balsam et al., 2011) and to estimate paleo-rainfall with complete reliability (Lü et al., 1994). Therefore, additional variables need to be considered and a greater number of well-dated paleo-rainfall reconstructions based on independent proxies need to be obtained in the future.
5. Conclusions We measured the magnetic susceptibility of surface soils on transects spanning a rainfall gradient across the Mu Us Desert. The transects were partitioned into a bedrock-dominated erosional zone and an accumulation zone. In the former, the climate was dry and windy and the values of vfd% were low and there was no relationship with rainfall. We determined that the lower limit for
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pedogenic production of magnetic minerals was around 340 mm/ yr. In the accumulation zone, the vfd% values were higher and we used the results to develop a transfer function between vfd% and regional MAP, which was then applied to three aeolian sections from the southern Mu Us desert to produce a regional record of paleo-rainfall during the Holocene. The results demonstrate that intervals of paleosol development correspond to intervals of higher rainfall. Our results provide an improved understanding of the relationship between environmental factors and the production of secondary ferrimagnetic minerals, especially in arid and semiarid environments. In addition, they constitute a feasible approach for quantitative paleoclimatic reconstruction in the study region, together with an improved understanding of the main sand provenance in the Mu Us Desert. Acknowledgments We thank Professor Gao Shangyu and Cheng Hong for detailed and constructive suggestions and Jan Bloemendal for English improvement. Special thanks to the anonymous reviewers and editors who have significantly improved this paper. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41330748, 41571184) and the Graduate Student Foundation of the MOE Key Laboratory of Environmental Change and Natural Disaster of Beijing Normal University of China (Grant No. 2015jzhz08). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aeolia.2017.04.003. References An, Z.S., Liu, T.S., Lu, Y.C., Porter, S.C., Kukla, G., Wu, X.H., Hua, Y.M., 1990. The longterm paleomonsoon variation recorded by the loess-paleosol sequence in central China. Quat. Int. 7–8, 91–95. http://dx.doi.org/10.1016/1040-6182(90) 90042-3. An, Z.S., Kukla, G.J., Porter, S.C., Xiao, J.L., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quat. Res. 36 (1), 29–36. http://dx.doi.org/10.1016/0033-5894(91)90015-W. Balsam, W.L., Ellwood, B.B., Ji, J.F., Williams, E.R., Long, X.Y., Hassani, A..El., 2011. Magnetic susceptibility as a proxy for rainfall: worldwide data from tropical and temperate climate. Quat. Sci. Rev. 30 (19–20), 2732–2744. http://dx.doi. org/10.1016/j.quascirev.2011.06.002. Blundell, A., Dearing, J.A., Boyle, J.F., Hannam, J.A., 2009. Controlling factors for the spatial variability of soil magnetic susceptibility across England and Wales. Earth Sci. Rev. 95 (3–4), 158–188. http://dx.doi.org/10.1016/j. earscirev.2009.05.001. Chen, Y.Y., Lu, H.Y., Yi, S.W., Zhang, E.L., Xu, Z.W., Yu, K.F., Mason, J.A., 2015. A preliminary quantitative reconstruction of precipitation in southern Mu Us sandy land at margin of Asian monsoon-dominated region during late Quaternary. J. Geogr. Sci. 25 (3), 301–310. http://dx.doi.org/10.1007/s11442015-1169-8. Department of Geography of Peking University, Commission for Integrated Survey of Natural Resources of Chinese Academy of Sciences, Lanzhou Institute of Desert Research of Chinese Academy of Sciences, & Lanzhou Institute of Glacier and Frozen Soil of Chinese Academy of Sciences, 1983. Natural conditions and its improvement and utilization in the Mu Us Sandland, Beijing, China. Ding, Z.L., Derbyshire, E., Yang, S.L., Sun, J.M., Liu, T.S., 2005. Stepwise expansion of desert environment across northern China in the past 3.5 Ma and implications for monsoon evolution. Earth Planet. Sci. Lett. 237 (1), 45–55. http://dx.doi.org/ 10.1016/j.epsl.2005.06.036. Du, S.H., Li, B.S., Chen, M.H., Zhang, D.D., Xiang, R., Niu, D.F., Wen, X.H., Qu, X.J., 2012. Kiloyear-scale climate events and evolution during the Last Interglacial, Mu Us Desert, China. Quat. Int. 263 (10), 63–70. http://dx.doi.org/10.1016/j. quaint.2012.01.004. Evans, M.E., Heller, F., 2003. Environmental Magnetism: Principles and Applications of Enviromagnetics. Academic Press, San Diego. Fassbinder, J.W.E., Stanjek, H., Vali, H., 1990. Occurrence of magnetic bacteria in soil. Nature 343, 161–163. http://dx.doi.org/10.1038/343161a0. Gedye, S.J., Jones, R.T., Tinner, W., Ammann, B., Oldfield, F., 2000. The use of mineral magnetism in the reconstruction of fire history: a case study from Lago di Origlio, Swiss Alps. Palaeogeogr. Palaeoclimatol. Palaeoecol 164, 101–110. http://dx.doi.org/10.1016/S0031-0182(00)00178-4.
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Magnetic susceptibility of surface soils in the Mu Us Desert and its environmental significance Xiaokang Liu a,b,c, Ruijie Lu a,b,c,⇑,1, Zhiqiang Lü a,b,c, Jing Du a,b,c, Feifei Jia d, Tengfei Li a,b,c, Lu Chen a,b,c, Yongqiu Wu a,b,c a
State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China School of Geography, Beijing Normal University, Beijing 100875, China Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China d School of Urban and Environmental Sciences, Liaoning Normal University, Dalian 116029, China b c
a r t i c l e
i n f o
Article history: Received 30 October 2016 Accepted 5 April 2017
Keywords: Modern surface soils Frequency-dependent magnetic susceptibility Regional MAP Transfer function Paleo-rainfall reconstruction
a b s t r a c t Magnetic susceptibility has been widely used as a climatic proxy in paleoclimatic research. In arid and semi-arid regions, the magnetic properties of modern surface soil are significantly influenced by precipitation. This is demonstrated by observed positive correlations between percentage frequency-dependent magnetic susceptibility (vfd%), which reflects the presence of fine-grained (superparamagnetic, SP) grains produced during weathering and pedogenesis, and regional mean annual precipitation (MAP). To further investigate this relationship, we measured the magnetic properties of 104 surface soil samples collected along two transects (AA and BB) spanning a rainfall gradient across the Mu Us Desert in northern China. There were no systematic trends in magnetic properties along transect BB; the vfd% values remained relatively low and stable, probably reflecting weak pedogenesis and the domination of the magnetic properties by lithology. In contrast, along transect AA there was a significant positive correlation (p < 0.01) between vfd% and regional MAP. From this relationship, we developed a transfer function (P = 274.1 + 1424.4 vfd%) and used it to produce quantitative estimates of paleo-precipitation within three Holocene aeolian sections located in the southern the modern Mu Us Desert. The results show that the variations of reconstructed precipitation are consistent with those of lithological properties, and they also confirm previous conclusions that paleosol development in the study area is dominated by precipitation. Overall the results further demonstrate the feasibility of using frequency-dependent magnetic susceptibility to quantitatively reconstruct regional paleo-precipitation, including within a geographically diverse desert area. In addition, they provide an improved understanding of the main sand provenance in Mu Us Desert. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Magnetic susceptibility (MS) measurements, which reflect how easily a material is magnetized in a weak magnetic field, have many applications in environmental research (e.g., Evans and Heller, 2003; Liu et al., 2012). In addition, the availability of relatively inexpensive and sensitive MS meters, which enable the rapid acquisition of large volumes of data, have resulted in the widespread use of MS measurements as a paleoclimatic indicator e.g.,
⇑ Corresponding author at: State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, No. 19, Xinjiekouwai Street, Haidian District, Beijing, China. E-mail address: [email protected] (R. Lu). 1 This author contributed equally with the first author to this work. http://dx.doi.org/10.1016/j.aeolia.2017.04.003 1875-9637/Ó 2017 Elsevier B.V. All rights reserved.
(Herb et al., 2013; Kukla et al., 1988; Liu and Deng, 2009; Verosub et al., 1993). Modern soils are formed under different climatic regimes and in some environments the magnetic properties of soils are considered to directly reflect climate, especially precipitation (Liu et al., 1995; Maher et al., 1994b). For regions of uniform parent material, numerous studies have used MS measurements of surface soils for the development of climofunctions (Balsam et al., 2011; Guo et al., 2011; Porter et al., 2001; Song et al., 2014; Xia et al., 2012) which can be used to reconstruct past climatic conditions, especially paleo-precipitation (Lü et al., 1994; Maher et al., 1994b). However, other studies have shown that arid or semi-arid regions, where soil parent material and topography are complex and varied, are unsuitable for using magnetic susceptibility to estimate variations in rainfall amount (Wei et al., 2008; Xia et al., 2007).
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Frequency-dependent magnetic susceptibility (vfd%) reflects the content of superparamagnetic (SP) grains produced during weathering and pedogenesis (Liu et al., 1990; Zhou et al., 1990). In addition, vfd% is very sensitive to low amplitude climatic fluctuations (Heller et al., 1991; Liu et al., 1990) and in arid zones is well correlated with regional MAP (Xia et al., 2007). The Mu Us Desert in northern China is located in a climatically sensitive zone and is therefore well suited for studies of climatic change (Ding et al., 2005; Sun et al., 1999). Significant progress has been made in investigating late Quaternary aeolian activity (He et al., 2010; Jia et al., 2015; Lu et al., 2005; Miao et al., 2016; Xu et al., 2015) and climate change (Du et al., 2012; Liu et al., 2014; Lu et al., 2011) in the Mu Us Desert; however, there have been few attempts to reconstruct the paleo-rainfall during the Holocene (Chen et al., 2015). Here, we present the results of an investigation of the relationship between measurements of the vfd% of surface soils and regional MAP along a precipitation gradient in the Mu Us Desert. We then use the resulting climofunction to reconstruct past precipitation variations within three Holocene aeolian sections from the same region. The findings provide insights into the relationship between environmental factors and the production of secondary ferrimagnetic minerals, enable a preliminary quantitative reconstruction of paleo-rainfall in the study region, and provide information on sediment provenance. 2. Material and methods 2.1. Study area The modern Mu Us Desert (Fig. 1), with an area of about 39,000 km2, is located on the Ordos Plateau with the Loess Plateau immediately to the south. It lies on the margin of the area impacted by the East Asian monsoon. The mean annual rainfall is 250–440 mm and the mean annual temperature is 6.0–8.5 °C. Warm and humid air brought by the East Asian summer monsoon delivers more than 60% of the annual precipitation, which falls mainly from June to August. The present vegetation consists mainly of Artemisia ordosica Krasch., Tamarix chinensis Lour., and Hippophae rhamnoides Linn. Regional landscape types are varied and include stabilized, semi-stabilized and active sand dunes, grassland, and interlinked sequences of lakes and swamps (‘bead s-on-a-string’) (Zhou et al., 1996). It is noteworthy that in the northwestern Mu Us Desert there is a bedrock-dominated zone consisting of weakly-cemented Cretaceous purple sandstone with occasional Jurassic celadon mudstone strata (Department of Geography of Peking University et al., 1983; Kapp et al., 2015; Yue et al., 2007). 2.2. Sampling and laboratory measurements In April 2015, we collected a total of 104 surface soil (including sand) samples along two transects (AA and BB) spanning a modern rainfall gradient in the Mu Us Desert (Fig. 1). The samples were collected from sites as far as possible from cultivated land and human habitation and with a natural vegetation cover to avoid possible magnetic contributions from anthropogenic sources. The depth interval from 2–5 cm was sampled from typical regional soil units including sand dunes and grassland, and observations were made of landscape type and soil and vegetation conditions around the sampling site (see Supplementary information). The main soil types sampled were aeolian sandy soil in the case of sand dunes and meadow soil in the case of grassland. In addition, three natural exposures of aeolian strata, sections ZBT (38°190 4700 N, 109°430 5600 E, altitude 1108 m.a.sl.), CC
(37°400 2700 N, 108°180 5300 E, altitude 1350 m.a.sl.), and LZ (37°360 0200 N, 108°210 2400 E, altitude 1363 m.a.sl.) were chosen for study. The ZBT section is a sand-paleosol sedimentary sequence and detailed lithological description and dating results can be found in Jia et al. (2015). The CC and LZ sections are paleo-dunes, with two sandy paleosol layers and one sandy paleosol layer, respectively. Lithological descriptions of the LZ and CC sections are given in Table 1. Five samples of organic sediments were collected from the CC and LZ sections for AMS 14C dating, and the ages were converted to calendar ages using the INTACAL 13 calibration (Reimer et al., 2013). In the laboratory the samples were air-dried, disaggregated and packed in 8 cm3 plastic boxes. Measurements of low (0.47 kHz) and high frequency (4.7 kHz) magnetic susceptibility (vlf and vhf ) were made using a Bartington Instruments MS2 meter and MS2B sensor in the MOE Key Laboratory of Environmental Change and Natural Disasters, Beijing Normal University. Percentage frequency-dependent magnetic susceptibility was calculated as vfd% =ðvlf vhf Þ=vlf 100% to estimate the relative content of superparamagnetic (SP) particles. Rainfall along the transect was estimated using data from 1981–2010 from 16 meteorological stations (http://data.cma.cn/). The rainfall at each sampling site was estimated using a kriginginterpolation model (based on the Nearby Principle method) implemented in GIS 10.2 and Surfer 8.
3. Results 3.1. Variation of magnetic susceptibility and its frequency dependence and construction of the climofunction The vlf values range from 8.17–174.15 108 m3/kg, with an average of 45.69 108 m3/kg. The vfd% values range from 0– 4.62%, with an average of 1.41%, and exhibit a roughly increasing trend from northwest to southeast. In terms of the various landscape types illustrated in Fig. 2, vlf is relative high in semistabilized or semi-active dunes and lower in the stabilized sand dunes. This may be the result of contrasts in dune source material source in this geographically diverse desert area. vfd% values are higher in stabilized sand dunes and grassland and low in active sand dunes; this clearly reflects the production of fine-grained secondary ferrimagnetic minerals weathering and pedogenesis. The vegetation of the sand dunes is dominated by Artemisia ordosica Krasch., Caragana korshinskii Kom. and that of the grassland is dominated by Gramineae (see Supplementary information). The variation of vfd% and precipitation along transect BB is illustrated in Fig. 3. In the bedrock-dominated zone in the northern part of the transect (blue triangles) it is clear that there is no systematic trend with the vfd% values remaining relatively low (less than 2%) and stable. The bedrock in this zone consists of Cretaceous purple sandstone (Fig. 1A) and Jurassic celadon mudstone (Fig. 1B) and it is possible that the distinctive lithology is responsible for the low and invariant vfd% values. In the southern part of the transect, in the sand dune zone (blue dotted arrows) where the rainfall exceeds 340 mm/yr, there is a positive correlation between vfd% and regional MAP. Due to the relatively constant vfd% values along much of the transect (the reasons for which are discussed in Section 4.2, below) we chose another transect AA (Fig. 1) which also follows the steepest modern precipitation gradient; however, there are no significant variations in lithology and the temperature is almost constant. Prior to the analyses, we removed 5 samples (Fig. 1 – orange dots and see Supplementary information) whose vfd% values were more than two times larger than the neighboring points (Song et al., 2014). The occurrence of these values may be
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Fig. 1. Left: location of the study area in China, and major atmospheric circulation systems. Upper right: map of regional precipitation based on data from local meteorological stations from 1981–2010 (http://data.cma.cn/) and the distribution of bedrock derived from the Digital Library of NGAC (http://en.ngac.org.cn/Map/List). Also shown are the locations of surface soil sampling transects AA and BB. Precipitation along the transects ranges from 260 mm/yr in the near-desert conditions in the northwest to over 380 mm/yr in the southeast sandland environment, a distance of about 150 km. Lower right: examples of landscapes, landforms and lithologies in the Mu Us Desert, including A – purple sandstone; B – celadon mudstone; C – barchan; D – fixed dune; E – sand dunes and marshy grassland; F – grassland.
Table 1 Lithological descriptions of the LZ and CC Sections. Section
Depth (cm)
Lithology
Munsell color
LZ
0–40
Sod horizon with abundant modern plant roots Loose fine sand layer
Very pale brown (10YR7/4)
40–370
370–476
476–620 CC
0–30
30–85 85–112 112–268 268–370
Sandy paleosol layer with white mycelia, compacted Loose coarse sand layer Sod horizon with abundant modern plant roots Weakly-developed sandy paleosol layer Loose fine sand layer Sandy paleosol layer, compacted Loose fine sand layer
Very pale brown (10YR7/4), light yellowish brown (10YR6/4) Dark yellowish brown (10YR4/4) Very pale brown (10YR7/4) Light yellowish brown (10YR6/4) Yellowish brown (10YR5/4) Light yellowish brown (10YR6/4) Brown (10YR4/3) Light yellowish brown (10YR6/4)
explained by heteroscedasticity in which a population of relatively higher vfd% values is mixed with a population of lower values ones. The different populations may reflect different sand sources in the area. A scatter plot of vfd% and regional MAP for the 23 remaining
Fig. 2. Average values of vlf and vfd% for different landscape types. G: grassland; SS-SA: semi-stabilized or semi-active dunes; S: stabilized sand dunes; A: active sand dunes.
samples is illustrated in Fig. 4. Application of linear regression analysis produced the following line of best fit which was statistically significant which can be approved by the t-testing and F-testing at 0.01 level by SPSS:
P ¼ 274:1 þ 1424:4 vfd %; R2 ¼ 0:77 ðp < 0:01Þ where P is mean annual precipitation.
ð1Þ
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Fig. 3. Topographic profile along transect BB (along the red arrowed profile in Fig. 1) and the relationship between precipitation and ᵡfd%.
developed soil, while CC-P1 exhibits the weakest pedogenic development. The reconstructed precipitation records based on the climofunction (Eq. (1)) were smoothed by moving averaging. The results (Fig. 6) are in accordance with lithological evidence for intensified pedogenesis, hence precipitation. From the early Holocene to the late Holocene, the reconstructed precipitation curves for the three sections exhibit a similar trend of variation with an initial increase and subsequent decrease. Maximum reconstructed precipitation occurred in the mid-Holocene. The paleo-precipitation varied within a relatively broad range (more than 100 mm) during the Holocene. It is worth noting that the range of application of our transfer function may only be limited in the Mu Us Desert and adjacent regions in consideration of the rainfall inflection point between the magnetic susceptibility and regional MAP (Long et al., 2011). Fig. 4. Scatter plot of fd% versus regional MAP for transect AA with line of best fit shown in green. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Discussion 4.1. Environmental controls on the soil magnetic properties
3.2. Aeolian sections in the southern Mu Us Desert: Sedimentary characteristics of the paleosol layers and application of the transfer function We applied the transfer function to the magnetic data from Holocene aeolian sections in the same region to reconstruct paleo-rainfall. The studied sections, ZBT, CC and LZ, are located on the southeastern or southern boundary of the modern Mu Us Desert (Fig. 1). The dating results for the CC and LZ sections are listed in Table 2. The ages are in chronological order with no inversions. The sections are of Holocene age. Photos of the sections are shown in Fig. 5. All three sections contain a paleosol layer(s) reflecting the early/mid-Holocene Asian summer monsoon precipitation maximum. In section CC, the upper paleosol layer (CC-P1) is loose yellowish brown (10YR5/4) weaklydeveloped with small pseudomycelia, root tissues and bio-pores. The lower paleosol layer (CC-P2) is compacted and brown in color (10YR4/3) with abundant pseudomycelia. In section LZ, there is a sandy paleosol layer (LZ-P) which is dark yellowish brown (10YR4/4), compacted and with occasional pseudomycelia. In the lithological description of section ZBT (Jia et al., 2015), there are three paleosol layers: the upper is dark yellowish brown (ZBTP1), compacted, with abundant bio-pores and white mycelia; the middle paleosol layer (ZBT-P2) is thin, dark yellowish brown with abundant root tissues and white mycelia; and the lower paleosol layer (ZBT-P3) is light yellowish brown, sandy and weaklydeveloped. Overall, we consider that paleosol ZBT-P1 is the best
The magnetic properties of soils are affected by a wide range of factors, including parent material, duration of soil formation, vegetation, topography, and fire history (Blundell et al., 2009; Gedye et al., 2000; Maher, 1998). However, the nature of the parent material is the primary factor (Wei et al., 2008), with pedogenesis as a secondary factor (An et al., 1991) which is dominantly affected by climate (Maher et al., 1994a). Precipitation is one of the five soil forming factors and has a major effect on the magnetic characteristics of soils (Tite and Linington, 1975). With abundant precipitation, flourishing vegetation and well-drained soils, strong weathering and pedogenesis result in the production of significant amounts of SP grains. Pedogenic iron oxides mainly consist of hematite, goethite and maghemite and the content of ferromagnetic maghemite dominates the magnetic susceptibility of soils (Long et al., 2016). We divide the area along transect BB in the Mu Us Desert into two zones, an erosion zone and an accumulation zone. In the bedrock-dominated zone where the bedrock is exposed, in the northern part of the transect, erosion is dominant under the influence of strong winds, relatively low precipitation and sparse vegetation coverage. Consequently, we designate it the ‘erosion zone’. In the downwind area along transect BB where the sand is transported from upwind, there is greater precipitation which results in vegetated sand dunes and sediment accumulation is dominant. Hence, we designate it the ‘accumulation zone’. A conceptual model of the factors influencing soil magnetic properties in the study area is illustrated in Fig. 7. In the erosion
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X. Liu et al. / Aeolian Research 25 (2017) 127–134 Table 2 AMS14C dates for the CC and LZ sections. Beta Lab No.
Sample No.
Depth (cm)
Material
387098 387099 387102 387100 387101
LAA4C01 LAA4C05 CC14C13 CC14C02 CC14C05
386–388 468–470 60–65 126–128 260–262
Organic Organic Organic Organic Organic
sediment sediment sediment sediment sediment
d13C
Radiocarbon age (yr BP)
Cal age (2r, cal yr BP)
21.7 22.6 22.2 22.1 22.1
3190 ± 30 5210 ± 30 1280 ± 30 4560 ± 30 6350 ± 30
3410 ± 45 5935 ± 20 1230 ± 55 5195 ± 125 7285 ± 35
Fig. 5. Photograph of the CC, LZ and ZBT sections.
Fig. 6. Quantitative precipitation reconstructions for the LZ (a), CC (b), ZBT (c) sections. The reconstruction for the ZBT section is from Chen et al. (2015) (d). The reconstructions are based on the climofunction shown in Eq. (1).
zone, climatic factors, including precipitation, temperature and strong winds, lead to rock weathering and weak pedogenesis, and in addition the soil magnetic properties may be diluted by the influx of coarse particles. In the accumulation zone, fine dusts are blown from upwind areas and precipitation is the major factor influencing pedogenesis. In addition, to the effect of intrinsic site and soil factors, the accumulation of magnetic minerals derived from subaerial dust falls and magnetic bacteria may also contribute to the soil magnetic properties.
4.2. The characteristics of the different zones and regional comparisons In the northwest part of the study area erosion is dominant because of the high ground surface, aridity (MAP of 260– 340 mm) and strong winds (Kapp et al., 2015) and the underlying bedrock is widely exposed. The relatively weakly-developed soils with a low content of SP grains in this zone are analogous to the topsoils which have a coarse grain-size, weak pedogenic development and local sediment provenance at the northern foot of the
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Fig. 7. Conceptual model of the factors influencing soil magnetic properties in the study area. The dominant factors influencing soil magnetic properties are ① – precipitation, ② – temperature, ③ – intrinsic site and soil factors (including altitude, pH, and clay organic matter content), ④ – accumulation of magnetic minerals derived from subaerial dust falls, and ⑤ – living magnetic bacteria. (I) Rock weathering, (II) pedogenesis, (III) dilution effect, (IV) vfd% depends on the content of superparamagnetic (SP) grains.
Tien Shan Mountains (Guo et al., 2011; Jia et al., 2013). Similarly, in arid or hyper-arid Egypt there is virtually no relationship between magnetic susceptibility and rainfall (Balsam et al., 2011). Another possible interpretation for these observations is that the pedogenic magnetic signal is diluted by the influx of coarse dust from the sediment source area (An et al., 1991). All of the foregoing factors may be responsible for the limited pedogenic development and weak soil magnetic properties in this part of the study area. However, our field observations indicate that pockets of aeolian accumulation close to the bedrock-dominated erosion zone due occur, but due to the unfavorable environmental conditions the process of soil formation is extremely slow and magnetic enhancement is very weak. In addition, it is possible that any fine magnetic grains of pedogenic origin produced in such locations are removed by the strong wind activity. In the ‘accumulation-dominated’ zone pedogenesis is the major control of soil magnetic properties. If the effect of variations in the characteristics of the protolith in the accumulation zone can be eliminated, a strong positive relationship between vfd% and regional MAP may be observed, as is the case of the surficial magnetic enhancement of soil profiles in the relatively warm and moist Northern Californian coastal region (Singer et al., 1996). Precipitation amount is the factor most likely to determine the degree of pedogenesis (Liu et al., 1995), and vfd% as a measure of the contribution of SP grains to the magnetic susceptibility of samples (Liu et al., 1991, 1992; Zhou et al., 1990) is a proxy for the intensity of pedogenesis (Liu et al., 1990; Maher et al., 1994b; Maher and Thompson, 1995; Torrent et al., 2007). In summary, in the ‘accumu lation-dominated’ zone in the Mu Us Desert, the variation of vfd% is likely to be dominated by the intensity of the production of pedogenic magnetic minerals, which depends on the gradient of the regional MAP. Temperature has a secondary and much less significant effect on vfd% (Song et al., 2014). Along transect AA, temperature is relatively constant (Fig. 1) and therefore the effect of this factor on pedogenesis can be eliminated. Other soil-forming factors (including altitude, pH and clay and organic matter content) (Long et al., 2011), the accumulation of magnetic minerals derived from subaerial dust falls from other sources, such as volcanic ash, fine desert dust, and dust raised from forest fires during soil forming periods
(Kukla et al., 1988) and living magnetic bacteria (Fassbinder et al., 1990), may also have a secondary influence on vfd%.
4.3. Implications for material sources of Mu Us Desert Magnetic measurements are useful for detecting signals associated with various environmental processes (Liu et al., 2012). Our results highlight some of the complexities involved in using magnetic properties to reconstruct regional-scale paleo-rainfall in a complex desert environment. One factor is provenance-related differences in source material, and other factors include climate, and specifically different combinations of rainfall. With regard to the provenance of the soil parent materials of the Mu Us Desert, a previous hypothesis (Department of Geography of Peking University et al., 1983) proposed that they were sourced from the underlying Cretaceous aeolianite and Jurassic mudstone. However, recent research concluded that although the underlying Cretaceous and Quaternary formations are important sources (Sun, 2000), material supplied from distant sources is also significant (Stevens et al., 2013), especially eroded material from Northeastern Tibet (Nie et al., 2015). In fact, the materials of the modern Mu Us Desert may be derived from multiple sources, with the western part heavily influenced by sediment transported from Tibet via the Yellow River, and the eastern part controlled more by recycling from local sources and the underlying bedrock (Stevens et al., 2013). The absence of a systematic relationship between vfd% and regional MAP along transect BB has implications for understanding the original sand source of the Mu Us Desert. In the northern part along transect BB, where the rainfall amount does not exceed 340 mm/yr, the widespread exposed bedrock and ‘erosion-domina ted’ environment may play an important role in the soil formation process. We speculate that the bedrock-exposed eroding uplands in the northwestern Mu Us Desert are one of main material sources for sandy sediments based on the erosion-accumulation and ‘from source to sink’ relationship. Compared with the ‘accumulation-do minated’ zone where pedogenesis is relatively strong under much higher rainfall, in the northern ‘erosion-dominated’ zone pedogenesis is extremely limited which leads to the relatively low and constant vfd% values.
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4.4. Reliability of the reconstructed of paleo-rainfall record The changes in reconstructed precipitation from the three aeolian sections in the Mu Us Desert are consistent with changes in lithological properties and they confirm the view of previous studies that paleosol development in North China was dominated by variations in monsoonal precipitation (An et al., 1990). During the Holocene climatic optimum, the prevailing paleosol and sandy paleosol development in the Mu Us region indicates a warm-moist climate with flourishing vegetation growth and that the majority of the dunes were stabilized (Xu et al., 2015). Our reconstructed paleo-precipitation curves (Fig. 6) are generally consistent with the textural and other characteristics of the paleosol layers. The increased precipitation promoted pedogenesis of the dune sands, increased the frequency of paleosol and sandy paleosol development (Jin et al., 2001), and resulted in an increase in the pedogenic production of SP grains and the increased vfd% values. Comparison of our results with a stable carbon isotope record from organic matter and a paleo-rainfall record from the ZBT section of Chen et al. (2015) reveals considerable consistency (Fig. 6d). The two quantitative precipitation reconstructions for the southeastern margin of the Mu Us Desert both exhibit a trend of increasing precipitation in the early Holocene, high and uniform precipitation in the middle Holocene and decreasing precipitation in the late Holocene. Both the overall trends and relative amplitudes of the features in the two curves are consistent. In addition, two intervals of possible increased aeolian activity during the middle Holocene, indicated by the presence of sand layers and thus decreased precipitation, are quite consistent. However, unlike the ZBT section, in the early Holocene part of the LZ and CC sections (Fig. 5) there is an obvious aeolian sand layer. This difference may reflect differences in altitude, temperature and moisture conditions (Lu et al., 2011; Qiang et al., 2016, 2013). In the middle Holocene, there are some differences between the sections; however, they all exhibit either a paleosol or sandy paleosol and the reconstructed precipitation also exhibits peak values. This may indicate that during the Holocene climatic optimum most of the sand dunes were stabilized, and pedogenesis occurred under a relatively warm and humid climate. In the late Holocene, the LZ section exhibits evidence of sand dune reactivation after 3.4 ka BP; while in the CC section a sandy paleosol developed at around 1.2 ka BP, which corresponds to the Medieval Warm Period. It is likely that a variety of factors influenced the development of the study sections, including differences in landscape, sediment supply and regional environmental conditions (Telfer and Hesse, 2013) which make detailed correlations difficult. The regional comparisons demonstrate the utility of our climofunction for reconstructing paleo-precipitation in the study region (Lü et al., 1994). However, because of the numerous influences on soil development, in the region it is unlikely that a transfer function based on a single variable can be used to express the full variability of this complex soil system (cf. Balsam et al., 2011) and to estimate paleo-rainfall with complete reliability (Lü et al., 1994). Therefore, additional variables need to be considered and a greater number of well-dated paleo-rainfall reconstructions based on independent proxies need to be obtained in the future.
5. Conclusions We measured the magnetic susceptibility of surface soils on transects spanning a rainfall gradient across the Mu Us Desert. The transects were partitioned into a bedrock-dominated erosional zone and an accumulation zone. In the former, the climate was dry and windy and the values of vfd% were low and there was no relationship with rainfall. We determined that the lower limit for
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pedogenic production of magnetic minerals was around 340 mm/ yr. In the accumulation zone, the vfd% values were higher and we used the results to develop a transfer function between vfd% and regional MAP, which was then applied to three aeolian sections from the southern Mu Us desert to produce a regional record of paleo-rainfall during the Holocene. The results demonstrate that intervals of paleosol development correspond to intervals of higher rainfall. Our results provide an improved understanding of the relationship between environmental factors and the production of secondary ferrimagnetic minerals, especially in arid and semiarid environments. In addition, they constitute a feasible approach for quantitative paleoclimatic reconstruction in the study region, together with an improved understanding of the main sand provenance in the Mu Us Desert. Acknowledgments We thank Professor Gao Shangyu and Cheng Hong for detailed and constructive suggestions and Jan Bloemendal for English improvement. Special thanks to the anonymous reviewers and editors who have significantly improved this paper. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41330748, 41571184) and the Graduate Student Foundation of the MOE Key Laboratory of Environmental Change and Natural Disaster of Beijing Normal University of China (Grant No. 2015jzhz08). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aeolia.2017.04.003. References An, Z.S., Liu, T.S., Lu, Y.C., Porter, S.C., Kukla, G., Wu, X.H., Hua, Y.M., 1990. The longterm paleomonsoon variation recorded by the loess-paleosol sequence in central China. Quat. Int. 7–8, 91–95. http://dx.doi.org/10.1016/1040-6182(90) 90042-3. An, Z.S., Kukla, G.J., Porter, S.C., Xiao, J.L., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quat. Res. 36 (1), 29–36. http://dx.doi.org/10.1016/0033-5894(91)90015-W. Balsam, W.L., Ellwood, B.B., Ji, J.F., Williams, E.R., Long, X.Y., Hassani, A..El., 2011. Magnetic susceptibility as a proxy for rainfall: worldwide data from tropical and temperate climate. Quat. Sci. Rev. 30 (19–20), 2732–2744. http://dx.doi. org/10.1016/j.quascirev.2011.06.002. Blundell, A., Dearing, J.A., Boyle, J.F., Hannam, J.A., 2009. Controlling factors for the spatial variability of soil magnetic susceptibility across England and Wales. Earth Sci. Rev. 95 (3–4), 158–188. http://dx.doi.org/10.1016/j. earscirev.2009.05.001. Chen, Y.Y., Lu, H.Y., Yi, S.W., Zhang, E.L., Xu, Z.W., Yu, K.F., Mason, J.A., 2015. A preliminary quantitative reconstruction of precipitation in southern Mu Us sandy land at margin of Asian monsoon-dominated region during late Quaternary. J. Geogr. Sci. 25 (3), 301–310. http://dx.doi.org/10.1007/s11442015-1169-8. Department of Geography of Peking University, Commission for Integrated Survey of Natural Resources of Chinese Academy of Sciences, Lanzhou Institute of Desert Research of Chinese Academy of Sciences, & Lanzhou Institute of Glacier and Frozen Soil of Chinese Academy of Sciences, 1983. Natural conditions and its improvement and utilization in the Mu Us Sandland, Beijing, China. Ding, Z.L., Derbyshire, E., Yang, S.L., Sun, J.M., Liu, T.S., 2005. Stepwise expansion of desert environment across northern China in the past 3.5 Ma and implications for monsoon evolution. Earth Planet. Sci. Lett. 237 (1), 45–55. http://dx.doi.org/ 10.1016/j.epsl.2005.06.036. Du, S.H., Li, B.S., Chen, M.H., Zhang, D.D., Xiang, R., Niu, D.F., Wen, X.H., Qu, X.J., 2012. Kiloyear-scale climate events and evolution during the Last Interglacial, Mu Us Desert, China. Quat. Int. 263 (10), 63–70. http://dx.doi.org/10.1016/j. quaint.2012.01.004. Evans, M.E., Heller, F., 2003. Environmental Magnetism: Principles and Applications of Enviromagnetics. Academic Press, San Diego. Fassbinder, J.W.E., Stanjek, H., Vali, H., 1990. Occurrence of magnetic bacteria in soil. Nature 343, 161–163. http://dx.doi.org/10.1038/343161a0. Gedye, S.J., Jones, R.T., Tinner, W., Ammann, B., Oldfield, F., 2000. The use of mineral magnetism in the reconstruction of fire history: a case study from Lago di Origlio, Swiss Alps. Palaeogeogr. Palaeoclimatol. Palaeoecol 164, 101–110. http://dx.doi.org/10.1016/S0031-0182(00)00178-4.
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