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Paleoclimatic evolution of Holocene loess and discussion of the sensitivity of magnetic susceptibility and median diameter
Quaternary International 296 (2013) 160e167
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Paleoclimatic evolution of Holocene loess and discussion of the sensitivity of magnetic susceptibility and median diameter Guoyong Zhao a, Xiuming Liu a, b, c, *, Qu Chen a, Bin Lü a, Hewen Niu a, Zhi Liu a, Pingyuan Li a a
Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Research School of Arid Environment & Climate Change, Lanzhou University, Lanzhou 730000, China b School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China c Department of Environment and Geography, Macquarie University, Sydney NSW 2109, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 23 June 2012
Climate changes are closely related to peoples’ livelihoods. Holocene climate changes are therefore particularly important to humans. It is known that eolian loess deposits in China are one of the best geological records for paleoclimatic change. As it is the top layer of loess sequence, Holocene loess may be easily disturbed by human activities. For that reason, it has not been studied thoroughly. Recently, a continuously deposited, high-resolution and well-preserved Holocene loess section has been found in Baicaoyuan, Huining County, Gansu Province, located in the northwest edge of the Chinese Loess Plateau. The magnetic and grain size signals of the section reflect Holocene paleoclimate variations and are comparable to the stalagmite oxygen isotope records of Sanbao and Hulu caves. They consistently record the general evolution process of Holocene paleoclimate in Eastern Asia. During the period 14.2e12.8 ka BP, Last Glacial Maximum turned into Holocene transition phase. During 12.8e11.5 ka BP, the climate was cold and dry, dominated by the Younger Dryas. Between 11.5 and 10.5 ka BP, the climate rapidly turned warm and wet, and 10.5e5.2 ka BP was the Holocene Optimum Period, during which some relatively dry-cold events could be observed. Since 5.2 ka BP, the climate slowly became dry and cold. In summary, the climate showed a general drying trend from the early Holocene to the late Holocene, which was controlled by variation of solar radiation. Compared with magnetic susceptibility, the median diameter curve fluctuates frequently. Some climate events were recorded by the median diameter curve, but not observed in the magnetic susceptibility curve. It is suggested that the grain size parameter is more sensitive to paleoclimate changes than susceptibility at the Baicaoyuan section during the Holocene. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Climate change is a hot topic nowadays, which has turned from the pure scientific field to include international political, economic, and diplomatic issues (Fang et al., 2011). The IPCC in 2007 reported that the temperature had a rise of 0.74 C (0.56 C to 0.92 C) during 1906e2005 A.D. (IPCC, 2007). The 0.74 C was derived from instrumental determination, which raises a question how the climate changed before the period without instrument records. In addition, the causes for the increase in temperature, undoubtedly, included interaction between human activities and nature. To evaluate the relative contributions of the two factors to the modern climate change requires separating them. Despite the influence of * Corresponding author. Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University, 222 Tianshui South Road, Chengguan District, Lanzhou 730000, China. E-mail address: [email protected] (X. Liu). 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2012.06.015
human activities on climate change, researchers need to seek answers from geological records. The Holocene is the most recent geological epoch, attracting much attention of geologists. Many researchers focused on the Holocene climate change (Liu et al., 2000; Haug et al., 2001; Wang et al., 2005; Yu et al., 2006; Huang et al., 2007), with the purpose of better understanding paleoclimate changes, and simultaneously predicting future climate trends. Loess is regarded as one of the best geologic records for paleoclimatic research, along with ice cores and marine oxygen isotopes (Heller and Liu, 1984; Liu, 1985; Kukla et al., 1988; Deng et al., 2007). In recent years, the researchers mainly endeavored to establish a perfect Quaternary geologic calendar, while relatively little attention has been paid to the paleoclimatic significance of Holocene loess. Holocene loess is the uppermost layer of the loess deposition and can be easily disturbed by human activities, which thus causes a hiatus or inaccuracy of the paleoclimatic records. Furthermore, compared with the stalagmite and lacustrine records, loess has relatively low resolution as
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a paleoclimatic archive. However, the Chinese Loess Plateau has a large area and the pedogenesis strength and sediment rates of loess deposits systematically change with various climate conditions from the northwest to the southeast. Recently, a loess profile with continuous deposition and high-resolution paleoclimatic records was found at Baicaoyuan, Huining County, Gansu Province, China. The data is comparable to the stalagmite records and is ideal for the study on Holocene climate variability. 2. Samples and methods The area of the Chinese Loess Plateau (CLP) is vast. From southeast to northwest, the thickness of loess deposits increases, the pedogenic degree decreases, and the climate signal intensifies (Liu et al., 1990). In the southeast CLP, soil is developed, the accumulation rate is low, so the resolution is low. In the northwest CLP, soil is weakly developed, the accumulation rate and resolution is high, but the recorded signal of summer monsoon is weak. Baicaoyuan section (3614.3900 N, 105 8.0320 E, 2089 11 m a.s.l.) is located in Huining County, Gansu Province, in the northwest edge of CLP, and to the west of the Liupan Mountains. Baicaoyuan has a mean annual temperature of 8.5 C and a mean annual precipitation of 320 mm (Deng, 2008). The section is close to the source area with high deposition rate, and in the transitional zone of monsoon and non-monsoon areas, and thus sensitive to paleoclimate changes, which makes it ideal for paleoclimatic study (Fig. 1). Baicaoyuan Holocene loess is described as follows: the total thickness of the section sampled is 2.1 m, and 105 samples were sampled at 2 cm intervals. The thickness of L0 is 1 m, loose, structureless, pale yellow, many grass roots and wormholes. The thickness of S0 is 0.84 m, black- brown, hard texture, white calcium nodules, and many grass roots and wormholes. The thickness of L1 is 0.26 m, with many pores that are looser than L0 and color lighter than that of L0. The samples were dried by natural air indoors, then were ground and put into small nonmagnetic plastic boxes with mass of 7.5 g,
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sealed by adhesive tape. Magnetic susceptibility (MS) was measured at low frequency (470 Hz, clf) and high frequency (4700 Hz, chf) using a Bartington MS2 meter with dual frequency sensor. Frequency-dependent magnetic susceptibility (cfd) is calculated by cfd ¼ clf chf. Anhysteretic remanent magnetization (ARM) was measured using a Minispin Magnetometer after being magnetized with a DTECH AF demagnetizer. The peak AF field used was 100 mT and the DC bias was 0.05 mT. ARM was then normalized by the bias field to obtain ARM susceptibility (cARM). Isothermal remanent magnetization (IRM) was acquired in progressively increasing magnetic fields up to 1T with a MMPM10 pulse magnetizer and the induced remanence after each field was measured using a Minispin Magnetometer. The IRM acquired in the maximum field of 1T was defined as the saturation isothermal remanent magnetization (SIRM), then remanent coercivity (Bcr), the ratio of saturation isothermal remanent magnetization and ARM susceptibility (SIRM/ cARM) and S-ratio (S-ratio ¼ IRM300 mT/SIRM) was calculated. The grain size was analyzed with a Malven Mastersizer 2000 laser grain size analyzer, which has a measurement range of 0.02e2000 mm. All parameters were measured in the Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University. 3. Results and analysis The results of magnetic and particle size parameters are shown in Figs. 2 and 3, respectively. Baicaoyuan section can be divided into 3 units as L0, S0 and L1 according to traditional loess stratigraphic division from top to bottom, which is consistent with the observation in the field. 3.1. Magnetic parameters Magnetic susceptibility (MS) is a physical quantity determined by the magnetization in a weak field (Liu and Deng, 2009). MS is usually positively related to the intensity of pedogenesis, which is used to indicate variation of summer wind strength (Heller and Liu,
Fig. 1. Loess Plateau with location of study section. Shading indicates the areal extent of the Chinese loess deposits and the dashed contour lines indicate present-day rainfall (in mm/year), adapted from Zhao and Roberts (2010).
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Fig. 2. Parameters of magnetism versus depth of Huining Holocene loess. B: Location and age of thermoluminescence sample (Wang et al., 2000).
1984; An et al., 1990, 1991; Liu et al., 1992). High MS is indicative of strong summer winds with high temperature and precipitation. Soil development and neoformation of fine ferrimagnetic minerals were responsible for High MS in paleosols (Zhou et al., 1990). In contrast, during the periods when the summer wind weakened and the winter wind intensified, soils were weakly developed with coarser grain size and low MS. High MS in S0 suggests mild climate, low MS in L0 reflects that the climate was dry and cold (Fig. 2a). Frequency-dependent magnetic susceptibility (cfd) is sensitive to superparamagnetic (SP) and single-domain (SD) magnetite (Thompson and Oldfield, 1986). These magnetic particles are created by soil development (Zhou et al., 1990; Liu et al., 1992, 2005). The value of cfd can reflect pedogenetic intensity, and higher values are indicative of stronger pedogenesis. cfd values of S0 are all above 8 * 108 m3/kg (Fig. 2b), suggesting that the climate was suitable and soils were strongly developed. cfd values are relatively low in L0 (Fig. 2b), which implies that the climate was cold and dry and soils were weakly developed. cARM/c can be used to estimate magnetic particle size (Liu et al., 2004), which is sensitive to fine magnetic particles (SD and fine PSD). c is sensitive to SP particles and coarse magnetic particles
(coarse PSD and MD). High values of cARM/c suggest fine magnetic particle size, while low values suggest coarse magnetic particle size. High cARM/c values in S0 (Fig. 2c) reflect that the climate was suitable and favored soil development; low cARM/c values in L0 (Fig. 2c) suggest that the climate was not as good as that recorded in S0. cARM/c increases dramatically in L0/S0 transitional horizon (Fig. 2c), which is consistent with fieldwork observations. It is suggested that cARM/c reflects the degree of soil development. After they are given a 1T magnetization, the samples were then given stepwise backfield IRMs (isothermal remanent magnetization), the opposite magnetic field at which the remanent magnetization decreases to zero is called residual magnetism coercive force (Bcr) (Thompson and Oldfield, 1986). Bcr is used to judge relative content of hand or soft magnetic components. High Bcr values are indicative of a large component of hard magnetic minerals; conversely, low Bcr values are indicative of a large component of soft magnetic minerals. S0 shows low Bcr values (Fig. 2d), which suggest that the samples contain more soft magnetic minerals and the climate was suitable and favored soil development; L0 demonstrates high Bcr values (Fig. 2d) suggesting more hard magnetic minerals, and a cold-dry climate, this could be
Fig. 3. Grain size parameters versus depth of Huining Holocene loess. B: The location and age of thermoluminescence date (Wang et al., 2000).
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ascribed to magnetic minerals of eolian origin and relatively weak development of soils. SIRM/cARM is especially sensitive to magnetic particles that are slight larger than SP particles, because the ratio is low within this grain size range (Thompson and Oldfield, 1986). High ratio implies that the samples contain more coarse magnetic minerals. Value of SIRM/cARM is low in S0 (Fig. 2e), suggesting more fine magnetic minerals that result from favorable climate and soil development; value of SIRM/cARM is high in L0 (Fig. 2e), revealing that the less developed loess contains coarse magnetic minerals of eolian origin that formed during cold-dry climate. SIRM/cARM suddenly increases at L0/S0 transitional horizon, which is similar to cARM/c (Fig. 2c). S-ratio is equal to the absolute value of the backfield IRM at 300 mT divided by 1T IRM (Pan and Zhu, 1996). High S-ratio that is close to 1 reveals that the magnetic minerals with low coercive force dominant (Ao and Deng, 2007). Conversely, low S-ratio reveals that magnetic minerals with high coercive force are dominant. Though the S-ratio is generally high through the section, low values are observed at L0 and part of S0 (Fig. 2f). The value increases dramatically in S0 (Fig. 2f), indicative of a large component of magnetic minerals with low coercive force. The highest values of Sratio correspond with part of Holocene Optimum Period (see the following sections), which suggests that the climate during middle Holocene was mild and pedogenesis intensified. 3.2. Particle size parameters Generally, there is positive correlation between the winter monsoon and particle size (Ding et al., 1994), with stronger winter monsoon corresponding with coarser particle size. The Loess Plateau source is the desert area in the northwest of China, whose southeast front retreated and expanded with the paleomonsoon variation. Sun et al. (1998) pointed out the deserts retreated westward about 1000 km during the Holocene Optimum period relative to the position of the Last Glacial Maximum period. Particle size can be also correlated to the distance between source area and deposition area. Shorter distance from the source area corresponds to coarser particle sizes (Liu, 1985). The distance during the S0 stage was longer than that during the L0 stage between Baicaoyuan section and the desert, and the grain size of S0 was finer than that of L0 (Fig. 3). The median diameter (Md) is the median value of different particle size fractions. MS and particle size are the most sensitive proxies in loess (Heller and Liu, 1984; An et al., 1990, 1991; Liu et al., 1992; Ding et al., 1994). Md is a widely accepted particle size parameter. MS (Fig. 2a) and Md (Fig. 3a) show a negative relationship. MS demonstrates high values in S0, while Md demonstrates low values, which strongly indicates that the summer monsoon dominated during this period, and favored pedogenesis; conversely, MS shows low values, and Md shows high values in L0, which implies that the winter monsoon strengthened and was unfavorable to pedogenesis during this period. Fig. 3beg shows the percentages of different grain size fractions versus depth. Fig. 3bed shows that S0 has a larger component of fine particles than L0 and Md demonstrates opposite distribution. Fig. 3eeg shows that L0 has a larger component of coarse particles than S0 and Md shows the same distribution as the coarse fractions. This suggests that Md mainly reflects the coarse components. The variation trends of magnetic and particle size parameters during the Last Glacial period are similar to those recorded in L0. In S0, the peaks of high values of c, cfd, cARM/c, and S-ratio correspond with a large component of fine grain size fractions (<16 mm (%)), and low values of Bcr, SIRM/cARM, Md and the coarse grain size fractions (>16 mm (%)). In L0, the correlations are
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opposite. All these parameters indicate that during the period when S0 deposited, grain size was relative fine, suggesting the climate was characterized by high temperature and precipitation and led to soil development and neoformation of ultrafine ferrimagnetics. This accounts for the high values of c, cfd, cARM/c, S-ratio and low values of Bcr and SIRM/cARM. During the period when L0 deposited, loess particle size coarsened, the climate was dry and cold, and the soils were weakly developed. This accounts for the decrease in c, cfd, cARM/c, S-ratio values, and the increase in Bcr and SIRM/cARM values. 4. Discussion 4.1. Age of Baicaoyuan Holocene loess Baicaoyuan, Lijiayuan and Jingyuan sections are located in the western edge of the Loess Plateau, Lijiayuan is about 20 km southeast to Baicaoyuan direction and Jingyuan about 50 km northwest to Baicaoyuan direction (Fig. 1). Ding et al. (1998) made detailed studies on the magnetic susceptibility and grain size of Lijiayuan section during the past 130,000 years and held that the Holocene loess deposit is about 2 m thick. Wang et al. (2012) studied the black wood charcoal record of Lijiayuan section since the Last Glacier Maximum and suggested that the thickness of the Holocene loess deposit is about 2.2 m. The thickness of the loess deposit in Baicaoyuan is 2.1 m, which is approximate to that of the Holocene loess in Lijiayuan section. The grain size variations of Baicaoyuan and Jingyuan sections show similar trends (Fig. 4). Sun et al. (2010, 2012) reported a detailed optically stimulated luminescence (OSL) dating of Jingyuan section since the Last Glacial and offered a reliable chronology for paleoclimatic research. Therefore, a detailed comparison between the grain size variation of Baicaoyuan section and that of Jingyuan section reported by Sun et al. (2010, 2012) was made, and 11 points were correlated as age controls (Fig. 4). The age of each sample of Baicaoyuan section was obtained by using the magnetic susceptibility age model (Kukla et al., 1988) and interpolation method. It is suggested that the basal age of Baicaoyuan section is about 14.2 ka BP and the age of Holocene loess is about 11.5 ka BP, which is in line with the base of the Holocene (Gradstein et al., 2005) and the thermoluminescence age of the Lijiayuan Holocene loess (11e10 ka BP) attained by Wang et al. (2000) (Fig. 3). 4.2. Are Baicaoyuan section deposits continuous? The Baicaoyuan section deposits are considered to be continuous. First, Baicaoyuan is located in the western CLP, with lower annual mean precipitation (320 mm) and higher sedimentation rate than in the southern CLP, it is unlikely eroded by water and wind deflation (Sun et al., 2010). Second, the paleoclimatic evolution recorded by Baicaoyuan section can be compared with stalagmite oxygen isotope of Sanbao and Hulu caves in the southern part of China (as discussed further in Section 4.3). Third, the curve of age-depth model is smooth, there is no dramatic variation (Fig. 4c), indicating that there is no hiatus in the section. The higher sedimentation rate of L0 than that of S0 indicates the dry and cold climate of L0 than that of S0. Fourth, many scientists think that loess in CLP deposited continuously, which provides continuous records of paleoenvironmental variation during the whole Quaternary (Liu, 1985; Kukla et al., 1988; Kukla and An, 1989; Ding et al., 1994; Lu et al., 1999; An, 2000; Lai and Wintle, 2006; Sun et al., 2010). Yuanbao and Jingyuan sections, situated at the northwest edge of CLP, were shown to be deposited continuously (Lai and Wintle, 2006; Sun et al., 2010, 2012). In summary, it is
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Fig. 4. Comparison of grain size variations between Baicaoyuan (a) and Jingyuan (b) sections and the age-depth model of Baicaoyuan section(c).
strongly suggested that Baicaoyuan section also accumulated continuously. 4.3. The evolution of Holocene paleoclimate Based on the multi-proxy studies, the Holocene paleoclimatic evolution is divided into five stages (Fig. 5). I 14.2e12.8 ka BP, the Last Glacial Maximum turned into the Holocene transition phase. MS increased, Md and d18O values decreased. This strongly suggests that the climate became warm and wet during this period.
Fig. 5. The relationship between magnetic susceptibility, median diameter, oxygen isotopes of stalagmites (Wang et al., 2001, 2008) and July insolation at 65 N (Berger and Loutre, 1991).
II 12.8e11.5 ka BP, Younger Dryas event. MS is low, grain size is coarse (compared to the Holocene Optimum Period (Fig. 5)), and the d18O values are high (Fig. 5c), indicative of weak summer monsoon and strong winter monsoon with low temperature and precipitation (Wang et al., 2001, 2008; Shao et al., 2006). However, there was a warm and wet episode during this stage. Zhou et al. (2001) suggested that climate was dry and cold at the beginning of the Younger Dryas, then warm and wet, and then drier and colder than at the beginning, based on the research of the loess-paleosol sequence of Dongxiang, Shenmu and wind-blown sand/lacustrine deposit sequence of Jingbian during the period of 11,70012,800 cal BP. Greenland Ice Sheets Project 2 (GISP2) (Lee and Slowey, 1999) provides another piece of evidence, the paleoclimatic record of which can be correlated to these results. Studies on paleoclimatic records in other areas, including the Midiwan peat profile located at the desert/loess transitional belt, China (Zhou et al., 1998), the Zhengbeitai loess profile located at Yulin, Shanxi Province, China (Zhou et al., 1998), and the lacustrine deposit of Zalairoer in Inner Mongolia of China (Wang et al., 1994), showed the same variation pattern. III 11.5e10.5 ka BP, rapidly change stage. MS rapidly increased (Fig. 5a) and the Md rapidly decreased (Fig. 5b). The value of stalagmite oxygen isotopes rapidly decreased (Fig. 5c). It is implied that the dry-cold climate rapidly changed into warmwet climate, with higher precipitation and temperature than before (Young Dryas period). IV 10.5e5.2 ka BP, the Holocene Optimum Period. MS is at maximum over the whole Holocene (Fig. 5a), and grain size is minimum (Fig. 5b), corresponding to the black loessial soil S0 over the CLP; at the same time, the stalagmite oxygen isotope is the lowest in south of China, suggesting that climate was much warmer and wetter during this period. Relatively high temperatures and precipitations favoured plant growth and biological activities, so this stage is called the Holocene Optimum Period. There is no agreement on the range of Holocene Optimum Period. Shi et al. (1994) held that Holocene Megathermal is 8.5e3 ka BP in China; Zhou et al. (1978) suggested that the Warm Time in Beijing is 7.5e2.5 ka BP;
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Kalis et al. (2003) argued that the Holocene Optimum Period in middle Europe is 9e5.5 ka BP. In fact, the discrepancy is mainly due to the different determination of warmth periods (Fig. 5), there is no difference essentially. However, the climate of Holocene Optimum Period was unstable, the median diameter curve fluctuates frequently during the 9e7 ka BP stage with two obvious peaks (7.8 ka, 8.2 ka drycold event) (Fig. 5). The error of Md is 0.46 w 0.46 mm (the maximum of Md is 23 mm, and the error of grain size analyzer is 2%), and separated the low-frequency and highfrequency components. The high-frequency amplitude is larger enough than the error of Md. Two obvious peaks (7.8 ka and 8.2 events) are observed in high-frequency amplitude curve (Fig. 6). There were 8.2 ka, 8.6 ka, 9.3 ka, 10.2 ka drought events in stalagmite records (Shao et al., 2006). The 8.2 ka BP event was also observed in other records (Alley et al., 1997; Bond et al., 1997; Grafenstein et al., 1998; Dean et al., 2002; Thomas et al., 2007). According to MS, Md of Baicaoyuan Holocene loess and the stalagmite oxygen isotope record of Sanbao and Hulu caves, it is revealed that the Holocene Optimum Period lasted about 5 ka (10.5e5.2 ka BP). V Since 5.2 ka BP, the climate slowly became dry and cold. Magnetic susceptibility decreased and median diameter increased (Fig. 5), stalagmite oxygen isotope value increased slowly (Fig. 5c). This period (5.2 ka BP to present) can be divided into two parts, with about 3 ka BP as the boundary line: the climate changed more dramatically since 3 ka BP than during 5.2e3 ka BP. Some other geological records show the similar paleoclimate variations, the climate changed to be dry-cold from 5.5 ka cal. BP in Holocene peat of Zoige area (Yu et al., 2006); temperature and humidity of Qinghai Lake achieved the maximum at 6.5 ka cal. BP and began to drop gradually after 4.5 ka cal. BP (Shen et al., 2005; Zhao et al., 2009). Baicaoyuan, Sanbao and Hulu are located in the East Asia monsoon region and the climate is controlled by the East Asia monsoon. The various proxies of susceptibility, grain size and oxygen isotope records are consistent and display similar evolution of Holocene climate in East Asia. In summary, the climate showed a general drying trend from the early Holocene to the late Holocene. Though the resolution of loess is lower than the stalagimite, they show very comparable evolution process of Holocene paleoclimate,
Fig. 6. The high-frequency amplitude of median diameter of Baicaoyuan section.
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implying that Baicaoyuan loess is a reliable Holocene climatic record. MS, Md and stalagmite oxygen isotope curves demonstrate good correlation with the July insolation at 65 N (Fig. 5). The insolation was high, and the climate was warm and wet during the period from 10.5 to 5.2 ka BP, which is defined as the Holocene Optimum Period; Since 5.2 ka BP, insolation decreased slowly and climate turned to be dry-cold, suggesting the evolution of Holocene paleoclimate was controled by insolation variation. During the Younger Dryas period, solar radiation shows the second highest value and the climate deteriorated dramatically in comparison with the Holocene Optimum Period, which is possible due to the effects of the Last Glacial Maximum called the climate return event (Xiong et al., 1999). 4.4. Which is more sensitive to paleoclimatic change in the northwest edge of Chinese Loess Plateau, median diameter or magnetic susceptibility? There is a negative relationship between MS and Md. They consistently record the evolution of Holocene paleoclimate. However, the curve of MS is smoother than that of Md. Some peaks and valleys are observed in the Md curve, which are absent in the MS curve. Especially during the Holocene Optimum Period, the MS curve is smooth with some insignificant peaks and valleys, whereas the Md curve fluctuates frequently with large amplitude. There are two obvious peaks during 9e7 ka BP (7.8 ka and 8.2 ka events) (Fig. 5), which implies the climate became dry-cold. For the stages except the Holocene Optimum Period, the Md curve fluctuates more frequently than MS curve, which is in line with the highfrequency climate oscillations (Sun et al., 2010). Other functions can affect grain size analysis, including surface mixing and pedogenesis, which are two possible post-depositional processes that may influence the fidelity of loess proxies used to trace high-frequency climate oscillations (Sun et al., 2010). The pedogenesis of Baicaoyuan section, which is located in the northwest edge of CLP with high sedimentation rate and low precipitation, is weak and has little influence on the grain size analysis and the interpretation of rapid monsoon oscillations. It is suggested that surface mixing acts as a low-pass filter and will attenuate the amplitude of high-frequency signals when increasing the thickness of surface mixing layer, while the amplitude of low-frequency signals will remain relatively unaltered. If Jingyuan section has a surface mixing depth similar to those from Xifeng and Weinan section (w5 cm), Sun et al. (2010) concluded that surface mixing has no significant impact on the rapid monsoon changes of the Last Glacial on millennial timescale, at the Jingyuan section, western part of CLP with a high sedimentation rate. Surface mixing may have no significant impact on the rapid monsoon changes in Baicaoyuan, which is also marked by a high sedimentation rate. Surface mixing and pedogenesis may have no significance on highfrequency climate oscillations in Baicaoyuan. The position of the desert southeastern front is related to grain size variation and deposition rate (Liu, 1985). From S0 to L0 stage in Baicaoyuan profile, the distance between Baicaoyuan and desert became less (Sun et al., 1998), resulting in coarser grain size and higher sedimentation rate and resolution. The position of the desert can affect climate reconstruction, which is more sensitive in the northwest edge of CLP than in the southeast edge. MS can only record the evolution trend of paleoclimate, which is sensitive only to the low-frequency climate signals. Md is sensitive not only to the low frequency signals but also the high frequency signals of climate changes (Fig. 5). One reason is that the shorter distance from the desert resulted in a high deposition rate, which makes grain size a high-resolution proxy for paleoclimatic research.
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Another reason is that winter monsoon is controlled by Siberian Mongolian High, with the northwest winds weakening southeastwards. The area in the northwest edge of the Loess Plateau is more influenced by the winter winds and more sensitive to the wind strength variations than that in the southeast edge. The stronger the winds are, the coarser the grain sizes are. So Md is more sensitive to paleoclimate changes than MS. Sun et al. (2010) also made the same conclusion based on Jingyuan section, the western part of CLP: mean grain sizes are more sensitive to environmental fluctuations than MS. However, the MS can also record the rapid climatic change events in the northwest part of CLP during the Last Glacial (Chen et al., 1997; Fang et al., 1999). 5. Conclusions Baicaoyuan Holocene loess is located in the northwest edge of Chinese Loess Plateau and has the advantages of high resolution and continuous deposition, which can be comparable to Sanbao and Hulu caves stalagmite oxygen isotopes with respect to recording the evolution of Holocene paleoclimate. It is a good Holocene paleoclimatic archive. The loess and stalagmite under study are located in the East Asia monsoon region. They record the similar general evolution process of Holocene paleomonsoon in East Asia. During 14.2e12.8 ka BP, the Last Glacial Maximum turned into Holocene transition phase; during 12.8e11.5 ka BP, the Younger Dryas event resulted in drycold climate; between 11.5 ka BP and 10.5 ka BP, the climate rapidly changed to be warm and wet; during 10.5e5.2 ka BP, the Holocene Optimum Period was characterized by high temperature and precipitation, although the climate was not stable and some dry-cold events were detected; during the last 5.2 ka BP, the climate slowly turned dry-cold. The fluctuations of magnetic parameters, grain size parameters and stalagmite oxygen isotope records are controlled by the variation of insolation. The comparison of MS and Md in recording paleoclimate indicates that MS curve is smooth and Md curve fluctuates frequently. Especially, there are two obvious dry-cold events in Md curve during 9e7 ka BP, which are absent in the MS curve. However, these events are observed in other paleoclimatic records. The conclusion is that the grain size parameter can be more sensitive than MS with respect to paleoclimate changes at the Baicaoyuan section during the Holocene. Acknowledgments This research was supported by National Natural Science Foundation of China (Grants 41072124, 40830105 & 40721061). We are grateful to the anonymous reviewers for their insightful comments and suggestions to improve the manuscript. We also like to thank Allan Grey for his careful checking of this manuscript and Deng Yang for his help in analyzing the data. References Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a prominent, widespread event 8200 yr ago. Geology 25 (6), 483e486. 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. Quaternary International 7-8, 91e95. An, Z.S., Kukla, G.J., Porter, S., Xiao, J.L., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quaternary Research 36 (1), 29e36. An, Z.S., 2000. The history and variability of the East Asian paleomonsoon climate. Quaternary Science Reviews 19, 171e187. Ao, H., Deng, C.L., 2007. Review in the identification of magnetic minerals. Progress in Geophysics 22 (2), 432e442 (in Chinese with English abstract).
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Paleoclimatic evolution of Holocene loess and discussion of the sensitivity of magnetic susceptibility and median diameter Guoyong Zhao a, Xiuming Liu a, b, c, *, Qu Chen a, Bin Lü a, Hewen Niu a, Zhi Liu a, Pingyuan Li a a
Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Research School of Arid Environment & Climate Change, Lanzhou University, Lanzhou 730000, China b School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China c Department of Environment and Geography, Macquarie University, Sydney NSW 2109, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 23 June 2012
Climate changes are closely related to peoples’ livelihoods. Holocene climate changes are therefore particularly important to humans. It is known that eolian loess deposits in China are one of the best geological records for paleoclimatic change. As it is the top layer of loess sequence, Holocene loess may be easily disturbed by human activities. For that reason, it has not been studied thoroughly. Recently, a continuously deposited, high-resolution and well-preserved Holocene loess section has been found in Baicaoyuan, Huining County, Gansu Province, located in the northwest edge of the Chinese Loess Plateau. The magnetic and grain size signals of the section reflect Holocene paleoclimate variations and are comparable to the stalagmite oxygen isotope records of Sanbao and Hulu caves. They consistently record the general evolution process of Holocene paleoclimate in Eastern Asia. During the period 14.2e12.8 ka BP, Last Glacial Maximum turned into Holocene transition phase. During 12.8e11.5 ka BP, the climate was cold and dry, dominated by the Younger Dryas. Between 11.5 and 10.5 ka BP, the climate rapidly turned warm and wet, and 10.5e5.2 ka BP was the Holocene Optimum Period, during which some relatively dry-cold events could be observed. Since 5.2 ka BP, the climate slowly became dry and cold. In summary, the climate showed a general drying trend from the early Holocene to the late Holocene, which was controlled by variation of solar radiation. Compared with magnetic susceptibility, the median diameter curve fluctuates frequently. Some climate events were recorded by the median diameter curve, but not observed in the magnetic susceptibility curve. It is suggested that the grain size parameter is more sensitive to paleoclimate changes than susceptibility at the Baicaoyuan section during the Holocene. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Climate change is a hot topic nowadays, which has turned from the pure scientific field to include international political, economic, and diplomatic issues (Fang et al., 2011). The IPCC in 2007 reported that the temperature had a rise of 0.74 C (0.56 C to 0.92 C) during 1906e2005 A.D. (IPCC, 2007). The 0.74 C was derived from instrumental determination, which raises a question how the climate changed before the period without instrument records. In addition, the causes for the increase in temperature, undoubtedly, included interaction between human activities and nature. To evaluate the relative contributions of the two factors to the modern climate change requires separating them. Despite the influence of * Corresponding author. Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University, 222 Tianshui South Road, Chengguan District, Lanzhou 730000, China. E-mail address: [email protected] (X. Liu). 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2012.06.015
human activities on climate change, researchers need to seek answers from geological records. The Holocene is the most recent geological epoch, attracting much attention of geologists. Many researchers focused on the Holocene climate change (Liu et al., 2000; Haug et al., 2001; Wang et al., 2005; Yu et al., 2006; Huang et al., 2007), with the purpose of better understanding paleoclimate changes, and simultaneously predicting future climate trends. Loess is regarded as one of the best geologic records for paleoclimatic research, along with ice cores and marine oxygen isotopes (Heller and Liu, 1984; Liu, 1985; Kukla et al., 1988; Deng et al., 2007). In recent years, the researchers mainly endeavored to establish a perfect Quaternary geologic calendar, while relatively little attention has been paid to the paleoclimatic significance of Holocene loess. Holocene loess is the uppermost layer of the loess deposition and can be easily disturbed by human activities, which thus causes a hiatus or inaccuracy of the paleoclimatic records. Furthermore, compared with the stalagmite and lacustrine records, loess has relatively low resolution as
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a paleoclimatic archive. However, the Chinese Loess Plateau has a large area and the pedogenesis strength and sediment rates of loess deposits systematically change with various climate conditions from the northwest to the southeast. Recently, a loess profile with continuous deposition and high-resolution paleoclimatic records was found at Baicaoyuan, Huining County, Gansu Province, China. The data is comparable to the stalagmite records and is ideal for the study on Holocene climate variability. 2. Samples and methods The area of the Chinese Loess Plateau (CLP) is vast. From southeast to northwest, the thickness of loess deposits increases, the pedogenic degree decreases, and the climate signal intensifies (Liu et al., 1990). In the southeast CLP, soil is developed, the accumulation rate is low, so the resolution is low. In the northwest CLP, soil is weakly developed, the accumulation rate and resolution is high, but the recorded signal of summer monsoon is weak. Baicaoyuan section (3614.3900 N, 105 8.0320 E, 2089 11 m a.s.l.) is located in Huining County, Gansu Province, in the northwest edge of CLP, and to the west of the Liupan Mountains. Baicaoyuan has a mean annual temperature of 8.5 C and a mean annual precipitation of 320 mm (Deng, 2008). The section is close to the source area with high deposition rate, and in the transitional zone of monsoon and non-monsoon areas, and thus sensitive to paleoclimate changes, which makes it ideal for paleoclimatic study (Fig. 1). Baicaoyuan Holocene loess is described as follows: the total thickness of the section sampled is 2.1 m, and 105 samples were sampled at 2 cm intervals. The thickness of L0 is 1 m, loose, structureless, pale yellow, many grass roots and wormholes. The thickness of S0 is 0.84 m, black- brown, hard texture, white calcium nodules, and many grass roots and wormholes. The thickness of L1 is 0.26 m, with many pores that are looser than L0 and color lighter than that of L0. The samples were dried by natural air indoors, then were ground and put into small nonmagnetic plastic boxes with mass of 7.5 g,
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sealed by adhesive tape. Magnetic susceptibility (MS) was measured at low frequency (470 Hz, clf) and high frequency (4700 Hz, chf) using a Bartington MS2 meter with dual frequency sensor. Frequency-dependent magnetic susceptibility (cfd) is calculated by cfd ¼ clf chf. Anhysteretic remanent magnetization (ARM) was measured using a Minispin Magnetometer after being magnetized with a DTECH AF demagnetizer. The peak AF field used was 100 mT and the DC bias was 0.05 mT. ARM was then normalized by the bias field to obtain ARM susceptibility (cARM). Isothermal remanent magnetization (IRM) was acquired in progressively increasing magnetic fields up to 1T with a MMPM10 pulse magnetizer and the induced remanence after each field was measured using a Minispin Magnetometer. The IRM acquired in the maximum field of 1T was defined as the saturation isothermal remanent magnetization (SIRM), then remanent coercivity (Bcr), the ratio of saturation isothermal remanent magnetization and ARM susceptibility (SIRM/ cARM) and S-ratio (S-ratio ¼ IRM300 mT/SIRM) was calculated. The grain size was analyzed with a Malven Mastersizer 2000 laser grain size analyzer, which has a measurement range of 0.02e2000 mm. All parameters were measured in the Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University. 3. Results and analysis The results of magnetic and particle size parameters are shown in Figs. 2 and 3, respectively. Baicaoyuan section can be divided into 3 units as L0, S0 and L1 according to traditional loess stratigraphic division from top to bottom, which is consistent with the observation in the field. 3.1. Magnetic parameters Magnetic susceptibility (MS) is a physical quantity determined by the magnetization in a weak field (Liu and Deng, 2009). MS is usually positively related to the intensity of pedogenesis, which is used to indicate variation of summer wind strength (Heller and Liu,
Fig. 1. Loess Plateau with location of study section. Shading indicates the areal extent of the Chinese loess deposits and the dashed contour lines indicate present-day rainfall (in mm/year), adapted from Zhao and Roberts (2010).
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Fig. 2. Parameters of magnetism versus depth of Huining Holocene loess. B: Location and age of thermoluminescence sample (Wang et al., 2000).
1984; An et al., 1990, 1991; Liu et al., 1992). High MS is indicative of strong summer winds with high temperature and precipitation. Soil development and neoformation of fine ferrimagnetic minerals were responsible for High MS in paleosols (Zhou et al., 1990). In contrast, during the periods when the summer wind weakened and the winter wind intensified, soils were weakly developed with coarser grain size and low MS. High MS in S0 suggests mild climate, low MS in L0 reflects that the climate was dry and cold (Fig. 2a). Frequency-dependent magnetic susceptibility (cfd) is sensitive to superparamagnetic (SP) and single-domain (SD) magnetite (Thompson and Oldfield, 1986). These magnetic particles are created by soil development (Zhou et al., 1990; Liu et al., 1992, 2005). The value of cfd can reflect pedogenetic intensity, and higher values are indicative of stronger pedogenesis. cfd values of S0 are all above 8 * 108 m3/kg (Fig. 2b), suggesting that the climate was suitable and soils were strongly developed. cfd values are relatively low in L0 (Fig. 2b), which implies that the climate was cold and dry and soils were weakly developed. cARM/c can be used to estimate magnetic particle size (Liu et al., 2004), which is sensitive to fine magnetic particles (SD and fine PSD). c is sensitive to SP particles and coarse magnetic particles
(coarse PSD and MD). High values of cARM/c suggest fine magnetic particle size, while low values suggest coarse magnetic particle size. High cARM/c values in S0 (Fig. 2c) reflect that the climate was suitable and favored soil development; low cARM/c values in L0 (Fig. 2c) suggest that the climate was not as good as that recorded in S0. cARM/c increases dramatically in L0/S0 transitional horizon (Fig. 2c), which is consistent with fieldwork observations. It is suggested that cARM/c reflects the degree of soil development. After they are given a 1T magnetization, the samples were then given stepwise backfield IRMs (isothermal remanent magnetization), the opposite magnetic field at which the remanent magnetization decreases to zero is called residual magnetism coercive force (Bcr) (Thompson and Oldfield, 1986). Bcr is used to judge relative content of hand or soft magnetic components. High Bcr values are indicative of a large component of hard magnetic minerals; conversely, low Bcr values are indicative of a large component of soft magnetic minerals. S0 shows low Bcr values (Fig. 2d), which suggest that the samples contain more soft magnetic minerals and the climate was suitable and favored soil development; L0 demonstrates high Bcr values (Fig. 2d) suggesting more hard magnetic minerals, and a cold-dry climate, this could be
Fig. 3. Grain size parameters versus depth of Huining Holocene loess. B: The location and age of thermoluminescence date (Wang et al., 2000).
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ascribed to magnetic minerals of eolian origin and relatively weak development of soils. SIRM/cARM is especially sensitive to magnetic particles that are slight larger than SP particles, because the ratio is low within this grain size range (Thompson and Oldfield, 1986). High ratio implies that the samples contain more coarse magnetic minerals. Value of SIRM/cARM is low in S0 (Fig. 2e), suggesting more fine magnetic minerals that result from favorable climate and soil development; value of SIRM/cARM is high in L0 (Fig. 2e), revealing that the less developed loess contains coarse magnetic minerals of eolian origin that formed during cold-dry climate. SIRM/cARM suddenly increases at L0/S0 transitional horizon, which is similar to cARM/c (Fig. 2c). S-ratio is equal to the absolute value of the backfield IRM at 300 mT divided by 1T IRM (Pan and Zhu, 1996). High S-ratio that is close to 1 reveals that the magnetic minerals with low coercive force dominant (Ao and Deng, 2007). Conversely, low S-ratio reveals that magnetic minerals with high coercive force are dominant. Though the S-ratio is generally high through the section, low values are observed at L0 and part of S0 (Fig. 2f). The value increases dramatically in S0 (Fig. 2f), indicative of a large component of magnetic minerals with low coercive force. The highest values of Sratio correspond with part of Holocene Optimum Period (see the following sections), which suggests that the climate during middle Holocene was mild and pedogenesis intensified. 3.2. Particle size parameters Generally, there is positive correlation between the winter monsoon and particle size (Ding et al., 1994), with stronger winter monsoon corresponding with coarser particle size. The Loess Plateau source is the desert area in the northwest of China, whose southeast front retreated and expanded with the paleomonsoon variation. Sun et al. (1998) pointed out the deserts retreated westward about 1000 km during the Holocene Optimum period relative to the position of the Last Glacial Maximum period. Particle size can be also correlated to the distance between source area and deposition area. Shorter distance from the source area corresponds to coarser particle sizes (Liu, 1985). The distance during the S0 stage was longer than that during the L0 stage between Baicaoyuan section and the desert, and the grain size of S0 was finer than that of L0 (Fig. 3). The median diameter (Md) is the median value of different particle size fractions. MS and particle size are the most sensitive proxies in loess (Heller and Liu, 1984; An et al., 1990, 1991; Liu et al., 1992; Ding et al., 1994). Md is a widely accepted particle size parameter. MS (Fig. 2a) and Md (Fig. 3a) show a negative relationship. MS demonstrates high values in S0, while Md demonstrates low values, which strongly indicates that the summer monsoon dominated during this period, and favored pedogenesis; conversely, MS shows low values, and Md shows high values in L0, which implies that the winter monsoon strengthened and was unfavorable to pedogenesis during this period. Fig. 3beg shows the percentages of different grain size fractions versus depth. Fig. 3bed shows that S0 has a larger component of fine particles than L0 and Md demonstrates opposite distribution. Fig. 3eeg shows that L0 has a larger component of coarse particles than S0 and Md shows the same distribution as the coarse fractions. This suggests that Md mainly reflects the coarse components. The variation trends of magnetic and particle size parameters during the Last Glacial period are similar to those recorded in L0. In S0, the peaks of high values of c, cfd, cARM/c, and S-ratio correspond with a large component of fine grain size fractions (<16 mm (%)), and low values of Bcr, SIRM/cARM, Md and the coarse grain size fractions (>16 mm (%)). In L0, the correlations are
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opposite. All these parameters indicate that during the period when S0 deposited, grain size was relative fine, suggesting the climate was characterized by high temperature and precipitation and led to soil development and neoformation of ultrafine ferrimagnetics. This accounts for the high values of c, cfd, cARM/c, S-ratio and low values of Bcr and SIRM/cARM. During the period when L0 deposited, loess particle size coarsened, the climate was dry and cold, and the soils were weakly developed. This accounts for the decrease in c, cfd, cARM/c, S-ratio values, and the increase in Bcr and SIRM/cARM values. 4. Discussion 4.1. Age of Baicaoyuan Holocene loess Baicaoyuan, Lijiayuan and Jingyuan sections are located in the western edge of the Loess Plateau, Lijiayuan is about 20 km southeast to Baicaoyuan direction and Jingyuan about 50 km northwest to Baicaoyuan direction (Fig. 1). Ding et al. (1998) made detailed studies on the magnetic susceptibility and grain size of Lijiayuan section during the past 130,000 years and held that the Holocene loess deposit is about 2 m thick. Wang et al. (2012) studied the black wood charcoal record of Lijiayuan section since the Last Glacier Maximum and suggested that the thickness of the Holocene loess deposit is about 2.2 m. The thickness of the loess deposit in Baicaoyuan is 2.1 m, which is approximate to that of the Holocene loess in Lijiayuan section. The grain size variations of Baicaoyuan and Jingyuan sections show similar trends (Fig. 4). Sun et al. (2010, 2012) reported a detailed optically stimulated luminescence (OSL) dating of Jingyuan section since the Last Glacial and offered a reliable chronology for paleoclimatic research. Therefore, a detailed comparison between the grain size variation of Baicaoyuan section and that of Jingyuan section reported by Sun et al. (2010, 2012) was made, and 11 points were correlated as age controls (Fig. 4). The age of each sample of Baicaoyuan section was obtained by using the magnetic susceptibility age model (Kukla et al., 1988) and interpolation method. It is suggested that the basal age of Baicaoyuan section is about 14.2 ka BP and the age of Holocene loess is about 11.5 ka BP, which is in line with the base of the Holocene (Gradstein et al., 2005) and the thermoluminescence age of the Lijiayuan Holocene loess (11e10 ka BP) attained by Wang et al. (2000) (Fig. 3). 4.2. Are Baicaoyuan section deposits continuous? The Baicaoyuan section deposits are considered to be continuous. First, Baicaoyuan is located in the western CLP, with lower annual mean precipitation (320 mm) and higher sedimentation rate than in the southern CLP, it is unlikely eroded by water and wind deflation (Sun et al., 2010). Second, the paleoclimatic evolution recorded by Baicaoyuan section can be compared with stalagmite oxygen isotope of Sanbao and Hulu caves in the southern part of China (as discussed further in Section 4.3). Third, the curve of age-depth model is smooth, there is no dramatic variation (Fig. 4c), indicating that there is no hiatus in the section. The higher sedimentation rate of L0 than that of S0 indicates the dry and cold climate of L0 than that of S0. Fourth, many scientists think that loess in CLP deposited continuously, which provides continuous records of paleoenvironmental variation during the whole Quaternary (Liu, 1985; Kukla et al., 1988; Kukla and An, 1989; Ding et al., 1994; Lu et al., 1999; An, 2000; Lai and Wintle, 2006; Sun et al., 2010). Yuanbao and Jingyuan sections, situated at the northwest edge of CLP, were shown to be deposited continuously (Lai and Wintle, 2006; Sun et al., 2010, 2012). In summary, it is
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Fig. 4. Comparison of grain size variations between Baicaoyuan (a) and Jingyuan (b) sections and the age-depth model of Baicaoyuan section(c).
strongly suggested that Baicaoyuan section also accumulated continuously. 4.3. The evolution of Holocene paleoclimate Based on the multi-proxy studies, the Holocene paleoclimatic evolution is divided into five stages (Fig. 5). I 14.2e12.8 ka BP, the Last Glacial Maximum turned into the Holocene transition phase. MS increased, Md and d18O values decreased. This strongly suggests that the climate became warm and wet during this period.
Fig. 5. The relationship between magnetic susceptibility, median diameter, oxygen isotopes of stalagmites (Wang et al., 2001, 2008) and July insolation at 65 N (Berger and Loutre, 1991).
II 12.8e11.5 ka BP, Younger Dryas event. MS is low, grain size is coarse (compared to the Holocene Optimum Period (Fig. 5)), and the d18O values are high (Fig. 5c), indicative of weak summer monsoon and strong winter monsoon with low temperature and precipitation (Wang et al., 2001, 2008; Shao et al., 2006). However, there was a warm and wet episode during this stage. Zhou et al. (2001) suggested that climate was dry and cold at the beginning of the Younger Dryas, then warm and wet, and then drier and colder than at the beginning, based on the research of the loess-paleosol sequence of Dongxiang, Shenmu and wind-blown sand/lacustrine deposit sequence of Jingbian during the period of 11,70012,800 cal BP. Greenland Ice Sheets Project 2 (GISP2) (Lee and Slowey, 1999) provides another piece of evidence, the paleoclimatic record of which can be correlated to these results. Studies on paleoclimatic records in other areas, including the Midiwan peat profile located at the desert/loess transitional belt, China (Zhou et al., 1998), the Zhengbeitai loess profile located at Yulin, Shanxi Province, China (Zhou et al., 1998), and the lacustrine deposit of Zalairoer in Inner Mongolia of China (Wang et al., 1994), showed the same variation pattern. III 11.5e10.5 ka BP, rapidly change stage. MS rapidly increased (Fig. 5a) and the Md rapidly decreased (Fig. 5b). The value of stalagmite oxygen isotopes rapidly decreased (Fig. 5c). It is implied that the dry-cold climate rapidly changed into warmwet climate, with higher precipitation and temperature than before (Young Dryas period). IV 10.5e5.2 ka BP, the Holocene Optimum Period. MS is at maximum over the whole Holocene (Fig. 5a), and grain size is minimum (Fig. 5b), corresponding to the black loessial soil S0 over the CLP; at the same time, the stalagmite oxygen isotope is the lowest in south of China, suggesting that climate was much warmer and wetter during this period. Relatively high temperatures and precipitations favoured plant growth and biological activities, so this stage is called the Holocene Optimum Period. There is no agreement on the range of Holocene Optimum Period. Shi et al. (1994) held that Holocene Megathermal is 8.5e3 ka BP in China; Zhou et al. (1978) suggested that the Warm Time in Beijing is 7.5e2.5 ka BP;
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Kalis et al. (2003) argued that the Holocene Optimum Period in middle Europe is 9e5.5 ka BP. In fact, the discrepancy is mainly due to the different determination of warmth periods (Fig. 5), there is no difference essentially. However, the climate of Holocene Optimum Period was unstable, the median diameter curve fluctuates frequently during the 9e7 ka BP stage with two obvious peaks (7.8 ka, 8.2 ka drycold event) (Fig. 5). The error of Md is 0.46 w 0.46 mm (the maximum of Md is 23 mm, and the error of grain size analyzer is 2%), and separated the low-frequency and highfrequency components. The high-frequency amplitude is larger enough than the error of Md. Two obvious peaks (7.8 ka and 8.2 events) are observed in high-frequency amplitude curve (Fig. 6). There were 8.2 ka, 8.6 ka, 9.3 ka, 10.2 ka drought events in stalagmite records (Shao et al., 2006). The 8.2 ka BP event was also observed in other records (Alley et al., 1997; Bond et al., 1997; Grafenstein et al., 1998; Dean et al., 2002; Thomas et al., 2007). According to MS, Md of Baicaoyuan Holocene loess and the stalagmite oxygen isotope record of Sanbao and Hulu caves, it is revealed that the Holocene Optimum Period lasted about 5 ka (10.5e5.2 ka BP). V Since 5.2 ka BP, the climate slowly became dry and cold. Magnetic susceptibility decreased and median diameter increased (Fig. 5), stalagmite oxygen isotope value increased slowly (Fig. 5c). This period (5.2 ka BP to present) can be divided into two parts, with about 3 ka BP as the boundary line: the climate changed more dramatically since 3 ka BP than during 5.2e3 ka BP. Some other geological records show the similar paleoclimate variations, the climate changed to be dry-cold from 5.5 ka cal. BP in Holocene peat of Zoige area (Yu et al., 2006); temperature and humidity of Qinghai Lake achieved the maximum at 6.5 ka cal. BP and began to drop gradually after 4.5 ka cal. BP (Shen et al., 2005; Zhao et al., 2009). Baicaoyuan, Sanbao and Hulu are located in the East Asia monsoon region and the climate is controlled by the East Asia monsoon. The various proxies of susceptibility, grain size and oxygen isotope records are consistent and display similar evolution of Holocene climate in East Asia. In summary, the climate showed a general drying trend from the early Holocene to the late Holocene. Though the resolution of loess is lower than the stalagimite, they show very comparable evolution process of Holocene paleoclimate,
Fig. 6. The high-frequency amplitude of median diameter of Baicaoyuan section.
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implying that Baicaoyuan loess is a reliable Holocene climatic record. MS, Md and stalagmite oxygen isotope curves demonstrate good correlation with the July insolation at 65 N (Fig. 5). The insolation was high, and the climate was warm and wet during the period from 10.5 to 5.2 ka BP, which is defined as the Holocene Optimum Period; Since 5.2 ka BP, insolation decreased slowly and climate turned to be dry-cold, suggesting the evolution of Holocene paleoclimate was controled by insolation variation. During the Younger Dryas period, solar radiation shows the second highest value and the climate deteriorated dramatically in comparison with the Holocene Optimum Period, which is possible due to the effects of the Last Glacial Maximum called the climate return event (Xiong et al., 1999). 4.4. Which is more sensitive to paleoclimatic change in the northwest edge of Chinese Loess Plateau, median diameter or magnetic susceptibility? There is a negative relationship between MS and Md. They consistently record the evolution of Holocene paleoclimate. However, the curve of MS is smoother than that of Md. Some peaks and valleys are observed in the Md curve, which are absent in the MS curve. Especially during the Holocene Optimum Period, the MS curve is smooth with some insignificant peaks and valleys, whereas the Md curve fluctuates frequently with large amplitude. There are two obvious peaks during 9e7 ka BP (7.8 ka and 8.2 ka events) (Fig. 5), which implies the climate became dry-cold. For the stages except the Holocene Optimum Period, the Md curve fluctuates more frequently than MS curve, which is in line with the highfrequency climate oscillations (Sun et al., 2010). Other functions can affect grain size analysis, including surface mixing and pedogenesis, which are two possible post-depositional processes that may influence the fidelity of loess proxies used to trace high-frequency climate oscillations (Sun et al., 2010). The pedogenesis of Baicaoyuan section, which is located in the northwest edge of CLP with high sedimentation rate and low precipitation, is weak and has little influence on the grain size analysis and the interpretation of rapid monsoon oscillations. It is suggested that surface mixing acts as a low-pass filter and will attenuate the amplitude of high-frequency signals when increasing the thickness of surface mixing layer, while the amplitude of low-frequency signals will remain relatively unaltered. If Jingyuan section has a surface mixing depth similar to those from Xifeng and Weinan section (w5 cm), Sun et al. (2010) concluded that surface mixing has no significant impact on the rapid monsoon changes of the Last Glacial on millennial timescale, at the Jingyuan section, western part of CLP with a high sedimentation rate. Surface mixing may have no significant impact on the rapid monsoon changes in Baicaoyuan, which is also marked by a high sedimentation rate. Surface mixing and pedogenesis may have no significance on highfrequency climate oscillations in Baicaoyuan. The position of the desert southeastern front is related to grain size variation and deposition rate (Liu, 1985). From S0 to L0 stage in Baicaoyuan profile, the distance between Baicaoyuan and desert became less (Sun et al., 1998), resulting in coarser grain size and higher sedimentation rate and resolution. The position of the desert can affect climate reconstruction, which is more sensitive in the northwest edge of CLP than in the southeast edge. MS can only record the evolution trend of paleoclimate, which is sensitive only to the low-frequency climate signals. Md is sensitive not only to the low frequency signals but also the high frequency signals of climate changes (Fig. 5). One reason is that the shorter distance from the desert resulted in a high deposition rate, which makes grain size a high-resolution proxy for paleoclimatic research.
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Another reason is that winter monsoon is controlled by Siberian Mongolian High, with the northwest winds weakening southeastwards. The area in the northwest edge of the Loess Plateau is more influenced by the winter winds and more sensitive to the wind strength variations than that in the southeast edge. The stronger the winds are, the coarser the grain sizes are. So Md is more sensitive to paleoclimate changes than MS. Sun et al. (2010) also made the same conclusion based on Jingyuan section, the western part of CLP: mean grain sizes are more sensitive to environmental fluctuations than MS. However, the MS can also record the rapid climatic change events in the northwest part of CLP during the Last Glacial (Chen et al., 1997; Fang et al., 1999). 5. Conclusions Baicaoyuan Holocene loess is located in the northwest edge of Chinese Loess Plateau and has the advantages of high resolution and continuous deposition, which can be comparable to Sanbao and Hulu caves stalagmite oxygen isotopes with respect to recording the evolution of Holocene paleoclimate. It is a good Holocene paleoclimatic archive. The loess and stalagmite under study are located in the East Asia monsoon region. They record the similar general evolution process of Holocene paleomonsoon in East Asia. During 14.2e12.8 ka BP, the Last Glacial Maximum turned into Holocene transition phase; during 12.8e11.5 ka BP, the Younger Dryas event resulted in drycold climate; between 11.5 ka BP and 10.5 ka BP, the climate rapidly changed to be warm and wet; during 10.5e5.2 ka BP, the Holocene Optimum Period was characterized by high temperature and precipitation, although the climate was not stable and some dry-cold events were detected; during the last 5.2 ka BP, the climate slowly turned dry-cold. The fluctuations of magnetic parameters, grain size parameters and stalagmite oxygen isotope records are controlled by the variation of insolation. The comparison of MS and Md in recording paleoclimate indicates that MS curve is smooth and Md curve fluctuates frequently. Especially, there are two obvious dry-cold events in Md curve during 9e7 ka BP, which are absent in the MS curve. However, these events are observed in other paleoclimatic records. The conclusion is that the grain size parameter can be more sensitive than MS with respect to paleoclimate changes at the Baicaoyuan section during the Holocene. Acknowledgments This research was supported by National Natural Science Foundation of China (Grants 41072124, 40830105 & 40721061). We are grateful to the anonymous reviewers for their insightful comments and suggestions to improve the manuscript. We also like to thank Allan Grey for his careful checking of this manuscript and Deng Yang for his help in analyzing the data. References Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a prominent, widespread event 8200 yr ago. Geology 25 (6), 483e486. 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. Quaternary International 7-8, 91e95. An, Z.S., Kukla, G.J., Porter, S., Xiao, J.L., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quaternary Research 36 (1), 29e36. An, Z.S., 2000. The history and variability of the East Asian paleomonsoon climate. Quaternary Science Reviews 19, 171e187. Ao, H., Deng, C.L., 2007. Review in the identification of magnetic minerals. Progress in Geophysics 22 (2), 432e442 (in Chinese with English abstract).
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