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Spatial Variations of Magnetic Susceptibility of Chinese Loess For

Spatial Variations of Magnetic Susceptibility of Chinese Loess For

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, B12101, doi:10.1029/2005JB003765, 2005

Spatial variations of magnetic susceptibility of Chinese for the last 600 kyr: Implications for monsoon evolution Qingzhen Hao and Zhengtang Guo1 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Received 5 April 2005; revised 2 September 2005; accepted 27 September 2005; published 2 December 2005.

[1] We examine spatial variations in magnetic susceptibility (MS) over the in China based on 50 sections in order to identify spatial changes in monsoon climate at key glacial, interglacial, and interstadial intervals for the last 600 kyr. The results indicate strong coherence between MS variations during the interglacial periods and present-day precipitation and temperature patterns. This suggests that the strength of the summer monsoon had a dominant influence on the MS signals in , through modulating pedogenic intensity. The distribution of MS during glacial periods is characterized by weak S-N gradients and rough W-E zonal patterns, indicating a negligible effect of the summer monsoon. Interstadial patterns are intermediate between the glacial and interglacial ones. Interglacial patterns for the last 600 kyr are essentially similar, suggesting that the climate regime during these periods has not undergone significant changes and that the east Asian summer monsoon has remained the main moisture carrier. Our estimates of the relative amplitudes of climate oscillations during these time slices are consistent with earlier paleoclimate studies. These data, associated with the available susceptibility-based climofunctions, may be used to estimate spatial changes of paleorainfall and paleotemperature for these key periods and hence to test climate models. Citation: Hao, Q., and Z. Guo (2005), Spatial variations of magnetic susceptibility of Chinese loess for the last 600 kyr: Implications for monsoon evolution, J. Geophys. Res., 110, B12101, doi:10.1029/2005JB003765.

1. Introduction key time intervals, such as marine oxygen isotope stage (MIS) 11 that is usually regarded as an astronomical [2] The spatially correlative Quaternary loess- analogue of the [Loutre and Berger, 2003], and sequences in the Chinese Loess Plateau (Figure 1) have MIS 13 representing a warm, humid climate extreme when long been regarded as near-continuous records of the east compared with the record over the last 2.6 Myr [Guo et al., Asian monsoon climate [Liu, 1985; Kukla and An, 1989; An 1998]. Examination of the spatial climate changes for a et al., 1990a], and the oldest loess-soil sequences have been longer time span would significantly improve our under- dated back to 22 Ma [Guo et al., 2002]. The alternations standing of the east Asian monsoon climate, and would also between soils and loess are commonly interpreted as an provide useful boundary conditions for climate models. indication of the waxing and waning of the east Asian [4] Magnetic susceptibility (MS) in Chinese loess is monsoon circulation, with the soil-forming periods widely used as a climate proxy [An et al., 1990a, 1991] corresponding to a strengthened summer monsoon and loess and is also a stratigraphic marker [Kukla and An, 1989]. Its deposits to a strengthened winter monsoon [An et al., value is higher in soils than in the intervening loess layers 1990a]. [Liu, 1985; An et al., 1990a]. Fine-grained minerals (mag- [3] The temporal changes of the monsoon climate in the netite and maghemite) of pedogenic origin are thought to be Loess Plateau have been reconstructed using various prox- responsible for the higher values in soils [e.g., Zhou et al., ies [e.g., An et al., 1990a; Guo et al., 1991, 1998, 2000a; 1990; Maher and Thompson, 1991]. This interpretation is Heller et al., 1993; Maher et al., 1994; Lu¨etal., 1994, also supported by most recent rock magnetic studies [Maher 1999; et al., 2001; Ding et al., 2002]. However, only a et al., 2003; Deng et al., 2004]. Analyses of various mineral few studies, mainly centered on the last two or younger magnetic parameters, such as temperature-dependent sus- climate cycles, have been carried out to examine spatial ceptibility and hysteresis properties [Deng et al., 2004], changes in the climate patterns [Sun et al., 1996; Nugteren indicate that the magnetic properties of the paleosols youn- and Vandenberghe, 2004], and the spatial patterns for older ger than 600 ka are similar. The temporal changes of MS in time slices remain to be addressed. These include several the sequences of the last 600 kyr are similar to other proxies of pedogenic intensity, such as the chemical weathering 1 Also at State Key Laboratory of Loess and Quaternary Geology, index (FeD/FeT) [Guo et al., 2000a], Rb/Sr ratio [Chen et Institute of Earth Environment, Chinese Academy of Sciences, Xian, China. al., 1999] and soil micromorphology [Guo et al., 1998]. Copyright 2005 by the American Geophysical Union. Because pedogenic intensity in the Loess Plateau is mainly 0148-0227/05/2005JB003765$09.00 controlled by the strength of the east Asian summer mon-

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Asian summer and winter monsoons [L. X. Chen et al., 1991]. In winter, the cold dry northwest winter monsoon of subarctic origin prevails across the . In summer, the region is under the control of the warm humid southeast summer monsoon of tropical/subtropical origin, leading to abundant precipitation. In spring, the summer monsoon begins to penetrate progressively further inland, and its northern front constitutes an important rain belt. From early June to early July of each year, this belt usually reaches and is contained in the Yangtze–Huaihe River basin between 26–34 N in eastern China, resulting in a long and continuous rainy season (the so-called ‘‘Meiyu belt’’ or ‘‘plum rain belt’’). Later, as the monsoon strengthens, the front penetrates into northern China, including the Loess Plateau, where most of the annual rainfall is concentrated in July, August and September [L. X. Chen et al., 1991]. At present, there is a strong climate gradient across the plateau (Figure 1). [7] In Quaternary loess-soil sequences in China, major interglacial soils and interbedded loess units of the last 600 kyr are labeled as S0, L1, S1, L2, S2, ..., and S5 from the top to the bottom according to Liu [1985]. Some soil units are indeed polycyclic, and can be subdivided into subunits [Guo et al., 1996]. For example, S2 consists of two subsoils, named S2SS1 and S2SS2, and S5 consists of three subunits (S5SS1, S5SS2 and S5SS3). The Holocene soil S0 differs from the older soils by its dark color, weak rubifi- cation, and the absence of clear calcitic horizons [Guo et al., 1991, 1994]. In contrast, the older soils are generally characterized by a brownish or reddish color with an angular blocky structure. Loess units usually have a light brown or yellow brown color with a massive structure. The Figure 1. Chinese Loess Plateau, location of the studied boundaries between loess and soil units are well reflected by sites, (top) mean annual temperature (C, dashed line) and MS (Figure 2). The stratigraphy is correlative across the whole Loess Plateau and can be readily correlated with the (bottom) mean annual precipitation (mm, dashed line) for 18 the interval 1951–1990. Solid (half shaded) circles marine d O record [Kukla, 1987], as was also confirmed by represent the newly measured (published) sections. The the eolian dust record from the North Pacific [Hovan et al., codes of sections and cities are as follows: BX, Binxian; 1989]. The loess-soil sequences above the L6 unit are CW, Changwu; DB, Dingbian; DX, Dingxi; HN, Huning; correlative with marine oxygen isotope stages (MIS) 1 through 15 [Kukla, 1987], and the correlation scheme with HX, Huanxian; JB, Jingbian; LC, Luochuan; LF, Linfeng; LT, 18 Lantian; LZ, ; JX, Jixian; SMX, ; the SPECMAP d O record [Imbrie et al., 1984] is shown in XA, Xi’an; XN, Xining; XF, Xifeng; YC, Yinchuan; Figure 2. YCH, Yichuan; YL, Yulin; YS, Yongshou. [8] Here we use MS data from 50 sections (Figure 1) for the study of the spatial distribution of MS during selected time slices; 29 of the sections were newly measured for this soon, the temporal and spatial changes of MS should study, and data for the other 21 sections were taken from primarily reflect the effects of the summer monsoon [An published literature (Table 1). Among these, 15 sections et al., 1990a, 1991]. provide MS records younger than L3. These sites cover a [5] On the basis of the MS data from 50 sections large area with the annual mean precipitation (MAP) spanning the last 600 kyr, this study aims (1) to address varying from 300 to 650 mm and the annual mean the spatial characteristics of MS variations in the Loess temperature (MAT) from 6 to 13C (Figure 1). Among Plateau region and their relationship with present-day cli- the 29 newly measured sections, seven (Xining, Xifeng, mate conditions; (2) to examine the spatial changes in the Changwu, Weinan, Yichuan, Dali and Linfen) were contin- monsoon climate during glacial, interglacial and interstadial uously sampled at 10 cm intervals. For the other 22 sections, times; and (3) to estimate the approximate amplitude of we first measured the MS in the field at 10 cm intervals displacement of the climate belts during different time using a portable susceptibility meter, in order to locate the slices. positions with maximum (for soils) and minimum MS (for loess units), and then took three parallel samples at these positions for measurements of mass MS in the laboratory. 2. General Setting and Methods The Holocene soils at Dali, Puxian, Weizhuang, Xixian and [6] The climate in the Loess Plateau is mainly controlled Yichuan were not successfully sampled due to strong by two seasonally alternating monsoon circulations: the east disturbance by agricultural activity.

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Figure 2. Magnetic susceptibility (MS) records of the loess-soil sequences of the last 600 kyr from Xifeng, Changwu, and Weinan, and correlation with the SPECMAP d18O record [Imbrie et al., 1984]. The major soil and loess units and the marine oxygen isotope stages are labeled. The scheme of land-sea correlation follows Kukla [1987]. The maxima and minima of MS used for the contour maps are marked with solid circles.

[9] Magnetic susceptibility was measured on air-dried and L2SS2), corresponding to MIS 3c and 6e (Figure 2), are samples using a Bartington Instruments MS2 susceptibility selected to represent interstadial situations. meter. Data from the literature were also obtained using the [11] In order to provide an independent characterization same instrument. We use the maximum MS value within of the climate gradients, the ratio of CBD (citrate-bicarbon- each soil unit or subsoil unit, and the minimum MS values ate-dithionite)-extractable free Fe2O3 (FeD) versus the total within the upper subunit of each major loess unit to compile Fe2O3 (FeT) for L1LL1 and S1 was also measured on the MS contour maps. Some of the glacial susceptibility 22 samples from selected sites along a NW-SE transect data for Shimao [Sun et al., 1999], Jingbian [Ding et al., and a W-E transect and compared with gradients estimated 1999] and Yulin [Sun and Ding, 1997], all from the loess/ from MS data. Sites on the NW-SE transect include transition zones, were not used because they contain Huanxian, Xifeng, Changwu, Bingxian, Yongshou and layers of sand. The interstadial soils within L1 at Lantian, and those on the W-E transect include Lanzhou, Dingbian and Mizhi [Sun and Ding, 1997] are not recog- Huining, Xifeng, Luochuan, Jixian and Linfeng (Figure 1). nizable from the susceptibility data, and were not used in the The FeD/FeT ratio is an effective indicator of pedogenic contour maps, in order to avoid any potential uncertainties. intensity [Guo et al., 2000a] as the CBD-extractable Fe2O3 For the new sections (see auxiliary material1), an average of (FeD) is mainly of pedogenic origin [Singer et al., 1992]. three measurements was used. Thus the MS contour maps FeD was extracted using the method of Mehra and represent the conditions during glacial maxima, interstadial Jackson [1960] and total Fe2O3 (FeT) was measured using periods, or interglacial climate optima for the time slices a WFD-Y2 atomic adsorption unit. The analytical preci- studied. sion is 11% for FeD and 0.4% for FeT. [10] We select the L1LL1, L2LL1, L3LL1, L4LL1 and L5LL1 loess units to represent glacial times, corresponding 3. Spatial Patterns of Magnetic Susceptibility to MIS 2, 6b, 8b, 10b and 12b, respectively (Figure 2). Interglacial times are represented by S0, S1, S2SS1, S3SS2, [12] The contour maps of MS for the time slices studied S4 and S5SS1, correlative to MIS 1, 5, 7, 9, 11 and 13, are shown in Figures 3 and 4. Glacial periods (Figure 3) respectively (Figure 2). Two interstadial soil units (L1SS2 overall are characterized by low MS values across the Loess Plateau, ranging from 15 Â 10À8 m3 kgÀ1(SI) to 90 SI. 1Auxiliary material is available at ftp://ftp.agu.org/apend/jb/ Another prominent and consistent feature is the weak S-N 2005JB003765. gradients. The spatial distribution is characterized by a W-E

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Table 1. Location of the Loess Sections Studied and Data Sources present-day annual precipitation and temperature on the Section Latitude, N Longitude, E References Loess Plateau (Figure 1). However, the latitudinal location Binxian 35.0 108.1 this study of the same isoline for different interglacial periods varied Changwu 35.2 107.8 this study considerably. Chunhua 34.8 108.5 this study [14] The steeper climate gradients during interglacial Dali 34.9 109.7 this study times, as shown by the MS spatial distribution, are also Dingxi 35.6 104.6 this study confirmed by the FeD/FeT, a chemical weathering index Fuping 34.7 109.0 this study Guanzhuang 34.6 105.9 this study [Guo et al., 2000a]. Figure 5 shows the change of MS and Heyang 35.2 110.1 this study FeD/FeT along two transects for S1 and L1LL1. Both Huanxian 36.6 107.2 this study proxies indicate comparable climate gradients. According Huining 36.3 105.1 this study to the linear fits, the gradient is roughly twice as steep for Jingyang 34.5 108.8 this study Jixian 36.1 110.7 this study S1 as for L1LL1 along the NW-SE transect (Figures 5a and Lantian 34.2 109.2 this study 5b), and over 5 times steeper along the W-E transect Linfen 35.9 111.5 this study (Figures 5c and 5d). Luochuan 35.8 109.4 this study [15] The interstadial distribution of MS (Figure 4g and Puxian 36.4 110.9 this study 4h), as represented by L1SS2 (MIS 3c) and L2SS2 (MIS Qinan 35.1 105.4 this study 34.6 105.7 this study 6e), is intermediate between the glacial and interglacial 35.1 109.1 this study patterns. However, the clear SE-NW gradient of the MS Weinan 34.3 109.5 this study values and the convexity of the MS isolines toward the NW Weizhuang 34.9 109.9 this study are similar to those of the interglacial times (Figure 4). Xifeng 35.8 107.8 this study Xining 36.6 101.8 this study [16] The origin of magnetic susceptibility changes in Xixian 36.7 110.9 this study Chinese loess-soil sequences is still somewhat controver- Yangling 34.3 108.1 this study sial. One view attributes the higher MS in soils to magnetic Yaoxian 34.9 109.0 this study minerals of pedogenic origin [e.g., Zhou et al., 1990; Maher Yichuan 36.0 110.1 this study and Thompson, 1991]. An alternative view postulates con- Yongshou 34.7 108.1 this study Yuzhong 35.9 104.2 this study trol of the MS values by depositional processes through the Baimapu 35.9 107.7 Yang and Ding [2003] diluting effect of the nonmagnetic minerals [Kukla, 1987], Baishui 35.4 106.9 Xiong et al. [2002] or changes of dust composition for a given depositional site Baoji 34.4 107.1 Yue and Xue [1996] [Sun and Liu, 2000]. The similarity between the spatial Baxie 35.7 103.4 An et al. [1990b] Beiyuan 35.6 103.2 An et al. [1991] patterns of MS in the last six major interglacial periods Chaona 35.1 107.2 Song et al. [2000] (Figures 4a–4f) and those of the present-day annual rainfall Dingbian 37.1 107.4 Sun and Ding [1997] and temperature (Figure 1), as demonstrated in this study, Hongde 36.8 107.2 Yang and Ding [2003] confirms the dominant role of moisture and temperature, Huanglong 35.6 109.8 Sun and Ding [1997] Jiaodao 36.0 109.5 Vidic et al. [2000] which are strongly dependent on the strength of the summer Jingbian 37.6 108.8 Ding et al. [1999] monsoon circulation. Our data support a mainly pedogenic Jingchuan 35.3 107.4 Ding et al. [2001] origin for the higher MS values in the interglacial paleosols Jiuzhoutai 36.1 103.8 F. H. Chen et al. [1991] over the last 600 kyr. Otherwise, we would expect the Lingtai 35.0 107.5 D. H. Sun et al. [1998] spatial pattern of the MS isolines to be convex toward the Mizhi 37.9 110.1 Sun and Ding [1997] Mubo 36.4 107.5 Yang and Ding [2003] SE or in parallel with the latitudinal direction, as is the cases Sanmenxia 34.6 111.2 Zhao et al. [2000] for the spatial variations in eolian grain size for both Shimao 37.9 110.0 Sun et al. [1999] glacial and interglacial times, due to the northwesterly Wubao 37.6 110.7 Sun and Ding [1997] dust-carrying winds and the location of the sources areas Xunyi 35.1 108.3 Xue et al. [2003] Yulin 38.2 109.8 Sun and Ding [1997] [Nugteren and Vandenberghe, 2004]. The spatial differen- tiation of eolian grain size from NW to SE appears to be more rapid during glacial periods than for interglacial ones, yielding stronger grain size gradients [Yang and Ding, zonal pattern. For L1LL1 (MIS 2) and L3LL1 (MIS 8b), 2004]. In contrast, the glacial gradients of the MS values slightly higher MS values (up to 70–90 SI) and greater are much weaker than for interglacials, also indicating a gradients are observed for the southeastern part of the Loess subordinate role of sedimentary processes in the MS varia- Plateau. tions in Chinese loess. [13] The interglacial soils (Figures 4a–4f) show much [17] The spatial patterns, which clearly show the MS higher MS values, ranging from 40 SI to 340 SI. The 70 SI isolines as convex toward the NW during all the interglacial isoline of MS, located in the southernmost part of the and interstadial periods, indicate that the southeast east Loess Plateau during glacial times, moved to the north- Asian summer monsoon is the main moisture carrier for ernmost part during interglacial times, and even beyond most of the Loess Plateau region during the soil-forming the plateau (e.g., S5SS1). The spatial distribution also intervals of the past 600 kyr. The similarity between the greatly differs from the glacial patterns, characterized by spatial patterns of MS in the last six major interglacial much stronger gradients from SE to NW. The MS isolines periods indicates that the atmospheric circulation patterns for all the studied interglacial times are convex toward the have been broadly similar during each. NW. The difference in the MS values between the eastern [18] Several susceptibility-based climofunctions have and western Loess Plateau is also much greater. These been developed for the loess-soil sequences in China features are highly consistent with the distribution of [e.g., Maher et al., 1994; Lu¨etal., 1994; Liu et al.,

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Figure 3. Magnetic susceptibility contour maps for major glacial times of the last 600 kyr. Solid circles represent the location of the loess sections used in the contour maps.

1995; Han et al., 1996]. Using our MS data and these according to the climofunction of Lu¨etal.[1994] and Han climofunctions, spatial variations of paleorainfall and et al. [1996]. paleotemperature can be quantitatively reconstructed for [20] During glacial times, the overall low values of MS climate models. (Figure 3) indicate the rather weak influence of the summer monsoon in the Loess Plateau region. During the deposition of L2LL1 (MIS 6b), L4LL1 (MIS 10b) and L5LL1 (MIS 4. Spatial Changes of the Monsoon Climate 12b), MS values in the Loess Plateau are generally lower [19] The spatial variations of MS across the Loess Plateau than 50 SI, indicating a negligible effect. The northern front can therefore be used to characterize spatial changes in the of the summer monsoon would have retreated southward monsoon climate, and particularly changes in the east Asian beyond the edge of the Loess Plateau, even during the summer monsoon circulation. According to an earlier in- summer season. This represents a southward movement of vestigation, the 50 SI isoline of MS for modern surface the climate belts of more than 250 km along the 108 line samples is roughly consistent with the 400 mm isohyet in of longitude (Figure 6a), and a rainfall decrease of more northern China [Sun et al., 1996]. This isohyet roughly than 150 mm in the southernmost part of the Loess coincides with the present-day boundary between the loess Plateau, according to the climofunction of Lu¨etal.[1994] and the [Zhu et al., 1980], and the boundary is also and Han et al. [1996], when compared with the present day considered, by some authors, to be roughly the northern (Figure 1). The rough W-E zonal patterns and the weak boundary of the present-day summer monsoon front [L. X. gradients of MS during these episodes are mainly attribut- Chen et al., 1991]. Thus the 50 SI isoline of MS can be used able to the latitudinal effects of climate and depositional to estimate the relative changes of the summer monsoon causes [Zheng et al., 1991; Chen et al., 1995]. front of the past 600 kyr (Figure 6a). Since the susceptibility [21] The northern front of the summer monsoon would isolines of the soils during the interglacial periods exceed have penetrated into the southeastern part of the Loess 50 SI, we use the 100 SI isoline of MS to compare the Plateau during the Last Glacial Maximum (LGM), as latitudinal displacement of monsoon circulation in intergla- evidenced by the higher MS values in this region cial times (Figure 6b). The 50 SI and 100 SI correspond (Figure 3a). This is consistent with the pedological features respectively to 409 mm and 629 mm of annual rainfall of L1LL1 near Weinan, characterized by a higher content of

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Figure 4. Magnetic susceptibility contour maps for (a–f) interglacial times and (g–h) two interstadial periods during the last 600 kyr. The solid circles represent the location of loess sections used in the contour maps. organic matter and a spongy microstructure, indicative of loess-desert boundary and had penetrated into the conditions [Guo et al., 1994]. Thus the strength of currently occupied by deserts. the east Asian summer monsoon appeared to be stronger [23] For the mid-Holocene paleosol S0, the 100 SI isoline during the LGM than for several earlier glacial periods of MS is located near 36.5N, 80 km north of the values during the past 600 kyr. Phytolith [Lu¨etal., 1999] and from modern surface soil samples [Sun et al., 1996]. This is terrestrial mollusk assemblages [Wu et al., 2001] also consistent with earlier observations of the distribution of revealed drier conditions for L2LL1 than for L1LL1. Such mid-Holocene soils in the present-day deserts [Dong et a temporal pattern is in conflict with the larger global ice al., 1996; J. M. Sun et al., 1998; Guo et al., 2000b]. volume during the LGM as indicated by the marine d18O However, the Holocene MS values and spatial gradients record [Imbrie et al., 1984] (Figure 2). are significantly lower than for the other interglacial times [22] The overall high MS values of the soil units studied (Figures 4a–4f), indicating a weaker summer monsoon. The (Figures 4a–4f) indicate a significantly strengthened sum- 100 SI isoline of MS in the mid-Holocene retreated south- mer monsoon on the Loess Plateau during interglacial times. ward 75 km compared with S1 (MIS 5) and 135 km The eastern plateau was under the strong control of the east compared with S5SS1 (MIS 13) (Figure 6b). A generally Asian summer monsoon, indicated by the MS values mostly weaker Holocene summer monsoon than for earlier inter- exceeding 80 SI. The northern front of the summer mon- glacial times was also inferred from soil properties [Guo et soon would have been located well beyond the present-day al., 1991, 1993, 1994], chemical weathering intensity

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Figure 5. Changes in magnetic susceptibility and paleoweathering intensity (FeD/FeT) of S1 and L1LL1 units along (a–b) the NW-SE transect and (c–d) the W-E transect. FeD and FeT of each site are averages of two subsamples.

[Guo et al., 2000a], pollen [Sun et al., 1997] and for MIS 13 based on observations of sapropel formation in phytolith [Lu¨etal., 1999] evidence. the eastern Mediterranean [Rossignol-Strick et [24] The S5SS1 soil (MIS 13) (Figure 4f) represents the al., 1998]. This exceptional climate event has no clear strongest summer monsoon over the past 600 kyr. The counterpart in the marine d18O record (Figure 2), but is position of the 100 SI isoline of MS (Figure 6b) suggests clearly imprinted in marine d13C records [Raymo et al., a 135 km northward displacement compared with the 1990], and was explained by a strengthened deep-water mid-Holocene. The climofunction of Lu¨etal.[1994] formation in the North Atlantic (NADW) [Guo et al., suggests a maximum rainfall increase of 325 mm for 1998] that influenced interhemispheric heat transport S5SS1 in the central Loess Plateau region compared to through the conveyor belt [Broecker, 1991]. The recent the present day (Figure 1). The strongest pedogenic EPICA ice core data [EPICA Community Members, 2004] intensity and the extreme warm humid climate in China revealed cooler-than-average interglacial conditions for during MIS 13 were also revealed by pedogenic indicators MIS 13 in , appearing to support the role of in northern [Guo et al., 1998] and southern China [Qiao et NADW in monsoon circulation. al., 2003]. Pollen assemblages from the central Loess [25] Interstadial patterns (MIS 3c and 6e, Figure 4g and Plateau suggest a greater proportion of broadleaf trees 4h) are more similar to interglacial cases, but the MS values than during younger interglacial periods [Wu et al., 2004]. suggest only a moderate strength for the summer monsoon. An unusually strong African monsoon was also reported The influence of the summer monsoon was restricted to the

Figure 6. Position of (a) the 50 SI isoline of magnetic susceptibility in glacial periods and (b) the 100 SI isoline of magnetic susceptibility in interglacial periods along the 108E longitude line.

7of10 B12101 HAO AND GUO: SPATIAL CHANGES OF CHINESE LOESS B12101 eastern Loess Plateau while the low MS values in the southernmost part of the Loess Plateau, even in the summer western Loess Plateau indicate a weak monsoon effect. season. Interstadial patterns are intermediate between those The effects of the Asian summer monsoon on northwest of the glacial and interglacial ones. China during MIS 3 are controversial. The high lake level [30] These results also allow estimates of the relative during MIS 3 in northwestern China has been attributed to latitudinal amplitude of the summer monsoon for different intensified monsoon rainfall [Shi et al., 2001], enhanced time slices. In terms of climate belts, glacial and interglacial glacial meltwater [Wu¨nnemann et al., 1998], or stronger contrast between the mid-Holocene and the LGM represents westerly circulation [Yang et al., 2004]. Our results do not 670 km along the 108E longitude transect. The Holocene support the hypothesis invoking heavier southeast monsoon monsoon was significantly weaker compared with the rainfall during MIS 3, because of the moderate MS values earlier major interglacials. The strongest summer monsoon throughout the Loess Plateau. is observed for soil S5SS1, corresponding to MIS 13, during [26] Our results also allow estimation of the glacial- which the monsoon circulation appears to have penetrated interglacial climate contrasts, in terms of climate parame- 135 km further north than during the mid-Holocene, ters and changes in the spatial extent of climatic zones. indicating an unusual climate period in northern China. Earlier studies showed that soil formation during the mid- Tentative estimates of paleorainfall and paleotemperatures Holocene, indicating the position of northern monsoon based on these data using the available climofunctions [e.g., front, extended into the deserts at 40–42N along the Maher and Thompson, 1994; Lu¨etal., 1994; Liu et al., 105–110E longitude belt [Dong et al., 1996; J. M. Sun et 1995; Han et al., 1996], might be used as boundary al., 1998]. Assuming the 50 SI isoline of MS for L1LL1 conditions in climate models. represents the northern monsoon front during the LGM, the difference between the mid-Holocene and the LGM [31] Acknowledgments. This study was supported by the Chinese would be 670 km along the 108E longitude transect, Academy of Sciences (KZCX2-SW-133 and KZCX3-SW-139), the National Natural Science Foundation of China (projects 40202033, slightly smaller than earlier estimate of 760 km by Dong 40231001 and 20021202), and 973 project 2004CB720203. We are et al. [1996]. Quantitative estimates according to Lu¨etal. grateful to Jan Bloemendal, Frank Oldfield and the editors for their [1994] yield a rainfall difference of 250 mm between the constructive reviews and for language improvement. We thank X. H. Chen, mid-Holocene and the LGM for the southern Loess S. Z. Peng, X. F. Yao, W. X. Wu, and X. Zhou for field assistance. Plateau (e.g., Xian). [27] Climate contrasts between the other glacial-intergla- References cial couplets over the last 600 kyr (S1/L2, S2/L3, S3/L4, S4/ An, Z. S., T. S. Liu, Y. C. Lu, S. C. Porter, G. Kukla, X. H. Wu, and Y. M. Hua (1990a), The long-term paleomonsoon variation recorded by the L5) are even greater, because of the stronger-than-Holocene loess-paleosol sequence in central China, Quat. Int., 7–8, 91–95. monsoon during interglacial times and the weaker-than- An, Z. S., X. H. Wu, Y. C. Lu, D. E. Zhang, X. J. Sun, and G. R. Dong LGM monsoon during glacial times (Figures 3 and 4a–4f). (1990b), A preliminary study on the paleoenvironment change of China during the last 20000 years (in Chinese), in Loess, Quaternary For example, compared with the LGM (L1LL1), the 50 SI Geology and Global Change, edited by T. S. Liu, pp. 1–26, Sci. isoline of MS showed a southward retreat of 155 km for Press, Beijing. L2LL1 and 100 km for L4LL1 (Figure 6a). In contrast, the An, Z. S., G. Kukla, S. C. Porter, and J. L. Xiao (1991), Magnetic suscept- 100 SI isoline of MS showed a northward advance of 75 km ibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years, Quat. Res., 36, 29–36. for S1, and 135 km for S5SS1 compared with the mid- Broecker, W. S. (1991), The great ocean conveyor, Oceanography, 4,79– Holocene (Figure 6b). Our study suggests a maximum 89. contrast in the southern Loess Plateau (e.g., Xian) between Chen, F., J. Li, and W. 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