Quaternary International 263 (2012) 63e70

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Quaternary International

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Kiloyear-scale climate events and evolution during the Last Interglacial, Mu Us , China

Shuhuan Du a, Baosheng Li b,c,*, Muhong Chen a, David Dian Zhang d, Rong Xiang a, Dongfeng Niu c, Xiaohao Wen c, Xianjiao Ou e a Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China b State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China c Department of Geography, South China Normal University, Guangzhou 510631, China d University of Hong Kong, Pokfulam Road, Hong Kong e School of Geography and Tourism, Jiaying University, Meizhou 514015, China article info abstract

Article history: The fifth segment of the Milanggouwan stratigraphical section (MGS 5) in the Mu Us Desert provides Available online 9 January 2012 high-resolution geological information on environmental variations during the Last Interglacial. The analysis of grain content (<50 mm), organic content, SiO2,Al2O3, TOFe, and SiO2e(Al2O3 þ TOFe) ratios in the MGS 5 suggest that there were 17 kiloyear-scale climate fluctuations in the Last Interglacial, including 9 warm events (W1eW9) and 8 cold events (C1eC8), dominated by the East Asian summer monsoon and winter monsoon respectively. The analysis also suggests that the Eemian interglacial was unstable, with 3 warm events (W7eW9) and 2 cold events (C7eC8), indicating that climate fluctuations affected the East Asian monsoon in the Mu Us Desert during the Last Interglacial. The change cycles and the nature of the kiloyear-scale climate events have a close temporal relationship with the ice-core oxygen isotope data, suggesting that the climate forming mechanism was affected by polar weather, North Atlantic sea ice, range of the Eurasian ice front, and movement of the frontal, all of which affect the intensity of the SiberianeMongolian high pressure region through the movement of the cold air mass. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction from a more recent study by the North Greenland Ice Core Project (NGRIP) indicates that the climate of the Eemian interglacial period during the last interglacial period or MIS 5 was relatively stable (NGRIP members, 2004). The examination of (marine or d18O1 to d18O5 in the continental glaciers) is the benthic foraminifera and the oxygen isotope obtained from the a topic in palaeoclimate study that has attracted considerable drilling conducted in northern Denmark indicated that there were attention. Researchers from around the world including China have two cold events during the Eemian (Solveig and Knudsen, 1997). conducted studies on kiloyear-scale climate fluctuations based on Similarly, the pollen and marine isotope data obtained from the ocean sediments (Wang and Yang, 1995; Tu et al., 2001; Sun and drilling conducted at ODP site 1059 off the southeast coast of the Luo, 2001), polar ice cores (Dansgaard et al., 1993; Grootes et al., USA indicate climate instability (Heusser and Oppo, 2003). 1993; Yao et al., 1997), and terrestrial sediments (An et al., 1991, However, both the terrestrial mollusc record from France (Rousseau 1995; Fang et al., 1996, 1999; An and Porter, 1997; Chen et al., and Puisségur, 1999) and examination of the pollen obtained from 2000, 2003). However, there are conflicting reports about the the lakes in northwestern Germany (Gapers et al., 2002) indicate climate conditions of the MIS 5e sub-segment (Eemian interglacial that climate conditions in the MIS 5e period were quite stable. period). The Greenland Ice-core Project (GRIP) data indicates that There are different records of kiloyear-scale climate events during the climate of MIS 5e was unstable, with rapid changes within the last interglacial in China. The stalagmite records examined by a short time-period (GRIP Members, 1993). On the other hand, data Yuan et al. (2004) suggest that the East Asian summer monsoon was weakest in sub-segment 5d of MIS 5, fluctuated between strengthening and weakening in sub-segment 5c, and remained

* relatively stable in sub-segment 5e. Guan et al. (2007) discovered Corresponding author. Department of Geography, South China Normal fi University, No. 55, Zhongshan Road, Guangzhou 510631, China. the occurrence of ve events during MIS 5 in which the summer E-mail address: [email protected] (B. Li). monsoon strengthened and four events in which the winter

1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.01.004 64 S. Du et al. / Quaternary International 263 (2012) 63e70 monsoon strengthened based on their research on two sections in layers from 61LS to 84S. These layers include 10 layers of aeolian eastern and western Liupanshan. An and Porter (1997) studied palaeo-mobile sands and palaeo-fixed to semi-fixed dune quartz and grains from the S1 palaeosols of China’s Luochuan Potou sands, 5 layers of palaeosols, and 9 layers of fluvio-lacustrine and Xiaheimu sections and discovered that during the last inter- facies, which overlap among the strata. If a dune sand and its glacial, including MIS 5e, multiple climate fluctuations along with overlying fluvio-lacustrine facies or/and palaeosol are assumed to nine dust events occurred. Fang et al. (1996, 1999) studied the Last represent a sedimentary cycle, then these data suggest that 8.5 Interglacial loess in Lanzhou and Linxia, and discovered that during sedimentary cycles occurred during this stage (because the MIS 5e, there occurred three events in which the East Asian bottom of segment 84S accounts for only half a cycle; Fig. 2). For summer monsoon suddenly strengthened and two events in which convenience, the palaeo-mobile dune sands, palaeo-fixed to semi- the winter monsoon strengthened. fixed dune sands, fluvial facies, lacustrine facies, and palaeosols The Salawusu River valley, located south of China’s Mu Us Desert are labelled in this paper as D, FD, FL, LS, and S, respectively, in in a low-lying part of the southeastern , provides Fig. 2. extensive geological information on environmental variations during the Late Quaternary. Li et al. (2000, 2005) reported evidence 3. Materials and methods of kiloyear-scale climate fluctuations in segment 5 of the Milang- gouwan stratigraphical section that corresponded to MIS 5 segment Geochemical analyses of 105 samples taken at 15 cm intervals of the Last Interglacial in the Late Quaternary, and suggested a close were performed using an X-ray fluorescence spectrometer (Type temporal relationship with the mainland ice-core climate data. 3070, Rigaku International Corp., Tokyo, Japan) in the Cold and Arid Based on these studies, the MGS 5 segment was examined further Regions Environment and Engineering Research Institute of the in search of new clues to the regional palaeoclimate. The relation- Chinese Academy of Sciences, with measuring range from 1 ppm to ship between the East Asian monsoons and evolution of the desert 99.99%. Samples were dried, ground, and sifted through a 200- environment, and the climate formation mechanism, was investi- mesh screen, crushing them into smaller pieces (30 mm in diam- gated by determining ages, and proxy palaeoclimatic indices [i.e., eter) for analysis. The test results were compared to GSD9 grain-size, organic content, SiO2,Al2O3, TOFe (total iron), and and GSS1, which are the national standards, and relative deviation e þ SiO2 (Al2O3 TOFe) ratios]. and relative errors were both calculated to be less than 5%. A total of 310 samples was obtained at intervals of 5 cm (a few 2. Regional setting and study site samples were obtained at 3 or 6 cm intervals) to measure the grain size and organic content. The Malvern Mastersizer 2000 M laser The Milanggouwan section (3745047.200 N, 10833005.400 E) is grain-size analyzer (measuring range 0.02e2000 mm) was used to located on the left bank of the Salawusu River’s middle reaches analyze grain-size because the experiment involved the analysis of (Fig. 1). The MGS 5 stratigraphic segment is a series of sedimentary the loess grain-size (Lu and An, 1997). The data was obtained sequences with depths of 36.95e51.93 m (Fig. 2), with a total of 24 through computer processing.

Fig. 1. Location and geological characteristics of the region surrounding China’s Milanggouwan stratigraphical section. S. Du et al. / Quaternary International 263 (2012) 63e70 65

Fig. 2. Sequence of the last interglacial period and its time-scale in the Milanggouwan stratigraphic section.

The organic matter content was measured using the standard oil including the 62S, 70FD, 76S, 79D, 81D, and 83D strata in MGS 5 bath K2Cr2O7 method (Walkley and Black, 1934) following the and the overlying 60D stratum and underlying 85FD stratum. experimental steps according to the national standard for deter- Quartz grains smaller than 10 mm were collected from the samples. mination of organic matter content of soil (GB 9834-88). The The TL age determination was conducted by Liangcai Lu using detection limit of this measurement is 0e8%, and the error is less a Type 711 age determination instrument (Littlemore Scientific than 0.05%. Instrument Corporation, Oxford, UK) in the TL laboratory of the Guangzhou Institute of Geochemistry, Chinese Academy of 4. Results Sciences, whereas OSL was conducted by Hua Zhao using the Daybreak 2200 optically stimulated luminescence instrument 4.1. Chronology (USA) in the OSL laboratory of the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences. To date, 8 thermoluminescence (TL) and 2 optically stimulated The age and the relevant parameters of each sample are listed in luminescence (OSL) dates have been obtained from eight layers, Tables 1 and 2.

Table 1 Results of the thermoluminescence (TL) dating and related parameters for strata in the MGS5 segment.

Horizon and lab record number Depth (m) U (ppm) Th (ppm) K (%) Annual dose (m Gy) Total dose (Gy) TL age (kyr BP) 60 D-TGD 784 36.95 0.11 0.01 10.06 0.20 2.10 0.02 2.48 0.08 186.20 13.96 75.08 7.40 62 S-TGD 785 37.60 1.07 0.05 12.09 0.24 2.03 0.016 2.31 0.07 190.00 14.25 82.25 8.50 70FD-TGD 786 42.75 0.75 0.04 6.50 0.13 1.99 0.016 2.02 0.06 193.80 14.53 95.90 9.46 76S-TGD 794 46.37 1.49 0.08 12.29 0.25 2.13 0.017 2.65 0.08 288.80 23.10 109.00 12.00 81D-TGD 787 48.47 0.42 0.02 11.04 0.22 1.94 0.016 2.27 0.07 266.76 20.00 117.50 11.70 83 DT-TGD 788 49.74 1.59 0.08 6.01 0.12 1.97 0.016 2.32 0.07 287.28 21.54 123.83 12.00 83 DM-XAT 601 50.29 1.50 0.04 6.10 0.15 1.94 0.018 2.66 0.10 332.27 35.56 124.94 15.84 85 FD-TGD 604 54.18 0.62 0.03 6.94 0.14 2.42 0.019 4.36 0.13 646.80 40.75 148.00 12.50

Table 2 Results of the optically stimulated luminescence (OSL) dating and related parameters in the MGS5 segment.

Horizon and lab record number Depth/m M/10 6 Th/10 6 K/% Total dose /E.D (Gy) Annual dose/(m Gy) Water content/% OSL/ka BP 62S-09G288 37.90 1.91 9.26 1.92 300.21 12.25 3.58 12.79 83.9 4.6 79D-09G289 47.44 2.13 8.93 2.05 4265 10.88 3.74 0.65 114.0 5.4 66 S. Du et al. / Quaternary International 263 (2012) 63e70

There is a well-defined functional relationship between the ages and depth of the samples (Fig. 2), which is evident from the fact that the linear regression correlation coefficient (r) is 0.9865. The sample ages obtained from the data and the segmentation sedi- Cycle No. mentation rates (Fig. 2) were used to determine the MGS 5 time- scale. The dating results show that the age of the bottom strata of 60D, which belongs to MGS 4 and overlies MGS 5, is 75.08 7.40 ka BP. Climate events No. This corresponds to the age of the boundary between MGS 4 and MGS 5, which is also the interface between 60D and 61LS. The w 134 W9 8.5 w 128 C8 w 122 W8 8 w 118 C7 lower limit of the sequence, at the interface between 84S and 85FD, w 117 W7 7 128 122 118 117 114 corresponds to the boundary between MGS 5 and MGS 6: however, Cycle age (ka BP) direct age data for this interface is not available at present. On the basis of TL dating of the strata that overlie and underlie the 84S, the age of 84S is estimated to be between 148 and 125 ka BP. Addi- tionally, assuming an average sedimentary rate of 16.87 cm/ka 84S 82LS 83D 81D during this period, the age of the interface is estimated to be Sedimentary cycle 80LS 134.7 ka BP. Thus, the ages within MGS 5 are consistent with the period from 132 to 73 ka BP in MIS 5 (Martinson et al., 1987), which “ ” 134.7 is generally referred to as the Last Interglacial . w To compare the climatic changes in MGS5 with the division 5e 115.1 scheme in the GRIP ice cores and the MIS 5 marine cores, MGS 5 age (ka BP) was divided into five stratigraphic segments on the basis of the known ages and the ages calculated using regression lines for sedimentation rates (Fig. 2). Then, each subsegment was dated on the basis of different sedimentation rates and calculated the age Cycle No. Subsegment limits for each stratum by means of linear interpolation (Table 3). The results reveal 8.5 sedimentary cycles during MGS 5 with fluc- tuations on a kiloyear-scale. Climate events No.

4.2. Proxy palaeoclimatic indices w 114 C6 w 113 W6 6 w 110 W5 5 w 111 C5 w 105 C4 Analysis of the grain size [Mz (f)] of MGS 5 was conducted using w 102 W4 4 113 111 110 102 100 the formula developed by Folk and Ward (1957): Cycle age (ka BP) Mz ¼ (f16 þ f50 þ f84)/3. The results of the analysis show that the Mz of the grain size in the MGS 5 ranges from 2.07 to 5.76f, where f the grain size of the dune sands ranges from 2.07 to 3.66 ,aver- 76S 105 aging 2.56f; the fluviolacustrine ranges from 2.07 to 4.554,aver- e 74S 79D 78LS 77D 73D 72FL aging 3.55f; and the palaeosol ranges from 2.86 to 5.76f, averaging Sedimentary cycle 4.17f. The analysis of the grain content (<50 mm), which includes silt and clay, to obtain the grain size distribution of MGS 5 indicates w 115.1 that the grain content changed to withstand the intensity of the w 110.4 East Asian winter and the summer monsoons. A sediment 5d 110.4 5c 100.2 composition with primarily coarser sand and a low grain content age (ka BP) (<50 mm) indicates a dry, cold, and windy climate at the time of its formation, whereas a sediment composition with primarily silt and clay indicates a mildly windy, warm, and relatively wet climate. The Cycle No. Subsegment trend of grain content (<50 mm) is similar to that of the Mz (f)in the MGS 5: a valley in the dune sands and a peak in the overlying fluviolacustrine or/and palaeosol. Therefore, the grain size of the dune sands and the overlying fluviolacustrine or/and the palaeosol Climate events No. ranges from coarse to fine, creating a sedimentary cycle of changing coarseness. The organic content in MGS 5 ranges from 0.023% to 0.567%, w 88 W2 2 w 100 C3 w 92 W3 3 w 91 C2 w 84 C1 w 83 W1 1 91 88 83 with an average of 0.122%, and the trend of organic content is Cycle age (ka BP) consistent with that of the grain content (<50 mm) (Fig. 3), with low content in the dune sands and high content in the overlying flu-

violacustrine or/and the palaeosol, creating an organic content 66LS 84 71D 92 e change cycle. e 69D 68LS 64LS 67FD 63FD Sedimentary cycle Analysis of the major elements, SiO2,Al2O3, TOFe, and the 61LS,62S 75 SiO2e(Al2O3 þ TOFe) ratios of the MGS 5 segment is shown in Table 4. SiO2 is the major constituent of the samples, ranging from

49.78% to 88.58% with an average of 74.68%. The second most w 89.3 w 100.2 abundant component is Al2O3, ranging from 4.75% to 12.68% with 5b 89.3 Subsegment age (ka BP) 5a 75.08

an average of 8.58%. TOFe ranges from 1.51% to 13.34% with an Table 3 Ages of 5 subsegments, 8.5 sedimentary cycles and the climate events record in the MGS5 segment. S. Du et al. / Quaternary International 263 (2012) 63e70 67

Fig. 3. Sedimentary cycles records by changes curves of grain content (<50 mm), organic content, SiO2,Al2O3, TOFe and SiO2e(Al2O3 þ TOFe) ratios in the MGS5 segment.

Table 4

Range and average of grain content (<50 mm) (%), organic content (%), SiO2 (%), Al2O3 (%), TOFe (%) and SiO2e(Al2O3 þ TOFe) ratios of different sedimentary facies in the MGS5 segment.

Sediment type Grain content (<50 mm) (%) Organic content (%) SiO2 (%) Al2O3 (%) TOFe (%) SiO2e(Al2O3 þ TOFe) ratios Dune sands Range 0e26.43 0.023e0.256 75.0e88.58 5.56e9.28 1.53e2.58 6.54e11.61 Average 3.76 0.082 80.37 7.65 1.97 8.52

Fluviolacustrine facies Range 0e55.66 0.062e0.465 49.78e83.46 4.75e10.54 1.51e13.34 3.39e8.18 Average 26.73 0.136 71.96 8.82 3.07 5.66

Paleosols Range 13.02e85.93 0.101e0.567 58.23e79.26 7.56e12.68 2.05e4.49 2.62e11.48 Average 43.79 0.19 69.74 9.70 3.08 6.38

average of 2.64%. The curves of SiO2,Al2O3, TOFe, and the SiO2e(Al2O3 þ TOFe) ratios show that SiO2 and SiO2e(Al2O3 þ TOFe) ratios are the major constituents of the dune sands and are relatively scarce in the fluviolacustrine and the palaeosol. On the other hand, Al2O3 and TOFe display an opposite trend, being relatively scarce in the dune sands and major constituents in the overlying fluviolacustrine or/and palaeosol. The grain content (<50 mm), organic content, SiO2,Al2O3, TOFe, and SiO2e(Al2O3 þ TOFe) ratios appear to follow the sedimentary cycle: The grain content (<50 mm), organic content, Al2O3 and TOFe content vary from a low level in dune sands to a high level in the overlying fluviolacustrine or/and palaeosol facies, whereas the SiO2 and SiO2e(Al2O3 þ TOFe) ratios vary from high to low, following a multiple peak-valley alternating irregular curve (Fig. 3).

5. Discussion and conclusions

Based on the field investigation of the strata, the MGS 5 dune sands are distributed evenly, with a structure similar to the modern aeolian sands. If the major chemical elements, i.e., SiO2,AL2O3 and TOFe in the palaeo-dune sands are compared with those in the modern , the comparison would confirm the similarity. According to the analysis of the 16 samples of modern dune sands e þ Fig. 4. Scatter plot of SiO2 Al2O3 TOFe of ancient dune sands in MGS5. obtained from the Mu Us Desert, the contents of SiO2,Al2O3, and 68 S. Du et al. / Quaternary International 263 (2012) 63e70

Fig. 5. Comparison of Milanggouwan records with other records during the last interglacial. a, Percent CaCO3 in V27-116 from locations west-southwest and west of Ireland. (Bond et al., 1992); b, Potou section >40 mm quartz size fraction content (An and Porter, 1997); c, Grain content (<50 mm) of MGS5 segment; d, Sanbo cave speleothem d18O records (Wang et al., 2008); e, NGRIP d18O records (North Greenland Ice Core Project members, 2004); f, NHSI ( summer insolation, mean July insolation) at 65N. DE3 to DE9 indicates six dust events intervals, respectively, recorded in Potou section ;W1eW9, the warm climate events recorded in the MGS5 segment; C1eC8, the cold climate events recorded in the MGS5 segment; A20eA25, the Chinese interstadial events recorded in the Saobao cave speleothem; 20e25, the DansgaardeOeschger events recorded in the GRIP ice cores.

TOFe range from 75.20% to 90.22%, 4.62% to 9.13%, and 1.42% to 81.90%, with an average of 72.18%, and most samples had quartz 2.96%, respectively, averaging 81.55%, 7.85%, and 2.23%. Thus, the contents greater than 60%. The second highest constituent was distributions of the three chemical elements in the palaeo and feldspar. The proportion of feldspar ranged from 5.80 to 23.40% modern dune sands are highly consistent. The scattering plots of with an average of 16.66%, and most samples had feldspar contents SiO2eAl2O3 þ TOFe in Fig. 4 show that the contents of SiO2 and greater than 13%. Although heavy minerals were rare, minerals Al2O3 þ TOFe in palaeo dune sands range between 75% to 88.58% such as common hornblendes, pyroxenes, and epidotes accounted and 7.18% to 11.47%, respectively. These scattering plots are similar for 78.07% of the total of the silicate minerals excluding quartz. to those of the modern dune sands, indicating that the palaeo-dune Analysis of the minerals in Pleistocene palaeo-aeolian dune sands sands were sediments deposited by the ancient desert in a manner (18 samples) showed that detrital minerals had the highest quartz similar to the deposition of dune sands in a modern desert content of 82.10% with the average being higher than 66.00%. The environment. second highest constituent was didymolite (>10%), and the other To illustrate the characteristics of palaeo and modern dune constituents are trace distributions of heavy minerals (<1%), such sands, the composition of both detrital minerals was analyzed. The as common hornblende, pyroxene, and delphinite. results of the analysis of 14 samples of modern aeolian sands show Highly consistent distributions of SiO2,Al2O3, and TOFe contents that the major constituent was quartz, ranging from 51.30 to and that of quartz and the silicate minerals, the source of SiO2 S. Du et al. / Quaternary International 263 (2012) 63e70 69 content in both sands, in the current and MGS 5 ancient aeolian strengthening formed fluviolacustrine facies and palaeosols, and sands suggested that these palaeo dune sands were aeolian sands, winter monsoon strengthening developed dune sands. and the sedimentary environment under which the palaeo-aeolian Both fluviolacustrine facies and palaeosols correspond to rela- dune sands were deposited was similar to that under which the tively warm and wet climates. The difference between them is the modern sands are deposited. This indicates that there were many sedimentary site. Fluviolacustrine facies and palaeosols show facies periods in the past when the East Asian winter monsoon was change frequently. The lakes at that time were shallow and small, prevailing. The climate conditions at that time can be estimated allowing fluviolacustrine sediments to be deposited, while palae- from the climate conditions during the winter and spring monsoon osols formed on the exposed areas between the lakes. in the modern Mu Us Desert or the strong wind and sand activity in The kiloyear-scale climate events due to the winter and summer the Ordos Plateau. This was the only well-developed climatic monsoons in the Milanggouwan section during the last interglacial situation during the last interglacial in terms of periodic activities are listed in Table 3, with the events dominated by the winter of wind and sand, which were caused by the repetition of climatic monsoon abbreviated as C1eC8, and the events dominated by the environments. In contrast, a strong summer monsoon prevailed summer monsoon abbreviated as W1eW9. Such climatic fluctua- over the Mu Us Desert during the summer and autumn, resulting in tions in MGS5 subsection appeared 17 times, forming 8.5 climate a warm and wet climate, which led to the formation of rivers and cycles. The time sequence of variations in the East Asian winter and lakes and fixed the positions of mobile dunes. Thus, the decrease in summer monsoons in the MGS 5a-5d subsegments not only the SiO2 content and a corresponding increase in the Al2O3 and correlates well with the DansgaardeOeschger climate variations TOFe content in MGS 5 fluviolacustrine facies and palaeosols bore obtained from the analysis of ice-core oxygen isotope contents at a strong relationship with the strong and repeated summer high latitudes (NGICP members, 2004), but also is in accord with monsoon activity at the time of their deposition. During the period short climate events recorded by speleothems in China (Wang et al., of dominance by the summer monsoon, environmental conditions 2008)(Fig. 5). The layers that formed mainly under the influence of limited the amount of windblown sand, thereby decreasing the the summer monsoon, W1eW6, correspond to the DO20e25 supply of SiO2 and increasing the dissolution and migration of sequence recorded by the NGRIP ice cores and A20e25 registered soluble elements, creating conditions favourable for the accumu- by Sanbo cave speleothem in Hubei province. C1eC6 correspond to lation of the relatively stable Al2O3 and TOFe. This hypothesis stadials (Fig. 5). There are some climate fluctuations in MGS 5e, explains the changes in the SiO2,Al2O3, and TOFe contents and the which can been seen as three peaks (W7, W8, W9) and two valleys SiO2e(Al2O3 þ TOFe) ratios in the dune sands and fluviolacustrine (C7, C8) in Fig. 5c. Currently, the nature of climate in the Eemian facies and palaeosols of MGS 5. The results also explain the distri- warm period (MIS 5e) is controversial, because the Eemian recor- bution curves of SiO2,Al2O3, and TOFe contents in each layer, ded in the NGRIP ice cores (NGRIP members, 2004) and stalagmites showing the growth and the corresponding decline. (Wang et al., 2008) was relatively stable (Fig. 5d, e), but that Therefore, the dune sands layer can be considered the product recorded in North Atlantic pelagic deposits (Fig. 5a) and the Loess of East Asian winter monsoon, while the fluviolacustrine facies Plateau (Fig. 5b) was a period with climate fluctuations. Significant and palaeosols can be regarded as the products of East Asian fluctuations of 5e have been recorded in North Atlantic areas, for summer monsoon, indicating a relatively warm and wet climate. example in core V28e56 (Kellogg, 1977) and V27e116 (Bond et al., This is why MGS5a, 5c and 5e developed more fluviolacustrine 1992). Similarly, the record in the Loess Plateau as dust accumu- facies and palaeosols, and MGS5b and 5d are characterized by lation events DE3eDE9 (An and Porter, 1997), with 5e divided by dune sands, consistent with the MIS5a-5e climates (GRIP, 1993). DE8 and DE9, correspond well to MGS5e (Fig. 5). Thus, the The grain content (<50 mm), organic content, Al2O3,andTOFein millennium-scale climate fluctuations indicated in the MGS 5e dune sands change from low to high while SiO2 and confirm the instability in East Asian monsoon climate during the SiO2e(Al2O3 þ TOFe) ratios change from high to low. This indicates Eemian warm period. climate cycles in which dominance by the East Asian winter The 65N curve corresponding to the Last Interglacial in Fig. 5 monsoon was followed by summer monsoon predominance. The consists of three peaks separated by two valleys (Berger, 1978; Salawusu River valley and the Mu Us Desert have experienced Berger and Loutre, 1991). The ages of the peaks are 81 ka, 103 ka, eight winter monsoon and nine summer monsoon climate fluc- and 126 ka, and the ages of the valleys are 93 ka and 114 ka. The tuations during the Last Interglacial, forming 8.5 winter and warm peaks of grain content (<50 mm) corresponded with the five summer monsoon alternating cycles, which indicate high reso- peaks in the 65N curve, and the cold valleys of grain content lution kiloyear-scale climatic cycles. (<50 mm) also corresponded to two valleys of the 65N curve. A period of summer monsoon strengthening does not neces- Changes in Earth’s orbit on a 10 ka-scale ultimately determined the sarily mean that local climate was dominated by the summer strength of the East Asian winter and summer monsoons, and monsoon: during this period the winter monsoon was weaker than triggered the evolution of the desert environment. Additionally, the summer monsoon. In spring and winter, the local accumulated 9 warm events (peaks W1eW9) and 8 cold events (valleys C1eC8) dune sands weathered at the same time. Once the stronger summer recorded in MGS 5 overlapped with the orbital periods, where most monsoon came, the warm and wet climate made dune sand peaks and valleys correspond to the D/O cycles recorded using pedogenesis greater than accumulation. Over a long time, local oxygen isotopes in the North Atlantic deep-sea sediments and the development of palaeosols resulted. Similarly, a period of winter Greenland ice cores. This suggests that the alternation between the monsoon strengthening does not imply a dominance of the local desert periods and the inter-desert periods that formed mainly climate by winter monsoon. During winter monsoon strengthening under the influence of the winter and summer monsoons, as when the summer monsoon was weaker, dune sands accumulated. indicated in the MGS 5 segment, probably resulted from the global Nevertheless, some calcareous plates can be found under certain climate changes during those periods. The monsoon environment dune sand strata, which may be due to summer monsoon rain. In changes were vitally dependent on the sea ice of the Polar-North Fig. 3, it can be seen that the whole trend of 84S represents the peak Atlantic (Li et al., 1998). Therefore, the ice coverage and tempera- value, but fluctuation with low values may indicate the winter ture in Eurasia changed with the movement of the North Pole front monsoon. The trend for 83D is a trough with some crests. Even- and thereby affected the strength of the Siberian-Mongolian high tually, the results of lithological characters and sedimentary facies pressure region due to the cold air mass (An et al., 1995). Keeping can been seen in the field, a period of summer monsoon this in mind, the monsoon cycles that corresponded to D/O events, 70 S. Du et al. / Quaternary International 263 (2012) 63e70 as indicated by grain content (<50 mm) in the MGS5 segment Gapers, G., Merke, J., Müller, H., 2002. The Eemian interglaciation in Northwestern e during the Last Interglacial, imply that climate variations in the Germany. Quaternary Research 58, 49 52. Greenland Ice-cord Project Members, 1993. Climate instability during the last North Atlantic and Greenland were related. Therefore, it is clear interglacial period recorded in the GRIP ice-cord. Nature 364, 203e207. from the comparison between the valleys C1eC8 in the MGS5 Grootes, P.M., Stuiver, M., White, J.W.C., Johnsen, S., Jouzel, J., 1993. Comparison of segment and DE3 to DE9 in the Loess Plateau that the six oxygenisotope records form the GISP2 and GRIP Greenland ice cores. Nature 366, 552e554. millennium-scale dust events were consistent with six cool events Guan, Q.Y., Pan, B.T., Gao, H.S., Li, P.Y., Wang, J.P., Su, H., 2007. Instability charac- (An et al., 1991), as confirmed by the high content of foraminifera teristics of the East Asian Monsoon recorded by high- resolution loess sections Neogloboquadrina pachyderma (S.) in drill holes V29 to V191 from the last interglacial (MIS5). Science in China, Series D 50, 1067e1075. Heusser, L., Oppo, D., 2003. Millennial-and orbital-scale climate variability in (McManus et al., 1994) in the North Atlantic. Southeastern United States and in the subtropical Atlantic during Marine Isotope Stage 5: evidence from pollen and isotope from ODP site 1059. Earth and Planetary Science Letters 214, 483e490. Acknowledgements Kellogg, T.B., 1977. Paleoclimatology and paleo-oceanography of the Norwegian and Greenland Seas: the last 450,000 years. Marine Micropaleontology 2, 235e249. This paper was funded by National Basic Research Program of Li, B.S., Zhang, D.D., Jin, H.L., Wu, Z., Yan, M.C., Sun, W., Zhu, Y.Z., Sun, D.H., 2000. Palaeo-Monsoon activities of Mu Us Desert, China since 150ka-A study of the China, Grant Number: 2010CB833405; National Natural Science stratigraphic sequences of the Milanggouwan section, Salawusu River area. Foundation of China, Grant Number: 40772118, 49971009; Palaeogeography, Palaeoclimatology, Palaeoecology 162, 1e16. Research Grants Council Grant of the Hong Kong Special Adminis- Li, B.S., Zhang, D.D., Wen, X.H., Dong, Y.X., Zhu, Y.Z., Jin, H.L., 2005. A multi-cycles of fl trative Region, Grant Number: HKU 7243/04H. We thank Jingzhao climatic uctuation in the last Interglacial period: typical stratigraphic section in the Salawusu River Valley on the Ordos Plateau, China. 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