ARTICLE IN PRESS. Quaternary Science Reviews 26 (2007)

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1 Quaternary Science Reviews 6 (7) Phytoliths as quantitative indicators for the reconstruction of past environmental conditions in China II: palaeoenvironmental reconstruction in the Loess Plateau Hou-Yuan Lu a,, Nai-Qin Wu a, Kam-Biu Liu b, Hui Jiang c, Tung-Sheng Liu a a Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 19, China b Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 783, USA c State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 6, China Received 9 April 6; received in revised form September 6; accepted 13 October 6 Abstract Quantitative reconstruction of the climatic history of the Chinese Loess Plateau is important for understanding present and past environment and climate changes in the Northern Hemisphere. Here, we reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) trends during the last 136 ka based on the analysis of phytoliths from the Weinan loess section (3414 N, 1913 E) near the southern part of the Loess Plateau in northern China. The reconstructions have been carried out using a Chinese phytolith climate calibration model based on weighted averaging partial least-squares regression. A series of cold and dry events, as indicated by the reconstructed MAT and MAP, are documented in the loess during the last glacial periods, which can be temporally correlated with the North Atlantic Heinrich events. Our MAT and MAP estimations show that the coldest and/or driest period occurred at the upper part of L unit (Late MIS 6), where MAT dropped to ca 4.4 1C and MAP to ca 1 mm. Two other prominent cold-dry periods occurred at lower Ll-5 (ca 77 6 ka) and L1-1 (ca ka) where the MAT and MAP decreased to about C and mm, respectively, ca C and 4 mm lower than today. However, the highest MAT (average C, max C) and MAP (average 757 mm, max. 1 mm) occurred at Sl interval (MIS 5). During the interstadial of L1-4 L1- (MIS 3) and during the Holocene warm-wet period, the MAT was about 1 1C and MAP 1 15 mm higher than today in the Weinan region. The well-dated MAT and MAP reconstructions from the Chinese Loess Plateau presented in this paper are the first quantitatively reconstructed proxy record of climatic changes at the glacial interglacial timescale that is based on phytolith data. This study also reveals a causal link between climatic instability in the Atlantic Ocean and climate variability in the Chinese Loess Plateau. r 6 Elsevier Ltd. All rights reserved. 1. Introduction Understanding long-term climatic change requires consideration of spatial as well as temporal aspects of climate dynamics (Cheddadi et al., 1998; Rousseau and Kukla, ; Ding et al., ). Therefore, accurate reconstructions of regional temperature and precipitation patterns are needed in order to trace the response of regional environment to climatic changes and to assess global Corresponding author. Tel.: ; fax: address: houyuanlu@mail.iggcas.ac.cn (H.-Y. Lu). circulation models on a regional scale (COHMAP Members, 1988; Seppa et al., 4). The loess palaeosol record in northern China is one of the most significant natural archives of environmental changes in the continental warm temperate and semihumid areas. The loess section is more than 3 m thick in some places, consisting of a sequence of loess and palaeosol layers that records climate variations over the last.6 Ma (Heller and Liu, 198; Liu, 1985; Kukla and An, 1989; An et al., 199, 1991; Rutter, 199; Ding et al., 1994, 1995, ; Xiao et al., 1995; Guo et al., 1996, 1998; An and Porter, 1997; Sun et al., 1997; Liu and Ding, 1998; Rousseau et al., ; Porter, 1). Reconstructions of palaeoclimate in the Loess Plateau have long been based /$ - see front matter r 6 Elsevier Ltd. All rights reserved. doi:1.116/j.quascirev.6.1.6

2 76 ARTICLE IN PRESS H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) primarily on traditional physical and geochemical methods such as magnetic susceptibility (MS) (Heller and Liu, 198; An et al., 1991; Lu et al., 1994; Liu et al., 1995b; Maher and Thompson, 1995), grain size (Ding et al., 1994, 1995, ), isotopes (Gu et al., 1996, 3; Ding and Yang, ; Jiang et al., ; Vidic and Montanez, 4), and weathering index (Guo et al., 1996, 1b). In order to quantitatively reconstruct past temperature and precipitation patterns, several researchers have attempted to extend the use of magnetic measurements as a quantitative proxy for reconstructing palaeoprecipitation (e.g. Lu et al., 1994; Maher et al., 1994; Liu et al., 1995b; Maher and Thompson, 1995; Porter et al., 1). Such reconstruction is based on observations of statistically significant relationships between magnetic properties, principally MS, of modern Loess Plateau soils and modern climatic parameters. There is a broad agreement among various magnetic-based palaeoprecipitation estimates for the last glacial interglacial cycle in the central part of the Loess Plateau (Evans and Heller, 1). However, several workers have reported inconsistencies between the magnitude of coeval MS peaks at sites in the northern part (with a drier modern climate) and the southern part (more humid) of the Loess Plateau (Guo et al., 1a; Bloemendal and Liu, 5). In addition, Lu et al. (1994) and Han et al. (1996) noted distinctly non-linear relationships between climatic parameters and MS signals in a large set of modern Chinese soils. Pollen-analytical results are commonly interpreted in a more qualitative and descriptive way. Recently, several numerical inference models have been developed that enable quantitative climate reconstructions from fossil pollen assemblages (e.g. Overpeck et al., 1985; Guiot et al., 1993; Davis, et al., 3; Seppa et al., 4; Rousseau et al., 6). However, because of the difficulty of extracting sufficient numbers of pollen grains from loess deposits, palynology has contributed very little to the quantitative reconstruction of the vegetation and climate history of the Loess Plateau (Sun et al., 1997). However, the accuracy of precipitation and temperature reconstructions using other quantitative proxies still needs to be tested for the last glacial interglacial cycle in the Loess Plateau. Here, we demonstrate the usefulness of phytoliths as such a palaeoclimatic proxy. Phytoliths are microscopic silica bodies that are produced in the tissue of grasses and some other plants (Piperno, 1988, 6; Mulholland, 1989; Wang and Lu, 1993; Alexandre et al., 1997; Fredlund and Tieszen, 1997; Runge, 1999; Pearsall, ), and they occur abundantly in loess palaeosol sediments as relicts of previous vegetation communities (Liu et al., 1996; Lu et al., 1996; Madella, 1997; Blinnikov et al., ). Among the many proxy indicators of loess palaeosol environments, sedimentary remains of phytolith are emerging as a promising source of information to quantify past climate changes (Wu et al., 1995b; Lu et al., 1996). The aim of the present study is to reconstruct temperature and precipitation variations at the glacial interglacial timescale using quantitative phytoliths inference models (Lu et al., 6) and to document the climatic oscillations during the last 136, years in the southeastern part of the Loess Plateau.. Study sites The Weinan loess section (3414 N, 1913 E) is located in the southern part of the Loess Plateau, about 55 km east of Xi an city (Fig. 1). This loess section has been studied in detail (Liu et al., 1994; Guo et al., 1994, 1996, 1998; Zhu et al., 1994; Ding et al., 1995; Liu et al., 1995a; Wu et al., 1995a, b, 1999, ; Gu et al., 1996, 1997; Sun et al., 1997; Liu and Ding, 1998; Diao and Wen, 1999; Lu et al., 1999; Wang et al.,, 5). The climate is continental warm temperate and semi-humid, with large seasonal differences in temperature and precipitation (Qian, 1991). The study area has a mean annual precipitation (MAP) of mm (AD , min. 317, max. 93 mm) and mean annual temperature (MAT) of C (AD , min. 11 1C, max C) (calculated from the modern climatic database maintained by the China Meteorological Administration). Although this area is situated in the warm-temperate forest zone (Wu, 198), no natural forests remain. Most of the land cover over the Loess Plateau has been converted to cultivated fields, primarily for wheat and maize. Field edges and slopes are covered by herbs, such as Carex lanceolata, Stipa bungeana, Cleistogenes chinensis and Dendranthema indicum, with local presence of Selaginella sinensis (Sun et al., 1997). In the Loess Plateau, the MAT generally decreases from 16 1C in the south to 4 1C in the north (Fig. a). Rainfall decreases from SE to NW along a gradient of decreasing influence from the Southeast Summer Monsoon (Sun et al., 1997). In the south of the Plateau along the Qinling Mountains MAP is approximately 8 mm, while in the north, MAP is less than 15 mm (Fig. b). 3. Materials and methods The Weinan section contains 1.8 m of loess palaeosol sequences from the L-1 loess to the Holocene palaeosol. According to thermoluminescence (TL) and AMS 14 C ages and sedimentation characteristics (Liu et al., 1994; Guo et al., 1996; Wang et al., ), the stratigraphy from the surface to base is as follows: (1) S, Holocene palaeosol, 1.43 m, brown to dark brown silt. () L1-1 (marine isotope stage (MIS) ), loess, m, brown silt. (3) L1- (upper MIS 3), weakly pedogenic palaeosol, m, brown to dark-brown silty clay or clayey silt. (4) L1-3 (middle MIS 3), loess, m, brown clayey silt. (5) L1-4 (lower MIS 3), weakly pedogenic palaeosol, m, silty clay or clayey silt. (6) L1-5 (MIS 4), loess, m, brown silt. (7) S1 (MIS 5a/5e), palaeosol, m, dark brown silty clay or clayey silt and dark red-brown mud. (8) L (MIS 6), loess, m, brown silt (Guo et al., 1996; Sun et al., 1997).

3 H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) o E 11 o E 8 o 1 o 1 o 5 o 4 o Yellow River 3 o 4 o N Beijing 9 o 1 o 11 o 1 o MU US Desert Taiyuan Xining LOESS PLATEAU Lanzhou Luochuan 35 o N 35 o N Yellow River Xi an Weinan Zhengzhou 1 o E 11 o E Major Loess Plateau Desert Sample location Principal cities Fig. 1. Sketch map showing the Loess Plateau and location of the Weinan section. A total of 159 samples, each weighing about 5 g, were collected for phytolith analysis from a depth of 34 cm to a depth of 18 cm. The sequence was cleaned before the sampling to prevent any pollution. No samples were taken from the top 34 cm due to disturbances from human activity. The upper loess palaeosol unit from 34 to cm was sampled at 4 cm intervals, and the lower loess palaeosol unit from to 18 cm was sampled at 1 cm intervals. An age model was developed using polynomial regression on the absolute dating results (Guo et al., 1996), using both TL and AMS 14 C ages. According to the sedimentation rate calculated from the age model, the time resolution was approximately 4775 years per sample during the Holocene and about years per sample during the ka interval. The samples were prepared by a procedure slightly modified from Piperno (1988) and Runge (1999). It consists of sodium pyrophosphate (Na 4 P O 7 ) deflocculation, treatment with 3% hydrogen peroxide (H O ) and cold 15% hydrochloric acid (HCl), zinc bromide (ZnBr, density.35 g/cm 3 ) heavy liquid separation, and mounting on a slide with Canada Balsam. Phytolith counting and identification were performed using a Leica microscope with phase-contrast at 4 magnification. More than 35 phytolith grains were counted in each sample. Phytolith abundance was expressed as percentages of all phytoliths counted. Identification was aided by the use of reference materials (Lu, 1998; Lu and Liu, 3a, b, 5; Lu et al., 6) and published keys (Piperno, 1988; Mulholland and Rapp, 199; Kondo et al., 1994; Piperno and Pearsall, 1998; Runge, 1999; Pearsall, ). In this study, phytoliths were divided into 1 types, according to the classification systems of Lu et al. (6) which are modified from the classification of Twiss et al. (1969), Wang and Lu (1993) and Kondo et al. (1994), as follows: (1) Pteridophyte types; () Gymnosperm types; (3) broad-leaf-type; (4) palmaceae phytolith; (5) panicoid (dumbbell and cross); (6) long saddle; (7) short saddle; (8) wavy-trapezoid; (9) wavy-narrow-trapezoid; (1) rondel; (11) fan-bamb; (1) fan-reed; (13) fan; (14) square; (15) rectangle; (16) board-elongate; (17) sinuate-elongate; (18) smooth-elongate; (19) long-point; () short point; (1) gobbet (nubby-irregular shape) (Fig. 3) (Lu et al., 6, Fig. 3). Quantitative inferences of temperature and precipitation were based on a surface sediment calibration set of 43 sites distributed along broad ecological and climatic gradients over China (Lu et al., 6). The environmental variable

4 76 ARTICLE IN PRESS H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) o 1 o 1 o 5 o MAT. The WA-PLS is a robust calibration method for quantitative palaeoenvironmental reconstruction based on phytolith data (Lu et al., 6). 4 o N 3 o N b 4 o N 3 o N o 1 o 11 o 1 o 4 8 o Xining Lanzhou Xi an Weinan 1 o E 11 o E 1 o 1 o E Xining 1 o o 1 o 11 o 1 o Sample location 3 4 Taiyuan Yellow accounting for the greatest amount of variation in the distribution of phytolith types among the surface sediments was MAP, followed by MAT. A unimodal response model, based on weighted averaging partial least squares regression and calibration (WA-PLS) (ter Braak and Juggins, 1993), was used for inferring the MAP (R -boot ¼.9, rootmean-square-error of prediction (RMSEP) ¼ 148 mm) and MAT (R -boot ¼.84, RMSEP ¼.5 1C) using C version 1.3 (Juggins, 3). These results confirm that phytoliths can provide reliable and robust estimates of the MAP and 4 o 3 o o 4 o 3 o 1 Lanzhou Sample location Xi an 4 River Zhengzhou Principal cities 16 5 Taiyuan Weinan Zhengzhou 8 11 o E 1 Principal cities Fig.. (a) Map of isotherms showing mean annual temperature (1C) in the Loess Plateau, China (1961 ); (b) Map of isohyets showing mean annual precipitation (mm) in the Loess Plateau, China (1961 ). Yellow River 4. Results The phytolith morpho-types and their abundance variations against depth are summarized in Fig. 4 and the palaeoprecipitation and palaeotemperature reconstructions are given in Fig. 5. The phytolith assemblages have been divided into six major zones (period) according to the variations in main phytolith types. The major phytolith types include the wavy-narrow-trapezoid, rondel, fan, square, rectangle, smooth-elongate, Long-point, short point, and gobbet. These six major zones are marked on the right-hand side of the diagram (Figs. 4 and 5) P6 zone (upper L and lower S1, m; ka) The basal phytolith assemblages are dominated by Gobbet type ( %), with low percentages of square, smooth-elongate, and short point types. Trapezoid and rondel types are present but infrequent in this zone (Fig. 4). At present, gobbet type (nubby-irregular shape), mainly from Chenopodiaceae taxa (Lu et al., 6), is very abundant in the surface soils of arid-cold areas of northwest China (Wang and Lu, 1993). Reconstructed MAT and MAP are C and mostly o15 mm, respectively, during this period (Fig. 5). 4.. P5 zone (middle-upper S1, m; ka) A gradual decline in the Gobbet type and a rapid rise in the square, fan, rectangle, and smooth-elongate phytoliths define this zone. The square ( %) and rectangle ( %) types peak in this zone. The fan type is also common ( %) (Fig. 4). In modern soil samples collected by the authors from different vegetation regions across China, the fan and square types are the most important and abundant types in warm-wet regions, reaching maximum abundance in southern China (Lu et al., 6). In the upper S1 ( m), pollen records document a greater proportion of herbaceous pollen, such as Gramineae, Chenopodiaceae, Cyperaceae, Leguminosae, and Artemisia, but there is no singularly dominant pollen type (Sun et al., 1997). Tree pollen percentages and influx are comparatively high, especially between 9.5 and 9.3 m (Sun et al., 1997). Sun et al. (1997) suggested that the comparatively high pollen influx during this period indicated that the vegetation was richer than the preceding zone and indicates a mild climate and higher precipitation than today. During the period ka, the reconstructed MAT and MAP were slightly higher, reaching from 7.3 to 15 1C and from 3 to 83 mm, respectively. There was a short cooling at 11.6 ka (one sample only), which rapidly

5 H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) Fig. 3. Some representative phytolith types from the Weinan loess section, China. (a, b) Short saddle; (c) wavy-trapezoid; (d) panicoid (dumbbell); (e) broad-leaf-type; (f, g) wavy-narrow-trapezoid; (h) Pteridophyte types; I sinuate-elongate; (j) rondel; (k) long-point; (l) short point; (m) fan-reed; (n) fan; (o) square; (p) gobbet (nubby-irregular shape); (q) smooth-elongate. returned to a MAT of 16 1C ( C) and a MAP of 48 mm ( mm) between 11 and 77 ka (Fig. 5) P4 zone (L1-5, m; ka) The Gobbet type also dominates this zone, except for a short decline in the middle of the zone ( m; 6 54 ka). Compared with the last zone, this zone is marked by lower percentages of fan (1.74.4%), square ( %), rectangle ( %), and smooth-elongate ( %) types, but in the middle of the zone ( m), the fan type increases to its maximum (3.5%) and other phytolith types such as square, rectangle, point and rondel are also abundant. From 77 to 6 ka the reconstructed MAT and MAP are lowered by about 5 1 1C and 4 6 mm than in the previous zone (Fig. 5). During this period, the MAT and MAP also fluctuated frequently and significantly, with two marked drying and cooling periods at 7 ka (6.1 1C, 13 mm) and 65 ka (6.6 1C, 48 mm), respectively. Both MAT and MAP increased again to C and mm, respectively, between 6 and 54 ka, with a prominent warm-wet stage around 61 ka. MAT and MAP decreased abruptly between 54 and 48 ka. The changes in

6 764 ARTICLE IN PRESS H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) Depth cm Zone MIS Pteridophyte Gymnosperm Broad-leaf-type Dumbbell Long-saddle Short-saddle Wavy-trapezoid Wavy-naarow Rondel Fan-bamboo Fan-reed Fan Square Rectangle Board-elongate Sinuate-elongate Smooth-elongate Long-point Short-point Gobbet 1 S p1 1 L1-1 3 p 4 5 L1- L1-3 p3 3 6 L1-4 7 L1-5 8 p S1 p L p (%) Fig. 4. Phytolith diagram from the Weinan section, Shanxi Province, China (1913 E, 3414 N, 65 m a.s.l.).

7 H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) Age ka MAP mm the reconstructed MAT and MAP are generally consistent with those inferred from the pollen data (Sun et al., 1997) P3 zone (L1-4 L1-, m; 46 3 ka) The phytolith assemblages in this zone are marked by a gradual decline in Square, Short point, and Gobbet types. Smooth-elongate and long-point phytoliths increased significantly during 3 3 ka. Percentages of board-elongate, fan, and rectangle types fluctuate somewhat but are generally high throughout this zone. The fan-reed phytoliths increased distinctly after 3 ka. Higher MAT ( C) and MAP ( mm) values generally occurred during ka, though a period of lower values is present from 4 to 38 ka. After 34 ka, MAT and MAP progressively increase, though interrupted by several lower values at 3.4, 9.4, 7.3, and 3.5 ka P zone (L1-1, m; ka) The lower and middle parts of this zone are characterized by maximum values of smooth-elongate ( %) and long-point (11.77.%) phytoliths. Wavy-narrow and rondel types rapidly increase at ca 17 ka. A gradual decline in square and rectangle types and a notable rise in the gobbet type define the middle part of this zone. Dumbbell and sinuate-elongate types are present continuously in the middle and upper part of this zone. MAT Zone Fig. 5. Time-series of reconstructed mean annual precipitation (MAP) and mean annual temperature (MAT) for the last 136 ka based on the phytolith sequences at the Weinan section. The vertical gray line shows the values of modern annual mean precipitation and temperature at Weinan region. o C p1 p p3 p4 p5 p6 The reconstructed MAT and MAP are mostly o9.5 1C and o55 mm, corresponding to the coldest and driest interval of the Lateglacial Maximum (LGM). The lowest MAT (ca 6.5 1C) and MAP (ca 37 mm) occurred at ka, some 6 7 1C and 5 3 mm lower than the modern observed values in this region P1 zone (L1-1, m; ka) This zone is characterized by an increase in dumbbell, short saddle, square, and rectangle phytoliths. Fan, rondel, smooth-elongate, and long-point are also abundant types. This zone is also characterized by a sharp decline in gobbet type and the appearance of Pteridophyte, broad-leaf, and long saddle types. Because of the low phytolith production of broadleaved deciduous trees, the sporadic appearance of broadleaved deciduous tree and bamboo phytoliths suggests that there were patches of broadleaved deciduous forest containing bamboos growing in a warm-humid environment during some intervals. Comparatively high pollen influx in this zone indicates that the vegetation was more luxuriant than the preceding zone, which is consistent with a milder climate and higher precipitation than at present (Sun et al., 1997). The MAT and MAP at 1,5 75 ka were C (maximum C) and mm (maximum 87 mm), respectively, and then declined slightly after 7.5 ka. The general declining trend was punctuated by several major drops to much drier and colder conditions at ca 6.4, 5., and 3.1 ka. 5. Discussion 5.1. Magnitude of palaeoclimatic changes in the Chinese Loess Plateau Our MAT and MAP reconstructions show that temperature and rainfall were lower during glacial times (L, Ll-5 lower, and L1-1), while higher MAT and MAP occurred during more humid and warmer interglacial (Sl, S) and interstadial (Ll- to L1-4) periods. During the upper L time interval, the MAT and MAP dropped to ca 4.4 1C and ca 15 mm, respectively, some 8.3 1C and 5 mm lower than today (Table 1). This zone corresponds to the interval of lowest weathering intensity (LW-7) at the top of the L loess (Guo et al., 1996). The very low MAT and MAP during the upper L indicate that the East Asian summer monsoon did not control the climate in this area during the coldest stage of the glacial period, as the modern MAP in the northern margin of the Asian summer monsoon is generally 43 4 mm (Qian, 1991). Importantly, the highest MAT (average C, max C) and MAP (average 757 mm, max. 1 mm) occurred during Sl (MIS 5; ka). During 77 6 ka (lower L1-5), the reconstructed MAT and MAP dropped to 1.1 1C and 445 mm respectively, approximately.6 1C

8 766 ARTICLE IN PRESS H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) Table 1 Reconstructed values of mean annual temperature (MAT) and mean annual precipitation (MAP) from Weinan in the Loess Plateau, China Stratigraphy Age (ka) MAT (1C) MAP (mm) Phytolith zone Min. Max. Average7SD Min. Max. Average7SD S P1 L P L1-/L P3 L1-5 upper P4 upper L1-5 lower P4 lower S P5 L P6 Modern (A.D.) (max C) and 14 mm (max. 46 mm) lower than at present at the Weinan site. These values are comparable with results from other proxies including pollen (Sun et al., 1997), weathering intensity (Guo et al., 1996), grain size (Xiao et al., 1995; Ding et al., 1995, ), and 1 Be (Gu et al., 1997) obtained from other loess palaeosol sequences, indicating that major changes in palaeoclimate occurred in this part of the Chinese Loess Plateau. The variations in MAT and MAP during the period of 6 3 ka (upper L1-5 to L1-) consist of two cycles. The first cycle is marked by a rapid increase around 6 ka followed by a gradual decline towards a minimum around 49 ka, with a prominent warm-wet stage around 61 ka (ca 16 1C, ca 9 mm). The second cycle started with another rapid increase around 49 ka rising to a maximum around 4 ka (ca 17 1C, ca 85 mm), followed by a progressive decline towards the Last Glacial Maximum (Fig. 5, Table 1). These results suggest that MAT and MAP in the warmest and wettest intervals during the interstadial of L1-4 to L1- (MIS 3) were about ca 1 1C and ca 1 15 mm higher than those values occurring even during the Holocene optimum period in this region. These episodically high temperature and precipitation records within L1- and L1-4 (corresponding to marine d 18 O stage 3) are therefore consistent with an intensification of summer monsoon, which was recorded almost across the entire Tibetan Plateau (Shi et al., 1). In addition, the records from large lakes at the same time in the Tengger and Badain Jaran Deserts showed higher precipitation and greatly enhanced summer monsoon, which greatly exceeded the temperature and precipitation conditions of mid Holocene (Shi et al., 1; Zhang et al., ). The ice core from the Tibetan Plateau also indicated that the climate during this period was 4 1C higher than the present (Yao et al., 1994; Shi et al., 1). All lines of the evidence consistently suggest that an exceedingly strong summer monsoon climate existed over this region, and in turn indicate that Indian summer monsoon, bringing moisture from the low-latitude ocean to the plateau, was highly enhanced during this period (Shi et al., 1; Clemens and Prell, 3). During the LGM between 3 and 1.5 ka, the reconstructed MAT and MAP decreased to about 6.5 1C (average 9.5 1C) and 37 mm (average 55 mm), respectively, about ca 6. 1C and ca mm lower than today. The MAT and MAP increased again to C (max C) and 4745 mm (max. 87 mm) during the Holocene period, with a short cooling at around ka and ka. Wu et al. () made quantitative estimates of the MAT and MAP for the LGM at Weinan using mollusc records. Their results suggest that this region during the LGM was at least 3 5 1C colder and precipitation about 1 3 mm lower than today, which are generally consistent with our phytolith estimates. The relatively high MAT and MAP during the LGM compared with the penultimate glacial period (L) indicate that the East Asian summer monsoon did affect this area even in the coldest stage of the last glacial period. Rather than being uniformly cold or dry as previously thought, there were periodic warm-humid air incursions to the Loess Plateau in the LGM period. Our results are consistent with pollen data for this period obtained from the same sequence, which indicate a moist meadow environment composed of many types of Compositae, Gramineae, and Polygonum (Sun et al., 1997). 5.. Correlation of the Loess Plateau palaeoclimatic record with Heinrich events During the last 1 ka, the Heinrich events (H events) were the dominant signal of past climate variability over Greenland and the North Atlantic as well as other areas in the world (Guo et al., 1996; Rousseau et al., 6). Understanding the impact of the Heinrich events outside the main belt of ice rafting is crucial for recognizing the underlying causes of these abrupt climatic events. To reveal the impact of the H events on the Chinese Loess Plateau, we focused our study on the palaeo-climatic variability in the Weinan loess palaeosol sequences from the upper L loess to the Holocene palaeosol (Fig. 6). The reconstructed MAT and MAP records during the glacial period are characterized by large variabilities with abrupt

9 H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) Age ka GRIP 18 O a GIK * IRD b MAP Weinan d MAT c Heinrich Events** H1 H H3 H4 H5a H5b? H6 H7? H8? ( vs. PDS) (%) (mm) ( o C) Fig. 6. Correlation of high-frequency climate variability between the Greenland ice-core (GRIP), IRD of the northern North Atlantic (GIK3415-9), and phytolith-based MAP and MAT reconstructions from the Weinan section for the last 136 ka. (a) Greenland GRIP Oxygen Isotopes (d 18 O) ice record (GRIP Members., 1993; Grootes et al., 1993); (b) Ice rafted debris of sediment core GIK3415-9, Northern North Atlantic (*Weinelt et al., 3); (c) MAP estimates using phytoliths transfer function at Weinan section; (d) MAT estimates using phytoliths transfer function at Weinan section. **H1 H8 indicate northern North Atlantic Heinrich events, basically coinciding with the greater climate variability in Loess Plateau. cold-dry spells centered around 16, 4.5, 6, 3, 4, 49, 53, 65 and 7 75 ka. (Fig. 6). Fig. 6b shows the ice rafted debris (IRD) of sediment core GIK in the northern North Atlantic (Weinelt et al., 3), which can be used to identify detrital material input attributed to the H events. A comparison between the IRD record with our phytolith record suggests that all the IRD events (H1-H8?) roughly coincide with strong and sharp cold-dry epochs in the Loess Plateau, even though some minor discrepancies exist due to chronological uncertainties in estimating the loess sedimentation rates. The H7 and H8 events have been reported based on the Fe/Ca ratio of core GeoB 391-1from the tropical Atlantic region (Jennerjahn et al., 4), and also called cold events C19 and C by McManus et al. (1994), which might correspond to the 7 and 75 ka colddry intervals, respectively, at Weinan region. If the events in the North Atlantic and those in the Chinese Loess Plateau were, in fact, linked as we believe, then our results, in parallel with the Luochuan grain-size data (Porter and An, 1995; Xiao et al., 1995, 1999; Xiao and An, 1999) and the Weinan chemical index of weathering (Guo et al., 1996), suggest that these events, which were possibly related to the Heinrich events, were synchronous in both regions, and therefore related to the East Asian monsoons. Our well-dated MAT and MAP records show for the first time and provide further quantitative proof that climatic changes in the Chinese Loess Plateau were strongly affected by these North Atlantic cooling events Comparison with other data As mentioned above, the Weinan loess palaeosol section has been studied extensively. Detailed grain size and MS data at cm intervals for this sequence are documented by Ding et al. (1995) and Guo et al. (1998), as presented in Fig. 7a and b. Fig. 7c shows the reconstructed MAP by Lu et al. (1994) and Maher et al. (1994) based on the MS approach that used a regression analysis between the MS of the present-day soil and local rainfall data at several sites across the Loess Plateau. The variations in grain size and MS provide reference data for comparison of reconstructed palaeoclimates. The phytoliths-based quantitative MAP reconstructions are approximately consistent with those of the MS estimated in the glacial interglacial time scale. However, there are higher-frequency fluctuations in the phytoliths-based quantitative MAP than the MS estimations during the

10 768 H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) Weinan Luochuan Age ka 1 Depth (m) 1 3 S L1-1 Grain size (</>1 µm) Magnetic susceptibility (SI units) MS MAP (mm) Phytoliths MAP (mm) Depth (m) 1 3 QMD (µm) a b c d e Phytoliths zone p1 p L1- L p L1-4 L1-5 S p4 p5 p6 L Fig. 7. Comparison between the grain sizes, MS (magnetic susceptibility), MAP reconstructions records from the Weinan section, and quartz grain sizes record from the Luochuan section: (a) grain sizes; (b) MS (magnetic susceptibility); (c) MAP reconstructions records, (1) by climofunction of Lu et al. (1994) and Han et al. (1996), () by climofunction of Maher et al. (1994), vertical gray shard line shows the values of modern mean annual precipitation at Weinan region; (d) MAP estimates from phytoliths transfer function using the weighted averaging partial least-squares regression, vertical gray shard line shows the values of modern mean annual precipitation at Weinan region; (e) the median grain size values of monomineralic quartz (QMD) from the loess paleosol section at Luochuan in the central Loess Plateau (Xiao et al., 1995, 1999; Porter and An, 1995). past 136 ka. Most notably, the MAP values obtained from phytolith estimates are lower than those of the MS estimates for MIS 6, the early part of MIS 4, and MIS (Fig. 7). This discrepancy may be caused by the different sensitivities of different proxies to various environmental parameters (Feng et al., 4). The reconstructed MAP based on the MS may reflect the mean values during a longer time due to the fact that the MS are only an approximate index of the Asian monsoon strength, because it takes a long time for weathering processes to produce pedogenic clay and new magnetic minerals to a certain depth in the loess and palaeosols (Xiao et al., 1995; Feng et al., 4). On the other hand, the phytolith-based MAP reconstructions are more sensitive to short episodes of climatic change (Lu et al., 6), because vegetation communities respond to climatic changes more quickly than do weathering processes, and phytoliths (generally between 4 mm) do not translocate within the soil profiles. The quartz component of loess and palaeosols is largely unaffected by weathering processes and therefore constitutes a more reliable proxy index of monsoon wind strength than MS signals (Xiao et al., 1995, 1999; Xiao and An, 1999). Xiao et al. (1995, 1999) and Porter and An (1995) presented the median grain size values of monomineralic quartz (QMD) from the loess palaeosol section at Luochuan in the central Loess Plateau. Our phytolithbased MAP reconstructions in the Weinan section, characterized by high-frequency and high-amplitude variations, is more comparable with the QMD record (Fig. 7) than the MS record, which implies rapid and significant changes in palaeoclimatic conditions that affect dust transport and vegetation changes. It is noteworthy that the lower boundary of the S1 palaeosol (ca 1.16 m depth), which was traced and identified based on the preserved characteristics observed in the field, is inconsistent with the prominent changes in the entire proxy index including grain size, MS, and phytoliths (Fig. 7). Several authors (Bronger and Heinkele, 1989; Guo and Liu, 1993; Kemp, 1995; Kemp et al., 1995, 1997; Guo et al., 1996; Feng et al., 4) have recognized the unusual characteristics of the soil forming processes of the last interglacial S1 palaeosol in the Chinese Loess Plateau. They suggested that the lower-portion of the S1 was developed on the underlying older and coarser loess L so that it inherited the physical and chemical properties of the parent materials. On the other hand, the use of phytolith data also has problems and weaknesses that may limit their useness as a palaeoclimatological proxy. For example, (a) As reviewed by Piperno (1988), the biases that affect phytolith assemblages are mainly the result of differential production,

11 H.-Y. Lu et al. / Quaternary Science Reviews 6 (7) transport, and preservation. Both wind and water transport are likely to lead to sorting of phytoliths of differing size, shape, and specific gravity, which may impact vegetation inference. (b) Similarly, taphonomic response of different phytolith morphologies to the chemical and mechanical soil environment can affect the phytolith record (Bartoli and Wilding, 198). Very fragile and small phytolith morphotypes, such as silicified mesophyll fragments, which abound in modern plant reference samples, are generally absent from loess assemblages, even from soil assemblages (Piperno, 1988). (c) Although all assemblages were diverse, containing elements of most of the major phytolith categories in our loess records, it should be noted that when the extremity dry cold epochs (upper L), certain assemblages only have a few of phytolith types. (d) The differences in preservation of diagnostic features may lead to an underestimation or overestimation of certain morphotypes. The existence of these taphonomic filters will therefore widen the range of climatic tolerances, blur the optima of critical phytolith taxa, and increase potential biases in the inference model. Further study of the physical chemical properties and reactivity of phytoliths of different origins should improve our knowledge of the dissolution of silicon in various terrestrial environments. 6. Conclusions This study is the first to quantitatively reconstruct Late Quaternary climatic changes using phytolith analysis in conjunction with transfer function techniques on a glacial interglacial timescale. Our phytolith-based reconstructions from Weinan have produced MAT and MAP values for the southern part of the Chinese Loess Plateau comparable with those derived from MS, grain sizes, pollen, and locally based environmental inferences at the glacial interglacial timescale, but our estimates show higher-frequency fluctuations on a millennium time scale, suggesting that phytoliths are a more sensitive proxy for quantitative palaeoclimatic reconstructions than MS and other pedogenesis-related proxies. A series of cold and/or dry events were indicated by the reconstructed MAT and MAP as documented in the loess and palaeosol sequence of the last glacial period, which can be temporally correlated with the North Atlantic Heinrich events. This correlation has also been reported by Porter and An (1995) based on the quartz grain-size data from the Luochuan loess section and the chemical weathering index from the Weinan loess section (Guo et al., 1996, 1998), suggesting that the Heinrich events are synchronous in both regions and are therefore climatically related to the East Asian monsoons. Our well dated and quantitatively derived MAT and MAP records, for the first time, provide further proof that the Chinese Loess Plateau was strongly affected by these North Atlantic cooling events. Our MAT and MAP reconstructions show that temperature and rainfall decreased during glacial times (L, lower Ll-5, and L1-1). The coldest and driest period occurred during the interval of the upper L (Late MIS 6), when the MAT and MAP dropped to ca 4.4 1C and ca 15 mm, respectively. Two other prominent cold-dry periods, occurred during the intervals of lower Ll-5 (77 6 ka) and L1-1 (3 1.5 ka). During these periods the reconstructed MAT and MAP were about C and mm, respectively, about C and 4 mm lower than today. On the other hand, higher MAT and MAP occurred during the interglacial (Sl, S) and interstadial (Ll-, L1-4) periods. The highest MAT (average C, max C) and MAP (average 757 mm, max. 1 mm) occurred in the interglacial of Sl (MIS 5) and in the interstadial of L1-4 to L1- (MIS 3), being ca 1 1C and ca 1 15 mm higher than the present interglacial including even the Holocene optimum in the region. Acknowledgments We are grateful to Dr. Freea Itzstein-Davey for her valuable comments on an earlier version of this manuscript. We greatly appreciate the valuable comments from two anonymous reviewers. We are also indebted to Liu Jiaqi and Chen Tiemei for providing the chronological controls, and to Prof. Sun Xiangjun, Song Changqing, Ding Zhongli, Guo Zhengtang, and Yang Xiangdong for their helpful discussion. 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