Stacked 2.6-Ma grain size record from the Chinese loess based on five sections and correlation with the deep-sea D 18 O record

Transcription

1 PALEOCEANOGRAPHY, VOL. 17, NO. 3, /2001PA000725, 2002 Stacked 2.6-Ma grain size record from the Chinese loess based on five sections and correlation with the deep-sea D 18 O record Z. L. Ding, 1 E. Derbyshire, 2 S. L. Yang, 1 Z. W. Yu, 3 S. F. Xiong, 1 and T. S. Liu 1 Received 19 October 2001; revised 10 April 2002; accepted 10 April 2002; published 6 August [1] This study aims to establish a stacked climate record of the Quaternary period from the Chinese loess sequence and to address the forcing mechanisms for the regional climate history of the Loess Plateau by correlating the stacked record with a composite d 18 O record in deep-sea sediments. A total of 18,352 samples were obtained from five loess sections, located at Baoji, Lingtai, Jingchuan, Puxian, and Pingliang in the southern and middle Loess Plateau. These yielded high-resolution grain size records. Between-section correlation of these shows that although small depositional hiatuses are present in places within a single section, most parts of the sections display near-continuous dust deposition throughout the Quaternary. The grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the Earth s orbit. The grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series, termed the Chiloparts record. This resolves most of the orbital timescale paleoclimate events buried in the loess-soil sequences of the southern and middle Loess Plateau and can be used as a regional archive of the Pleistocene climate history in the Loess Plateau. Comparison of the Chiloparts record with a composite marine d 18 O record shows that for the past 1.8 Ma, the loess-paleosol record can be correlated almost cycle by cycle with the marine record. Several discrepancies in the climatic events between the two records have also been identified, implying that regional forcing mechanisms may have played a part in the climatic evolution of the Chinese Loess Plateau. INDEX TERMS: 4899 Oceanography: Biological and Chemical: General or miscellaneous; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 1099 Geochemistry: General or miscellaneous; KEYWORDS: Chinese loess, grain size, orbital timescale, continental climate record, land-sea climate correlation 1. Introduction [2] In the past two decades, studies of the loess-paleosol sequence in the Loess Plateau of north central China have provided plentiful information about regional and global climate changes during the Quaternary [Heller and Liu, 1982; Liu, 1985; Burbank and Li, 1985; Kukla, 1987; Kukla and An, 1989; Rutter et al., 1991; An et al., 1991a; Ding et al., 1994; Porter and An, 1995; Derbyshire et al., 1995; Liu and Ding, 1998; An et al., 2001]. In the Loess Plateau, loess deposits with a thickness ranging from several tens to more than three hundred meters cover more than 273,000 km 2 in Shanxi, Shaanxi and Gansu Provinces [Liu, 1985]. The loess, accumulated in the past 2.6 Ma, was transported mainly by northwesterly winter monsoon winds over East Asia from the vast deserts and desert margins directly to the north and northwest of the Plateau [Liu, 1985; An et al., 1991b; Derbyshire et al., 1998; Liu and Ding, 1998]. Typical loess sections are characterized by the alternation of pale yellow loess units and brownish or reddish soil 1 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 2 Centre for Quaternary Research, Department of Geography, Royal Holloway, University of London, Egham, UK. 3 Department of Resource Exploitation and Engineering, China University of Mining and Technology, Beijing, China. Copyright 2002 by the American Geophysical Union /02/2001PA horizons; more than thirty of these loess-soil couplets have been identified in many thick loess sections [Liu, 1985; Kukla and An, 1989; Ding et al., 1993]. The loess horizons are interpreted as having accumulated during dry, cold, windy glacial periods with a significant weakening of the Asian summer monsoon, whereas the soils formed in interglacial times when the intensity of the summer monsoon was much greater [Kukla and An, 1989; An et al., 1991a; Liu and Ding, 1998]. [3] Various studies of the loess have shown that signatures of regional environmental changes can be retrieved by multiple proxy indicators such as magnetic susceptibility [An et al., 1991a; Zhou et al., 1990; Heller et al., 1994; Verosub et al., 1993; Maher and Thompson, 1995; X. M. Liu et al., 1995], grain size [An et al., 1991b; Ding et al., 1994; Vandenberghe et al., 1997], chemical weathering indexes [T. S. Liu et al., 1995; Gu et al., 1997; Guo et al., 1998; Chen et al., 1999; Han et al., 1998], pedogenic micromorphology [Bronger and Heinkele, 1989; Guo et al., 1991; Rutter and Ding, 1993], pollen [Sun et al., 1997], phytolith [Lu et al., 1996] and snail assemblages [Rousseau and Wu, 1997], and carbon and oxygen isotopic ratios [Han et al., 1997]. Some of these proxy records display forcing at precessional (23 ka and 19 ka) and obliquity (41 ka) periodicities of the Earth s orbit throughout the Quaternary, and at the 100 ka ice volume-associated cycles in the late Pleistocene [Ding et al., 1994, 1995]. These results imply that global forcing factors were a major influence upon the climatic history recorded in the loess. Thus the loess record 5-1

2 5-2 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS provides a valuable means of improving understanding of the forcing mechanisms that drove Pleistocene changes in the climate system. Reliable reconstruction of the climatic history of the Loess Plateau is central to the achievement of this aim. [4] However, a critical problem in the study of the Chinese loess remains to be overcome, namely determining to what degree the loess sequence provides a continuous record of climatic changes. As a terrestrial sediment type, hiatuses of varying magnitude might well be expected during long-term accumulation of subaerial dust. Previous studies, based on correlation of loess-soil units, have shown that the major stratigraphic units can be correlated between different sites in the Loess Plateau, suggesting an approximate continuity of loess deposition [Kukla and An, 1989; Ding et al., 1993]. However, this does not imply that complete, high-resolution climatic records can be obtained with any certainty from a single standard section. Furthermore, there is no doubt that the processes of loess transport and sedimentation have been affected by locally varying factors such as geomorphological conditions, the configuration of loess-desert distribution, and wind trajectories, which are liable to induce substantial differences in the loess records at different sites [Liu, 1985]. It follows that a comprehensive study of multiple loess sections is essential before any paleoclimatic reconstruction can be regarded as representative of the whole loess region. [5] The development of reliable timescales is the key to the generation of high-resolution climatic records from the loess-soil sequence. In the past, loess chronology has been established using two main approaches. The first assumes constant deposition of magnetic minerals [Kukla et al., 1988] or the ratio of some specific grain size fractions [Vandenberghe et al., 1997] between paleomagnetic reversals, and the other is based on tuning paleoclimatic proxy records to theoretical changes in the elements of the Earth s orbit [Ding et al., 1994; Lu et al., 1999; Heslop et al., 2000]. Subsequent studies have shown that variations in the magnetic susceptibility of the loess-soil sequence may be controlled by multiple factors such as pedogenesis, grain size, and organic matter content [Zhou et al., 1990; Meng et al., 1997; Sun and Liu, 2000a]. The assumption of constant deposition of magnetic minerals from remote sources [Kukla et al., 1988] may be not valid. Over the loess region, each of the loess-soil units can be interpreted as a specific, essentially synchronous climatic event. Resulting timescales based on the assumption of a constant ratio of selected specific grain size fractions would thus be influenced by changes in loess thickness, dust input from local sources and so on. In comparative terms, the orbital tuning approach provides more identical age constraints for loesssoil units than other methods, even where thickness of a specific loess or soil unit varies significantly from one section to another. An earlier version of the loess orbital timescale, developed on the basis of the Baoji grain size curve [Ding et al., 1994], was constrained using a paleomagnetic polarity timescale which has proved systematically younger than a more recent one [Cande and Kent, 1995]. A revised version of the orbitally tuned timescale is therefore needed to advance such studies of the Chinese loess record. [6] In the study reported here, we generated five highresolution grain size records from five loess sections located in different places on the Loess Plateau. Intersectional correlation of the grain size records allows us to recognize small depositional hiatuses within a single loess sequence, and to identify all of the orbital-scale climatic events that affected the Loess Plateau in the last 2.6 Ma. The grain size records were then tuned to changes in the Earth s orbital parameters, and the established orbital timescales were used to generate a stacked grain size time series for the Loess Plateau. Finally, this stacked set was correlated with a composite marine d 18 O record. 2. Setting and Stratigraphy [7] The five loess sections used in the present study are located at Baoji, Lingtai, Jingchuan, Puxian, and Pingliang (Figure 1). Each is situated within a loess yuan, that is a flat, broad, high tableland, a landform widely regarded as having preserved a relatively complete set of mineral dust deposits [Liu, 1985]. All five sections show a Pleistocene loess-soil sequence underlain by Tertiary red clay of varying thickness. The red clay in the Loess Plateau has also been shown to have an eolian origin [Liu et al., 1988; Ding et al., 1998a]. The Baoji loess section (34.43 N, E), about 161 m thick, is located in the southernmost part of the Loess Plateau, and has been under study for over a decade [Rutter et al., 1991; Ding et al., 1994; Liu et al., 2000]. This section was re-sampled in the present study, in order to generate climate records with a higher temporal resolution than those previously obtained. The Lingtai (35.06 N, E), Jingchuan (35.29 N, E) and Pingliang (35.58 N, E) loess sections, with thicknesses of about 176 m, 199 m and 108 m, respectively, are located in the middle-western part of the Plateau where loess deposits appear to be relatively better preserved than elsewhere. Only the upper portion of the Pleistocene loess sediment is clearly exposed at Pingliang. The 151-m thick Puxian (36.44 N, E) loess section was only recently discovered. It is the only known section in the middle eastern part of the Loess Plateau with a relatively complete set of loess-soil units. [8] In general, loess deposits consist of two major stratigraphic units, namely loess and paleosol. Loess horizons are labeled Li and soils Si. Paleosols are defined as those units having the appearance and a degree of pedogenic development similar to or greater than the Holocene soil in the same area [Ding et al., 1993]. Thirty-seven paleosol units have been identified at Baoji [Rutter et al., 1991]. Within some thick loess horizons, one or more relatively weakly developed soils can also be recognized particularly in the southern part of the Loess Plateau. In the Lingtai, Jingchuan and Puxian sections, all thirty-seven major paleosols can be distinguished from the loess units, whereas only those above L13 can be seen clearly at Pingliang. Some of the weak soils within loess horizons, however, are not so readily distinguishable in these sections compared to the situation at Baoji. In some earlier loess studies, the Luochuan and Xifeng sections (Figure 1) were regarded as

3 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-3 Figure 1. Schematic map showing location of the studied loess sections in the Chinese Loess Plateau. The Mu Us Desert to the north of the Loess Plateau and the mountains along and within the Loess Plateau are also indicated. standards for the Chinese loess [Liu, 1985; Kukla and An, 1989]. However, the paleosols below loess unit L15 in both of these sections are difficult to distinguish from the overlying and underlying loess horizons because of disturbance and mixing by unknown processes. In all the sections used in the present study, each of the paleosols can be clearly defined in the field, thus making them more suitable than the Luochuan and Xifeng sections for Pleistocene climatic reconstruction. [9] In the field, we took samples at 5 cm intervals at Jingchuan, Puxian and Pingliang and at 3 5 cm intervals at Lingtai and Baoji. A total of samples were collected for grain size analyses. In addition, we collected paleomagnetic samples from those stratigraphic parts of the sections where earlier work had indicated the presence of paleomagnetic events. Magnetic remanence was measured with a 2G three-axis cryogenic magnetometer in the Institute of Geology and Geophysics, CAS, grain size being determined with a SALD-3001 laser diffraction particle analyzer. The grain size analytical procedure has been described by Ding et al. [1999a]. [10] The paleomagnetic polarity records of the Baoji, Lingtai and Jingchuan sections have already been published [Rutter et al., 1991; Ding et al., 1999b, 2001a]. The positions of magnetic events at Puxian and Pingliang are essentially similar to those at the other three sites. The Brunhes/Matuyama magnetic reversal is located in the lower or middle part of loess unit L8, and the upper and lower reversals of the Jaramillo subchron are defined, respectively, within L10 (between S9 and S10) and L12 (between S11 and S12). The Olduvai subchron is detected between the middle part of L25 and the lower part of S26 or the uppermost part of L27. The Matuyama/Gauss reversal occurs within the oldest loess unit (L33). Below L33, Tertiary red clay sediments with very fine grain size distributions are preserved in the sections. The loess-soil

4 5-4 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS sequences are therefore regarded as having a basal age of about 2.6 Ma. 3. Correlation of Grain Size Records [11] Generation of high-resolution grain size records enables us to correlate climatic variations in detail between the five loess sections. Many climate proxy indexes have been used in loess studies, but our principal reason for selecting grain size to develop a representative climate record for the Chinese loess was that down-section grain size variability displays a comparatively spatial stability, as will be shown below. In recent years, loess grain size has been widely used as a proxy indicator of the intensity of the Asian winter monsoon [An et al., 1991b; Ding et al., 1994; Porter and An, 1995; Vandenberghe et al., 1997]. This contention is essentially adopted from studies of the eolian fraction in oceanic sediments. However, recent work has strongly suggested that grain size changes in Chinese loess may be closely associated with the episodic advance and retreat of dust source regions during glacial cycles [Ding et al., 2001b]. In glacial periods, the aridity of the dust source regions across northern China may have been greatly enhanced by two major processes. (1) The lowered global sea level gave rise to extensive exposure of the continental shelves around the West Pacific marginal seas [Wang, 1998], thus leading to an increase in the continentality of the climate in northern China. (2) As a likely response to the expansion of both polar ice sheets and sea ice cover, the atmospheric high-pressure cell over Siberia was enhanced, so drawing more cold, dry air masses into the mid-latitudes. This resulted in the winter monsoon dominating the climate of northern China. As the geological records suggest, the Chinese deserts may have advanced several hundred kilometers to south and east during the last glacial maximum, compared to their extent in the Holocene optimum [Sun et al., 1998]. The effect of this was a varying distance between the Loess Plateau and the deserts, thus accounting for known grain size variations in loess deposited at specific sites in the Loess Plateau. Therefore overall increases in mean grain size in loess horizons may be explained, at least in part, by diminishing distance between the Loess Plateau and the source regions during glacial periods. [12] Figure 2 shows the grain size records for those parts of the five sections above the S8 paleosol. The thickness of the S0-S8 part of the section at Jingchuan, Lingtai, Puxian, Baoji, and Pingliang is 67.5 m, 60 m, 65 m, 63.5 m and 84 m, respectively. Soil unit S2 consists of two discrete soils (S2-1 and S2-2) and S5 contains three soils (S5-1, S5-2 and S5-3). Therefore eleven major soils are recognizable in this part of all five of the profiles. Relatively thick loess beds separate most of the paleosol units. Median grain sizes are consistently greater in the loess horizons compared to the paleosols. Within loess horizons such as L1, L2, L6 and L7, one or two weak soils can be seen in the field, all of which show finer grain size distributions (Figure 2). However, no weak soils corresponding to the two grain size minima within L5 (see Figure 2) have been recognized in the field. Grain size variability is very similar from one section to another, so that a cycle-to-cycle grain size correlation is readily made. Small discrepancies are present in only three aspects. (1) The three individual soils within S5 are not clearly expressed in the Puxian grain size record. (2) While the S4-L4 couplet shows a gradual increase in median grain size in most of the records, S4-L4 at Pingliang shows a broader range of variation. (3) The grain size increase in the upper part of L2 at Pingliang is not as prominent as in other records, suggesting the presence of a minor depositional hiatus. [13] Correlation of the five grain size records for the S8- S15 portion is shown in Figure 3. In the Pingliang section, samples were taken only down to the base of S12. The depths of this stratigraphic part at Jingchuan, Lingtai, Puxian, Baoji, and Pingliang are from 64.6 to m, 56.9 to 93.3 m, 61.4 to 91.8 m, 60 to 92.7 m and 80 to m, respectively. The L9 and L15 loess horizons, traditionally known as the Upper and Lower Sandy Loess, have long been used as stratigraphic markers in loess studies [Liu, 1985]. Both show exceptional thickness and coarse particle sizes in the field. Between L9 and L15, paleosols are closely spaced except for the relatively thick loess of L13. The S9 soil unit is composed of two discrete soils, termed S9-1 and S9-2 [Rutter et al., 1991; Ding et al., 1993]. A total of seven paleosols are recognizable between L9 and L15. Grain size variability is relatively small between L9 and L15, but a cycle-to-cycle correlation is clearly seen between the sections (Figure 3). The two grain size minima within L9 are all prominent in all five sections. The S9-2 soil and the L15 loess are exceptionally thin in the Puxian section, suggesting the presence of depositional hiatuses within both units. [14] Figure 4 shows grain size correlations for the S15- S26 part of the sequence at the Jingchuan, Lingtai, Puxian, and Baoji sections. Thicknesses for this part are 53.2 m (Jingchuan), 45.5 m (Lingtai), 31.3 m (Puxian), and 39.2 m (Baoji). The thick loess unit L24 is also a stratigraphic marker, within which one or two weak soils are identifiable in the field [Rutter et al., 1991; Ding et al., 1993]. Between L15 and L24, there are nine paleosols with intercalated thin loess horizons. The S26 soil unit is about 3 m thick in most sections and, with its strong reddish color, is another stratigraphic marker [Rutter et al., 1991; Ding et al., 1993]. One relatively weakly developed soil is recognizable within both L25 and L26. As shown in Figure 4, alternation of these loess-soil units is clearly expressed in the grain size records particularly at Jingchuan, and most of the grain size cycles are readily correlated between the four sections. However, small differences in grain size variability do exist. For example, there are two grain size minima within L24 at Jingchuan, Puxian and Baoji, but grain sizes in this horizon are much more variable at Lingtai. At Baoji, the grain size record shows that the two paleosols S20 and S21 appear to merge, and the S17 grain size minimum is not so prominent as in other records. [15] The grain size record of the lower part of the sequence (from S26 to the uppermost part of the Tertiary red clay) can be seen in Figure 5. Units L27 and L32 are two thick loess layers that have been used as stratigraphic markers [Rutter et al., 1991; Ding et al., 1993]. In the middle of L27, three closely spaced soils are readily recognizable in the field, although they are not clearly

5 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-5 Figure 2. Median grain size records of the Jingchuan, Lingtai, Puxian, Baoji, and Pingliang loess sections above S8. The lithological column is shown on the left of the grain size record for each section. The loess and paleosol units are labeled using the L i and S i system. The position of the B/M paleomagnetic reversal is indicated in each loess-paleosol sequence. The depositional hiatuses and discrepancies suggested by intersite correlation of the grain size records are marked with?. expressed in the grain size records. The thickness of L32 is over 8 m in most of the loess sections, within which there are two discrete but weakly developed soils. Between L27 and L32, the five paleosols are closely spaced except for the relatively thick loess of L29. The soil complex S32, mostly thicker than 3 m, is among the reddest paleosols in the entire loess sequence and can be regarded as one of the bestdeveloped paleosols [Rutter and Ding, 1993]. The Matuyama/Gauss magnetic reversal is found within the oldest loess unit L33. This horizon is relatively thick, being made up of rather coarse silts. The red clay, with its generally fine grain size, is found below L33. All stratigraphic features are clearly shown by the grain size records, grain size variability being quite readily correlated between the four sections (Figure 5). [16] These grain size correlations demonstrate that all five loess sections, although located in different parts of the Loess Plateau, have a very similar stratigraphy that is clearly reflected in the grain size records. Although it is inherently unlikely that the loessic dust deposition over the past 2.6 Ma is completely preserved in any single loess section, the fact that any substantial depositional hiatus can be detected by grain size correlation makes possible the reconstruction of an essentially complete sequence of climatic events using the grain size records. In addition, the down-section variation in the five grain size curves illustrated above shows a very similar pattern, although grain size distributions in the southern part of the Loess Plateau are generally finer than in the northern part. Such spatially consistent grain size variability implies that the

6 5-6 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS Figure 3. Median grain size records of the Jingchuan, Lingtai, Puxian, Baoji, and Pingliang loess sections between S8 and S15. The lithological column is shown on the left of the grain size record for each section. The loess and paleosol units are labeled using the L i and S i system. The positions of the upper and lower boundaries of the Jaramillo Chron (J) are shown in each loess-paleosol sequence. The depositional hiatuses and discrepancies suggested by intersite correlation of the grain size records are marked with?. transport and deposition of loess particles across the Loess Plateau may be controlled by regional winds, and that dust inputs into the loess strata from local sources within the Plateau may be minor. Such characteristics recommend loess grain size as a good index for use in paleoclimatic reconstruction. 4. Orbital Timescale [17] In order to construct a representative grain size time series for the Chinese Loess Plateau, an age model for each of the grain size records we have generated must be developed. It has long been known that oscillations in Quaternary climate were primarily forced by variations in the Earth s orbital parameters [e.g., Milankovitch, 1941; Hays et al., 1976; Imbrie et al., 1984]. Theoretically calculated time series of the Earth s orbital geometry can thus be used to fine-tune the timescales of climatic proxy records. This approach has achieved great success in developing climatic timescales for deep-sea and loess sedimentary series covering the entire Quaternary period [Ruddiman et al., 1989; Raymo et al., 1989; Shackleton et al., 1990; Ding et al., 1994; Heslop et al., 2000]. As shown in Figures 2, 3, 4, and 5, most of the climatic cycles recorded by grain size data can be readily correlated between different loess sections, thus suggesting that such loess sequences contain a

7 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-7 Figure 4. Median grain size records of the Jingchuan, Lingtai, Puxian, and Baoji loess sections between S15 and S26. The lithological column is shown on the left of the grain size record for each section. The loess and paleosol units are labeled with the L i and S i system. The positions of the upper and lower boundaries of the Olduvai Chron (O) are shown in each loess-paleosol sequence. The depositional hiatuses and discrepancies suggested by intersite correlation of the grain size records are marked with?. record of almost continuous dust deposition. This satisfies the requirements for the development of an orbital timescale for each of the sections. [18] We employed the orbital tuning method modified by Yu and Ding [1998]. This involves the simultaneous tuning of the grain size records to the calculated changes in obliquity and precession of the Earth s orbit [Berger and Loutre, 1991] in the following steps. First, as loess chronological studies cannot provide accurate age constraints, we assumed an in-phase change for the winter monsoon and/or the loess-desert distance monitored by the grain size records with respect to global ice volume variations. This assumption is made on the basis of results from previous studies indicating that the East-Asian monsoon changes registered in the Chinese loess record are tightly linked to the cyclic growth and decay of the

8 5-8 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS Figure 5. Median grain size records of the Jingchuan, Lingtai, Puxian, and Baoji loess sections from S26 to the upper part of the Tertiary red clay. The lithological column is shown on the left of the grain size record for each section. The loess and paleosol units are labeled using the L i and S i system. The position of the M/G paleomagnetic reversal is indicated in each loess-paleosol sequence. Northern Hemisphere ice sheets [Ding et al., 1995; Shackleton et al., 1995a]. In the tuning targets, therefore, the obliquity curve is lagged by 8 ka and the precessional index curve by 5 ka, both values being used in the reconstruction of the Specmap d 18 O record [Imbrie et al., 1984]. In the second step, we established an initial timescale for each of the grain size records, using paleomagnetic reversals as time controls, and with reference to the previous orbital timescale of the Baoji section [Ding et al., 1994]. In the third step, we repeatedly used two phase-free digital filters with a central period of 41 ka and 22 ka to extract these frequency components from the grain size records on the initial timescale, and matched the phase of each cycle of the filtered curves to the phase of the corresponding cycle in the target curves by manually adding new time control points and by assuming a uniform depositional rate between these points. In so doing, the paleomagnetic boundaries were consistently displaced upward in the loess profiles because of the lock-in effect that has recently been identified in loess studies [Zhou and Shackleton, 1999; Heslop et al., 2000]. This tuning results in an intermediate timescale for the grain size records. Finally, we applied the Dynamic Optimization method mentioned by Yu and Ding [1998] to add automatically new time control points to the grain size records on the intermediate timescale. This method iteratively adjusts

9 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-9 Figure 6. Grain size time series of the Puxian loess section, and filtered precession and obliquity signals (solid lines) versus lagged theoretical orbital precession (reversed) and obliquity records (dashed lines). The filtered precession and obliquity signals from the grain size time series have a central period of 22 and 41 ka, respectively. The theoretical orbital data are from Berger and Loutre [1991]. The filtered signals and orbital records are all normalized. each of the grain size data points in order to achieve the highest correlation coefficients between the orbital frequency curves filtered from the grain size records and the target curves [Yu and Ding, 1998]. [19] Figures 6, 7, and 8 show the final versions of the tuned Puxian, Baoji, and Pingliang grain size timescales, and provide a comparison of the filtered 41 ka and 22 ka frequency components (solid lines) with the obliquity and

10 5-10 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS Figure 7. Grain size time series of the Baoji loess section and filtered precession and obliquity signals (solid lines) versus lagged theoretical orbital precession (reversed) and obliquity records (dashed lines). The filtered precession and obliquity signals from the grain size time series have a central period of 22 and 41 ka, respectively. The theoretical orbital data are from Berger and Loutre [1991]. The filtered signals and orbital records are all normalized. Figure 8. (opposite) Grain size time series of the Pingliang loess section, and filtered precession and obliquity signals (solid lines) versus lagged theoretical orbital precession (reversed) and obliquity records (dashed lines). The filtered precession and obliquity signals from the grain size time series have a central period of 22 and 41 ka, respectively. The theoretical orbital data are from Berger and Loutre [1991]. The filtered signals and orbital records are all normalized.

11 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-11

12 5-12 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS Figure 9. Coherency and variance spectra calculated from the Puxian, Baoji, and Pingliang grain size and theoretical orbital records. Two signals have been processed for each diagram: (1) ETP, a signal formed by normalizing and adding variations in orbital eccentricity, obliquity, and precession (reversed), and (2) the loess grain size record on the tuned orbital time scales. Top: variance spectra (solid line: cross spectrum between the two signals; dashed line: ETP variance spectrum) plotted on arbitrary log scales. Bottom: coherency spectrum plotted on a hyperbolic arctangent scale and provided with a 5% significance level. Frequencies are in cycles per thousand years.

13 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-13 Figure 10. Correlation of the Jingchuan, Lingtai, Puxian, Baoji, and Pingliang grain size records plotted on the orbital timescales for the interval Ma. Each grain size time series is normalized to an interval ranging from 1 to 1. The major loess and soil units are labeled. The shaded areas indicate the parts that are excluded in the construction of the stacked grain size record shown in Figure 12.

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15 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-15 precession records (dashed lines) of the Earth s orbit [Berger and Loutre, 1991]. The Pingliang grain size record was tuned only down to the base of the S9-1 soil. In these diagrams, both the filtered and theoretically calculated orbital signals are normalized. For the obliquity signal, almost all the cycles filtered from the Puxian, Baoji and Pingliang grain size time series fit the theoretical version, with the modulations being similar to astronomical forcing except for a few small parts of the Baoji section, which may be a result of the relatively low temporal resolution of the Baoji grain size record. The correlation coefficient between the filtered and theoretical obliquity records is (n = 2601) at Puxian, (n = 2601) at Baoji and (n = 961) at Pingliang. The correlation coefficient between the filtered and theoretical curves at the precession frequency band is (n = 2601) at Puxian, (n = 2601) at Baoji, and (n = 961) at Pingliang. The correlation between the filtered and theoretical records at the obliquity frequency band is better than that at the precession frequency band for each of the sections. The modulations also show a poorer fit at the precessional frequency band, particularly in the lower part of the sections. Nevertheless, the precessional signals filtered from the grain size records on the orbital timescales fit the theoretical record almost cycle by cycle (Figures 6, 7, and 8), thus suggesting that the three grain size orbital timescales are well constrained. Those for the Jingchuan and Lingtai sections will be published elsewhere, but it should be mentioned here that both the filtered obliquity and precession signals from both the Jingchuan and Lingtai orbital timescales superimpose well on the theoretical versions of the two orbital elements. The correlation coefficient between the filtered and theoretical records at the obliquity frequency band is at Jingchuan and at Lingtai, and the correlation coefficient at the precession frequency band is and (n = 2601), respectively. [20] Cross-spectral comparison of the resulting grain size time series with orbital data was then used to evaluate our grain size timescales. Figure 9 shows the results of the crossspectral analyses between the grain size data plotted on the Baoji, Puxian and Pingliang timescales and the theoretical orbital record (ETP curve). The ETP curve was formed by stacking the normalized eccentricity and obliquity curves together with the normalized and reversed precession index curve [Berger and Loutre, 1991]. Cross-spectral analyses show that for the last 2.6 Ma, both the Baoji (Figure 9, left) and Puxian (Figure 9, middle) grain size time series closely match the orbital signals over the obliquity (41 ka) and precession (23 and 19 ka) periodicities. The coherency is about 0.98 at the 41 ka and 19 ka periodicities and about 0.93 at the 23 ka periodicity, these values being well above the 5% significance level. The coherency over the eccentricity periodicity (100 ka), however, is insignificant for both the Baoji and Puxian grain size time series. The Pingliang grain size time series matches well the ETP curve at the 41 ka and 23 ka periodicities, with a coherency of more than 0.9, whereas the coherency over the 100 ka-eccentricity and the 19 ka-precession periodicities is insignificant (Figure 9, right). In both the Jingchuan and Lingtai sections, the coherencies are higher than 0.95 over the 41 ka and 23 ka periodicities and are about 0.94 over the 19 ka periodicity for the past 2.6 Ma (will be published elsewhere). These crossspectral analyses further suggest that our tuned grain size timescales have been tightly constrained by both the theoretical obliquity and precession records. 5. Stacked Grain Size Record [21] Correlation of the resultant Jingchuan, Lingtai, Puxian, Baoji, and Pingliang grain size time series is shown in Figures 10 and 11. As has been shown above, grain size cycles for the major loess-paleosol units are readily matched from one section to another, but certain discrepancies occur particularly within some major units. To stack a representative grain size record from the sections, it is crucial to determine which parts in a single record should be excluded. In the record of the past 1.3 Ma (Figure 10), we recognized five places in the Puxian and Pingliang records that differ substantially from other records. At Pingliang, the uppermost part of L2 is evidently lacking, and grain size variation in the S4-L4 couplet is significantly different from other sections. The latter discrepancy may be the result of a distinctive regional sedimentation history, since we have not recognized any depositional hiatuses in this couplet at Pingliang. At Puxian, as on the other sections, the S5 soil complex is composed of three discrete soils. Although clearly visible in the field, however, these are not expressed clearly in the grain size record, perhaps because of disturbance by some unknown processes. The S9-2 soil and L15 loess at Puxian are both exceptionally thin compared to their counterparts in the other sections, a situation possibly attributable to depositional hiatuses. For these reasons, the uppermost part of L2 at Pingliang, and the S5, S9-2 and L15 units at Puxian were excluded from the stacking of a representative grain size record. [22] For the time interval Ma (Figure 11), correlation of the grain size records reveal a further three candidates for exclusion from the stacking of a representative record. The record in soil units S17 and S20-S21 at Baoji is significantly different from that found in the other sections. At Lingtai, grain size variability within L24 has a much higher frequency than elsewhere, although whether this represents real climatic signals or is attributable to other processes is not known. Considering the proximity of Lingtai to both the Jingchuan and Baoji sections (Figure 1), we have chosen to discard the grain size data of this part of the Lingtai sequence. In addition, it should be noted that the grain size variability of the thick loess bed L32 at Puxian is very different from all other records considered here. Specifically, the upper part of this unit of the Puxian Figure 11. (opposite) Correlation of the Jingchuan, Lingtai, Puxian, and Baoji grain size records plotted on the orbital timescales for the interval Ma. Each grain size time series is normalized to an interval ranging from 1 to 1. The major loess and soil units are labeled. The shaded areas indicate the parts that are excluded in the construction of the stacked grain size record shown in Figure 12.

16 5-16 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS Figure 12. Correlation of the Chiloparts grain size record with a composite marine oxygen isotope record. The major loess-paleosol units and the marine oxygen isotope stages are labeled. profile shows much coarser grain sizes than in other sections. Given the great distance between Puxian and the other sections, it might be argued that regional depositional processes explain these differences and, therefore, the use of the data from L32 is admissible. [23] Following the recognition of the small depositional hiatuses and the cases of inconsistent grain size variability over these short periods, the five grain size records were then stacked together according to the following procedures. First, each grain size time series was normalized (as shown in Figures 10 and 11) in order to give the same weighting to each record in the stack. The normalized records were then interpolated linearly at 1 ka intervals. Finally, the grain size data at each time level were averaged to form a new time series. Figure 12 shows the resulting stacked grain size series, for which we propose the acronym Chiloparts (Chinese loess particle timescale). For obvious reasons, the stacked grain size curve is much smoother than the single grain size curves shown in Figures 10 and 11, but all of the major grain size cycles recognized in the individual grain size records are clearly expressed. It should be noted that during the normalization of the grain size data, each of the records was reversed such that finer grain size data correspond to higher values in the Chiloparts record (Figure 12). [24] The stacked Chiloparts record also enables us to calculate the ages of the 37 major paleosol units (Table 1). It should be understood that the paleosol ages estimated on the basis of the previous Baoji grain size timescale [Ding et al., 1994] are mostly younger than those listed in Table 1, because the timescale of the Cenozoic magnetic reversals previously used has proved to be substantially younger than the more recent one [Cande and Kent, 1995]. We thus recommend that these revised soil ages (Table 1) be used in future studies. 6. Correlation to Deep-Sea D 18 O Record [25] It has long been recognized that the pelagic sediments of the open oceans can provide the most continuous and reliable signatures of Quaternary global climate changes, since the d 18 O records of benthic foraminifera tests derived from different oceans correlate closely and display good spatial stability. Although the continuity of marine sediments is markedly affected by factors such as core-drilling processes and bioturbation, a complete oxygen isotope record such as Specmap [Imbrie et al., 1984] is obtainable by stacking signals from different sites. Recently, Shackleton and coworkers constructed a composite marine d 18 O record down to the late Miocene, based on the records from V19-30 [Shackleton and Pisias, 1985], ODP Site 677 [Shackleton et al., 1990] and ODP Site 846 [Shackleton et al., 1995b]. The timescale of this composite record was established by astronomical tuning and constrained by the revised paleomagnetic polarity timescale [Cande and Kent,

17 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS 5-17 Table 1. Time Duration of Major Paleosol Units in the Chinese Loess Sequence, Estimated From the Chiloparts Timescale Unit Age, ka S0 top 0 S0 bottom 1.1 S1 top 7.3 S1 bottom 12.8 S2-1 top 190 S2-1 bottom 219 S2-2 top 234 S2-2 bottom 245 S3 top 307 S3 bottom 336 S4 top 360 S4 bottom 412 S5-1 top 479 S5-1 bottom 531 S5-2 top 549 S5-2 bottom 579 S5-3 top 585 S5-3 bottom 621 S6 top 684 S6 bottom 710 S7 top 760 S7 bottom 787 S8 top 819 S8 bottom 865 S9-1 top 943 S9-1 bottom 958 S9-2 top 971 S9-2 bottom 989 S10 top 1018 S10 bottom 1049 S11 top 1061 S11 bottom 1076 S12 top 1102 S12 bottom 1120 S13 top 1158 S13 bottom 1208 S14 top 1220 S14 bottom 1240 S15 top 1263 S15 bottom 1281 S16 top 1297 S16 bottom 1318 S17 top 1350 S17 bottom 1365 S18 top 1390 S18 bottom 1411 S19 top 1441 S19 bottom 1453 S20 top 1467 S20 bottom 1492 S21 top 1505 S21 bottom 1525 S22 top 1540 S22 bottom 1571 S23 top 1588 S23 bottom 1648 S24 top 1711 S24 bottom 1734 S25 top 1801 S25 bottom 1828 S26 top 1891 S26 bottom 1946 S27 top 2089 S27 bottom 2119 S28 top 2130 S28 bottom 2146 S29 top 2177 S29 bottom 2217 S30 top 2249 S30 bottom 2260 S31 top 2271 S31 bottom 2307 S32 top 2493 S32 bottom ]. According to a recent study, the marine oxygen isotope signal is actually a composite of changes in both deep-water temperature and global ice volume [Shackleton, 2000]. [26] Climate correlation between the Chinese loess and deep-sea sediments has been a major and recurrent concern of loess researchers [Liu, 1985; Kukla, 1987; Kukla and An, 1989; Liu and Ding, 1998; Liu et al., 2000]. However, these earlier studies concentrate mainly on similarities between the two sets of archives. As new records are generated for both loess and marine sediments, renewed scrutiny of matches and discrepancies between the two sets is justified. A comparison of the composite marine d 18 O curve and our terrestrial Chiloparts record is shown in Figure 12. Above paleosol S8 in the loess, or above marine oxygen isotope (MOI) stage 21, variations in both loess grain size and marine d 18 O signals are characterized by relatively low frequencies and high amplitudes. Spectral analysis (not shown) displays the dominance of a 100-ka climatic period in both records. Each of the glacial-interglacial climatic cycles shows a close match between the two records, with discrepancies present only within some specific glacial or interglacial periods. For example, the oxygen isotope record clearly shows three peaks within MOI stage 5, but there is little grain size variability within its counterpart (S1). In the Chiloparts record, clearly expressed second-order grain size cycles are evident in the thick loess units such as L2, L5 and L6, all of which show two grain size maxima and three minima. These cycles represent the influence of orbital precessional cyclicity on the climate in northern China. However, these second-order cycles are not prominent in the deep-sea isotope record, implying that loess grain size variability may be more sensitive to precessional forcing than the marine d 18 O record. The grain size peak indicating the Holocene soil (S0) is exceptionally low compared to other soil units in the Chiloparts record and the marine counterpart (MOI stage 1), perhaps reflecting partial erosion of the top-soil on the Loess Plateau. [27] From MOI stage 21 down to stage 64, climatic variability recorded by both the Chiloparts and isotope records is mainly dominated by the 40-ka obliquity rhythms with relatively low amplitude. An almost cycle-to-cycle correlation is evident between the two records (Figure 12). The major discrepancy between the Chiloparts and the oxygen isotope records is found in loess units L9 and L15. These are exceptionally thick and coarse grained right across the Loess Plateau. They have generally been regarded as reflecting the coldest and driest periods in the entire Quaternary [Liu, 1985]. However, evidence for these two extreme climatic events is not so prominent in the composite d 18 O record, suggesting that the remarkable coarsening and thickening of the L9 and L15 horizons is a product of extreme regional climatic events. [28] Throughout the interval Ma, the composite d 18 O record can be subdivided into 40 stages (MOI stage 64 to 104), with the 41-ka obliquity climatic cycles remaining dominant. Over this time interval, eight major paleosol units (S25-S32) and eight major loess units (L26-L33) have been identified [Rutter et al., 1991; Ding et al., 1993]. However, the thickness of the loess layers varies dramatically, some

18 5-18 DING ET AL.: STACKED QUATERNARY CLIMATE RECORD FROM CHINESE LOESS being very thin (i.e., L28, L30 and L31) and some being extremely thick (i.e., L27 and L32). There are three closely spaced weak soils within L27, and two or three discrete, weak soils within L32. One weak soil within L26 is also recognizable. Most of the major loess-soil units and the weak soils within thick loess beds are clearly expressed in the Chiloparts grain size record (Figure 12). Nevertheless, grain size variability does not show as uniform amplitude as the oxygen isotope record, mainly because of the presence of some particularly coarse-grained loess horizons such as L33, the uppermost part of L32 and L29. Except for L15, L33 is the coarsest loess bed in the entire sequence. The extreme climate on the Loess Plateau, suggested by this loess unit, is totally absent from the isotope record. Thus, distinctive loess stratigraphy and grain size variability make correlation between the Chiloparts and the oxygen isotope records much poorer for the interval Ma than that is the case for Ma. [29] In summary, the Chiloparts and the composite d 18 O records show good correlation in the glacial-interglacial climatic oscillations, the two records matching almost cycle by cycle throughout the interval Ma, and both records document a major shift in dominant climatic periodicity from 41 ka to 100 ka at about Ma. The major discrepancies are as follows: (1) the precessional signals in some glacial periods are not so well expressed in the d 18 O record as in the Chiloparts record; (2) the extreme climatic conditions marked by units L9, L15 and L33 are not so prominent in the d 18 O record; and (3) correlation between the two records throughout the interval Ma is generally poor. In addition, the d 18 O signal shows a tendency to increase from 2.6 Ma to about 0.9 Ma, suggesting a trend toward increasing global ice volume and/or a decrease in marine deep-water temperature. This trend is not so evident in the Chiloparts grain size time series (Figure 12). 7. Discussion and Conclusions [30] It has long been known that pelagic sediments are usually continuous, thus allowing substantially complete retrieval of the preserved paleoclimatic signals. This unique characteristic is mainly attributable to the relative simplicity of the sediment accumulation processes in the open oceans. In general, pelagic sediments consist essentially of far-traveled atmospheric dust and biogenic detritus, with few materials transported by rivers and with few turbulent flows being involved. Sedimentological processes tend to be more complicated in the case of terrestrial sediments such as lacustrines, because of the influence of water level fluctuations, shifting transport channels and so on. Such processes tend to cause depositional hiatuses, so complicating the development of age models and reconstruction of long-term paleoclimatic history. Among terrestrial deposits, however, the loess of China may be regarded as an exception. Several studies [Liu, 1985; Kukla and An, 1989; Rutter et al., 1991; Ding et al., 1993] have demonstrated the almost continuous nature of the loesspaleosol accumulation in some classic sections including Luochuan, Xifeng, and Baoji (Figure 1). The results presented here tend to fortify this view in that the grain size records from five quite widely dispersed sections correlate closely, strongly suggesting that the atmospheric dust deposited in the Quaternary has been well preserved. [31] While the general completeness and continuity of the Chiloparts record is confirmed, questions might reasonably be raised concerning its spatial representativeness and its temporal resolution. It has been reported that dust sedimentation rates in the northwestern part of the Loess Plateau are several times higher than in the southern part [Burbank and Li, 1985]. Figure 13 shows the median grain size records above S2 at Lijiayuan and Xinzhuangyuan in the northwestern part of the Plateau (Figure 1). The thickness of the S0-S2 portion in the two sections is about 43 m and 63 m, respectively, being about 3 times thicker than in the southern and central Loess Plateau. Samples from both sections have been taken at 2-cm intervals and analyzed [Ding et al., 1998b, 1999a]. Both records clearly show three individual soils (S1-1, S1-3 and S1-5) and two loess units (S1-2 and S1-4) within the last interglacial soil of S1. It is noteworthy, however, that such a stratigraphic sequence is not evident in the grain size records from Baoji or the other sections (Figure 2). This strongly suggests that while the Chiloparts grain size time series evidently displays climatic signals on orbital timescales, parts of the signals (particularly in the soils) have been damped. Within the glacial loess horizons of L1 and L2, the general trend of grain size variability in the Lijiayuan and Xinzhuangyuan sections is similar to that in sections in the central and southern Loess Plateau (Figure 2). For example, there are two grain size lows and three highs in L2, which is readily explained as a response to orbital precessional forcing. However, superimposed on this trend, there are numerous grain size oscillations on millennial timescales (Figure 13) within both L1 and L2. An earlier study showed that the millennial scale grain size oscillations of L1 in the loess sections from the northern and northwestern Loess Plateau generally correlate with the GISP2 record of the Greenland ice sheet [Ding et al., 1998b]. It thus appears highly likely that climate records with a temporal resolution higher than orbital timescales will be reconstructed for the northern and northwestern Loess Plateau in future. In this context, the Chiloparts time series should be seen as representative of orbital scale climatic changes recorded only in the southern and central Loess Plateau. [32] Correlation between the Chiloparts and the composite d 18 O records (Figure 12) demonstrates that in the past 1.8 Ma, the two records can be correlated cycle by cycle and that both records document a major shift in the dominant climatic periodicity from 41 ka to 100 ka at about Ma. This may have an important bearing on the forcing mechanisms for loess-soil alternations on the Loess Plateau. Spectral analyses of long-term climatic records long ago identified the periodicity shift at about Ma [Pisias and Moore, 1981; Prell, 1982; Ruddiman et al., 1986, 1989; Ding et al., 1994]. However, the cause of this mid-pleistocene climatic transition remains a puzzle in paleoclimatology [Raymo et al., 1997; Clark et al., 1999]. Although variation in the eccentricity of the Earth s orbit has a distinct 100 ka periodicity, it plays only a minor role in modulating changes in the Earth s incoming insolation [Berger and Loutre,