Reinterpreting climate proxy records from late Quaternary Chinese loess: A detailed OSL investigation

Earth-Science Reviews 80 (2007) 111 – 136 www.elsevier.com/locate/earscirev

Reinterpreting climate proxy records from late Quaternary Chinese loess: A detailed OSL investigation Thomas Stevens a,⁎, David S.G. Thomas a , Simon J. Armitage a,1 , Hannah R. Lunn a , Huayu Lu b a b

Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 3QY, UK Institute of Geography and Ocean Sciences, Nanjing University, Nanjing 210093, China Received 26 May 2006; accepted 8 September 2006 Available online 30 October 2006

Abstract Numerous authors have utilised physical properties of Chinese loess and red clay deposits to develop apparently detailed and continuous past climate records from the Miocene into the Holocene. Many of these studies have further suggested that the principal climatic agent responsible for the aeolian emplacement and diagenesis of Chinese loess, the East Asian Monsoon, has fluctuated rapidly on millennial to sub-millennial timescales, in concert with dramatic changes in the North Atlantic (Dansgaard–Oeschger cycles and Heinrich events) and the Western Pacific (El Niño Southern Oscillation). Much of this evidence is based on reconstructions and age models that are tied to assumptions concerning the nature of loess sedimentation and diagenesis, for example, the belief that loess sedimentation can be viewed as essentially continuous. Some authors have however, cast doubt on these assumptions and suggest that the application of radiometric techniques may be required to determine their validity. Recent studies utilising Optically Stimulated Luminescence (OSL) methods have reinforced these doubts and here, OSL dates obtained at 10 cm intervals from three sites along a transect across the Chinese Loess Plateau have been used, in combination with climate proxy evidence, to test the existing assumptions that underpin many palaeoclimatic reconstructions from loess. In this way, the first time-continuous and independently dated late Quaternary climate reconstruction is developed from loess. The data indicate that sedimentation is episodic and that once emplaced, loess is prone to pedogenic disturbance, diagenetic modification and in some cases erosion. The relationships between proxies and sedimentation rates are also assessed and climatic interpretations based on different age models compared. The implications of these findings for reconstructions of climate from loess are explored and comparisons are made between the developed palaeoclimate records and evidence from ice and ocean cores. This exercise also highlights important information concerning the relative influence of forcing mechanisms behind East Asian Monsoon change over the late Quaternary. © 2006 Elsevier B.V. All rights reserved. Keywords: luminescence dating; Chinese loess; Quaternary; East Asian Monsoon; sedimentation; diagenesis

⁎ Corresponding author. Fax: +44 1865 275 885. E-mail addresses: [email protected] (T. Stevens), [email protected] (D.S.G. Thomas), [email protected] (S.J. Armitage), [email protected] (H.R. Lunn), [email protected] (H. Lu). 1 Present address: Department of Geography, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK. 0012-8252/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2006.09.001

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1. Introduction Sediments on the Chinese Loess Plateau have accumulated and weathered as red clay, loess and palaeosols throughout the Neogene Period (Guo et al., 2002). It is generally held that the Middle and Upper Pleistocene loess–palaeosol successions (hereafter referred to as Chinese loess) preserve a continuous and highly resolved record of late Quaternary climate change, unparalleled in other terrestrial sediments (Liu and Ding, 1998; An, 2000; Porter, 2001; Sun and An, 2006). Various attempts have been made to correlate the Chinese loess record with the astronomically tuned oceanic δ18O record (Imbrie et al., 1984; Prell et al., 1986; Martinson et al., 1987) with dating provided through proxy-based methods (Kukla et al., 1990), statistical correlation to the δ18O record (Bloemendal et al., 1995), independent astronomical tuning (Ding et al., 2002) and direct correlation to the δ18O record (Balsam et al., 2005). However, recent work using Optically Stimulated Luminescence (OSL) dating (Singhvi et al., 2001; Lai and Wintle, 2006; Stevens et al., 2006) has raised significant questions about the validity of such approaches. In this paper we first review previous approaches to Chinese loess analysis, and then detail how the application of OSL dating to samples from key sections on the Chinese Loess Plateau (Fig. 1) is both raising significant doubts about previous Quaternary climate change interpretations, and is providing a new

Fig. 1. Map of sites dated in this paper using OSL. Modern isohyets and potential dust transport directions are shown as broken lines and arrows respectively. Map of China (inset) shows Chinese Loess Plateau (light shading), potential source regions (dark shading), major rivers and study area (box) (after Stevens et al., 2006).

Fig. 2. Normalized stacked grain-size record (Chiloparts) for last 2.6 Ma (after Ding et al., 2002).

interpretation of the history of loess accumulation and palaeosol development. 2. Proxy records in loess Analyses of long sequences in the Chinese loess record have been used to provide unrivalled terrestrial records of climate change in the late Cenozoic, extending back over 2.5 Ma (Kukla et al., 1990). Inferred changes in the strength and persistence of the East Asian Monsoon have been used to propose global and regional mechanisms of climate forcing, as well as teleconnections and feedbacks within the climate system (Ding et al., 1995; Sun et al., 1998a; Guo et al., 2000; Nugteren et al., 2004; Sun, 2004). A key assumption of such interpretations is that, once deposited, the resolution of the climate signal in the loess record has not been markedly impacted upon by post-depositional diagenesis. As such, a long and continuous record of palaeoclimate conditions at deposition can be gained from the loess itself and from the palaeosols that have developed during times when soilforming conditions were enhanced. Research has also focused on the potential for rapid climate change signals to be recorded in loess, often linking high temporal resolution variation in loess proxies to Heinrich events (Bond et al., 1992), Dansgaard– Oeschger cycles (Dansgaard et al., 1993) and the late glacial oscillations (Alley et al., 1993). Numerous authors have suggested that cyclical oscillations in loess proxies at sub-orbital timescales are indicative of millennial scale climate variation, linked to the North Atlantic (Chen et al., 1995; Porter and An, 1995; Xiao et al., 1995; Guo et al., 1996a; Lu et al., 1996; Zhou et al., 1996; An and Porter, 1997; Chen et al., 1997; Zhang et al., 1997; Ding et al., 1998; Liu and Ding, 1998; Zhou et al., 1998; Ding et al., 1999; Fang et al., 1999; Lu et al., 1999; Heslop et al., 1999; Lu et al., 2000; Rousseau and Kukla, 2000; Vandenberghe and Nugteren, 2001; Zhou et al., 2001;

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Fig. 3. Visual correlation of SPECMAP δ18O (Imbrie et al., 1984) with Huanxian high sensitivity low frequency bulk susceptibility (10− 8 m3 kg− 1) and marine oxygen-isotope stage.

Xiong et al., 2002; Zhang et al., 2002; Huang et al., 2003; Liu et al., 2005). As these records extend back for more than 2.5 Ma, they potentially provide a terrestrial record of Quaternary climate change that is unrivalled for its duration and resolution (Fig. 2). There have been two broad views taken in producing chronologies of deposition for the loess record. The first view, held by many researchers, is that aeolian loess deposition has essentially been continuous in the Pleistocene, preserves a continuous record of globallysignificant climate changes across the Loess Plateau as a whole, and has the resolution to record millennial scale climate change in a manner that parallels the marine isotope record (e.g. Liu and Ding, 1998). The second view holds that, in common with other terrestrial sediments, loess deposition is episodic, spatially discontinuous and that disconformities are common in the preserved record (e.g. Singhvi et al., 2001). Until recently, the first view has informed attempts to date and extract palaeoclimate records from Chinese loess. We now review the implications of these two views for the ways in which chronologies of loess deposition have been developed.

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boundaries are then applied to the loess record in a stratigraphically ‘counting from the top’ manner (Fig. 3). In the first method, ages for proxy record changes between the MIS-equivalent boundaries have been determined by linear interpolation (Chen et al., 1995; Xiao et al., 1995). It is assumed that down-section variations in sedimentation rate are insignificant between ‘known age’ tie points such as MIS boundaries. Reference section calibration, the second approach to determining sediment ages between MIS boundaries, has been more extensively used on Chinese loess (Porter and An, 1995; Lu et al., 1996; An and Porter, 1997; Chen et al., 1997; Zhang et al., 1997; Fang et al., 1999; Lu et al., 2000; Xiao et al., 2002; Vidic and Montañez, 2004; Sun et al., 2006). Initially, the age of sediments at a reference section or site is calculated through correlation and peak matching with the oceanic δ18O record (e.g. Vandenberghe et al., 1997). Based on this age model, a relationship between loess property changes and sedimentation rate is developed (Fig. 4). This relationship is then extrapolated to include the whole sequence or further sites and thus sedimentation rate, and consequently age, can be established simply by analysis of the relevant loess property. Grain-size records have been used in this way, under the assumption that the climatic influences on grain-size and sedimentation rate are the same, or that the relationship between them has been unchanging (Porter and An, 1995; Vandenberghe et al., 1997; Lu et al., 1999; Vandenberghe and Nugteren, 2001; Nugteren and Vandenberghe, 2004; Nugteren et al., 2004). Similarly, Kukla et al. (1988, 1990) suggested that magnetic susceptibility changes are related to fluctuations in magnetic signal ‘dilution’ resulting from varying loess depositional rates. The extent of this dilution could be used to calculate sedimentation rates, again once a reference section or site was established. Recently, dating has also been refined through extrapolating geomagnetic

2.1. Dating the Chinese loess record by proxy The Chinese loess record has for the most part been dated using methods tied to the astronomically tuned oceanic δ18O record (Imbrie et al., 1984; Prell et al., 1986; Martinson et al., 1987). Two main types of methodology have been applied, based on the supposed broad similarity between the oceanic δ18O record and magnetic susceptibility or grain-size variation in loess. Both approaches regard stratigraphic boundaries or major changes in proxies as being synchronous with marine oxygenisotope stage (MIS) boundaries. The ages of the MIS

Fig. 4. The relationship between grain-size and sedimentation rate measured by Vandenberghe et al. (1997) for reference layers in the last two glacial–interglacial cycles (after Vandenberghe et al., 1997).

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Fig. 5. Regional estimates of Mass Accumulation Rate (MAR) for MIS 1 using stratigraphic boundary age assignment, correlation of magnetic susceptibility to marine oxygen-isotope records and radiometric methods (after Kohfeld and Harrison, 2003). Central point is median value and bars denote the range of data.

axial dipole intensity changes from deep-sea composite records (Liu et al., 2005). In addition to the above, direct orbital tuning has also been employed to date loess (Liu and Ding, 1998; Yu and Ding, 1998; Sun and Huang, 2006). However, all such methods assume an intrinsic similarity between loess proxy records and a target timescale (e.g. the SPECMAP curve) and also that the climatic mechanism behind proxy variations in loess responds in a linear fashion to orbital forcing. This approach also necessarily obliterates potentially important phase relationships between loess deposition and the target timescale. 2.2. Assessing loess proxy chronologies These methods rely on two underpinning assumptions: that aeolian deposition has been uninterrupted, and that once formed, the proxy signals within the sediments have not been disturbed by further erosional or diagenetic processes. Increasingly, new evidence is suggesting that these assumptions may not in fact be valid, while alternative direct dating methods may provide more robust chronologies and palaeoenvironmental interpretations of these extensive deposits. Kohfeld and Harrison (2003) highlight discrepancies between mass accumulation rates calculated through proxy-based and absolute age models (Fig. 5), suggesting that poor correlation of magnetic susceptibility records to marine isotope stratigraphy, and spatial variations in sedimentation and diagenesis, may account for this. Importantly, an analysis of existing luminescence ages derived from Chinese loess by Singhvi et al. (2001) provides direct support for the contention that not only has loess sedimentation been episodic in nature, but also that extended hiatuses in the record are common.

It had in fact been noted by Derbyshire et al. in 1997 that some Chinese loess lithostratigraphic sequences might be incomplete as evidenced through ‘welding’ and truncation of palaeosols and the existence of erosion surfaces. Through micromorphological investigation, Guo et al. (1996b) have demonstrated that deflation of sediments due to wind action has led to the presence of disconformities. This suggests that the widely assumed relationships between loess proxy and marine isotope records may be far from straightforward, impacting on the utility of loess for identifying rapid climate changes and linking them to the North Atlantic. Lu et al. (2004a) have proposed that some sections of loess sequences are affected by diagenesis and discontinuous sedimentation to such an extent that age resolution is dramatically reduced, in some cases to a degree that obscures orbital cycles. Pedogenic overprinting has considerable implications for palaeoclimatic interpretations based on loess stratigraphy. Feng et al. (2004a,b) and Liu et al. (2004) have recently shown that pedogenic processes during the formation of the last interglacial soil extended into the underlying penultimate glacial loess (Fig. 6). Thus stratigraphic boundaries may not be indicative of a temporal distinction between glacial and interglacial age loess, impacting on the preserved climate record, a point confirmed by recent OSL applications to deposits from NW China by Lai and Wintle (2006) and Stevens et al. (2006). Kemp and Derbyshire (1998) indicate that many palaeosols on the Loess Plateau are accretionary, although with individual soils indicating distinct climatic episodes ‘welded’ together into a single palaeosol stratum. In addition, Rokosh et al. (2002) state that local variations in the balance between sedimentation rate and soil

Fig. 6. Conceptual model for pedogenic overprinting on previously emplaced loess. (1) As sediment is deposited, loess builds up. (2) Climate changes and sedimentation rate decreases with soil formation on both newly deposited sediment and pre-existing loess. (3) Sedimentation rates increase again and pedogenesis is reduced. In this way the climatic boundary when soil forming began has been obscured (after Liu et al., 2004).

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development may impair the validity of using proxies from individual sites to infer regional or global palaeoclimatic change. In contrast however, Huang et al. (2003) use luminescence dating to suggest that last glacial age Malan Loess formation does not serve as parent material for the Holocene soil. However, it is unclear how these luminescence dates were obtained. Finally, some authors have suggested that a traditional interpretation of loess as purely an aeolian sediment may be in error, and that colluvial, alluvial and mass wasting processes contribute to accumulation (e.g. Fang et al., 1994). The significance of these complications is compounded by the role stratigraphic and proxy boundaries have played, as ‘tie points’, in correlating deposits, and climatic interpretations, across the Loess Plateau. These boundaries have been assigned ages from MIS stratigraphy assuming that ice volume and East Asian Monsoon changes are synchronous and that boundaries represent the palaeosurface at times of monsoon change (Fig. 3). The first assumption has never been empirically demonstrated and recent evidence suggests that monsoon intensity changes may have been asynchronous across China (An et al., 2000, 2005; He et al., 2004) causing the loess proxy record to lead or lag ice volume and marine δ18O changes. When added to the problem of pedogenic overprinting, as noted above (e.g. Feng et al., 2004b), age assignments based on loess–MIS boundary correlations from individual sites and reference sections become extremely unreliable. Indeed, direct orbital tuning (e.g. Yu and Ding, 1998) is further limited by the unclear response of the East Asian Monsoon to orbital forcing. Further difficulties in the interpretation of climate proxy records from loess stem from the uncertain nature of current sedimentation in relation to past rates and the relative influences of different source regions. Modern patterns of loess deposition suggest that dust storms emanating from the north and northwest (Fig. 1) of the Loess Plateau are responsible for much of the current air fall (Derbyshire et al., 1998; Lu and Sun, 2000). This pattern is consistent with a winter monsoon controlled wind-transporting agent, although the precise rates and timing of sedimentation as well as the specific synoptic mechanisms responsible for dust transport and deposition remain unclear (Derbyshire et al., 1998; Sun, 2002; Zhang et al., 2003). Part of the difficulty lies in the uncertainties associated with the relative influence of source regions. Current understanding of the source of modern dust emplaced on the Loess Plateau is limited by the significant content of anthropogenic Pb, which makes the fingerprinting of source regions difficult (Jahn et al., 2001). However, Sun (2002) has recently demonstrated that mountain processes have produced the vast amount of

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material that has subsequently formed Quaternary loess. It is stated that Gobi desert areas immediately to the north of the Loess Plateau (Fig. 1) have acted as vast silt holding areas rather than source regions sensu stricto. However, certain authors (Bowler et al., 1987; Liu et al., 1994; Honda et al., 2004) have argued for a more northwestern source (Fig. 1). Such uncertainties require careful consideration when interpreting loess records from China. 3. Independent dating of loess Given the difficulties underpinning chronologies of loess deposition based on the extrapolation of records from proxy indicators, and the growing availability of a wide suite of radiometric dating methods, it has been suggested that independent direct dating of loess is the only means by which an effective chronology of deposition and climate history may be gained (Derbyshire et al., 1997; Singhvi et al., 2001; Kohfeld and Harrison, 2003; Lu et al., 2004a). However, relatively few studies have utilised independent dates obtained at the resolution required to address issues of loess sedimentation and many independent dates published prior to recent methodological advances may contain inaccuracies (Kohfeld and Harrison, 2003). In this section we evaluate the use and impact, as well as the potential, of independent dating of the loess record from China. 3.1. Non-radiometric independent dating of the Chinese loess record Given the great thicknesses and potential ages of Chinese loess, reversals in the geomagnetic dipole field have been used to date deposits well into the Miocene (Guo et al., 2002). However, the last major polarity shift is the Brunhes–Matuyama transition, at 780 ka (Cande and Kent, 1995). It remains controversial as to whether shorter-lived fluctuations of the dipole field, which would increase the potential application of the technique in the Late Pleistocene, are recorded in loess (Fang et al., 1997; Zhou and Shackleton, 1999). Therefore, for dating loess deposition over time periods within the last 7–8 glacial cycles, other methods are required. Amino acid geochronology has also been utilised to uncover the relative ages of loess units in China (Oches and McCoy, 2001). However, the technique is at present only able to resolve loess units/sub-units and therefore not exploitable for analysis of rapid changes in sedimentation. Further age models have been based on isotopes or elemental concentrations. Based on the discovery that concentrations of the cosmogenic isotope 10Be in loess from Luochuan (35°45′ N, 109°25′ E; Fig. 7) are highly

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Fig. 7. Map of China showing location of sites mentioned in text. Box indicates area of Fig. 1.

coherent with the marine δ18O record, Shen et al. (1992) created a loess age model using recorded variations in 10 Be values. However, in addition to the unquantified effects of 10Be mobility and complex depositional processes, such an age model also has the same limitations as those constructed through tuning other proxies to the marine δ18O record. An age model based on Rb concentrations in loess at Luochuan has also been proposed by Chen et al. (2000) (Fig. 8). Again however, the technique relies on tuning part of the loess record to marine δ18O stratigraphy to develop a relationship between Rb concentrations and sedimentation rate. It therefore seems that the only methods suitable for dating loess at the resolution required to test the assumptions made about sedimentation and diagenesis may be radiometric dating techniques.

from Lac du Bouchet in France and correlated it with that obtained from a loess profile near Caoxian (36°33′ N, 104°38′ E; Fig. 7), at similar latitude in China. As this age model relies on correlation of geomagnetic variation that occurs contemporaneously at different regions, through reference to a radiocarbon dated lake record, it is independent of tuning to records that have no determined physical basis for covariance. However, because of the uncertain way that geomagnetic secular variation is recorded in loess (Zhou and Shackleton, 1999) this age model may also potentially be in error. U-series dating has had very limited application to loess. It has been used to date the formation of calcrete nodules at Luochuan and Liujiapo (34°20′ N, 109°20′ E; Fig. 7) giving very precise results (Rowe and Maher, 2000). However, calcrete nodules generally only develop below soils and appear to post-date their formation (Rowe and Maher, 2000) thus

3.2. Radiometric dating of the Chinese loess record Radiometric dating offers many advantages over ages extrapolated from relationships between climate proxy variables in loess and other global chronologies. Only independent radiometric dating can highlight leads and lags in loess deposition relative to changes in the global climate system. Detailed dating of key profiles also offers a means to examine both the applicability of models of loess deposition relying upon the use of tie points to correlate between sites, and any regional variations in sedimentation rates, diagenetic effects and pedogenic impacts. Various radiometric dating methods have been applied to the Chinese loess record. Heslop et al. (1999) utilised a radiocarbon dated record of geomagnetic secular variation

Fig. 8. Relationship between Rb concentration and calculated sedimentation rate (mm a− 1) from loess and palaeosol units at Luochuan (35°45′ N, 109°25′ E; Fig. 7; after Chen et al., 2000).

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U-series cannot be used to examine sedimentation rates within the system. Radiocarbon dating has also been applied directly to organic material within loess deposits, often to date highfrequency events (Guo et al., 1996a; Zhou et al., 1996, 1998, 2001; Porter and Zhou, 2006). For example, Zhou et al. (1996, 1998, 2001) used radiocarbon dating on many sections across the Loess Plateau (Fig. 9) to suggest that the East Asian Monsoon exhibited dramatic fluctuations around the same time as the Younger Dryas. However, the relative timing of the fluctuations in the summer monsoon compared to North Atlantic variation was considered still unclear. Porter and Zhou (2006) have recently suggested, using the appearance of radiocarbon dates (indicating pedogenesis) in many sections across a transect of the Loess Plateau, that East Asian Holocene climate may have varied considerably and is intrinsically

Fig. 9. Uncalibrated radiocarbon dates and stratigraphy at Midiwan (37°39′ N, 108°37′ E; Fig. 7) indicating stratigraphic changes dated as late glacial (after Zhou et al., 1998).

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Fig. 10. Radiocarbon age-depth relationship for Biandukou (38°13′ N, 100°52′ E; Fig. 7), northwest of the Chinese Loess Plateau (after Yu et al., 2006).

linked to the North Atlantic. The information from such an exercise is extremely important for determining the extent of, and regional variation in, soil formation during deposition of younger loess. However, correlations to such fine scale individual events in the North Atlantic as proposed by Bond et al. (2001) may somewhat exceed the limits of the technique. A similar proposal for climatic teleconnections between the East Asian Monsoon, the North Atlantic and also the Western Pacific has been made by Yu et al. (2006). Holocene millennial scale variability is suggested from radiocarbon-dated loess, (Biandukou; 38°13′ N, 100°52′ E; Fig. 7), and palaeolake sequences to the northwest of the Loess Plateau. In this case, radiocarbon dates from the loess sequence have been used to develop an age-depth relationship using linear interpolation (Fig. 10). Although the low number of dates limits the findings, the use of radiometric dating has allowed detailed investigation of the forcing mechanisms behind the East Asian Monsoon. Changes in the region are proposed as synchronous, with global climate shifts during Termination 1 forced by a mechanism involving the El Niño Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO). During El Niño and negative NAO conditions, the East Asian Winter Monsoon is enhanced by westerly through flow and the summer monsoon weakened by southward migration of the ITCZ and a cooler Western Pacific Warm Pool. The opposite situation occurs during La Niña and positive NAO. However, radiocarbon dates do have limitations when considering changes in sedimentation rate. Those derived from organic matter in loess are often difficult to interpret due to incorporation of older or younger carbon (Wang et al., 2003). Also radiocarbon methods cannot date loess directly and this limits the applicability of the technique in loess, especially in the arid northwest. Some results are consistent with the stratigraphy (Zhou et al., 2001) but if

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this is misleading due to mixing or pedogenic overprinting, incorrect interpretations may result. In addition, radiocarbon dating is limited to material younger than ∼30 ka and can only be calibrated with any certainty back to 26 cal ka BP (Reimer et al., 2004). Thus additional techniques are required to date actual loess sedimentation during the entire late Quaternary, an understanding of which is essential for testing the two conflicting hypotheses of loess development. 3.3. Luminescence dating The potential for luminescence dating of quartz from aeolian sediments has been increasingly realized since the 1980s (Aitken, 1998; Stokes, 1999). Many minerals, including quartz and most feldspars, store energy within their crystal structure that has been deposited by ionising radiation from the environment and in addition, cosmic rays. The amount of stored energy increases with the amount of radiation to which a crystal has been exposed (Duller, 2004). Luminescence dating provides direct ages of the time that has elapsed since this trapped charge was last zeroed, either thermally, or as in the case for loess, from exposure to sunlight (Aitken, 1998). By measuring the stored energy in a crystal through sample stimulation in the laboratory, an estimate of the dose of radiation a sample has experienced since zeroing can be obtained. By dividing this value by the measured annual dose rate at the sample's location, an age in years since sample burial can be obtained. The potential for using this method to establish not only depositional ages but also rates of sedimentation through appropriate within-site applications has made luminescence techniques particularly suitable for application to the Chinese loess record. Early work on loess was conducted using thermoluminescence (TL) methods involving measurement of the proportion of energy emitted as light when aliquots are heated under laboratory conditions (Wintle, 1981, 1982; Li and Sun, 1985). This measured signal can be used as an estimate of the palaeodose, the dose of ionising radiation a sample has received since burial, commonly termed the equivalent dose (De). As it can be difficult to establish the extent to which a TL signal was bleached by sunlight prior to burial, applications to loess need to account for any residual signal that has not been removed during sediment transport (Lu et al., 1988). Applications of TL during the infancy of the technique greatly enhanced the potential to date loess deposition directly, though TL ages in this pioneering work (Wintle, 1981, 1982; Li and Sun, 1985) may suffer errors not only due to issues surrounding the measurement of a nonsunlight bleachable TL signal, but also due to the un-

known influences of signal depletion in feldspars (anomalous fading) and signal saturation in quartz (Wintle and Huntley, 1982; Debenham, 1985; Wintle, 1985, 1987). Despite these issues and significant differences in the laboratory protocols employed by different researchers (Wintle, 1990), important applications of TL to loess were made and are summarised in Table 1. Optically Stimulated Luminescence (OSL) obtains a De from a sample during exposure to light, as opposed to heat (TL), under controlled laboratory conditions, essentially reproducing the conditions that occurred during sunlight bleaching of sediment. Thus OSL measures only the sunlight bleachable signal, removing the need to subtract an optically stable residual signal, as is the case with TL. First applied to Chinese loess by Musson et al. (1994), OSL has come to dominate luminescence dating applications, with developments in laboratory protocols (e.g. Murray and Wintle, 2000; Lai et al., 2003; Roberts and Duller, 2004; Lai, 2006; Wang et al., 2006; Stevens et al., in press) increasingly refining the use of the method and the precision and accuracy of the ages that it produces. A number of studies have utilised various OSL methods of sample preparation and stimulation (Table 2) with the use of quartz grains stimulated by blue light appearing to yield the most consistent results (Roberts et al., 2001; Roberts and Wintle, 2001, 2003; Watanuki et al., 2003; Küster et al., 2006). 3.4. OSL applications to the Chinese loess record Current OSL protocols offer the potential to obtain dates for loess deposition from intensively sampled sediment exposures in order to test assumptions of continuous sedimentation and limited diagenesis (Roberts et al., 2001; Singhvi et al., 2001; Kohfeld and Harrison, 2003; Lai, 2006; Maher and Hu, 2006; Stevens et al., 2006, in press). At Duowa (35°39′ N, 102°38′ E; Fig. 7) on the western Loess Plateau, Roberts et al. (2001) sampled an upper Holocene section at 10–75 cm intervals (Table 2). Applying OSL to the 4–11 μm grain-size fraction, their ages suggested almost continuous sedimentation, with enhanced rates after 2.5 ka due to anthropogenic inputs. Also noteworthy is that the rate of sedimentation does not decrease during soil development. This suggests that both pedogenesis and sedimentation rate, linked to different aspects of the East Asian Monsoon (Liu and Ding, 1998), can be enhanced at the same time. The ages have recently been used to develop a time-continuous reconstruction of Holocene rainfall (Maher and Hu, 2006). Lai and Wintle (2006) and Lai (2005) have recently applied OSL dating to samples collected at 33–50 cm

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Table 1 Examples of TL ages from Chinese loess, 1980–2006 Age range

Location

Grain type and size

240 17.8–84 17.7–84 22–60 174 ± 14 74–390 24.3–137.1 25.1–141.4 2–290 9.95–17.2 27–56 15.8–55.1 61–792 70.0 ± 7.4 69 ± 7 40.9–85.6 74.5–123.2 10–134 41–152 64–149 10.1–128.1 16 ± 0.6

Yuanguo Zhaitang Zhaitang Liadong Wushen Bannar Kansu and Lujiaowan Lantian Beiyuan Luochuan Baxie Off Bohai Sea Liujiapo Shandong Caijiagou Fuxian Yuanbo Jiuzhoutai Caojiagou and Shimao Lanzhou Tuxiandao and Yuanbo Weinan Zhengjiang

Quartz, n/a Quartz, 90–120 Polyminerals, 4–11 n/a n/a n/a Polyminerals, 4–11 n/a Polyminerals, 4–11 Polyminerals, 4–11 Polyminerals, 4–11 Polyminerals, 4–11 n/a n/a n/a n/a n/a Polyminerals, 4–11 Polyminerals, 4–11 n/a Polyminerals, 4–11 Polyminerals, 4–11

10.9–101.1

Tuxiandao

n/a

(ka)

Authors

Comments

Li and Sun, 1985 Lu et al., 1987b Lu et al., 1987a Jiao et al., 1987 Zheng, 1988 Wen and Zheng, 1988 Lu et al., 1988 Li et al., 1990 Forman, 1991 Zhou et al., 1992 Li and Zhou, 1993 Musson et al., 1994 Zheng et al., 1994 Sun et al., 1995a Sun et al., 1995b Chen et al., 1997 Fang et al., 1997 Sun et al., 1998b Frechen, 1999 Chen et al., 2000 Wang et al., 2000 Watanuki and Tsukamoto, 2001 Lu et al., 2004b

Likely age underestimate

(μm)

intervals from loess over the Holocene–Pleistocene and MIS 2–3 boundaries from Yuanbo (35°38′ N, 103°08′ E; Fig. 7; Table 2). Sedimentation was found to be essen-

Difficult to interpret ages Difficult to interpret ages Older ages likely inaccurate

Older ages likely inaccurate

Older ages likely inaccurate

Difficult to interpret ages Uncertain origin for ages Different methods tested

Age overestimate shown through comparisons to OSL Infrared and thermal stimulation

tially continuous across the stratigraphically inferred MIS 2–3 boundary (Fig. 11), and where rate variations were identified, they did not coincide with mean sample grain-

Table 2 Examples of OSL ages from Chinese loess, 1990–2006 Age range

Location

(ka)

Grain type and size

Authors

Comments

First OSL (infrared) dating of loess Green and infrared (IR) stimulation IR stimulation Green and IR stimulation Green, IR and post-IR blue stimulation Post-IR blue stimulation IR stimulation Tested blue light, IR and post-IR blue light stimulation Tested IR and post-IR blue light stimulation No information given Infrared and thermal stimulation

(μm)

15.5–55.1 10.8–80.2 50.6–109 1–16.8 3.1–4.4 0.41–12.03 9.2–90.4 14.9–94.6

Liujiapo Zhengzho Lanzhou Midiwan Zhengjiang Duowa Xujiachun Urumqi and Zhengjian

Polyminerals, 4–11 Polyminerals, 4–11 Polyminerals, 4–11 Quartz/feldspar, 125–150 Quartz/polyminerals, 4–11 Polyminerals, 4–11 Polymineral, 4–11 Quartz/polyminerals, 4–11

Musson et al., 1994 Zhao et al., 1998 Frechen, 1999 Lai et al., 1999 Watanuki and Tsukamoto, 2001 Roberts et al., 2001 Lai et al., 2001 Watanuki et al., 2003

0.07–52.8

Hexi Corridor

Polyminerals, 4–11

Stokes et al., 2003

2.1–11.7 10.9–101.1 3.1–11.5 4.1–11.7

Wulipu Tuxiandao Majiayuan Shiyou He Baiyong He

n/a n/a Polyminerals, 4–11 Quartz, 90–160

Huang et al., 2003 Lu et al., 2004b Huang et al., 2004 Küster et al., 2006

0.5–50.0 0.2–29.6

Yuanbo Beiguyuan Xifeng and Shiguanzhai

Quartz, 45–63 Quartz, 40–63

Ages compared to IR stimulated polymineral ages Lai and Wintle, 2006; Lai, 2005 Blue stimulation Stevens et al., 2006 Blue stimulation

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susceptibility (e.g. He et al., 2004) and grain-size variation (e.g. Vandenberghe and Nugteren, 2001) upheld by the detailed OSL chronologies? Given the two contrasting viewpoints in the literature (e.g. Derbyshire et al., 1997; Liu and Ding, 1998), these questions require urgent attention if significant progress is to be made in the utilisation of loess deposits for understanding past climate. 4. Analysis of loess sections at Shiguanzhai, Xifeng and Beiguoyuan

Fig. 11. OSL dates as a function of depth and sedimentation rates for Yuanbo section (35°38′ N, 103°08′ E; Fig. 7; after Lai, 2005).

size variations or the previously assumed MIS 2–3 boundary. These results demonstrate that it is not possible to construct an age model for loess using a simple relationship between grain-size and sedimentation rate (e.g. Vandenberghe et al., 1997). Further, this suggests that variations in deposition rates and processes may have been independent of simple relationships with major marine-record derived changes in the global climate system. In addition, the stratigraphic boundary between the Holocene soil and last glacial loess was shown not to be synchronous with the MIS 2–1 boundary. In the remainder of this paper, we utilise a detailed sampling strategy implemented by Stevens et al. (in press) at two previously studied sites (Kukla et al., 1988; Maher and Thompson, 1991; Liu and Ding, 1993; Rousseau and Kukla, 2000) and one unstudied location close to the well-studied Lantian section (e.g. Lu et al., 1988) on the Chinese Loess Plateau (Fig. 1) to generate a suite of OSL ages, first presented in Stevens et al. (2006), as well as proxy data. These data, and the location of the three sites, allow three critical questions raised in the literature to be addressed: how did sedimentation, diagenesis and preservation potential vary across the Loess Plateau over the late Quaternary; do ages support previous age models based on correlation of proxy variation to marine records and orbital tuning (e.g. Porter and An, 1995; Vandenberghe et al., 1997; Vidic and Montañez, 2004); and are stratigraphies and climatic interpretations based on the proxy records of magnetic

Across the modern SE to NW climatic and depositional gradient (Lu and Sun, 2000; Sun, 2002) there is significant facies variation in the upper Quaternary Malan Loess (L1 unit) and Black Loam (S0 unit) formations associated with different environmental, sedimentary and diagenetic contexts. Analysis of the three sites, Shiguanzhai, Xifeng and Beiguoyuan (Fig. 1), across this gradient by applying OSL dating as well as magnetic susceptibility and grain-size methods at high vertical sampling resolution (10 cm) enables the questions highlighted above to be directly addressed. It should be noted that the Malan Loess (L1) and Black Loam (S0) formations have traditionally been assigned a Late Pleistocene and Holocene age, respectively (e.g. Liu and Ding, 1998). Beiguoyuan comprises two sections located near Huanxian, Gansu Province on the northern Loess Plateau (Fig. 1). The section with the most complete Holocene record (36° 37′ 21.3″N, 107° 17′ 12.2″E, 1523 m a.s.l) lies several km NW of the main section (36° 37′ 36.2″N, 107° 16′ 57.4″E, 1545 m a.s.l) where a longer record of loess deposition is preserved. Sampling concentrated on the full Holocene section, entirely composed of Black Loam and the upper 3 m of the main section containing the Black Loam and upper Malan Loess. A new Xifeng, Gansu Province, exposure (35° 32′ 09.4″N, 107° 43′ 13.5″E, 1281 m a.s.l) was sampled over the upper 4.30 m to include the entire Black Loam and upper Malan Loess. At the Shiguanzhai site (34° 10′ 22.2″N, 109° 11′ 45.5″E, 708 m a.s.l), Shaanxi Province, on the south-eastern fringe of the plateau, the upper 1.75 m of the section was sampled to include the upper Malan Loess, Black Loam palaeosol and a distinct loess layer lying above, here termed L0. Full details of the analytical techniques used in age determination are presented in Stevens et al. (in press) with age data presented in Stevens et al. (2006). Coarse quartz silt (40–63 μm) was used for equivalent dose (De) determination using the SAR procedure (Murray and Wintle, 2000) and through construction of standard growth curves (Roberts and Duller, 2004) using a Risø

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TL-DA-15 TL/OSL reader. Dose rates were calculated using U, Th and K contents measured using ICP-MS and -AES. Cosmic dose rates were calculated using present day burial depth (Prescott and Hutton, 1994). Samples for magnetic susceptibility analysis were pretreated by air drying, disaggregation and sieving at 2 mm prior to weighing and measurement in 25 × 25 mm diamagnetic pots. Measurements were taken using high (4.7 kHz) and low (0.47 kHz) frequency fields at high sensitivity (0.1) using a Bartington MS2 magnetic susceptibility meter. Bulk low frequency susceptibility (χlf) and high frequency susceptibility (χhf) measurements are given in units of m3 kg− 1 and % frequency dependence (χfd) is defined as (χlf −χhf) /χlf. Frequency dependence is intended to estimate the relative contribution of fine viscous grains at the border between superparamagnetic and single domain to the total ferromagnetic assemblage (Chen et al., 1995; Dearing et al., 1996). It has been suggested that such grains are the most sensitive indicator of the East Asian Summer Monsoon (Maher and Thompson, 1991). Grain-size measurements were made using a Cilas 920 laser granulometer after preparation according to that outlined in Sun et al. (2002). Multiple grain-size statistics were calculated through the GRADISTAT program (Blott and Pye, 2001) using the Folk and Ward (1957) and moments methods. The reproducibility of grain-size and

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magnetic susceptibility methods is given in Table 3, estimated by repeating machine measurements 20 times on two samples. 4.1. Patterns of sedimentation, preservation and disturbance OSL dates and calculated sedimentation rates (±1σ) for the sites are displayed in Fig. 12 with full interpretations outlined in Stevens et al. (2006). Although errors on the dates and calculated sedimentation rates limit analysis to timescales of centennial resolution or lower, the fine sampling interval and regularity of the OSL dates bring to light information pertaining to the nature of sedimentation and subsequent preservation and diagenesis of late Quaternary loess. At the studied sites, sedimentation on the Loess Plateau appears to be more episodic than continuous and sedimentation rates can vary by an order of magnitude over very short timescales. For example, at Beiguoyuan main section, a significant change in sedimentation rate occurs at around 19 ka (Fig. 12B). The greatest changes in sedimentation rate appear to occur in the NW with less variation further to the SE. The large change in the upper Malan Loess formation of Shiguanzhai (Fig. 12D) is only explicable by invoking the influence of anthropogenic sediment addition. General sedimentation rates are highest in the NW of the study region (c.f.

Table 3 Reproducibility measurements from repeat measurements of selected samples. The mean is arithmetic, calculated using the method of moments, sorting is determined using the Folk and Ward (1957) method and c.v. denotes the coefficient of variation. Errors for χfd are calculated through taking the mean and standard deviation of individual samples, not through propagating errors of averaged χlf and χhf measurements Magnetic susceptibility CH04/1/2 (n = 20) χhf (10− 8m3kg− 1) χlf (10− 8m3kg− 1) χfd (%)

Mean

S.D.

470.8 451.5 4.1

0.5 2.3 0.5

CH04/1/25 (n = 20) c.v. (%) 0.1 0.5 12.1

Total (n = 40)

Mean

S.D.

c.v. (%)

Average c.v. (%)

348.6 343.4 1.5

0.9 1.2 0.5

0.2 0.4 30.3

0.2 0.4 21.2

Grain-size CH04/2/2 (n = 20)

Mean (μm) Sorting Median (μm) N63 μm (%) b4 μm (%) 63–16 μm (%) 16–4 μm (%) N31 μm (%) b16 μm (%) N31:16–4 μm 63–16:16–4 μm

CH04/4/10 (n = 20)

Mean

S.D.

c.v. (%)

33.0 3.0 31.1 11.8 5.7 59.5 17.7 49.7 28.7 2.8 3.4

0.1 0.0 0.2 0.1 0.1 0.5 0.6 0.3 0.5 0.1 0.1

0.4 0.3 0.7 1.0 1.9 0.9 3.2 0.7 1.7 3.6 3.8

Total (n = 40)

Mean

S.D.

c.v. (%)

Average c.v. (%)

21.7 2.9 19.0 1.3 6.6 55.8 28.7 27.4 72.6 1.0 1.9

0.7 0.0 0.8 0.5 0.2 0.6 0.7 1.9 1.9 0.1 0.1

3.4 0.5 4.2 38.3 3.0 1.1 2.5 7.1 2.7 9.6 3.6

1.9 0.4 2.5 19.7 2.4 1.0 2.9 3.9 2.2 6.6 2.9

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Fig. 12. Dated portions of (A) Holocene and (B) main sections at Beiguoyuan as well as (C) Xifeng and (D) Shiguanzhai. L0, S0 and L1 refer to the loess–palaeosol units with all of Fig. 12a in S0. Sedimentation rates and errors (±1σ) calculated through linear regression are also shown (after Stevens et al., 2006).

Kohfeld and Harrison, 2003), consistent with the suggestion that sediment source regions and transport pathways are to the north of the Loess Plateau (Fig. 1; Lu and Sun, 2000; Sun, 2002). High temporal variability suggests that the transporting agents or sediment source were highly changeable. Assuming aeolian deposition and given that sediment supply was not likely to be the limiting factor (Zhang et al., 2003), it is inferred that winter monsoon winds and the frequency of dust storms associated with depressions have varied in a non-linear fashion, implying that the winter monsoon has the capacity to shift dramatically between different states. The interpretation of this is complicated further by the discovery of hiatuses in the record. At Beiguoyuan a significant hiatus occurs between 10 and 13 ka (Fig. 12A; B). Given the likely conditions at the time, combined with the otherwise fairly unbroken sedimentation, this is best interpreted as an erosional unconformity associated with deflation during a period of enhanced winter monsoon strength. The corresponding aridity and enhanced wind speed would convert Beiguoyuan into a source region rather than a sediment trap. This provides further evidence that the winter monsoon was highly variable but also implies that secondary or reworked loess will be found to the SE on the Loess Plateau. In addition, it highlights the possibility that sediments deposited at sites in the north of the Loess Plateau have a lower overall preservation potential than sites to the SE. Clearly, the likelihood of erosion surfaces in loess of China must not be overlooked. Possible hiatuses also occur in the sequences further to the south, although intense pedogenic

disturbance or non-aeolian deposition in the record limits interpretations (Fig. 12C; D). These are more likely due to breaks in sedimentation associated with reduced winter monsoon strength or erosion surfaces from sheetwash processes during enhanced summer monsoon activity. The short, millennial scale duration of these breaks and unconformities further suggests that the winter monsoon does not respond in a linear fashion to external forcing. Apparent pedogenic disturbance or non-aeolian deposition at Xifeng and Shiguanzhai is manifest in the OSL age distributions down section (Fig. 12C; D). At the base of the Holocene soil a sequence of the strata yields highly variable OSL ages with multiple age inversions of up to 10 ka. This variation is only reasonably accounted for by the possibility of extensive and enhanced bioturbation during a break or severe reduction in sediment accumulation or by emplacement of sediments through alluvial, colluvial or mass wasting processes. In the bioturbation scenario, during periods of landscape stability, burrowing, soil forming processes and development of krotovina through rooting of plants will have cycled, surfaced and reburied significant volumes of sediment. During this process, exposure of sediments to light would have rezeroed any pre-existing signal that had accumulated since initial deposition. Bateman et al. (2003) demonstrated that OSL ages can indeed be significantly altered by pedoturbation and where sampling is not systematic this may lead to errors in the interpretation of dates. Thus in this stratum, OSL dates are recording not initial sediment emplacement, but the last time the sediments were exposed at the surface. In the central and

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Fig. 13. Comparisons at Beiguoyuan main section of 1996 and 2003χlf data by depth.

southern Loess Plateau, in strata that experienced nonaccretionary soil formation, this will mean that the age of sediment deposition and most recent signal zeroing may not be coincident. In the non-aeolian scenario, colluvial, alluvial and mass wasting processes would account for the anomalous dates. In the case of alluvial sedimentation, there is an increased likelihood of partial bleaching of the luminescence signal prior to deposition. In mass wasting or colluvial processes, there will be a mixing of different age sediments. Both these scenarios provide further support for the interpretation of highly variable and episodic loess sedimentation in China and have significant implications for the use of proxy records for climatic reconstruction. Clearly if sediments were emplaced by non-aeolian processes or have experienced the level of pedogenic disturbance required to lead to OSL age inversions of up to 10 ka, the record of climatic variation contained within will be obscured to this extent also. Thus, where such variability in the OSL dates occurs, it implies that the proxy record of past climate will also be obliterated. The suggestion, based on luminescence dates by Huang et al. (2003), that there has been no pedogenesis on glacial age loess even in the humid south of the Loess Plateau, in contrast to the findings above, can also be explained by effects of bioturbation. At Shiguanzhai, in the south of the study region (Fig. 1) there are anomalously young ages at the top of the Malan Loess formation (Fig. 12D), similar to those presented by Huang et al. (2003). Systematic and high-resolution dating reveals these not to be true depositional ages, as explained above, but artefacts of bioturbation. Thus, the young ages of the upper Malan Loess formation presented in Huang et al. (2003) are also likely explained in this way. A further significant outcome of dating sections that have been intensively vertically sampled is the suggestion that site-specific influences are more important than has previously been believed. Comparison of overlapping records from the two sections at Beiguoyuan (Fig. 12A; B)

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highlights large differences in sedimentation rates during the late glacial. This suggests that local climatic, geomorphologic or ecological conditions directly impart a significant influence on either sedimentation or preservation potential, further implying that generalized regional interpretations from single sites will be problematic. To further expand on this, two records of magnetic susceptibility from the main section at Beiguoyuan in 1996 and 2003 are presented in Fig. 13. It is apparent that while the χlf records generally match, some parts are significantly different (see Table 3 for error limits). This suggests that facies differences are significant even over very small lateral distances. The large differences in χlf values above 40 cm may indicate reworking of the upper Holocene soil and consequent lateral differences in reworking of magnetic minerals. This suggests that where reworking is common, the proxy record may be highly variable at a site over distances of a few meters, making palaeoclimatic interpretations difficult. These findings concerning changes in sediment emplacement, pedogenic disturbance and erosion down and across section as well as across the Loess Plateau prompt us to consider the implications for the interpretation of climate proxy records from Chinese loess. High sampling resolution OSL dating uncovers, for the first time, episodic sedimentation, erosional events, pedogenic disturbance and site-specific lateral facies variation in Chinese loess. These data favour the interpretation of the loess record propounded by Derbyshire et al. (1997) and Singhvi et al. (2001) amongst others. Such a development has profound implications for the understanding of past climate records obtained from loess. 4.2. OSL age and proxy variation In order to consider the implications of the above findings for the interpretation of the proxy climate record,

Fig. 14. χfd and N31 μm:16–4 μm grain-size variation by OSL-based age model for Beiguoyuan Holocene section.

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grain-size and magnetic susceptibility records are plotted here by OSL ages. Figs. 14–17 show the grain-size ratio of N 31 μm to 16–4 μm and χfd records plotted by OSL age for Beiguoyuan Holocene, Beiguoyuan main, Xifeng and Shiguanzhai sections respectively (Fig. 1). Age models were constructed using the OSL ages in Fig. 12 to develop an age–depth relationship. Sedimentation rate appears constant over certain portions of the studied sections (Fig. 12); hence minimum and maximum OSL ages are taken for these portions, with ages interpolated between. Each stratum of the sections with a differing sedimentation rate, calculated in this case from the maximum and minimum ages, is stacked together by age, including hiatuses. Proxy records are plotted using this age–depth relationship and where intense pedogenic disturbance or non-aeolian sedimentation is evident from the OSL ages, records are abandoned as misleading. In this way, for the first time, late Quaternary proxy records from loess taken at standard depth intervals are replotted using a completely independent timescale. However, there are likely to be inaccuracies in this timescale that prevent fine scale interpretation of proxy variation, due to the averaging of OSL ages that themselves have relatively large uncertainties. In addition, average proxy errors calculated through repeat measurements (Table 3) are also plotted. The developed chronologies outlined below provide palaeoclimatic information free from any assumptions concerning sedimentation outlined in Section 2. Analysis of such records provides valuable insight into the effect of incorrect assumptions on the reconstructed climate record, but also important new information about past East Asian Monsoon changes. Fig. 14 shows proxy variation by OSL age for the Beiguoyuan Holocene section. Despite the entire section comprising the Black Loam (S0) formation (previously assumed to be Holocene in age), a record from the latest

Pleistocene is also preserved. High grain-size ratio values and low χfd values are shown during the latest Pleistocene, with some millennial scale grain-size changes in the late glacial. As shown by the relatively small spacing between proxy data points, sedimentation rate is also rather high at this point (∼17 cm/k.y.). Unfortunately, the Holocene– Pleistocene boundary age loess is absent. Above this disconformity, χfd values show a gradual increasing trend to peak values between 4 and 2 ka. In addition, although the change to lower grain-size ratio values is more abrupt, values are at a minimum between 2 and 5 ka. However, sedimentation rates were higher over that interval (∼17 cm/k.y.) with reduced rates (10 cm/k.y.) occurring earlier, between 10 and 5 ka. Fig. 15 shows proxy variation with OSL age for the main section at Beiguoyuan. A record including the last glacial maximum is preserved with many rapid changes in the summer and winter monsoon proxies (magnetic susceptibility and grain-size respectively) observed over the interval between 22 and 19 ka. Grain-size ratios and sedimentation rates are also greatly enhanced (40–50 cm/

Fig. 15. χfd and N31 μm:16–4 μm grain-size variation by OSL-based age model for Beiguoyuan main section.

Fig. 17. χfd and N31 μm:16–4 μm grain-size variation by OSL-based age model for Shiguanzhai.

Fig. 16. χfd and N31 μm:16–4 μm grain-size variation by OSL-based age model for Xifeng.

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k.y.). At 18.5 ka sedimentation rates drop rapidly by an order of magnitude to 5 cm/k.y. until around 13.5 ka. This is accompanied by a gradual reduction in grain-size ratio, although no real change in χfd. Following this period there is again a hiatus in the record that coincides with that in the Holocene section. This suggests that the gaps in the record at the two Beiguoyuan sections were caused by the same, regional event, supporting the idea of a greatly enhanced winter monsoon causing deflation in the area. This disconformity at Beiguoyuan main section therefore again encompasses the Holocene–Pleistocene boundary. Sedimentation commences again at around 10 ka (10–16cm/k.y.) at a similar rate to that of the Holocene section but there is no preserved record later than 7.5 ka. Early Holocene grain-size ratios are generally lower than those around 20 ka but are similar to late glacial values. χfd values are significantly higher than glacial values. Fig. 16 shows proxy variation with OSL age at Xifeng. A record extending to 29 ka is preserved. Because of the effects of intense pedogenesis or non-aeolian deposition on the OSL dates, the interval between ∼19 and 6 ka is not suitable for palaeoclimatic reconstruction. Between 29 and 28.5 ka, sedimentation rates are extremely high (N 100 cm/k.y.) although the grain-size ratio is low. χfd values are large over this high sedimentation rate interval and rapidly drop at the same time as a dramatic reduction in sedimentation rate at 28.5 ka (11–13 cm/k.y.). Between 28.5 and 19 ka this sedimentation rate is maintained, with a steady decline in χfd values and a steady increase in grain-size ratio. After the heavily bioturbated/non-aeolian sequence, the climate record and OSL age model becomes resolvable again between 6 and 0.25 ka. Sedimentation rates are similar to those of between 28.5 and 20 ka between 0.25 and 4.5 ka (10–12 cm/k.y.) and are higher (18–20 cm/k.y.) between 4.5 and 6 ka. Grain-size ratios are reduced between 2 and 5 ka, coincident with elevated χfd values. Towards the end of the Holocene, grain-size and χfd values increase and decrease respectively. Fig. 17 shows proxy variation with OSL age for Shiguanzhai. Unfortunately, because of the low sedimentation rates and heavy bioturbation or non-aeolian deposition in the sequence, only a small part of the palaeoclimate record from Shiguanzhai is resolvable, although a Mid and Upper Holocene record is present. The small grain-size ratios and large χfd values associated with this site are expected due to the distal location of its sediment source and to its relatively moist climate (Fig. 1). Lower χfd values and higher grain-size ratios are evident between 30 and 23 ka, although the resolution is low due to low sedimentation rates. The Holocene record becomes resolvable from around 9 ka,

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but the addition of sediment, probably due to agricultural activities, prevents a climate proxy record being preserved for the latest 0.6 ka. A minimum in grain-size ratio occurs between 6 and 3.5 ka although χfd values do not appear to vary much in the Holocene. Sedimentation rates in the Holocene are relatively constant at around 3.5–4 cm/k.y., up until emplacement of the L0 unit associated with anthropogenic sediment addition. 4.3. Interpretation of the proxy record changes A number of important issues concerning the dynamics and nature of the East Asian Monsoon over the late Quaternary, as well as how changes are recorded during loess emplacement and diagenesis, arise from these reconstructions. In general, the trends suggested in the literature of lower sedimentation rates and grain-size against higher magnetic susceptibility values during the Holocene, and vice versa during the cold periods of the Pleistocene (e.g. Liu and Ding, 1998), are here confirmed. It can be inferred from this that the East Asian Winter Monsoon was in general strengthened during the cold periods of the Pleistocene and that the summer monsoon was considerably weakened. The Holocene age Black Loam formation does appear to show a trend towards strengthened values of the summer monsoon and a weaker winter monsoon. However, further examination of the records yields information relevant to the interpretation of climate proxy information from loess. In the first instance, it will be noted that errors on the proxy records limit detailed analysis. This indicates that records such as χhf or individual grain-size categories with lower associated errors may allow more secure palaeoclimate interpretations. Further important issues to consider are the extent to which the sediments undergo diagenetic alteration that can obscure the climate record or the influence of non-aeolian deposition. At Xifeng and Shiguanzhai, nearly half the dated records are not resolvable due to one or both of these processes. Furthermore, low sedimentation rates in large parts of the loess sequences greatly reduce resolution, prohibiting detailed reconstruction of the past monsoon. However, the existence of very high sedimentation rate, or ‘pulsed sedimentation’, units may allow very highresolution records of past monsoon dynamics to be reconstructed. Due to the precision attainable using OSL dating, the temporal resolution of these pulsed units is unfortunately not yet obtainable. However the stratum at Beiguoyuan centred on 20 ka (Fig. 15) clearly contains a record that far exceeds the resolution of that in ocean basins. Indeed, the existence of these pulsed sedimentation units confirms the rapid nature of monsoon change in

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China. Detailed analysis of these units is required so as to utilise their full potential. The climatic mechanisms behind proxy change can also be investigated using the reconstructions outlined above. A striking finding is that there is no simple relationship between grain-size and sedimentation rate. Despite a lack of empirical evidence, conventional understanding suggests that source aridity controls sedimentation rate while wind speed is the dominant control on grain-size, with both factors being controlled by the winter monsoon (Xiao et al., 1995; Vandenberghe and Nugteren, 2001; Sun, 2004; Sun et al., 2004; Sun and An, 2006; Sun et al., 2006). However, at Xifeng between 29 and 28.5 ka, sedimentation rates were extremely high (N100 cm/k.y.) although grain-size was relatively small (Fig. 16). Conversely, during the emplacement of the pulsed sedimentation unit at Beiguoyuan, the grain-size values are relatively large. This demonstrates either that source aridity and wind speed are not linked in a linear fashion, or that the climatic influences hypothesized as the mechanisms behind grain-size and sedimentation rate are incorrect. It seems likely that aridity and wind speed may be linked through the Siberian High Pressure system. However, their changes are not necessarily coeval and the absence of definitive proof either way precludes quantitative reconstructions. Thus, extreme caution should be used when trying to separate the influence of wind speed and source aridity on grain-size and sedimentation rate. In addition, the evidence that increased winter monsoon strength may have eroded loess material at Beiguoyuan indicates that regional wind speed, not just source aridity, has an influence on sedimentation rate, in contrast to the suggestions of many authors (e.g. Sun and An, 2006). Examining the relative timing of changes in different proxies can shed light on the interactions and covariation of their climatic causes. Again however, no straightforward relationship can be seen in the data. Changes in the proxies are not always in anti-phase, and on occasion they covary. This indicates that long-term palaeomonsoon change is more complex than the simple anti-phase relationship between summer and winter monsoon change that has been assumed previously (e.g. Xiao et al., 1995). On a finer scale, the new age models suggest that proxies vary in and out of phase over sub-millennial timescales. While it is uncertain as to whether these changes actually reflect the past dynamics of the East Asian Monsoon, the data suggest that loess has the capacity to record centennial scale variability in deposited grain properties. Whether these reflect local influences, or rapid and substantial shifts of East Asian Monsoon regimes, is at present unclear.

From the above records, some conflicting evidence is apparent concerning the timing of the reported ‘MidHolocene Optimum’ (An et al., 2000; Feng et al., 2004c; He et al., 2004; An et al., 2006). At Beiguoyuan Holocene section, a reduction in sedimentation rates in the early Holocene coincides with the timing of the Holocene Optimum suggested by An et al. (2000) and He et al. (2004) in this region. However, grain-size and χfd values indicate that reduced winter monsoon activity and an enhanced summer monsoon occurred much later, between 5 and 2 ka. If it is assumed that sedimentation rate indicates source area aridity, then the results indicate that source regions were least arid in the early Holocene, with decreasing wind speeds and increasing precipitation during the mid to late Holocene in the northern Loess Plateau. However, problems may lie in the uncertain climatic and environmental mechanisms behind the various proxy measures. If the grain-size and magnetic susceptibility values indicate enhanced summer monsoon conditions with reduced winter monsoon influence, then this climatic ‘optimum’ is dated as being considerably more recent than has been suggested (An et al., 2000; He et al., 2004). This interpretation is supported by evidence at Xifeng (Fig. 16). A trough in grain-size values and peak in χfd is dated to between 2 and 5 ka and, in contrast with Beiguoyuan, corresponds to a reduction in sedimentation rate. Further detailed dating of Holocene sections is required to resolve this issue. It is suggested that sedimentation rates are influenced by a range of factors such as vegetation and geomorphology at the site, source aridity, wind speed and regional climate. It is therefore difficult to assign a particular climatic cause to its variation and as such, the timing of grain-size and χfd changes are taken as indicative of specific aspects of the East Asian Monsoon. Therefore the OSL dates support a more recent climatic ‘optimum’ during the Holocene than has been suggested by other authors (An et al., 2000; He et al., 2004). A further issue at Xifeng is the relatively high χfd values and low grain-size ratios towards the base of the section, potentially caused by wetter and less windy conditions during MIS 3. However, the stratigraphic boundary between L1LL1 and L1SS1 occurs ∼2 m below the dated MIS 2–3 boundary (Martinson et al., 1987). As such, if as is suggested by the OSL age model, the proxy changes are indicative of East Asian Monsoon variation during the transition between MIS 3 and 2, then there is no change in visual stratigraphy accompanying this MIS boundary. The evidence presented above indicates that the assumptions employed when examining the past climate recorded in loess from China may have led to

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considerable errors in the interpretation of that record. Critically, these results suggest that loess cannot be considered to hold a continuous and highly resolved climate record. The implications of such a suggestion are far reaching and require further analysis. Prior to outlining these it is necessary to determine the effects of incorrect assumptions on previous age models. 4.4. Discrepancies in previous age models Errors in age models will affect the palaeoclimatic interpretation of a proxy record significantly. Age models utilised in loess research have often relied on assumptions of continuous sedimentation, an unchanging relationship between grain-size and sedimentation rate and age equivalence of stratigraphic or proxy boundaries with marine oxygen-isotope stage changes (e.g. Porter and An, 1995). The above results indicate that these assumptions are incorrect and that this error would propagate into the age model so that only very general trends in climate would be accurately constrained. Significant breaks in deposition and probable erosional events that manifest themselves as disconformities are not readily apparent from the stratigraphic or proxy record, and combined with the rapid changes in sedimentation rate, this undermines the assumption of continuous sedimentation. The apparent lack of relationship between changes in grain-size and sedimentation rate, contrary to the findings of Ding et al. (2001), undermine the grainsize-based age models utilised previously (e.g. Porter and An, 1995; Vandenberghe et al., 1997; Nugteren and Vandenberghe, 2004). In addition, the OSL ages outlined above demonstrate that the S0–L1 stratigraphic boundary is not synchronous with the Holocene–Pleistocene boundary (Fig. 12). Significant pedogenesis on glacial

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age loess occurred at all sites, with age differences between the Holocene–Pleistocene boundary and the stratigraphic S0–L1 boundary being 7.5–8.5 ka. An offset also occurs at the MIS 2–3 boundary at Xifeng although no such ‘pedogenic overprinting’ explanation is possible for this discrepancy. Presumably pedogenesis during late MIS 3 was not strong enough to produce a palaeosol. The discrepancies identified between mass accumulation rates calculated through so called ‘correlation dating’ and radiometric techniques (Kohfeld and Harrison, 2003) may be partially reduced by incorporating these errors into the former of these techniques. Construction of age models based on tying stratigraphic or proxy boundaries, defined as the midpoint of proxy change, to MIS boundaries allows a direct comparison to the OSL-based age model. Fig. 18 shows two such age models constructed for Xifeng assuming a) that MIS 1–2 and 2–3 boundaries (Martinson et al., 1987) are time equivalent to the loess stratigraphic S0–L1LL1 and L1LL1–L1SS1 boundaries, or b) that the mid-point of a grain-size change is coeval with the MIS boundaries. While it was not possible to construct age models using the grain-size and sedimentation rate relationship method suggested by Nugteren and Vandenberghe (2004) or through tuning to orbital frequencies, due to the relatively short age range covered in this dataset, the principle of using ‘knownage’ boundaries, inherent in these techniques, is tested using the above methods. The apparent continuity of the proxy record at Xifeng using the MIS tied age models is an artefact of the inability of the methods applied to uncover pedogenic disturbance or non-aeolian sedimentation in the section. The stratigraphy-based age model (Fig. 18A) underestimates the extent of the record preserved by around

Fig. 18. χfd and N31 μm:16–4 μm grain-size variation by (A) stratigraphic boundary-based age model and (B) grain-size boundary age model for Xifeng section.

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8 ka, while conversely, the grain-size-based age model overestimates the age of the lowermost sediment by over 6 ka. This is due to the extremely high sedimentation rates occurring at around 28 ka not being uncovered by the grain-size model. The age offset between the S0–L1 and Holocene–Pleistocene boundary is highlighted using the grain-size-based age model but not using the stratigraphic age model. Both MIS tied age models also fail to recognize the large changes in sedimentation rate observed using the OSL ages. Utilisation of either MIS tied age model would lead to a considerably different climate interpretation to that obtained using the OSL age model due to offsets in apparent monsoon variation of up to 10 ka, although larger offsets are apparent in the stratigraphy-based model (Fig. 18A). Using either MISbased age model suggests the apparent age of the Holocene ‘Optimum’ is shown as between 5 and 9 ka, which is more consistent with the interpretations of An et al. (2000) and He et al. (2004). However, this age is not consistent with the OSL dating, suggesting that the age of the Holocene ‘Optimum’ on the Chinese Loess Plateau has previously been overestimated. Assuming that the OSL ages used are correct, the above results demonstrate that using MIS tied dating techniques will cause considerable error in age models, and as a consequence, climatic interpretation. The use of these age models, and related age models resting on similar assumptions, to develop so called continuous records and to date rapid climate shifts will lead to misinterpretation. This issue requires addressing urgently; it is recommended that research on past climates reconstructed from loess should be more conservative in the choice of assumptions. Independent radiometric dating is at present the only method that can highlight the true

nature of loess deposition, preservation and diagenesis. It is therefore of critical importance that future studies utilise these techniques in order to reinterpret the palaeoclimate record contained within loess. 5. Reinterpretation of the loess record Fig. 19 shows a) χfd and July insolation changes (W m− 2) for 65° N (Berger and Loutre, 1991) and b) grain-size ratio variations by age for all four sections. As the age models are entirely independent, the data can be compared without incorporating circular reasoning. Proxy variations show broad similarities over the study region, although interpretations are limited by the considerable gaps in the records. Important differences occur in millennial scale events and the specific onset of broad patterns. There seems to be more intersite variation exhibited in the grain-size values. This perhaps indicates that site-specific conditions impact upon the general synoptic signal recorded in grain-size variations to a greater extent than in magnetic susceptibility. The general trend in χfd is shown to be similar in shape to the July insolation record at 65° N, although there appears to be a significant lag in the onset of peak Holocene soil forming conditions. χfd (Fig. 20) and grain-size (Fig. 21) values for the studied sites (except the poorly resolved record of Shiguanzhai) can be compared to proxy records from various archives. Comparison of our records to the stacked SPECMAP curve (Imbrie et al., 1984) highlights a general similarity between records, perhaps most clearly seen in χfd (Fig. 20A). The higher χfd and lower grain-size values at the base of the studied Xifeng section seems to correspond broadly with MIS 3 in the

Fig. 19. (A) χfd (%) with July insolation changes (W m− 2) for 65° N (Berger and Loutre, 1991) and (B) N31 μm:16–4 μm grain-size variation by age for all studied sections. Dark squares indicate Beiguoyuan Holocene section, dark diamonds indicate Beiguoyuan main section, light triangles indicate Xifeng and light squares indicate Shiguanzhai section. This key is repeated for Figs. 20–22.

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Fig. 20. χfd (%) with (A) SPECMAP stacked δ18O (Imbrie et al., 1984), (B) NGRIP δ18O values (NGRIP, 2004), (C) methane (ppb) recorded in GISP (Brook et al., 1996) and (D) carbon dioxide (ppm) in Vostok (Petit et al., 1999) for all studied sections except Shiguanzhai. Key is given in the caption of Fig. 19.

SPECMAP record but the response is apparently much larger. This broad similarity reinforces the idea that the summer and winter East Asian Monsoons are linked to changes in ice volume (Ding et al., 1995) and further reinforces the interpretation presented above that no stratigraphically evident pedogenesis occurred during the emplacement of late MIS 3 sediments. Comparisons to δ18O values from the NGRIP ice core (NGRIP, 2004) again highlight broad similarities, especially with grainsize changes (Figs. 20B; 21B). Due to disconformities and errors in the loess age models, it is difficult to correlate specific events in the North Atlantic with changes in the East Asian Monsoon, as has been attempted in the past (e.g. Porter and An, 1995). However, the grain-size record at Beiguoyuan seems to indicate significant oscillations in the East Asian Winter Monsoon around the same time as the dramatic shifts in climate associated with the late glacial oscillation in the NGRIP core. Further, a possible reason for the nonlinear response in grain-size and χfd records of Xifeng around 28 ka may be highlighted in the NGRIP record.

There appears to be broad correspondence between a substantial shift in the monsoon proxies and Greenland interstadials 3 and 4. Unfortunately the record ends at 29 ka so this cannot be substantiated. Extending the Xifeng age model will be crucial aspect in understanding this change. A difference between the NGRIP δ18O values and the loess records is the lack of evidence for a Holocene ‘Optimum’ over Greenland (Fig. 20B; 21B). This suggests that while the East Asian Monsoon is linked to ice volume and North Atlantic variation, an insolation or other forcing mechanism still significantly influences the system. Figs. 20 and 21, C) and D) show loess proxy variation plotted against CH4 (Brook et al., 1996) and CO2 (Petit et al., 1999) respectively. Again, both loess proxy records show a broad similarity to the trends in CH4 and the oscillations during Termination 1 are also apparent. However, much of the millennial scale detail is different, most apparently in the Holocene where CH4 levels tend to show reduced values during the midHolocene. The resolution of the Vostok CO2 record is

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Fig. 21. N31 μm:16–4 μm grain-size ratios with (A) SPECMAP stacked δ18O (Imbrie et al., 1984), (B) NGRIP δ18O values (NGRIP, 2004), (C) methane (ppb) recorded in GISP (Brook et al., 1996) and (D) carbon dioxide (ppm) in Vostok (Petit et al., 1999), all inversed, for all studied sections except Shiguanzhai. Key is given in the caption of Fig. 19.

significantly lower than the Greenland CH4 record, and as such comparisons on the millennial scale are difficult. However, as with CH4, there is a broad similarity in the trends of atmospheric CO2 and the general trends of the East Asian Monsoon. Fig. 22 shows continental components of the Ca flux in the GRIP ice core (Fuhrer et al., 1993) against a)

grain-size and b) sedimentation rate calculated from linear regression (Stevens et al., in press). Both grainsize increases and sedimentation rate increases broadly correspond with increases in aerosol flux over Greenland. Some differences are evident, such as the lack of Holocene variability and greater last glacial variability in the Greenland record. This may in part be attributable

Fig. 22. Flux of continental Ca (ppb) in GRIP ice core (Fuhrer et al., 1993) against (A) N31 μm:16–4 μm grain-size ratios and (B) sedimentation rate (cm/k.y.) for all sites. Key for A) is given in the caption of Fig. 19.

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to gaps in the record caused predominantly by deflation at Beiguoyuan. The high sedimentation rate from 28 ka at Xifeng occurs close to a peak in dust flux in the Fuhrer et al. (1993) record, although there are other peaks in the Greenland record that are not accompanied by changes in sedimentation rate. It therefore appears that dust flux over Greenland is related to changes in sedimentation and grain-size over the Chinese Loess Plateau. However, the relationship may not be straight forward, as indicated by discrepancies. Whether this is a causal relationship, with changes in wind speed and source aridity both contributing, remains unclear. Certainly, the great volumes of dust mobilised over the Loess Plateau will have been widely distributed, potentially having a significant effect on global climate (Kohfeld and Harrison, 2001). In all the records shown in Figs. 20–22 it is apparent that while grain-size changes appear to follow the general trends shown in atmospheric carbon dioxide and methane, North Atlantic climate, Greenland dust, and global ice volume, χfd changes lag these records by some thousands of years. This may be due to a delay in χfd signal acquisition but may also be explained in part by a lagged response of the East Asian Summer Monsoon to external forcing mechanisms. It appears that the East Asian Winter Monsoon and consequently the Siberian High Pressure system are more directly connected to the external forcing mechanisms of insolation, global ice volume and atmospheric carbon. This further suggests that variations in the Siberian High will be influenced by abrupt changes in ice volume and North Atlantic climate. Direct confirmation of this from loess, in light of the evidence presented above is lacking, although changes in the East Asian Summer Monsoon have been reconstructed from speleothem evidence and oceanic records (Wang et al., 2001; Yuan et al., 2004; Oppo and Sun, 2005; Wang et al., 2005). These records suggest that the summer monsoon has been forced by solar variation, insolation changes and North Atlantic climate. While correlating specific events is difficult, even with high precision 230 Th dates, there is strong evidence linking the East Asian Monsoon to insolation changes and a wide range of other forcing factors. Indeed, radiocarbon dating has been used to suggest that monsoon climate is directly forced by North Atlantic variation and ENSO (Yu et al., 2006). The evidence presented above for loess is complementary to this argument. There appear to be a number of forcing mechanisms responsible for East Asian Monsoon change, operating to varying degrees over different time frames. It is interesting to note that over the last glacial and the Holocene, East Asian Summer Monsoon changes in

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the speleothem record from Dongge Cave in China are in phase with, or slightly lag, insolation changes, with transitions shown as abrupt (Yuan et al., 2004). This trend is similar to that shown in the loess record, although the change between the Holocene and Pleistocene may be less abrupt in loess and exhibits a larger lag. The record of past monsoon change in loess may be complicated by the varying penetration of the monsoon front and also delay in magnetic susceptibility signal acquisition. Thus, while the evidence presented above supports previous interpretations from speleothem and ocean sediment evidence, a great deal of more work is required to elaborate on the relative influence of various forcing factors on the East Asian Monsoon. 6. Summary and implications The analysis of OSL dates presented in this paper suggests that many interpretations of loess climate proxy data have been based on assumptions about loess sedimentation that are untenable. The use of highresolution OSL dating has enabled many of these assumptions to be tested for the first time. From such examination, it is suggested that the reservations of certain authors (e.g. Derbyshire et al., 1997; Singhvi et al., 2001; Kohfeld and Harrison, 2003; Lu et al., 2004a) have been upheld. The loess record is fragmentary, prone to site-specific influences, and sedimentation is highly variable. Intense bioturbation and non-aeolian sedimentation may have obscured substantial portions of the past climate record and there are large discrepancies in the relative timing of MIS and epoch boundaries compared to stratigraphic delineations in loess. This implies that an urgent refocusing of research goals and methodology used in loess study is needed. An approach more consistent with the analysis of other terrestrial sediments is required, appreciating that while the loess record is unique in resolution over some periods of time, it is also prone to the same limitations that afflict other terrestrial deposits. Highly resolved sections of the loess record may yield data unparalleled in resolution, but their utilisation relies on the systematic and detailed use of radiometric dating. Furthermore, the investigation of highly bioturbated strata and nonaeolian deposition remains vital in the understanding of proxy datasets. Based on this new interpretation, some important information concerning the past dynamics and forcing agents behind the East Asian Monsoon has been uncovered. It appears that although trends in grain-size and sedimentation rate tend to follow broadly similar

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patterns, their relationship is not constant. This suggests that they are controlled by different forcing factors, possibly different manifestations of winter monsoon variation, and implies that the use of grain-size changes for estimating sedimentation rates based on their relationship in a MIS correlated sample section may lead to significant error. The East Asian Monsoon appears to change broadly in line with the patterns of Northern Hemisphere insolation changes. However, the timing and much of the finer detail of the proxy records cannot be explained in this way, suggesting that the East Asian Monsoon exhibits complex and non-linear response to a variety of forcing mechanisms operating over different time periods. Increased proxy variation during the late glacial suggests a dynamic teleconnection to the North Atlantic and to other climate systems, possibly operating through the Siberian High and the Western Pacific Warm Pool. A discrepancy in the timing of the Holocene ‘Optimum’ over the Loess Plateau requires further investigation. The results presented in this paper indicate that the Holocene ‘Optimum’ lags peak summer insolation values, contrary to previous findings (An et al., 2000; He et al., 2004). This has also been suggested for the Mu Us desert, north of the Loess Plateau, using OSL dates on dune sequences (Lu et al., 2005). Using independent dates, the limiting pre-condition of tying East Asian Monsoon change to global ice volume can be discarded and independent investigation into the timing of variations in the East Asian Monsoon conducted. This represents the next step in Chinese loess research and will shed light on forcing mechanisms behind the East Asian Monsoon, how they operate, and the climatic manifestations of variations in their relative influence. It is becoming clear that the idea of a simple oscillation between relative dominance of the summer and winter monsoons in China masks the complexity of this regional climate system. Obtaining a fundamental understanding of monsoon dynamics and driving forces represents an even more pressing challenge given the human and climatic consequences of a future shift in monsoon intensity. Acknowledgements The authors thank Yi Shuangwen and Sun Xuefeng for their help in the field, NERC for access to their ICP-MS and -AES facilities (Award OSS/279/0205) and the National Natural Sciences Foundation of China (Grant 40325007, 40121303) for fieldwork support. TS thanks Jesus College, Oxford University Centre for the Environment and Dr. Stephen Stokes for financial support.

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