Pedogenic modification of loess: significance for palaeoclimatic reconstructions

Earth-Science Reviews 54 Ž2001. 145–156 www.elsevier.comrlocaterearscirev

Pedogenic modification of loess: significance for palaeoclimatic reconstructions Rob A. Kemp ) Centre for Quaternary Research, Department of Geography, Royal Holloway, UniÕersity of London, Egham, Surrey TW20 0EX, UK Received 8 October 1999; accepted 15 December 2000

Abstract This review considers the role of pedogenic processes in modifying wind-blown dust Žloess., concentrating particularly on the ways that resulting properties may be interpreted as indicators of past climatic conditions and changes. Emphasis is placed on the sequences of palaeosols developed within loess deposits that are frequently regarded as some of the best terrestrial equivalents of marine-sediment records of long-term global climatic change. A palaeosol is generally interpreted in terms of the broad pedogenic processes and environments assumed to be currently responsible for that type of soil forming at the present surface. Even the very presence of a palaeosol may have palaeoclimatic significance, however, in that it is often taken to indicate a period of relative land surface stability and warmer andror moister conditions between cold andror arid phases of loess accumulation. In reality, it may be more useful to consider many loess–palaeosol sequences in terms of changing balances between pedogenesis and loess accumulation over geological time. In most regions, it seems that the balance swings towards pedogenesis during interglacials or interstadials when sediment supply and transport are limited and the climate is warmer andror wetter. Where accumulation rates are still appreciable during these ‘soil-forming intervals’, however, the soils and palaeosols may be accretionary with surface build-up keeping pace with pedogenesis. Welding may also occur where covering sediments are insufficiently thick to isolate an underlying palaeosol from the effects of pedogenesis active at a new land surface. Further complications occur due to reworking of palaeosols and syndepositional pedogenic alteration of loess units. Generally, such pedocomplexes can only be deciphered if the different pedogenic, geomorphic and sedimentary processes are identified and ordered within a pedosedimentary reconstruction. A recent trend has been to treat some loess–palaeosol sequences as quasi-continuous time series, particularly when comparing depth functions of climatic-proxy properties such as magnetic susceptibility and grain size with marine and ice-core isotope curves. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Loess; Palaeosol; Palaeoclimate; Pedosedimentary processes; Pedocomplex

1. Introduction Loess can be defined simply as a terrestrial clastic sediment, composed predominantly of silt-size parti)

Fax: q44-1784-4728-36. E-mail address: [email protected] ŽR.A. Kemp..

cles, which is formed essentially by the accumulation of wind-blown dust ŽPye, 1995, p. 653.. Periglacial, perimontane and peridesert forms are often identified, their differentiation mainly based upon supposed source areas ŽPye, 1995.. While disputing Ž1990. assertion that wind-blown dust necesPecsi’s ´ sarily has to undergo ‘loessification’ before it can be

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considered as loess sensu stricto, Pye Ž1995. recognizes that most accumulations are modified to some degree by syn- or post-depositional processes. Many of these processes are pedogenic in nature and are controlled to a considerable extent by climatic factors. The aim of this review is to consider the role of pedogenic processes in modifying loess, concentrating particularly on the ways that resulting properties and features may be interpreted as indicators of past climatic conditions and changes. No attempt is made to provide a complete geographical coverage as loess deposits extend across vast areas of Central Asia, China, North America, South America, Europe, North Africa and New Zealand, and the associated literature base is immense ŽPye, 1995.. Similarly, it is not the intention to concentrate specifically on the merits of different techniques of analysis, nor the significance of specific properties Že.g. magnetic susceptibility, stable isotopes or micromorphology.. These aspects are covered elsewhere in this journal issue Že.g. Evans and Heller, in press; Zhou, in press. and in other publications Že.g. Catt, 1990; Kemp, 1998, 1999; Maher, 1998.. Instead, the main emphasis is on the general approaches undertaken to unlock climatic signals held within such materials. Selected examples are used to illustrate the range of complexities frequently encountered. Initially, the paper follows a somewhat ‘static’ view by treating palaeosols within loess covers as discrete units; more ‘dynamic’ pedosedimentary approaches are discussed later in the review when considering reworked, welded and accretionary palaeosols as well as syndepositional alterations of so-called loess units. Finally, brief mention is made of the use of high-resolution loess–palaeosol sequences as proxy records of global sub-millennial climatic changes.

2. Palaeoclimatic significance of palaeosol units in loess Vertical exposures within loess covers, whether periglacial, perimontane or peridesert in origin, often reveal alternating horizontal or sub-horizontal zones, varying in colour, texture or other properties, which

are differentiated and designated as loess and palaeosol units. Such are their thickness and apparent age span, these loess–palaeosol sequences are frequently regarded as some of the best terrestrial equivalents of marine-sediment records of long-term global climatic change ŽKukla, 1987; Kemp and Derbyshire, 1998.. Indeed, it is common to see direct correlations with oxygen isotope curves from marine cores, the palaeosols and loess units being matched to warm and cold stages Žor substages., respectively Že.g. Morrison, 1978; Kukla, 1987; Bronger et al., 1998a.. The traditional viewpoint of a palaeosol within a loess sequence is that it marks a break in deposition, during which time the surface was stable and covered in vegetation, thus, encouraging the development of a soil ŽGerasimov, 1973.. This has often been assumed to take place during interglacials and interstadials when loess was not being deposited ŽGerasimov, 1973; Morrison, 1978.. Thus, following this concept, the very presence of a palaeosol has palaeoclimatic significance in that it is taken to indicate a period of warmer andror moister conditions between cold andror arid phases of loess accumulation ŽMorrison, 1978.. Frequently, the only evidence of the specific climatic conditions during this depositional hiatus may be provided by the palaeosol itself ŽCatt, 1991.. A buried palaeosol is generally interpreted in terms of the broad pedogenic processes and environments assumed to be currently responsible for that type of soil forming at the present surface ŽPawluk, 1978.. Even if this assumption is correct, however, there may be doubts over the uniqueness of some relationships between soil properties Žor features., soil processes and macroenvironments ŽKemp, 1999.. We should not discount the possibility that different local and regional combinations of pedogenic processes andror environments may produce similar pedological properties or horizons ŽBrewer, 1972; Pawluk, 1978; Semmel, 1989.. A particular problem lies in the differentiation of the influence of climate from the other classic soil-forming factors Žorganisms, relief, parent material and time. of Jenny Ž1941.. This issue has been debated extensively in the literature with the general conclusion that, provided the assumptions and constraints are taken into account, there are situations where it is justifiable and feasible

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to utilize contemporary relations and deduce some information on the climate during the formation of a palaeosol ŽCatt, 1990; Bronger et al., 1998a; Tsatskin et al., 1998; Birkeland, 1999.. Catt Ž1991. suggested that soils formed in loess are particularly suitable for such an approach as they tend to form in relatively uniform sediments on almost level surfaces, thus, removing two of the main variables Žparent material and topography. from the soil-forming equation. He noted that most interglacial palaeosols formed in central European loesses were argillic brown earths, comparable in character to the overlying Holocene soils and, therefore, presumably indicative of similar climatic conditions to the present. More-developed rubified argillic brown earths and less-developed chernozems were attributed to warmerrwetter and shorterrcooler conditions, respectively. Equivalent-age palaeosols further to the east have different forms, a reflection of geographical differences in climate during the Quaternary ŽCatt, 1991.. For example, in southern Siberia, interglacial chernozems formed in continental parkland–steppe settings contrast with interstadial gley and cryogenic soils developed under cold periglacial tundra conditions ŽChlachula et al., 1997.. Attempts have been made to relate features within some cryogenic soils Že.g. ice-wedge casts, soil wedges. to specific minimum temperature conditions on the basis of contemporary associations ŽHuijzer, 1993., thus, opening the possibility for more quantitative estimates of climatic conditions during past periods of cryogenic pedogenesis ŽVandenberghe and Pissart, 1993.. However, although the presence of cryogenic features in palaeosols is generally accepted as one of the more reliable broad climatic proxies ŽCatt, 1991., Huijzer Ž1993. and Vandenberghe and Pissart Ž1993. emphasized the dangers of attempting to derive too specific climatic data from their individual forms as there are numerous uncertainties involved, notably the relationships between ground and air temperatures, the effects of varying snow and vegetation covers as well as moisture contents, and even the genesis of some features. Field-based assessments of palaeosols may be supplemented by laboratory data that provide further insights into particular types or rates of pedogenic processes and associated climatic controls. For instance, Bronger et al. Ž1998b. used similarities in

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clay-mineral suites Žand inferred weathering intensities. between palaeosols and soils at the present surface in Tadjikistan to support their assertion that climates during Pleistocene interglacials and the Holocene were broadly comparable in this region. Liu et al. Ž1985. matched the field and analytical characteristics of a series of palaeosols down one sequence in China with taxonomically similar soils forming at the present surface in specific climatic zones. Using these assumed direct relationships, they proposed detailed estimates of mean annual temperature and precipitation during each soil-forming interval, and highlighted significant differences in climate between some interglacials. A similar approach was taken by Tsatskin et al. Ž1998. and Mestdagh et al. Ž1999. for loess–palaeosol sequences in the Ukraine and Tadzhikistan, respectively. Quantitative climatic reconstructions from palaeosols normally depend on the establishment of climofunctions, mathematical relationships between climatic variables and measured properties of soils forming at the present surface ŽCatt, 1991.. Probably the best-known example of a climofunction in this respect is that derived by Maher et al. Ž1994., who correlated present mean annual precipitation with magnetic susceptibility differences between B and C horizons of modern soils distributed across the Chinese Loess Plateau ŽFig. 1a.. By putting the appropriate magnetic susceptibility values of each palaeosol unit at different sites across the plateau into the climofunction, Maher and Thompson Ž1995. were able to reconstruct palaeoprecipitation distribution maps for different time intervals ŽFig. 1b.. This relationship assumes that the magnetic susceptibility signal reaches equilibrium rapidly within a particular climatic regime ŽFig. 2, curve a., rather than gradually increasing over time ŽFig. 2, curve b.. If it is a continuously developing soil property, or one that only reaches a maximum after tens or hundreds of thousands of years, the differences in magnetic susceptibility values between palaeosols could simply be a function of the varying durations of soil-forming intervals ŽMaher, 1998.. Separating the influences of the climate and time factors is a ubiquitous problem faced by palaeopedologists when attempting to interpret the significance of the extent of soil development from bulk properties. Bronger et al. Ž1998b., for instance, con-

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Fig. 1. Ža. The magnetic susceptibilityrrainfall climofunction derived for the Chinese Loess Plateau by Maher et al. Ž1994.. Žb. Reconstruction of the regional distribution of rainfall across the Loess Plateau during the last interglacial, based on the magnetic susceptibility climofunction, compared to the present-day distribution Žafter Maher and Thompson, 1995..

cluded that the greater degree of pedochemical weathering and clay mineral formation in the Middle Pleistocene F6 palaeosol in Yugoslavia compared to the Holocene soils in the same area reflects a longer period of pedogenesis, rather than an earlier subtropical climate as had been previously suggested.

Similarly, red colours are frequently attributed to pedogenic processes requiring Mediterranean-type climates or specific minimum temperature and moisture conditions, whereas often differences in degree of redness may reflect the length of soil-forming intervals as much as any climatic factor ŽCatt, 1991..

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Fig. 2. Range of theoretical chronofunctions indicating different potential soil development pathways. Curve Ža.: rapid establishment of equilibrium between degree of soil development and climate. Curve Žb.: gradual increase in degree of soil development with time before reaching steady state. Curve Žc.: stepped and regressive changes in degree of soil development with time, caused by intrinsic thresholds being exceeded or changes in climate andror erosionrdeposition.

Making allowance for the time variable may be even more difficult if soil development has proceeded episodically or as a step function ŽFig. 2, curve c., rather than linearly or curvilinearly. This tendency has been explained in terms of pedogenic thresholds, whereby limits of stability of particular features or properties are exceeded by changes in intrinsic or extrinsic controls ŽMuhs, 1984.. It could conceivably result in periods of regressive pedogenesis ŽJohnson et al., 1990., whereby property development is suppressed or even reversed ŽFig. 2, curve c.. For instance, Žextrinsic. addition or erosion of material at the surface during the soil-forming interval might have regressive effects on soil formation, though influxes of aeolian dust could even increase development rates by rejuvenating profiles ŽJohnson et al., 1990., thereby complicating apparent soil– climate relationships. A further example is provided by Van Vliet-Lanoe¨ Ž1990., who suggested that illuvial clay coatings in many argillic soils at the current land surface in western Europe are relict features that mainly accumulated under Late-glacial interstadial boreal climates, thus, questioning the temperate Žin-

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terglacial. status often routinely attached to such coatings in palaeosols Že.g. Bronger et al., 1998a.. Her argument assumes that the pedoclimate and physico-chemical condition of the recently deposited loess parent-material were optimal at this time for fine clay translocation. As the climate and vegetation Žextrinsic factors. changed through the Holocene and the soils became more leached Žintrinsic factor., clay translocation processes were inhibited. As well as emphasizing the potential dangers of assigning to narrow a palaeoclimatic significance to pedological properties in isolation, this lack of consensus over a fundamental soil feature highlights the possibility that some characteristics of surface soils are not necessarily in equilibrium with the present environment and, therefore, should be used cautiously as climatic proxies when comparing with palaeosols ŽKemp, 1999..

3. Climatic change recorded within palaeosols Significant changes in climate during past soilforming intervals might have important consequences for the use of palaeosols as proxies of past climatic conditions. Where climatic changes were sufficient to affect types, rates or combinations of pedogenic processes, the bulk morphology or properties of a palaeosol might reflect either an amalgam of conditions or just the impact of the most recent phase, earlier features having been masked or destroyed. Earlier features are often only preserved where the latest processes were less intense; for instance, horizons of secondary Žreprecipitated. carbonate might be retained if the climate became more arid leading to a reduction in depth of leaching ŽBirkeland, 1999.. With mid-latitude palaeosols sensitive to the extreme climatic fluctuations associated with interglacialrglacial cycles, emphasis has often been placed on the recognition of microstratigraphic associations of macro- and micromorphological features assumed to have formed under succeeding temperate and periglacial conditions ŽCatt, 1991.. One of the earliest examples of this approach was provided by Fedoroff and Goldberg Ž1982., who reconstructed a pedogenic development sequence responding to in-

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Fig. 3. Pedogenic reconstruction of a palaeosol developed over a glacial–interglacial–glacial period in the Paris Basin Žafter Fedoroff and Goldberg, 1982.. Following deposition of the silty parent material ŽPhase 1. and formation of a fragipan ŽPhase 2. during a glacial stage, the land surface was stabilized with an intergrade derno-podzolic soil initially forming under cold, moist conditions of the glacial–interglacial transition ŽPhase 3.. Illuvial brown coarse clay and silty clay coatings of this phase were succeeded by yellow fine clay coatings typical of a sol brun lessive´ during temperate conditions of the interglacial ŽPhase 4.. Climatic deterioration towards the end of the interglacial and into the next glacial stage was marked by accumulation of poorly sorted silty clay coatings Žgrey forest soil; Phase 5. and silt coatings, and then freeze–thaw disruption of the illuvial features ŽPhase 6. prior to burial of the soil by loess ŽPhase 7..

ferred climatic changes over a glacial–interglacial– glacial period in the Paris Basin ŽFig. 3..

4. Erosion and r or reworking of palaeosols and loess Soils subjected to extreme climatic changes during transitions between interglacial and glacial stages were often prone to erosion or reworking by solifluction processes. Indeed, the absence of A or E horizons above B horizons in many palaeosols is generally attributed to erosion prior to burial by loess ŽBronger et al., 1998a.. Sometimes the whole solum may be removed, mixed with sediment and transported to another location where it may form the parent material for a new soil ŽFedoroff and Gold-

berg, 1982; Leigh et al., 1989; Semmel, 1989; Cremaschi et al., 1990.. Numerous studies, particularly in western Europe Že.g. Mucher, 1986; Cremaschi et al., 1990; Huijzer, ¨ 1993; Vandenberghe et al., 1998., have shown that many loess units are themselves the product of postor syndepositional reworking of wind-blown dust. Apart from cryoturbation which may be considered as a Žcryo.pedogenic rather than a sedimentary process ŽHuijzer, 1993., there are a number of possible transport mechanisms including solifluction, rain splash, rain or snow-melt wash and overland water flow, each being inferred from diagnostic macro- and micromorphological structures and fabrics. Sometimes the reworking simply involved localized or more widespread movement and redistribution of the loess components: in other situations, materials of

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extraneous composition and particle size were incorporated. It is important to recognize the evidence for palaeosol and loess reworking as it can provide crucial insights into how the landscape developed in response to environmental controls and changes between, and even during, so-called ‘soil-forming intervals’ ŽLeigh et al., 1989; Semmel, 1989; Cremaschi et al., 1990; Huijzer, 1993; Vandenberghe et al., 1998..

5. Diagenetic modification and welding of palaeosols The apparent absence of an A horizon in a palaeosol should not be automatically attributed to erosion. Post-burial oxidation of organic matter may modify colour patterns, thus, making recognition of such horizons difficult ŽMcDonald and Busacca, 1992.. Other possible diagenetic changes that occur

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include compaction, loading and rejuvenation by bases leached down from an overlying palaeosol ŽGerasimov, 1973; Catt, 1991; Tsatskin et al., 1998.. The last, particularly where associated with precipitation of secondary carbonate from the leachate, is a relatively simple example of welding ŽOlson and Nettleton, 1998. when covering sediments are insufficiently thick to isolate an underlying palaeosol from the effects of pedogenesis active at a new land surface. A horizons may be converted into B horizons by this mechanism as the surface and effective zone of biological activity is raised and illuvial components Že.g. clay, carbonate. accumulate and are superimposed upon previously formed biotic fabrics ŽMcDonald and Busacca, 1992; Kemp and Zarate, 2000.. ´ Numerous exposures of several of the major soil stratigraphic units of Europe Že.g. Rocourt Soil. and the USA Že.g. Sangamon Soil. have been shown to be pedocomplexes comprising welded interstadial

Fig. 4. Reconstruction of the pedosedimentary stages responsible for the development of a buried pedocomplex at Attenfeld in Bavaria Žafter Kemp, 1999.. Following deposition of reworked Tertiary deposits and Loess Loam A, the land surface was stabilized and an interglacial argillic soil developed. Climate deterioration during the succeeding glacial resulted in cryogenic disruption of the clay coatings and erosion of the uppermost eluvial horizons ŽStage 1.. Deposition of Loess Loam B was followed by renewed land surface stability and pedogenic clay translocation during an interglacial with clay coatings accumulating in the Loess Loam B and lower down in the Loess Loam A where they were superimposed upon the disrupted argillic fabric of the earlier-formed soil. In the next glacial stage, the climate deteriorated once again and many of the illuvial features in the Loess Loam B were disrupted by cryogenic activity and overlying eluvial horizons were eroded ŽStage 2.. The loess that then accumulated was sufficiently thick to isolate the pedocomplex from the effects of pedogenesis active at the next major land surface ŽStage 3..

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and interglacial palaeosols ŽPaepe et al., 1990; Rodbell et al., 1997.. Yet other pedocomplexes Že.g. F6, Yugoslavia. are two or more interglacial palaeosols welded upon each other. Failure to recognize their mode of development over long time periods may lead to erroneous palaeoclimatic assertions being made on the basis of strongly developed bulk properties ŽBronger et al., 1998b.. Pedocomplexes are even more complicated where erosional and reworking processes have been active at some stageŽs. during their development, especially if they span major Žinterglacialrglacial. cycles of climatic change ŽTsatskin et al., 1998.. Generally, such pedocomplexes can only be deciphered if the different pedogenic, geomorphic and sedimentary processes are identified and ordered within a pedosedimentary reconstruction ŽFedoroff and Goldberg, 1982; Leigh et al., 1989; Cremaschi et al., 1990; Mestdagh et al., 1999.. Thin sections can often help in such situations as illustrated by Kemp et al. Ž1994. and Kemp Ž1999., who used the depth distri-

bution and microstratigraphic relationships of a series of micromorphological features Žclay coatings, fragmented clay coatings, silt infillings and rounded aggregates. to reconstruct the pedosedimentary and associated palaeoclimatic history of a pedocomplex from Bavaria ŽFig. 4..

6. Syndepositional alteration of loess Much of the secondary carbonate and clay within some arid and semi-arid soils of southwestern United States results from the alteration and redistribution of wind-blown calcareous dust added to the surface as the soils develop ŽMcFadden, 1988.. Furthermore, significant rates of loess accumulation have been recorded during the Holocene within surface soils in other places as diverse as Alaska ŽBeget, ´ 1996., Washington State ŽMcDonald and Busacca, 1998. and China ŽKemp and Derbyshire, 1998.. All these examples appear to be in conflict with the classic

Fig. 5. Schematic model illustrating the development of a loess–palaeosol sequence in response to changing dominances of depositional and pedogenic processes Žafter Kemp and Derbyshire, 1998.. Accretionary phases, involving rapid accumulation and weak pedogenic alteration, alternate with periods of reduced dust inputs and more substantial pedogenic alteration associated with establishment of relatively stable land surfaces. At no stage, however, are pedogenic or depositional processes completely inactive. Where deposition is slower andror pedogenic alteration is deeper, welded palaeosols or pedocomplexes may develop.

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notion of soil formation occurring during interglacial phases of landscape stability ŽMorrison, 1978..

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In reality, it may be more useful to consider many loess–palaeosol sequences in terms of changing bal-

Fig. 6. Pedosedimentary log based on the vertical changes in relative influences of different post- and syn-depositional processes inferred from thin sections spaced at close vertical intervals through the L1rS1rL2 sequence in Lanzhou, on the western edge of the Chinese Loess Plateau Žafter Kemp et al., 1999.. Note that there is evidence of significant alteration even within the so-called ŽL. loess units.

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ances between pedogenesis and loess accumulation over geological time ŽFig. 5.. McDonald and Busacca Ž1998. suggested that the most important factor controlling the degree of soil development in the Palouse of Washington State is sediment supply, which may be out of phase with climate changes, thus, calling into question the interglacial status automatically attached to well-developed palaeosols. In most regions, it seems that the balance swings towards pedogenesis during interglacials or interstadials when sediment supply and transport are limited and the climate is warmer andror wetter, thus, ensuring the development of the major palaeosol units during interglacials and interstadials. Where accumulation rates are still appreciable during these ‘soil-forming intervals’, however, the soils and palaeosols may be accretionary with surface build-up keeping pace with pedogenesis Že.g. Kemp et al., 1996, 1997.. Even in regions such as western and central Europe or the mid-continent USA, where it is assumed that periglacial sources were absent and, therefore, dust inputs effectively zero during interglacials, it is generally accepted that many of the interstadial palaeosols are accretionary in nature reflecting some degree of syndepositional pedogenesis ŽMarkewich et al., 1998; Vandenberghe et al., 1998; Mestdagh et al., 1999.. Even when they do not merge or weld with other palaeosols in pedocomplexes, it is not always straightforward to derive palaeoclimatic interpretation from the bulk properties of accretionary palaeosols as allowance must be made for the rejuvenating or even diluting effect of fresh material into an evolving profile ŽMcFadden, 1988.. The model illustrated in Fig. 5 has a further ramification in that during periods when the pedosedimentary balance swings towards loess accumulation, loess units may be subjected to relatively minor syndepositional pedogenic alteration that may leave some kind of weak imprint, Že.g. root channels, cryogenic structures.. This notion is partly reflected in reports of ‘weathered loesses’ in China ŽKukla, 1987. or ‘incipient soils’ in eastern Europe ŽTsatskin et al., 1998. and the USA ŽHayward and Lowell, 1993.. In reality, however, these are often subjective separations of a continuum—when does ‘loess’ become a ‘soil’? An argument could be made for consideration of complete loessŽ –palaeosol. sequences as massive pedosedimentary complexes

which reflect varying interactions of pedogenic and sedimentary Žgeomorphological. processes over time. A high-resolution micromorphological study of a thick sequence in western China, for instance, has shown that even apparently massive units of loess contain suites of pedological features which provide proxy records of quite subtle climatic changes during periods of high sediment accumulation ŽFig. 6. ŽKemp et al., 1999.. Similar types of records were obtained from sequences in the Netherlands by Huijzer Ž1993., though the pedosedimentary reconstructions also included phases of significant reworking of the primary loess material.

7. Matching of loess–palaeosol sequences to marine- and ice-core oxygen-isotope records The identification of discrete palaeosol and loess units are clearly necessary for communication and correlation purposes. From a pedosedimentary and even palaeoclimatic viewpoint, however, the use of artificial boundaries across what is very often a continuum must be questioned. While early correlations of palaeosol and loess units to marine oxygen isotope stages and substages fully utilized these distinctions Že.g. Kukla, 1987; An et al., 1991., it is noticeable that more recent comparison of high-resolution depth functions of climatic-proxy properties with marine and ice-core isotopic records have tended to reduce the significance of individual units and instead treat the sequences more as quasi-continuous time series. In China, for instance, grain-size fluctuations reflecting variations in wind-transport energy during the Last Glacial have been correlated to subMilankovitch climatic oscillations recorded in marine- and ice-cores ŽPorter and An, 1995.. Other properties, such as magnetic susceptibility, calcium carbonate content, and carbonroxygen isotope ratios in soil organic matter and secondary carbonate, have also been plotted against continuous timescales converted from depths with the aim of identifying global sub-millennial climatic changes ŽChen et al., 1997; Fang et al., 1998; Wang et al., 1998.. Further examples have been discussed from Europe ŽHatte´ et al., 1999. and Alaska ŽOches et al., 1998.. These are very exciting developments, yet such approaches require very tight chronological controls and the

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assumption that hiatuses do not occur in the records ŽLu et al., 1999.. Clearly we need to ensure that wiggle-matching does not become just an exercise in statistics. While emphasis appears to be on the identification and understanding of global, or at least hemispheric, mechanisms linking these various high resolution records, there is still a need to pay attention to the nature of the local signal. From a loess– palaeosol viewpoint, in particular, it is important to differentiate the primary sedimentological signal from that produced by syn- andror post-depositional pedogenic modifications. Acknowledgements I thank Justin Jacyno for drafting all the figures in this paper and the two referees for their thoughtful comments. References An, Z.S., Kukla, G.J., Porter, S.C., Xiao, J., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of Central China during the last 130,000 years. Quaternary Research 36, 29–36. Beget, ´ J.E., 1996. Tephrochronology and paleoclimatology of the last interglacial cycle recorded in Alaskan loess deposits. Quaternary International 34–36, 121–126. Birkeland, P.W., 1999. Soils and Geomorphology. Oxford Univ. Press, Oxford. Brewer, R., 1972. The basis of interpretation of soil micromorphological data. Geoderma 8, 81–94. Bronger, A., Winter, R., Heinkele, Th., 1998a. Pleistocene climatic history of East and Central Asia based on paleopedological indicators in loess–paleosol sequences. Catena 34, 1–17. Bronger, A., Winter, R., Sedov, S., 1998b. Weathering and clay mineral formation in two Holocene soils and buried paleosols in Tadjikistan: towards a Quaternary paleoclimatic record in Central Asia. Catena 34, 19–34. Catt, J.A., 1990. Paleopedology manual. Quaternary International 6, 1–95. Catt, J.A., 1991. Soils as indicators of Quaternary climatic change in mid-latitude regions. Geoderma 51, 167–187. Chen, F.H., Bloemandal, J., Wang, J.M., Li, J.J., Oldfield, F., 1997. High-resolution mutli-proxy climate records from Chinese loess: evidence for rapid climatic changes over the last 75 kyr. Palaeogeography, Palaeoclimatology, Palaeoecology 130, 323–335. Chlachula, J., Rutter, N.W., Evans, M.E., 1997. A late Quaternary loess–paleosol record at Kurtak, southern Siberia. Canadian Journal of Earth Sciences 34, 679–686. Cremaschi, M., Fedoroff, N., Guerreschi, A., Huxtable, J.,

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