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The position of paleopedology in geosciences and agricultural sciences
Pergamon
Quaternary International Vols 51/52, pp. 87-93, 1998. © 1998Publishedby 1NQUA/ElsevierScienceLtd All rightsreserved. Printedin GreatBritain, PII: S1040-6182(97)00035- 9 1040-6182/98/$19.00
THE POSITION OF PALEOPEDOLOGY IN GEOSCIENCES AND AGRICULTURAL SCIENCES A. Bronger* and J.A. Cattt *Geographisches Institut, Christian-Albrechts Universitat zu Kiel, D24098 Kiel, Germany tlACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK
Paleopedologyis a relatively young science with roots in various older geosciences.At present it suffersfrom a lack of clear definitionsand fundamental principles. Nevertheless it has considerable potential for applications in stratigraphy,paleontology,sedimentology,archeology, paleoclimatology,seismology,geomorphology,soil classificationand agriculture.Consequentlyit could play a major coordinatingrole in soil science, especiallyin contributing to a natural, strongly genetic systemof soil classification.We suggest that such a system,with emphasis on developmentof soil properties in relationto current and past environmentalfactors,wouldbe of greater valuein all thesesubjectsthan the current systemsdesignedprincipallyfor agriculturalinterpretation. © 1998 Publishedby INQUA/ElsevierScienceLtd. All rights reserved. INTRODUCTION
reviewing the importance of paleopedology in geosciences and agricultural sciences. Justification is certainly required, because we are aware of considerable ignorance of, and even prejudice against, paleopedology among other earth scientists, including some soil scientists.
As Tandarich and Sprecher (1994) have described, agricultural interests in the fledgling sciences of chemistry and geology developed almost simultaneously in the late eighteenth and early nineteenth centuries. The chemical interests were based on analyses of soils, especially their humic components, and of plant ashes, to clarify the factors influencing growth of plants and the production of crops. The geological interests were in processes of soil formation, such as weathering of minerals and rocks, and in reasons for the geographical variability of soil. The International Society of Soil Science developed out of this agricultural geology in the 1920s, though the independent science of Bodenkunde (soil knowledge or soil science) had earlier arisen from European chemical interests (Sprengel, 1837). The realisation that soils need to be studied from many points of view (geological, botanical, chemical, geographical, physical, mineralogical) had also led to the independent and more multidisciplinary science of pedology (Fallou, 1862), though unfortunately this is now often seen in a narrower sense as a branch of soil science focussing on soil genesis and classification (Buol et al., 1989, pp. 3-4). Surprisingly, these contrasts between the geological, agrochemical and multidisciplinary approaches in soil science persist to the present day. Paleopedology clearly has an important role near the geological end of the spectrum, especially now that paleosols even as old as Precambrian have been recognised throughout the geological column. But it is also important for a proper understanding of the agrochemical and agroenvironmental behaviour of surface (non-buried) soils, and it plays a vital role in the multidisciplinary (pedological) study of soils. We therefore see paleopedology as occupying an important position in soil science (Fig. 1) with considerable potential for a major coordinating role through a genetic system of classification. In this paper we summarise evidence to justify these statements by
P A L E O P E D O L O G Y AS A SUBDISCIPLINE OF GEOLOGY Paleopedology may be defined as the study of paleosols, which have in turn been somewhat loosely defined as soils 'formed on a landscape during the geologic past' (Ruhe, 1956; Yaalon, 1971). Paleosols are buried beneath younger deposits (buried paleosols), were once buried but have subsequently been re-exposed by erosion (exhumed paleosols) or have persisted on the land surface to the present day (non-buried or relict paleosols). We will discuss later the problem of how long a soil must persist on the land surface before it can be called a non-buried paleosol. In geology buried paleosols have been recognised for over a century in many countries, such as U.S.A., U.K. and Germany. The first to be recognised were fairly young soils (Quaternary, Tertiary or Mesozoic), which are still identifiable because they had not been strongly altered by metamorphism or advanced diagenesis. However, modern petrographic and geochemical techniques have allowed older and often quite strongly metamorphosed soils to be recognised and used to interpret past climates, terrestrial plant and animal assemblages, geomorphological features and even changes in the composition of the primeval atmosphere (Retallack, 1990; Feakes et al., 1989). The importance of buried paleosols to geologists results from several fundamental properties of soils: 1. Many take fairly long periods (103-107yr) to form, and consequently represent major episodes of land surface stability, with little or no erosion or 87
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A. Bronger and J.A. Catt Agricultural
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.Soil seoiments
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Se Sediments -- I'Geology, ~ Geomorphology,etcI
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Buriedpaleosols (fossilsoils)
Non-buriedpaleosols (relict soils) Paleopedology?
FIG. 1. The position of paleopedologywithingeosciencesand agriculturalsciencesand internationalscientificunions. deposition. This allows breaks in sedimentation to be recognised, which are especially useful in subdividing and correlating long sequences of unfossiliferous and lithologically monotonous sediments. The breaks often constitute unconformities because the land surface, beneath which the soil formed, originated by emergence following uplift and tilting; unconformities are often sources of valuable ores and minerals, and are important in understanding the tectonic history of a region. 2. Their profile characteristics (the properties, thickness and sequence of horizons) are influenced mainly by the five groups of soil-forming factors - - climate, organisms, relief, parent material and time (Dokuchaev, 1883; Jenny, 1 9 4 1 ) - lateral variations of which give rise to characteristic catenary changes of soil properties. This not only allows past environmental factors (climate, biota and geomorphology) to be assessed, but may also indicate the length of the episode/s of land surface stability. In addition it often distinguishes true paleosols from local sequences that superficially resemble soil profiles but are in fact successions of different lithostratigraphic units or products of diagenetic processes such as mobilisation and redeposition of calcium carbonate or iron oxides (Valentine and Dalrymple, 1976). 3. Soils are habitats for most terrestrial life-forms, and buried paleosols often contain body fossils (e.g. mollusc shells, phytoliths, seeds, pollen grains) or trace fossils (e.g. root traces, burrows) of plants and animals living on the land at the time the soil developed. These are often better than any other evidence for such life forms, and for some early periods it may be the only evidence; e.g. for the earliest land animals in the Late Ordovician of Pennsylvania (Retallack and Feakes, 1987). Buried Quaternary paleosols often contain human artefacts or even hominid fossils such as bones and teeth; these are of
considerable value in archeology, because they are more likely to be in situ than those found in sediments, and the properties of the surrounding soil provide evidence for the environment in which man was living at the time. In addition many Holocene buried soils provide evidence for human impacts on the landscape, such as deforestation, erosion and other types of soil degradation, and knowledge of these can help predict the effects of modern soil use or misuse. This range of potential uses of paleosols has greatly expanded their study by geologists, archeologists and other earth scientists over the past decade or two. However, because these workers have often lacked detailed pedological knowledge, there have been examples of incorrect interpretations and failures fully to exploit the scientific potential of temporary exposures of buried paleosols. One of the most important recent uses of buried Quaternary paleosols has been the paleoclimatic interpretation of the multiple soils in the loess sequences of eastern Europe, China, Soviet Central Asia and other regions. These sequences have provided detailed records of climatic change for the last 2.4 million years, which are potentially at least as detailed as the oxygen isotope record obtained from deep oceanic cores (Bronger and Heinkele, 1989). Like the oceanic cores, they also show a strong statistical relationship to the variations in solar radiance resulting from perturbations of the earth's orbit, the so-called Milankovitch curves. Paleosols have undoubted importance for understanding the processes driving natural climatic change, not just through the Quaternary (glacials, interglacials, interstadials) but throughout geological time, because the Milankovitch curves have now been identified in cyclic sediment sequences of many preQuaternary periods. The same processes will undoubtedly control future climatic change, though the
The Position of Paleopedology in Geosciences and Agricultural Sciences more immediate effects of atmospheric pollutants (greenhouse gases) may be superimposed on those related to the Milankovitch curves. The main advantage of the loess-paleosol sequences in reconstructing past global climate change is that they cover all or much of the Quaternary period in many different geographical regions. When the paleosols are adequately dated and fully interpreted, they will allow geographical variation of past climates to be assessed more reliably and in more detail than is possible by any other method known at present. Another recent use of buried soils with considerable economic importance is in estimating the timing and frequency of tectonic events, with the intention of calculating lengths of periods between past episodes and thus predicting when future earthquakes will occur. For example, Gerson et al. (1993) described soils displaced by past vertical fault movements that were buried by colluvial wedges accumulating on the downthrown side soon after the movement. They found that the time of burial can be estimated from radiocarbon assays of their organic matter, thermoluminescence studies or the extent of soil development on the surface of the colluvial wedge. Dating colluvial wedges adjacent to fault scarps by calculating soil development indices (e.g. Harden, 1982; Harden and Taylor, 1983) is especially useful in arid regions, where organic matter and other datable materials are rare or absent. The dates of successive soils buried in this way therefore indicate the approximate periods between episodes of earthquake activity. Also, sequences of buried soils can sometimes be traced across fault zones; differences in the displacement of successive soils (again dated by radiocarbon or other methods) then indicate the frequency of past movements, and the age of the oldest undisplaced soil gives the minimum period of time since the last episode of movement.
GEOMORPHOLOGICAL SIGNIFICANCE OF PALEOSOLS Another branch of geoscience in which the study of paleosols has brought significant advances is the dating of land surfaces and clarification of the geomorphological history of an area. This relies upon the identification and absolute or relative dating of surface paleosols. The longer a soil remains unburied on the land surface the more it develops and, assuming other soil forming factors (climate, organisms, relief and parent material) have been constant, a chronosequence of soils on land surfaces of increasing age (e.g, a flight of river terraces) shows increasing development of properties such as profile depth, weathering of primary minerals, humus incorporation, clay illuviation, etc. In areas where the environmental factors have remained fairly constant, such as the Merced River valley in California (Harden, 1982), these properties can be combined into a soil development index that is strongly correlated with time. Such combined indices minimise the problem of
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properties that do not increase linearly with time; e.g. those that initially increase slowly, then change more rapidly and finally slow down or even cease to change over long periods. However, in many mid- and high-latitude humid regions environmental factors have not remained constant for long. The repeated large glacial-interglacial climate changes during the later Quaternary, and the associated biotic, geomorphological and sedimentological changes, have greatly complicated the relationships between soil properties and time. In such regions simple chronofunctions can only be used to date soils and the land surfaces on which they occur when they are no older than 10,000yr (i.e. the last major change in climate). Despite this limitation some thick, strongly weathered surface soils in both low and mid-latitudes are thought to have begun development in pre-Quaternary periods. Despite this limitation, some advances in dating soils and land surfaces have been made in mid-latitude regions by using the characteristic properties of soils that develop under climatic extremes. For example, in many regions soils containing typical periglacial features, such as ice-wedge casts or possibly fragipans, can only occur on land surfaces that have persisted without significant modification since at least the last major cold stage in the Late Weichselian (Wisconsinan). If this type of information is combined with knowledge of the age of later Quaternary parent materials of the soils, it provides a powerful tool for dating the land surface beneath which the soil occurs (Semmel, 1989). At least it eliminates the possibility of a major episode of Late Weichselian or Holocene erosion. An extension of this approach relies on the recognition of interglacial soil features, indicating that the land surface has persisted without significant erosion or deposition since at least 125,000yr ago. For example, the paleo-argillic soils of England and Wales (Avery, 1980), occurring on older river terraces and upland plateaux in S. England, have redder colours (usually as discrete mottles) and more extensive segregations of birefringent clay in their Bt horizons than any (Holocene) soils formed on Devensian (=Weichselian) parent materials. Periglacial soil features are often superimposed on the paleo-argillic Bt horizon, which is also almost always overlain by thin (< 1 m) Late Devensian loess (containing typical Holocene soil features), thus providing evidence for a long history of pedogenetic and minor depositional events on these ancient land surfaces. Micromorphological studies of some paleo-argillic Bt horizons have suggested even longer pedogenetic histories, spanning two or more interglacials, distinguished by argillans of different colour or composition, different relationships to hydromorphic features or different extents of disruption by frost in the intervening periglacial periods (Bullock and Murphy, 1979; Chartres, 1980; Kemp, 1987a, Jongmans et al., 1991). However, some soils on very old river terraces have not shown the expected complexity of micromorphological
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features (Kemp, 1987b), possibly because intense interglacial pedogenesis may have obliterated earlier evidence for mild interglacial or periglacial microfeatures (Kemp et al., 1993). Soils which have persisted at the surface since the Last Interglacial or even since the end of the Weichselian have clearly formed at least partly "on a landscape during the geologic past' and so must qualify as non-buried paleosols (G6ze, 1947; Hunt and Sokoloff, 1950) even though, following Erhart (1932), 'paleosol" was originally used as equivalent only to buried soil (Johnson and Hole, 1994). In this sense non-buried paleosols are approximately equivalent to the Reliktboden of Ramann (1911, p. 526; 1928, p, 37), the secondary soil formations and two-stage soils of Polynov (1927), the polygenetic soils of Bryan and Albritton (1943) and the surface soils 'out of harmony with the environment' of Nikiforoff (1943). All these terms were based upon the concept of soils showing 'two or more sets of properties which can be related to different combinations of soil-forming factors through sets of often incompatible soil-forming processes', implying 'two or more environmentally different periods of soil development' (Bronger and Catt, 1989). The main problem with distinguishing non-buried paleosols in this way is that fairly young soils qualify as paleosols in mid- and high-latitudes (Catt, t989), where Quaternary climatic changes had their greatest effect, whereas much older soils in low latitudes may not. Even within quite small areas, soils of the same age may or may not qualify depending on whether or not they have been influenced by human activities, because these are usually included as part of the "organisms' group of environmental soil-forming factors. Indeed, because of the increasingly widespread influence of human activities on soils and the possibility of even quite small recent (mid- or late-Holocene) climatic changes influencing soils, there is a strong possibility that almost all soils now qualify as non-buried paleosols. This problem of definition will have to be resolved in due course, but for the moment it serves the purpose of emphasising the necessity of relating soil properties to either present environmental conditions or the profile's past history.
PALEOSOLS IN SOIL CLASSIFICATION SYSTEMS Classification of soils is a necessary subdiscipline of pedology; it is important for communication between pedologists, for transferring results of field trials, laboratory or glasshouse experiments to other areas and for portrayal of soil variation on maps of various scales. In the last few decades numerous national and two international systems have been developed, but none is yet universally accepted. Of the two international systems, the very detailed American Soil Taxonomy (Soil Survey Staff, 1975, 1992) is used for soil surveys at large scales (e.g. 1 : 20,000), whereas the FAO-Unesco system
(FAO-Unesco, 1974, 1988), now the World Reference Base (Spaargaren, 1994), serves mainly as a legend for the 1 : 5,000,000 Soil Map of the World. Although the American system includes some 'Pale'-great groups to distinguish strongly developed profiles, neither system really attempts to relate soil properties to landscape history. Consequently neither is suitable for classifying paleosols or for encouraging studies aimed at understanding soil genesis, and neither is a natural genetic system. Both are in fact compromises aimed principally at agricultural applications and based on minimum necessary analytical investigations and brief assessments of field (e.g. stratigraphic) relationships. A true natural genetic system of classification, relating all soil properties to a past history of climatic and other environmental factors, might prove more universally acceptable and useful in various other applications as well as in agriculture. One particular deficiency of all soil classification systems known to us is the failure to distinguish soil sediments at any taxonomic level. Sediments formed by mass movement of soil materials with minimal dispersion of the constituent particles often resemble the original soils, especially in terms of particle size distribution and agricultural behaviour, and are consequently never distinguished from them on soil maps. However, the distinction is essential for correct geomorphological and stratigraphical interpretation, as soil sediments often occur in very different situations from the original soils, even if they have only been transported for short distances. If the geomorphological or stratigraphic context of a soil is misinterpreted, so also is its paleoclimatic or other paleoenvironmental significance. Recognition of soil sediments and the paleoenvironmental interpretation of the in situ soils from which they were derived is especially important in the cave environment. Here they are often associated with deposits rich in mammalian remains and human artefacts, which are important for understanding evolutionary changes and early human activities in relation to the history of Quaternary climatic change. The soil sediments provide the only means of relating sequences within the protected environment of the cave to climatic changes outside. A full and correct understanding of landscape history as a basis for soil classification and nomenclature is essential for a proper appreciation of many modern environmental problems. For example, the soil cover of central and south India consists mainly of Vertisols ('Black Soils') and Lixisols (Rhodustalfs or 'Red Soils'), both of which are non-buried paleosols because they formed in an earlier period of wetter climate than the present. In the present semi-arid conditions (<1000 mm annual rainfall and very high evaporation) soilforming processes such as deep weathering have almost ceased; instead secondary carbonate is accumulating in the saprolite and lower parts of the Lixisol Bt horizons (Bronger and Bruhn, 1989), and under arable agriculture soil erosion is becoming a severe problem. It is
The Position of Paleopedology in Geosciences and Agricultural Sciences widely accepted that soil erosion can be tolerated from a crop production viewpoint provided soil formation keeps pace with it to compensate for the losses. However, in semiarid India the soil development processes have changed and the rate of compensatory regeneration of soil is in effect zero. The erosion there is consequently a permanent loss of one of the country's most important natural resources, but it has not been recognised as such because the soils were not identified as paleosols. The erosion now occurring in many other countries (Oldeman et al., 1990) is probably a similar irreplaceable loss of resources inherited from earlier geological periods.
AGRICULTURAL SIGNIFICANCE OF PALEOSOLS Many soil properties influencing agricultural production are paleosol features inherited from earlier periods when climate or other environmental conditions were different from those of the present. In midlatitude regions they can be divided into relict features of cold Quaternary stages and those inherited from interglacials (Catt, 1992). A subsoil feature that often results from periglacial soil disturbance (Van Vliet and Langohr, 1981) is the brittle, compact fragipan horizon, formed in loamy subsoils by growth of ground-ice lenses and laminae. These have bulk densities of 1.6-2.0 g/cm 3, and can present almost impenetrable barriers to roots and severely limit downward percolation o f water (Smeck and Ciolkosz, 1989), especially where their upper surfaces are further hardened by precipitation of iron oxides or silica from groundwater slow to penetrate the layer. Fragipans usually occur at depths of 50-100 cm, that is just below the original active layer of the periglacial environment. Consequently they mainly affect trees, which often suffer windthrow because of shallow rooting, especially in wet weather when the soil above the fragipan is saturated and easily deformed. The wide range of soil structures formed by frost disturbance in periglacial conditions, such as ice wedge casts, involutions, stripes, pingos, thermokarst and patterned ground features, are an important cause of short-range variation in soil depth and particle size distribution. Efficient farmers can usually adapt their management techniques (choice of crops, timing and rates of fertiliser applications, etc.) to obtain maximum productivity and profit or to reduce environmental pollution from different soil types. This is easy where the different soils form large areas, especially if soil boundaries correspond with field boundaries, but with small, intricately shaped areas within a field differential management is difficult, at least with present farm machinery. The farmer must then choose between treatments that are suited to one or other of the soil types present, or management that is a compromise between them. Either approach can lead to significant
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loss of yield, up to 50% in some years (Evans and Catt, 1987), or to increased leaching losses, for example where too much nitrogen is applied to very permeable soils. The deeply penetrating cracks formed by ice wedge casts, which are often filled with very permeable material, such as coversand, also form preferential flowpaths through less permeable soil for loss of fertiliser or pesticide to groundwater. Interglacial features that can influence soil performance in agriculture include the strong enrichment of Bt horizons with illuvial clay, the increased content of iron oxides produced by stronger weathering of primary minerals and deep acidification. In lower latitudes strong clay enrichment of Bt horizons often decreases subsoil permeability because the clay is deposited in the voids that conduct water (Jongmans et al., 1991). This produces pseudogley or stagnogley profiles in which root development may be limited by seasonally anaerobic conditions. At higher latitudes, where non-buried interglacial paleosols were disturbed by frost action in subsequent periglacial periods, the illuvial clay bodies (argillans) were disrupted and the fragments (papules) incorporated into the soil matrix. This disruption created new structural units (peds) separated by a fresh system of voids, which increased the Bt horizon permeability. The clay-enriched paleo-argillic soils of S. England are consequently often subject to by-pass (fissure) flow and are more permeable than Holocene soils containing much less clay. The longterm stability of peds in these soils is probably maintained in part by the large amounts of iron oxides produced by interglacial weathering. The greater amounts of oxides also increase the phosphate fixation capacity of interglacial paleosols; larger amounts of phosphate fertiliser are consequently required to achieve the same initial yield increase as on less weathered soils. However, the fixed phosphorus can subsequently be released slowly, especially if nitrogen is added without further phosphorus to stimulate growth of cereals. In the long term some non-buried paleosols therefore have better phosphorus economies and lose less phosphate by leaching than younger, lessweathered soils. Pedogenesis in humid regions results in progressive acidification, initially of the topsoil and later of subsoil horizons. In many mid- and high-latitude regions the Holocene was probably not long enough for subsoil horizons to become very acid, even where the soil parent material is weakly- or non-calcareous. In contrast, non-buried interglacial paleosols are often completely decalcified to a depth of 5 m or more, and subsoil horizons to depths of 2-3 m commonly have pH values less than 4.5. Surface liming rapidly increases topsoil pH, but may take many years, sometimes even centuries, to affect the subsoil. Consequently, where acidity extends below the depth to which lime can readily be incorporated (20-30cm), surface liming brings little or no improvement in the growth of deeprooting crops, such as bushes and trees; this is often because root development is restricted by the increased
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availability of aluminium and manganese in strongly acid conditions. Finally, in some regions that have become arid by late Holocene climatic change (e.g.N. Africa) earlier Holocene soils with well-preserved Ah horizons have been buried by aeolian sand or colluvium resulting from soil erosion. With modern engineering technology and improved understanding of water conservation and nutrient cycling, these buried paleosols could be reclaimed with considerable benefit to local agricultural production.
CONCLUSIONS The range of fundamental and applied sciences in which paleosols are currently being used shows that paleopedology occupies a central position of considerable importance in the broad subject of soil science, or pedology in the sense of Fallou (1862). The needs of stratigraphers, geomorphologists, paleontologists, paleoclimatologists and others to recognise and interpret paleosols in various ways places an increasing responsibility on soil scientists to provide clearer guidelines for the identification and interpretation of buried soils, which can only be achieved by better fundamental studies (mineralogical, micromorphological, chemical, etc.) of surface soils, leading to better understanding of the development of their properties in relation to current and past environments. Proper communication between and within these disciplines will demand that the results of the fundamental soil property and profile studies are reflected in a more natural genetic classification of soils no longer dominated by the essentially chemical and physical demands of agriculture. Once the need for a fully genetic classification is recognised, paleopedology will automatically play a major formative role within it.
REFERENCES Avery, B.W. (1980). Soil Classificationfor England and Wales (Higher Categories), Soil Survey Technical Monograph, Vol. 14. Rothamsted Experimental Station, Harpenden. Bronger, A. and Bruhn, N. (1989). Relict and recent features in tropical Alfisols from South India. Catena Supplement, 16, 107-128. Bronger, A. and Catt, J.A. (1989). Paleosols: problems of definition, recognition and interpretation. Catena Supplement, 16, 1-7. Bronger, A. and Heinkele, T. (1989). Paleosol sequences as witnesses of Pleistocene climatic history. Catena Supplement, 16, 163-186. Bryan, K. and Albritton, C.C. (1943). Soil phenomena as evidence for climate changes. American Journal of Science, 241, 469-490. Bullock, P. and Murphy, C.P. (1979). Evolution of a paleo-argillic brown earth (Paleudall) from Oxfordshire, England. Geoderma, 22, 225-252. Buol, S.W., Hole, F.D. and McCracken, R.J. (1989). Soil Genesis and Classification (3rd edn.). Iowa State University Press, Ames. Catt, J.A. (1989). Relict properties in soils of the central and northwest European temperate region. Catena Supplement, 16, 41-58. Catt, J.A. (1992). Quaternary environments and their impact on British soils and agriculture. Quaternary Proceedings, 2, t7-24. Chartres, C.J. (1980). A Quaternary soil sequence in the Kennet Valley, central southern England. Geoderma, 23, 125-146.
Dokuchaev, V.V. (1883). Russian chernoziom. St. Petersburg, Russia (In Russian; English translation by N. Kaner, Israel, Program for Scientific Translation, Jerusalem). Erhart, H. (1932). Sur la nature et la gen6se des pal6o-sols du loess ancien d'Alsace. Comptes Rendus l'Academie des Sciences Paris. 194, 554-556. Evans, R. and Catt, J.A. (1987). Causes of crop patterns in eastern England. Journal of Soil Science, 38, 309-324. Fallou, F.A. (1862). Pedologie oder allgemeine und besondere Bodenkunde. Schonfeld, Dresden. Feakes, C.R., Holland, H.D. and Zbinden, E.A. (1989). Ordovician paleosols at Arisaig, Nova Scotia, and the evolution of the atmosphere. Catena Supplement, 16, 207-232. FAO-Unesco (1974). Soil Map of the Worm 1 : 5,000,000: Vol. 1 Legend. UNESCO, Paris. FAO-Unesco (1988). Soil Map of the World. Revised legend, World Soil Resources Report 60. FAO, Rome. Gerson, R., Grossman, S., Amit, R. and Greeubaum, N. (1993). Indicators of faulting events and periods of quiesence in desert alluvial fans. Earth Surface Processes and Landforms, 18, 181-202. G6ze, B. (1947). Paleosols and modern soils. In: Comptes Rendus Conference Pedologie Mediterranban, pp. 210-219. AlgerMontpellier, Paris. Harden, J.W. (1982). A quantitative index of soil development from field descriptions: examples from a chronosequence in central California. Geoderma, 28, 1-28. Hunt, C.B. and Sokoloff, V.P. (1950). Pre-Wisconsin soil in the Rocky Mountain Region, a progress report. US Geological Survey Professional Paper 221G, 109-123. Jenny, H. (1941). Factors of Soil Formation. McGraw-Hill, New York. Johnson, D.L. and Hole, F.D. (1994). Soil formation theory: a summary of its principal impacts on Geography, Geomorphology, Soil-Geomorphology, Quaternary Geology and Paleopedology. In: Amundson, R., Harden, J. and Singer, M. (eds), Factors of Soil Formation. A Fiftieth Anniversary Retrospective, pp. 111-126. Soil Science Society of America Special Publication, Vol. 33. Jongmans, A.G., Feijtel, T.C.J., Miedema, R., Van Breemen, N. and Veldkamp, A. (1991). Soil formation in a Quaternary terrace sequence of the Allier, Limagne, France. Macro- and micromorphology, particle size distribution, chemistry. Geoderma, 49, 215-239. Kemp, R.A. (1987a). Genesis and environmental significance of a buried Middle Pleistocene soil in eastern England. Geoderma, 41, 49-77. Kemp, R.A. (1987b). The interpretation and environmental significance of a buried Middle Pleistocene soil near Ipswich Airport, Suffolk, England. Philosophical Transactions of the Royal Society, London B, 317, 365-391. Kemp, R.A., Whiteman, C.A. and Rose, J. (1993). Paleoenvironmental and stratigraphic significance of the Valley Farm and Barham Soils in eastern England. Quaternary Science Reviews, 12, 833-848. Nikiforoff, C.C. (1943). Introduction to paleopedology. American Journal of Science, 241, 194-200. Oldeman, L.R., Hakkeling, R.T.A. and Sombroek, W.G. (1990). World Map of the Status of Human-induced Soil Degradation: Global Assessment of Soil Degradation (GLASOD). ISRIC/UNEP/ Winand Staring Centre/ISSS/FAO/ITC. Polynov, V.V. (1927). Contributions of Russian scientists to paleopedology. Russian Pedol. Invest. Part 8. Academy of Sciences, Moscow. Ramann, E. (1911). Bodenkunde, 3rd edn. Springer, Berlin. Ramann, E. (1928). Evolution and classification of soils, Trans. C.L. Whittles. Heifer and Sons, Cambridge. Retallack, G.J. (1990). Soils of the Past. An Introduction to Paleopedology. Unwin Hyman, Boston. Retallack, G.J. and Feakes, C.R. (1987). Trace fossil evidence for Late Ordovician animals on land. Science, 235, 61-63. Ruhe, R.V. (1956). Geomorphic surfaces and the nature of soils. Soil Science, 82, 441- 455. Semmel, A. (1989). Paleopedology and geomorphology: examples from the western part of central Europe. Catena Supplement, 16, 143-162. Smeck, N.E. and Ciolkosz, E.J. (eds) (1989). Fragipans; their Occurrence, Classification, and Genesis, Soil Science Society of America Special Publication, Vol. 24, SSSA, Madison. Soil Survey Staff (1975). Soil Taxonomy. A Basic System of Soil Classification for Making and Interpreting Soil Surveys, US Department of Agriculture, Agriculture Handbook 436. Government Printing Office, Washington, DC.
The Position of Paleopedology in Geosciences and Agricultural Sciences Soil Survey Staff(1992). Keys to Soil Taxonomy, 5th edn. Soil Management Support Services Technical Monograph 19. Pocahontas Press, Blacksburg. Spaargaren, O.C. (ed.) (1994). World Reference Base for Soil Resources - - Draft. ISSS-ISRIC-FAO, Wageningen. Sprengel, C.S. (1837). Die Bodenkunde oder die Lehre yore Boden nebst einer vollstandioen Anleitung zur Chemischen Analyse der Ackereden.
I. Muller, Leipzig. Tandarich, J.P. and Sprecher, S.W. (1994). The intellectual background for the Factors of Soil Formation. In: Amundsen, R., Harden, J. and Singer, M. (eds.), Factors of Soil Formation: A
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Fiftieth Anniversary Retrospective, pp. 1-13. Soil Science Society
of America Special Publication, Vol. 33. Valentine, K.W.G. and Dalrymple, J.B. (1976). Quaternary buried paleosols: a critical appraisal. Quaternary Research, 6, 209-220. Van Vliet, B. and Langohr, R. (1981). Correlation between fragipans and permafrost with special reference to silty Weichselian deposits in Belgium and northern France. Catena, $, 137-154. Yaalon, D.H. (1971). Soil-forming processes in time and space. In: Yaalon, D.H. (ed.), Paleopedolooy: origin, nature and dating of paleosols, pp. 29-39. International Soil Science Society and Israel Universities Press, Jerusalem.
Quaternary International Vols 51/52, pp. 87-93, 1998. © 1998Publishedby 1NQUA/ElsevierScienceLtd All rightsreserved. Printedin GreatBritain, PII: S1040-6182(97)00035- 9 1040-6182/98/$19.00
THE POSITION OF PALEOPEDOLOGY IN GEOSCIENCES AND AGRICULTURAL SCIENCES A. Bronger* and J.A. Cattt *Geographisches Institut, Christian-Albrechts Universitat zu Kiel, D24098 Kiel, Germany tlACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK
Paleopedologyis a relatively young science with roots in various older geosciences.At present it suffersfrom a lack of clear definitionsand fundamental principles. Nevertheless it has considerable potential for applications in stratigraphy,paleontology,sedimentology,archeology, paleoclimatology,seismology,geomorphology,soil classificationand agriculture.Consequentlyit could play a major coordinatingrole in soil science, especiallyin contributing to a natural, strongly genetic systemof soil classification.We suggest that such a system,with emphasis on developmentof soil properties in relationto current and past environmentalfactors,wouldbe of greater valuein all thesesubjectsthan the current systemsdesignedprincipallyfor agriculturalinterpretation. © 1998 Publishedby INQUA/ElsevierScienceLtd. All rights reserved. INTRODUCTION
reviewing the importance of paleopedology in geosciences and agricultural sciences. Justification is certainly required, because we are aware of considerable ignorance of, and even prejudice against, paleopedology among other earth scientists, including some soil scientists.
As Tandarich and Sprecher (1994) have described, agricultural interests in the fledgling sciences of chemistry and geology developed almost simultaneously in the late eighteenth and early nineteenth centuries. The chemical interests were based on analyses of soils, especially their humic components, and of plant ashes, to clarify the factors influencing growth of plants and the production of crops. The geological interests were in processes of soil formation, such as weathering of minerals and rocks, and in reasons for the geographical variability of soil. The International Society of Soil Science developed out of this agricultural geology in the 1920s, though the independent science of Bodenkunde (soil knowledge or soil science) had earlier arisen from European chemical interests (Sprengel, 1837). The realisation that soils need to be studied from many points of view (geological, botanical, chemical, geographical, physical, mineralogical) had also led to the independent and more multidisciplinary science of pedology (Fallou, 1862), though unfortunately this is now often seen in a narrower sense as a branch of soil science focussing on soil genesis and classification (Buol et al., 1989, pp. 3-4). Surprisingly, these contrasts between the geological, agrochemical and multidisciplinary approaches in soil science persist to the present day. Paleopedology clearly has an important role near the geological end of the spectrum, especially now that paleosols even as old as Precambrian have been recognised throughout the geological column. But it is also important for a proper understanding of the agrochemical and agroenvironmental behaviour of surface (non-buried) soils, and it plays a vital role in the multidisciplinary (pedological) study of soils. We therefore see paleopedology as occupying an important position in soil science (Fig. 1) with considerable potential for a major coordinating role through a genetic system of classification. In this paper we summarise evidence to justify these statements by
P A L E O P E D O L O G Y AS A SUBDISCIPLINE OF GEOLOGY Paleopedology may be defined as the study of paleosols, which have in turn been somewhat loosely defined as soils 'formed on a landscape during the geologic past' (Ruhe, 1956; Yaalon, 1971). Paleosols are buried beneath younger deposits (buried paleosols), were once buried but have subsequently been re-exposed by erosion (exhumed paleosols) or have persisted on the land surface to the present day (non-buried or relict paleosols). We will discuss later the problem of how long a soil must persist on the land surface before it can be called a non-buried paleosol. In geology buried paleosols have been recognised for over a century in many countries, such as U.S.A., U.K. and Germany. The first to be recognised were fairly young soils (Quaternary, Tertiary or Mesozoic), which are still identifiable because they had not been strongly altered by metamorphism or advanced diagenesis. However, modern petrographic and geochemical techniques have allowed older and often quite strongly metamorphosed soils to be recognised and used to interpret past climates, terrestrial plant and animal assemblages, geomorphological features and even changes in the composition of the primeval atmosphere (Retallack, 1990; Feakes et al., 1989). The importance of buried paleosols to geologists results from several fundamental properties of soils: 1. Many take fairly long periods (103-107yr) to form, and consequently represent major episodes of land surface stability, with little or no erosion or 87
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A. Bronger and J.A. Catt Agricultural
Sciences
Geosciences
I,
I
SoilScience i
.Soil seoiments
Pedology
Se Sediments -- I'Geology, ~ Geomorphology,etcI
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Soils
Buriedpaleosols (fossilsoils)
Non-buriedpaleosols (relict soils) Paleopedology?
FIG. 1. The position of paleopedologywithingeosciencesand agriculturalsciencesand internationalscientificunions. deposition. This allows breaks in sedimentation to be recognised, which are especially useful in subdividing and correlating long sequences of unfossiliferous and lithologically monotonous sediments. The breaks often constitute unconformities because the land surface, beneath which the soil formed, originated by emergence following uplift and tilting; unconformities are often sources of valuable ores and minerals, and are important in understanding the tectonic history of a region. 2. Their profile characteristics (the properties, thickness and sequence of horizons) are influenced mainly by the five groups of soil-forming factors - - climate, organisms, relief, parent material and time (Dokuchaev, 1883; Jenny, 1 9 4 1 ) - lateral variations of which give rise to characteristic catenary changes of soil properties. This not only allows past environmental factors (climate, biota and geomorphology) to be assessed, but may also indicate the length of the episode/s of land surface stability. In addition it often distinguishes true paleosols from local sequences that superficially resemble soil profiles but are in fact successions of different lithostratigraphic units or products of diagenetic processes such as mobilisation and redeposition of calcium carbonate or iron oxides (Valentine and Dalrymple, 1976). 3. Soils are habitats for most terrestrial life-forms, and buried paleosols often contain body fossils (e.g. mollusc shells, phytoliths, seeds, pollen grains) or trace fossils (e.g. root traces, burrows) of plants and animals living on the land at the time the soil developed. These are often better than any other evidence for such life forms, and for some early periods it may be the only evidence; e.g. for the earliest land animals in the Late Ordovician of Pennsylvania (Retallack and Feakes, 1987). Buried Quaternary paleosols often contain human artefacts or even hominid fossils such as bones and teeth; these are of
considerable value in archeology, because they are more likely to be in situ than those found in sediments, and the properties of the surrounding soil provide evidence for the environment in which man was living at the time. In addition many Holocene buried soils provide evidence for human impacts on the landscape, such as deforestation, erosion and other types of soil degradation, and knowledge of these can help predict the effects of modern soil use or misuse. This range of potential uses of paleosols has greatly expanded their study by geologists, archeologists and other earth scientists over the past decade or two. However, because these workers have often lacked detailed pedological knowledge, there have been examples of incorrect interpretations and failures fully to exploit the scientific potential of temporary exposures of buried paleosols. One of the most important recent uses of buried Quaternary paleosols has been the paleoclimatic interpretation of the multiple soils in the loess sequences of eastern Europe, China, Soviet Central Asia and other regions. These sequences have provided detailed records of climatic change for the last 2.4 million years, which are potentially at least as detailed as the oxygen isotope record obtained from deep oceanic cores (Bronger and Heinkele, 1989). Like the oceanic cores, they also show a strong statistical relationship to the variations in solar radiance resulting from perturbations of the earth's orbit, the so-called Milankovitch curves. Paleosols have undoubted importance for understanding the processes driving natural climatic change, not just through the Quaternary (glacials, interglacials, interstadials) but throughout geological time, because the Milankovitch curves have now been identified in cyclic sediment sequences of many preQuaternary periods. The same processes will undoubtedly control future climatic change, though the
The Position of Paleopedology in Geosciences and Agricultural Sciences more immediate effects of atmospheric pollutants (greenhouse gases) may be superimposed on those related to the Milankovitch curves. The main advantage of the loess-paleosol sequences in reconstructing past global climate change is that they cover all or much of the Quaternary period in many different geographical regions. When the paleosols are adequately dated and fully interpreted, they will allow geographical variation of past climates to be assessed more reliably and in more detail than is possible by any other method known at present. Another recent use of buried soils with considerable economic importance is in estimating the timing and frequency of tectonic events, with the intention of calculating lengths of periods between past episodes and thus predicting when future earthquakes will occur. For example, Gerson et al. (1993) described soils displaced by past vertical fault movements that were buried by colluvial wedges accumulating on the downthrown side soon after the movement. They found that the time of burial can be estimated from radiocarbon assays of their organic matter, thermoluminescence studies or the extent of soil development on the surface of the colluvial wedge. Dating colluvial wedges adjacent to fault scarps by calculating soil development indices (e.g. Harden, 1982; Harden and Taylor, 1983) is especially useful in arid regions, where organic matter and other datable materials are rare or absent. The dates of successive soils buried in this way therefore indicate the approximate periods between episodes of earthquake activity. Also, sequences of buried soils can sometimes be traced across fault zones; differences in the displacement of successive soils (again dated by radiocarbon or other methods) then indicate the frequency of past movements, and the age of the oldest undisplaced soil gives the minimum period of time since the last episode of movement.
GEOMORPHOLOGICAL SIGNIFICANCE OF PALEOSOLS Another branch of geoscience in which the study of paleosols has brought significant advances is the dating of land surfaces and clarification of the geomorphological history of an area. This relies upon the identification and absolute or relative dating of surface paleosols. The longer a soil remains unburied on the land surface the more it develops and, assuming other soil forming factors (climate, organisms, relief and parent material) have been constant, a chronosequence of soils on land surfaces of increasing age (e.g, a flight of river terraces) shows increasing development of properties such as profile depth, weathering of primary minerals, humus incorporation, clay illuviation, etc. In areas where the environmental factors have remained fairly constant, such as the Merced River valley in California (Harden, 1982), these properties can be combined into a soil development index that is strongly correlated with time. Such combined indices minimise the problem of
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properties that do not increase linearly with time; e.g. those that initially increase slowly, then change more rapidly and finally slow down or even cease to change over long periods. However, in many mid- and high-latitude humid regions environmental factors have not remained constant for long. The repeated large glacial-interglacial climate changes during the later Quaternary, and the associated biotic, geomorphological and sedimentological changes, have greatly complicated the relationships between soil properties and time. In such regions simple chronofunctions can only be used to date soils and the land surfaces on which they occur when they are no older than 10,000yr (i.e. the last major change in climate). Despite this limitation some thick, strongly weathered surface soils in both low and mid-latitudes are thought to have begun development in pre-Quaternary periods. Despite this limitation, some advances in dating soils and land surfaces have been made in mid-latitude regions by using the characteristic properties of soils that develop under climatic extremes. For example, in many regions soils containing typical periglacial features, such as ice-wedge casts or possibly fragipans, can only occur on land surfaces that have persisted without significant modification since at least the last major cold stage in the Late Weichselian (Wisconsinan). If this type of information is combined with knowledge of the age of later Quaternary parent materials of the soils, it provides a powerful tool for dating the land surface beneath which the soil occurs (Semmel, 1989). At least it eliminates the possibility of a major episode of Late Weichselian or Holocene erosion. An extension of this approach relies on the recognition of interglacial soil features, indicating that the land surface has persisted without significant erosion or deposition since at least 125,000yr ago. For example, the paleo-argillic soils of England and Wales (Avery, 1980), occurring on older river terraces and upland plateaux in S. England, have redder colours (usually as discrete mottles) and more extensive segregations of birefringent clay in their Bt horizons than any (Holocene) soils formed on Devensian (=Weichselian) parent materials. Periglacial soil features are often superimposed on the paleo-argillic Bt horizon, which is also almost always overlain by thin (< 1 m) Late Devensian loess (containing typical Holocene soil features), thus providing evidence for a long history of pedogenetic and minor depositional events on these ancient land surfaces. Micromorphological studies of some paleo-argillic Bt horizons have suggested even longer pedogenetic histories, spanning two or more interglacials, distinguished by argillans of different colour or composition, different relationships to hydromorphic features or different extents of disruption by frost in the intervening periglacial periods (Bullock and Murphy, 1979; Chartres, 1980; Kemp, 1987a, Jongmans et al., 1991). However, some soils on very old river terraces have not shown the expected complexity of micromorphological
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features (Kemp, 1987b), possibly because intense interglacial pedogenesis may have obliterated earlier evidence for mild interglacial or periglacial microfeatures (Kemp et al., 1993). Soils which have persisted at the surface since the Last Interglacial or even since the end of the Weichselian have clearly formed at least partly "on a landscape during the geologic past' and so must qualify as non-buried paleosols (G6ze, 1947; Hunt and Sokoloff, 1950) even though, following Erhart (1932), 'paleosol" was originally used as equivalent only to buried soil (Johnson and Hole, 1994). In this sense non-buried paleosols are approximately equivalent to the Reliktboden of Ramann (1911, p. 526; 1928, p, 37), the secondary soil formations and two-stage soils of Polynov (1927), the polygenetic soils of Bryan and Albritton (1943) and the surface soils 'out of harmony with the environment' of Nikiforoff (1943). All these terms were based upon the concept of soils showing 'two or more sets of properties which can be related to different combinations of soil-forming factors through sets of often incompatible soil-forming processes', implying 'two or more environmentally different periods of soil development' (Bronger and Catt, 1989). The main problem with distinguishing non-buried paleosols in this way is that fairly young soils qualify as paleosols in mid- and high-latitudes (Catt, t989), where Quaternary climatic changes had their greatest effect, whereas much older soils in low latitudes may not. Even within quite small areas, soils of the same age may or may not qualify depending on whether or not they have been influenced by human activities, because these are usually included as part of the "organisms' group of environmental soil-forming factors. Indeed, because of the increasingly widespread influence of human activities on soils and the possibility of even quite small recent (mid- or late-Holocene) climatic changes influencing soils, there is a strong possibility that almost all soils now qualify as non-buried paleosols. This problem of definition will have to be resolved in due course, but for the moment it serves the purpose of emphasising the necessity of relating soil properties to either present environmental conditions or the profile's past history.
PALEOSOLS IN SOIL CLASSIFICATION SYSTEMS Classification of soils is a necessary subdiscipline of pedology; it is important for communication between pedologists, for transferring results of field trials, laboratory or glasshouse experiments to other areas and for portrayal of soil variation on maps of various scales. In the last few decades numerous national and two international systems have been developed, but none is yet universally accepted. Of the two international systems, the very detailed American Soil Taxonomy (Soil Survey Staff, 1975, 1992) is used for soil surveys at large scales (e.g. 1 : 20,000), whereas the FAO-Unesco system
(FAO-Unesco, 1974, 1988), now the World Reference Base (Spaargaren, 1994), serves mainly as a legend for the 1 : 5,000,000 Soil Map of the World. Although the American system includes some 'Pale'-great groups to distinguish strongly developed profiles, neither system really attempts to relate soil properties to landscape history. Consequently neither is suitable for classifying paleosols or for encouraging studies aimed at understanding soil genesis, and neither is a natural genetic system. Both are in fact compromises aimed principally at agricultural applications and based on minimum necessary analytical investigations and brief assessments of field (e.g. stratigraphic) relationships. A true natural genetic system of classification, relating all soil properties to a past history of climatic and other environmental factors, might prove more universally acceptable and useful in various other applications as well as in agriculture. One particular deficiency of all soil classification systems known to us is the failure to distinguish soil sediments at any taxonomic level. Sediments formed by mass movement of soil materials with minimal dispersion of the constituent particles often resemble the original soils, especially in terms of particle size distribution and agricultural behaviour, and are consequently never distinguished from them on soil maps. However, the distinction is essential for correct geomorphological and stratigraphical interpretation, as soil sediments often occur in very different situations from the original soils, even if they have only been transported for short distances. If the geomorphological or stratigraphic context of a soil is misinterpreted, so also is its paleoclimatic or other paleoenvironmental significance. Recognition of soil sediments and the paleoenvironmental interpretation of the in situ soils from which they were derived is especially important in the cave environment. Here they are often associated with deposits rich in mammalian remains and human artefacts, which are important for understanding evolutionary changes and early human activities in relation to the history of Quaternary climatic change. The soil sediments provide the only means of relating sequences within the protected environment of the cave to climatic changes outside. A full and correct understanding of landscape history as a basis for soil classification and nomenclature is essential for a proper appreciation of many modern environmental problems. For example, the soil cover of central and south India consists mainly of Vertisols ('Black Soils') and Lixisols (Rhodustalfs or 'Red Soils'), both of which are non-buried paleosols because they formed in an earlier period of wetter climate than the present. In the present semi-arid conditions (<1000 mm annual rainfall and very high evaporation) soilforming processes such as deep weathering have almost ceased; instead secondary carbonate is accumulating in the saprolite and lower parts of the Lixisol Bt horizons (Bronger and Bruhn, 1989), and under arable agriculture soil erosion is becoming a severe problem. It is
The Position of Paleopedology in Geosciences and Agricultural Sciences widely accepted that soil erosion can be tolerated from a crop production viewpoint provided soil formation keeps pace with it to compensate for the losses. However, in semiarid India the soil development processes have changed and the rate of compensatory regeneration of soil is in effect zero. The erosion there is consequently a permanent loss of one of the country's most important natural resources, but it has not been recognised as such because the soils were not identified as paleosols. The erosion now occurring in many other countries (Oldeman et al., 1990) is probably a similar irreplaceable loss of resources inherited from earlier geological periods.
AGRICULTURAL SIGNIFICANCE OF PALEOSOLS Many soil properties influencing agricultural production are paleosol features inherited from earlier periods when climate or other environmental conditions were different from those of the present. In midlatitude regions they can be divided into relict features of cold Quaternary stages and those inherited from interglacials (Catt, 1992). A subsoil feature that often results from periglacial soil disturbance (Van Vliet and Langohr, 1981) is the brittle, compact fragipan horizon, formed in loamy subsoils by growth of ground-ice lenses and laminae. These have bulk densities of 1.6-2.0 g/cm 3, and can present almost impenetrable barriers to roots and severely limit downward percolation o f water (Smeck and Ciolkosz, 1989), especially where their upper surfaces are further hardened by precipitation of iron oxides or silica from groundwater slow to penetrate the layer. Fragipans usually occur at depths of 50-100 cm, that is just below the original active layer of the periglacial environment. Consequently they mainly affect trees, which often suffer windthrow because of shallow rooting, especially in wet weather when the soil above the fragipan is saturated and easily deformed. The wide range of soil structures formed by frost disturbance in periglacial conditions, such as ice wedge casts, involutions, stripes, pingos, thermokarst and patterned ground features, are an important cause of short-range variation in soil depth and particle size distribution. Efficient farmers can usually adapt their management techniques (choice of crops, timing and rates of fertiliser applications, etc.) to obtain maximum productivity and profit or to reduce environmental pollution from different soil types. This is easy where the different soils form large areas, especially if soil boundaries correspond with field boundaries, but with small, intricately shaped areas within a field differential management is difficult, at least with present farm machinery. The farmer must then choose between treatments that are suited to one or other of the soil types present, or management that is a compromise between them. Either approach can lead to significant
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loss of yield, up to 50% in some years (Evans and Catt, 1987), or to increased leaching losses, for example where too much nitrogen is applied to very permeable soils. The deeply penetrating cracks formed by ice wedge casts, which are often filled with very permeable material, such as coversand, also form preferential flowpaths through less permeable soil for loss of fertiliser or pesticide to groundwater. Interglacial features that can influence soil performance in agriculture include the strong enrichment of Bt horizons with illuvial clay, the increased content of iron oxides produced by stronger weathering of primary minerals and deep acidification. In lower latitudes strong clay enrichment of Bt horizons often decreases subsoil permeability because the clay is deposited in the voids that conduct water (Jongmans et al., 1991). This produces pseudogley or stagnogley profiles in which root development may be limited by seasonally anaerobic conditions. At higher latitudes, where non-buried interglacial paleosols were disturbed by frost action in subsequent periglacial periods, the illuvial clay bodies (argillans) were disrupted and the fragments (papules) incorporated into the soil matrix. This disruption created new structural units (peds) separated by a fresh system of voids, which increased the Bt horizon permeability. The clay-enriched paleo-argillic soils of S. England are consequently often subject to by-pass (fissure) flow and are more permeable than Holocene soils containing much less clay. The longterm stability of peds in these soils is probably maintained in part by the large amounts of iron oxides produced by interglacial weathering. The greater amounts of oxides also increase the phosphate fixation capacity of interglacial paleosols; larger amounts of phosphate fertiliser are consequently required to achieve the same initial yield increase as on less weathered soils. However, the fixed phosphorus can subsequently be released slowly, especially if nitrogen is added without further phosphorus to stimulate growth of cereals. In the long term some non-buried paleosols therefore have better phosphorus economies and lose less phosphate by leaching than younger, lessweathered soils. Pedogenesis in humid regions results in progressive acidification, initially of the topsoil and later of subsoil horizons. In many mid- and high-latitude regions the Holocene was probably not long enough for subsoil horizons to become very acid, even where the soil parent material is weakly- or non-calcareous. In contrast, non-buried interglacial paleosols are often completely decalcified to a depth of 5 m or more, and subsoil horizons to depths of 2-3 m commonly have pH values less than 4.5. Surface liming rapidly increases topsoil pH, but may take many years, sometimes even centuries, to affect the subsoil. Consequently, where acidity extends below the depth to which lime can readily be incorporated (20-30cm), surface liming brings little or no improvement in the growth of deeprooting crops, such as bushes and trees; this is often because root development is restricted by the increased
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availability of aluminium and manganese in strongly acid conditions. Finally, in some regions that have become arid by late Holocene climatic change (e.g.N. Africa) earlier Holocene soils with well-preserved Ah horizons have been buried by aeolian sand or colluvium resulting from soil erosion. With modern engineering technology and improved understanding of water conservation and nutrient cycling, these buried paleosols could be reclaimed with considerable benefit to local agricultural production.
CONCLUSIONS The range of fundamental and applied sciences in which paleosols are currently being used shows that paleopedology occupies a central position of considerable importance in the broad subject of soil science, or pedology in the sense of Fallou (1862). The needs of stratigraphers, geomorphologists, paleontologists, paleoclimatologists and others to recognise and interpret paleosols in various ways places an increasing responsibility on soil scientists to provide clearer guidelines for the identification and interpretation of buried soils, which can only be achieved by better fundamental studies (mineralogical, micromorphological, chemical, etc.) of surface soils, leading to better understanding of the development of their properties in relation to current and past environments. Proper communication between and within these disciplines will demand that the results of the fundamental soil property and profile studies are reflected in a more natural genetic classification of soils no longer dominated by the essentially chemical and physical demands of agriculture. Once the need for a fully genetic classification is recognised, paleopedology will automatically play a major formative role within it.
REFERENCES Avery, B.W. (1980). Soil Classificationfor England and Wales (Higher Categories), Soil Survey Technical Monograph, Vol. 14. Rothamsted Experimental Station, Harpenden. Bronger, A. and Bruhn, N. (1989). Relict and recent features in tropical Alfisols from South India. Catena Supplement, 16, 107-128. Bronger, A. and Catt, J.A. (1989). Paleosols: problems of definition, recognition and interpretation. Catena Supplement, 16, 1-7. Bronger, A. and Heinkele, T. (1989). Paleosol sequences as witnesses of Pleistocene climatic history. Catena Supplement, 16, 163-186. Bryan, K. and Albritton, C.C. (1943). Soil phenomena as evidence for climate changes. American Journal of Science, 241, 469-490. Bullock, P. and Murphy, C.P. (1979). Evolution of a paleo-argillic brown earth (Paleudall) from Oxfordshire, England. Geoderma, 22, 225-252. Buol, S.W., Hole, F.D. and McCracken, R.J. (1989). Soil Genesis and Classification (3rd edn.). Iowa State University Press, Ames. Catt, J.A. (1989). Relict properties in soils of the central and northwest European temperate region. Catena Supplement, 16, 41-58. Catt, J.A. (1992). Quaternary environments and their impact on British soils and agriculture. Quaternary Proceedings, 2, t7-24. Chartres, C.J. (1980). A Quaternary soil sequence in the Kennet Valley, central southern England. Geoderma, 23, 125-146.
Dokuchaev, V.V. (1883). Russian chernoziom. St. Petersburg, Russia (In Russian; English translation by N. Kaner, Israel, Program for Scientific Translation, Jerusalem). Erhart, H. (1932). Sur la nature et la gen6se des pal6o-sols du loess ancien d'Alsace. Comptes Rendus l'Academie des Sciences Paris. 194, 554-556. Evans, R. and Catt, J.A. (1987). Causes of crop patterns in eastern England. Journal of Soil Science, 38, 309-324. Fallou, F.A. (1862). Pedologie oder allgemeine und besondere Bodenkunde. Schonfeld, Dresden. Feakes, C.R., Holland, H.D. and Zbinden, E.A. (1989). Ordovician paleosols at Arisaig, Nova Scotia, and the evolution of the atmosphere. Catena Supplement, 16, 207-232. FAO-Unesco (1974). Soil Map of the Worm 1 : 5,000,000: Vol. 1 Legend. UNESCO, Paris. FAO-Unesco (1988). Soil Map of the World. Revised legend, World Soil Resources Report 60. FAO, Rome. Gerson, R., Grossman, S., Amit, R. and Greeubaum, N. (1993). Indicators of faulting events and periods of quiesence in desert alluvial fans. Earth Surface Processes and Landforms, 18, 181-202. G6ze, B. (1947). Paleosols and modern soils. In: Comptes Rendus Conference Pedologie Mediterranban, pp. 210-219. AlgerMontpellier, Paris. Harden, J.W. (1982). A quantitative index of soil development from field descriptions: examples from a chronosequence in central California. Geoderma, 28, 1-28. Hunt, C.B. and Sokoloff, V.P. (1950). Pre-Wisconsin soil in the Rocky Mountain Region, a progress report. US Geological Survey Professional Paper 221G, 109-123. Jenny, H. (1941). Factors of Soil Formation. McGraw-Hill, New York. Johnson, D.L. and Hole, F.D. (1994). Soil formation theory: a summary of its principal impacts on Geography, Geomorphology, Soil-Geomorphology, Quaternary Geology and Paleopedology. In: Amundson, R., Harden, J. and Singer, M. (eds), Factors of Soil Formation. A Fiftieth Anniversary Retrospective, pp. 111-126. Soil Science Society of America Special Publication, Vol. 33. Jongmans, A.G., Feijtel, T.C.J., Miedema, R., Van Breemen, N. and Veldkamp, A. (1991). Soil formation in a Quaternary terrace sequence of the Allier, Limagne, France. Macro- and micromorphology, particle size distribution, chemistry. Geoderma, 49, 215-239. Kemp, R.A. (1987a). Genesis and environmental significance of a buried Middle Pleistocene soil in eastern England. Geoderma, 41, 49-77. Kemp, R.A. (1987b). The interpretation and environmental significance of a buried Middle Pleistocene soil near Ipswich Airport, Suffolk, England. Philosophical Transactions of the Royal Society, London B, 317, 365-391. Kemp, R.A., Whiteman, C.A. and Rose, J. (1993). Paleoenvironmental and stratigraphic significance of the Valley Farm and Barham Soils in eastern England. Quaternary Science Reviews, 12, 833-848. Nikiforoff, C.C. (1943). Introduction to paleopedology. American Journal of Science, 241, 194-200. Oldeman, L.R., Hakkeling, R.T.A. and Sombroek, W.G. (1990). World Map of the Status of Human-induced Soil Degradation: Global Assessment of Soil Degradation (GLASOD). ISRIC/UNEP/ Winand Staring Centre/ISSS/FAO/ITC. Polynov, V.V. (1927). Contributions of Russian scientists to paleopedology. Russian Pedol. Invest. Part 8. Academy of Sciences, Moscow. Ramann, E. (1911). Bodenkunde, 3rd edn. Springer, Berlin. Ramann, E. (1928). Evolution and classification of soils, Trans. C.L. Whittles. Heifer and Sons, Cambridge. Retallack, G.J. (1990). Soils of the Past. An Introduction to Paleopedology. Unwin Hyman, Boston. Retallack, G.J. and Feakes, C.R. (1987). Trace fossil evidence for Late Ordovician animals on land. Science, 235, 61-63. Ruhe, R.V. (1956). Geomorphic surfaces and the nature of soils. Soil Science, 82, 441- 455. Semmel, A. (1989). Paleopedology and geomorphology: examples from the western part of central Europe. Catena Supplement, 16, 143-162. Smeck, N.E. and Ciolkosz, E.J. (eds) (1989). Fragipans; their Occurrence, Classification, and Genesis, Soil Science Society of America Special Publication, Vol. 24, SSSA, Madison. Soil Survey Staff (1975). Soil Taxonomy. A Basic System of Soil Classification for Making and Interpreting Soil Surveys, US Department of Agriculture, Agriculture Handbook 436. Government Printing Office, Washington, DC.
The Position of Paleopedology in Geosciences and Agricultural Sciences Soil Survey Staff(1992). Keys to Soil Taxonomy, 5th edn. Soil Management Support Services Technical Monograph 19. Pocahontas Press, Blacksburg. Spaargaren, O.C. (ed.) (1994). World Reference Base for Soil Resources - - Draft. ISSS-ISRIC-FAO, Wageningen. Sprengel, C.S. (1837). Die Bodenkunde oder die Lehre yore Boden nebst einer vollstandioen Anleitung zur Chemischen Analyse der Ackereden.
I. Muller, Leipzig. Tandarich, J.P. and Sprecher, S.W. (1994). The intellectual background for the Factors of Soil Formation. In: Amundsen, R., Harden, J. and Singer, M. (eds.), Factors of Soil Formation: A
93
Fiftieth Anniversary Retrospective, pp. 1-13. Soil Science Society
of America Special Publication, Vol. 33. Valentine, K.W.G. and Dalrymple, J.B. (1976). Quaternary buried paleosols: a critical appraisal. Quaternary Research, 6, 209-220. Van Vliet, B. and Langohr, R. (1981). Correlation between fragipans and permafrost with special reference to silty Weichselian deposits in Belgium and northern France. Catena, $, 137-154. Yaalon, D.H. (1971). Soil-forming processes in time and space. In: Yaalon, D.H. (ed.), Paleopedolooy: origin, nature and dating of paleosols, pp. 29-39. International Soil Science Society and Israel Universities Press, Jerusalem.