Quaternary buried paleosols: A critical review

QUATERNARY

6,209-222

RESEARCH

Quaternary

(1976)

Buried

Paleosols:

A Critical

Review

K. W. G. VALENTINE AND J. B. DALRYMPLE Agriculture Canada, Soil Research Institute, 6660 N. W. Marine Drive, Vancouver, British Columbia V6T 1X2, Canada; and Department of Soil Science, University of Reading, Berkshire, England Received

May 5, 1975

A review of work on buried paleosols in the disciplines of pedology, Quaternary geology, and archaeology is presented under the headings of (1) the problems of identification, (2) techniques of study, (3) buried paleosols and Quaternary stratigraphy, (4) archaeological stratigraphy and dating, (5) layered soils, and (6) past environment from buried paleosols. It is suggested that future pedological research of interest to Quaternary studies should concentrate on clarifying what is a soil as opposed to a weathered sediment, what processes and features are peculiar to pedogenesis as opposed to diagenesis, and what are the relationships between soil-site conditions and soil characteristics.

INTRODUCTION A paleosol is a soil which has formed on a landscape of the past (Ruhe 1965, Working Group on the Origin and Nature of Paleosols, 1971). Such a soil may be on either a buried or an exposed surface. Buried paleosols occur where the land surface has been covered by younger sediments. Paleosols on exposed surfaces are termed relict. Some of their features have been derived from former climates or topographic and drainage conditions. In rare cases a buried landsurface has been reexposed by denudation and the paleosols here are called exhumed. Although studies of paleosols in various forms have been made since the end of the nineteenth century it is only within the last 20 years that a coherent field of research has developed with its associated concepts and definitions. Reviews of works in individual countries have appeared (Polynov, 1927; Thorp et al., 1951), but no overall review has been published. At first, the present writers attempted a complete review of Quaternary paleopedology. However, it soon became apparent that it would be impossible to deal with the complete topic

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within the confines of a single paper. Moreover, the methods of study and problems of definition, recognition, and interpretation of buried and relict paleosols are sufficiently different to warrant separate treatment. This review will therefore deal with buried paleosols. It is hoped to discuss relict paleosols in a later paper.

THE PROBLEMS OF IDENTIFICATION Views on the ease with which buried paleosols can be recognized range from the optimism of Ruhe, “There is no problem in recognizing a buried soil as a paleosol” (1965, p. 755), to the pessimism of Ruellan, “the identification of a buried soil in a single section is rarely simple and irrefutable” (1971, p. 9). Opinions will vary with experience derived from different geographic areas, statement is probably but Ruellan’s nearer the truth. The difficulties stem from (1) the looseness with which such terms as soil, weathered zone, and sediment have been used, (2) the ephemeral nature and susceptibility to change of many soil features, and (3) the similarity

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of soils to sediments that have suffered diagenesis. A wide range of weathered materials will have been buried in Quaternary landscapes which have suffered periodic erosion and deposition. The nature of these weathered materials will depend on the time elapsed and their erosion and transportation history prior to burial. As Gerasimov (1971) points out, they will vary from soils formed in situ, through transported soil material where the fabric is retained (usually slope-wash), to raw sediments where the original arrangement of the particles has been destroyed by the transporting agency. It therefore becomes important in the identification of buried paleosols to define what is meant by a soil. Implicit in most work is the acceptance of the pedological concept of soil as a natural body formed in the surface materials of the earth under the influence of climate, biota, topography, and time. It will have vertically differentiated layers due to the relative intensities of biological, chemical, and physical weathering and the translocation of the products. A vertical section through these layers constitutes a soil profile. This concept is derived from the original Russian idea of soil, which was little known in the rest of the world until Glinka’s work on soil science was translated by Marbut (Glinka, 1927). Prior to this time a slightly different concept of a “weathering zone” or “profile” had grown out of geological studies of weathered tills in North America. Kay (1916a) first described the various layers of weathering, and Leighton and MacClintock (1930) gave them numbers. The weathering profile includes the soil profile but extends down into the zone of chemical weathering below the limit of carbonate leaching. Ruhe (1969, p. 14) shows the relationship between the weathering profile and soil profile. The use of the two

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terms implies a distinction between soil and rock weathering that is very difficult to make in practice. Nevertheless, it is important to realize in reading the literature on buried paleosols (especially the American literature) that sometimes the soil profile is meant (Simonson 1941), sometimes the weathering profile is meant (Brophy, 1959), and sometimes they are used interchangeably (Morrison and Frye, 1965). The problem of identification not only involves the recognition of either of these two types of profiles but also their differentiation from the original sediments. Sediments are a diverse group of deposits made up of discrete solid particles with intervening pore spaces that have been laid down by water, wind, ice, or gravity on the surface of the earth under conditions of temperatures that are normal to the surface (Twenhofel, 1926). They will be the parent materials of the soils developed on their upper surfaces by soil forming processes. In fact, in some cases, such as a colluvial slope or alluvial fan, the soil may be an integral part of the sediment. Further, at depth, sediments are subject to a group of processes, known collectively as diagenesis, which operate between the time of deposition and metamorphism. This group of processes includes the weathering of minerals by oxidation and reduction, hydrolysis and solution, the biological action of anaerobic bacteria, compaction, cementation, recrystallization, and the lattice alteration of clays by the expulsion of water and ion exchange. It is the similarity of the processes and products of pedogenesis to those of diagenesis that is one of the major causes of confusion in the recognition of buried paleosols. At our present state of knowledge there are very few features that are unique to terrestrial soils as opposed to sediments. Even the clay skins of argillic horizons, which are

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often quoted as features unique to soils, have been reported from purely sedimentary sites (Bullock and Mackney, 1970) or as a result of biotite weathering in situ (Mermut and Pape, 1971). The transitory nature of many soil features and their liability to postburial alteration adds another difficulty. Yaalon (1971) separates the features of paleosols into three groups according to the reversibility of the processes forming them. In his easily altered group are mollic and salic horizons and mottles. On the other hand, he suggests cambic, umbric, spodic, and calcic horizons are relatively persistent and oxic, placic, argillic, and natric horizons plus concretions are very resistant to alteration. However, it is important to define the conditions of preservation when constructing and using a scale like this. Yaalon implies this by including many features in more than one group. An example is the humus of mollic horizons which is particularly liable to destruction if the conditions of burial allow the continued activity of microorganisms. However, Stevenson (1969) points out that under wet or stagnant environments organic compounds may be retained almost indefinitely as the lack of oxygen limits aerobic organisms, and toxic metabolic waste products will accumulate. The same author also states that the dry, cold environment of terrestrial glacial sediments can be conducive to humus retention. In addition to the destruction of existing features, new features can be inherited after burial. Ruhe (1956) has shown how a soil buried by calcareous loess can be secondarily enriched with bases, carbonates and iron oxides by the downward percolation of soil solutions. On a more drastic scale, Beckmann (1963) describes the alteration by heat, pressure and solution of the upper horizons of paleosols covered by lava in Sometimes existing features Hawaii.

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continue to develop, such as the gleyed clay pans in volcanic ash soils buried on colluvial foot slopes, as described by Dalrymple (1967). All these forms of postburial alteration to buried paleosols raises the question of how deep is “buried.” Ruellan (1971) suggests that two types of buried paleosols should be distinguished: firstly, those “which are deeply buried beneath the present zone of direct biological action” (p. 8), and secondly “soils of shallow burial which continue to evolve under the direct action of pedogenesis” (p. 9). The latter form is very similar to a relict paleosol and may be very difficult to recognize unless the old groundsurface can be seen. TECHNIQUES OF STUDY The difficulty of recognizing buried soils is illustrated by the range of approaches and methods reported by the Working Group (1971). This report emphasized the necessity of combining field with laboratory techniques. Field evidence such as color, structure, decalcification, clay skins, concretions, and roots has been used. For example, Simonson (1954) identified the Yarmouth Grey Brown Podzolic in the Kansan drift by its distinctive structure and cementation and clay and sesquioxide accumulation in the B2b horizon. Raeside (1964) used color, structure, clay skins, and root traces among other features to identify five buried soils in loess in South Island, New Zealand. Evidence of decalcification has been one of the most common field tests for buried paleosols. Leighton (1923) separated the Wisconsin and Illinoian drift sheets in Illinois on this basis and Zeuner (1945) reports its use for identifying weathered zones and soils and therefore time discontinuities between loess sheets in France and Germany. Gile and Hawley (1966) reported not only decalcified

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horizons but also the occurrence of distinctly pedogenic Cca and Bt horizons in correlating a number of buried arid zone soils in New Mexico. Color also has been commonly used, for example, to identify the red and gray soil horizons in aeolian and fluvial deposits in southeastern Australia (Butler, 1958), the red B horizons of buried soils in loess (Thor-p, 1935), or Brown B horizons between ash layers in Japan (Kobayashi, 1965). However, color alone is not always a reliable guide. It may be inherited from the parent material, or color variations may be produced by deep subsurface weathering associated with ground-water movement in layered sediments. Thus, Valentine and Dalrymple (1975) have suggested that a pseudoferric podzol profile has been produced by the redistribution of iron oxides by water percolating through an organic lacustrine mud many meters below the present surface. Among the laboratory techniques applied, the routine analyses of particle size, pH, organic content, cation exchange capacity, and nitrogen, potassium, and phosphorous are regularly carried out, but these are often more for description than identification, as the Working Group (1971) points out. Only in the clearest cases can a buried soil be identified from a single exposure with one profile using such routine analyses. Recently, special techniques have been suggested to identify the products of the combined biological, chemical, and physical weathering that is peculiar to soils and thereby recognize buried land surfaces in layered sediments. Thus, Goh (1972) uses amino acid analysis, Dormaar (1967) has used the infrared spectra of humic acids in buried Ah horizons, and Runge et al. (1974) suggest the identification of particular phosphorous compounds. However, these techniques have yet to be tried on a wide range of soils, and for their ultimate test they should be tried on organic-rich buried

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sediments which are known not to be terrestrial soils. This is necessary in order to confirm that results from soils and sediments are actually different. Mineralogical analyses of the various particle size fractions have been applied widely to buried paleosols, but again they are more often used to characterize the layered sediments than to identify the paleosol. Thus, Kobayashi and Shimizu (1962) used the heavy and light minerals of the sand fraction to define the nonconformity in volcanic ash deposits under which a buried soil occurred in Japan, and Beckmann (1963) used the clay mineralogy to distinguish layers of lava, volcanic ash, tuff, and alluvium plus the degree of weathering in their soils in Hawaii. Only rarely is a buried soil identified using mineralogy as the principal tool. Brophy (1959) has used heavy minerals to separate weathering profiles in Sangamon till and outwash, and Matsui et al. (1970) identify soils by their clay mineralogy in volcanic ash in Japan. Micromorphology is probably the most useful single technique as it allows the identification of the distinctive arrangement of particles and voids that constitutes a soil fabric as opposed to a sediment fabric. Dalrymple (1958, 1964) was able to identify paleosolic fabrics in archaeological sites and volcanic ash layers. Morozova (1963) used it on soils buried under loess in Russia; it has been used to elucidate the Quaternary development of paleosols in loess in France (Federoff, 1969), and Brewer (1972) gives some examples of its contribution to the study of soils in layered sediments in Australia. Lastly, techniques from other disciplines have been applied to buried paleosols such as the analysis of soil pollen (Dimbleby, 1952), land mollusca (Kerney, 1963), and opal phytoliths (Dormaar and Lutwick, 1969), but the purpose of this work is usually to recon-

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struct the past environment via the paleobotany of the paleosol rather than to identify the paleosol itself. A review of the literature then, confirms that the identification of a buried paleosol is “rarely simple and irrefutable,” as Ruellan (1971) suggests. Emphasis must be placed on the recognition of stable characteristics that are peculiar to soils. Nikiforoff (1943) suggested the mineral framework of the soil itself plus hardpans and concretions, and Simonson (1954) added sesquioxides and structure. Yaalon (1971) in his group of features formed by irreversible processes includes oxic, argillic, and natric horizons as well as soil crusts like laterite, plinthite and silcrete. In addition, there are the products of soil biochemical reactions such as the amino and humic acids mentioned above. But many more detailed studies are needed of the features that are peculiar to soils or sediments in contrast to features that are common to both. For example Siuta and Motowicka-Terelak (1969) have described the forms of ferriginous precipitates in Quaternary sediments and modern soils, Yehle (1954) has discussed the confusion between soil tongues and frost wedges, and Meade (1960) described the production of oriented fabric in clay sediments. However, ultimately as Brewer (1972) points out, buried paleosols must be recognized within the hypothesis of soil formation rather than from the occurrence of individual features. In other words, it is the vertical distribution down a profile or, more importantly, the horizontal variation of soil properties across a landscape that must be recognized. Valentine and Dalrymple (1975) have suggested that the crucial test of a buried soil landscape must be the existence of a paleocatena which they define as a group of paleosols on the same buried land surface whose original soil properties differ owing to their different original landscape position and soil-

water regimes. agenesis cannot three-dimensional tures .

213 Sedimentation and direproduce this logical pattern of soil fea-

BURIED PALEOSOLS AND QUATERNARY STRATIGRAPHY Much of the initial interest in buried paleosols was derived from their early use as stratigraphic marker beds in geological sections. The recognition and use of such paleosols seems to have occurred independently in various parts of the world in the latter part of the nineteenth century. Polynov (1927) describes how in the 1870s Feofilaktov observed “deep loess humus coloured horizons” in the Lubny district of the Poltava government, Russia. In North America, weathered layers in till sheets were recognized on the basis of zones of oxidation and the leaching of carbonates (Leverett, 1898) and in New Zealand Hardcastle (1889) described buried soils in loess which appeared as dark bands with root and worm channels. Since these initial studies, an enormous amount of literature has been published. Buried paleosols have been used either to separate deposits of different ages within vertical sections or to correlate deposits of the same age from one section to another by associating soils laterally across the buried landscape. Without doubt, the most extensive stratigraphic application of buried paleosols has been in the loess landscapes of the world. The early work in Russia was concentrated in the Ukraine. Pedologists and geologists attempted to use the buried humus horizons in loess to subdivide the Quaternary deposits into three or four stages parallel with the Alpine stages in Europe (work by Laskarev and Krokos, reported by Polynov, 1927). However, difficulties arose over the contentious origins of the loess and the irregular occurrence of humus within it. The generally accepted idea of loess as a

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weathering product (Berg, 1926) even though derived from stratified material was difficult to reconcile with the presence of buried soils. Moreover, these buried soils, which were supposedly Chernozems that had developed under interglacial steppe conditions, often had less organic matter than the intervening loess. More recently the acceptance of the aeolian origin of loess has allowed its detailed subdivision and at least six major soils (some of which are recognized as not being Chernozems) now separate the various loesses in the Russian Plain (Morozova, 1963). In North America, similar soils have been described throughout the Midwest the Mississippi-Missouri valley and (Schultz and Stout, 1945; Frye and Leonard, 1949; Ruhe, 1969). The most extensive is the Sangamon soil developed in Loveland loess during the IllinoianWisconsin interglacial. In the classic loess areas of northwest China numerous leached red-brown soil layers within deep sections of yellow calcareous loess were also recognized by Thorp (1935) in early geological work. The subdivision of the Quaternary stratigraphy in Europe owes much to the identification and dating of buried soils in loess. Brunnacker (1964) described three buried soils below the Wiirm loess at Regensberg, Bavaria and dated them as the Gunz/Mindel, Mindel/ Riss, and Riss/Wiirm interglacials. Similarly, Fink (1954) described the Gottweig, Paudorf, and Stillfried B soils as representing interstadials within the Alt Wiirm loess of Austria. Elsewhere buried paleosols in loess have been described by Prozek and Lozek (1957) in Czechoslovakia, Pecsi (1967) in Hungary, and Paepe (1968) in Belgium. Recent work on loess landscapes and glacial chronology has called into question some of the assumptions made in the early stratigraphic interpretations of buried paleosols in loess. Firstly, a relatively constant change was assumed from

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major glacial to interglacial conditions. Thus, the occurrence of three buried paleosols in a section was assumed to represent three periods of interglacial weathering. The actual periods were determined by simply counting back. However, it is now becoming apparent that there were many smaller climatic oscillations within the major changes. For example, Suggate (1974) discusses the present confusion over the exact progression of cooling at the end of the Last Interglacial. Three buried paleosols could therefore represent oscillations within one interglacial rather than three full interglacials. Secondly, the original work on loess landscapes viewed the development of the buried soils as a clear-cut alternation of loess deposition and soil formation, although as early as 1927, Polynov warned of the complications arising from changes in the relative rates of the two processes. Recent work in North America and Russia reported by Ruhe et al. (1971) and Gerasimov (1973) shows that loess deposition and weathering is complex. Rates of loess deposition vary enormously, but even the maximum calculated by Ruhe and his co-workers of 0.3 m in 121 years or by Gerasimov of 0.5 mm/year could hardly engulf a living soil. Obviously, the soil surface was often able to move upwards with loess additions, and this is why the top of such a buried soil is often so difficult to determine. Soil formation and loess deposition were contemporaneous processes and loess stratigraphy is dependent on their relative not absolute rates. Buried paleosols also have been used in the subdivision of glacial tills, notably in North America. Leighton and MacClintack (1962) have discussed the development of the early work in the Midwest. Extensive studies of the drift sheets and their weathered zones were carried out by Kay and Apfel(1929) in Iowa and by Leighton and MacClintock (1930) in Illinois. Kay (1916b) coined the term

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“gumbotil” for the weathered zone of till, and its origin has been disputed ever since. In an early pedological study, Simonson (1941) presents morphological and chemical data to suggest it is the B horizon of a Planosol. The main opposing view is that it is an accretion gley that has accumulated by lateral wash into depressions (Frye et al., 1960). Similar stratigraphic studies have been made in the Rockies by, among others, Hunt and Sokoloff (1950) and Richmond (1962), who describes six soils on materials of four glacial stages. There seems to have been little comparable work on tills outside North America. Buried paleosols have also been used in stratigraphy of nonglaciated landscapes. Vucetich and Pullar (1963) and Dalrymple (1967) have separated volcanic ash layers via their intervening soils in New Zealand. Extensive work has also been done on ash layers in Japan (Matsui, 1967). Kobayashi (1965) uses soils to separate three ash layers in the Wtirm and by adding pollen and archaeological data, he establishes a chronology from the Riss/Wtirm interglacial to the Holocene. In other landscapes, Butler’s work (1958) on fluvial and parna (eolian clay) layers in southeastern Australia depended on the recognition of buried soils. In a rather different field, periodic fluctuations in the accumulation and decomposition of organic deposits due to climatic variations have been shown by the presence of “recurrence surfaces” (Godwin 1954), which may be regarded as buried organic paleosols. A weakness of much of this work is its concentration on single profiles. In only a few studies has there been an attempt to describe the variation of the paleosol across the buried landscape. One of the earliest references to a buried paleocatena is found in Zeuner (1945, p. 112) where a brown-earth occurs on the upper slopes of the Younger Loess 1 and a black-earth in the depressions, near

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St. Pierre-les-Elberf, France. Similarly, Brunnacker (1967) described a Parabraunerde that varies laterally to a Terra Fusca and Pseudogley in loess in southern Germany. In Utah, Richmond (1962) shows how buried soils change from a Brown Podzolic at the highest altitudes through Brown Forest and Brown soils to Sierozems at the lowest levels. The recognition of the full variation of the buried paleosol becomes extremely important when lateral correlation is attempted. Without this knowledge difficulties arise when, as Barriere (1971) points out, genetically different soils occur at the same stratigraphic level and genetically similar soils occur at different levels. In the Midwest of North America, the distinctive Sangamon soil with its Planosol profile on the crests and humic gley profile in the hollows has allowed the separation of Wisconsin and Illinoian A slightly difloess (Ruhe, 1969). ferent approach has been taken by workers attempting long-distance correlations where paleocatenas do not match. In this case, the universality of Quaternary climatic changes is assumed and the soils with the greatest degree of development are then matched. On this basis, Morrison and Frye (1965) correlate the Coccoon soil of the Great Basin of Nevada with the Sangamon of the Midwest in North America. Richmond (1970) attempts intercontinental correlation using the degree of paleosol development and suggests that the Sacagawea Ridge/Bull Lake interglacial in the Rockies, the Sangamon in the Midwest, and the Mindel/Riss in Europe are all contemporaneous. There are serious objections to this extensive correlation at our present stage Quite apart from the of knowledge. necessity of a complete stratigraphy in each area, there are many pitfalls in the simple correlation of the degree of soil development with time. Pedologists have

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attempted such correlations only where they are confident that all other factors such as parent material, soil water regime, soil forming processes, and climate can be held constant within a small area (e.g., Chandler, 1942). Moreover, time itself is often difficult to determine as the age of even the same land surface, such as a pediment, may vary considerably from one place to another (Mulcahy, 1961). The use of buried paleosols as stratigraphic units has been put on a formal basis in the United States of America (American Commission on Stratigraphic Nomenclature, 1961) where a soil stratigraphic unit is defined as “a soil with physical features and stratigraphic relations that permit its consistent recognition and mapping.” Other studies of the stratigraphic use of paleosols have been made by Morrizon (1967), who proposed the term “Geosol,” and Brewer et al. (1970), who review previous concepts and suggest that a “type transect” and not a “type site” should be described. ARCHAEOLOGICAL STRATIGRAPHY AND DATING Archaeology is the other principal discipline where buried soils have been used as stratigraphic units. Much work has been done, especially in the U.S.S.R. and Europe, on the correlation between buried soils in river terraces or loess deposits and the remains of early man. Unfortunately, very little of the material is available in English, but Markov et aE. (1965) provide (in Russian) examples of buried soils associated with Paleolithic sites when they discuss studies by (among others) Velichko in the central Desna basin (p. 342), Ivanova in the central Dnestr valley (p. 358), and Gromov near Krasnoyarsk (p. 381). One of the very few English summaries is given by Moskvitin (1961) for European

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U.S.S.R., Poland, and Czechoslovakia. Zeuner (1945) used buried paleosols to distinguish different loess deposits and their associated archaeological levels in France and Germany. On a more detailed scale, it is often of interest to distinguish the organic horizons of a paleosol from the human occupation levels within a single excavation. Using micromorphology, Dalrymple (1958) has shown how the distinctive humus forms of the organic matter or the fabric of the mineral material of a paleosol may be identified. The absolute dating of paleosols for stratigraphic purposes in geology and archaeology has been attempted by the radiocarbon dating of both organic and inorganic carbon from the soil. The dating of soil organic matter is not simple, as different humus fractions will be of different age, there will be constant recycling, and more recent carbon will be added by the filtration of humic acids or root penetration (Campbell et al., 1967; Scharpenseel, 1971). The true age of the soil (when organic matter started to accumulate) will be impossible to determine. Instead, the date will represent the Mean Residence Time (Campbell et al.; 1967) of the various organic fractions plus, in the case of buried soils, the time elapsed since burial The complications inherent in using inorganic carbonates have been discussed by Bowler and Polach (1971). Postpedogenic exchange of modern 14C in moist soils can give dates markedly younger than dates from organic carbon. In dry environments, on the other hand, inorganic carbonate dates are usually older as postformation 14C exchange is minimal and there is an initial low 14C/12 C ratio in the soil carbonate (Williams and Polach, 1971) In spite of these difficulties, intelligible dates have been obtained by comparing results from different materials. Thus, Vogel and Zagwijn (1967) used charcoal, humus, and inorganic carbonates to ob-

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tain dates for the Gottweig and Paudorf soils of Fink (1954). Runge et al. (1973) compared the dates from clay-organic complexes in buried mineral soils and from overlying peat. Satisfactory dates were obtained from the peat by dating fulvic, humic, and residence fractions separately. An approximate chronology was obtainable from the clay-organic complexes if subsequent organic contamination could be accounted for. An additional complication is encountered in dating buried soils in archaeological sections where the date is required in calendar years rather than radiocarbon years. Recently, a calibration of about the last 8500 calendar years has been achieved by radiocarbon dating wood of known calendar age from living and dead samples of the bristlecone pine in California (Ferguson, 1968).

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associated pedological discontinuities with depth. In Australia, Churchward (1961) recognized layers within fluvial and aeolian soils by the discordant relationships with depth of pedality, clay skins, and porosity, and coherence, and Walker (1962) used a similar approach to identify three superposed soil systems on hillslopes. In North Africa, many soils with thin surface veneers of allochthonous materials are described as buried paleosols (Ruellan, 1971), and in midlatitude proglacial areas it is becoming increasingly apparent that numerous soils have a considerable amount of loess in the top 50 cm. Avery et al. (1959) give an example from southern England and Price et al. (1975) use quartz/felspar ratios among other techniques to identify discontinuities in loess soils from southwestern Kentucky. These are only two isolated examples from a voluminous literature. Finally, LAYERED SOILS the problems of defining “buried” beIt has been pointed out above that it is comes absurd with recent suggestions very difficult to set a minimum depth that aerosolic dust may have been added limit below which a paleosol may be re- to the surface horizons of soils on garded as truly buried. Hence, it is pos- a global scale during the Quaternary sible to make a case for a gradation of (Syers et al., 1969). This raises the posburied paleosols from those covered by sibility that all soils are layered! many meters of material to those covTHE PAST ENVIRONMENT FROM ered by only a few centimeters, as long BURIED PALEOSOLS as the original ground surface is still The interpretation of past climate or Into this shallow category discernible. vegetation patterns from the morphology fall the composite or layered soils which of buried paleosols is dependent on the have been recognized as extremely comconcepts of zonality and uniformitarianmon features of the earth’s surface since ism in soil science. Unless it is true that soil genesis studies have placed considerparticular soils are frequently associated able emphasis on establishing the uniwith particular environments and that formity of the soil parent materials. A similar past and present soils are the refull discussion of such soils will not be sult of similar processes, such interpreattempted as many workers would prefer tations are impossible. Scholtes et al. to regard them as special forms of relict paleosols. Regardless of how they are (1951) have discussed the possibilities of and emphasize that classified, they occur over wide areas such interpretations wherever such processes as slopewash or care must be taken to choose soils from similar topographic positions when comaeolian deposition have produced thinly paring buried and surface soils. It is also layered sediments. They are usually when using the concept of identified by showing lithological and important

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zonality to make sure that the soils being studied are zonal. In spite of these difficulties, many interpretations have been made and often correlated with evidence from other disciplines. Ruhe and Scholtes (1956) contrast the paleo-Gray Brown Podzolic soils found on buried ridges with the present surface Brunizems in parts of Iowa and suggest that the present grassland was formerly forested. Morozova (1962) also infers changes in bioclimatic conditions in the Russian plain from the morphological and chemical characteristics of buried soils. A detailed study of paleosols and paleoclimates has been attempted by Sorenson (1973) in northern Canada by developing a statistical correlation between present soil variables and present climatic variables. The same soil variables from buried paleosols are then used to recreate the paleoclimate. On this basis, it is suggested that the frequency of cold dry conditions in summer was slightly less at times in the late Holocene and that forests occurred 100 km north of their present limit. Archaeology’s main interest in buried paleosols stems from the possibility they afford of determining the landscape conditions in the immediate vicinity of excavated sites. Reconstructions have been attempted via either the morphology, micromorphology, or paleoecology of the soils. Cornwall (1953) attempted to interpret some climatic aspects of the Bronze Age from soils buried under Dahymple monuments of that time. (1958) identified braunerde with sol lessiue’ fabric in loess soils from the last interglacial and suggested that the climate was somewhat warmer and drier than today. Although soil science is under great pressure to furnish environmental evidence, it is debatable whether we understand the interaction of the soil-forming processes with the site and environmental factors well enough yet to make con-

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fident extrapolations. Much more direct evidence of the environment is given by paleoecological data from pollen, mollusca, or opal phytoliths in buried paleo~01s. Havinga (1963) has reconstructed the Boreal and Atlantic vegetation of parts of Holland from the pollen spectra in podzols beneath cover sands. Kerney (1963) used the variations in molluscan assemblages associated with buried soils on chalk to determine the late-glacial climatic oscillations in southeastern England, and Dormaar and Lutwick (1969) showed the dominance of grasslands in southern Alberta in postglacial times by opal phytolith counts from buried Ah horizons. However, more studies are needed on the production, preservation, dispersion, and method of incorporation into the soil if extrapolations from present to past conditions are to be made. Moreover, it is important to make sure that the buried soil under study was representative of a predominant part of the landscape. Evans and Valentine (1974) have shown that the molluscan assemblage from a soil on the slopes of a buried chalk valley in southern England indicated cleared pasture conditions by about 3900 BP, whereas the assemblage from a small exposure of wetter soils in the valley bottom indicated woodland conditions. An unrepresentative picture of the environment may be obtained from paleoecological data from a soil of only local importance which has been fortuitously buried beneath an earthwork. CONCLUSIONS The interest in buried paleosols on the part of Quaternary geologists and archaeologists has put a lot of pressure on soil science in general and pedology in particular. These other disciplines have inevitably posed questions which the young science of pedology has sometimes found difficult to answer. The main problems center on what is meant

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by a “soil,” what features distinguish it (if it is different) from a weathered sediment, and what clues to its formative environment do these features offer? Future pedological research on buried paleosols should be concentrated along two main lines: 1. To propose an acceptable definition of a soil as opposed to a sediment and in conjunction with this to identify the processes and features that are peculiar to soils. The definition should probably concentrate on the particular combination of biological, chemical, and physical weathering in soils at the surface of the earth, and the way these processes produce a logical variation in properties according to landscape position. Enough is probably known already of this three-dimensional variation in soil properties to allow buried paleosols to be identified in long sections. However, they are found more commonly in single profiles in small exposures. This is where a clear understanding of which features connote pedogenesis and which connote diagenesis is necessary. Such an understanding will be derived only from studies of buried examples of known soil and nonsoil materials and the determination of the relative stability of their features under various conditions. 2. To clarify the relationships between soil-site conditions and soil characteristics. Until particular soil features, or more probably a combination of soil features, can be associated confidently with various environments, it will be extremely difficult to interpret past environments from single exposures of buried paleosols. Pedology must decide whether a particular soil profile is a product of one main soil process or whether it is a product of a particular balance of all soil processes. If the latter case is true, it will be much more difficult to particular

ascribe a single environment

soil profile as it implies

to a that

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all soil processes are operative all over the world and although there may be a particular balance of processes in each region, under extremely local conditions it is possible for almost any one to predominate. ACKNOWLEDGMENTS The writers acknowledge the help of Dr. N. F. Alley and Dr. J. F. Dormaar in criticizing the original manuscript.

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