Soil Profiles as Instructional Aids in the Introduction of Climatic Geomorphology

Donald L. Plondke

Cities Service Oil and Gas Corporation

ABSTRACT

The teaching of geomorphic pro­cesses in American education has his­torically neglected the interrelationships of climate and landforms. Particularly at the introductory level of physical geog­raphy courses, there is a need to accen­tuate that current landscape features and residual or buried morphologies, includ­ing the soil mantle, contain evidence of climatic change. Climatic processes, sur­face biota, underlying geology, and the locally-dominant set of geomorphic pro­cesses coalesce in the surface layer, known as the solum, to reveal the recent geomorphic history. Soil profiles are useful illustrative devices in introducing the concept of morphogenetic regions and in comparing between regions the dynamic interplay of climate, vegeta­tion, surface material, and slope. In fact, an introduction to soil morphology and genesis may be the best pedagogical means of making the transition from cli­matological to geomorphic topics in in­troductory physical geography courses.

KEY WORDS : soils, climatic geomorphol­ogy, morphogenesis, geomorphology, Pleistocene, teaching methods.

INTRODUCTION

It is common for introductory physi­cal geography courses at any level to be topically partitioned, acquiring a struc­ture that reflects the logical arrange­ment of major sub-disciplines covered in texts and syllabuses. As a result, the novice physical geography student is confronted with logical and seemingly cohesive subunits in his course of study: I-global geometry and mensuration of the earth's surface ; II-the atmosphere and global circulation; III-climatology/ meteorology; IV-soils and natural veg­etation ; V-the earth's crust and geo­logical principles; VI-landforms (usu­ally organized by major processes and geomorphic agents) ; and VII-geo­morphic regions. This seventh and last unit may encompass some sort of in­tended regional synthesis based on cli­mates, prevalent landforms, or relative continental location. The student's first exposure to geomorphology empha-

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sizes form and process, quite necessar­ily, but this first impression may lack an elementary understanding of the mean­ing of geochronology, climatic change, and regional history. To overcome some of this overly systematic bias in teaching the essentials of geomorphology, it is suggested that exemplary models or ac­tual field-derived soil profiles be utilized in the early stages of classroom courses in order to accentuate the role of cli­matic change, Pleistocene history, and biotic activity in landform evolution, and to impart to the student a greater intel­lectual sensitivity to the integrated na­ture of processes operating in the at­mosphere, biosphere, and lithosphere.

If our objective as educators is, in es­sence, to make geographers out of stu­dents more accustomed to systematized laboratory approaches in the study of physical science, it wquld seem prudent to put forth classical geograph ic meth­odologies, especially empiricism and re­gional sythesis. Soil is perhaps the best physical representation of interactive processes and of the notion of synthe­sized evolution. Geomorphology has fo­cused historically on that thin veneer of the earth 's surface that is in a constant phase of dynamic flux am idst varying intensities of chemical , biological , tec­tonic, and , most importantly, human agencies. Profile development in con­temporary surface and buried soils is an expression of polygenetic processes and illustrates tangibly the interaction of cli­mate, vegetation, and geology. The oc­currence of horizons in soils and, even more dramatically, in paleosols, ampli­fies the dimensions of t ime and change in the environmental m ilieu and in the evolution of surface morphology. An ad­equate appreciation of geomorphic pro­cesses cannot be gained without a con­ceptual awareness of events in Pleisto­cene climatic history.

AN HISTORICAL PERSPECTIVE ON CLIMATIC GEOMORPHOLOGY

As a science , geomorphology was developed by researchers interested foremost in the humid, temperate re­gions of North America and Europe. The

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late n ineteenth century witnessed the emergence of a more systematic geo­morphology by those actively engaged in surveying semi-arid lands in the west­ern Un ited States. Arid lands, polar en­vi ronments, and subtropical investiga­tions followed, resulting in modified models of erosion cycles and definitions of geomorphic regions. William Morris Davis himself modelled the arid and gla­cial erosion cycles which later gave im­petus to the definition of climatically-re­lated "morphogenetic" regions that were first introduced in North America by Pel ­tier (1950). The interest in arid environ­ments and Pleistocene glaciation was instrumental in elevating climatic geo­morphology which , as Sparks (1972) has noted, has become an important sub­discipline in Europe, particularly in Ger­many and France.

It is well known that since 1900, the Russian view of zonal soil evolution, first inspired by V. V. Dokuchaiev, has been predominant in stressing the relation­ship between climates and soil devel­opment. Dokuchaiev's 1900 classifica­tion most directly related soil types to major vegetation reg ions which are, in turn, regional surface expressions of zonal climatic differences. Since Doku­chaiev, some authors have overstated the climatic influence, and their hypotheses have had to be tempered by geologic reason. One would be mistaken to attri­bute current landscape morphology ex­clusively to present processes alone, or to suggest that particular cl imates in­variably produce characteristic soil types, regardless of other factors. Furthermore, the influence of climate must be consid­ered in a geohistorical context ; i.e. soils are contemporary or relict expressions of alternating glacial and periglacial pe­riods in the Pleistocene (Fig . 1). In this light, climat ic geomorphology as re­vealed in the laboratory of soil science must rely significantly on the discovery and correlation of paleosols in order to reconstruct a true climatic history of any pedogenetic regime. The primary objec­tive of paleopedologic analysis is to de­fine the degree of preservation of soil profile features, a definition needed to facilitate inferences about the prevailing

RECENT

1 Late phase WISCONSIN Glaciation <--I Middle phase

1 Early phase

PEORIAN Interglacial

IOWAN Glaciation

SANGAMON Interglacial

ILLINOIAN Glaciation

YARMOUTH Interglacial

KANSAN Glaciation

AFTONIAN Interglacial

NEBRASKAN Glaciation

Figure 1. Pleistocene: North American divisions.

environments that controlled soil gene­sis.

Fortunately, geographers have con­tributed their sensitive empiricism to the study of climatic change and soil evo­lution. They are more apt to adopt schemes of classification based on multi­dimensional genetic criteria rather than on simple "factors" of genetic influence. Geography has been a leader on the frontier of Pleistocene reconstruction and has scrutinized the nature of alternating genetic processes that induce differen­tiation in soil horizons, including addi­tions, removals, and transformations that occur repeatedly through variegated cli­matic milieux. As scientists, however, we must be ever-vigilant not to disregard limiting or moderating influences that restrict our attempts to directly link cli­mate, soil development, and landform evolution. Meso-scale climatology is not sufficiently refined in a spatial context

so as to make determinate climatic gen­eralizations about zonal or intrazonal soils. Thorn (1982) has pointed out that the field of climatic geomorphology is "a resilient assertion awaiting elevation to hypothesis standing." There needs to be greater recognition of the disparity that does exist between the prevailing meteorological conditions and microcli­mates occurring at the ground surface.

As is common in many geographic problems, climatic classification of soil­forming regimes is inhibited by the lim­itations of precise regionalization . Pure climatic regions are not independently derivative; typically, regional climates have been based on vegetation classifi­cation. The solum is a weathering do­main with its own microclimate, and it is particularly difficult to quantify and correlate climatological data with essen­tial weathering activity. Furthermore, we are limited by our sketchy knowledge of

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the temperature and moisture require­ments of the complex chemical pro­cesses operating in profiles.

Pleistocene Soil Markers and Climatic Change

It is critical to understanding soils as climatic-geomorphic products that the student be aware of the dynamic inter­change between macroscale and mi­croscale effects in the weathering pro­file. Climatic classifications are only valid for soils when other pedogenetic effects are modelled as constants. In light of this, it is important to emphasize to students the distinctions in process and the rel­ative dominance of competing soil­forming factors between zonal, intra­zonal, and azonal soils. In a similar way, geomorphological features are often­times contradictory in the prevailing cli­matic environment. Geomorphic cycles are more frequently interrupted or su­perposed than completed , and local morphology is, particularly in temperate zones, a melange of juxtaposed paleo­morphic surfaces. Representative soil profiles are microcosms of the reality of geomorphological discontinuity. Most cultivated soils in the midwestern United States are polygenetic, often expressed locally as profile discontinuities.

Soil morphological evidence of Pleis­tocene change is not restricted to tem­perate latitudes, but is useful for illus­trating change in most climatic zones. The fertile soils of the midwestern U.S. are classic examples of the relatively quick succession of climates where relict forms abound. The glaciated regions of the upper Midwest reveal vertical se­quences of alternating mature profile development and abrupt interspersion of uniform loess deposits. The interstadial periods between glacial advances in the Holocene, investigated by Dean, et al. (1984) and by Alford (1985), brought ex­tensive eolian transport, indicative of re­gional droughts. Multiple periods of eolian deposition are apparent for nu­merous soil series that cover the Central Plains. B-horizons show varying degrees of argillic development, controlled in large part by the amount of time lapsing between multiple episodes of dune de-

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position concurrent with moderating cli­mates and changes in the availability of moisture and organic material. Muhs (1985) did a comparative study of dune morphology and composition in north­eastern Colorado and the Nebraska sand hills. He characterized the late Holocene on the central Great Plains as drought­ridden, induced by increases in resi­dence time of dry, Pacific-originating air and strong zonal circulation east of the Rockies. Holocene eolian sands in the Colorado front range underlie parabolic dunes, show evidence of multiple epi­sodes of deposition, and reveal an in­terspersion of developed paleosols. Un­conformities in the Nebraska Typic Ustipsamments are difficult to detect, but the profiles are characteristic: poorly-de­veloped A/Ac/C horizons, lack of B-ho­rizon development, and no translocation of clay. Locally, profile development was limited by deflation, the absence of veg­etation due to drought or grazing, and by frequent superposition of eolian sands during both waxing and waning glacial conditions.

By regionally correlating these eolian soils with the more well-developed soils having differentiated B-horizons and in­creased clay content, it is possible to de­lineate geologic and climatic Pleistocene boundaries. The Holocene climates left evidence of rapid transitions between dry and moist regimes through a long-term trend toward prairie conditions. The ef­forts by Dean and others (1984) to de­rive a midwestern Holocene climatic model have been based on the obser­vation of cyclical variations and abrupt transitions evident in varved sediments of changing mineral and organic com­position.

The observable physical properties of soil profiles are direct reflections of the pedogenetic environment and, there­fore, are also indicators of the local bal­ance of geomorphological agents during the period of soil formation. But before generalizations about the climatic re­gime and resulting geomorphological impact can be stated, the seasoned stu­dent must resolve questions of: 1) the likelihood of multiple pedogenetic pe­riods affecting composition of the pro-

file; 2) relative age of the profile vis a vis currently-operating weathering activity and organic processes; and 3) relative balance among microclimatic influ­ences, situational topography, prevailing regional climate, activity of organisms, and man-land symbiosis. By modelling profiles, it is possible to idealize the cli­matic influence while holding other fac­tors constant.

tation of fossil or buried soils. The zo­nation of horizons in a paleosol (Fig. 2) may represent an historical complex of pedogenetic events ; the relict soil no longer represents dynamic evolution controlled by the regional climate. But the utility of paleosols in establish ing re­gional datums for reconstruction of cli ­matic history is not diminished as long as there is a focus on the sharper phys­ical manifestations of transitions ; e.g. superimposed loess, inversion of zones of eluviation, and truncated profiles. In­vestigations of climatic changes and their

Paleosols

A more cautious but usually fruitful approach must be taken in the interpre-

HORIZON

Wisconsin Loess

A 1b

A 2b

B 1b

B 21b

B 22b

B 3b

C 1b

C b

DEPTH (in.) DESCRIPTION

0-90 Gray-Brown Podzol from loess cover.

0-2 Light gray to white , friable silt loam with medium platy structure; some black and brown iron concretions.

2-5 White, very friable silt loam with moderately coarse platy structure; some sand grains and iron concretions.

5-7 Transitional horizon; more like one below.

7-15 strong brown to dark yellowish-brown clay; plastic and sticky; moderate to strong medium subangular blocky structure.

15-21 Mottled yellowish-brown and reddish­yellow clay with moderate to weak medium subangular blocky structure.

21-30

30 - 54

54-84

Transitional horizon of mottled pale yellow, light gray and reddish­yellow clay-loam with weak very coarse blocky structure.

Leached and oxidized till consisting of clay loam - dominantly pale yellow with few mottles; massive in place but friable when removed.

Oxidized but unleached till, pale yellow to light yellowish-brown clay loam; massive in place but friable when removed.

Figure 2. Typical profile : Gray-brown Podzolic Yarmouth paleosol.

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influence on landforms are best con­ducted in areas where sharp transit ions appear and where the original pedoge­netic properties archived in the solum are resistant to diagenetic alteration . Paleo­sols offer the most promising area for research into soil profile indicators of climatic processes, but visual observa­tion, by itself, is only an initial step in the reconstruction of regional geo ­morphic history. Since changing envi­ronmental conditions at least surficially efface the original genetic character of paleosols, the discovery of residual soil properties that suggest characteristics of past cl imates requires even closer ob­servation of soil structure and compo­sition. To draw paleoenvironmental con­clusions from paleosols, controlled testing must be undertaken, including detection of the position and concentra­tion of calcium carbonate nodules, lo­cation of iron compound accumulations, and laboratory analysis of transforma­tion in clay minerology in buried B-ho­rizons. Examination of B-horizons is es­pecially worthwhile because they can reveal much about the local topography and drainage conditions at the time of pedogenesis. Strongly developed B-ho­rizons are common in depression soils that escaped erosion through the cli­matic periodicity of the Pleistocene.

Preserved paleosol profiles are ex­tremely valuable stratigraphic analysis tools when they can be chronologically identified in a region. Leopold (1964) has emphasized that as marker horizons, these soils are instrumental in terrace correlation and reconstruction of fluvial history.

SOIL PROFILES AS CLiMATO­GEOMORPHIC MODELS

Precision in measurement of pro­cesses involved in soil pedogenesis has been elevated through quantitative modelling of inter-horizon activity. Kirkby (1985) recently has modelled processes in the interaction of soil profile and hill ­slope development. By means of multi­variate sub-models of: 1) organic mat­ter; 2) the inorganic profile associated with nutrient cycling ; and 3) the weath­ering profile, he demonstrated the fea-

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sib i lity of quantify ing the varied pro­cesses affecting the soil ' s weathering milieu and its propensity for horizon dif­ferentiation . But in a more practical ed­ucational context, it is not difficult for in­structors to acqu ire exemplary profiles that typify broad climatic classes. The older and more mature soils with clearly observable zonation, as they continue to develop, can be shown to be increas­ingly influenced by the climatic situation and the cover of vegetation . Soil texture and color suggest major illuviation char­acteristics which, by themselves, make reasonable climatic indicators. Well-de­veloped A/ B/ C profiles in Pleistocene so ils, suggesting favorable, stabilized paleocl imates, co-exist side-by-side with weakly -horizoned A / Ac / C profiles in eolian / alluvium sediments of the Cen­tral Plains. Any abrupt boundary indi­cates the likelihood of sudden and sometimes dramatic climatic events in the past (e.g. flooding). Hardened crusts or other homogeneous zones of concen­trated composition illuminate a particu­lar climatic environment that may have dominated (e.g. calcrete as a signature of semi -arid conditions).

Each major climatic region possesses a classic set of illustrative landforms and a modelled geomorphic cycle. The dy­namic interrelationship between climate, vegetation, and soils as they evolve un­der changing temperature and moisture conditions can be graphically presented to introductory level students (compare Figs. 3, 4 and 5). Groupings of pedoge­netic processes that result in model pro­files which represent major soil classes can be matched to general ized climates and vegetation categories. Podzols, for example, are typified in the literature as occurring in coniferous forest areas in humid continental climates, such as are found in the northeastern United States. There needs to be emphasis on dynamic change and locational patterns in any explanation of the major processes of soil development, such as podzolization, lat­erization, and calcification . Cold cli­mates, temperate continental zones, and tropical environments each have differ­ent equilibria of processes at the air / ground cover/ soil interface that call for

Cold

1

1

1

1

1

1

1

1 1 1 1

V

Hot

Dry --------------------------------------->

1

1 1

1

Polar Climates

Humid microthermal climates

A RID 1 ______________________________________ _

1

1 Climates 1

1

Humid mesothermal climates

1------------------------------------1

1

1 1

Tropical climates

--------_1-----------------------------------Dry --------------------------------------->

Wet

Cold

1 1 1 1

1

1 1

1

1

1

1

V

Hot

Wet

Figure 3. Climate as a function of temperature and moisture (after Gabler et al. 1975).

Dry

Cold

1

1

1

D 1

1

E 1

1 s 1

1

E 1

1 R 1

1 V T 1

1 Hot 1

Dry

---------------------------------------> Perpetual snow and ice

Mid-latitude grassland

Tropical

grassland

Tundra

Coniferous forest

Mid-latitude forest

other tropical forest

Tropical rain forest

--------------------------------------->

Wet

Cold

V

Hot

Wet

Figure 4. Vegetation as a function of temperature and moisture (after Gabler et al. 1975).

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Dry ---------------------------------------> Wet

Cold Perpetua l snow and i ce Cold

Tundra

C Podzol h e I C

D s I h P Gray-brown podzolic e t I e r s n I r a e u I n i r t I 0 r Latosolic-podzolic t & I z i

B I e e r I m 0 I

V w I Tropical latosols V n I

Hot I Hot

Dry ---------------------------------------> Wet

Figure 5. Soils as a function of temperature and moisture (after Gabler et al. 1975).

generalized modelling . By understand­ing the structure and composition of soils common to these contrasting climatic regions, the student of geomorphology can gain an initial perspective on the en­vironmental limitations that affect the complexity of geomorphological action .

CONCLUSION

Explanations of processes should not be detached from a broader understand­ing of climatic regions. Geomorphology should seek to be a science of synthesis more than a rigorous discipline of pro­cess description. Exemplary regional soil profiles taken from markedly contrasting climatic regimens can aid in giving a vi ­sual impression of regional synthesis. Soil-forming processes such as podzol­ization and laterization are, in them ­selves, descriptions of the interactions between climate, surface cover, and par­ent material. Color photographs or pic­toral models which clearly demarcate horizons in regional soil profiles can be shown to students at strategic transi ­tional points in a course of study to el ­evate the notion of interplay between the lithosphere, atmosphere, and biosphere,

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and to emphasize the role of climatic factors in the evolution of landscapes.

REFERENCES

Alford, J . 1982. Glacial Outwash Loess as a Climatic Indicator. Annals of the Associa­tion of American Geographers, 72 : 138-140.

Dean, W., Bradbury, J ., Anderson, R. and Bar­nosky, C. 1984. The Variability of Holocene Climate Change : Evidence from Varved Lake Sediments. Science, 226 (4679) : 1191-1194.

Gabler, R., Sager, R. , Brazier, S. and Pourciau, J. 1975. Introduction to Physical Geog­raphy. San Francisco : Rinehart Press, Holt, Rinehart and Winston.

Kirkby, M. 1985. A Basis for Soil Profile Mod­elling in a Geomorphic Context. Journal of Soil Science, 36: 97-121.

Leopold, L., Wolman, M., and Miller, J . 1964. Fluvial Processes in Geomorphology. San Francisco and London : W. H. Freeman and Company.

Muhs, D. 1985. Age and Paleocl imatic Signif­icance of Holocene Sand Dunes in North­eastern Colorado. Annals of the Associa­tion of American Geographers, 75: 566-582.

Peltier, L. 1950. The Geographic Cycle in Per­iglacial Regions As It Is Related to Climatic

Geomorphology. Annals of the Associa­tion of American Geographers, 40: 214-236.

Sparks, B. 1972. Geomorphology. London: Longman Group Limited.

Thorn, C. 1982. Bedrock Microclimatology and the Freeze-Thaw Cycle : A Brief Illustration. Annals of the Association of American Geographers, 72 : 131-137.

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