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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