Reference Work Entry

Sedimentology

Part of the series Encyclopedia of Earth Science pp 1271-1277

Weathering, soils, and paleosols

  • Gregory J. RetallackAffiliated withDepartment of Geological Sciences, University of Oregon

Soils are fundamental to life on this planet. The mineral nutrients provided by weathering within soil and the degree of drainage of soil control what kind of life can thrive in a particular place. On the other hand, living creatures with their roots, jaws, and other means of acquiring nutrients do much to determine the nature of soil. Soils include billions of bacteria, millions of nematodes and a few plants in just about every square centimeter. Soil's diverse microbes and internal absorptive surfaces of clay neutralize poisons and purify water. By fueling photosynthesis, soil regulates the composition of the atmosphere. Through the engine of soil, life has far-reaching effects on land, water, and air. The intimate interrelationship between soil, life, and surface environments also has a long fossil record in the form of fossil soils, or paleosols. These remains of soils of the past are now known as old as 3,500 million years on Earth. Even more ancient soils and paleosols are now known on the Moon and Mars, and certain kinds of meteorites may be fragments of paleosols as old as 4,600 million years.

Weathering

Weathering is the relentless alteration of sediments and rocks by a variety of chemical, biological, and physical agents at the surface of planetary bodies. In humid forested environments of good soil fertility, as in the oak forests of northern Europe and eastern North America, the general weathering regime is a soil-forming process called lessivage (Figure W1). Chemically, lessivage is destruction of primary minerals, such as feldspars, by weak acids, such as carbonic acid, which liberate into soil solution cationic nutrients such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+). This chemical reaction, known as hydrolysis, is not simple dissolution of minerals, but incongruent dissolution which leaves a residue of silica (Si) and sesquioxides (Al3+, Fe3+) in clays, such as smectite (q.v.), and oxide and hydroxide minerals (q.v.), such as goethite. The process of lessivage thus creates soil with fewer feldspar grains and more reddish clay. Biologically, hydrolysis is important because base cations are fundamental plant nutrients: calcium for cell walls, magnesium as a critical component of chlorophyll and potassium and sodium as cell electrolytes. Plants promote hydrolysis in many ways. Their root respiration of carbon dioxide (CO2) enriches soil water in carbonic acid (H2CO3). Other acids and chelates produced by plants and their associated microbes accelerate release of nutrients by weathering. Physically, plants hold the soil together with their roots, and regulate water loss by means of ground cover. Earthworms, squirrels, and gophers physically churn the soil.
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Figure W1

Common weathering regimes.

A variety of weathering regimes is found in different parts of the world (Figure W1). In humid regions of high water table, such as swamps, the principal soil building process is accumulation of plant debris in peat, within which decay is suppressed by lack of oxygen and acidic leachates from plants. Chemically reducing soil below the peat can be leached of iron in its ferrous (Fe2+) ionic form which is green to gray in color and more soluble than red, oxidized (Fe3+) iron. The process by which reddish brown soils are turned gray by leaching of reduced iron is called gleization. In humid regions of conifer forests on sediments and rocks rich in quartz, plants produce unusual amounts of acid, with the result that even clay is destroyed in the soil, which builds a subsurface horizon reddened by sesquioxides and humus in a process called podzolization. Humid tropical rain forests grow in soils of low fertility because of long and deep weathering of nutrient cations promoted by abundant moisture and warmth. They have thick soils rich in oxide minerals, such as hematite, and clay minerals, such as kaolinite (q.v.). Their chemical enrichment in iron (Fe3+) and aluminum (Al3+) is a soil forming process called ferallitization. The deep and thorough weathering of tropical soils can enrich sesquioxides (Fe3+ and Al3+) and silicon (Si) elements to the grade of economically valuable ores of iron (laterite, q.v.), aluminum (bauxite, q.v.) and silica (silcrete, q.v.). In deserts (q.v.) there is limited water for hydrolysis and mobilization of nutrient cations. Calcium, and also some magnesium, hydrolytically released from feldspars and other minerals, is not washed from the profile or taken up by plants. It accumulates by calcification in a subsurface horizon as hard, white, nodules, or bands, called caliche (q.v.) or calcrete, mainly fine-grained, low-magnesium calcite (CaCO3), but rarely dolomite [CaMg(CO3)2]. Over time, large nodules grow and coalesce into thick layers. In very dry climates hydrolytic weathering is very limited, and there is insufficient moisture to leach away the cations, which accumulate in the soil as crystals of rock salt (NaCl) or gypsum (CaSO4): a process called salinization.

The result of these various weathering processes is to produce a weathering profile of alteration that diminishes in intensity down from the surface to the parent material below. Weathering profiles are tens of meters thick in tropical humid regions but only centimeters thick in desert playas. The upper part of a weathering profile is commonly called soil or solum. It represents that part of the profile most altered by roots, burrows, and other processes that completely obscure structures of the parent material. In deeply weathered profiles there is additional alteration well below the soil, in a thick horizon called saprolite. Typically this is clayey, reddish, or yellowish, soft material in which folding, crystal outlines, schistosity, or bedding from the parent material remain visible. Although saprolitic weathering may appear to be largely chemical because it is beyond the reach of most roots and burrows, there is a biological component to this alteration from termites, fungi, and other microbes. Regolith is another term used for deep weathering profiles, but regolith includes sediments as well as soil and saprolite. The terms weathering profile and soil refer to alteration in place, without transport. Soil is commonly eroded and the material redeposited. Sediment of recognizable soil clasts is called a pedolith or soil sediment. A redeposited laterite is the original and remains the best example, because clasts so rich in kaolinite and hematite are distinctive. Although most sediments are derived from soils, it is best not to extend the term pedolith to widespread and far-travelled alluvium, but restrict it to distinctive soil materials. The distinction between sedimentation and soil formation is fundamental because soils develop in profiles from the top downward, but sediments accumulate in sequences from the bottom upward. The theory and practice of soil science (pedology) and sedimentary geology (sedimentology) are very different, and have further diverged because of the traditional association of pedology with agricultural studies and sedimentology with geological studies.

Soils

Soil profiles are the tangible and diverse products of weathering. We classify them in order to understand the processes that form them, and to manage their use. The US Soil Conservation Service recognizes a dozen soil orders (Figure W2), which are the largest units in a comprehensive, hierarchical soil classification that is widely used throughout the world. The basis for soil classification is the sequence and development of horizons. The process of lessivage, described above, produces a soil with a leached, sandy, quartz-rich surface horizon (E or eluvial) over a red, clayey subsurface (Bt or argillic) horizon. The classification has strict definitions of the degree of alteration needed for specific horizons, for example at least 8 percent additional clay is needed for an argillic horizon, with some exceptions. If the soil has an argillic horizon that is relatively fertile (with abundant available base cations) it is an Alfisol, but a soil with an argillic horizon impoverished in base cations is an Ultisol. Soils can be identified using a dichotomous key (Figure W3), but the following paragraphs convey the gist of each soil order and some of the critical terminology in their definition.
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Figure W2

Cartoons of climate, vegetation, and profile form of various orders of soils defined by the US soil taxonomy.

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Figure W3

A key for recognition of the soil orders of the US soil taxonomy, with emphasis on features that can be recognized in outcrop and in petrographic thin sections of paleosols.

Entisols (incipient soils) are immature, showing little weathering beyond the initial growth of pioneering plants. They develop on flood deposits, landslides, and other geologically young surfaces. These are important soils for market gardening.

Inceptisols (young soils) have recognizable horizons, but none of these are developed to the extent found in other soil orders because of a short time of development or conditions hostile to soil formation. Because they are an early stage in the development of other soil orders, Inceptisols are extremely diverse. Typically their subsurface shows only minor accumulation of clay, carbonate, or iron stain in what is termed a cambic horizon (Bw in soil science shorthand). Inceptisols are important soils for crops, orchards, and pasture.

Andisols (volcanic ash soils) are formed on volcanic ash rich in glassy shards that weather to noncrystalline colloids with low bulk density and very high fertility. In profile form, they are generally similar to Inceptisols. These soils are very important for cropping in tropical regions, where other soils are deeply weathered of plant nutrients.

Histosols (peaty soils) are soils in the sense that they support bald cypress swamps, papyrus marshes and other wetland vegetation. Their surface layer (histic epipedon or O horizon), is what a sedimentary geologist would call peat (q.v.). It forms by successive increments of plant material, with decay suppressed by stagnant water conditions, in a way analogous to the accumulation of sediments. They are best left alone to preserve water quality, but some Histosols are logged for specialty lumber and dried for domestic fuel.

Spodosols (sandy forest soils) have attractive profiles with white (eluvial) surface horizons contrasting with reddish brown (spodic or Bs) subsurface horizons enriched in iron, aluminum, and organic matter. They are quartz-rich, clay-poor, acidic, and infertile. They support conifer forests and heath that can tolerate such low fertility, and are used mainly for softwood lumber production and water quality preservation.

Alfisols (fertile forest soils) have a subsurface horizon enriched in clay (argillic or Bt horizon) that is rich in nutrient cations. Such clays are typically smectite and illite. These soils support broadleaf forest vegetation, but are widely cleared for crops and grazing.

Ultisols (base-poor forest soils) have a clayey subsurface horizon that is poor in nutrient cations and usually dominated by kaolinitic clay (kandic and Bt horizon). Other than this mineral and chemical difference, they appear generally similar to Alfisols. Mixed conifer-broadleaf forest is typical. Once cleared these soils are fertile enough for cattle grazing, orchards, and vineyards, but are best left forested for lumber and water quality preservation.

Oxisols (deeply weathered tropical soils) are thick, nutrient poor, and highly aluminous and ferruginous (oxic or Bo horizon). A common micromorphology is sand-sized clods of hematite and quartz. These soils support tropical rain forest, but are used for sugar cane, as well as tropical tree crops such as cocoa, mango, and papaya.

Vertisols (swelling clay soils) are rich in smectite clays, which have the physical property of swelling when wet and then cracking as they shrink and dry. They form in climates with pronounced seasonality of rainfall. The most distinctive physical feature of these soils is a pattern of troughs or pits a few to several decimeters deep (gilgai microrelief) between the pressure ridges around the deep cracks. At depth, the criss-crossing slickensided planes and deformed pressure ridges create a characteristic thinning and thickening of soil horizons (mukkara structure). Vertisols support mainly grassland and wooded grassland, and are used primarily for grazing. Their physical instability is very destructive of roads, fences, and buildings.

Mollisols (grassland soils) have a thick, dark, clayey surface of high fertility (mollic epipedon or A horizon). This consists of small rounded clayey crumbs enriched and bound with finely decayed organic matter. Mollisols are widely used for grazing, as well as herbaceous crops such as corn and wheat.

Aridisols (desert soils) are little weathered, clay-poor, and have nodules or layers of calcite or dolomite (calcic or Bk horizon) within a meter of the surface. Salts such as gypsum may also be found at depth. Some Aridisols are irrigated for crops, but water flushes salts to the surface with disastrous results. Others are used for grazing at low stocking densities, but most Aridisols remain unused.

Gelisols (permafrost soils) have ground ice or other permafrost features (gelic materials) within a meter of the surface. Ice wedges, stone stripes and other deformation features are characteristic. Some Gelisols support taiga forest at high latitudes and krummholz at high altitudes. Other Gelisols support tundra and alpine fellfield. A short growing season limits agricultural use of Gelisols, although forestry, reindeer herding and caribou hunting does support some human activity.

Another way of looking at soils useful in understanding how they form such diversity is analyzing the factors that create them: climate, organisms, topographic relief, parent material and time for formation. The diversity of soils can be considered a vast natural laboratory of concurrent experiments in soil formation. If we wish to study one of the factors in soil formation such as time, then we should find a group of soils of comparable climates, vegetation, geomorphic setting and parent materials but varied time for formation. Such a suite of soils is called a chronosequence, and mathematical relationships between soil properties and time derived from such a group of soils is called a chronofunction. Examples include soils of a flight of alluvial terraces excavated by a river cutting lower into its valley during uplift, or soils of moraines left behind by retreating glaciers. Chronofunctions may quantify the accumulation of clay in argillic horizons, of carbonate in calcic horizons and of peat in histic epipedons with the progress of time. Soils of the chronosequence may have been dated by radiocarbon, human artifacts or fossils, but the chronofunctions derived from them can be used to assess the age of soils nearby that lack dateable materials. Landscape histories reconstructed in this way are important for siting permanent facilities such as bridges, dams, and nuclear power plants. In addition to chonofunctions, comparable approaches can be used to quantify the role of climate (climofunctions), organisms (biofunctions), topographic relief (topofunctions) and parent material (lithofunctions). In this way it is possible to investigate the process of soil formation with rigor.

Paleosols

Soils have a fossil record as paleosols. Most of these are fossilized by burial in flood deposits or volcanics (Figure W4), but some are still at the surface, either by exhumation or by outlasting the conditions that formed them. Lateritic Ultisols and Oxisols from the middle Miocene (16 Ma) thermal maximum are widespread surface paleosols at high latitudes, and are easily recognized because they are so much more deeply weathered, kaolinitic, and ferruginous than associated soils. Paleosols also are commonly preserved at major geological unconformities (Figure W5).
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Figure W4

A modern grassland soil, and two comparable paleosols, all Mollisols, buried in volcanic sandstones at the middle Miocene (14 Ma) site of Fort Ternan, Kenya. Hammer for scale has a handle 25 cm long. These are the earliest known well-drained grassland soils in Africa.

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Figure W5

Diagrammatic sketch of Precambrian paleosols at major unconformities, including pre-Torridonian (810 Ma) profiles from near Sheigra, Scotland (above), and pre-Huronian profiles from near Elliot Lake, Ontario, Canada (below). All the paleosols were well drained, as indicated by corestones and clay formation, but the Scottish profiles were more oxidized and indicate higher levels of atmospheric oxygen than the Canadian profiles.

The fossilization process of soils is a hindrance to interpretation of paleosols because it alters many of their features, including those important to their classification and reconstruction of past soil-forming processes. Three common alterations after burial of soils in sedimentary successions are: (1) decomposition of organic matter such as leaf litter and roots at the surface of the soil; (2) mobilization of reduced iron (Fe2+) during organic matter decomposition under low oxygen conditions; and (3) dehydration of yellow iron-hydroxides, such as goethite, to red iron oxides, such as hematite. These three processes alone can convert a dark brown to yellowish brown soil to a carbon-lean claystone, brick-red in color, and riddled with green-gray alteration haloes that coalesce toward the top of the profile. Such substantial changes in appearance have hindered the recognition of many paleosols. With deep burial, paleosols are compacted from the weight of overburden and cemented by deep groundwater. Metamorphism can further obscure soil features and minerals by the development of cleavage and metamorphic minerals.

Despite problems of burial alteration, many features of soils remain in paleosols and can be used to recognize them within sedimentary or volcanic sequences. Features most diagnostic of paleosols are: (1) tubular structures that branch and thin irregularly downward or show anatomy of fossilized root traces; (2) gradational alteration down from a sharp lithological contact like that of a land surface and soil horizons; and (3) the complex patterns of cracks and mineral replacement like those of soil clods (peds) and planar features (cutans). It is also possible to classify paleosols within systems devised for soils, provided allowance is made for burial alteration. Geochemical analysis, X-ray diffraction, and thin section petrography are particularly helpful in the classification of lithified paleosols. Tests of soil fertility used to distinguish Alfisols from Ultisols do not work with paleosols because the originally reactive soil surfaces have been compacted, cemented, and recrystallized during burial. However Alfisols have molar ratios of alumina/bases less than 2, smectite, and illite as dominant clays, and thin section fabrics with scattered highly birefringent clay streaks when viewed under crossed nicols. Ultisols by comparison have alumina/bases more than 2, consist mainly kaolinite and retain a fabric dominated by highly oriented and highly birefringent clay. Each of the US soil orders has a characteristic petrographic appearance (Figure W3) and a long fossil record (Figure W6).
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Figure W6

Geological range of different weathering products and soil orders of the US taxonomy. The diversification of soils and weathering products through time reflects evolutionary changes in the atmosphere and terrestrial ecosystems.

Soils and paleosols of the and planets provide new challenges for Earth-bound classifications of soil, and the geological history of soil. Conditions on our water-less and atmosphere-less Moon are most unlike Earth. The most important soil forming process on the Moon is continual bombardment of the surface by sand-sized micrometeoroids. These pulverize and locally melt the soil, darkening the surface with added metal. Dark metal-rich horizons found in lunar cores represent fossil soils buried by the debris of exceptionally powerful impacts. Each of these thin paleosols, like the surface soil, took many millions of years to form (see Lunar Sediments).

On there is a thin atmosphere of carbon dioxide and evidence from shorelines and paleochannels of water in the distant geological past. Soils analyzed there by robotic landers are rich in iron and swelling-clay (smectite), with subsurface hardpans of salts. These soils are similar to salty Aridisols now forming in the Dry Valleys of Antarctica. Such soils require warmer temperatures and more water than currently available on Mars, and they may be relict paleosols dating back to the time of fossil channels on Mars more than 2,000 million years ago.

Some scientists have also suggested that certain kinds of meteorites may be parts of paleosols. Like Martian paleosols, carbonaceous chondrites are rich in iron and smectite, cracked, and stained, and veined with salts. By this interpretation, some carbonaceous chondrites may be pieces of the oldest paleosols in the solar system, some 4,600 million years old.

The most ancient known paleosols on Earth, some 3500 years old, are thick, deeply weathered and green gray in color, unlike ancient soils and paleosols of the Moon, Mars, and meteorites. They may represent an extinct soil order, for the moment informally called “Green Clays”. Soils on Earth were alive with microbes at least back 2,700 million years, judging from the life-like carbon isotopic composition of paleosols. Microbial crusts also would explain landscape stabilization, deep weathering, and microtextures of paleosols back to 3,500 million years. Nevertheless, Precambrian paleosols do not fit easily within modern soil classifications and may reflect an early atmosphere rich in carbon dioxide but with very low amounts of oxygen (Figure W4). Red and oxidized paleosols including Oxisols and weathering products such as laterites that indicate the rise of oxygen in the atmosphere do not appear until about 2,200 million years ago. With the appearance of the first large continents by amalgamation of smaller island arcs at this time, came the first paleosols recognizable as Vertisols and Aridisols of dry climate. Gelisol paleosols also have been found among the tillites and striated pavements remaining from late Precambrian ice ages.

Land plants and animals have left traces in Late Ordovician Entisols, Inceptisols, and Andisols. Histosols do not appear until the advent of substantial land vegetation during the Early Devonian. Forested soils such as Alfisols are not known earlier than Late Devonian. Spodosols and Ultisols may be as old, but are currently not known among paleosols older than Carboniferous. Mollisols of grasslands appeared relatively late in geological history with the Tertiary rise of grasses and grazers. The role of grassland ecosystems in the evolution of humans and the emergence of agriculture and civilizations also has been investigated using fossil soils associated with human fossils and artifacts.

The long fossil record of soils is complimentary to evidence of fossils and sediments for the history of life and environments on Earth in the geological past. Soils and life have both diversified though geological time, and paleosols record this fundamental part of terrestrial ecosystems.

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© Dowden, Hutchinson & Ross, Inc. 1978
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