Paleopedology (palaeopedology in the United Kingdom) is the discipline that studies
soil
Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...
s of past geological eras, from quite recent (
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...
) to the earliest periods of the Earth's history. Paleopedology can be seen either as a branch of
soil science
Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, soil classification, classification and Soil survey, mapping; Soil physics, physical, Soil chemistry, chemical, Soil biology, biologica ...
(
pedology
Pedology (from Greek: πέδον, ''pedon'', "soil"; and λόγος, ''logos'', "study") is a discipline within soil science which focuses on understanding and characterizing soil formation, evolution, and the theoretical frameworks for modelin ...
) or of
paleontology
Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure ge ...
, since the methods it uses are in many ways a well-defined combination of the two disciplines.
History
Paleopedology's earliest developments arose from observations in
Scotland
Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the 19th century there were many other finds of former soils throughout Europe and North America. However, most of this was only found in the search for animal and/or plant fossils, and it was not until
soil science
Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, soil classification, classification and Soil survey, mapping; Soil physics, physical, Soil chemistry, chemical, Soil biology, biologica ...
first developed that buried soils of past geological ages were considered of any value.
It was only when the first relationships between soils and climate were observed in the
Eurasian Steppe
The Eurasian Steppe, also called the Great Steppe or The Steppes, is the vast steppe ecoregion of Eurasia in the temperate grasslands, savannas and shrublands biome. It stretches through Manchuria, Mongolia, Xinjiang, Kazakhstan, Siberia, Europea ...
that there was any interest in applying the finds of former soils to past ecosystems. This occurred because, by the 1920s, some soils in Russia had been found by
K.D. Glinka that did not fit with present climates and were seen as relics of warmer climates in the past.
In 1892
Eugene W. Hilgard had related soil and climate in the United States in the same manner, and by the 1950s analysis of Quaternary
stratigraphy
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy has three related subfields: lithost ...
to monitor recent environmental changes in the northern hemisphere had become firmly established. These developments have allowed soil fossils to be classified according to
USDA soil taxonomy
USDA soil taxonomy (ST) developed by the United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate Soil classification, classification of soil types according to several parameters (most commonly their p ...
quite easily with all recent soils. Interest in earlier soil fossils was much slower to grow but has steadily developed since the 1960s owing to the development of such techniques as
X-ray diffraction
X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...
which permit their classification. This has allowed many developments in
paleoecology
Paleoecology (also spelled palaeoecology) is the study of interactions between organisms and/or interactions between organisms and their environments across geologic timescales. As a discipline, paleoecology interacts with, depends on and informs ...
and
paleogeography
Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term pale ...
to take place because
soil chemistry
Soil chemistry is the study of the Chemistry, chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and Environment (biophysical), environmental factors. In the early 1870s a consulting chemist to the R ...
can provide a good deal of evidence as to how life moved onto land during the
Paleozoic
The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
era.
Finding soil fossils and their structure
Remains of former soils can either be found under
deposited sediment in unglaciated areas or in extremely steep cliffs where the old soil can be seen below the younger present-day soil. In cases where
volcano
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most oft ...
es have been active, some soil fossils occur under the
volcanic ash
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...
. If there is continued deposition of sediment, a sequence of soil fossils will form, especially after the retreat of
glacier
A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...
s during the
Holocene
The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...
. Soil fossils can also exist where a younger soil has been eroded (for instance by wind), as in the
Badlands
Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded."Badlands" in '' Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 2, p. 47. They are characterized by steep slopes, ...
of
South Dakota
South Dakota (; Sioux language, Sioux: , ) is a U.S. state, state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota people, Dakota Sioux ...
. (One must exclude areas where present-day soils are relics of former wetter climates, as with
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
and
southern Africa
Southern Africa is the southernmost region of Africa. No definition is agreed upon, but some groupings include the United Nations geoscheme for Africa, United Nations geoscheme, the intergovernmental Southern African Development Community, and ...
. The soils of these regions are proper ''
paleosols
In geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science.
In geology and paleon ...
''.)
Soil fossils, whether buried or exposed, suffer from alteration. This occurs largely because almost all past soils have lost their former vegetative covering, and the organic matter they once supported has been used up by plants since the soil was buried. However, if remains of plants can be found, the nature of the soil fossil can be made a great deal clearer than if no flora can be found because roots can nowadays be identified with respect to the plant group from which they come. Patterns of ''root traces'' including their shape and size, is good evidence for the vegetation type the former soil supported. Bluish colours in the soil tend to indicate the plants have mobilized nutrients within the soil.
The
horizons of fossil soils typically are sharply defined only in the top layers, unless some of the
parent material
Parent material is the underlying geological material (generally bedrock or a superficial deposits, superficial or drift (geology), drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from th ...
has not been obliterated by
soil formation
Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order ( anisotropy) within soils. These alteration ...
. The kinds of horizons in fossil soils are generally the same as those found in present-day soils, allowing easy classification in modern taxonomy of all but the oldest soils.
Analysis

Chemical analysis of soil fossils generally focuses on their
lime
Lime most commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Bo ...
content, which determines both their pH and how reactive they will be to dilute acids. Chemical analysis is also useful, usually through
solvent extraction
A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...
to determine key minerals. This analysis can be of some use in determining the structure of a soil fossil, but today X-ray diffraction is preferred because it permits the crystal structure of the former soil to be determined.
With the aid of X-ray diffraction, paleosols can now be classified into one of the 12 orders of
soil taxonomy (
Oxisols
Oxisols are a soil order in USDA soil taxonomy, best known for their occurrence in tropical rain forest within 25 degrees north and south of the Equator. In the World Reference Base for Soil Resources (WRB), they belong mainly to the ferralso ...
,
Ultisols
Ultisol, commonly known as red clay soil, is one of twelve soil orders in the United States Department of Agriculture soil taxonomy. The word "Ultisol" is derived from "ultimate", because Ultisols were seen as the ultimate product of continuous ...
,
Alfisols
Alfisols are a soil order in USDA soil taxonomy. Alfisols form in semi-arid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility. "Alf" refers to aluminium (Al) and iro ...
,
Mollisols
Mollisol is a soil type which has deep, high organic matter, nutrient-enriched surface soil (A horizon), typically between 60 and 80 cm (24-31 in) in depth. This fertile surface horizon, called a mollic epipedon, is the defining diagnosti ...
,
Spodosols
Podzols, also known as podosols, spodosols, or espodossolos, are the typical soils of coniferous or boreal forests and also the typical soils of eucalypt forests and heathlands in southern Australia. In Western Europe, podzols develop on heathland ...
,
Aridisols
Aridisols (or desert soils) are a soil order in USDA soil taxonomy. Aridisols (from the Latin ''aridus'', for "dry", and ''solum'') form in an arid or semi-arid climate. Aridisols dominate the deserts and xeric shrublands, which occupy about on ...
,
Entisols
Entisols are soils, as defined under USDA soil taxonomy, that do not show any profile development other than an A-horizon (or “A” horizon). Entisols have no diagnostic horizons, and are unaltered from their parent material, which could ...
,
Inceptisols
Inceptisols are a soil order in USDA soil taxonomy. They form quickly through alteration of parent material. They are more developed than Entisols. They have no accumulation of clays, iron oxide, aluminium oxide or organic matter. They have an ...
,
Gelisols
Gelisols are an order in USDA soil taxonomy. They are soils of very cold climates which are defined as containing permafrost within of the soil surface. The word "Gelisol" comes from the Latin meaning "to freeze", a reference to the proce ...
,
Histosols
In both the World Reference Base for Soil Resources (WRB) and the USDA soil taxonomy, a Histosol is a soil consisting primarily of organic materials. They are defined as having or more of organic soil material starting within 40 cm from the so ...
,
Vertisols
A vertisol is a Soil Order in the USDA soil taxonomy and a Reference Soil Group in the World Reference Base for Soil Resources (WRB). It is also defined in many other soil classification systems. In the Australian Soil Classification it is c ...
and
Andisols
In USDA soil taxonomy, andisols are soils
Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific defi ...
). Many
Precambrian
The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...
soils, however, when examined do not fit the characteristics for ''any'' of these soil orders and have been placed in an order called green clays. The green colour is due to the presence of certain unoxidised minerals found in the primitive Earth because
O2 was not present. There are also some forest soils of more recent times that cannot clearly be classified as alfisols or as spodosols because, despite their sandy horizons, they are not nearly acidic enough to have the typical features of a spodosol.
Importance
Paleopedology is an important scientific discipline for the understanding of the ecology and evolution of ancient ecosystems, both on Earth and the emerging field of exoplanet research, or
astropedology. In
geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
, a knowledge of the structure of former soils is also valuable to understand the composition of paleo continents.
Models
The different definitions applied to soils are indicative of the different approaches taken to them. Where farmers and engineers concentrate on certain soil properties, soil scientists have a different view. Essentially, these differing views of the definition of soil are different theoretical bases for their study.
Soils can be thought of as
open systems in that they represent a boundary between the Earth and the atmosphere where materials are transported and are changed. There are four basic types of flux: additions, subtractions, transfers, and transformations. Examples of addition can include mineral grains and leaf litter, while subtractions can include surficial
erosion
Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
of minerals and of
organic matter
Organic matter, organic material or natural organic matter is the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come fro ...
. Transfers include the movement of a material within a
soil profile
A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
, and transformations are the change of composition and form of the materials within a soil.
Soils can also be considered to be energy transformers in that they are physical structures of material that are modified by naturally occurring processes. The Sun constitutes the primary energy source for soils and significantly outweighs any heat generated by
radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
flowing up from deep within the
Earth's crust
Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...
. The deposition of sediment, or the addition of
groundwater
Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...
or rain, can also be considered an energy gain because new minerals and water can alter preexisting materials within the soil. These processes, coupled with the amount of energy available to fuel them, are what create a soil profile.
Another way to view soils is that they are environmental products that are molded over a period of time from the materials available to them. The large amount of influences that effect the formation of soils can be simplified to five main factors: climate, organisms, topographic relief, parent material, and time. These five factors can be easily remembered using the acronym "CLORPT". These categories are useful for mentally considering that aspects that occurred during the formation of a soil or paleosol. More importantly however, CLORPT allows for a theoretical framework when creating natural experiments for the study of soil formation.
Climate
When soil science was first founded, climate was considered one of the most important factors regarding the formation of soil. For instance, temperate regions have widespread acidic sand spodosols, and in tropical regions red clayey oxisols are common. The tendency to use climatic data for the classification of soils has been challenged by efforts to base the classification of soil on observable features within the soils. However, paleoclimates cannot be interpreted from paleosols identified using paleoclimatic data. The identification of paleosols using climatic data is changing. For example, aridisols have been redefined as soils that possess a calcic horizon of less than 1 meter in depth.
Soil climate is also a special kind of microclimate. It refers to the moisture, temperature, and other climatic indicators that are found within the pores of soil. For example, in well-drained soils, the soil climate is a somewhat subdued version of the regional climate. In waterlogged soils, soil climate is not related to regional climate because the temperature and oxygenation of waterlogged soils is more dependent on local groundwater paths and rates than on atmospheric conditions. Estimates of other types of soil climate are now beginning to find their way into the classification of soils, the models for soil formation, and into the study of
soil biology
Soil biology is the study of microbial and faunal activity and ecology in soil.
Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a so ...
.
The classification of climate from paleosols can be related using climatically sensitive features of soils that are sensitive to particular climatic variables, but even the best of these features lack precision. This is because soils are not as sensitive as meteorological instruments for recording climatic conditions. However, in a fairly broad category, climate can be interpreted from the sensitive features found in soils. One of the most large-scale influences regarding the classification of climate was created in 1918, then modified over two decades by the German meteorologist
Vladimir Köppen. He proposed there are five main climate groups (
Köppen climate classification
The Köppen climate classification divides Earth climates into five main climate groups, with each group being divided based on patterns of seasonal precipitation and temperature. The five main groups are ''A'' (tropical), ''B'' (arid), ''C'' (te ...
), each corresponding to the main types of terrestrial vegetation. Each climate type is designated by letters, with upper-case letters referring to the main climate groups and lower-case letters referring to subsidiary climatic features.
Organisms
Large plants are only part of the organisms that play a role in soil formation. For example,
fungi
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
are closely associated with the roots of many
vascular plants
Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified tissue ( ...
by making available nutrients like nitrogen and phosphorus in a way that their host plants can utilize, and play an important role in returning organic matter to the soil by decomposing leaf litter. The list of organisms that interact with and affect soil is extensive, and it is these interactions that allow for the presence of paleosols to be inferred.
Not only can particular organisms be interpreted from paleosols, but also ancient ecosystems. The soil interaction of plants is different from community to community. They each have distinct patterns of root traces, soil structure, and overall profile form. Identifying these features is useful for providing an overall assessment of the influence past organisms had on any particular paleosol. However, qualifying these general effects of organism activity can be difficult because the level of their expression is as related to their nature as it is to the amount of time available for soil formation. Even when fossils that are found in paleosols are understood, much more can be learned regarding their preservation, ecology, and evolution by studying the paleosols they inhabited.
A fossilized footprint,
burrow
file:Chipmunk-burrow (exits).jpg, An eastern chipmunk at the entrance of its burrow
A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of Animal lo ...
, or
coprolite
A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...
(fossil feces), are examples of trace fossils
(ichnofossils). These trace fossils do not represent any physical part of an organism, but rather are evidence of an organism's activity within its environment. Whereas a bone, leaf, or stem might provide enough information to positively identify a particular species, trace fossils rarely allow for such a precise identification. However, unlike fossilized body parts which can be affected by many variables, trace fossils are not often transported away and are usually found in the place where the organism lived. This advantage makes trace fossils in paleosols especially important because they allow for interpretation of the animal's behavior in its natural environment. A great example of this is the simple shallow fossilized burrows of solitary bees that make their homes in soil.
Just as fossilized footprints, burrows, and coprolites represent trace fossils or organisms, paleosols can be considered trace fossils of an ancient ecosystem. Much like the small percentage of species that are fossilized, very few species within an ecosystem leave any discernible trace in paleosols. However, their more general effects within a paleosol may be preserved. A good example of this is root traces. Analyzing the pattern of root traces, the sequence of soil horizons, and other features can help identify the type of vegetation that was present during the formation of the soil.
General features such as stature and spacing determine what botanists call a "plant formation." Distinct from a community or association, plant formation is not defined by any particular species. Examples of plant formation include forests, woodlands, and grasslands. Because it may not be possible to determine whether a particular plant was an oak, eucalyptus, or other species, plant formations in paleosols make it possible to identify an ancient woodland ecosystem from an ancient grassland ecosystem.
Topographic relief
The nature of soils will vary with
topography
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary sci ...
, which can be understood by comparing the thin rocky soils of mountain tops to the thick fertile soils of grass-covered lowlands. Even in a featureless lowland, the nature of a soil will vary greatly depending on whether or not it is well drained; although the drainage of soil is not completely independent because vegetation, microclimate, and the age of the land surfaces will vary within a given landscape. However, in smaller areas, the limiting factors may be so extensive that a variation in soils across a landscape will constitute a true topographical sequence, and the features within these soils can yield reliable topographic functions.
Bold landscapes like alpine ridges and peaks can be resolved based on distinct slope-related processes. For example, steep alpine slopes have sparse vegetation with soils that are eroded by snow melt, agitated by
frost heave
Frost heaving (or a frost heave) is an upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface, upwards from the depth in the soil where freezing temperatures have penetrated int ...
, and impacted by rock fall. These processes create thin, shallowly rooted, lightly weathered and rocky soils that are indicative of a mountain slope environment. The size and degree of these processes do not allow for strict analysis as topographic functions because of the extensive variation in climate, vegetation, parent materials, and land surface age at different elevations on a mountainside.
Parent material
The rock or sediment associated with a soil's development is referred to as its
parent material
Parent material is the underlying geological material (generally bedrock or a superficial deposits, superficial or drift (geology), drift deposit) in which soil horizons form. Soils typically inherit a great deal of structure and minerals from th ...
, which is the starting point for the process of soil formation. During early formation, soils are not so different from their parent materials. With time however, soils will contain less features of their original parent material. In order to make an accurate assessment of the amount of soil formation that has occurred, the parent material must be known to establish a base line, or starting point in the soil's formation.
In most instances, parent material is independent of soil formation. The formation of
igneous rock
Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
The magma can be derived from partial ...
s and
metamorphic rock
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...
s occurs in locations and by processes below the surface of the Earth. These rocks are often the parent material for soils and are sometimes derived from soils, but the degree of sedimentary sorting and distribution varies so widely that these are also considered to be independent of soils.
Very few parent materials associated with soils are entirely uniform in their composition or structure. Frequently, there is some degree of irregularity including foliation, veining, jointing, or layering that in some cases helps with soil formation and in other cases hampers it. For example, some sedimentary layering promotes the formation of soil such as a
silt
Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...
y cover on bedrock, or a sandy cover on a clayey
alluvium
Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...
layer. In both of these cases, a
friable
In materials science, friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under stress or contact, especially by rubbing. The opposite of friable is indurate.
Substances tha ...
surface material has been established by nonpedogenic instances. Other instances of sedimentary surface cementation, or fine interbedded sequences of clay and sand, could be considered to be not conducive to the formation of a soil. Nonuniform parent materials may be difficult to find in soils and paleosols, although deviations from normally found minerals could lend clues to the original parent material. If grains of primary materials are not found in the parent material, it can be inferred that later additions occurred. For example,
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
is not found in
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic
phonolite
Phonolite is an uncommon shallow intrusive or extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a var ...
, and
olivine
The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
is not found in
granite
Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...
.
The role of parent material is best understood from studies of soils that formed under similar conditions on different parent materials or lithosequences (differing soil profile characteristics because of differing parent materials). This provides a starting point for understanding what role the parent material played during the formation of the soil. The generalized relationships obtained from these studies can be used to determine what effects the parent material had on the paleosol during its formation. The difficulty lies with the fact that the parent material no longer exists, and therefore its nature can only be estimated using nearby materials.
These estimates are typically based on four critical assumptions that should be recognized as assumptions and thus assessed cautiously when evaluating soils and paleosols. These four simplifying assumptions allow for a detailed analysis of the changes that occur during the formation of a soil and the burial of a soil.
# The first assumption is that the parent material is fresh. This means that the parent material assumed to be a proxy for the original parent material must be both chemically and physically similar to that original material. For example,
saprolite
Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops, its color comes from ferric compounds. Deeply weathered profiles are widespread ...
cannot be considered to be an accurate representation of a parent material derived from a forested soil on granite, but it could be representative of a cultivated soil formed after a
clear-cut
Clearcutting, clearfelling or clearcut logging is a forestry/logging practice in which most or all trees in an area are uniformly cut down. Along with shelterwood and seed tree harvests, it is used by foresters to create certain types of for ...
and
erosion
Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
of a forested soil.
# The second assumption is that the parent material had a uniform composition within the soil profile. If the properties of the material found below the profile are to be considered representative of the parent material of the entire profile, this must be true. However, this is difficult considering that few rocks or sediments are uniform enough to be considered an accurate representation of the original parent material. For example, it is extremely difficult to detect a thin layer of windblown dust on top of granite within a thick clayey soil.
# The third assumption is that at least one of the constituents of the parent material is unaltered by weathering and is still present. The main problem with this is that no constituents are fully immune to the broad
weathering
Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...
processes that exist in nature. For example, quartz is a fairly stable mineral in soils with pH>9, while
alumina
Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...
(AlO) is stable in between pH 4.5 and 8 (mostly in clay). Trace elements that are usually stable in soils over a wider range of environmental conditions include
lead
Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...
(Pb) and
zirconium
Zirconium is a chemical element; it has Symbol (chemistry), symbol Zr and atomic number 40. First identified in 1789, isolated in impure form in 1824, and manufactured at scale by 1925, pure zirconium is a lustrous transition metal with a greyis ...
(Zr) but are not always sufficiently present to be useful.
# The fourth assumption is that volume change is proportional to thickness and density. This states that the loss of soil volume and the degree of compaction during burial are related to their density or thickness change. Although common sense suggests that volume and density are three dimensional and thickness is one dimensional, observations on various materials, including fossil plants of known shape
show that while under conditions of static vertical load, soils and fossils are maintained by pressure at the side.
See also
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Cutans Cutans are the modification of the soil texture, or soil structure, at natural surfaces (particle, pore, or ped) in soil materials due to illuviation. Cutans are oriented deposits which can be composed of any of the component substances of the soil ...
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Paleosols
In geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science.
In geology and paleon ...
References
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Subfields of paleontology
Pedology