Paleopedology
Encyclopedia
Paleopedology is the discipline that studies soil
s of past geological eras, from quite recent (Quaternary
) to the earliest periods of the Earth
's history. Paleopedology can be seen either as a branch of soil science
(pedology
) or of paleontology
, since the methods it uses are in many ways a well-defined combination of the two disciplines.
circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the nineteenth 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 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 steppe
s of Russia
and Kazakhstan
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 relic of warmer climates in the past.
Eugene W. Hilgard
, in 1892, had related soil and climate in the United States
in the same manner, and by the 1950s analysis of Quaternary stratigraphy 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
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 which permit their classification. This has allowed many developments in paleoecology
and paleogeography to take place because the soils' chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic
.
in unglaciated areas or in extremely steep cliffs where the old soil can be seem below the young present-day soil. In cases where volcano
es have been active, some soil fossils occur under the volcanic ash. If there is continued deposition of sediment, a sequence of soil fossils will form, especially after the retreat of glacier
s during the Holocene
. Soil fossils can also exist where a younger soil has been eroded (for instance by wind), as in the Badlands of South Dakota
. (One must exclude areas where present-day soils are relics of former wetter climates, as with Australia
and Southern Africa
. The soils of these regions are proper paleosols.)
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 mobilised nutrients within the soil.
The horizons of fossil soils typically are sharply defined only in the top layers, unless some of the parent material has not been obliterated by soil formation. The kinds of horizons in fossil soils are, though, generally the same as those found in present-day soils, allowing easy classification in modern taxonomy of all but the oldest soils.
content, which determines both their pH and how reactive they will be to dilute acid
s. Chemical analysis is also useful, usually through solvent extraction 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 exact 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, Ultisols
, Alfisols
, Mollisols
, Spodosols, Aridisols
, Entisols, Inceptisols
, Gelisols
, Histosols, Vertisols and Andisols
). Many Precambrian
soils, however, when examined do not fit the characteristics for any of these soil orders and have been placed in a new 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, there are not nearly acidic enough to have the typical features of a Spodosol.
In geochemistry
, a knowledge of the structure of former soils is also valuable to understand the composition of certain continental rocks laid down many years ago.
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s of past geological eras, from quite recent (Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
) to the earliest periods of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's history. Paleopedology can be seen either as a branch of soil science
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
(pedology
Pedology
Pedology may refer to:*Pedology *Pedology *Pediatrics...
) or of paleontology
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, 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 ScotlandScotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
circa 1795 whereby it was found that some soils in cliffs appeared to be remains of a former exposed land surface. During the nineteenth century there were many other finds of former soils throughout Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. However, most of this was only found in the search for animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
and/or plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
fossils and it was not until soil science 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
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
were observed in the steppe
Steppe
In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...
s of Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...
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 relic of warmer climates in the past.
Eugene W. Hilgard
Eugene W. Hilgard
Eugene Woldemar Hilgard was a German-American expert on pedology...
, in 1892, had related soil and climate in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
in the same manner, and by the 1950s analysis of Quaternary stratigraphy 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
USDA Soil Taxonomy developed by United States Department of Agriculture and the National Cooperative Soil Survey provides an elaborate classification of soil types according to several parameters and in several levels: Order, Suborder, Great Group, Subgroup, Family, and Series.- Example of...
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 which permit their classification. This has allowed many developments in paleoecology
Paleoecology
Paleoecology uses data from fossils and subfossils to reconstruct the ecosystems of the past. It involves the study of fossil organisms and their associated remains, including their life cycle, living interactions, natural environment, and manner of death and burial to reconstruct the...
and paleogeography to take place because the soils' chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
.
Finding soil fossils and their structure
Remains of former soils can either be found under deposited sedimentSediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
in unglaciated areas or in extremely steep cliffs where the old soil can be seem below the young present-day soil. In cases where volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
es have been active, some soil fossils occur under the volcanic ash. If there is continued deposition of sediment, a sequence of soil fossils will form, especially after the retreat of glacier
Glacier
A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s during the Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
. Soil fossils can also exist where a younger soil has been eroded (for instance by wind), as in the Badlands of South Dakota
South Dakota
South Dakota is a state located in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux American Indian tribes. Once a part of Dakota Territory, South Dakota became a state on November 2, 1889. The state has an area of and an estimated population of just over...
. (One must exclude areas where present-day soils are relics of former wetter climates, as with Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and Southern Africa
Southern Africa
Southern Africa is the southernmost region of the African continent, variably defined by geography or geopolitics. Within the region are numerous territories, including the Republic of South Africa ; nowadays, the simpler term South Africa is generally reserved for the country in English.-UN...
. The soils of these regions are proper paleosols.)
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
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
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 mobilised nutrients within the soil.
The horizons of fossil soils typically are sharply defined only in the top layers, unless some of the parent material has not been obliterated by soil formation. The kinds of horizons in fossil soils are, though, 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 limeCalcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
content, which determines both their pH and how reactive they will be to dilute acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s. Chemical analysis is also useful, usually through solvent extraction 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 exact 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, Ultisols
Ultisols
Ultisols, commonly known as red clay soils, are one of twelve soil orders in the United States Department of Agriculture soil taxonomy. They are defined as mineral soils which contain no calcareous material anywhere within the soil, have less than 10% weatherable minerals in the extreme top layer...
, Alfisols
Alfisols
Alfisols are a soil order in USDA soil taxonomy. Alfisols form in semiarid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility. "Alf" refers to aluminium and iron . Because of their productivity and abundance, the...
, Mollisols
Mollisols
Mollisols are a soil order in USDA soil taxonomy. Mollisols form in semi-arid to semi-humid areas, typically under a grassland cover. They are most commonly found in the mid-latitudes, namely in North America, mostly east of the Rocky Mountains, in South America in Argentina and Brazil, and in...
, Spodosols, Aridisols
Aridisols
Aridisols are a soil order in USA soil taxonomy. Aridisols form in an arid or semi-arid climate. Aridisols dominate the deserts and xeric shrublands, which occupy about one third of the Earth's land surface...
, Entisols, Inceptisols
Inceptisols
Inceptisols are a soil order in USDA soil taxonomy. They form quickly through alteration of parent material. They are older than entisols. They have no accumulation of clays, Iron, Aluminum or organic matter. They have an Ochric or Umbric horizon and a cambic subsurface horizon....
, Gelisols
Gelisols
Gelisols are an order in USDA soil taxonomy. They are soils of very cold climates which are defined as containing permafrost within two meters of the soil surface...
, Histosols, Vertisols and Andisols
Andisols
In USDA soil taxonomy, Andisols are soils formed in volcanic ash and defined as soils containing high proportions of glass and amorphous colloidal materials, including allophane, imogolite and ferrihydrite...
). Many Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
soils, however, when examined do not fit the characteristics for any of these soil orders and have been placed in a new order called green clays. The green colour is due to the presence of certain unoxidised minerals found in the primitive earth because O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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, there are not nearly acidic enough to have the typical features of a Spodosol.
Uses
Paleopedology is a very important discipline today for the understanding of the ecology of ancient ecosystems because it gives a clue as to what soils conditions past animals and plants were required to live under and how the plants obtained essential nutrients.In geochemistry
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
, a knowledge of the structure of former soils is also valuable to understand the composition of certain continental rocks laid down many years ago.