Paleopedological record
Encyclopedia
The paleopedological record is, essentially, the fossil record of soil
s. The paleopedological record consists chiefly of paleosols buried by flood
sediment
s, or preserved at geological unconformities, especially plateau
escarpments or sides of river
valley
s. Other fossil
soils occur in areas where volcanic activity
has covered the ancient soils.
The keys to recognising fossils of various soils include:
. With the exception of some exceedingly old soils which have a clayey, grey-green horizon that is quite unlike any present soil and clearly formed in the absence of O2, most fossil soils can be classified into one of the twelve orders recognised by this system. This is usually done by means of X-ray diffraction
, which allows the various particles within the former soils to be analysed so that it can be seen to which order the soils correspond.
Other methods for classifying soil fossils rely on geochemical analysis of the soil material, which allows the minerals in the soil to be identified. This is only useful where large amounts of the ancient soil are available, which is rarely the case.
, when life on land was precluded by a very thin or nonexistent ozone layer
, soils were subject to much more rapid erosion and most fossils from this period are of undeveloped Entisol
s or Inceptisols
. Vertisol
s and Aridisols
have a continuous fossil record from Paleoproterozoic
continent
s onwards (though little is known about when they were first vegetated), whilst a few Andisol
fossils are known from the Mesoproterozoic
and more abundantly from the Ordovician
just before land vegetation began to emerge. Other major Andisol fossils can be found in the middle Jurassic
of Siberia
.
Oxisol
s, deeply weathered tropical soils, have a rich fossil record from the Paleoproterozoic onwards. Outside of ice age
s, Oxisols have generally been the dominant soil order in the paleopedological record. This is because soil formation, after which Oxisols take more weathering to form than any other soil order, has been almost non-existent outside eras of extensive continental glaciation. This is not only because of the soils formed by glaciation itself, but also because mountain building, which is the other critical factor in prodcing new soil, has always coincided with a reduction in global temperatures and sea levels. This is because the sediment formed from the eroding mountains reduces the atmospheric
CO2
content and also cause changes in circulation linked closely by climatologists
to the development of continental ice sheet
s. Oxisols were not vegetated until the late Carboniferous, probably because microbial evolution
was not before that point advanced enough to permit plants to obtain sufficient nutrients from soils with very low concentrations of nitrogen, phosphorus, calcium and potassium.
Owing to their extreme climatic requirements, Gelisol
fossils are confined to the few periods of extensive continental glaciation - the earliest being 900 million years ago in the Neoproterozoic
. However, in these periods fossil Gelisols are generally abundant, notable finds coming from the Carboniferous
in New South Wales
.
The earliest land vegetation is found in early Silurian
Entisols and Inceptisols, and with the growth of land vegetation under a protective ozone layer several new soil orders emerged. The first, Histosol
s, emerged in the Devonian
but are rare as fossils because most of their mass consists of organic materials that tend to decay quickly. Alfisols
and Ultisols
emerged in the late Devonian and early Carboniferous, and have a continuous, though not rich, fossil record in eras since then. Spodosols are known only from the Carboniferous and from a few periods since that time - though less acidic soils otherwise similar to Spodosols are known from the Mesozoic
and Tertiary
and may constitute an extinct suborder.
During the Mesozoic the paleopedological record tends to be poor, probably because the absence of mountain-building and glaciation meant that most surface soils were very old and were constantly being weathered of what weatherable materials remained. Oxisols and Orthent
s are the dominant groups, though a few more fertile soils have been found, such as the extensive Andisols mentioned earlier from Jurassic Siberia. Evidence for widespread deeply weathered soils in the Paleocene
can be seen in abundant Oxisols and Ultisols in now-heavily glaciated Scotland
and Antarctica. Mollisols
, the major agricultural
soils of the present, are unique in their geological youth, being known from the Eocene
but common only from the Miocene
, as grasslands evolved. The most abundant paleopedological record is that of the Quaternary
with few soils different from types widely found today.
An important difference between the paleopedological record and the fossil record of plant
s and animal
s is that very few of the soils found are extinct types. Despite the difficulties of identification mentioned earlier, this makes paleopedology
(the study of fossil soils) potentially very useful to understanding the ecological relationships in past ecosystems.
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. The paleopedological record consists chiefly of paleosols buried by flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
sediment
Sediment
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....
s, or preserved at geological unconformities, especially plateau
Plateau
In geology and earth science, a plateau , also called a high plain or tableland, is an area of highland, usually consisting of relatively flat terrain. A highly eroded plateau is called a dissected plateau...
escarpments or sides of river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
valley
Valley
In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...
s. Other fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
soils occur in areas where volcanic activity
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...
has covered the ancient soils.
Problems of recognition
After burial, soil fossils tend to be altered by various chemical and physical processes. These include:- Decomposition of organic matter that was once present in the old soil. This hinders the recognition of vegetationVegetationVegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
that was in the soil when it was present. - Oxidation of iron from Fe2+ to Fe3+ by O2OxygenOxygen 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...
as the former soil becomes dry and more oxygen enters the soil. - Drying out of hydrous ferric oxides to anhydrous oxides - again due to the presence of more available O2 in the dry environment.
The keys to recognising fossils of various soils include:
- Tubular structures that branch and thin irregularly downward or show the anatomy of fossilised rootRootIn vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
traces - Gradational alteration down from a sharp lithological contact like that between land surface and soil horizons
- Complex patterns of cracks and mineralMineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
replacements like those of soil clods (peds) and planar cutanCutanCutan is one of two polymers which occur in the cuticle of some plants. The other and better-known polymer is Cutin. Cutan is believed to be a hydrocarbon polymer, whereas cutin is a polyester, but the structure and synthesis of cutan are not yet fully understood...
s.
Classification
Soil fossils are usually classified by USDA soil taxonomyUSDA 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...
. With the exception of some exceedingly old soils which have a clayey, grey-green horizon that is quite unlike any present soil and clearly formed in the absence of O2, most fossil soils can be classified into one of the twelve orders recognised by this system. This is usually done by means of X-ray diffraction
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
, which allows the various particles within the former soils to be analysed so that it can be seen to which order the soils correspond.
Other methods for classifying soil fossils rely on geochemical analysis of the soil material, which allows the minerals in the soil to be identified. This is only useful where large amounts of the ancient soil are available, which is rarely the case.
Records of the various soil groups
During the PrecambrianPrecambrian
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...
, when life on land was precluded by a very thin or nonexistent ozone layer
Ozone layer
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...
, soils were subject to much more rapid erosion and most fossils from this period are of undeveloped Entisol
Entisol
In USA soil taxonomy, Entisols are defined as soils that do not show any profile development other than an A horizon. An Entisol has no diagnostic horizons, and most are basically unaltered from their parent material, which can be unconsolidated sediment or rock...
s or 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....
. Vertisol
Vertisol
In both the FAO and USA soil taxonomy, a vertisol is a soil in which there is a high content of expansive clay known as montmorillonite that forms deep cracks in drier seasons or years. Alternate shrinking and swelling causes self-mulching, where the soil material consistently mixes itself, causing...
s and 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...
have a continuous fossil record from Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
s onwards (though little is known about when they were first vegetated), whilst a few Andisol
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...
fossils are known from the Mesoproterozoic
Mesoproterozoic
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...
and more abundantly from the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
just before land vegetation began to emerge. Other major Andisol fossils can be found in the middle Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
of Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
.
Oxisol
Oxisol
Oxisols are an order in USDA soil taxonomy, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator. Some oxisols have been previously classified as laterite soils.-Formation:...
s, deeply weathered tropical soils, have a rich fossil record from the Paleoproterozoic onwards. Outside of ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
s, Oxisols have generally been the dominant soil order in the paleopedological record. This is because soil formation, after which Oxisols take more weathering to form than any other soil order, has been almost non-existent outside eras of extensive continental glaciation. This is not only because of the soils formed by glaciation itself, but also because mountain building, which is the other critical factor in prodcing new soil, has always coincided with a reduction in global temperatures and sea levels. This is because the sediment formed from the eroding mountains reduces the atmospheric
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
content and also cause changes in circulation linked closely by climatologists
Climatology
Climatology is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences...
to the development of continental ice sheet
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...
s. Oxisols were not vegetated until the late Carboniferous, probably because microbial evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
was not before that point advanced enough to permit plants to obtain sufficient nutrients from soils with very low concentrations of nitrogen, phosphorus, calcium and potassium.
Owing to their extreme climatic requirements, Gelisol
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...
fossils are confined to the few periods of extensive continental glaciation - the earliest being 900 million years ago in the Neoproterozoic
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
. However, in these periods fossil Gelisols are generally abundant, notable finds coming from the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
in New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
.
The earliest land vegetation is found in early Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
Entisols and Inceptisols, and with the growth of land vegetation under a protective ozone layer several new soil orders emerged. The first, Histosol
Histosol
In both the FAO soil classification and the USA soil taxonomy, a histosol is a soil consisting primarily of organic materials. They are defined as having or more of organic soil material in the upper . Organic soil material has an organic carbon content of 12 to 18 percent, or more, depending on...
s, emerged in the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
but are rare as fossils because most of their mass consists of organic materials that tend to decay quickly. 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...
and 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...
emerged in the late Devonian and early Carboniferous, and have a continuous, though not rich, fossil record in eras since then. Spodosols are known only from the Carboniferous and from a few periods since that time - though less acidic soils otherwise similar to Spodosols are known from the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
and Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...
and may constitute an extinct suborder.
During the Mesozoic the paleopedological record tends to be poor, probably because the absence of mountain-building and glaciation meant that most surface soils were very old and were constantly being weathered of what weatherable materials remained. Oxisols and Orthent
Orthent
In USDA soil taxonomy, Orthents are defined as Entisols that lack horizon development due to either steep slopes or parent materials that contain no permanent weatherable minerals . Typically, Orthents are exceedingly shallow soils. They are often referred to as "skeletal soils" or, in the FAO soil...
s are the dominant groups, though a few more fertile soils have been found, such as the extensive Andisols mentioned earlier from Jurassic Siberia. Evidence for widespread deeply weathered soils in the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...
can be seen in abundant Oxisols and Ultisols in now-heavily glaciated Scotland
Scotland
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...
and Antarctica. 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...
, the major agricultural
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
soils of the present, are unique in their geological youth, being known from the Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
but common only from the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
, as grasslands evolved. The most abundant paleopedological record is that of the 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...
with few soils different from types widely found today.
An important difference between the paleopedological record and the fossil record of 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...
s and 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...
s is that very few of the soils found are extinct types. Despite the difficulties of identification mentioned earlier, this makes paleopedology
Paleopedology
Paleopedology is the discipline that studies soils of past geological eras, from quite recent to the earliest periods of the Earth's history...
(the study of fossil soils) potentially very useful to understanding the ecological relationships in past ecosystems.