Paleopedological record

Last updated September 19, 2019

The paleopedological record is, essentially, the fossil record of soils. The paleopedological record consists chiefly of paleosols buried by flood sediments, or preserved at geological unconformities, especially plateau escarpments or sides of river valleys. Other fossil soils occur in areas where volcanic activity has covered the ancient soils.

Soil mixture of organic matter, minerals, gases, liquids, and organisms that together support life

Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Earth's body of soil, called the pedosphere, has four important functions:

A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrology and are of significant concern in agriculture, civil engineering and public health.

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation. If buried, they may eventually become sandstone and siltstone through lithification.

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 vegetation that was in the soil when it was present.
Oxidation of iron from Fe²⁺ to Fe³⁺ by O₂ 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.

Vegetation is an assemblage of plant species and the ground cover they provide. 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 than the term flora which refers to species composition. Perhaps the closest synonym is plant community, but vegetation can, and often does, refer to a wider range of spatial scales than that term does, including scales as large as the global. Primeval redwood forests, coastal mangrove stands, sphagnum bogs, desert soil crusts, roadside weed patches, wheat fields, cultivated gardens and lawns; all are encompassed by the term vegetation.

Oxygen is the chemical element with the symbol O and atomic number 8, meaning its nucleus has 8 protons. The number of neutrons varies according to the isotope: the stable isotopes have 8, 9, or 10 neutrons. Oxygen is a member of the chalcogen group on the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O^₂. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up almost half of the Earth's crust.

The keys to recognising fossils of various soils include:

Tubular structures that branch and thin irregularly downward or show the anatomy of fossilised root traces
Gradational alteration down from a sharp lithological contact like that between land surface and soil horizons
Complex patterns of cracks and mineral replacements like those of soil clods (peds) and planar cutans .

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. Roots can also be aerial or aerating, that is, growing up above the ground or especially above water. Furthermore, a stem normally occurring below ground is not exceptional either. Therefore, the root is best defined as the non-leaf, non-nodes bearing parts of the plant's body. However, important internal structural differences between stems and roots exist.

A mineral is, broadly speaking, a solid chemical compound that occurs naturally in pure form. Minerals are most commonly associated with rocks due to the presence of minerals within rocks. These rocks may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are usually excluded, but some minerals are often biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.

Classification

Soil fossils are usually classified by USDA soil taxonomy. 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.

USDA soil taxonomy (ST) 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. The classification was originally developed by Guy Donald Smith, former director of the U.S. Department of Agriculture's soil survey investigations.

X-ray crystallography (XRC) is a technique used to determine the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their crystallographic disorder, and various other information.

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 Precambrian, 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 entisols or inceptisols. Vertisols and aridisols have a continuous fossil record from Paleoproterozoic continents 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.

The Precambrian 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 the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains high concentration of ozone (O₃) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometers (9.3 to 21.7 mi) above Earth, although its thickness varies seasonally and geographically.

In USDA 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. Entisols are the second most abundant soil order, occupying about 16% of the global ice-free land area.

Oxisols, deeply weathered tropical soils, have a rich fossil record from the Paleoproterozoic onwards. Outside of ice ages, 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 producing 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 causes changes in circulation linked closely by climatologists to the development of continental ice sheets. 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, histosols, 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 orthents 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 plants and animals 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.

Related Research Articles

The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago (Mya), to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō ("coal") and ferō, and was coined by geologists William Conybeare and William Phillips in 1822.

Oxisol A soil type known for occurring in tropical rain forests

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. In the World Reference Base for Soil Resources (WRB), they belong mainly to the Ferralsols, but some are Plinthosols or Nitisols. Some Oxisols have been previously classified as laterite soils.

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 iron (Fe). Because of their productivity and abundance, the Alfisols represent one of the more important soil orders for food and fiber production. They are widely used both in agriculture and forestry, and are generally easier to keep fertile than other humid-climate soils, though those in Australia and Africa are still very deficient in nitrogen and available phosphorus. Those in monsoonal tropical regions, however, have a tendency to acidify when heavily cultivated, especially when nitrogenous fertilizers are used.

Mollisol soil type in USDA soil taxonomy

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 (Pampas) and Brazil, and in Asia in Mongolia and the Russian Steppes. Their parent material is typically base-rich and calcareous and include limestone, loess, or wind-blown sand. The main processes that lead to the formation of grassland Mollisols are melanisation, decomposition, humification and pedoturbation.

Ultisols, commonly known as red clay soils, are 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 weathering of minerals in a humid, temperate climate without new soil formation via glaciation. 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 of soil, and have less than 35% base saturation throughout the soil. Ultisols occur in humid temperate or tropical regions. While the term is usually applied to the red clay soils of the Southern United States, Ultisols are also found in regions of Africa, Asia, and South America.

In the geosciences, paleosol can have two meanings. The first meaning, common in geology and paleontology, refers to a former soil preserved by burial underneath either sediments or volcanic deposits, which in the case of older deposits have lithified into rock. In Quaternary geology, sedimentology, paleoclimatology, and geology in general, it is the typical and accepted practice to use the term "paleosol" to designate such "fossil soils" found buried within either sedimentary or volcanic deposits exposed in all continents as illustrated by Rettallack (2001), Kraus (1999), and other published papers and books.

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 classification, as lithosols. The basic requirement for recognition of an orthent is that any former soil has been either completely removed or so truncated that the diagnostic horizons typical of all orders other than entisols are absent.

In soil classification, an Umbrisol is a soil with a dark topsoil and in which organic matter has accumulated within the mineral surface soil—in most cases with low base saturation—to the extent that it significantly affects the behaviour and utilization of the soil. Umbrisols are the counterpart of comparable soils with a high base saturation.

Páramo high-altitude wet tundra in South America

Páramo can refer to a variety of alpine tundra ecosystems. Some ecologists describe the páramo broadly as "all high, tropical, montane vegetation above the continuous timberline". A more narrow term classifies the páramo according to its regional placement in the northern Andes of South America and adjacent southern Central America. The páramo is the ecosystem of the regions above the continuous forest line, yet below the permanent snowline. It is a "Neotropical high mountain biome with a vegetation composed mainly of giant rosette plants, shrubs and grasses". According to scientists, páramos may be "evolutionary hot spots" and among the fastest evolving regions on Earth.

This is an index of articles relating to soil.

Paleopedology is the discipline that studies soils 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.

The Huronian glaciation was a glaciation that extended from 2.4 billion years ago (Gya) to 2.1 Gya, during the Siderian and Rhyacian periods of the Paleoproterozoic era. The Huronian glaciation followed the Great Oxygenation Event (GOE), a time when increased atmospheric oxygen decreased atmospheric methane. The oxygen combined with the methane to form carbon dioxide and water, which do not retain heat as well as methane does.

The late Paleozoic icehouse, formerly known as the Karoo ice age, was the climate state 360–260 million years ago (Mya) in which large land-based ice-sheets were present on Earth's surface. It was the second major glacial period of the Phanerozoic. It is named after the tillite found in the Karoo Basin of South Africa, where evidence for this ice age was first clearly identified in the 19th century.

Steven's Creek Heritage Preserve is a nature preserve in the US State of South Carolina. It is managed by the South Carolina Department of Natural Resources. It covers 434 acres (1.76 km²) in McCormick County and Edgefield County, and was established to protect rare animal and plant species. Webster's Salamander and Miccosukee gooseberry are among the species which have few populations outside the preserve. The nearest population center is Clarks Hill, about two road miles (3–4 km) to the southwest.

Paleontology in Tennessee refers to paleontological research occurring within or conducted by people from the U.S. state of Tennessee. During the early part of the Paleozoic era, Tennessee was covered by a warm, shallow sea. This sea was home to brachiopods, bryozoans, cephalopods, corals, and trilobites. Tennessee is one of the best sources of Early Devonian fossils in North America. During the mid-to-late Carboniferous, the state became a swampy environment, home to a rich variety of plants including ferns and scale trees. A gap in the local rock record spans from the Permian through the Jurassic. During the Cretaceous, the western part of the state was submerged by seawater. The local waters were home to more fossil gastropods than are known from anywhere else in the world. Mosasaurs and sea turtles also inhabited these waters. On land the state was home to dinosaurs. Western Tennessee was still under the sea during the early part of the Cenozoic. Terrestrial portions of the state were swampy. Climate cooled until the Ice Age, when the state was home to Camelops, horses, mammoths, mastodons, and giant ground sloths. The local Yuchi people told myths of giant lizard monsters that may have been inspired by fossils either local or encountered elsewhere. In 1920, after local fossils became a subject of formal scientific study, a significant discovery of a variety of Pleistocene creatures was made near Nashville. The Cretaceous bivalve Pterotrigonia thoracica is the Tennessee state fossil.

A Nitisol in the World Reference Base for Soil Resources (WRB) is a deep, red, well-drained soil with a clay content of more than 30% and a blocky structure. Nitisols correlate with the Kandic Alfisols, Ultisols and Inceptisols of the USDA soil taxonomy.

The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.

References

Retallack, Gregory John; Soils of the past: An introduction to paleopedology, (2nd edition), 2001, Blackwell Science; Malden, Massachusetts.

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