Paleosol

In geoscience, paleosol 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 paleontology, a paleosol is 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 sedimentary and volcanic deposits exposed in all continents.
In soil science the definition differs slightly: paleosols are soils formed long ago that have no relationship in their chemical and physical characteristics to the present-day climate or vegetation. Such soils are found within extremely old continental cratons, or in small scattered locations in outliers of other ancient rock domains.

Properties

Because of the changes in the Earth's climate over the last 50 million years, soils formed under tropical rainforest have become exposed to increasingly arid climates which cause former oxisols, ultisols or even alfisols to dry out in such a manner that a very hard crust is formed. This process has occurred so extensively in most parts of Australia as to restrict soil development—the former soil is effectively the parent material for a new soil, but it is so unweatherable that only a very poorly developed soil can exist in present dry climates, especially when they have become much drier during glacial periods in the Quaternary.
In other parts of Australia and in many parts of Africa, drying out of former soils has not been so severe. This has led to large areas of relict podsols in quite dry climates in the far southern inland of Australia and to the formation of torrox soils in southern Africa. Here, present climates allow, effectively, the maintenance of the old soils in climates under which they could not have formed from the parent material during the Mesozoic and Paleocene.
Paleosols in this sense are always exceedingly infertile soils, containing available phosphorus levels orders of magnitude lower than in temperate regions with younger soils. Ecological studies have shown that this has forced highly specialised evolution amongst Australian flora to obtain minimal nutrient supplies. The fact that soil formation is not occurring makes ecologically sustainable management even more difficult. However, paleosols often contain the most exceptional biodiversity due to the absence of competition.

Taxonomic classification

The record of paleosols extends into the Precambrian in Earth's history, with rare paleosols older than 2.5 billion years. Geology, biology, and the atmosphere all changed significantly over that time, with dramatic shifts at the Great Oxidation Event and during the Paleozoic, when complex animals and land plants proliferated.
Consequently, our modern soil classification system cannot be readily applied to paleosols. For example, a modern alfisol—broadly defined as a forest soil—would not have existed prior to the evolution of trees. More problematically, it is specifically defined by chemical properties that would not be preserved in the rock record. While modern soil orders are often used to describe paleosols in a qualitative sense, a paleosol-specific naming scheme has been proposed, although it is only used sporadically in the literature.
Until a paleosol-specific naming scheme is fully adopted, many paleo-pedologists have stuck to using the taxonomic classification of soils provided by the United States Department of Agriculture. The USDA soil taxonomy attempts to use the measurable properties and objective features within soils to classify them. The methodology developed a hierarchical structure among the different soil taxa, classifying the soils initially at a general level, then assigning soils to progressively more limited subdivisions.
The USDA soil taxonomy does come with drawbacks, including an emphasis on observable features, new nomenclature, and hierarchical organization. The emphasis on observable features can make the soil taxonomy similar in appearance to a legal document. The hierarchical structure cannot be applied more deeply than the order level regarding paleosols. However, despite these drawbacks, the USDA soil taxonomy is still the most comprehensive and influential soil classification system to date. To distinguish and identify paleosols from one another, certain diagnostic horizons and features need to be taken into account. For instance, all paleosols have an A horizon, but histosols have an O horizon above the A horizon.

Identification

Rye & Holland laid out five criteria for identifying a paleosol. While this was prompted by the need for more stringent identification of Precambrian paleosols, it is applicable to paleosols of any age. The criteria are:

Formed in situ on bedrock,
soft-sediment deformation at the top of the profile, and
up-profile changes in chemistry, texture, and mineralogy consistent with terrestrial weathering processes.

In the field, physical signs of a paleosol include evidence of horizonation, bedrock incorporated into a finer overlying lithology, and evidence of surface processes.

Soil types

Below is a list of soils and some of their diagnostic features that provides a framework for telling these paleosols, or even modern soils, apart:

Entisol (incipient soil)

Horizons : A & C
This soil has a very slight degree of soil formation. Original crystalline, metamorphic, or sedimentary features of the parent material experienced little alteration from soil formation. Most are found on young geomorphic surfaces such as flood plains and on steep slopes where erosion removes material as the soil forms. Signs of early successional vegetation of grasses and other herbs and shrubs. Root traces are diagnostic of this type of paleosol because of the small amount of alteration from their parent material in other respects. However, for Entisols of Ordivician age or older, a peak in magnetic susceptibility is indicative of an Entisol.

Inceptisol (young soil)

Horizons: A, sometimes E, Bw, & C
These soils represent a stage of formation beyond Entisols, but not to the degree of development in other soil orders. Typically can be imagined as having a light-colored surface horizon over a moderately weathered subsurface horizon. Forms in low-rolling parts of landscapes in and around steep mountain fronts. Shrubby woodlands of pole trees that form during recolonization of disturbed ground by forests are particularly characteristic of this paleosol. Open woodlands and wooded grasslands are also characteristic of this paleosol.

Andisol (volcanic ash soil)

Horizons: A, Bw, & C
These are soils of volcanic ash of siliceous nature, consisting of bubbles or shards of volcanic glass with a high internal surface area. This soil weathers rapidly to imogolite and smectite. Thus they are highly fertile, rich in organic matter, and have particularly low bulk density. These properties and the aforementioned weathering products typically alter during burial, sometimes to distinctive minerals like celadonite and clinoptilolite. At least 60% recognizable pyroclastic fragments in thin sections are characteristic of this paleosol. This paleosol forms in and around volcanoes.

Histosol (peaty soil)

Horizons: O, A, sometimes Bg, & C
Organic-rich soils with thick peaty horizons, that form in cool, well drained localities or low-lying, permanently waterlogged areas. The primary formation process is accumulation of peat, meaning organic matter is produced faster than it can decompose in the soil. The leaching or formation of gley minerals overprinting prior soil or sedimentary features is associated with peat accumulation.

Spodosol (forest soil)

Horizons: A, E, sometimes Bh, Bs, & C
A subsurface horizon enriched with iron and aluminum oxides or organic matter is characteristic of Spodosols. Displays opaque cements that form distinctive radially cracked, concretionary rims to abundant quartz grains in thin sections. Spodosols form on hilly bedrock or low, rolling quartz-rich sediments. Found principally in humid climates in which clay and soluble salts are dissolved and washed out of the profile and most common in temperate regions. Characteristic vegetation are conifer forests and other kinds of evergreen woody vegetation that can tolerate low nutrient levels and high soil acidity.

Alfisol (fertile forest soil)

Horizons: A, sometimes E, Bt, sometimes Bk, & C
Base-rich forested soils that have a light-colored surface horizon over a clayey subsurface horizon, rich in exchangeable cations. If paleosols contain nodules of carbonate in a horizon deep within the profile, such base saturation can be assumed. If lacking in carbonate nodules, Alfisols can be distinguished by the abundance of base rich clays or by molecular weathering ratios of alumina/bases of less than 2. These soils are not found at the poles or on high mountain tops.

Ultisol (base-poor forest soil)

Horizons: A, sometimes E, Bt, & C
Base-poor forest soils that are similar to Alfisols at first glance. However, Ultisols are more deeply weathered of mineral nutrients. There should not be any calcareous material anywhere within an Ultisol profile and have molecular weathering ratios of alumina/bases of more than 2. Kaolinite and highly weathered aluminous minerals such as gibbsite are common in the profile. Low-base status is attributed to a long formation time. Form mostly on older parts of landscapes, such as rolling hills of bedrock, high alluvial terraces, and plateau tops. Natural vegetation consists of coniferous or hardwood forests.

Oxisol (tropical deeply-weathered soil)

Horizons: A, Bo, sometimes Bv, & C
Deeply weathered soils with texturally uniform profiles. Dominated by kaolinitic clays or other base-poor oxides such as gibbsite or boehmite. Contains molecular weathering ratios of alumina/bases of 10 or more. These soils have deeply weathered mottled horizons. Characteristic of this type of paleosol is a stable microstructure of sand-sized spherical micropeds of iron-stained lay. Very old, often amounting to tens of millions of years. Found on stable continental locations on gentle slopes of plateaus, terraces, and plains. The natural vegetation for Oxisols is a rainforest.