Палеопедология: изучение почв прошлого

Discipline studying soils of the past eras
Paleopedology (palaeopedology in the United Kingdom) 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.

Paleopedology's earliest developments arose from observations in Scotland 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 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 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 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 soil chemistry can provide a good deal of evidence as to how life moved onto land during the Paleozoic era.

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 volcanoes 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 glaciers 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 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 has not been obliterated by soil formation. 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.

Chemical analysis of soil fossils generally focuses on their lime content, which determines both their pH and how reactive they will be to dilute acids. 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 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 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.

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, a knowledge of the structure of former soils is also valuable to understand the composition of paleo continents.

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 of minerals and of organic matter. Transfers include the movement of a material within a soil profile, 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 flowing up from deep within the Earth's crust. The deposition of sediment, or the addition of groundwater 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. 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. 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), 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. For example, quartz is a fairly stable mineral in soils with pH>9, while alumina (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 (Pb) and zirconium (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.