Көне топырақтар: қалыптасуы, ерекшеліктері және экологиялық маңызы

Soil buried under sediment or not representative of current environmental conditions

In geoscience, paleosol (palaeosol in Great Britain and Australia) 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 (alluvium or loess) or volcanic deposits (volcanic ash), 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.

Because of the changes in the Earth's climate over the last 50 million years, soils formed under tropical rainforest (or even savanna) 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 (where temperate rainforest was formerly dominant) and to the formation of torrox soils (a suborder of oxisols) 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.

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 (2.42 billion years ago) 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 (USDA). 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.

Palaeosols are frequently used as palaeoclimatological tools for gauging the climate in which they formed. Because rates and styles of weathering are dependent on climatic factors, paleosols can be used to reconstruct variables of past climate. Mean annual precipitation (MAP) and air temperature (MAAT) are two commonly reconstructed variables which, along with seasonality and in conjunction with other paleoenvironmental tools, can be used to describe past terrestrial climates. A suite of paleoclimatic proxies exist and while they vary in focus, many rely on changes in chemical composition throughout a soil profile that occur during weathering, burial, and post burial processes. Their use depends on factors such as post burial alteration, parent material, and soil order; not every proxy is applicable to every paleosol. Most proxies are applicable to Phanerozoic paleosols (not older), as landscape processes changed dramatically after the rise of land plants. Seasonality (the presence and strength of seasons) requires a more nuanced reconstruction approach. Proposed seasonality proxies primarily rely on a soil wetting/drying process, during which pedogenic carbonate can form; like other proxies, this tool is continually being tested and refined.

Soils form in near constant contact with the atmosphere, so their chemical composition is affected by the composition of the atmosphere through both direct and indirect pathways. The oxidation of paleosols has been used as an indicator of atmospheric oxygen, which has risen over Earth's history. Paleosols have also been used to reconstruct atmospheric carbon dioxide levels, based on modern studies of soil carbon gas exchange, carbon isotopes in pedogenic carbonate nodules, and mass balance approaches taking multiple atmospheric gases (typically carbon dioxide, oxygen, and methane) into account. These methods are being actively developed in the field of early Earth research.

Paleosols are an important archive of information about ancient ecosystems and various components of fossil soils can be used to study past plant life. Paleosols often contain ancient plant materials such as pollen grains and phytoliths, a biomineralized form of silica produced by many plants such as grasses. Both pollen and phytolith fossils from different plant species have characteristic shapes that can be traced back to their parent plants. Over long geological time scales, phytoliths may not necessarily be preserved in paleosols due to ability of the poorly crystalline silica to dissolve. Another indicator of plant community composition in paleosols is the carbon isotopic signature. The ratio of different carbon isotopes in organic matter in paleosols reflects the proportions of plants using C3 photosynthesis, which grow in cooler and wetter climates, versus plants using C4 photosynthesis, which are better adapted to hotter and drier conditions. Other methods for detecting past plant life in paleosols are based on identifying the remains of leaf waxes, which are slow to break down in soils over time.

As records of previous Earth surfaces that can be stacked on one another, paleosol sequences are also useful in the field of paleoseismology.