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Thepaleopedological record is, essentially, thefossil record ofsoils. The paleopedological record consists chiefly ofpaleosols buried byfloodsediments, or preserved at geological unconformities, especiallyplateau escarpments or sides ofrivervalleys. Otherfossil soils occur in areas wherevolcanic activity has covered the ancient soils.
After burial,soil fossils tend to be altered by various chemical and physical processes. These include:
The keys to recognising fossils of various soils include:
Soil fossils are usually classified byUSDA 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 ofX-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.
During thePrecambrian, when life on land was precluded by a very thin or nonexistentozone layer, soils were subject to much more rapid erosion and most fossils from this period are of undevelopedentisols orinceptisols.Vertisols andaridisols have a continuous fossil record fromPaleoproterozoiccontinents onwards (though little is known about when they were first vegetated), whilst a fewandisol fossils are known from theMesoproterozoic and more abundantly from theOrdovician just before land vegetation began to emerge. Other major andisol fossils can be found in the middleJurassic ofSiberia.
Oxisols, deeply weathered tropical soils, have a rich fossil record from the Paleoproterozoic onwards. Outside ofice ages, oxisols have generally been the dominant soil order in the paleopedological record. This is becausesoil formation, after which oxisols take moreweathering to form than any other soil order, has been almost non-existent outside eras of extensive continentalglaciation. 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 theatmosphericCO2 content and also causes changes in circulation linked closely byclimatologists to the development of continentalice sheets. Oxisols were not vegetated until the late Carboniferous, probably because microbialevolution was not before that point advanced enough to permit plants to obtain sufficientnutrients from soils with very low concentrations ofnitrogen, 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 theNeoproterozoic. However, in these periods fossil gelisols are generally abundant, notable finds coming from theCarboniferous inNew South Wales.
The earliest land vegetation is found inEarly 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 theDevonian but are rare as fossils because most of their mass consists of organic materials that tend to decay quickly.Alfisols andultisols 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 lessacidic soils otherwise similar to spodosols are known from theMesozoic andTertiary 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 andorthents 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 thePaleocene can be seen in abundant oxisols and ultisols in now-heavily glaciatedScotland andAntarctica.Mollisols, the majoragricultural soils of the present, are unique in their geological youth, being known from theEocene but common only from theMiocene, as grasslands evolved. The most abundant paleopedological record is that of theQuaternary with few soils different from types widely found today.
An important difference between the paleopedological record and the fossil record ofplants andanimals is that very few of the soils found are extinct types. Despite the difficulties of identification mentioned earlier, this makespaleopedology (the study of fossil soils) potentially very useful to understanding the ecological relationships in past ecosystems.