Paleopedological record
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.
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 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.
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 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.
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