Tutorial:Buried Soils

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Buried Soils
Buried soils, as their name implies, are soil profiles that have been buried beneath later sediment. Soils may be buried beneath natural deposits such as alluvial, aeolian, or colluvial sediments, or beneath anthropogenic deposits and structures such as middens, cultivation deposits, burial mounds, walls and banks. The term palaeosol has also been used to describe buried soils, but this is a less specific term as it also includes unburied relict soils, and exhumed soils. There is also an implied age limit to the term palaeosol, which it has been suggested falls logically at 10,000 years BP (Catt, 1987). The dark, organic A horizon of the buried soil is also sometimes referred to as a "turf line".

Importance of buried soils
Buried soils are relatively common in archaeological contexts and are an important source of archaeological and environmental information. As soils only develop in stable landsurfaces, buried soils are important as they represent former ground surfaces for human occupation. They also provide the basis for agricultural systems, hence buried soils are important repositoried of information regarding land management practices such as tillage and manuring.

At the field (macroscopic) level ard marks and rigs preserved in the buried soils offer direct evidence of agricultural activity, whilst the nature of the buried soil profile, as defined by the sequence of soil horizons, offer evidence of past environmental conditions and vegetation. A range of analytical techniques may also be applied to buried soils to elucidate the anthropogenic and natural processes involved in their formation. These include:
 * Soil pH
 * Phosphate analysis
 * Thin section micromorphology
 * Earthworm granules

Preservation of buried soils
Burial helps preserve the characteristics of old land surfaces by isolating them from the atmosphere effectively halting soil formation and isolating them from the effects of later soil development processes. However, isolation requires deep, rapid burial. If a soil is buried slowly through the gradual accumulation of deposits, in the early phases of burial the old soil profile may not be sufficiently deeply buried to be isolated form the effects of soil formation at the accumulating ground surface. The depth at which the soil is completely isolated from the surface depends on the texture of the overburden material, the nature of the soil forming processes, and the depth of the water table. The required depth of burial below heavy clay deposits is much lower than below light sandy soils (Wilson, 2000) and likewise if the watertable is close to the surface this may help isolate the buried soil from the effects of surface processes, in particular the penetration of oxygen which fuels microbes and decomposition processes. It should also be remembered that once buried preservation conditions can change to the detriment of the archaeology. For example, soil erosion can lower the groundsurface and bring the buried soil into the zone of influence of surface processes such as ploughing, rooting and bioturbation. However, whilst protecting the buried soil from further soil development, burial itself creates a whole new set of chemical, phyiscal,and biological conditions (the burial environment). Typical changes associated with this environment include the formation of iron pans in the buried organic horizons, compaction, waterlogging and the formation of iron and manganese nodules, decomposition of organic matter, mineral weathering, and new mineral formation (e.g. vivianite).

Identification of buried soils
The identification of buried soils requires the presence of a characteristic vertical set of soil horizons. The actual sequence of horizons will depend on the soil type prior to burial, but usually the upper most horizon is a darker, more organic horizon that may preserve a crumb structure and which represents the topsoil or A horizon of the old soil profile. Below this the soils tend to become less organic, rooting frequency declines, and stoniness may increase, there may also be evidence of soil features such as iron nodules, clay coatings and crystals of calcite in underlying horizons.

More information about typical horizon sequences can be found here.

More information about the identification of buried soils in the field can be found here.