Tutorial:Burial Environment

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The burial environment
Besides increased physical pressures, chemical and biological conditions in buried deposits are likely to be very different to those in an equivalent deposit at the surface, which is exposed to the air.

Chemical changes result from isolation from the atmosphere and the influence of groundwater. This can affect soil pH and oxygen availability (redox potential). This is important for archaeological preservation because ‘redox potential and pH define threshold conditions beyond which materials are unstable.’ (Banwart, 1998).

Soil moisture is often higher in buried deposits as they are flooded seasonally or permanently by groundwater. Even relatively shallowly buried soils can become waterlogged if a perched water table develops. Together with isolation from the atmosphere, this can result in the development of anaerobic conditions as oxygen is quickly used up by soil micro-organisms feeding on organic matter. Anaerobic conditions are usually beneficial for archaeological preservation as most soil organisms can’t survive. Although anaerobic microbes can thrive the overall rate of degradation tends to be slowed.

Soil pH can fluctuate widely both spatially as the composition of the soil is heterogeneous, but also over time. Wetting and drying, and microbial activity in the burial environment are likely to increase acidity (lower soil pH), which can result in increased chemical weathering and solubility of some chemical compounds. Soil pH is also influenced by the overlying deposits. For example at Beeston Castle, Cheshire where the pH of a buried soil had been raised by the down profile leaching of base rich soil waters from the overlying mortar-rich deposits (Macphail, 1987).

The soil biota differs from that typically found in surface horizons. Tap roots and deep dwelling earthworms may still penetrate, but the biota is dominated by micro-organisms, typically those that can survive or thrive in anaerobic conditions. Where channels and roots do penetrate oxygen levels can be raised resulting in localised areas of degradation. Likewise if watertable levels fall, the exposed deposits may begin to degrade again as the soil become aerated.

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