Analytical Methods:Redox Monitoring

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Degradation processes linked to soil redox potential
The availability of oxygen in the soil is an important variable in the preservation of archaeological artefacts, organic matter and stratigraphy as anaerobic conditions inhibit microbial and worm activity. The oxidation potential (or conversely reduction potential) is measured as redox potential and is affected by isolation from the atmosphere, soil drainage/wetness (waterlogging produces anaerobic conditions), organic matter content and biological activity (particularly microbes and roots). Dewatering of deposits through drainage, irrigation, or mineral extraction is particularly damaging as previously anaerobic deposits are exposed to oxygen resulting in increased microbial attack of fragile organic remains.

In general, it is assumed that a reducing environment is favourable for archaeological preservation. However, Hopkins (2004) warns that a low redox potential can mask degradation processes as anaerobes can operate in reducing environments and oxygen could still be reaching the deposit where it is rapidly utilised by aerobes.

Approaches to monitoring soil redox potential
Measurement of redox potential can be problematical as any disturbance of deposits can result in oxygen penetration; in-situ measurements are preferable to sampling. The redox conditions of a soil can be estimated from the soil morphology, but this is an insensitive measure as relict features of previous oxidation states can be preserved in soil. In-situ redox probes can be installed in soil profiles to measure soil characteristics, however, this involves excavation which is undesirable in areas of high archaeological potential and they provide localised measurements of this spatially and temporally variable property.

Deep dipwells with sampling or divers could be used to measure dissolved oxygen concentrations in groundwater. Dissolved oxygen concentration does not equate directly with redox potential, but it could be useful as a proxy measurement at a national or regional scale. However, dipwells cannot measure directly conditions in soil above the ground watertable or in perched watertables.

Redox measurements should be interpreted alongside watertable fluctuations, soil pH and soil organic matter content in order to model the likely response of the cultural heritage resource to changes in soil quality.

Sampling strategies
Soil redox potential can vary greatly over very short distances depending on soil moisture, roots, microbial activity, depth and organic matter content. Organic archaeological remains have the potential to create localised redox conditions. The meausrement of site wide redox, therefore, can be problematical. A vertical array of probes or manual measurements can give an indication of how redox changes with depth and by taking measurements alongside dipwells the effect of watertable fluctuations can be better understood. However, generally the placing of redox sampling points should be made on a site by site basis (Smit et al., 2006) and the number of sampling points is likely to be constrained by cost.