Field Analysis:Is this deposit gleyed
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[edit] Has this deposit been affected by waterlogging (gleying)?
When soils and sediments are saturated with water, the air filling voids in the soil is replaced with water. In this situation oxygen may be used up by bacteria and fungi in the soil quicker than it is replaced by diffusion from the surface leading to the onset of anaerobic conditions. In anaerobic conditions, microbial activity is slowed down, resulting in decreased decomposition rates for organic remains and increased preservation. Anaerobic conditions also result in the chemical reduction of iron in the soil turning it from its relatively immobile ferric form to the more soluble ferrous form. In permanently waterlogged sites the result is the loss of iron from the soil and the development of grey and blue-grey soil colours.
Fluctuation of the watertable level over time, however, results in a zone of temporary waterlogging with alternating conditions of oxidation and reduction. The result is the movement and reprecipitation of iron within the soil producing a mottled soil colour (grey matrix with orangey, pinkish or reddish mottles), and discrete concretions (nodules and pans) or iron.
The importance of this for field archaeology is two-fold:
- Gleying is an indicator of good conditions for organic preservation, at least under current hydrological soil conditions.
- Gleying results in distinct changes in soil colour. Soil colour is such an obvious indicator of changes in soil properties that it is used to help define context boundaries. Gleyic changes can make it hard to identify sedimentary boundaries and overprinting by mottles can even be mistaken for different contexts.
[edit] Indicators of gleying
When attempting to identify the effects of gleying in the field a checklist of features that can be looked for/considered are:
- Waterlogged soil conditions - waterlogging generates the biochemical conditions necessary for gleying. Waterlogging may occur either because the deposit is affected by groundwater or because this or an underlying deposit is poorly drained (compacted, heavy clay texture, low porosity etc.) leading to the development of a perched watertable.
- Saturated soil - free water clearly visible in the soil is an indication of saturated soil conditions which may promote gleying. The absence of saturation, however, does not necessarily mean that the deposit has not been affected by gleying because alternating saturated / dry conditions can also lead to gleyic overprinting.
- The typical gley colour is blue-grey as this indicates reducing conditions.
- Mottles and nodules - Alternating reducing - oxidising conditions lead to the development of organge, pink, or reddish mottles in a greyish soil matrix. Typically these will be found in the centre of aggregates (soil peds) or around voids, roots, and organic matter. Where there has been strong segregation these mottles can develop into discrete cemented nodules or pans of iron (reddish, brownish or black in colour.
- Organic preservation - Good organic preservation in non-dessicating environments is an indication of anaerobic conditions.
- Change in soil colour on drying - Grey reduced soils and sediments in gley soils may develop a pinkish or reddish colour if left exposed to the air. This can take several hours to develop and will only occur if there is free ferrous iron remaining in the soil (see follow-on analyses).
[edit] Uncertainties in identifying gleying
- A number of natural and anthropogenic materials normally have grey colouration including ash and the bleached grey Ea horizon of podzolic soils. A change in colour on drying to a more pinkish or brownish hue is a clear indication that the deposit was gleyed, but a lack of change does not indicate that it was not. For distinguising gleying from podzolisation look at the identifying podzolisation page accessible by clicking here.
- Gley features are something that develop in response to current soil conditions (namely waterlogging). The stability of these features in response to changing conditions differs greatly. In many soils the features associated with gleying will at least partially disappear. However, in some cases (e.g. iron has been leached from the soil or where there are very strongly impregnated features) the features may remain even though the waterlogged anaerobic conditions that created them no longer persist.
- Mottling - soils can appear mottled for reasons other than gleying. For example, where there are multiple soil material types which have become imperfectly mixed together, or where artefacts or stones in the soil have degraded or partially degraded. Examining the texture, composition and mineralogy of the mottles and comparing it to main soil material can help to distinguish between these different mottle types.
[edit] How does the SASSA Field Tool make this interpretation?
A printable recording sheet for SASSA's 'Has this deposit been affected by waterlogging (gleying)?' can be found here
A high score tends to confirm that this context / deposit has been affected by gleying. To score highly you need:
- A poorly drained soil or one that has a poorly drained layer underlying it.
- A grey or greyish soil colour with the following (a reddish colour can score highly if the parent material is also red):
- A saturated or wet moisture content.
- Mottles of a pinkinh or reddish hue and of the same general composition as the surrounding soil.
- Nodules or pans of cemented iron.
- Exhibit a colour change on being exposed to air.
[edit] Follow-on analysis
In the event of an inconclusive score the following field analyses may help to provide an answer to this query.
