Analytical Methods:Micromorphology

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Micromorphology
In contrast to bulk soil analyses, soil micromorphology allows a trained user to examine the physical structure and organisation of the soil or sediment. The analogy commonly used is that if you take a pocket watch and dissolve it in acid you will find out what it is made of, but not what it was for, how it worked, or how it was made. To understand this you have to examine an intact watch.

Archaeological questions
Micromorphology can be an extremely useful technique for identifying formation processes for deposits and sequences of deposits. This may include pre-occupation soil formation, sedimentary processes (natural and anthropogenic) of deposition, and processes of post-depositional alteration. Micromorphology can be very helpful in identifying the individual components (natural and anthropogenic) of a deposit. It has proven to be very helpful in interpreting deposits where there is microstratigraphy such as alluvial deposits and in annually applied floor and wall surfaces. As such it can help to elucidate construction methods and analysis of floor layers can help interpretation of site use and identification of activity areas.

Examples of studies where micromorphology has been successfully used include:
 * Soil erosion at Marliani, Amorgos in Greece
 * Post-burial clay and iron movement in ring barrow sediments

Interpretation of thin sections relies on the presence and organisation of certain features in the soil. The formation and persistence of these features depends on the type of feature and the development of the soil since it was formed. Topsoils for example rarely produce useful information because bioturbation (mixing by earthworms) and ploughing destroy earlier features and structures. Your specialist will be able to advise on the likely usefulness of micromorphology for your site based on the questions you want to address. The draw back of micromorphology is that it can be time consuming and costly, talk to your specialist to ensure that it is the most appropriate technique. If, for example, you are more concerned with identifying a buried soil rather than interpreting the nature and management of that soil, quicker cheaper chemical analyses may give the same level of information.

Sampling
Micromorphology can be costly so it is a good idea to involve a specialist before samples are taken. They will be able to recommend a sampling pattern that minimises sample numbers whilst ensuring that sufficient are taken to address the questions that are being asked.

In general samples should be representative of the deposit in question. Sampling across boundaries can provide important information on depositional and soil forming processes and also reduces the total number of samples needed and hence the cost. However, it is not always appropriate; for example, when boundaries are very diffuse the sample may not provide a good picture of either deposit.

Micromorphology samples consist of undisturbed blocks of soil or sediment cut from the face of a profile. These samples must be supported to prevent disturbance during, sampling, transport and storage. Kubiena tins are commonly used for this purpose as they have detachable lids, are easily removed during sample processing, and being aluminium they don’t rust. These can usually be obtained from the processing laboratory or specialist.

Samples are cut from freshly cleaned section faces. One lid of the kubiena tin is removed and a knife is used to cut away soil materials outside the tin so that the tin can be gently pushed into the face. Once the tin is filled the orientation (an arrow showing which way is up) and sample number is marked on the tin, and it is cut out of the section, excess material is removed and the second lid fitted and secured using elastic bands and/or plastic food wrap. Samples should be processed as soon as possible, but if they must be stored they should be kept refrigerated.

Kubiena samples can also be taken from larger monolith samples. These are collected in the same way as kubiena samples using metal monolith tins or standard square profile plastic guttering cut to length and then wrapped in plastic food wrap.

Sample preparation and analysis
Specialist processing is required to produce a thin section; usually the methods of Murphy (1986) are followed. Processing involves drying the sample in acetone vapour and impregnating it with resin. The cured block is then sliced and bonded to a glass slide. The excess sample is cut away and the remaining sample is lapped leaving a 30 micron thick sample attached to the glass slide. This then needs polishing and coverslipping. Your specialist should be able to recommend a laboratory to prepare the samples.

Soil thin sections are examined using petrological microscopes, usually at magnifications of between x10 and x200. Descriptions follow the terminology of Stoops (2003) and a specialist (micromorphologist) is required to describe and interpret the sample. This is essentially a subjective or semi-quantitative process and the results are interpretations rather than statistically valid statements.

Other complimentary techniques such as can be used to give quantitative data where necessary. These include:
 * Point counting,
 * Image analysis,
 * Micro-analysis

More information on thin section manufacture and description can be found on the following websites:
 * Stirling University thin section laboratory website
 * North Caroline State University Micromorphology webpage

Data and Interpretation
Micromorphological descriptions usually consist of a written description and summary table or diagram. These in themselves are of little interest to an archaeologist as specialist terminology is used. It is the interpretation placed on the section by the specialist that is of interest to the archaeologist. For this reason it is important that the specialist has a clear idea of what are the key archaeological questions.

In many ways the interpretation of a set of thin sections is very similar to the stratigraphic interpretation of an archaeological site. In some instances there may be more than one possible interpretation. Micromorphology can be an extremely useful tool, but its success relies on the skill and experience of the specialist and the communication between specialist and archaeologist.

Related Techniques

 * Image Analysis
 * Micro-analysis

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