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Downloaded by MONASH UNIV on August 25, 2013 | http://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch010
Archaeological Soils and Sediments: Application of Microfocus Synchrotron X-ray Scattering, Diffraction, and Fluorescence Analyses in Thin-Section 1
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W . Paul Adderley , Ian A . Simpson , Raymond Barrett , and Timothy J. Wess 3
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School of Biological and Environmental Sciences, University of Stirling, Stirling FK9 4LA, Scotland, United Kingdom European Synchrotron Radiation Facility, BP220, 38043 Grenoble, Grenoble Cedex 9, France Biomaterials Centre, School of Optometry and Vision Sciences, University of Cardiff, King Edward VII Avenue, Cardiff CB10 3NB, Wales, United Kingdom 2
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Archaeological soils and sediments reflect the cultural environment in which they have been formed. Their analysis allows assessment of the nature and intensity of past events. With the results of such analyses playing an increasing role in forming archaeological interpretations, there is a need to verify optical analysis and interpretation of materials and to examine materials that are presently considered amorphous or unknown in conventional optical analyses. This paper discusses the use of microfocus sychrotron X-ray methods and the issues surrounding their application to archaeological soils and sediments.
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© 2007 American Chemical Society
In Archaeological Chemistry; Glascock, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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195 Soils and sediments are an integral part of archaeological sites. Materials range from soils found in field areas that have been managed in past times to the burial matrix of archaeological structures. Since such archaeological soils and sediments reflect the cultural environment in which they have formed, they can themselves be considered cultural artifacts. Examining these materials can be the key to understanding past agricultural and archaeological site activities thereby allowing interpretation of both the nature and intensity of past events. The study of soils and sediments has been developed using a variety of methodologies. Most common are procedures derived from geochemical prospecting methods: the elemental characterization of bulk sediment samples (7, 2) to geostatistical treatments of the resultant datasets (5). These methods, applicable at both site- and regional-scales (4), provide data that can be spatially related to large archaeological features. This has led to developments in relating chemical signatures to specific past activities (5) in a variety of contexts, particularly the use of elements considered relatively immobile in soils and sediments (6). The size of the samples used in these methods relative to the scale of the study can limit extension of these methods to more detailed contexts since, in such instances, analysis of bioarchaeological - sometimes called "ecofacts" - (e.g. bone, wood) and artifactual (e.g. metals, slags, clinker) materials found in these soils and sediments is commonly confounded. A t smaller scales, more refined signatures are generally required. There is also an increasing need to understand the nature of the soils and sediment matrix, in terms of spatial organization and of the preservation of the bioarchaeological and artifactual components. Analyses of microscale features of archaeological soils and sediments through optical thin-section micromorphology can achieve these more refined goals. The scale of analysis and the use of undisturbed samples allows the optical properties of bioarchaeological materials and artifactual materials to be characterized separately at a range of scales from ~5 to 1000 μπι. Furthermore, the spatial relationships between these components can be examined, thereby increasing the interpretative value of the methodology. Thin-section micromorphology has become an established method in archaeological studies and has been used in varied on-site contexts including: mixed materials in middens and during site formation (7-P); and occupation surfaces (10, 11). Past agricultural development surrounding archaeological sites has similarly been examined (72-75). Sample preparation requires undisturbed sampling of the soil or sediment, removal of water in the sample through acetone exchange, resin impregnation with polyester resin, and finally cutting, mounting, and polishing a thin-section on a glass slide. Optical examination is then performed using a polarizing microscope with descriptions and semi-quantitative frequency analyses following standardized international systems (16,17). Such studies sometimes use
In Archaeological Chemistry; Glascock, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
Downloaded by MONASH UNIV on August 25, 2013 | http://pubs.acs.org Publication Date: August 16, 2007 | doi: 10.1021/bk-2007-0968.ch010
196 complementary bulk chemical (18) and physical analyses (8) to confirm or augment initial optical microscope results. Such validation is essential, especially as the subject area of archaeological soil micromorphology develops, with the consequent expectation of increased interpretative value sought from the methodology. The logical extension of combining optical observation with chemical and physical measurements is to develop new understandings of the type and nature of the bioarchaeological and artifactual materials in the sediment matrix, extending from detailed chemical characterizations to process-based concepts of material alteration. Knowledge of the use of archaeological materials examined can be greatly enhanced by understanding the processes involved in their formation or degradation. Experimental archaeology methods, where material sources are examined for comparison with archaeological features, can be employed. For example, experimental combustion and examination o f the resultant ash and fuel residues has led to major advances in understanding changes in past resource utilization (7). This approach requires knowledge of the likely source of the materials studied and of the processes involved in their production. Where this knowledge is absent and materials seen in thin section are of unknown or speculative origin, or, alternatively, where the conditions required to generate the material are not practicable to replicate in a controlled experiment, alternative prospecting methodologies are required. High-resolution compositional measurements are possible through use of a variety of microanalytical methods. Ideally, these should be non-destructive, can be targeted on small areas of sample, and have low minimum detection limits. Electron-probe X-ray microanalysis ( E P X M A ) and proton-induced X-ray emission (PIXE) techniques have both been used successfully on archaeological sediment thin sections (19, 20). Both techniques yield elemental composition data for a range of elements. E P X M A has the advantage of being non destructive, whereas PIXE when used on thin-section samples is typically destructive; conversely the detection limit for PIXE is lower than E P X M A . Several other microanalytical methods in common use potentially have application on soil and sediments section samples. Laser-ablation inductively coupled plasma mass spectrometery (LA-ICP-MS) has been used on soil thinsections from a controlled field experiment (21) but required special resins in the preparation. There is presently (May 2006) no reported use of this method on archaeological soil samples. Likewise, for extremely fine-resolution studies (i.e.
