Tuesday, 11 July 2006

An Evaluation of Lead Isotope Analysis to Identify Anthropogenic Sources of Lead in Historical Farm Soils.

Clare A. Wilson1, Jeffrey R. Bacon2, Donald Davidson1, and Malcolm Cresser3. (1) University of Stirling, School of Biological and Environmental Sciences, Stirling, United Kingdom, (2) Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, United Kingdom, (3) University of York, Environment Department, York, YO10 5DD, United Kingdom

Multi-element analysis of soils and sediments from archaeological sites have shown that former human activity can impact the soil resulting in elevated element concentrations. In particular the results of a recent study by the authors have highlighted the correlation between enhanced levels of lead (Pb) and functional areas on post-medieval farm sites abandoned within the last 65-150 years. These signatures are an important resource potentially providing information on past management practices. However, even where local reference materials have been geochemically fingerprinted it is difficult to identify Pb sources because of the diversity of inputs (e.g. alloys, paints, glass, fuel materials), modern atmospheric deposition, and mixing of elemental signatures in the soil.

A preliminary study of the potential of lead isotope ratios to differentiate between Pb sources used TIMS analysis to determine ratios of 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 206Pb/207Pb, 208Pb/207Pb and 208Pb/206Pb in soils from the house, hearth, byre, kailyard, and arable fields of an abandoned croft in Shetland, Scotland. Local fuel (turf and coal), plaster, and soil parent materials (sand and till) were also analysed. The results show clear differences in Pb isotope ratios between the reference materials, and between the soils of the different contexts.

One the basis of their Pb isotope ratios, the soils from this site divide into two clear groupings (Figure 1), which can be modelled using linear mixing models. The first grouping consists of the hearth, house overburden and house floor soil (y= -0.77x + 2.79) and has end members of coal and turf and an unidentified input. The second consists of the byre, garden and arable soils (y= -0.49x + 2.19) and has end members of the hearth material and local wind blown sands. These two groupings reflect the known history of the site and expected material streams through these contexts.

These initial results are positive and suggest that more research is justified to fully evaluate the technique as a means of sourcing imported anthopogenic Pb in historical and archaeological soils as a means of better understanding former soil management practices.

Figure 1: Bilateral plot of 206Pb/207Pb against 208Pb/206Pb for soils and reference materials from Shetland croft.

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