The first systematic study of the relationship between enhanced soil phosphate and past human settlement was by Olaf Arrhenius in Sweden (Arrhenius, 1931, Arrhenius, 1934), although apparently the connection between phosphate and past settlement had been noted as early as 1911 by Hughes whilst working in Eygpt (Bethell and Máté, 1989). Arrhenius conducted systematic surveys in Skåne, southern Sweden, whilst working for a sugar beet company, and published his results relating to archaeology from the late 1920’s and on into the 1960’s. The method was employed and adapted elsewhere, for example by Walter Lorch in Germany, and although initial uptake was slow, by the 1960’s published studies from Europe and the U.S.A.
began appear thick and fast (Cook and Heizer, 1965). It is misleading to think that phosphate analysis at this time was the only chemical analysis employed on soils by archaeologists. As Cornwall (1958) details in Soils for the Archaeologist, there were a wide range of available wet chemistry techniques available to identify single element concentrations, however many were either qualitative, laborious, or both. In a short article by Lutz (1951), the enhancement of P, N, Ca and K over old settlements in Alaska was noted, however the sample number was small and the spacing between samples 50 feet (15.24 m), presumably to limit the time and cost of the analysis, but perhaps also as the research question was simply to measure the properties of the observed enhanced soil. Whilst phosphate analysis was also primarily qualitative, it was quick and affordable, and unlike other elements connected to human activity, offered a single element that could capture a wide range of activities with repeated success (Holliday and Gartner, 2007).
Methods improved, as well as the understanding of the factors that influence phosphate retention in soils. In an extensive and thorough article by Cook and Heizer (1965), based on numerous sites in the U.S.A. and Mexico, sampling was employed on different scales, on highly varied soils, and both vertical and horizontal retention of phosphate in the soils was related to archaeological evidence. In many of the case studies, several elements were quantified (Ca, P, C, N) as well as organic content. The interconnection between these factors, the different
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elements and the soil properties was stressed, suggesting that measuring just one variable, such as phosphate, could lead to misinterpretation. Entwistle et al. (1998) also found P alone an unreliable source for human settlement patterns. Phosphate, as the elements is present in a wide range of organic and inorganic materials utilised by humans, often cannot distinguish between past activities.
Published in 1973, Eidt’s short article on phosphate spot testing is widely referred to. The method is a rapid, qualitative test for soils using inexpensive reagents (hydrochloric acid, ammonium molybdate) and an ascorbic acid reducing agent in a two-step process (Eidt, 1973).
This method was an alternative to the previously widespread use of ammonium sulphate as a reagent, or the method used by Provan (1971) (see next section), which was also developed primarily for agriculture. Published in Science, in 1977, Eidt’s improved method offered both qualitative and quantitative methods for enhanced results, and specifically refers to the use of phosphate testing in archaeology (Eidt, 1977). The paper outlines extracting phosphate fractions retained by differing mechanisms within the soil, and also the alternative, qualitative, quick spot test that became widely used in archaeology as a means of understanding the past use of space on micro and macro scales.
Not all published studies undertook spot testing alone. Conway’s analysis of a Romano-British settlement used a complicated analysis to determine the proportions of extractable phosphates compared to total phosphates in occupation deposits, the results determined by colourimetry (Conway, 1983), and more recently, likewise Hutson et al. (2009) employed fractionated phosphate to determine potential sources.
Phosphate analysis, by the 1980’s, had decades of research and refinement, and as a result, a plethora of extraction and analytical methods had been applied to archaeological sites. The widely divergent approaches, from quick, in situ spot tests (Bakkevig, 1980), to Conway’s quantitative total extraction, mainly stems from the uncertainty in our knowledge of the phosphate cycle and its relation to archaeological samples in varied environmental conditions (Conway, 1983). Published in 1989, Bethell and Máté‘s thorough dissection of the topic is still very much relevant in the subject of archeologically geochemistry, and is still widely referred to (Linderholm, 2007, Oonk et al., 2009a). The problems identified in the critique are many, such as the lack of temporality in phosphate mapping results, particularly when used as a topsoil prospection technique. Multi-phase aspects of a site are lost or blurred, although attempts have been made to relate relative proportion of available and unavailable phosphate to chronological changes (Beach, 1998), and equally post occupation land use can affect results (Gjerpe and Samdal, 2005). In addition, despite the theory that organic phosphates added to the soil quickly
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mineralise and become ‘fixed’, there is a volume of evidence that suggests mobility is a problem in certain environmental conditions (Crowther, 1997, Craddock, 1989, Cannell, 2013). The paper by Bethell and Máté (1989) is now perhaps a little outdated, if, and only if, one considers multi-element analysis to have superseded single multi-element analysis. Clearly, not all deem this to be the case, as phosphate analysis continues in archaeology in commercial and research projects, which has ever expanded the number of applied analytical methods, which were thoroughly considered by Holliday and Gartner (2007). However, Bethell and Máté (1989) do state, that without multi-elemental approaches in the future, the development of feature specific geochemical interpretations are potentially limited. Unsurprisingly, therefore, Bethell and Smith (1989), published a multi-elemental approach the same year, based upon work on burials at Sutton Hoo in 1987.
Although Bethell and Smith (1989) was not the first multi-elemental application using a single instrument in archaeology, (e.g. Keeley et al., 1977), is remains an excellent example for integrated, planned sampling within an archaeological excavation strategy. The paper does not shy away from some of the key questions in archaeological geochemistry, such as the use of background sampling, sampling methods, elemental mobility, the effect of local environmental conditions, and inter-site comparability. In addition, because of its early use of ICP-AES, it inevitably had to include the discussion of appropriate extraction techniques for sample preparation. In the intervening years since Bethell and Smith (1989) published their Sutton Hoo study, the use of ICP, particularly ICP-MS, has increased, becoming most commonly used instrument for multi-elemental approaches. In the 1990’s, the number of published studies began to grow (Linderholm and Lundberg, 1994, Entwistle and Abrahams, 1997, Entwistle et al., 1998, Wells et al., 2000, Middleton, 1996, Rimmington, 1998, Aston et al., 1998a, 1998b), and the expansion continues to this day.
In tandem with the small, but growing number of published studies, the understanding of the differential retention mechanisms within soil improved. Certain elements seemed to be repeatedly enhanced in archaeological contexts, such as calcium (Ca), potassium (K), magnesium (Mg), and of course P (Middleton, 1996, Entwistle et al., 2000). In addition, strontium (Sr) was particularly associated with food preparation, alongside P and Ca (Middleton, 1996, Milek and Roberts, 2013), whilst hearths were associated with these elements and zinc (Zn), K, Mg. Copper (Cu), lead (Pb), barium (Ba), iron (Fe), aluminium (Al), and sodium (Na) appear to be less universal and more site specific indicators (Knudson et al., 2004, Wilson et al., 2007, Vyncke et al., 2011 Milek and Roberts, 2013). Rather than wade through what each element has been associated with by whom, it is more fruitful to consider that whilst a handful of elements (Ca, P, K, Sr) have more universal application, most sites need to be seen as unique. Figure 2.3 shows
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the complex relationship between input, retention and exchange within the soil, the dominance of the different systems, and the environmental dependence of retention mechanisms. Nor is it solely enhancement that is measured, relative depletion of certain elements has also been observed. For example, Oonk et al. (2009b) and Vyncke et al. (2011) highlight that relative depletion due to organic loading of the soil or high trafficked areas is also a factor that applies to archaeological contexts.
Figure 2. 3. Diagram of the various pathways and pools of trace elements within a soil, with the soil solution as the dominant means of various forms of elements entering and interacting within the soil. Modified from Tack (2010), figure 2.1.
In summary, in the many decades since archaeological geochemistry was instigated as a tool in archaeology for prospection and understanding settlement morphology, a plethora of extraction and instrumentation approaches have been tried. Starting with wet chemical extraction of available phosphate through to the use of ICP to simultaneous detect up to seventy elements, the method has been applied on a wide range of archaeological sites. That said, there are still gaps in our knowledge, technological issues and challenges facing the use of major and minor elemental concentrations’ in the soil as a window into past human activity. Further past and present challenges are considered in the next section and chapter 3.