2.4 Isotope Analysis

Stable isotope techniques are based on the principle that human and animal body tissues will reflect the isotopic composition of food and water ingested during their lifetimes – ‘You are what you eat’. Isotopes are different versions of the same elements, and characteristic patterns or isotope signatures can be useful in determining dietary habits, and the location or environment in which a human or animal inhabited during life. In archaeological case studies, analysis most commonly involves the samples of preserved hard tissues including bone, tooth enamel and dentine, but can also include soft tissues such as hair or fingernails in certain instances of good preservation. Analysis involves both the inorganic components of hard tissues (also known as bioapatite) but also preserved proteins such as collagen which may be preserved in bone or tooth dentine and can be extracted through the removal of the mineral component (demineralisation) and gelatinisation.

Carbon and nitrogen analysis of collagen (bone and dentine)

The most frequently used technique in archaeology involves the analysis of ratios of carbon and nitrogen isotopes in bone collagen (δ¹³C and δ¹⁵N). Data from controlled feeding studies and field experiments have confirmed that bone collagen and other body tissues, such as hair (keratin), reflect dietary inputs and that these techniques can provide a direct measurement of diet (Ambrose 1993). Bone remodels during life and therefore bone collagen can be used to determine long-term dietary trends prior to death (~5-10years) (Hedges et al. 2007). Tooth dentinal collagen can be used to assess short-term changes that occurred during childhood, as these tissues form in early life and undergo little remodelling (Gage et al. 1989). The relative abundance of the stable isotopes of carbon, ¹³C and ¹²C (δ¹³C), vary between different habitats, for example, between plants of different photosynthetic pathway such as C4 plants (tropical grasses including maize and sorghum) and C3 plants (including all trees, bushes and temperate grasses) (Smith and Epstein 1971; DeNiro and Epstein 1978). Carbon isotope ratios also vary between terrestrial and marine ecosystems (Schoeninger et al. 1983).

The stable isotope ratio of ¹⁵N and ¹⁴N (δ¹⁵N) increases by 3-5% with each step up the foodchain, and is therefore most commonly used to indicate the trophic level of protein consumed (Bocherens and Drucker 2003). Given that aquatic food chains are often far longer than terrestrial ones, nitrogen isotope data can also be used to distinguish between marine/freshwater and terrestrial diets (Richards et al. 2001). Nitrogen isotope values of animals at the same trophic level are also influenced by a range of other variables, and are often related to local environmental conditions (e.g. water availability, temperature, use of fertilisers, stocking rate, salinity, coastal proximity, biome) as well as physiology (see reviews in Hedges and Reynard 2007, Britton et al. 2008).

The analysis of the carbon and nitrogen isotope ratios in bone collagen involves the sampling of a small amount of bone or tooth dentine (<1g), sample cleaning and demineralisation to remove the mineral matrix. The sample is then gelatinised and purified through filtration. The gelatinised protein is dried, weighed into tin capsules (0.5-1mg in weight), and are analysed using mass spectrometry.

Strontium and oxygen analysis of bioapatite

Although less common than the analysis of bone collagen, the other most frequently used type of stable isotope analysis in bioarchaeology is the strontium and oxygen isotope analysis of dental enamel. Enamel, like bone, is formed of a material known as bioapatite, however, unlike bone, the densely packed crystalline structure of enamel makes it preferential for this type of isotope analysis as it less resistant to undergoing chemical changes in the burial environment (known as diagenesis) (Hoppe et al. 2003). Unlike bone, tooth enamel is not remodeled during life, and therefore the isotopic composition of tooth enamel is directly related to environment and diet during dental formation (i.e. during childhood). Therefore, isotope data determined from dental enamel can be compared to the local environmental and geological parameters, identifying indigenes and potential immigrants. The strontium (⁸⁷Sr/⁸⁶Sr) isotope ratio of enamel is directly related to the ⁸⁷Sr/⁸⁶Sr ratio of ingested plants and, ultimately, that of the soils and water available to the plants. This has been demonstrated to correlate with underlying geology and been used to explore human and animal mobility in the past (see review in Bentley 2006). ⁸⁷Sr/⁸⁶Sr analysis involves the sampling and dissolving of small section of tooth enamel (<20mg) in a strong nitric acid in a high-grade clean laboratory. A series of steps are then undertaken in order to isolate and extract dissolved strontium from the sample solutions. Samples are analysed using mass spectrometry. Another method, utilising laser ablation techniques, can be used on intact samples without the need to dissolve samples prior to analysis. This is virtually non-destructive, although measurements may not be as precise as with ‘traditional’ solution methods.

Oxygen (¹⁸O and ¹⁶O, or δ¹⁸O) isotope ratios of tooth enamel can also be used to help identify the place of origin of individuals, as the oxygen isotopic composition (δ¹⁸O) of body water is directly related to meteoric water consumed during life (D’Angela and Longinelli 1990), which is in itself closely related to local temperature (Dansgaard 1964). Therefore, when used in tandem, these methods can help to pinpoint likely areas of geographical (geological) and climatic origin of individuals. Given that teeth form incrementally, analysis of sub-samples of enamel for either oxygen or strontium isotopes can be used to gain time-series information (e.g. Balasse et al. 2005, Britton et al. 2009) Oxygen isotope ratios are commonly determined through the analysis of dental enamel, including the powdering of a sub-sample of enamel (<20mg) and subsequent reaction with ortho-phosphoric acid. This CO₂ produced is then analysed for carbon and oxygen isotope ratios using mass spectrometry, measuring the bulk oxygen in a sample. However, another method, in which the phosphate-only component of the bioapatite crystal is first isolated (through the formation of silver phosphate) and then analysed, may also be used. This method is sometimes favoured on the reasoning that the phosphate component of the bioapatite crystal is better protected against diagenetic alteration.

Archaeological applications

Isotope analysis is now playing a routine role in many archaeological projects and is a key method utilized for investigating human diets and subsistence strategies, animal husbandry practices, movements and migrations, and individual life histories (through the analysis of incrementally developed tissues). By far the most common application of isotope analysis to archaeological case studies is the analysis of the ratios of the stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) to archaeological bone collagen to reconstruct long-term dietary history of individuals. In other areas of the United Kingdom, this has been applied to human remains from the Palaeolithic (e.g. Richards et al. 2000, Schulting et al. 2005, Stevens et al. 2010), through to the Mesolithic (e.g. Schulting and Richards 2002a, Schulting and Richards 2002b), Neolithic (e.g. Richards and Hedges 1999, Richards 2000, Richards et al. 2003) and later Prehistoric periods through to the Roman (e.g. Chenery et al. 2010, Fuller et al. 2005, Jay et al. 2008, Redfern et al. 2010, Richards et al. 1998) and Medieval periods (e.g. Fuller et al. 2003, Muldner and Richards 2005, Muldner and Richards 2007b, Richards et al. 2002, Roberts et al. 2010). There have also been a small number of published diachronic studies, assessing dietary change at the same location through time (e.g. The City of York, Muldner and Richards 2007a). A small number of studies have also utilized these techniques to investigate other aspects of human life histories and population demography, such as the age of weaning in the late Medieval period (Fuller et al. 2003, Richards et al. 2002).

Although applications are growing, a far smaller number of studies have incorporated strontium isotope analysis (⁸⁷Sr/⁸⁶Sr) of bioapatite (normally tooth enamel) – a method utilized to explore geographical origins of individuals and movements they may have undertaken during their lifetimes (e.g. Chenery et al. 2010, Eckardt et al. 2009, Evans et al. 2006a, Evans et al. 2006b, Montgomery 2002). The reasons for the smaller number of such studies (compared to other isotope approaches) largely relates to analytical costs. Strontium isotope studies are often conducted alongside stable oxygen isotope analyses (δ¹⁸O), in order to provide complimentary climatic or palaeotemperature proxies to ⁸⁷Sr/⁸⁶Sr data to better establish geographical origins of individuals (e.g. Eckardt et al. 2009).

In addition to applications of isotope analyses to human remains, these techniques have also been applied to zooarchaeological materials in order investigate the palaeoecology and biogeography of archaeological-important prey species (Britton et al. 2009), as well as foddering and other husbandry practices employed by human populations while raising domesticates. This has included the successful identification of animal husbandry practices rarely found in modern contexts such as salt-marsh grazing (Britton et al. 2008) and the seasonal movements of animals between different types of pasture (Evans et al. 2007). Animal isotope data – whether wild or domesticate – is also a vital component of any human isotope study in order to establish a suitable ‘baseline’.

Previous Isotope Archaeology Studies in Scotland

In comparison to other areas of the UK, there are have been relatively few published stable isotope studies conducted on Scottish archaeological material, and even fewer that have been led by archaeological research groups within Scotland. Although there are some obvious issues with the availability of material from certain time-periods (e.g. the scarcity of skeletal material from the Palaeolithic and Mesolithic), and issues of preservation (due to often acidic soils), the amount of work conducted in Scotland is certainly not representative of the quantity of archaeological material excavated or adequately addressing the key issues of modern Scottish archaeological investigation. Much work to date has focused on the Islands, including Orkney and the Hebrides.

e.g.

• Mesolithic-Neolithic dietary transitions in Orkney (Mieklejohn et al. 2005, Richards 2004, Schulting and Richards 2002c)
• Seaweed foddering in Neolithic (and modern) sheep the Orkney Isles (Balasse et al. 2005, Balasse et al. 2006, Balasse et al. 2009)
• A single diachronic dietary study (Iron Age, Viking and Late Medieval) in Newark Bay, Orkney (Richards et al. 2006)
• Hebridean migrations including origins of Norse communities (Montgomery et al. 2003, Montgomery and Evans 2006)
• Strontium isotope mapping of the Isle of Skye (Evans et al. 2009)
• Isotope analysis of archaeological faunal material on South Uist (Mulville et al. 2009)

There has been a limited amount of published isotope work conducted on archaeological material from the Scottish mainland, including a study of the diets and origins of the Bishops at Medieval Whithorn (Muldner et al. 2009).

A recent project funded by the US National Science Foundation, headed by T. Douglas Price, involving Janet Montgomery (Bradford) amongst others, is utilizing Scottish material in a larger project looking at Viking migration in the North Atlantic. Some of this material (e.g. that from the Hebrides) has already been published, along with that from other areas of the North Atlantic (e.g. Iceland, Price and Gestsdóttir 2006).

Another recent large-scale AHRC-funded project, The Beaker Isotope Project, assessing mobility, migration and diet in the early Bronze Age, also incorporated a limited amount of Scottish material. To date, much of this material is being prepared for publication.

Future Areas of Focus

More studies

Given the potential of isotope methods to explore ancient life-ways, there is a clear need for more isotope studies on Scottish material. General awareness of the potential of isotope analysis of animal or human remains should be promoted, and there inclusion in bioarchaeological studies should be endorsed. Although it should be noted that such studies be conducted by specialists (i.e. isotope archaeologists), or in collaboration with those specialists, it is essential not only that scientific analytical techniques are robust, but also that data are interpreted within in a robust archaeological framework by those who thoroughly understand the limitations of isotope archaeology, understand the processes of archaeological investigation and appreciate the archaeological relevance/setting of the material under investigation.

Growth of isotope archaeology research groups, laboratories and collaborations within Scotland

England now hosts several isotope archaeology research groups with dedicated in-house analytical facilities at a number of universities (e.g. Oxford, Cambridge, Reading, Bradford). These, along with other global research centers such as those at Harvard University, University of British Columbia and the Max Planck Institute for Evolutionary Anthropology, Leipzig, are also testament to how successfully dedicated isotope archaeology facilities can produce research of international significance. No such research groups currently exist in Scotland. This is highlighted as an area of potential development, alongside further effective integration with isotope work at environmental science (SUERC) and agronomic research centers (e.g. University of Dundee), and the forging of international collaborations.

At SUERC, analysis of δ²H, δ¹³C, δ¹⁵N, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr. ²⁰⁶Pb/²⁰⁷Pb/²⁰⁸Pb isotopes for topics such as climate, diet, population movement, and source provenance is available. Analysis of δ³⁴S for dietary work is currently being developed and should be available by autumn 2012.

To date, of the published isotopic archaeology studies conducted on Scottish material, it is notable that few of these studies were undertaken by Scottish research teams or in Scottish laboratories.

Identification of key issues and research frameworks

In additional to the above, it is also clear that there are numerous aspects of Scottish archaeology that would be particularly benefit from isotopic investigations and areas for future focus, including:

Movements:The fundamental research carried out by Jane Evans (NERC Isotope Geoscience Laboratory, Keyworth) and others, has produced a general map of ‘bioavailble’ strontium for the British Isles and more detailed maps of certain regions of Scotland (Evans et al. 2009, Evans et al. 2010). These studies go beyond traditional geological mapping, and provide the framework in which archaeological investigations can take place. There is much potential in Scotland to utilize geographically/geologically-varying isotope systems (⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ³⁴S) to explore human and animal mobility. Given that bulk and incremental tooth samples can be taken, there is the potential for research to take place at a variety of scales, from within lifetime movements, landscape use (exploring aspects of animal husbandry and transhumance) and general population mobility to larger issues such as colonisation, and modern and ancient diasporas.

Diachronic changes in diet and health:In addition to site-specific dietary studies, there is a need for multi-period dietary studies in Scotland. Diachronic stable isotope studies (carbon and nitrogen) are very effective ways of assessing changes in diets and subsistence strategies through time, especially when combined with osteological and zooarchaeological approaches. Unlike other approaches, long-term (e.g. bone collagen) and short-term (e.g. dentine collagen) bulk dietary trends can be established using isotopic methods – in both individuals and on population levels. These methods lead to directly comparable geographically and temporally comparable data sets, and are an effective way of looking at population level changes through time – changes that can be correlated with larger scale social and political changes (e.g. mass population movements, land clearance, etc)

Animal husbandry and subsistence strategies:One growing area of isotope archaeology is a focus on ancient production systems, and the identification of animal husbandry and management practices. Analysis of zooarchaeological materials is not just essential for interpreting human dietary isotope data, but has become an established sub-discipline. Aside from a small number of notable examples (Balasse et al. 2005, Balasse et al. 2006, Balasse et al. 2009), there is a clear need for site/period specific projects, diachronic studies and broad-scale investigations in Scottish archaeology. Given the northern latitudinal extreme of the country, and the unique and contrasting nature of the landscape and ecosystems (e.g. islands, salt-marsh, mountains, coasts, heathland), Scotland represents a unique opportunity to investigate human adaptation and flexibility.

See also the ScARF Case Study: The Medieval Bishops of Whithorn