By Richard Madgwick
Isotope analysis represents one of the most exciting innovations in environmental archaeology in the last 30 years and is now a crucial tool for palaeoclimatologists, human osteologists, zooarchaeologists, malacologists and archaeobotanists. Initially pioneered by Michael DeNiro in the 1970s, the discipline is now one of the fastest developing in environmental archaeology, with techniques being constantly refined and new, innovative applications being developed.
In brief, isotope analysis involves the measurement of the ratio of certain isotopes of particular chemical elements. Mass spectrometry is used to measure ratios, through separating isotopes on the basis of their mass to charge ratio. Ratios are frequently expressed as delta values (δ), which are parts per thousand (‰) differences from an internationally accepted standard.
Isotopes of carbon (δ13C), nitrogen (δ15N), strontium (87Sr/86Sr) and oxygen (δ18O) have undergone the greatest research and are the most firmly established analytical tools. All four approaches are most frequently used for analysing human and animal remains (including mollusc shells), but are becoming an ever more potent tool for archaeobotanists. Carbon and nitrogen isotope analysis of collagen is an established method for reconstructing diet in past human and animal populations. The nitrogen ratio provides an indication of the trophic level at which an individual is feeding, with diets heavy in animal or marine protein being more enriched in the heavier isotope (15N). The carbon ratio establishes the degree of marine input in feeding and can also indicate whether an individual draws their food from an arid area. Although this is a gross oversimplification of the complexity behind variation in ratios, it is these variables that have the greatest impact. Analysis of the heavier isotopes of strontium is applied for establishing the geological origins of humans and animals. It relies on the principle that strontium isotope ratios from geology manifest themselves in the local ecosystem and are thus transferred, with little fractionation, to the humans and animals that feed in that area (although rainfall also affects ecosystem values). Due to susceptibility to diagenetic alteration this approach can only be employed for dental enamel, a much denser material than bone, and therefore the signal provides a geological snapshot from the early years of life, when teeth were developing. Oxygen isotope analysis can be applied to bone (phosphate), teeth (carbonate) and shell (carbonate) and is useful as a climatic indicator and to provide evidence for seasonal movements of animals and seasonal exploitation of molluscs. Lead isotope analysis, frequently used for establishing the source of artefacts, has also proved useful in certain instances for the analysis of provenance and cultural affinity in human remains but remains infrequently used in environmental archaeology.
Exciting developments in the discipline include the refinement of approaches to sulphur (δ34S) and hydrogen (δ2H) isotope analysis, both of which have ever improving interpretative potential. One new innovation is the application of hydrogen in conjunction with carbon and nitrogen isotope analysis to reconstruct the trade of wool in past societies. As more studies are undertaken, results from sulphur isotope analysis of bone collagen are becoming ever more insightful, providing indications of marine and freshwater diets and in some instances proving useful for identifying geological origins. Arguably the most exciting advancements are occurring in the analysis of plant remains, with more controlled experiments on modern material dramatically enhancing our understanding of the driving forces behind variation in signatures. Work is also underway to develop a method of strontium isotope analysis for provenancing archaeological plant remains and the future will doubtless hold many more innovations to address ever wider archaeological questions.
The number of submissions of isotope research papers to Environmental Archaeology has increased substantially in recent years. Publications have included methodological advancements made by experimental work on profiling cattle molars, which provided evidence for long-term averaging of strontium (Montgomery et al. 2010). Other studies have demonstrated innovative approaches to animal foddering, including seaweed for sheep in Neolithic Orkney (Balasse et al. 2009), winter leaf foddering for cattle and sheep in Neolithic France (Balasse et al. 2012) and household scraps and forest forage for pigs in late Bronze Age and early Iron Age Britain (Madgwick et al. 2012). Other research has revealed continuity in husbandry strategy at a time of agricultural change (Fisher and Thomas 2012). Papers are not confined to animal remains and an innovative study on a Late Classic Mexican population which revealed social differentiation in diet based on age and perhaps also social status (Lopez et al. 2011) represents a highlight of analyses on human bone.
The development of isotope analysis has unquestionably radically enhanced the interpretative potential of environmental remains. Its greatest value is the direct link between results and the material analysed. No longer do archaeologists have to rely solely on the seeds, bones and shells that are recovered to infer diet. Nor is it necessary to focus only on the typology of associated artefacts to infer origins and directions of mobility. Both can be addressed directly and at the level of the individual rather than the community through isotope analysis.
Balasse, M., Boury, L., Ughetto-Monfrin, J. and Tresset, A. 2012. Stable isotope insights (δ18O, δ13C) into cattle and sheep husbandry at Bercy (Paris, France, 4th millennium BC): birth seasonality and winter leaf foddering. Environmental Archaeology 17: 29-44.
Balasse, M., mainland, I and Richards, M. 2009. Stable isotope evidence for seasonal consumption of marine seaweed by modern and archaeological sheep in the Orkney archipelago (Scotland). Environmental Archaeology 14: 1-14.
Fisher, A. and Thomas, R. 2012. Isotopic and zooarchaeological investigation of later medieval and post-medieval cattle husbandry at Dudley Castle, West Midlands. Environmental Archaeology 17: 151-167.
López, C.M., Núñez, L.F., Morales, P., Cienfuegos, E., Otero, F. 2011. Diet and health at Chinikiha´, Chiapas, Mexico: some preliminary results. Environmental Archaeology 16: 82-96
Madgwick, R., Mulville, J. and Stevens, R.E. 2012. Isotopic analysis of fauna from British middens of the Late Bronze Age: Evidence for diversity in foddering strategy and herd management. Environmental Archaeology17: 126-140.
Montgomery, J., Evans, J.A. and Horstwood, M.S.A. 2010. Evidence for long-term averaging of strontium in bovine enamel using TIMS and LA-MC-ICP-MS strontium isotope intra-molar profiles. Environmental Archaeology 15: 32-42.