Science Notes: Using incremental strontium isotope analysis to recreate subsistence ranges

Strontium isotope analysis has become a standard in mobility research, featuring in many post-excavation analyses of human and animal remains. While ‘typical’ strontium analysis is able to tell us much about movement in the past, new techniques are being applied in ways that enable us to appreciate migration on even smaller timescales. One such methodology, which we are discussing in this month’s ‘Science Notes’, is laser ablation multicollector inductively coupled plasma mass spectrometry, or LA-MC-ICP-MS for short. This is used to assess incremental strontium isotope ratios within tooth enamel, which has the potential to identify mobility to a seasonal, or even monthly, resolution. With this data, it then becomes possible to recreate a given population’s subsistence range.

ICP-MS is used in many aspects of archaeological science to assess the composition of a sample in such a way that preserves the spatial distribution of each element/isotope. For example, the last time we discussed it in ‘Science Notes’, the technique was used to identify the geochemical composition of sarsen debitage at Stonehenge (see CA 367). To collect the sample, a focused laser beam is used to remove sequential layers of material and vaporise them into an aerosol, which is then analysed using a special mass spectrometer.

Gruta da Oliveira, located approximately 40m above the Almonda spring

For the technique to be able to detect small-scale mobility, the individual being sampled will have had to come from a region where the baseline strontium values have extreme variation over a relatively short distance. Such was the case for a recent study that tracked modern sheep and ibex in the Altai Mountains of Mongolia using GPS. Sequential strontium analysis of these animals was successfully able to reconstruct their monthly movements.

In testing whether this methodology can be used effectively on ancient human remains, an international team led by researchers from the University of Southampton looked at Palaeolithic samples from Portugal. Located 100km north-east of Lisbon, the Almonda karst system on the southern edge of the Mesozoic Central Limestone Massif of the Portuguese Estremadura has been found to have a high degree of geographical variation in 87Sr/86Sr values, and hence is a great location to test MC-ICP-MS’s ability to detect seasonal movement in the distant past.

Two of the teeth sampled, which are from Neanderthals, were from Gruta da Oliveira, located approximately 40m above the Almonda spring. The layers from which they came were dated using optically stimulated luminescence (OSL) and Bayesian modelling to 93,250±3,450 years ago. This date was reinforced by the presence of Levallois stone tool technology within the same context. The third sample, a human tooth from the Magdalenian period, came from the Galeria da Cisterna, located 5m above the spring and 35m below Gruta da Oliveira, and was directly dated to 11,122±34 BC. The team also sampled animal remains from both caves, as well as another cave nearby, to determine the probable subsistence ranges of these hominids’ prey.

While only three hominid samples were analysed, the results of this pilot study were intriguing. The two Neanderthals from Gruta da Oliveira had similar strontium isotope profiles. Both were found to have probably moved between four different geological areas, at least during the ages of 8.5-14.5 years old for the first individual – whose third molar was sampled – and the ages of 3.5-6.5 years old for the second individual – whose premolar was sampled (above). Conversely, the results from the Magdalenian individual from Galeria da Cisterna suggest that they oscillated between two geographically distinct areas, probably from the source of the Almonda River to the banks of the Tagus River.

To then extrapolate these results in order to determine the probable settlement/subsistence territory for each population, the area had to (a) contain the site of the sample, and (b) contain terrain with soil Sr values matching those measured in the teeth. For the Gruta da Oliveira samples, the team found this included an area of c.600km2 surrounding the cave, while for the Galeria da Cisterna sample it included an area c.300km2 – roughly half the size of their predecessors. This smaller range for the Magdalenian sample is supported by the materials found in the same archaeological context, approximately 80% of which come from sources within 300km2.

Crucially, the results from the animal bones suggested that the hominid’s main prey species did not have a major change in territory between the two periods, meaning that the observed decrease in the hominid subsistence range is unlikely to have been driven by changes in hunting patterns. It is more likely, the team argue, that this diminished range is due to increasing population density during the later Upper Palaeolithic, an argument supported by an observed increase in the frequency of archaeological sites dating from this period across the Iberian peninsula and south-west France.

The full results of the study were recently published in PNAS:

Image: João Zilhão / Text: : Kathryn Krakowka