Strontium (Sr) isotope analysis has been used in archaeology for several decades now to assess movement and migration at both the individual and population level. To be as accurate as possible, however, it requires a strong baseline knowledge of the biologically available strontium in the regions from which the samples are likely to have originated. This is not merely a reflection of the underlying geology but can be affected by such factors as the region’s proximity to a marine environment and/or human settlements, as well as precipitation and runoff. We benefit in this country from the work of the British Geological Survey (BGS), which has brought together all the known data on the variation of biologically available strontium across Great Britain in order to create detailed maps that can be used for analysis.
This work is not yet complete, with some areas having significantly more data points than others – meaning that the resolution of this map can be a bit patchy. The south-west of England, for instance, had previously relied on only 31 samples to determine its likely strontium isotopic composition. In this month’s ‘Science Notes’, we examine a recent project, led by researchers from the Universities of Reading and Exeter, as well as the British Geological Survey, which has sought to rectify this by more than tripling the number of samples we have for this region (Journal of Archaeological Science, https://doi.org/10.1016/j.jas.2022.105628). It shows that there are some areas which have much higher strontium values than originally thought.
Previously, most research had assumed that any sample with a ‘highly radiogenic’ strontium isotope value (i.e. over 0.714) is likely to have originated from Scotland or abroad, because areas of England and Wales that host biosphere 87Sr/86Sr values above this level appear to be extremely rare. This is due not only to the underlying geologies of these two countries, but also to the large amount of marine strontium – which has a low radiogenic value – brought inland by precipitation. There are a few localised places in Wales (e.g. the Malvern Hills) where the strontium values sometimes creep over 0.714, but these regions are quite small in area and their effect on the strontium levels of humans may be limited. Wales, on the whole, is still underrepresented, however, and this analysis could change with more data points, given that much of Wales is characterised by highly radiogenic bedrock.
The south-west of England is characterised by similar geologies to Wales, and could also host areas of high strontium, but with so few samples, knowledge of local variation has been limited. Recently, researchers were given the opportunity to redraw the map for this region, however. The team collected plant samples from across Devon and Cornwall, and compared them with a small number of animal bone samples from livestock with a known life-history (meaning it was known where they grazed and hence where they got their strontium levels from).
Overall, the new values were comparable to the previous ones and, considering that much of the old map estimated the likely 87Sr/86Sr values based on the underlying lithologies of the region, the map for the south-west holds up remarkably well – attesting to the accuracy of such extrapolation methods. One main difference, however, was found in areas defined by Cornubian granite bedrock. Large continuous areas of the north-west part of Dartmoor, which has both Cornubian granite bedrock as well as rubidium-rich soil, consistently reported sample values of strontium over 0.714. This area of Dartmoor is lower-lying than the moorlands, so the combination of runoff from the uplands with a high concentration of 87Rb in the soil, which decays into 87Sr, may explain these high values. It also means that, unlike the moorlands, this area was more likely to have seen human settlement, agriculture, and animal husbandry throughout history, which may have had a significant impact on the strontium-levels of the plants and animals living there.
Given this new evidence, the team suggests that previous strontium isotope research should be reconsidered. The highly radiogenic livestock remains found at Durrington Walls, for instance, may not have originated from Scotland. Instead, it may be more likely that the animals had come from the south-west of England (or possibly Wales – since more work needs to be done there). There have also been several Romano-British skeletons found to have very high strontium levels, which had been interpreted as people who grew up abroad. However, given the large amount of tin-mining that occurred in the south-west during this period, this might have to be reconsidered, too.
In addition to Dartmoor, there are other outcrops across Cornwall and the Isles of Scilly that have similar characteristics. The team was, however, unable to obtain enough samples from these regions, so it is unknown whether they have similarly high levels of strontium or whether they might be tempered by the high levels of marine strontium that are probably present there. As always, further research is needed.
TEXT: K Krakowka