Saturday, 3 June 2017

Two long-distance migrants in the eighth- to tenth-century Islamic necropolis at Tauste, Spain

The current brief post is primarily concerned with two women buried in the eighth- to tenth-century Islamic necropolis at Tauste, Spain, both of whom appear to be long-distance migrants from a significantly colder and/or mountainous location than Spain. What follows consists of a brief discussion of the oxygen isotope results from this site and the areas of Eurasia where those with unusual isotope result could have potentially spent their childhoods.

The geographic distribution of areas with rainwater/drinking-water oxygen isotope values below ‑12.0‰, shown in black; the location of the eighth- to tenth-century Islamic necropolis at Tauste, Spain, is marked in red. Image: C. R. Green, based primarily on the IAEA's 2013 RCWIP model. Click here for a larger version of this map.

The Islamic necropolis at Tauste (Zaragoza, Spain), close to the northern frontier of Al-Andalus, was discovered in 2010 and contains a minimum of 4,500 people, all of whom were placed on their right-hand side and aligned to face Mecca. The burial ground has been radiocarbon dated to between the eighth and the tenth centuries AD and the remains of 31 individuals from this cemetery have been additionally subjected to multiple isotope analysis, with the aim of looking at their geographic origins, mobility and nutrition. The results of this suggest that the majority of the population buried here (c. 84%) grew up locally in the Ebro basin of northern Spain. However, five individuals had results significantly outside the expected local ranges for oxygen and/or strontium isotopes, and these outliers are of considerable interest. Two of them, both male, had tooth enamel phosphate oxygen isotope levels—δ¹⁸Op—above 19.1‰, notably higher than those of the majority of the population investigated here (16.4–18.0‰ δ¹⁸Op). As has been discussed in a number of previous posts, such results are indicative of a childhood spent in a much warmer and perhaps more coastal environment than this area of Spain and would be consistent with an origin in the very southernmost parts of Iberia, large areas of North Africa, or parts of the Near East including the northern Arabian peninsula.

Even more interesting are the two individuals, both female, who have exceptionally low oxygen isotope results: 14.48‰ δ¹⁸Op (individual T-31, aged 16–20) and 14.25‰ δ¹⁸Op (individual T-15, aged 33–45). When considering these two individuals, it is worth recalling that—as Montgomery et al have recently emphasised—there are very few environmental, biological, or cultural processes that that can result in human tooth enamel oxygen isotope values that are lower than would be expected on the basis of the consumed drinking-water. So, the question is, where in Eurasia or Africa can be found drinking-water with sufficiently depleted drinking-water oxygen isotope values to produce the tooth enamel results that these two women possess?

Oxygen isotope values versus strontium isotope values in the tooth enamel of individuals from Tauste, showing the main 'local' group and the five outliers (image: Guede et al, 2017, fig. 7, used under PLOS's CC BY 4.0 license).

With regard to this, the authors of the isotope study of the Tauste Islamic necropolis comment only that the two women in question may come 'from a colder or higher altitude region' and 'a more mountainous geographical region', and don't investigate their potential origins any further. However, a significantly greater degree of precision than this is not only arguably possible in light of the above observations, but also actually produces some rather fascinating results. The reason for this is that human tooth enamel phosphate oxygen isotope levels as dramatically low as 14.25 and 14.48‰ δ¹⁸Owould, in light of the above, appear to require a drinking-water value of below -12.2‰ to -12.6‰ δ¹⁸Odw (using the 2008 Daux et al drinking-water equation; using the alternative 2010 revised Levinson equation it would require a drinking-water oxygen isotope value of below -13.7‰ to -14.2‰ δ¹⁸Odw), something that is notably rare across much of the globe.

As can be seen from the map included at the top of this post, areas with rainwater/drinking-water values below -12.0‰ are, in fact, only widely encountered in northern Scandinavia and Russia, and are otherwise really only found in parts of the Alps, the Caucasus Mountains region, and the Hindu Kush, Tibetan Plateau and Tian Shan area in the east. Needless to say, such a very restricted distribution of candidate areas strongly suggests that the two women spent their childhoods not simply in a 'higher altitude region' as the article indicates, but were instead very likely to be long-distance migrants to northern Iberia who had potentially travelled even further over their lifetimes than the two men noted above. Which of these possible childhood residences is more credible is obviously a matter of debate. The closest locale is, of course, the Alps, and if correct it would imply that the women in question had moved from an early medieval Christian milieu to an Islamic one at some point. Next closest is Scandinavia, which is at least worth noting as not only were the Vikings occasionally active around Iberia and in the Mediterranean from the ninth-century, but slaves from more northerly parts of Europe were certainly being traded south to Iberia in this era too (with the English perhaps playing a part in this, given the tenth-century Hudud al-'Alam's reference to trade between Al-Andalus and Britain). Finally, the women could well have grown up in the Islamic world and have simply travelled a significant distance to reach Al-Andalus, perhaps from either the Caucasus Mountains area or even the Hindu Kush. Whatever the case may be, these women are thus clearly of considerable interest.

In conclusion, the isotopic results from the large eighth- to tenth-century Islamic necropolis at Tauste are worth serious consideration, with around 13% of the sampled population having clearly non-local oxygen isotope values. Whilst pinning down the childhood locales of these people with any degree of certainty remains challenging, the published results do at the very least seem to offer yet more potential evidence for there having been a notable degree of medium- to long-distance mobility and contact in early medieval Europe.

Location of Muslim and Christian archaeological sites and the Upper March or Muslim northern frontier in western Europe during the ninth century, with the location of Tauste marked (image: Guede et al, 2017, fig. 9, used under PLOS's CC BY 4.0 license).

The content of this post and page, including any original illustrations, is Copyright © Caitlin R. Green, 2017, All Rights Reserved, and should not be used without permission.