Chapter 18
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Seeking the Local 87Sr/86Sr Ratio To Determine Geographic Origins of Humans James H. Burton* and T. Douglas Price Department of Anthropology, University of Wisconsin−Madison, 1180 Observatory Drive, Madison, Wisconsin 53706, U.S.A. *E-mail:
[email protected] Critical to the success of efforts to determine the geographic origins of humans using 87Sr/86Sr is an accurate knowledge of this ratio and its range for local humans. There can be significant inadequacies in trying to estimate this ratio using available proxies, whether geological (rock, soil, water) or biological (modern or ancient faunal proxies). Empirical examination of the human data, as prima facie evidence, coupled with additional proxy data along with any information about dietary catchments, can improve our resolution of the sought-for ‘local’ human 87Sr/86Sr ratio.
Use of Bone Data To Assess Locality The first publication of the use of 87Sr/86Sr to study human mobility is by Jonathon Ericson (1), who compared 87Sr/86Sr ratios in dental enamel, as a record of residency ‘prior to marriage’, to that of bone, reflecting ‘post-marital residence’. He proposed, one could compare variation in males to that in females and assess regional intermarriage patterns and suggested that the method could also be used to assess more general issues of human mobility and resource procurement as well as animal ecology. This follows from the fact that most dental enamel mineralizes shortly after birth, trapping the strontium present in the diet at that time, while bone chemically remodels throughout life. A difference between bone and tooth 87Sr/86Sr ratios implies that the individual must have moved during their lifetime to a region with a different 87Sr/86Sr from the place of birth.
© 2013 American Chemical Society In Archaeological Chemistry VIII; Armitage, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
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In fossil samples, however, diagenetic alteration commonly causes bones to yield an indeterminate mix of biological and geological strontium, precluding them from providing accurate biological 87Sr/86Sr data. From our observations of hundreds of such paired measurements, most bone values do appear to have 87Sr/86Sr close to the estimated local ratio. For obvious immigrants with quite different enamel ratios, the bone ratios tend to be between enamel and local ratios, but generally much closer to the local ratio than to the enamel. It is unlikely that all these individuals migrated at such an early age that bone would have enough time to biologically alter. It is far more likely that this represents post-depositional alteration. Nelson and others (2) examined bones of modern and fossil marine animals and found that fossil marine animal bones taken from terrestrial contexts had values matching terrestrial 87Sr/86Sr, not their biological 87Sr/86Sr ratio (i.e., that of modern seawater). Sealy and colleagues (3) likewise discovered significant diagenetic alteration of fossil bone 87Sr/86Sr ratios. Such diagenetic alteration toward the local 87Sr/86Sr happens because bone has an enormous capacity for diagenetic absorption of strontium through ion exchange with the soil and can pick up local soil strontium, regardless of how long the individual might have been in residence there (4). Although one can argue the case that diagenetically added strontium is local strontium so that bones are still a proxy for the place of death, soil ratios 87Sr/86Sr, like rock ratios, as discussed below, can vary greatly, typically more than the range in biological tissues by an order of magnitude or two; any particular diagenetically affected sample could have ratios either higher or lower than the sought for average exhibited by unaltered biological specimens. There are also methodological problems with this simplistic comparison. Theoretically, the time for bones to chemically remodel is on the order of tens of years (5) so that, even for immigrants coming from isotopically contrasting terrains, bone ratios need not be greatly different from enamel ratios, but should be somewhere in between the enamel ratio and the (unknown) local ratio. Cancellous tissues remodel fairly quickly (up to 25% per year) so that only 10% of the original strontium might be present after 8 years, but cancellous tissue is also the least likely to be preserved and most likely to be seriously contaminated. Cortical tissues on the other hand are somewhat more robust and more likely to be sampled, but remodel at the rate of approximately 2-3%/year (6); even after twenty years in a new location most of the strontium would be from the earlier place(s) of residence. There are cases in which bone ratios could equal enamel ratios without implying that the individual is local. In studies of sacrificial burials in which the victims are likely captives from other locations, place of death or burial might not be the same as that of last residence, even though bone values might match enamel values; ‘bone=tooth=local’ would fail to identify these as exotic. As an example, in our study of burials from the archaeological site of Aztalan, Wisconsin (7), one individual, MTMC 11, a distinctive bundle burial, had essentially identical ratios for two enamel samples and a bone sample. In the simplistic bone=tooth means local heuristic, this individual would be deemed local, but more intensive study revealed that other bone-teeth pairs have ratios that differed from this one, and they equal ratios of other materials believed to 310 In Archaeological Chemistry VIII; Armitage, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
be local to the area. This individual is an outlier whose bones and teeth had not equilibrated to the local ratio. This individual had apparently been transported as a bundle and reburied at Aztalan, explaining the anomaly. Although this case in probably an extraordinary one, it illustrates a hazard of the ‘bone=tooth=local’ paradigm. As Ericson explicitly acknowledged, the place of ‘post-marital’ residence need not be the same as the place of death, which need not be the place of burial.
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Use of Geological 87Sr/86Sr Measurements A common premise of the use of 87Sr/86Sr to determine geographic origins is that there is some pervasive ratio reflecting source material that passes into soil and plants then into the human diet and ultimately into human dental enamel without any change in the 87Sr/86Sr ratio, i.e., local geological 87Sr/86Sr = dietary 87Sr/86Sr = enamel 87Sr/86Sr. Ericson suggested as a starting point that one estimate from geological maps, based upon the age, location, and rock types - along with any existing isotope data - what the rock ratios might be and what contrast there might be among regions of interest. This underlying premise is conceptually useful as an explanatory heuristic but has little value beyond that because the variation, locally, in geological materials typically exceeds that of biological tissues by orders of magnitude. The main problem is that most rocks are complex, heterogeneous assemblages of minerals with very different 87Sr/86Sr ratios such that whole rock ratios vary according to subtle changes in the percentage of each mineral. For example, granites commonly have high-Sr/low-Rb feldspars, with accordingly relatively low 87Sr/86Sr ratios ≈ 0.7+, along with high-Rb/low-Sr micas that can be extremely radiogenic, with 87Sr/86Sr >10.0 (8). Naylor and others (9) measured 87Sr/86Sr in minerals of the Louis Lake batholith and found most ratios to be 0.709) and the 87Sr/86Sr of potential dietary items, we can begin to sort who is who. 318 In Archaeological Chemistry VIII; Armitage, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.
While a strong mode in the data - in the absence of large group migration - is a likely candidate for a ‘local’ ratio, the other lines of evidence discussed above should be compared with the human data, being aware of the possible limitations of each. Nonetheless, modal data itself will be problematic and probably insufficient when a significant percentage of the population is mobile. Congruence of human data with faunal results adds support to the assessment of a local ratio, while divergence indicates that further scrutiny is required.
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Conclusion Critical to the success of using strontium isotopes to assess human mobility is knowledge of the 87Sr/86Sr ratio for local humans. This has been estimated by measuring proxy materials, including geological materials (mineral, rock, soil, ground water, well water, soil and rock leachates) and modern and fossil biological materials, each of which in specific situations can yield inaccurate results for humans. Inspecting the human 87Sr/86Sr ratio as prima facie evidence, augmented by assessments of the dietary catchment, can improve our confidence that we are identifying local residents.
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