Exploring the Potential of the Strontium Isotope Tracing System in Denmark
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Danish Journal of Archaeology, 2012 Vol. 1, No. 2, 113–122, http://dx.doi.org/10.1080/21662282.2012.760889 Exploring the potential of the strontium isotope tracing system in Denmark Karin Margarita Frei* Danish Foundation’s Centre for Textile Research, CTR, SAXO Institute, Copenhagen University, Copenhagen, Denmark (Received 1 October 2012; final version received 14 November 2012) Migration and trade are issues important to the understanding of ancient cultures. There are many ways in which these topics can be investigated. This article provides an overview of a method based on an archaeological scientific methodology developed to address human and animal mobility in prehistory, the so-called strontium isotope tracing system. Recently, new research has enabled this methodology to be further developed so as to be able to apply it to archaeological textile remains and thus to address issues of textile trade. In the following section, a brief introduction to strontium isotopes in archaeology is presented followed by a state-of-the- art summary of the construction of a baseline to characterize Denmark’s bioavailable strontium isotope range. The creation of such baselines is a prerequisite to the application of the strontium isotope system for provenance studies, as they define the local range and thus provide the necessary background to potentially identify individuals originating from elsewhere. Moreover, a brief introduction to this novel methodology for ancient textiles will follow along with a few case studies exemplifying how this methodology can provide evidence of trade. The strontium isotopic system the strontium isotopic signatures are not changed in – from – Strontium is a member of the alkaline earth metal of a geological perspective very small time spans, due to Group IIA in the periodic table. Strontium is a trace the very slow decay of 87-Rubidium) and can be followed element whose ionic radius is 1.13 Å, very similar to through processes of weathering, and on into the food that of calcium 0.99 Å (Faure 1986). Due to their similar chain, through the diet as previously mentioned (Figure 1). ionic radii, strontium can replace calcium in mineral lat- Even though differences in the strontium concentrations may tices and thereby is incorporated into the skeletal tissues occur along such uptake chains (due to different partitioning and body through diet. Strontium is concentrated in of strontium into various media such as water, plants, bones, fi Calcium-bearing minerals such as hydroxyapatite, the and so on), strontium isotopes are not signi cantly fractio- mineral with which tooth enamel is composed. This is nated, and strontium isotopic compositions are not affected ’ ’ one of the characteristics that make strontium useful to and thus remain stable , making these signatures highly archaeologists. useful for tracing. In conclusion, the rate of production 87 87 86 Moreover, strontium has four naturally occurring of Sr is so slow that the Sr/ Sr ratio of a substance can isotopes 88Sr (82.53%), 87Sr (7.04%), 86Sr (9.87%), be considered invariant over archaeological timescales. Thus, and 84Sr (0.56%) (Faure 1986). Three of the four naturally in terms of strontium delivery to a plant, the groundwater 87 86 ’ occurring isotopes of strontium are ’stable’, whereas 87Sr inherits Sr/ Sr ratios characteristic of the soluble biologi- ’ is radiogenic and is therefore variable, as it is partially cally available fractions/components in a soil which, in turn, formed by the radioactive decay of naturally are transferred isotopically unchanged and unfractionated on occurring 87Rb (half-life of 48.8 billion years) (Faure to the plant (Benson et al. 2006) and throughout the food 1986). The strontium isotopic tracer system relies on the chain. Therefore, the skeletal tissues of, for example, a sheep, 87 fl use of two of the four ‘naturally occurring’ isotopes, Sr will re ect the bioavailable strontium isotope characteristics and 86Sr. The ratio of these isotopes, 87Sr/86Sr, is some- of its feeding ground. what related to their natural abundance and is often ~0.7 (~7% 87Sr/~10% 86Sr). In general, what needs to be kept in mind is that age The bioavailable strontium and the type/nature of rocks are parameters which control The strontium isotopic ratio which is incorporated in liv- the strontium isotopic composition of a geological base- ing organisms and thus characteristic of a particular catch- ment and its sedimentary derivates. These strontium iso- ment area can be affected by other exogenous factors topic properties are maintained in recent processes (i.e., which can contribute to the strontium budget of a soil *Email: [email protected] © The Partnership of the Danish Journal of Archaeology 2012 114 K.M. Frei 1–100 ppm 0.05–15 ppm 100–1000 ppm 50–500 ppm 0.2–20 ppm Modern seawater 8 ppm 0.7092 Soil 10–1000 ppm Bedrock 87Sr/86Sr >0.715 >>0.720 ~0.703 0.707–0.709 Sr >100 ppm >100 ppm 500 ppm 600–2000 ppm Figure 1. Diagram illustrating the strontium path from the geological strata to the human/animal hair (Frei 2010). (Price et al. 2002). The exogenous factors can be several: can contribute variably depending on the topogra- some of the most important are seawater spray, atmo- phy, wind direction, forest canopy, distance to the spheric dust, diet, and modern fertilizer (Frei and Frei shore, as well as the nature of the particles. 2011). Regrettably, there is little information available on this topic, thus a more detailed evaluation is (1) Seawater contains strontium with a strontium iso- needed to better estimate their potential effect. topic ratio of 87Sr/86Sr ratio of ~ 0.7092 (Figure 1), However, it is suspected that the influence will which has remained unchanged through the archae- be minimal, unless there is a large and continuous ological/historical/present timescale. Places like the deposition of atmospheric particles on the soil. Faroe Islands or Iceland which are surrounded by (4) Diet is also an important factor to consider when seawater have shown to be highly affected by this interpreting the results of the strontium isotopic factor (Price and Gestsdottir 2006, Frei et al. analysis, as it can be misleading when high 2009a, Frei 2010). This means that the sea-spray amounts of seafood have been consumed through- contributes to the total strontium budget of a soil out an individual’s childhood. Since seawater has when near the coast. Therefore, this is an important a 87Sr/86Sr value of ~0.7092, this will affect the issue that needs to be considered in Denmark. strontium isotopic composition of a person living (2) Fertilizers have also often been suspected as being off seafood. Thus, in prehistoric periods when potential contaminants of soils. Recent studies aim- seafood was known to constitute an essential part ing to show the actual effect that modern fertilizers of the diet, it is highly recommended to perform have on soils have demonstrated that concentrations δ13C measurements of the same individuals (same of strontium in fertilizers utilized in Denmark are tooth if possible) to estimate the percentage of often so small that their effect is minimal and often possible strontium deriving from seafood in rela- not even measurable (Frei and Frei 2011). tion to terrestrial food. (3) Atmospheric particles, for example, ash from vol- In conclusion, there is no doubt that some of these canic eruptions or sand particles from the Sahara exogenous factors may contribute in various degrees to the Danish Journal of Archaeology 115 bioavailable strontium isotopic composition of the soil or values and a baseline range for future successful tracing diet. Therefore, it is important to create bioavailable stron- investigations. The first study is based on strontium iso- tium baselines and not only consider the local basement tope values of surface waters, including a few samples geology. from Bornholm and northern Germany (Frei and Frei 2011). The second is based on strontium isotope analyses of archaeological and modern fauna from Denmark Baselines (Bornholm excluded) (Frei and Price 2012). The results There are several ways in which baselines of the bioavail- of these two studies are quite similar and can be used able strontium isotopic range of a place can be con- together to define the Danish local bioavailable strontium structed. For example, Evans et al. (2010) constructed a isotope range (Figure 3). The overall bioavailable stron- strontium isotope distribution ‘map’ across Britain based tium isotope range in Denmark, Bornholm excluded, is essentially on data from modern plant material. Other ~0.708–0.711. The lowest values come from the Limfjord studies considered the composition of surface and mineral area near Mors and Fur, most probably due to the geolo- waters (Montgomery et al. 2006, Voerkelius et al. 2010, gical outcrops of the Paleogene volcanic ash layers (Frei Frei and Frei 2011), others performed combinations of and Frei 2011). The highest values derive from the island modern and archaeological fauna to achieve the same of Zealand, which can be interpreted as a result of glacio- purpose (Frei and Price 2012). There is as yet no ideal genic silica-rich till topsoils. In general, it seems that the way to perform baselines; therefore, it is plausible that a lowest 87Sr/86Sr ratios are between 0.7078 and 0.7098 combination of several methods can provide the most which stem primarily from the western part of Denmark, accurate range. To produce such baselines is, however, whereas the higher values 87Sr/86Sr ratios are between time-consuming and costly, nevertheless they are crucial 0.7089 and 0.7108 and stem from the eastern part of for providing reliable information on mobility issues. Denmark, east of the Lillebælt (Little Belt) (Frei and Once local sites and potential target areas are sufficiently Price 2012).