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3. Material and Methods

3.1 Stable sulphur isotopes

Sulphur is incorporated into vertebrates through their diet. Experimental data has shown that it is mainly the protein portion of the diet that is reflected in collagen isotopic data (Ambrose and Norr 1993). Sulphur in collagen is present in only two amino acids, methionine and cysteine. Methionine is an essential amino-acid, which implies that it is derived directly from ingested protein, whereas the cysteine is non-essential, and synthesised either from the diet or from methionine (Bohinski 1979). There are four naturally occurring sources for sulphur: organic matter, minerals in rocks and soils, sea water, and atmospheric deposition of sulphuric gases. The δ³⁴S value in plants generally primarily mirrors that of its geological surroundings (Brady and Weil 1999). The terrestrial sulphur isotopic signature thus varies depending on the geological setting, and terrestrial δ³⁴S values are far more varied than in marine environments. In sedimentary rocks, the δ³⁴S values range from -40 to +40‰. European granitic rocks display δ³⁴S values between -4 and +9‰, mafic rocks have δ³⁴S values close to 0‰, and metamorphic rocks exhibit δ³⁴S values ranging from -20 to +20‰ (Krouse 1980; Faure and Mensing 2005). However, the bioavailable sulphur displays ranges that are levelled out compared to the ranges in bedrock. The δ³⁴S values for the oceans, by contrast, are rather uniform, averaging +21‰, with marine vegetation having δ³⁴S values between +17‰ and +21‰ (Peterson and Fry 1987). Due to the so-called sea spray effect (Kusakabe et al. 1976; Wadleigh et al. 1994), sulphur isotopes in plants growing close to the shore might be affected by marine sulphur to some extent. Freshwater systems are much more dispersed, the δ³⁴S values ranging from -22 to +20‰, as a result of the reduction of sulphate ions (SO_4¯) to hydrogen sulphide (H₂S) (Krouse 1980; Faure and Mensing 2005).

The isotopic fractionation between food and consumer is relatively small (-1‰ to +2‰), which means that the δ³⁴S value in bone or dentine collagen reflects the sulphur isotopic composition of the diet which, in turn, reflects the geology/locality where the food sources originated (Peterson et al. 1985; Bol and Pflieger 2002; Sharp et al. 2003; Richards et al. 2003; Fraser et al. 2006; Buchardt et al. 2007; Nehlich and Richards 2009; Nehlich et al. 2010). As a result, mobility in prehistoric populations can potentially be detected by comparing the human δ³⁴S values with values from local animals of approximately the same date.

Since Öland consists of sedimentary rocks, known to exhibit highly variable sulphur isotope signatures on a global scale, local faunal samples are crucial in order to establish the local terrestrial bioavailable δ³⁴S range. Like for other isotopes used in mobility studies, sulphur can only be used to exclude local values, not to pinpoint any specific region, that is, we can only positively identify non-local values. Hence, values falling within the established local terrestrial range, cannot with certainty be identified as local, although it will frequently be the most plausible interpretation (see Table 4). The analysis of both dentine and bone collagen from one individual, enables detection of change of residence during the lifetime of an individual, because the collagen is formed during different ages.

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