6 Stable isotope research: migration and diet
6.1 Migrationnext section
Stable isotopes of strontium (87Sr/86Sr), oxygen (δ18O), sulphur (δ34S) and lead (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of tooth enamel served to establish the area of childhood residence and therefore the provenance of individuals from Schipluiden and Swifterbant. Stable isotopes of these elements are bound to the local underground geology, influenced by altitude, precipitation and distance from the sea, and as such specific for a geographical area (Budd et al. 2004). The research was carried out in cooperation with the University of Durham and published elsewhere (Smits et al. 2008). Samples were taken from human enamel and associated soils horizons, animals and /or grave pits to establish the local isotopic make-up and to rule out possible diagenetic influences (postdepositional absorbtion). The Swifterbant population appeared to be more homogeneous and of local origin, than the population at Schipluiden, which was more heterogeneous. Two non-locally grown individuals could be pointed out for Schipluiden and one for Swifterbant in a total sample population of 20 individuals on the basis of the combined strontium, lead and oxygen evidence (see figure 6 for the combined Sr and O values). The result of the sulphur isotope study discriminates roughly between the two groups but interpretation in view of distance from the sea is hindered by possibly diagenetic influences and failing information on the local S values. The two Schipluiden non-locals have no deviating Sr values, but show oxygen signals pointing to origins in eastern (continental) and south-western (Atlantic) directions. One of these individuals showed high Pb values and a more terrestrial diet as well, supporting the interpretation as non-local. The Swifterbant ‘immigrant’ has been primarily separated on the basis of the non-local Sr values, supported by high Pb values and a distinct terrestrial 13C/15N isotopic signature. One of the presumed ‘local’ people at Schipluiden had, however, similar 13C/15N values. In all of the five Schipluiden burials only ‘locals’ were interred, while both ‘immigrants’ were identified among the three investigated isolated remains. This may be seen as an indication that mortuary practices for non-local individuals were different from those of local people at Schipluiden. We have to be careful with such a conclusion in view of the low numbers of analysed samples and the non-straightforward interpretation of the complex isotope data patterns. Future research should be especially directed to this presumed relationship of origin and mortuary practice.
As the local interred individuals were men and children one wonders whether this group was patrilocal and if women migrated from other regions, but as the skeletal remains of women are almost undetectable in the isolated remains this hypothesis cannot be researched here.
Fig. 10 Combined Sr and O values for Schipluiden and Swifterbant (data derived from Smits et al. 2008). The dashed lines indicates the range of local oxygen values, the arrows indicate immigrants. Nos 1 and 2 are children, no. 3 is an individual from a different site and possibly a different date.
6.2 Diet (by J.van der Plicht)
The isotopic content of materials is expressed in the so-called delta (δ) values, which is defined as the deviation of the rare to abundant isotope ratio from that of a reference material (Mook 2006):
For carbon, the reference material is the calcium carbonate in the shell of fossil belemnite from the PeeDee Formation (so-called PDB) in the USA. For Nitrogen, the reference is air (atmospheric N2 ). The stable isotope concentrations are measured by IRMS (Isotope Ratio Mass Spectrometry) at the Centre for Isotope Research in Groningen. The analytical error is 0.1 and 0.2 ‰ for δ 13Cand δ 15N, respectively. The absolute rare isotope contents of these standards can be found in Mook (2006).
The stable isotope ratio of carbon has been measured routinely by the 14C laboratories since the 1960’s. The reason is that this is used for the correction of 14C dates for isotopic fractionation, by using δ 14C= 2δ 13C. Thus, for both human and animal bone collagen, a wealth of data exist for this stable isotope ratio δ 13C for all materials dated, including bone collagen (Van Klinken et al. 2004). During the last two decades or so, this information has been supplemented with measurements of the stable isotope of nitrogen, δ 15N, in bone collagen.
In this section, new δ 13C and δ 15N stable isotope data are discussed for Hardinxveld and Schipluiden. It is in part an update from earlier work, which included Swifterbant (Smits et al. 2008). Here we discuss the data in terms of the paleodiet. The data will also be used in a study to determine the 14C reservoir effects for human skeletal material. We mention here only the Hardinxveld burial of an adult woman. Based on the δ 15N value, the assumption of a fluviatile diet and using the procedure developed by Cook et al. (2001) for the Iron Gates Mesolithic sites, the size of the 14C reservoir effect for this individual is preliminarily established as being no more than 350 years.
The δ 13C for humans and animals living on land are part of the terrestrial food chain. Typical δ 13C values for bone (based on a C3 diet) are around -21 ‰ (Lanting & van der Plicht, 1995/’96, 1998). The isotope content for organisms living in water (rivers or the sea) is different. The δ 13C starting point of the trophic chain in water is more positive. Where ‘terrestrial’ bone collagen shows on average δ 13C ≈ -21 ‰, ‘marine’ foods give a much higher δ 13C content: δ 13C ≈ -13 ‰. For freshwater, the situation is more variable. There the δ 13C may be more positive than in terrestrial systems, but can also be the same or even more negative.
The isotope δ 15N is in particular important for the observation of marine or fresh water dietary components. Terrestrial human bone collagen shows 15N values of typically 5-10 ‰, depending on the food source. The δ 15N values are much higher for a 100 % diet of marine or fresh water food: about 16-18 ‰ (Schoeninger et al. 1983). As food passes from the trophic level of the producer to the consumer, there is enrichment in both δ 13C and δ 15N. This applies to both the terrestrial and the marine environment. Producers/consumers can be for example plants/herbivores, or herbivores/carnivores. In bone collagen, there is a ≈1 ‰ increase in δ 13C and a ≈3 ‰ increase in δ 15N, per step in the trophic level. This observation spawned the intriguing title of a key review paper on paleodiets: “you are what you eat, plus a few permil” (Kohn 1999).
A series of δ 13C and δ 15N measurements were obtained for Schipluiden and Hardinxveld. Data were obtained for fossil bones from both sites, and from food residues in cooking vessels. The data are shown in tables 4, and 5 respectively. The list includes quality parameters like carbon content (% C), nitrogen content (% N) and (for the bones) the C/N ratio. The latter is a key quality parameter for bone collagen, used in particular for quality assessment of 14C dates. A plot of δ 13C versus δ 15N for the human bone samples from both sites is shown in figure 11. Also included in the figure, are earlier measurements for Swifterbant measured at Durham, UK (Smits et al. 2008). These Swifterbant data are obtained on dentine; in this case, no data are available for bone collagen.
Table 4 Stable isotope ratios for bones from Schipluiden and Hardinxveld.
Fig. 11 Stable isotope ratios δ 13C and δ 15N for human bones (Hardinxveld, Schipluiden; this paper) and published values for dentine from Swifterbant (Smits et al. 2008).
The region indicated in figure 11 by by a dashed circle comprises a total of about 200 δ 13C and δ 15N values for human bones from the Groningen database. These are human bones submitted for 14C dating, for which also δ 13C and δ 15 N has been measured, representing all regions in the Netherlands and 14C ages (Kuitems 2007).
The δ 13C and δ 15N values for Schipluiden, Hardinxveld and Swifterbant are clearly higher in 15N than the general values from the database. This is a strong indication of an aquatic food source for these populations. The δ 13C and δ 15N values for the three populations occupy different regions in the δ 13C/δ15N plot, as is tentatively indicated by the ellipses in figure 11.
The samples from Schipluiden and Hardinxveld show more elevated values for the stable isotope ratio δ 15N. This indicates that the aquatic dietary component is larger than for Swifterbant, the latter being (slightly) more terrestrial. Based on the stable isotope research for the human bones, Smits et al. (2008) concluded earlier that the Swifterbant population represents a mixed diet, and Schipluiden a more aquatic diet. The data from Hardinxveld form an extension of our dataset, confirming this observation. The Hardinxveld bones are slightly more depleted in δ 13C, and show elevated values for δ 15N, which is consistent with an aquatic food source. On the whole the Schipluiden group shows less depleted values of δ 13C, compared with the two other sites, indicating a distinct marine component in the diet.
Apart from these general conclusions, there is clearly one ‘outlier’ for each site, apparently individuals with a terrestrial diet. At Swifterbant; an isolated molar with a distinctly lower δ 15N value, belonged to an immigrant, based on other isotopic investigations (Smits et al 2008 and above). The Hardinxveld sample consists of an unarticulated bone, from which no additional isotopic information is available, nor an accurate age or sex diagnosis. The Schipluiden individual, man of c.25-40 years at death, had no divergent values for the other isotopes and should be considered as locally (i.e. in the region) grown up.
In order to investigate food sources for the prehistoric population, a series of stable isotope ratios 13C and 15N was measured for a relatively large spectrum of animals: otter, beaver, deer, dog, pig, wild boar, elk, seal, various water birds and sturgeon (fig. 12). These samples are a representative fauna in terms of associated bones found at both sites.
The 13C and 15N values for both this fauna and for the humans from the three locations are shown in figure 12.
Fig. 12 Stable isotope ratios δ 13C and δ 15N for bone collagen from various animals; also the human data (fig. 11) are included for comparison.
The three data points for otter cover a broad range in isotope ratio values for which no explanation is at hand. It is known that otters can live from either marine or freshwater food sources. Otters also show a broad range of reservoir effects in their 14C dates. The two data points for beaver are consistent in the herbivore regime. In the Groningen database, there are 5 stable isotope measurements for beaver from the province of Drenthe, the Netherlands. These show similar δ 13C values, but the δ 15N values range between 1 and 6 ‰. The results for the deer all plot in the herbivore region, with one exception; one roe shows an unexpectedly high δ 15N value. High δ 15N values are known for bones of herbivores living in arid areas and in coastal regions. This is a subject of further investigation. The values for the series of pigs and wild boars also show up in the herbivore region, generally ranging between -19.5 and -22 ‰ for δ 13C, and between 4 and 9 ‰ for 15N (Kuitems 2007). Two domesticated dog bones plot higher than the pigs and wild boars but lower than the humans, which is expected concerning their position in the trophic level. Values generally range between -21.0 and -23.0 ‰ for δ 13C, and between 9 and 15 ‰ for δ 15N (Kuitems 2007). There is one measurement for elk in the present dataset. The δ 15N values for this species are known to be low. The isotopic value falls in the range of a series of 17 animals from the Netherlands, with δ 13C values ranging between -21 and -23 ‰, and δ 15N values between 1.5 and 4 ‰ (Van Klinken 2005). The δ 13C and δ 15N values for only one fish bone (sturgeon) could be measured. A larger series of sturgeon bones did not produce good quality collagen or no collagen at all. The δ 15N value plots below the human values. Three bird bones yield δ 13C and δ 15N values: duck, swan and cormorant. They show extreme values on the stable isotopes chart. The duck has (δ 13C, δ 15N) = (-24.9 ‰, 2.8 ‰), the swan (δ 13C, δ 15N) = (-15.3 ‰, 11.7 ‰), and the cormorant (δ 13C, δ 15N) = (-14.1 ‰, 15.2 ‰). Modern (non-prehistoric) ducks are the subject of migration studies by biologists. The δ 13C and δ 15N values of their bones are not understood. For example, in Northern Canada the values of δ 13C range from -22 to -16 ‰, and the δ 15N values from 9 to 15 ‰, not depending on freshwater versus marine gradients as was expected, but on regional effects and biological factors such as the sex of the animal (Braune et al. 2005). The cormorant δ 15N value is very similar to those obtained from a stork from the province of Drenthe, the Netherlands, the Groningen database: (δ 13C, δ 15N) = (-18.59, 14.2) ‰. One data point for seal is obtained, with δ 13C and δ 15N values as expected.
In general, the stable isotopes shown in figure 12 are consistent with what is to be expected, concerning the trophic chain. The human bones are 3-5 ‰ in 15N higher than the food sources such as sturgeon.
Radiocarbon dates of food residues on pottery from Northern European inland areas are clearly influenced by freshwater reservoir effects, caused by fish and molluscs cooked in the pots. The food residues are shown to have possible 14C age excesses up to 300 years (Fischer & Heinemeier 2003).
In this study, we analysed food residues from Schipluiden for the stable isotopes δ 13C and δ 15N. We refer to food residues as charred, dark residue with a thickness up to a few mm, which adhere to the inner surface of cooking pottery. A total of 11 samples were analysed, following the procedure of Morton & Schwartz (2004). The values are presented in table 5 and depicted in figure 13. Values from Tybrind Vig, Denmark (Craig et al. 2007) are shown as well. The δ 15N values display similar values; the δ 13C values, however, are clearly less negative. This is possibly a coastal/inland effect. From Amose (Denmark), Fischer & Heinemeier (2003) analysed the food remains for δ 13C and 14C (dating), but included no δ 15N isotopes. They observe a change at a δ 13C value of -26 ‰ for Stone Age sites from Northern Europe. This is the average value of terrestrial food, and of some mixtures of marine and freshwater components. More negative δ 13C values correspond to marine samples, and less negative to freshwater samples (Fisher and Heinemeier, 2003). The δ 13C values from the coastal sites are generally more positive than those from the inland sites, which would imply that marine species formed a substantial part of the food cooked in the pots at coastal sites.
Following this observation from Denmark, we note that the majority of the Schipluiden food residues (8 out of 11) show δ 13C < -26 ‰ - suggesting a significant marine food component. The δ 15N values are elevated, with one exception, which supports this conclusion. The exception shows the lightest δ 13C value: δ 13C = -22 ‰, and also has the lowest δ 15N value: δ 15N = 5 ‰. Most likely, this vessel has been used for the cooking of (mainly) terrestrial food.
Table 5 Stable isotope ratios for food residues from Schipluiden.