Migration in the Bell Beaker period of central Europe.

Price, T. Douglas ; Grupe, Gisela ; Schroter, Peter 等

The movement of people in the past - via marriage, migration, conquest, colonization is an important topic in archaeology. The arrival of new people has often been used to explain the appearance of innovative features in the archaeological record. Examples are numerous and include the origins of modern humans, the spread of agriculture, the introduction of metals and many, many others. Arguments revolve around the effects of migration and diffusion vs independent invention (e.g. Adams et al. 1978; Anthony 1990; Champion 1992).

Rebuttal, rather than resolution, continues because movement and residential changes have been difficult to measure. Evidence is generally circumstantial; archaeologists have relied on indirect means, such as styles of pottery decoration, vessel form, architecture or other presumed signals of identity, to examine questions of mobility (e.g. Rouse 1986). Such proxy information is often suspect; materials may have moved through trade, exchange or other mechanisms without direct contact with the original producers.

A technique is described in this study for directly examining questions of prehistoric residential change, using strontium isotope ratios in human bone and tooth enamel. Differences in these ratios in the same individual indicate migration. Results from a study of Bell Beaker burials in Bavaria suggest that both human mobility in the Bell Beaker period and the potential of strontium isotope analysis are high.

The Bell Beaker period

The Bell Beaker is one of the more intriguing and lesser-known periods in European prehistory. Discussions of this time occupy surprisingly little space in texts and on library shelves, yet the evidence is fascinating. The term 'Bell Beaker' is used for a type of pottery and a group of people, as well as a period of time (Guillaine 1984; Luning 1994; Sangmeister 1972; Shennan 1986; Strahm 1997a; 1997b). The Bell Beaker period, appearing at the end of the Neolithic and the beginning of the Bronze Age, and dating from approximately 2500-1900 BC, is named after a distinctively shaped ceramic vessel, probably a drinking cup. The uniform pottery is found most commonly in graves that also contain certain, distinctive materials including jet and amber ornaments, some of the first gold and bronze objects in Europe, archery equipment, and occasional horse bones as well (Sherratt 1994). The Bell Beaker individuals found in these graves are frequently robust males with a distinctive 'short-headed' skull (e.g. Gerhardt 1976; 1978).

Bell Beaker materials are distributed irregularly from Denmark to Sicily and from Ireland to eastern Europe. Compilation and analysis of radiocarbon dates from this period suggests an origin in the Rhine delta shortly before 2500 BC in a Corded Ware context (Lanting & van der Waals 1976). In some regions such as the British Isles, the distribution of Bell Beaker materials is almost continuous, while in others the remains are very sparse. The end of the Bell Beaker is variable in these areas, depending on the date of appearance of Bronze Age materials. The Bell Beaker period lasts longer in Britain than elsewhere.

The exotic materials found in Bell Beaker graves, along with their patchy distribution, the general absence of settlement and the distinctive skeletal remains has constituted evidence for migration. In an attempt to explain Bell Beaker, Childe (e.g. 1950; 1957) used analogies with groups such as traders, prospectors, smiths, warriors, missionaries and a kind of gypsy folk. This interpretation has remained a standard view of Bell Beaker. Others, however, including Engelhard (1991), Harrison (1980) and Sherratt (1994), have suggested that increased social ranking, not population movement, was responsible for the adoption and spread of Bell Beaker materials as symbols of wealth and status. The question of the importance of mobility in the Bell Beaker period is unresolved. It has not been possible to determine if these materials were brought by their owners or imported.(1)

Bell Beaker folk in Bavaria

Bavaria was selected for this study in large part because of the high number of excavated burials from graves and cemeteries of the Bell Beaker period in this area (e.g. Engelhard 1991). More than 100 Bell Beaker sites have been excavated, with the majority found between the Danube and the Alps. The archaeological and anthropological evidence suggests that the closest connections to these Bavarian Bell Beaker materials are to the northeast or east, likely from southwest Poland, Hungary or Austria (Engelhard 1991; Gallay 1979; Menk 1979).

Human skeletal materials have been found either in small burial sites (10-30 individuals: Augsburg, Irlbach, Kunzing-Bruck, Landau SO, Osterhofen and Weichering) or as single or very small groups of burials (no more than five individuals: Altdorf, Landau, Manching, Pommelsbrunn, Straubing-Oberau and Tuckelhausen [ILLUSTRATION FOR FIGURE 1 OMITTED]. These sites have been excavated over the last 100 years and several are the result of recent rescue from construction (e.g. Kociumaka & Dietrich 1992; Weinig 1992). This material is housed in the State Anthropological Collection, Munich, Germany.

Bavaria can be divided into several zones geologically, split between north and south by the Danube River [ILLUSTRATION FOR FIGURE 1 OMITTED]. Zones to the north of the river are of mixed, non-uniform geological formations. The region northeast of the river is characterized by granitic sediments with isotopic ratios greater than 0.710, ranging to 0.750 and higher (Sollner, unpublished data). Sediments south of the Danube are largely glacial in origin, including loess deposits in a wide band along the river. The area between the river loess and the Alpine Foreland at the southern border of Bavaria consists primarily of glacially redeposited chalk sediments. The loess and marine carbonates have strontium isotope ratios ranging between 0.708 and 0.710. In terms of strontium isotope ratios, these areas are easily distinguishable analytically.

Strontium isotope analysis

The basic principles for the strontium isotope analysis are straightforward. Both elemental strontium content and its isotopic ratios in rock, groundwater, soil, plants and animals vary depending on local geology (Dasch 1969; Hurst & Davis 1981; Graustein 1989). The strontium isotope ratios in rock depend on the age and composition of the material (Faure & Powell 1972; Faure 1986).

Because materials in nature have variable strontium contents, variations in absolute 87Sr abundances strontium isotope compositions are expressed as isotope ratios. Variations in strontium isotope compositions in natural materials are conventionally expressed as 87Sr/86Sr ratios. The abundance of 86Sr is similar to that of 87Sr; the radiogenic isotope 87Sr comprises approximately 7.04% of total strontium and 86Sr is approximately 9.87% (Faure & Powell 1972). Thus the total global ratio of 87Sr/86Sr is approximately 0.71327 (7.04/9.87).

Elemental strontium in bedrock moves into soil and groundwater and into the food chain. In humans and other animals, the vast majority of strontium is deposited in hard tissue. The strontium isotope composition of human bones thus matches the diet, which in turn reflects the strontium isotope composition of the local geology.

These ratios serve as tracers of the geology of the areas where individuals grew up and where they died, respectively. Bone undergoes complete replacement or turnover of its inorganic phase (e.g. Jowsey & Gordon 1971; Parfitt 1983) so that measurements of bone strontium reflect the last years of the life of the individual. The enamel in teeth, on the other hand, forms during infancy and undergoes relatively little change during life. Enamel has very few internal organic structures and is thus considered inert tissue which does not recrystallize or remodel after formation (Steele & Bramblett 1988). Differences in strontium isotope ratios between bone and tooth of the same individual thus provide a record of mobility and residence change (Ericson 1985; 1989; Ezzo et al. 1997; Grupe et al. 1997; Price et al. 1994a; 1994b; Sealy 1989; Sealy et al. 1991; 1995).

TABLE 1. The number of burials, samples and immigrants from Bell
Breaker sites in Bavaria. A cut-off value of 0.001 is used to
identify immigrants in this table.

cemetery no. no. of migrants % migrants
 of samples @ 0.001

Altdorf 2 1 50.0
Augsburg 14 2 14.3
Irlbach 12 2 16.7
Kunzing-Bruck 6 0 0.0
Landau 9 4 44.4
Manching 3 1 33.3
Osterhofen 8 3 37.5
Pommelsbrun 1 0 0.0
Straubing-Oberau 5 2 40.0
Tuckelhausen 2 0 0.0
Weichering 7 2 28.6

total 69 17 24.6

Post-depositional contamination of bone and tooth (diagenesis) is not a significant problem in strontium isotope studies. Certainly, bone is more susceptible to diagenesis than the denser dental enamel (Molleson 1988; Vernois et al. 1988); cleaning techniques have been developed which remove much of the diagenetic contamination from bone (Price et al. 1992; Sillen 1989). Using such cleaning techniques, Sealy (1989) was successful in recovering the known, biological values from bone which was initially contaminated with diagenetic strontium of a different isotopic ratio. Other researchers (e.g. Staudigel et al. 1985; Schmitz et al. 1991; Koch et al. 1992) using such acid-cleaning procedures have had similar success obtaining 87Sr/86Sr measurements from fossils as old as the Palaeozoic era. It is also the case that diagenetic strontium will reflect local isotope ratios. In such situations, contamination can only mask, not provide, the evidence for migration and seems to be a minor problem.

Results and interpretation

The number of samples and the number of burials from each of the Bell Beaker sites in Bavaria is presented in TABLE 1. Samples of bone and/or tooth enamel were obtained from a total of 69 Bell Beaker individuals and prepared for analysis.(2) Strontium isotope ratios were measured in these materials on a thermal ionization mass spectrometer.(3) This is the largest sample of 87Sr/86Sr ratios in prehistoric human tissues available anywhere to date.(4) The results of the analyses are presented in FIGURE 2.

The basic expectation of the study was that differences in strontium isotope ratios between bone and tooth of the same individual would indicate residential change. There are clear differences between bone and tooth in the graph, but there is also a good bit of variation. It is not always obvious how to distinguish migrants, where to draw the line. Samples with high isotope ratio values clearly indicate residential shift, but what about the large number of intermediate values? In this paper we discuss two methods for determining the cut-off point:

1 a value based on geological differences in the area, and

2 a value based on bone strontium isotope ratios as an indicator of indigenous values.

1

The geochemical differences in 87Sr/86Sr ratios for the geology of Bavaria are highly significant. For example, migration from a granitic area (with values greater than 0.710) into a region dominated by carbonate-rich soils (with values averaging slightly more than 0.709) would be readily visible in the bone and enamel. A conservative cut-off value of 0.001 between the bone and enamel measurements should permit probable immigrants to be identified. Using this value, 16 of 68 tooth-bone pairs from the Bell Beaker period indicate immigrants. In addition, a Bell Beaker child from the site of Straubing has a very high Sr isotope ratio in tooth enamel of 0.716210 (no compact femoral bone was available). This child must have moved a substantial distance in the early years of its life. Thus, a total of 17 of the 69 Bell Beaker individuals, almost 25% (24.6%), changed residence to new geological regions during their lifetimes.

2

A second cut-off value was also used, based on isotope ratio values in bone, which provide a conservative estimate of immigrants to this area. The mean value +2 standard deviations was used; the bone mean +2 s.d. value was 0.7103; 97.5% of the bone values fall within this cut-off point. This bone value is used as a cut-off point in the enamel 87Sr/86Sr data to distinguish immigrants. This value substantially exceeds the maximum 87Sr/86Sr value for soil samples from the project area (0.70992). Using this criterion, 13 enamel samples (18.8 %) lie beyond the cut-off value of 0.7103.

Thus, our estimates for the number of immigrants in the Bell Beaker burials from southern Bavaria range between 18.8% and 24.6%. These values estimate the proportion of the burial population that moved into this area from places with a different geological composition, either elsewhere in Bavaria or central Europe. These are very conservative estimates for several reasons, including:

1 the geological zones of Bavaria are quite large and migration within the same geological region will not be discernible with strontium isotopes;

2 the numerical cut-off values are cautious as discussed above; and

3 the turnover rate in cortical bone is very slow.

It is very clear that not all individuals are changing residence; there are no indications of mass migration or demic diffusion. There are several generations of the population represented in the burials and some of these have not moved from outside the local geological regime in their lifetime. The strontium isotope data also provide other details. Examination of the chronology of the individual sites indicates a tendency for higher migration rates in the earlier part of the Bell Beaker period. The sites from this initial period are smaller and measured rates of migration vary from 33% to 50%. The larger cemeteries of Irlbach, Augsburg and Weichering are dated to the younger phase of the Bell Beaker period and contain more burials, 20-30 inhumations, perhaps an indication of a more sedentary population; migration rates are lower, 14.3% to 33%.

TABLE 2. Age distribution of Bell Beaker burials in this study.

age no. of burials

child 9
juvenile 8
adult 59
unknown 1

Age and sex information on the burials is summarized in TABLES 2 & 3. With regard to sex, females were somewhat more mobile than males. Many of the skeletons could be sexed either anthropologically or archaeologically; 38 were males and 24 were females. An equal number of males and females (8 each) in the total sample were indicated as mobile using the 0.001 cut-off value. Thus a higher proportion of females appear to be migrants; this pattern is seen at the individual sites as well. This higher proportion might be explained by the practice of female exogamy in marital residence. This practice results in the movement of females from an ancestral residence to the community of the male. At this point, however, the evidence is only suggestive; additional research on this pattern is needed.

It is possible to examine distances and direction of migration as well. The overall direction of migration for the Bell Beaker people, based on the strontium isotope data appears to be from northeast to southwest, i.e. from granitic to chalk and loess sediments. The variation seen in the strontium isotope ratios for tooth enamel suggests that migrants came from a number of different areas. We can roughly estimate the minimal distances traversed by migrant individuals. Among the sites investigated, the cemetery at Augsburg was at a maximum distance from the granitic deposits northeast of the Danube [ILLUSTRATION FOR FIGURE 1 OMITTED]. Graves 9 and 10 at Augsburg contained two migrants; if those two individuals travelled along the river valleys, first to the west and then to the south, the total distance involved was at least 220 km. Of course, these individuals may have come from further afield in Bavaria or elsewhere.

TABLE 3. Sex distribution of Bell Beaker burials in this study.

sex no. of burials

male 38
female 24
unknown 13

In sum

The application of strontium isotope analysis to human skeletal material from the Bell Beaker period has provided strong evidence that migration was substantial in this period. Strontium isotope analysis of human bone and teeth appears to be a very useful technique for the investigation of prehistoric population movement and residential change. The range of applications is wide - documentation of foreign rulers, questions of animal pastoralism and transhumance, evidence of conquest, marital residence change, and many other questions can be addressed. This list continues to grow and we anticipate a variety of reports utilising this new and important method in the coming years.

Acknowledgements. There are a number of individuals and institutions to be acknowledged in a project such as this. Funding for this research was supplied in large part by the Deutsche Forschungsgemeinschaft. Additional funding came from the US National Science Foundation (BNS-8702731 and BNS-9111680). A number of colleagues have contributed to this research including Brian Beard, James Burton, Bernd Herrman, Clark Johnson, Jan Lanting, Dr A.M. Parfitt, Steve Shennan, F. Sollner and C. Strahm. Sample preparation was done in Madison by Kathie Evans and Bill Middleton and in Munich by Dirk Weickmann.

1 One of the very few archaeometric studies of Bell Beaker materials involved the analysis of the chemical composition of both local and Bell Beaker ceramic vessels (Rehman et al. 1992). The results of the study indicated that Beaker and local pottery was similar, rather than different, in composition and the obvious inference was made that it was all produced locally. Given the results of the present study, it might be suggested that immigrant individuals may have been producing Bell Beaker vessels from local materials.

2 Laboratory procedures for preparation and analysis of bone and enamel samples are reported in Grupe et al. 1997.

3 The study was undertaken as a joint project between the University of Wisconsin-Madison (USA) and the University of Munich (Germany). Approximately half the samples were analysed at each locality. Several samples were analysed in duplicate at the two laboratories to insure comparability of results.

4 A preliminary report on this work appeared in Price et al. 1994a and a more detailed study in Grupe et al. 1997. The complete data-set for this study is reported in Grupe et al. 1997.

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