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Strontium isotope characterization in the study of prehistoric human ecology
Jonathon E. Ericson Program in Social Ecology, University of California, lrvine, California 92717, U.S.A. Keywords: strontium isotopes, residence patterns, catchment, Calitbrnia, characterization.
Strontium Isotope Characterization in the Study of Prehistoric Human Ecology Residence patterns provide keys to social structurc, information tlow, and pattcrns of material culture. A biogeochemical model has been formulated using tooth eruption sequence, elemental exchange rates in bone tissue, and the geochemistry of strontium isotopes--to examine patterns of human residence in the past. Strontium isotopes, characteristic of local geology, pass unmodified through the food chain, isotopic values of human second molar teeth, representative of the individual from ages six to twelve, and bone, representative of the last six years of life, characterize thc food chain of pre-marital and marital residences respectively. Marriage residence patterns are determined by stratifying age/sex data. The biogeochemical model and preliminary results are presented for two sites in California, Journal of Human Evolution (1985) 14, 503-514
1. I n t r o d u c t i o n My aim in this paper is to present a methodological approach using strontium isotopes as tracers for studying specific behavioral patterns (Ericson, 1980): (1) There as been a considerable interest in catchment analysis particularly in terms of thc prehistoric utilization of biotic resources (cf. Findlow & Ericson, 1980; Roper, 1979; Higgs & Vita-Finzi, 1972; Vita-Finzi & Higgs, 1970). In conjunction with other botanical analyses, the assay of strontium isotopes in h u m a n tissues can be used to identify the catchments used by prehistoric peoples. (2) There is a long, and generally unsuccessful, history of the investigation of the importance of marine resources for h u m a n subsistence (cf., Stark & Voorhies, 1978). Strontium isotope analysis of h u m a n bonc should provide some resolution of these questions. (3) Is it possible to accurately trace prehistoric individuals back to pre-marital places of residence? Over the past two decades, the analysis of prehistoric residence patterns has received serious attention. A n u m b e r of problems, however, currently limit the accuracy and effectiveness of these studies. Analysis of ceramic designs has been used to postulate particular forms of social organization (Longacre, 1964; Deetz, 1965; Hill, 1972), although there is insufficient information to understand which mechanisms arc responsible for such variation. Likewise, clusterings of discrete osteological traits in a population have been used to reconstruct genetic groups (Lane & Sublett, 1972) and, in turn, suggest residence rules (Birkby, 1982; Spence, 1974). Even with these advances we arc unable to specify the locations ofpre-marital residence for contemporaneous individuals in the past. A characterization procedure is needed which will routinely and accurately identify the pre-marital residence "catchments" of individuals. T h e pilot study described herein dcmonstratcs that strontium isotope ratios are an effective means for studying dietary input ti-om discrete geochemical environments. This method is drawn from the fields of geochronology and geochemistry. Strontium, originating in parent rock, passes through the food chain. There is sufficient natural variation in the strontium of thc parent rocks that it can be used to characterize and 0047-2484/85/050503 + 12 $0300/0
9 1985 Academic Press Inc. (London) Limited
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differentiate local environments. The proposed method uses the isotopic ratios of biogenic strontium as tracers and signatures of specific catchment areas. This study is a logical extension of chemical characterization investigations which have been used to trace tile sources of different artifhctual materials. The development of the proposed technique will significantly advance the forensic repertoire of anthropology. Many different catchments within specific geological provinces will be characterizable, eventually leading to the reconstruction of patterns of intermarriage within a region.
2. Characterization by Strontium Isotopes The fundamental assumption underlying the application of this technique is that strontium, originating in a local source material, passes through the tbod chain without significant fractionation of its isotopes. The biogenic record of strontium, as reflected by the diet of an individual, is incorporated into skeletal tissues and stored for long periods of time (Toots & Voorhies, 1965; Parker & Toots 1970; Parker, 1968; Schoeninger, 1979a,b; Sillen 1981a,b). In contrast--and not to be confused with strontium isotope research--elemental strontium relative to calcium does decrease as it passes through the food chain (e.g., Elias et al., 1982; Price, Schoeninger & Armelagos, this issue). The change in the strontium/calcium ratio at each trophic level has stimulated interest in paleonutritional studies. An excellent review of such studies is now available (Sillen & Kavanagh, 1982). In living systems, strontium as well as barium and lead tends to follow calcium as elemental constituents during dietary uptake, internal distribution within the body, and excretion (Schroeder et al. 1972; Underwood, 1977; Elias el al. 1982). Biopurification factors--ratios of non-nutrient metals to calcium in nutrients and consumer--are important to understand the transfer of strontium relative to calcium from soil to plants to humans it is difficult to suggest a mechanism in these passive or other elemental processes which would lead to the separation of strontium isotopes, i.e., tiactionation. Soil (Dasch, 1969) and plants (Hurst & Davis, 1981) are equilibrated with the local source rock and share similar isotopic ratios for strontium. Plants reach chemical equilibrium with strontium isotopes available in soil moisture. Recent work by Hurst & Davis (1981) on the absorption of strontium f?om coal flyash by plants indicates that the isotopes achieve rapid equilibrium with the flyash in the soil. Plants were also in isotopic equilibrium with the leachable strontium from soils without flyash. This indicates that bioavailable strontium in the environment will not be fractionated by the processes involved in the plant and animal uptake of strontium (Hurst & Davis, 1981; Straughan el al., 1981). Work by Warren & Spencer (1978), however, suggests that there is biological t?actionation of strontium isotopes in humans. Nevertheless, general geochemical data indicate that isotopes with atomic numbers (Z) above 40 do not fractionate under normal biological conditions (C. C. Patterson, pets. comm.). Strontium has Z numbers at 86 and 87. Observations by Warren & Spencer (1978) may reflect insufficient time tbr the ingested tracers to achieve equilibration with the major reservoir of strontium stored in bone. Geochemistry of Slrontium
Fortunately the geochemistry of strontium is well known. Strontium isotopes are used routinely to study the structure of the earth in terms of the origin and the age of rock (Faure & Powell, 1972). Strontium (Sr) and rubidium (Rb) are contained in amounts of generally
STRONTIUM
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505
less that 1% in most igneous, metamorphic and sedimentary rocks. Strontium substitutes for calcium in minerals such as plagioclase and apatite. Rubidium substitutes for potassium in minerals, particularly micas and potassium feldspars. Strontium and rubidium follow different magmatic pathways. Rubidium has one naturally occurring radioactive isotope, 87Rb, which decays to a stable isotope of strontium. Thus, rocks with more Rb will contain more 875r produced by radioactive decay than those rocks containing less rubidium. Hence, the isotopic composition of a rock is a function of the Rb/Sr ratio and length of time. As a result, both geological age and rock type are significant in creating variability in the proportion of strontium isotopes observed in rock. Although this ratio in granites, the most common continental rocks, varies from 0-700 to 0"737 (Figure 1), each granite carl be characterized by discrete isotopic values for strontium. The technical procedures, which require mass spectrometry and uhraclean laboratory preparation to avoid contamination, have been highly refined. The precision of isotopic measurement can be as low as +0'00005 depending upon the analytical system employed. Given the large natural variation in continental granites (0"037) and the precision of measurement (0"00005), the scale of discrimination among granites is 740 units. Figure i. Naturat variations of strontium isotopes in continental granites {dots) and ocean basahs over geologicaltime (after Faure & Powell, 1972).
-
0.735
-
0.730 0.725 ,_ 0.720 0.715 "5 r
0.710 o9
0 ~ 9 00
9 9
9
..
z.O00
3000 2000 1000 Time in millions of years
-
0.705 9
0.700 Present
It is important to note that the strontium values fbr continental granites are linearly correlated with geological age. This trend suggests that one can estimate the expected isotopic ratio for a granite if its geological age is known. Likewise, in some cases, the isotopic values for derived sedimentary and metamorphic rocks can be estimated if the parent granite and its geological age are known. This correlation will be usethl in modelling the isotopic differences between various continental rock types.
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In contrast, the mean isotopic composition of ocean basalts is relatively invariant, exhibiting a ratio of approximately 0"7037 + 0.0001 (Faure & Powell, 1972). Food chains, dependent solely upon ocean basalts for strontium nutrients, would be impossible to distinguish using this technique. However, in some coastal environments the discrete values for the marine (sea water with an isotopic value of 0.7091) and terrentrial (ocean basalt) food chains would greatly enhance the reconstruction of maritime subsistence strategies. Table 1 illustrates a hypothetical case for a Hawaiian coastal environment as a study of the dietary fractions using carbon and strontium isotope ratios. It is important to note that the isotopic values of sea water have changed over geological time as a long-term trend (Faure & Powell, 1972). These values will be retlected in certain marine sedimentary rocks like marine limestone. In summary, significant variation in strontium isotopes exists as a function of age and type of rock. Continental rocks such as granites have a greater range of variation than do ocean basalts. Since strontium isotopes, characteristic of local geology, pass unmodified through the food chain, they can be used as signatures of portions of the local landscape. Hypothetical Hawaiian catchment: estimation of dietary fractions of marine shellfish and terrestrial plants using results of carbon and strontium isotope analysis and mixing equations
Table 1
Analysis
H u m a n bone
M a r i n e shellfish
T e r r e s t r i a l plants
81:~C% ~TSr/~Sr
-20"6 0'7075
-15"0 0"7091"
-22'0 0"7037~
E q u a t i o n 1: -20.6 =
E q u a t i o n 2:
AP(-15.0) + BQ(-22-0)
AM(0.7091) + BN(0-7037) 0.7075 =
AP + BQ Where A = % Shellfish in diet P = C a r b o n in shellfish M = S t r o n t i u m in shellfish
A M + BN
B = % terrestrial plants in diet Q = C a r b o n in plants N = S t r o n t i u m in plants
Let P = Q: M = 300,000 p.p.m.; N = 200 p.p.m.; solvc simultaneously tot A and B. Ans: A = 20% shellfish; B = 80% plants * Value of ocean water; "~ Value of oceanic basalts (Faure & Powell, 1972, p. 41).
Identifying Residence First of all, the technique proposed here for identifying residence requires that catchments of contemporaneous settlements be characterizable by strontium isotopic ratios, i.e., there must be significant geological variation among the dilt~rent residence areas. Secondly, the h u m a n body produces a variety of biogeochemical records during the lifetime of an individual. Each secondary tooth, for example, contains the biogeochemical record of a specific period of growth and time of emplacement. Although a secondary tooth does absorb some strontium fi'om saliva, it is extremely thin--10-30 btm (Little & Barrett, 1976)--and can be removed by excoriation in sample preparation process. The interior enamel portion, however, is isolated biologically from further chemical exchange and can be used for analysis. This interior structure preserves the biogenic record of strontium
STRONTIUM ISOTOPE
CHARACTERIZATION
507
intake for long periods of time (Toots & Voorhies, 1965: Parker & Toots, 1970; Parker, 1968; Schoeninger, 1979a,b; Sillen, 1981a,b; Steadman et aL, 1958). The strontium isotope ratio of the secondary tooth, excluding the crown, reflects the local environment during emplacement of the tooth. If emplacement occurs prior to marriage, the tooth should contain a biogenic record of the environment of pre-marital residence. If we assume that the common age of marriage occurs after twelve years of age in both males and females, the second molar is a most suitable tooth representing pre-marital diet. Information on post-marital residence (as distinguished from the location of burial) requires that the pre-marital biogenic record be erased, diluted or effectively displaced within specific tissue. Fortunately, bone tissue--in contrast to teeth--is continuously remodeled. In a period of six years, bone undergoes complete replacement of its chemical constituents (Glimcher, 1976; Rasmussen & Bordier, 1974), thereby equilibrating with a new environment. The comparison of the isotopic records of secondary teeth and bone in an adult provides the basis for distinguishing pre- and post-marital residences, if the individual was relatively sedentary as a child and marries as an adolescent into a new community, the differences in the tbrmer and latter geochemical reservoirs will be detectable after six years by comparison of tooth and bone from that individual. By stratifying a cemetery population by age and Sex, the pattern of marital residence can be constructed. For example, if the residences of males--as measured by strontium ratios in the second molars--are non-local, different from and more variable than females, then a pattern ofmale exogamy is implied. Potentially, the technique is far more powerful. If each catchment of a region can be characterized by an isotopic signature, then each individual can be traced to his/her place of residence resulting in the determination of regional intermarriage patterns.
Some Recognized Problems There arc a number of recognized problems which affect the application of this technique. Some of these problems can be overcome, others cannot. (1) Geological variability. The proposed method requires that significant variation in strontium isotope ratios be present among difl~rent catchments. If not, application will be limited. Preliminary analysis of an area should include an estimate of its isotopic variability, which can be estimated from geological maps which provide initial data on the geological age, location, and variation in rock types and isotopic data bases. Compilation of these data should provide an estimate of the expected strontium isotope ratios within and between theoretical catchment zones. (2) Catchment defnition. The spatial definition of catchment zones to define major areas of food procurement is a critical problem. Initially, Vita-Finzi & Higgs (1972, p. 31) suggested "a radius of 5 km tbr the analysis of sedentary exploration territories and 10 km for mobile economies." Since it is useful to express these distances in terms of travel time to compensate for topographic variation, walking travel-times of one and two hours have used. More recently, "work space" has been proposed as an alternative means for evaluating energy expenditure on a complex surface (Ericson & Goldstein, 1980). Dennell (1980) has recently criticized simple catchment studies for sedentary groups, suggesting that landforms and soil types are more reliable guides to the prehistoric landscape than modern land-use data. defining group catchment remains a problem.
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If the population under study is extremely mobile, the biogeochemical records of teeth and bones will be very difficult to interpret, nevertheless, given prcscnt analytical technology, it is possible that mobility patterns can be discerned by measuring the isotopic values on a crystal-by-crystal basis following the growth patterns in teeth using an ion probe. (3) Catchment characterization. The characterization of the catchment is another major problem. There is no "magic circle" of containment that can be drawn around any group without a high degree of uncertainty. Two approaches may resolve some of this problem. Direct empirical study is preferred, involving the measurement of strontium isotope ratios in bone from a sufficient number of adults in the population to estimate an average Value. Secondly, soil and/or biological samples such as plants and/or rodents can be collected at grid points within the site catchment. Various sample mixtures can be made which are consistent with different theoretical or empirical catchment models. The strontium isotope value of each mixture could be compared with values from the human population. In cases of environmental pollution or geochemical change due to modern land use, parent rock samples could be substituted to establish the baseline for strontium isotope ratios. (4) Biogenic contamination. As noted earlier, strontium is exchanged on the surface of teeth. This reaction layer, as well as the interior surface at the dentine interface, can be removed in the laboratory. The remaining inner enamel is effectively isolated and most useful for analysis. (5) Diagenic contamination. Diagenic alteration is another major problem. Occasionally, the biogenic record in tooth and bone may be obliterated by soil moisture contamination. Soil contamination occurs more readily in bone than in tooth enamel (Ericson et al., 1979). The major contamination of bone, however, apparently comes from the remineralization of tissue with secondary calcite. Decontamination of tissue is best accomplished by mechanical excoriation, tissue separation, and phase separation by chemical reaction. The sample is broken, sieved, sorted, and reacted with acetic acid (Ericson et al., 1981; Sullivan & Krueger, 1981), preferably in an ultraclean environment to avoid contamination. Diagcnic change is a problem that can be alleviated by proper sample preparation. occasionally, however, samples simply cannot be used. The criteria and tests for the evaluation of sample contamination need to be defined. (6) Masking by high strontium foods. The consumption of sufficient quantities of high strontium foods will swamp or mask low strontium foods that are consumed in larger quantities. The ingestion of sufficient strontium hyper-accumulator plants (Coughlin & Ericson, 1981) or derivatives of specific hyper-accumulating tissue such as bark (Elias el al., 1982) or marine shellfish (Schoeninger & Peebles, 1982) can cause problems in interpreting the biogenic record. It is essential to consider such masking effects through the assay of strontium concentrations in foodstuffs.
3. Potential Applications Although a number of inherent problems have been identified, careful research will lead to an ultimate gain in knowledge. A few applications for this new technique are indicated below.
STRONTIUM
ISOTOPE
CHARACTERIZATION
509
(1) Human ecology and territoriality. We will be able to study prehistoric subsistence strategies and human territoriality through strontium isotopic characterization. We can compare observed isotopic values of human populations with expected values drawn from biologicaI data, grouped to simuiate theoretical catchment models. (2) Food sharing and exchange. We will be able to study isotopic variation reflecting dietary differences due to food sharing and/or exchange within and between groups. Categories for analysis can include age, gender, social status, household, and occupational/craft groups. (3) Migration and warfare. We will potentially be able to identify patterns of human migration by studying founding populations and regional catchment patterns. Likewise, the origin of "foreign" military groups who died in battle and were interred in foreign soil will be discernible. (4) Marital residence. As discussed above, it is proposed that we should be able to describe the prehistoric patterns of marital residence, which may be linked to social structure, as well as information, energy, and culture flow. (5) Animal ecology. The strontium isotope tracing technique can be extended to questions regarding the hunting of wild animals and general problems of animal ecology. With domestication, the home range of animals is more restricted, leading to more pronounced differences between wild and domesticated species. This regulating of" domestic animals can be examined directly. (6) Exchange of food. For the first time, we should be able to identify the origin and movement of certain non-local foodstuffs. We will be able to characterize foods and potentially identify their origin. In all, the strontium isotope tracing technique offers an important breakthrough for the study of past human ecology and social organization. At this preliminary stage, it is important to determine if biogenic records can be observed in human tissue and if these records conform to paleodietary and environmental data. The foIlowing pilot study documents these conditions.
4. A Pilot Study The prehistoric Chumash Indians who occupied a portion of the coast of the Pacific Ocean and the interior of the Santa Monica Mountains near Los Angeles, California, offer a useful population for study. Chumash society was the result of more than 7000 years of cultural development. This growth culminated in a society with heirarchically organized political systems and an active morley economy (King, 1982a). Ethographic and ethnohistoric data indicate that there were frequent migrations of families over long distances within the Chumash area (Hudson et al., 1981 ; Johnson el al., 1982; King, n.d.). Ethnohistoric data for the Santa Monica Mountain Chumash have not been thoroughly analyzed to determine patterns ofpre- and post-marital movement. Available data suggest that neolocal residence within the area was common and that exogamous marriages were ambilical (Johnson el al. 1982; King, n.d.; Edberg, 1982). The people who lived in the Malibu Canyon drainage, the study area, were closely related and most likely moved between villages with great frequency. Two sites along the Malibu Canyon drainage provided tooth and bone samples: (1) Malibu (CA-LAn-264) is the historic village of Humali, located on the coast along Malibu Lagoon, an estuary by the streams of Malibu Canyon. Two cemeteries from
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this site have been excavated. One, containing 129 burials, was used between approximately A.D. 1786 and 1816 during the period of Spanish mission recruitment. The other cemetery, dating from the late Middle Period (ca a.n. 700-1000), has yielded approximately 40 burials. Food remains recovered from the midden at this site indicate that fish, shellfish and sea mammals constituted an important part of the diet of the inhabitants. Remains from vegetable foods and land mammals document the contribution of terrestrial resources. Ethnographic and ethnohistoric information indicate that Humaliu, the largest village in the area, was the capital of the Santa Monica Mountain Chumash. (2) The Century Ranch sites (CA-LAn-225, 229, 227 and 840) are located in the canyon approximately 7-5 km inland from Malibu. Several cemeteries have been excavated in this area, including (a) a late Middle Period cemetery at CA-LAn-840, adjacent to Century Ranch, yielding 30 burials, and (b) a Late Period, phase 2B (ca a.D. 1650--1770), cemetery at CA-LAn-227, containing 22 burials. Recent excavations at the historic village of Talepop (CA-LAn-229) as well as ethnographic data, indicate the local plant and animal foods provided virtually all subsistence. At Talepop ocean fish were a more important part of the diet during the early occupation (ca A.D. 900-1200)--the period of concern--than later (ca A.D. 1650--1803) (King 1982b). Historical records indicate that several couples living at Talepop had been born at other Santa Monica Mountain villages and had established neolocal residence. There were strong kin ties between the villagers of Humaliu and Talepop (Edberg, 1982). Fortunately for this study, both Malibu and Century Ranch contain coeval cemeteries. The graves at Malibu Village on the coast date to 1000 B.P. The small cemetery at LAn-840 near Century Ranch is dated to approximately 1000 B.P. by the obsidian hydration method (C' Meighan, pers. comm.). The strontium isotopes of the second molars and metatarsals of a mate and female from Matibu and of one individual from Century Ranch were analyzed by ultraclean techniques and mass spectrometry at Lamont Doherty Observatory. The results are presented in Table 2.
5. Determination of Geochemical Endpoints The Malibu Creek study area at Century Ranch is part of a drainage basin underlain by the Topanga Formation. The lower and upper Topanga Formation members are composed of marine sediments while the middle member is the Miocene Conejo Volcanics suite. The Chumash population which lived in this area would have derived their food from soils developed as mixtures of the upper Topanga Formation member and the Conejo Volcanic suite. Evidence for this comes from: (1) the location of the village on the Conejo Volcanic suite, (2) the fact that the upper Topanga Formation member lies immediately to the north of the village, and (3) the path of the drainage of Malibu Creek, beginning in the upper Topanga Formation and proceeding to the south through the Conejo Volcanic suite. Geology, geochemistry and geomorphology for the area is well documented. Blackerby (1965), Higgins (1976), and Crowe el al. (1976) have investigated major element geochemistry and the general geology of the Malibu area as well as Santa Cruz Island, an extension of the Santa Monica Mountains. More detailed petrographic geologic and isotopic data are provided by Hurst (1978, 1979a,b, 1982, 1983) for these same areas. There
511
STRONTIUM ISOTOPE CHARACTERIZATION
is no evidence to suggest that d r a m a t i c climatic or g e o m o r p h o l o g i c a l changes h a v e modified the d r a i n a g e p a t t e r n s or direction of s e d i m e n t t r a n s p o r t over the last 1000 y e a r s (Hurst, pers. comm.). T h e soils within the c a t c h m e n t are locally derived,-the s u r r o u n d i n g ridges m i n i m i z e m a j o r influxes of extraneous sediments, and the d r a i n a g e is r e a d i l y t r a c e a b l e t h r o u g h the source rocks.
Human Strontium Isotope Ratios As a p r e l i m i n a r y test of the s t r o n t i u m isotope tracing technique, several samples of h u m a n bone a n d teeth were e x a m i n e d to see if s t r o n t i u m isotopes could indeed be used to d e d u c e d i e t a r y p a t t e r n s ( T a b l e 2). Briefly, the C h u m a s h test case indicates that the two coastal Table 2
Preliminary results of strontium and carbon isotope analysis of three Chumash individuals from Malibu (LAn-264) and Century Ranch (LAn-840)
Site
Provenience
Tissue
Sex
Age
aTSr/ll~Sr
LAn-264 LAn-264 LAn-264 LAn-264 LAn-840 Lan-840
Burial 9 Burial 9 Burial 18 Burial 18 Burial 7 Burial 7
M2 Tooth Metatarsal M2 Tooth Metatarsal M2 Tooth Bone
F F M M --
52-59 52-59 23 28 23-28 ---
0"70857_+ 10 0'70876+ 15 0"70841+ 1l 0"70892+--23 (/-70765 + i I --
61:~C
- 15"1 -14"8 - 16"9
i n d i v i d u a l s (aTSr/86Sr = 0'7088 a n d 0"7089) from M a l i b u village had a diet based heavily on m a r i n e foods ( s t r o n t i u m isotope ratios in sea water are 0'7091), consistent with the archeological evidence. T h e g e o c h e m i s t r y of the diet of the female, i n d i c a t e d by s t r o n t i u m isotope ratios, did not c h a n g e significantly from adolescence (second molar, 87Sr/aGSr = 0"7086) to a d u l t h o o d (metatarsal, 87Sr/86Sr = 0'7088). T h e isotopic ratio from her tooth indicates t h a t she resided in the s a m e or a n o t h e r coastal village or off-shore island as an adolescent a n d subsisted largely on m a r i n e tbods. As noted previously, s t r o n t i u m isotopes will likely not distinguish between coastal village areas. T h e i n l a n d i n d i v i d u a l (87Sr/8(~Sr = 0"7077) from the n e a r b y C h u m a s h site at C e n t u r y R a n c h a p p e a r s to have an elevated value for this ratio relative to the soil (87Sr/~6Sr = 0"7045) in the area. These soils are derived from a m i x t u r e of the Conejo Volcanics (STSr/a6Sr = 0'7032, H u r s t , 1983) a n d the T o p a n g a F o r m a t i o n (87Sr/a~Sr = 0'7088, H u r s t , n.d.). T h e bone s a m p l e ratio is higher than soil moisture (0-7045) as well, indicative o f a biogenic signal r a t h e r than diagenic c o n t a m i n a t i o n . T h e elevated value is consistent with e t h n o g r a p h i c d a t a r e g a r d i n g the C h u m a s h which indicate t h a t inland villagers spent the fall and winter in the i n l a n d villages but moved to the Pacific coast d u r i n g the spring a n d s u m m e r as well as c o n s u m e d fish during this period. If so, the terrestrial c o m p o n e n t of the diet of the C e n t u r y R a n c h individual is masked by the high c o n c e n t r a t i o n of s t r o n t i u m from shellfish (Schoeninger & Peebles, 1982). C a r b o n a n d nitrogen isotope ratios can be used to resolve the relative c o n t r i b u t i o n o f m a r i n e foods in the diet. C a r b o n isotope analyses of these three individuals ( T a b l e 2) were c o n d u c t e d to verif) the s t r o n t i u m results. T e r r e s t r i a l tbods, excluding non-Calvin p l a n t s have c a r b o n isotope values of a p p r o x i m a t e l y 813C = -22O/oo. M a r i n e shellfish have c a r b o n isotope values (8~:~C = - 150/oo). It is clear that the carbon isotope ratios in h u m a n bone of the two M a l i b u s a m p l e s are closer to the m a r i n e end of this spectrum. T h e p a t t e r n is
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consistent with ethnobotanical and faunal reconstructions. The Century Ranch case is more difficult to interpret since analyses were perlbrmed on different tissues. Nevertheless, both the carbon and strontium isotopic values are intermediate to the isotopic endpoints for marine and terrestrial foodstuffs which is internally consistent. 6. C o n c l u s i o n s
The strontium isotope characterization technique following further development will offer numerous possibilities for investigating various aspects of prehistoric lifeways. There arc significant natural variations in the isotopic ratio of these isotopes that are of value in the study of dietary patterns. The primary applications of the technique will be for characterizing catchment areas, estimating marine v s terrestrial contributions to the diet, and determining marital residence patterns. At this preliminary stage, the strontium isotope technique appears to offer an exciting new source of information for the study of prehistory. The author gratefully acknowledges the early encouragements of Professor C. C. Patterson, Department of Geochemistry, California Institute of Technology, and Professor G. R. Tilton, Department of Geological Sciences, University of California, Santa Barbara, Professors Clement Meighan and Gall Kennedy, Department of Anthroplogy, UCLA, provided skeletal samples for analysis. The author acknowledges useful discussions with Linda Bruncher, Department of Anthropology, University of California, Berkeley, Drs Alan Zindler and Humbert Stautigel, Department of Isotope Geology, Lamont-Doherty Geological Observatory, and Dr Chester King of Topanga, California and Professor R. Hurst, Department of Geology, California State University at Los Angeles for providing data notes and criticisms on an earlier version of the manuscript. The author acknowledges the support of the Milton Fund, Harvard Medical School, and Professor Crompton, Harvard Museum of Comparative Zoology. Dr H. W. Krueger, Geochron Laboratories, Cambridge, MA, kindly provided the three carbon isotope analyses. References
Blackerby, B. A. (1965). The Conejo Volcanics in the Malibu Lake Area of the Western Santa Monica Mountains, Los Angeles County, CA. Ph.D. dissertation, University of Calilbrnia, Los Angeles. Birkby, W. H. (1982). Biosocial interpretations from cranial nonmetric traits of Grasshopper Pueblo sketetat remains. In (W. A. Longacre, S. J. Holbrook, & M. W. Graves, Eds), Multidsciplinary Research at Grasshopper Pueblo Arizona, pp. 36-41. University of Arizona, Anthropological Papers, 40. Coughlin, E. & Ericson, J. E. (1981). Biogeochemical residues as ethnobotanical indicators. Harvard Botanical Museum Leaflets 28, 71-80. Browe, B. M., McLean, H., Howell, D. G. & Higgins, R. E. (1976). Petrography and major element chemistry of the Santa Cruz Island volcanics. In (D. G. Howell, Ed.) Aspects of the Geologic History of the California Continental Borderland, pp. 196-215. AAPG Misc. Pub. 24. Dasch, E. J. (1969). Strontium isotopes in weathering profiles, deep-sea sediments and sedimentary rocks. Geochimica Cosmochimica Acta 33, 1521-1552. Deetz, J. (1965). The dynamics of stylistic change in Arikan ceramics. Illinois Studies in Anthropology 4. Dennell, R. W. (1980). The use, abuse, and potential of site catchment analysis. In (F.J. Findlow &J. E. Ericson, Eds) Catchment Analysis, pp. 1-20. Anthropology, UCLA 10. Edberg, R. (1982). Ethnohistoric and historic overview of Talepop and the Rancho Las Virgenes. Archaeological Investigations at Talepop (LAn-229) (draft report). ()ffice of PuNic Archaeology, University of California, Santa Barbara. Elias, R. W., Hirao, Y. & Patterson, C. C. (t982). ri'he circumvention of the naturai biopurification of calcium along nutrient pathways by atmospheric inputs of industrial lead. Geochimica Cosmochimica Acta x, 000 000. Ericson, J. E. (1980). Strontium isotope characterization. Proceedingr of the Society for Archaeological Sciences (Abstract).
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Ericson,J. E. & Goldstein, R. (1980). Work space: A new approach to the analysis of energy expenditure within site catchments. In (F.J. Findlow &J. E. Ericson, Eds) Catchment Analysis, pp. 21 30. Anthropology, UCLA 10. Ericson,J. E., Shirahata, H. & Patterson, C. C. (1979). Skeletal concentrations of lead in ancient Peruvians. New England Journal of Medicine 300, 946-951. Ericson, J. E., Sullivan, C. & Boaz, N. T. (1981). Diets of Pliocene mammals from Omo deduced from carbon isotopic ratios in tooth apatite. Palaeogeography, Palaeoclimatology and Palaeoecology 36, 69-73. Faure, G. & Powell, J. L. (1972). Strontium Isotope Geology. New York: Springer-Verlag. Findlow, F.J. & Ericson, J. E. (Eds.) (1980). Catchment Analyses: Essays on Prehistoric Resource Space. Anthropology UCI,A 10. Glimcher, M.J. (1976). Composition, structure, and organization of bone and other mineralized tissues and the mechanism of calcification. In Handbook of Pbysiology, Endocrinology, Chapter 7. Baltimore: Williams & Wilkins. Higgs, E. S. & Vita Finzi, C. (1972). Prehistoric economies: A terretorial approach. In (E. S. Higgs, Ed.), Papers in Economic Prehistory, pp. 27-36. Cambridge: Cambridge University Press. Higgins, R. E. (1976). Major element chemistry of the Cenozoic volcanic rocks in the Los Angeles basin and vicinity. In (D. G. Howell, Ed.), Aspects of the Geologic History of the California Continental Borderland, pp. 216.227. AAPG Miscellaneous Publication 24. Hill, J. N. (1972). Inferring prehistoric social organization through ceramic pattern recognition. In Interdisciplinary Colloquium on Mathematics in Behavioral &iences, Colloquium Documents 1971-72, Western Management Service Institute, ULCA, 7.1-7.7. Hudson, I3. T., Blackburn, T., Timbrook, C. & Timbrook,.]. (Eds) ( 1981 ). The bye of the Flute: Chumash Traditional fIistory and Rituals as Told by Fernando Librado Kitsepawit to John P. Harrington, 2nd Edn. Banning: Malhi Museum Press. Hurst, R. W. (1978). The Conejo volcanics, western Santa Monica Mountains: Volcanogencsis at an oceanic-continental plate interface. Eos Transaction~ of the A GU 59, 1213. Hurst, R. W. (I979a). Geologic and geochemical investigations of'Miocene volcanics in the Transverse Ranges, southern California. In (J. M. Armentrout, M. R. Cole, & H. Terbest, Eds) CenozoicPaleogeographyofthe Western United States. Society of Economic Paleoentologists and Mineralogists, Pacific Coast Section, Paleogeography Symposium 3. Hurst, R. W. (1979b). Petrologic constraints on the differentiation of the Coneio volcanic suite, western Santa Monica Mountains. Eos Transactions of the AGU 60, 972. Hurst, R. W. (1982). Pctrogensis of the Coneio volcanic suite, southern Calilbrnia: Evidence for mid-ocean ridge-continental margin interactions. Geology 10, 267 272. Hurst, R. W. (1983). Volcanogensis contemporaneous with mid-ocean ridge subductiou and translation. In (S. S. Austithis, Ed.) Significance of Trace Elements in Petrogenetic Problems and Controversy. Athens: Theophrastus Publications. Hurst, R. W. & Davis, T. E. (1981). Strontium isotopes as tracers of airborne fly ash from coal-fire power plants. Environmental Geology 3, 363 367. Johnson, J. R., Warren, C. N. & Warren, S. E. (1982). Ethnohistoric overview of native culture. In (L. R. Wilcoxson, PI) Fznal Report Intensive Cultural Resources Survey jot the Goleta Flood Protection Program, Santa Barbara County, California, pp. 12-48. San Diego, CA: Archaeological Systems Management, lnc. King, C. (1982a). The evolution of Chumash society: a comparative study of artifacts used in social system maintenance in the Santa Barbara Chennel Region Before A.D. 1804. Ph.D. dissertation, University of California, Davis. King, C. (1982b). Summary and integration of research at Talepop. In Archaeological Investigations at Talepop (LAn-229), ch. 3 (draft report). Santa Barbara: Office of Public Archaeology, S.P.R.I. University of California, Santa Barbara. King, C. (n.d.). Draft; Appendix 1: Ethnohistoric background of Vandenberg Air Force Base of Cultural Resources Mitigation. Chambers Consultants and Planners. Lane, R. A. & Sublett, A.J. (1972). Osteology of social organization: residence patterns. American Antiquity 37, 186-201. Little, M. F. & Barrett, K. B. (1976). Trace element content of surface and subsurface enamel relative to caries prevalence on the west coast of the U.S.A. Archives of Oral Biology 21, 651-657. Longacre, W. A. (1964). Archaeology as anthropology: A case study. Science 144, 1454-1455. Parker, R. B. (1968). Electron microprobe analysis of fossil bones and teeth. Gealogical Society of America, Special Paper 101,415-416. Parker, R. B. & Toots, H. (1970). Minor elements in fossil bone. Geological Society of America Bulletin 81,925-932. Rasmussen, H. & Burdier, P. (1974). The Physiological and Cellular Basis of Metabolic Bone Disease. Baltimore: Williams & Wilkins. Roper, D. (1979). The method and theory of site catchment analysis: A review. In (M. SchillEr, Ed.), Advances in Method and Theory, H, pp. 119-140. New York: Academic Press. Schoeninger, M. (1979a). Dietary reconstruction dt Chalcatzingo, a formative period site in Morelos, Mexico. Technical Report 9, Contribution in Human Biology 2, Museum of Anthropology, University of Michigan.
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Schoeninger, M. (1979b). Diet and status at Chalcatzingo: Some empirical and technical aspects of strontium analysis, american ,Journal of Physical A nthropolow 51,295-310. Schoeninger, M . J . & Peebles, C. S. (1982). Effect of mollusc eating on human bone strontium level. Journal of Archaeological Science 8, 391 397. Schroeder, H. A., Tipton, I. H. & Nason, A. P. (1972). Trace metals in man: Strontium and barium. Journal of Chronic Diseases 95, 491. Sillen, A. (1981a). Strontium and diet at Hayonim Cave, Israel: An evaluation of the strontium/calcium technique for investigating prehistoric diets. Ph.l). dissertation, University of Pennsylvania, Philadelphia. Ann Arbor: University Microfilms. Sillen, A. (1981b). Strontium and diet at Hayonim Cave. AmericanJournal of Physical Anthropolog~ 56, 131 137. Sillen, A. & Kavanagh, M. (1982). Strontium and paleodietary researcb: a review. The Yearbook of Physical
Anthropology. Spence, M. W. (1974). Residential practices and the distribution of skeletal traits in Teotihuacan, Mexico. Man 9, 262-273. Stark, B. L. & Voorhies, B. (Eds) (1978). Prehistoric Coastal Adaptations: The Economyand Ecologyof Maritime Middle America. New York: Academic Press. Steadman, L. T., Brudevold, F. & Smith, F.A. (1958). Distribution of strontium ill teeth ti'om different geographic areas. Journal of the American Dental Association 57, 340 344. Straughan, I. R., Elseewi, A. A., Page, A. L., Kaplan, I. R., Hurst, R. W. & Davis, T. E. ( 1981 ). Fly ash-derived strontium as an index to monitor deposition from coal-fired power plants. Science 212, 1267 1269. Sullivan, C. H. & Krueger, H. W. (1981). Carbon isotope analysis of separate chemical phases of modern and fossil bone. Nature 292, 333 335. Toots, H. & Voorhies, M. R. (1965). Strontium in tlossil bones and the reconstruction of lbod chains. Science 149, 854-855. Vita-Finzi, C. & Higgs, E. (1970). Prehistoric economy in the Mt. Carmel area of Palestine: Site catchment analysis. Proceedings of the Prehistoric' Societp 36, 1 37. Warren, J. M. & Spencer, H. (1978). Comparative excretions of strontium isotopes, llealth Physic~ 34, 67 70.
Strontium Isotope Characterization in the Study of Prehistoric Human Ecology Residence patterns provide keys to social structurc, information tlow, and pattcrns of material culture. A biogeochemical model has been formulated using tooth eruption sequence, elemental exchange rates in bone tissue, and the geochemistry of strontium isotopes--to examine patterns of human residence in the past. Strontium isotopes, characteristic of local geology, pass unmodified through the food chain, isotopic values of human second molar teeth, representative of the individual from ages six to twelve, and bone, representative of the last six years of life, characterize thc food chain of pre-marital and marital residences respectively. Marriage residence patterns are determined by stratifying age/sex data. The biogeochemical model and preliminary results are presented for two sites in California, Journal of Human Evolution (1985) 14, 503-514
1. I n t r o d u c t i o n My aim in this paper is to present a methodological approach using strontium isotopes as tracers for studying specific behavioral patterns (Ericson, 1980): (1) There as been a considerable interest in catchment analysis particularly in terms of thc prehistoric utilization of biotic resources (cf. Findlow & Ericson, 1980; Roper, 1979; Higgs & Vita-Finzi, 1972; Vita-Finzi & Higgs, 1970). In conjunction with other botanical analyses, the assay of strontium isotopes in h u m a n tissues can be used to identify the catchments used by prehistoric peoples. (2) There is a long, and generally unsuccessful, history of the investigation of the importance of marine resources for h u m a n subsistence (cf., Stark & Voorhies, 1978). Strontium isotope analysis of h u m a n bonc should provide some resolution of these questions. (3) Is it possible to accurately trace prehistoric individuals back to pre-marital places of residence? Over the past two decades, the analysis of prehistoric residence patterns has received serious attention. A n u m b e r of problems, however, currently limit the accuracy and effectiveness of these studies. Analysis of ceramic designs has been used to postulate particular forms of social organization (Longacre, 1964; Deetz, 1965; Hill, 1972), although there is insufficient information to understand which mechanisms arc responsible for such variation. Likewise, clusterings of discrete osteological traits in a population have been used to reconstruct genetic groups (Lane & Sublett, 1972) and, in turn, suggest residence rules (Birkby, 1982; Spence, 1974). Even with these advances we arc unable to specify the locations ofpre-marital residence for contemporaneous individuals in the past. A characterization procedure is needed which will routinely and accurately identify the pre-marital residence "catchments" of individuals. T h e pilot study described herein dcmonstratcs that strontium isotope ratios are an effective means for studying dietary input ti-om discrete geochemical environments. This method is drawn from the fields of geochronology and geochemistry. Strontium, originating in parent rock, passes through the food chain. There is sufficient natural variation in the strontium of thc parent rocks that it can be used to characterize and 0047-2484/85/050503 + 12 $0300/0
9 1985 Academic Press Inc. (London) Limited
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differentiate local environments. The proposed method uses the isotopic ratios of biogenic strontium as tracers and signatures of specific catchment areas. This study is a logical extension of chemical characterization investigations which have been used to trace tile sources of different artifhctual materials. The development of the proposed technique will significantly advance the forensic repertoire of anthropology. Many different catchments within specific geological provinces will be characterizable, eventually leading to the reconstruction of patterns of intermarriage within a region.
2. Characterization by Strontium Isotopes The fundamental assumption underlying the application of this technique is that strontium, originating in a local source material, passes through the tbod chain without significant fractionation of its isotopes. The biogenic record of strontium, as reflected by the diet of an individual, is incorporated into skeletal tissues and stored for long periods of time (Toots & Voorhies, 1965; Parker & Toots 1970; Parker, 1968; Schoeninger, 1979a,b; Sillen 1981a,b). In contrast--and not to be confused with strontium isotope research--elemental strontium relative to calcium does decrease as it passes through the food chain (e.g., Elias et al., 1982; Price, Schoeninger & Armelagos, this issue). The change in the strontium/calcium ratio at each trophic level has stimulated interest in paleonutritional studies. An excellent review of such studies is now available (Sillen & Kavanagh, 1982). In living systems, strontium as well as barium and lead tends to follow calcium as elemental constituents during dietary uptake, internal distribution within the body, and excretion (Schroeder et al. 1972; Underwood, 1977; Elias el al. 1982). Biopurification factors--ratios of non-nutrient metals to calcium in nutrients and consumer--are important to understand the transfer of strontium relative to calcium from soil to plants to humans it is difficult to suggest a mechanism in these passive or other elemental processes which would lead to the separation of strontium isotopes, i.e., tiactionation. Soil (Dasch, 1969) and plants (Hurst & Davis, 1981) are equilibrated with the local source rock and share similar isotopic ratios for strontium. Plants reach chemical equilibrium with strontium isotopes available in soil moisture. Recent work by Hurst & Davis (1981) on the absorption of strontium f?om coal flyash by plants indicates that the isotopes achieve rapid equilibrium with the flyash in the soil. Plants were also in isotopic equilibrium with the leachable strontium from soils without flyash. This indicates that bioavailable strontium in the environment will not be fractionated by the processes involved in the plant and animal uptake of strontium (Hurst & Davis, 1981; Straughan el al., 1981). Work by Warren & Spencer (1978), however, suggests that there is biological t?actionation of strontium isotopes in humans. Nevertheless, general geochemical data indicate that isotopes with atomic numbers (Z) above 40 do not fractionate under normal biological conditions (C. C. Patterson, pets. comm.). Strontium has Z numbers at 86 and 87. Observations by Warren & Spencer (1978) may reflect insufficient time tbr the ingested tracers to achieve equilibration with the major reservoir of strontium stored in bone. Geochemistry of Slrontium
Fortunately the geochemistry of strontium is well known. Strontium isotopes are used routinely to study the structure of the earth in terms of the origin and the age of rock (Faure & Powell, 1972). Strontium (Sr) and rubidium (Rb) are contained in amounts of generally
STRONTIUM
ISOTOPE
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505
less that 1% in most igneous, metamorphic and sedimentary rocks. Strontium substitutes for calcium in minerals such as plagioclase and apatite. Rubidium substitutes for potassium in minerals, particularly micas and potassium feldspars. Strontium and rubidium follow different magmatic pathways. Rubidium has one naturally occurring radioactive isotope, 87Rb, which decays to a stable isotope of strontium. Thus, rocks with more Rb will contain more 875r produced by radioactive decay than those rocks containing less rubidium. Hence, the isotopic composition of a rock is a function of the Rb/Sr ratio and length of time. As a result, both geological age and rock type are significant in creating variability in the proportion of strontium isotopes observed in rock. Although this ratio in granites, the most common continental rocks, varies from 0-700 to 0"737 (Figure 1), each granite carl be characterized by discrete isotopic values for strontium. The technical procedures, which require mass spectrometry and uhraclean laboratory preparation to avoid contamination, have been highly refined. The precision of isotopic measurement can be as low as +0'00005 depending upon the analytical system employed. Given the large natural variation in continental granites (0"037) and the precision of measurement (0"00005), the scale of discrimination among granites is 740 units. Figure i. Naturat variations of strontium isotopes in continental granites {dots) and ocean basahs over geologicaltime (after Faure & Powell, 1972).
-
0.735
-
0.730 0.725 ,_ 0.720 0.715 "5 r
0.710 o9
0 ~ 9 00
9 9
9
..
z.O00
3000 2000 1000 Time in millions of years
-
0.705 9
0.700 Present
It is important to note that the strontium values fbr continental granites are linearly correlated with geological age. This trend suggests that one can estimate the expected isotopic ratio for a granite if its geological age is known. Likewise, in some cases, the isotopic values for derived sedimentary and metamorphic rocks can be estimated if the parent granite and its geological age are known. This correlation will be usethl in modelling the isotopic differences between various continental rock types.
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In contrast, the mean isotopic composition of ocean basalts is relatively invariant, exhibiting a ratio of approximately 0"7037 + 0.0001 (Faure & Powell, 1972). Food chains, dependent solely upon ocean basalts for strontium nutrients, would be impossible to distinguish using this technique. However, in some coastal environments the discrete values for the marine (sea water with an isotopic value of 0.7091) and terrentrial (ocean basalt) food chains would greatly enhance the reconstruction of maritime subsistence strategies. Table 1 illustrates a hypothetical case for a Hawaiian coastal environment as a study of the dietary fractions using carbon and strontium isotope ratios. It is important to note that the isotopic values of sea water have changed over geological time as a long-term trend (Faure & Powell, 1972). These values will be retlected in certain marine sedimentary rocks like marine limestone. In summary, significant variation in strontium isotopes exists as a function of age and type of rock. Continental rocks such as granites have a greater range of variation than do ocean basalts. Since strontium isotopes, characteristic of local geology, pass unmodified through the food chain, they can be used as signatures of portions of the local landscape. Hypothetical Hawaiian catchment: estimation of dietary fractions of marine shellfish and terrestrial plants using results of carbon and strontium isotope analysis and mixing equations
Table 1
Analysis
H u m a n bone
M a r i n e shellfish
T e r r e s t r i a l plants
81:~C% ~TSr/~Sr
-20"6 0'7075
-15"0 0"7091"
-22'0 0"7037~
E q u a t i o n 1: -20.6 =
E q u a t i o n 2:
AP(-15.0) + BQ(-22-0)
AM(0.7091) + BN(0-7037) 0.7075 =
AP + BQ Where A = % Shellfish in diet P = C a r b o n in shellfish M = S t r o n t i u m in shellfish
A M + BN
B = % terrestrial plants in diet Q = C a r b o n in plants N = S t r o n t i u m in plants
Let P = Q: M = 300,000 p.p.m.; N = 200 p.p.m.; solvc simultaneously tot A and B. Ans: A = 20% shellfish; B = 80% plants * Value of ocean water; "~ Value of oceanic basalts (Faure & Powell, 1972, p. 41).
Identifying Residence First of all, the technique proposed here for identifying residence requires that catchments of contemporaneous settlements be characterizable by strontium isotopic ratios, i.e., there must be significant geological variation among the dilt~rent residence areas. Secondly, the h u m a n body produces a variety of biogeochemical records during the lifetime of an individual. Each secondary tooth, for example, contains the biogeochemical record of a specific period of growth and time of emplacement. Although a secondary tooth does absorb some strontium fi'om saliva, it is extremely thin--10-30 btm (Little & Barrett, 1976)--and can be removed by excoriation in sample preparation process. The interior enamel portion, however, is isolated biologically from further chemical exchange and can be used for analysis. This interior structure preserves the biogenic record of strontium
STRONTIUM ISOTOPE
CHARACTERIZATION
507
intake for long periods of time (Toots & Voorhies, 1965: Parker & Toots, 1970; Parker, 1968; Schoeninger, 1979a,b; Sillen, 1981a,b; Steadman et aL, 1958). The strontium isotope ratio of the secondary tooth, excluding the crown, reflects the local environment during emplacement of the tooth. If emplacement occurs prior to marriage, the tooth should contain a biogenic record of the environment of pre-marital residence. If we assume that the common age of marriage occurs after twelve years of age in both males and females, the second molar is a most suitable tooth representing pre-marital diet. Information on post-marital residence (as distinguished from the location of burial) requires that the pre-marital biogenic record be erased, diluted or effectively displaced within specific tissue. Fortunately, bone tissue--in contrast to teeth--is continuously remodeled. In a period of six years, bone undergoes complete replacement of its chemical constituents (Glimcher, 1976; Rasmussen & Bordier, 1974), thereby equilibrating with a new environment. The comparison of the isotopic records of secondary teeth and bone in an adult provides the basis for distinguishing pre- and post-marital residences, if the individual was relatively sedentary as a child and marries as an adolescent into a new community, the differences in the tbrmer and latter geochemical reservoirs will be detectable after six years by comparison of tooth and bone from that individual. By stratifying a cemetery population by age and Sex, the pattern of marital residence can be constructed. For example, if the residences of males--as measured by strontium ratios in the second molars--are non-local, different from and more variable than females, then a pattern ofmale exogamy is implied. Potentially, the technique is far more powerful. If each catchment of a region can be characterized by an isotopic signature, then each individual can be traced to his/her place of residence resulting in the determination of regional intermarriage patterns.
Some Recognized Problems There arc a number of recognized problems which affect the application of this technique. Some of these problems can be overcome, others cannot. (1) Geological variability. The proposed method requires that significant variation in strontium isotope ratios be present among difl~rent catchments. If not, application will be limited. Preliminary analysis of an area should include an estimate of its isotopic variability, which can be estimated from geological maps which provide initial data on the geological age, location, and variation in rock types and isotopic data bases. Compilation of these data should provide an estimate of the expected strontium isotope ratios within and between theoretical catchment zones. (2) Catchment defnition. The spatial definition of catchment zones to define major areas of food procurement is a critical problem. Initially, Vita-Finzi & Higgs (1972, p. 31) suggested "a radius of 5 km tbr the analysis of sedentary exploration territories and 10 km for mobile economies." Since it is useful to express these distances in terms of travel time to compensate for topographic variation, walking travel-times of one and two hours have used. More recently, "work space" has been proposed as an alternative means for evaluating energy expenditure on a complex surface (Ericson & Goldstein, 1980). Dennell (1980) has recently criticized simple catchment studies for sedentary groups, suggesting that landforms and soil types are more reliable guides to the prehistoric landscape than modern land-use data. defining group catchment remains a problem.
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If the population under study is extremely mobile, the biogeochemical records of teeth and bones will be very difficult to interpret, nevertheless, given prcscnt analytical technology, it is possible that mobility patterns can be discerned by measuring the isotopic values on a crystal-by-crystal basis following the growth patterns in teeth using an ion probe. (3) Catchment characterization. The characterization of the catchment is another major problem. There is no "magic circle" of containment that can be drawn around any group without a high degree of uncertainty. Two approaches may resolve some of this problem. Direct empirical study is preferred, involving the measurement of strontium isotope ratios in bone from a sufficient number of adults in the population to estimate an average Value. Secondly, soil and/or biological samples such as plants and/or rodents can be collected at grid points within the site catchment. Various sample mixtures can be made which are consistent with different theoretical or empirical catchment models. The strontium isotope value of each mixture could be compared with values from the human population. In cases of environmental pollution or geochemical change due to modern land use, parent rock samples could be substituted to establish the baseline for strontium isotope ratios. (4) Biogenic contamination. As noted earlier, strontium is exchanged on the surface of teeth. This reaction layer, as well as the interior surface at the dentine interface, can be removed in the laboratory. The remaining inner enamel is effectively isolated and most useful for analysis. (5) Diagenic contamination. Diagenic alteration is another major problem. Occasionally, the biogenic record in tooth and bone may be obliterated by soil moisture contamination. Soil contamination occurs more readily in bone than in tooth enamel (Ericson et al., 1979). The major contamination of bone, however, apparently comes from the remineralization of tissue with secondary calcite. Decontamination of tissue is best accomplished by mechanical excoriation, tissue separation, and phase separation by chemical reaction. The sample is broken, sieved, sorted, and reacted with acetic acid (Ericson et al., 1981; Sullivan & Krueger, 1981), preferably in an ultraclean environment to avoid contamination. Diagcnic change is a problem that can be alleviated by proper sample preparation. occasionally, however, samples simply cannot be used. The criteria and tests for the evaluation of sample contamination need to be defined. (6) Masking by high strontium foods. The consumption of sufficient quantities of high strontium foods will swamp or mask low strontium foods that are consumed in larger quantities. The ingestion of sufficient strontium hyper-accumulator plants (Coughlin & Ericson, 1981) or derivatives of specific hyper-accumulating tissue such as bark (Elias el al., 1982) or marine shellfish (Schoeninger & Peebles, 1982) can cause problems in interpreting the biogenic record. It is essential to consider such masking effects through the assay of strontium concentrations in foodstuffs.
3. Potential Applications Although a number of inherent problems have been identified, careful research will lead to an ultimate gain in knowledge. A few applications for this new technique are indicated below.
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ISOTOPE
CHARACTERIZATION
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(1) Human ecology and territoriality. We will be able to study prehistoric subsistence strategies and human territoriality through strontium isotopic characterization. We can compare observed isotopic values of human populations with expected values drawn from biologicaI data, grouped to simuiate theoretical catchment models. (2) Food sharing and exchange. We will be able to study isotopic variation reflecting dietary differences due to food sharing and/or exchange within and between groups. Categories for analysis can include age, gender, social status, household, and occupational/craft groups. (3) Migration and warfare. We will potentially be able to identify patterns of human migration by studying founding populations and regional catchment patterns. Likewise, the origin of "foreign" military groups who died in battle and were interred in foreign soil will be discernible. (4) Marital residence. As discussed above, it is proposed that we should be able to describe the prehistoric patterns of marital residence, which may be linked to social structure, as well as information, energy, and culture flow. (5) Animal ecology. The strontium isotope tracing technique can be extended to questions regarding the hunting of wild animals and general problems of animal ecology. With domestication, the home range of animals is more restricted, leading to more pronounced differences between wild and domesticated species. This regulating of" domestic animals can be examined directly. (6) Exchange of food. For the first time, we should be able to identify the origin and movement of certain non-local foodstuffs. We will be able to characterize foods and potentially identify their origin. In all, the strontium isotope tracing technique offers an important breakthrough for the study of past human ecology and social organization. At this preliminary stage, it is important to determine if biogenic records can be observed in human tissue and if these records conform to paleodietary and environmental data. The foIlowing pilot study documents these conditions.
4. A Pilot Study The prehistoric Chumash Indians who occupied a portion of the coast of the Pacific Ocean and the interior of the Santa Monica Mountains near Los Angeles, California, offer a useful population for study. Chumash society was the result of more than 7000 years of cultural development. This growth culminated in a society with heirarchically organized political systems and an active morley economy (King, 1982a). Ethographic and ethnohistoric data indicate that there were frequent migrations of families over long distances within the Chumash area (Hudson et al., 1981 ; Johnson el al., 1982; King, n.d.). Ethnohistoric data for the Santa Monica Mountain Chumash have not been thoroughly analyzed to determine patterns ofpre- and post-marital movement. Available data suggest that neolocal residence within the area was common and that exogamous marriages were ambilical (Johnson el al. 1982; King, n.d.; Edberg, 1982). The people who lived in the Malibu Canyon drainage, the study area, were closely related and most likely moved between villages with great frequency. Two sites along the Malibu Canyon drainage provided tooth and bone samples: (1) Malibu (CA-LAn-264) is the historic village of Humali, located on the coast along Malibu Lagoon, an estuary by the streams of Malibu Canyon. Two cemeteries from
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this site have been excavated. One, containing 129 burials, was used between approximately A.D. 1786 and 1816 during the period of Spanish mission recruitment. The other cemetery, dating from the late Middle Period (ca a.n. 700-1000), has yielded approximately 40 burials. Food remains recovered from the midden at this site indicate that fish, shellfish and sea mammals constituted an important part of the diet of the inhabitants. Remains from vegetable foods and land mammals document the contribution of terrestrial resources. Ethnographic and ethnohistoric information indicate that Humaliu, the largest village in the area, was the capital of the Santa Monica Mountain Chumash. (2) The Century Ranch sites (CA-LAn-225, 229, 227 and 840) are located in the canyon approximately 7-5 km inland from Malibu. Several cemeteries have been excavated in this area, including (a) a late Middle Period cemetery at CA-LAn-840, adjacent to Century Ranch, yielding 30 burials, and (b) a Late Period, phase 2B (ca a.D. 1650--1770), cemetery at CA-LAn-227, containing 22 burials. Recent excavations at the historic village of Talepop (CA-LAn-229) as well as ethnographic data, indicate the local plant and animal foods provided virtually all subsistence. At Talepop ocean fish were a more important part of the diet during the early occupation (ca A.D. 900-1200)--the period of concern--than later (ca A.D. 1650--1803) (King 1982b). Historical records indicate that several couples living at Talepop had been born at other Santa Monica Mountain villages and had established neolocal residence. There were strong kin ties between the villagers of Humaliu and Talepop (Edberg, 1982). Fortunately for this study, both Malibu and Century Ranch contain coeval cemeteries. The graves at Malibu Village on the coast date to 1000 B.P. The small cemetery at LAn-840 near Century Ranch is dated to approximately 1000 B.P. by the obsidian hydration method (C' Meighan, pers. comm.). The strontium isotopes of the second molars and metatarsals of a mate and female from Matibu and of one individual from Century Ranch were analyzed by ultraclean techniques and mass spectrometry at Lamont Doherty Observatory. The results are presented in Table 2.
5. Determination of Geochemical Endpoints The Malibu Creek study area at Century Ranch is part of a drainage basin underlain by the Topanga Formation. The lower and upper Topanga Formation members are composed of marine sediments while the middle member is the Miocene Conejo Volcanics suite. The Chumash population which lived in this area would have derived their food from soils developed as mixtures of the upper Topanga Formation member and the Conejo Volcanic suite. Evidence for this comes from: (1) the location of the village on the Conejo Volcanic suite, (2) the fact that the upper Topanga Formation member lies immediately to the north of the village, and (3) the path of the drainage of Malibu Creek, beginning in the upper Topanga Formation and proceeding to the south through the Conejo Volcanic suite. Geology, geochemistry and geomorphology for the area is well documented. Blackerby (1965), Higgins (1976), and Crowe el al. (1976) have investigated major element geochemistry and the general geology of the Malibu area as well as Santa Cruz Island, an extension of the Santa Monica Mountains. More detailed petrographic geologic and isotopic data are provided by Hurst (1978, 1979a,b, 1982, 1983) for these same areas. There
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is no evidence to suggest that d r a m a t i c climatic or g e o m o r p h o l o g i c a l changes h a v e modified the d r a i n a g e p a t t e r n s or direction of s e d i m e n t t r a n s p o r t over the last 1000 y e a r s (Hurst, pers. comm.). T h e soils within the c a t c h m e n t are locally derived,-the s u r r o u n d i n g ridges m i n i m i z e m a j o r influxes of extraneous sediments, and the d r a i n a g e is r e a d i l y t r a c e a b l e t h r o u g h the source rocks.
Human Strontium Isotope Ratios As a p r e l i m i n a r y test of the s t r o n t i u m isotope tracing technique, several samples of h u m a n bone a n d teeth were e x a m i n e d to see if s t r o n t i u m isotopes could indeed be used to d e d u c e d i e t a r y p a t t e r n s ( T a b l e 2). Briefly, the C h u m a s h test case indicates that the two coastal Table 2
Preliminary results of strontium and carbon isotope analysis of three Chumash individuals from Malibu (LAn-264) and Century Ranch (LAn-840)
Site
Provenience
Tissue
Sex
Age
aTSr/ll~Sr
LAn-264 LAn-264 LAn-264 LAn-264 LAn-840 Lan-840
Burial 9 Burial 9 Burial 18 Burial 18 Burial 7 Burial 7
M2 Tooth Metatarsal M2 Tooth Metatarsal M2 Tooth Bone
F F M M --
52-59 52-59 23 28 23-28 ---
0"70857_+ 10 0'70876+ 15 0"70841+ 1l 0"70892+--23 (/-70765 + i I --
61:~C
- 15"1 -14"8 - 16"9
i n d i v i d u a l s (aTSr/86Sr = 0'7088 a n d 0"7089) from M a l i b u village had a diet based heavily on m a r i n e foods ( s t r o n t i u m isotope ratios in sea water are 0'7091), consistent with the archeological evidence. T h e g e o c h e m i s t r y of the diet of the female, i n d i c a t e d by s t r o n t i u m isotope ratios, did not c h a n g e significantly from adolescence (second molar, 87Sr/aGSr = 0"7086) to a d u l t h o o d (metatarsal, 87Sr/86Sr = 0'7088). T h e isotopic ratio from her tooth indicates t h a t she resided in the s a m e or a n o t h e r coastal village or off-shore island as an adolescent a n d subsisted largely on m a r i n e tbods. As noted previously, s t r o n t i u m isotopes will likely not distinguish between coastal village areas. T h e i n l a n d i n d i v i d u a l (87Sr/8(~Sr = 0"7077) from the n e a r b y C h u m a s h site at C e n t u r y R a n c h a p p e a r s to have an elevated value for this ratio relative to the soil (87Sr/~6Sr = 0"7045) in the area. These soils are derived from a m i x t u r e of the Conejo Volcanics (STSr/a6Sr = 0'7032, H u r s t , 1983) a n d the T o p a n g a F o r m a t i o n (87Sr/a~Sr = 0'7088, H u r s t , n.d.). T h e bone s a m p l e ratio is higher than soil moisture (0-7045) as well, indicative o f a biogenic signal r a t h e r than diagenic c o n t a m i n a t i o n . T h e elevated value is consistent with e t h n o g r a p h i c d a t a r e g a r d i n g the C h u m a s h which indicate t h a t inland villagers spent the fall and winter in the i n l a n d villages but moved to the Pacific coast d u r i n g the spring a n d s u m m e r as well as c o n s u m e d fish during this period. If so, the terrestrial c o m p o n e n t of the diet of the C e n t u r y R a n c h individual is masked by the high c o n c e n t r a t i o n of s t r o n t i u m from shellfish (Schoeninger & Peebles, 1982). C a r b o n a n d nitrogen isotope ratios can be used to resolve the relative c o n t r i b u t i o n o f m a r i n e foods in the diet. C a r b o n isotope analyses of these three individuals ( T a b l e 2) were c o n d u c t e d to verif) the s t r o n t i u m results. T e r r e s t r i a l tbods, excluding non-Calvin p l a n t s have c a r b o n isotope values of a p p r o x i m a t e l y 813C = -22O/oo. M a r i n e shellfish have c a r b o n isotope values (8~:~C = - 150/oo). It is clear that the carbon isotope ratios in h u m a n bone of the two M a l i b u s a m p l e s are closer to the m a r i n e end of this spectrum. T h e p a t t e r n is
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consistent with ethnobotanical and faunal reconstructions. The Century Ranch case is more difficult to interpret since analyses were perlbrmed on different tissues. Nevertheless, both the carbon and strontium isotopic values are intermediate to the isotopic endpoints for marine and terrestrial foodstuffs which is internally consistent. 6. C o n c l u s i o n s
The strontium isotope characterization technique following further development will offer numerous possibilities for investigating various aspects of prehistoric lifeways. There arc significant natural variations in the isotopic ratio of these isotopes that are of value in the study of dietary patterns. The primary applications of the technique will be for characterizing catchment areas, estimating marine v s terrestrial contributions to the diet, and determining marital residence patterns. At this preliminary stage, the strontium isotope technique appears to offer an exciting new source of information for the study of prehistory. The author gratefully acknowledges the early encouragements of Professor C. C. Patterson, Department of Geochemistry, California Institute of Technology, and Professor G. R. Tilton, Department of Geological Sciences, University of California, Santa Barbara, Professors Clement Meighan and Gall Kennedy, Department of Anthroplogy, UCLA, provided skeletal samples for analysis. The author acknowledges useful discussions with Linda Bruncher, Department of Anthropology, University of California, Berkeley, Drs Alan Zindler and Humbert Stautigel, Department of Isotope Geology, Lamont-Doherty Geological Observatory, and Dr Chester King of Topanga, California and Professor R. Hurst, Department of Geology, California State University at Los Angeles for providing data notes and criticisms on an earlier version of the manuscript. The author acknowledges the support of the Milton Fund, Harvard Medical School, and Professor Crompton, Harvard Museum of Comparative Zoology. Dr H. W. Krueger, Geochron Laboratories, Cambridge, MA, kindly provided the three carbon isotope analyses. References
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