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Ca in marine and terrestrial foodwebs in the Southwestern Cape, South Africa
Journal of Archaeological Science1988,15,4255438
Sr and Sr/Ca i&nMarine and Terrestrial Foodwebs in the Southwestern Cape, South Africa Judith C. Sealy” and Andrew Sillen” (Received 20 September 1987, revisedmanuscript
accepted 3 February 1988)
Strontium and calcium have been measured in a range of plants and animals (both marine and terrestrial) from the southwestern Cape of South Africa as part of an investigation of modern and prehistoric foodwebs in the region. First, the meat of marine molluscs and crustaceans are shown to have Sr and Sr/Ca values comparable to those of terrestrial plants. Thus, the consumption of these marine foods in this region cannot produce the markedly elevated Sr levels seen in archaeological human skeletons from coastal sites; such levels are shown to be a diagenetic phenomenon. Second,
reduction in Sr/Ca in higher trophic levels is seen only when predators are compared with their prey. However, individual herbivore or carnivore species cannot be taken to represent other animals in their respective trophic level. These data imply that Sr/Ca
is inappropriate for determining meat intake in complex prehistoric human diets in this region. The technique may be more useful in examining specific prey-predator relationships, including those in the early hominid fossil record. Keywords: STRONTIUM, DIAGENESIS.
CALCIUM,
DIET, BONE, FOODWEB, AFRICA,
Introduction The Sr concentrations of human bone (and/or the strontium-calcium ratio, SrjCa) have been used to study the relative proportions of meat and plant foods in the diets of prehistoric populations (Brown, 1973; Schoeninger, 1979, 1981, 1982; Katzenberg, 1983; Fornaciari et al., 1984; Price, 1985; Price et al., 1985; Sillen, 1986). The method stems from the demonstrated discrimination against Sr (in favour of Ca) in mammalian digestive tracts (Spencer et al., 1960, 1973; Kostial et al., 1969). In addition, mammalian kidneys excrete Sr more rapidly than Ca (Spencer et al., 1960; Walser & Robinson, 1963). Thus, meat foods are expected to contain much less Sr and have lower Sr/Ca than plant foods. Skeletons of individuals ingesting large quantities of meat are expected to contain relatively low levels of Sr. This discrimination against Sr, or biopurification of Ca, has been demonstrated in at least one natural foodweb, a remote sub-alpine ecosystem in the High Sierras incorporating plants, herbivores and carnivores (Elias et al., 1982). The trophic reduction in Sr/Ca seen in this study has been assumed to be universal and thus apply to other ecological settings. “Department Africa.
of Archaeology,
University
of Cape l’own,
Rondebosch
7700, South,
425 03054403/88/040425
+ 14 $03.00/O
0 1988 Academic
Press Limited
426
J. C. SEALY
AND
Figure 1. Geological geological substrates
A. SILLEN
map of the southwestern Cape showing from which animals were collected (Table
the four 1).
principal
Archaeological studies have used Sr and Ca measurements of the skeletons of animals from different trophic levels as reference points for herbivore and carnivore diets (Sillen, 1981a, b; Price et al., 1985); however, actual foodwebs have not been considered. For example, gazelles and foxes have been chosen to represent herbivores and carnivores, respectively, in Near Eastern sites (Sillen, 1981a, b, 1986), but foxes certainly do not consume gazelles. In this study, we examine the assumption that Sr/Ca is relatively invariant within trophic levels. This project is part of a larger programme of investigation into the diets of Late Stone Age hunter-gatherers and pastoralists in the southwestern Cape of South Africa using a number of archaeometric techniques, including Sr/Ca. The geology of this area is complicated, with a land surface composed of a mosaic of different rock types (Fig. 1). Possible differences in soil Sr/Ca between these areas constitute another potential complication in this undertaking (Toots & Voorhies, 1965). In addition, local prehistoric populations are known to have eaten seafoods, including shellfish. It has been suggested that mollusc eating produces elevated bone Sr levels (Bisel, 1980; Schoeninger & Peebles, 1981; Kyle, 1986) thus obscuring variations in bone Sr/Ca due to differential intake of meat and plant foods. To evaluate Sr/Ca as atrophic level indicator in this environment, we have conducted a survey of Sr/Ca in indigenous foodstuffs which were consumed by prehistoric people. In
Sr AND Sr/Ca IN MARINE AND TERRESTRIAL FOODWEBS 427
addition, we have sampled carnivores to make possible the examination of the distribution of these elements in the foodweb as a whole. Samples of terrestrial animals were collected from the different geological substrates, together with a range of edible plants and marine organisms, and their Sr/Ca measured. The results, presented in Part I of this article, lead us to question the belief that consumption of marine foods produces elevated bone Sr/Ca. In Part II, we further examine this issue and present data showing that elevated Sr/Ca in bone from marine shell middens is a diagenetic, not a dietary, phenomenon. Part I
Archaeological and ethnographic research in the area has provided a fairly detailed picture of the kinds of plants and animals that made up Late Stone Age diets (Parkington, 1976, 1984; Buchanan, 1986). However, the relative importance of the various items remains uncertain. Plant foods included a number of varieties of underground bulbs and corms, especially those of the family Iridaceae. Thesehave durable fibrous corm casings that constitute a large proportion of the identifiable archaeological plant food residues. The genera Watsonia and Moraea were particularly important. Seeds and berries were also consumed, as were certain soft fleshy leaves, stems and flowers, although these are probably severely under-represented in the archaeological record. Meat food, at least in the Late Holocene, was most often obtained from small animals such as rock hyrax, tortoises or small antelope such as duiker, steenbok and grysbok. At the coast, fish, stranded seals and whales, marine birds and shellfish were important items of diet. These organisms persist in the wild in the research area. One of us (J.S.) has already made an extensive collection of the foods eaten by prehistoric people for stable carbon isotope analysis (Sealy, 1986; Sealy & van der Merwe, 1986). We have drawn on this collection for the study described here, supplementing it with additional specimens where necessary. More than 80 specimens of plants and animals important in prehistoric diets in the southwestern Cape are analysed here, together with some carnivores. Carnivores were not food items, but their inclusion in the sample allows us to examine the distribution of Sr/Ca in the foodweb as a whole. The aim of this article is to provide an overview of the foodweb, rather than to examine Sr/Ca variability within species (see Sillen, 1988). The number of observations for each species is small, so the results should be regarded as cautionary rather than conclusive. However, our conclusions rest upon the demonstration of overlapping Sr/Ca values for different kinds of foods. We do not believe that these patterns would be substantially altered by larger sample sizes. Methods
Meat, plant and shellfish samples were cleaned of surface contaminants, freeze-dried and then ashed in a muffle furnace at 600 “C. Bone and shell samples were not ashed. Three to five milligrams of the resulting ash (or bone or shell chips) were weighed with a PerkinElmer AD-4 microbalance and digested in 100 ul concentrated HNO, (Aristar) in borosilicate glass culture tubes at 1.50“C. The samples were brought to dryness and redissolved in 1 ml of 0.2 N HNO,. These solutions provided stocks from which aliquot samples could be removed for analysis. The digestion procedure was performed in duplicate for each specimen. If the difference in Sr/Ca between duplicates was greater than lo%, repeat digestions of the sample, again in duplicate, were analysed. Concentrations of Ca in digestion stocks were measured with flame atomic absorption spectrophotometry employing a Varian model AA6 AAS, following methods described elsewhere (Sillen, 1981a, b, 1986). Concentrations of Sr were measured with a PerkinElmer model 5000 AAS fitted with an HGA graphite furnace. Recovery was found to be 104% when 500 ng Sr and 1000 ng Sr were added to samples before digestion (Sillen,
J. C. SEALY AND A. SILLEN
428
Table 1. SrjCa andSr of bones of terrestrial of the southwestern Cape
Cenozoic marine sands
Animal Porcupine (Hystrix Hyrax (Procavia
Malmesbury shales 1.7 (347)
ajiiicae-australis) capensis)
Hare (Lepus capensis) Tortoise (Chersina angulata)
Duiker (Sylvicapra grimmia) Steenbok/grysbok (Raphicerus sp.) Bat-eared fox (Otocyon megalotis)
3.6 (839) 52(1172) 3.0 (628) 1.4 (337) 6.1 4.3 5.9 54 1.5
d&erent
Table Mountain sandstones 3.0 (577) 3.8 (867) 4.1 (1037)
geological
regions
Witteberg/ Bokkeveld shales and quartzites 3.0 (642)
1.5 (388)
(1277) (999) (1320) (1161) (2.54)
4.7 (1054) 3.6 (840)
3.5 (813)
I.1 (267)
0.8 (179) 1.3 (285)
1.1 (266) 2.3(581) 2.5 (414)
The initial value is the Sr/Ca ratio; bone in ppm. Each pair of numbers
animalsfrom
2.3 (405)
the number in brackets represents the analysis
is the Sr content of one animal.
of the
1986). Some samples from this study were also analysed by an independent method, isotope dilution, and the Sr contents as measured by atomic absorption were found to be 104-105% of the values obtained by isotope dilution, thus confirming the recovery data. Ratios were calculated on the basis of ng Sr/ug Ca. Results
Bone Sr/Ca of modern herbivores and insectivores collected from the four principal geological substrates of the southwestern Cape are presented in Table 1 (also see Figure 1). Samples were collected in areas of undisturbed natural vegetation. The animals are all species known to have been eaten by Late Stone Age populations, except bat-eared foxes which are recent immigrants to the region (Hendey, 1983). (These are included because they are one of only two species we were able to obtain from a Malmesbury shale area. Such shales weather to produce fertile soil and, as a result, are today intensively cultivated.) Hyrax and tortoise are the two animals most frequently found in archaeological sites. From Table 1, it is clear that geological variation is not an important source of Sr/Ca variability in this region: the values for each species are broadly similar on all rock types. However, some species have consistently higher Sr and Sr/Ca than others: all the specimens of hyrax and tortoise measured yield Sr/Ca values between 3 and 6, whereas the ratios for hares and small bovids (duiker and steenbok/grysbok) are between 0.8 and 2. Hyrax, tortoise, duiker and grysbok are browsers. Hares and steenbok include a certain amount of grass in their diets. At least one early paper in this field suggested that the browsing/grazing status of an animal might affect its Sr/Ca (Toots & Voorhies, 1965). A recent study of northwestern Zimbabwe fauna has shown that’specialized grazers have lower Sr/Ca than other animals, although more generalized grazers do not (Sillen, 1988).
Sr AND Sr/Ca IN MARINE AND TERRESTRIAL
FOODWEBS 429
However, browsing/grazing habits do not fully explain the patterning in Sr/Ca amongst the herbivores in this study: hyrax and tortoise are browsers and have high Sr and Sr/Ca, while duiker, also a browser, has relatively low Sr and Sr/Ca. Table 2 contains the results of analyses of meat and bone from the same animal. In all cases, meat contains far less Sr than bone. It also contains correspondingly less Ca. Thus, absolute levels of Sr and Ca in these two tissues vary far more than Sr/Ca. Table 3 documents Sr/Ca of a number of terrestrial carnivores. UCT 729,730 and 1722 were collected in an area of coastal marine sands; the other animals all come from sandstone regions. They are therefore directly comparable to the herbivores in Table 1. Contrary to expectations, the Sr contents and Sr/Ca of the carnivores overlap values at the lower end of the herbivore range (especially those for small bovids). An explanation may lie in closer examination of the carnivore diets. Black-backed jackal and Cape fox are not consistent carnivores, since in addition to meat they eat considerable quantities of insects, arachnids, birds’ eggs, vegetable matter, etc. (Smithers, 1983). These could account for the relatively high Sr/Ca of two of the black-backed jackals (UCT 1718,729):’ Leopards, caracal and African wild cat prey principally on mammals, although the two smaller cats consume some reptiles and insects (Smithers, 1983). The values for the caracal and wild cat are at the lower end of the carnivore range, consistent with a largely meat diet. At first glance, the readings for leopards seem anomalous. We know that these animals consume almost nothing but meat, yet their Sr/Ca varies from 0.8 to 1.6, at the upper end of the carnivore range in this region even when compared with such mixed feeders as black-backed jackal. This anomaly is resolved by recognizing that the principal prey of leopards in the mountains of the western Cape is hyrax (Stuart, 1981). As demonstrated above, hyrax (and tortoise) have elevated Sr/Ca compared with other herbivores (Table 1). Thus, a reduction in Sr/Ca occurs in this circumscribed trophic relationship but is not apparent in generalized comparisons of herbivores with carnivores. These relationships are summarized in Figure 2. It is clearly essential to consider actual prey-predator relationships in order to observe reduced Sr/Ca in high trophic level organisms. In the sample of herbivores and carnivores from the southwestern Cape analysed here, there is no clear reduction in carnivore Sr or Sr/Ca compared with the values for many of the herbivores. However, if one compares hyrax with leopard, or fish with seals and cormorants (actual prey-predator pairs), then the reduction in Sr/Ca in the higher trophic level becomes apparent. Table4contains the Sr and Sr/Ca ofsome important indigenous food plants and a sample of wild honey, a favourite food of protohistoric local hunter-gatherers. Strontiumcalcium values for the plants vary from 4.1 to 10.8. These figures are rather higher, and the range is larger than that reported for plants from an experimental field in the Netherlands (Sr/Ca from 2.95 to 6.25) (Runia, 1987). The plants analysed here, however, include a greater range of plant parts and species (including a succulent and two aquatic species). The range of Sr/Ca values of the plants in Table 4 is approximately the same as that of the meat samples in Table 2, although the absolute abundances of both Sr and Ca are generally much higher in plants than in meat. Honey in this region has rather low Sr/Ca (Table 4). The Sr/Ca for edible tissue of the marine organisms listed in Table 5 varies from 1.Oto 15.8. Seafoods do not contain markedly more Sr than terrestrial foods; the Sr contents of all the marine foods measured fall within approximately the same range as that of the edible terrestrial plants. The consumption of marine foods would not appear to lead to elevated dietary Sr. Available studies on marine shellfish elemental composition report low concentrations of both Sr and Ca in the bodies of the animals, when compared to corresponding shells
769
766
767 '
741
730
879
878
771
137
728
Laboratory number
Hare (Lepus capensis) Eland (Taurotragus oryx) Tortoise (Chersina angulata) Hyrax (Procavia cape&s) Hyrax (Procavia capensis) Caracal (Felis caracal) Seal (Arctocephaluspusillus) Cormorant (Phalacrocorax capensisj Steenbras fish (Lithognathus lithognathus) Hottentot fish (Pachymetapon blochii)
Animal
Table 2. Sr/Ca
analyses
978
1179
186
132
82
839
1172
1054
79
337
Sr (bone) (Ppm)
of meat and bone from
219
223
221
193
234
234
225
225
291
240
Ca (bone) (PPt)
the same animal
4.5
5.3
0.8
0.7
a.3
3.6
5.2
4.1
0.3
1.4
Sr/Ca ( x 1000)
16
105
17
9
1
20
36
44
31
78
Sr (meat) @pm)
8
15
4
1
1
4
6
7
7
7
Ca (meat) @pt)
2.0
7.0
4.2
9.0
I.0
5.0
6.0
6.3
4.4
11.1
Sr/Ca (x 1000)
0.1
5.1
1.4
4.1
-
4.2
3.7
4.9
36
4.2
Meat Ash %
Sr
AND
Sr/Ca
Table 3. SrjCa Laboratory number 729 1717 1718 1716 1722 730 1887 1888 1889 1893
156 157 758 886 887 890 1045 888 & 1885
MARINE
of carnivore
AND TERRESTRIAL
bonesfrom
the southwestern
FOODWEBS
43 1
Cape Sr/Ca (x 1000)
Animal Black-backed jackal (Canis mesomelas) Black-backed jackal (Canis mesomelas) Black-backed jackal (Canis mesomelas) Cape fox (Vulpes chama) African wild cat (Felis lybica) Caracal (Felis caracal) Leopard (Pantherapardus) Leopard (Pantherapardus) Leopard (Pantherapardus) Leopard (Pantherapardus)
Table 4. SrjCa Laboratory number
IN
ofplantfoodsfrom
272
279
1.0
151
200
0.8
299
183
1.6
120
192
0.6
110
207
0.5
82
234
0.3
310
194
1.6
167
174
0.9
251
167
1.5
143
171
0.8
the southwestern
Cape Sr/Ca (x 1000)
Plant Moraea fugax corms Grielum humzjiisum roots Prionium serratum stems Wild onion bulbs Aponogeton distyachos flowers Hoodia sp. stem Oxalis spp. corms Honey
Ash %
226 1562 122 451 316
38 186 30 54 44
5.9 8.4 4.1 8.3 7.2
10.8 3.6 9.2 1.9 5.4
555 76 72
87 7 28
6.4 10.8 2.6
11.1 2.3 0.2
(Vinogradov, 1953; Burnett et al., 1978) (see also Table 6). Since both Sr and Ca are low in muscle, the Sr/Ca of this tissue may resemble that of the shell. However, species are highly variable in their shell Sr content. This is related to the proportion of calcite to aragonite in their skeletons (Vinogradov, 1953). For example, 10 species of marine mollusc from the Irish Sea had Sr contents in shell varying between 14 and 1300 ppm; bodies were far less variable. [Comparable values were obtained for a freshwater species (Segar et al., 1971).] Strontium concentrations in the meat of a wide variety of shellfish species are reported to be some two to IO-fold that of seawater (Black & Mitchell, 1952; Burnett et al., 1978), although in one instance a concentration factor of 140 has been reported (Kyle, 1986). In a study of radiostrontium distribution in a freshwater lake, “Sr levelsin the soft tissues of clams represented a concentration factor 730 times that of the lake water; values for the
432
J. C. SEALY
m---m
AND
Carnivore
A. SILLEN
bones
.
-
Herbivore
bones
-
Plants
SrKa
x 1000
Figure 2. The ranges of Sr/Ca in terrestrial plants compared herbivores and carnivores from the southwestern Cape. Note carnivore readings with the lower end of the herbivore range.
Table 5. Sr/Ca edible tissue) Laboratory number 741 740 761 786 818 766 769 1020 2320 1021 2293 2464 830
of marine foods from
Animal Seal (Arctocephaluspusillus) Penguin (Spheniscus demersus) Cape cormorant (Phalacrocorax capensis) Crayfish (Jasus lalandii) Crayfish (Jasus lalandii) Steenbras fish (Lithognathus lithognathus) Hottentot fish (Pachymetapon blochii) Limpet (Patella granatina) Limpet (Patella granatina) Limpet (Patella granularis) Limpet (Patella granularis) Black mussel (Choromytilus meridionalis) Seaweed (Ulva sp.)
the southwestern
Cape.
with the bones of the overlap of the
(AN readingsfor
Ash %
meat/
Sr/Ca (x 1000)
9
1
9.0
8
8
1.0
17
4
4.3
7.6
547
46
11.9
8.6
536
34
15.8
5.1
105
15
7.0
0.1
16
8
2.0
8.2
357
58
6.2
9.5
428
66
6.5
8.5
277
65
4.3
11.0
241
65
3.7
14.0
261
17
15-3
7.8
703
79
8.9
2464
2320
2293
Laboratory number
14.0
9.5 261
428
241
11.0
meat andshells Sr meat (PPm)
of shellfish
Ash % (meat)
of Sr and Ca contents
17
66
65
Ca meat tPPt)
The shellfish in this table are the three most common species in the southwestern refuse in middens for which data are available (Parkington, 1976; Robertshaw,
Limpet (Patella granularis) Limpet (Patella granatina) Mussel (Choromytilus meridionalis)
Shellfish
Table 6. Comparison
409 387
1226 1093
of the shell
2.8
3.0
2.8
Sr/Ca shell ( x 1000)
up over 70%
440
Ca shell (Ppt) 1219
Sr shell (mm)
Cape middens. Together, they make 1977, 1979; Buchanan, 1986).
15.3
6.5
3.7
Sr/Ca meat ( x 1000)
434
J. C. SEALY
AND
A. SILLEN
clam shells were not reported (Ophel, 1963). These data, concerning the radioactive nuclide “Sr rather than common Sr, are clearly anomalous when compared with literature on the common Sr contents of molluscs. They should not be used to infer common Sr contents of marine foods in reconstructing palaeodiets. Part II ( Elevated Sr levels have been reported in human skeletons recovered from both freshwater and marine shell middens (Schoeninger & Peebles, 1981; Kyle, 1986). Skeletons from middens in the southwestern Cape also contain large amounts of Sr. Since these values are not readily explicable in terms of the Sr/Ca of marine foods reported in Part I or in the literature, we believe they are likely to result from a post-depositional effect. To test this hypothesis, we examined bones from two archaeological human skeletons recovered from shell middens in the southwestern Cape by means of the solubility profile method (Sillen, 1986). Methods Solubility profiles rely on the differential solubility of diagenetic, when compared with biogenic apatite (Sillen, 1986). Bone specimens are freezer-milled and 50 mg of the resulting powder placed in an Eppendorf microcentrifuge tube. One millilitre of 100 mM acetic acid/sodium acetate buffer adjusted to pH 4.5 is added to the powder, and the preparation is placed in an ultrasonic bath for exactly 1 min, then centrifuged in an Eppendorf microcentrifuge. After centrifugation for 10 s, the supernatant is decanted and saved for elemental analysis (as described above). New buffer is added to the residue, and the procedure repeated at least 20 times. The series of supernatants thus represents a profile of soluble mineral: the first washes contain the most soluble mineral, washes 5-10 less soluble mineral, washes lo-15 even less soluble mineral, and so on. After 20 washes, the least soluble mineral is present as a residue in the centrifuge tube. Results The results of this procedure are presented in Figure 3. In both samples, Sr/Ca decreases rapidly in the first washes. After wash ~$11, however, Sr/Ca remains constant. In UCT 1683, the curve flattens to a value roughly 25% of the initial ratio (that of the first wash); for UCT 1738, the curve flattens to approximately 30% of the Sr/Ca in the initial wash. For comparison, the solubility profile of fresh bone is presented in this figure. In contrast to fresh bone, the archaeological specimens release considerable Sr from a highly soluble, diagenetic mineral compartment. Unlike whole specimen analyses, the Sr/Ca of washes 1l-20, and of the residues, fall within the range of biologically explainable values. Such diagenetic accumulation of Sr in bone results from high levels of Sr circulating in shell middens (see also Kyle, 1986). It bears restating that shells contain high levels of both Sr and Ca, in contrast to shellfish meat (Table 6). Moreover, the Sr/Ca of shells may not reflect the levels of free Sr and Ca in shell middens available for diagenetic uptake by the interred bone. In the geologically young matrices of interest to archaeologists, relatively more Sr (compared with Ca) may be available. This is because, in marine carbonates, Sr is more mobile than Ca, as demonstrated by the gradual reduction in matrix Sr over geological time (Kinsman, 1969; Shearman & Shirmohammadi, 1969). Discussion The data presented above have implications not only for the application of Sr/Ca to southwestern Cape archaeology, but also for the use of Sr/Ca’ relationships in other archaeological contexts.
Sr
AND
Sr/Ca
IN MARINE
AND TERRESTRIAL
FOODWEBS
435
6
0
4
8
IO Wash
16
20
24
number
Figure 3. Solubility profiles of two archaeological human bones and 1738) and a modern American sheep bone. The sheep bone is demonstrate the flat curve characteristic of biogenic Sr/Ca. Absolute and Ca are not comparable to those from the southwestern Cape. 0, + , UCT 1683; +, modern sheep.
(UCT 1683 included to values of Sr UCT 1738;
First, the data indicate that the consumption of seafood is unlikely to cause especially elevated skeletal Sr/Ca. This observation, coupled with the demonstration of highly soluble Sr in skeletons derived from marine shell middens, suggests that diagenesis is responsible for elevated Sr seen in these skeletons. Although our data do not directly address freshwater middens, we suspect a similar process may occur in such contexts. In at least one study, freshwater molluscs have been shown to be comparable in Sr/Ca to marine molluscs (Segar et al., 1971). This observation may require re-evaluation of some archaeological conclusions made previously. In an analysis of six individuals from the site of Lu”2.5 in northern Alabama, elevated Sr was found in Archaic when compared with Mississippian skeletons (Schoeninger & Peebles, 1981). This was thought to be due to higher Sr content of the Archaic diet, which (unlike the Mississippian) included freshwater molluscs. However, the Archaic skeletons were also buried within a shell-midden, whereas the agricultural Mississippians presumably were not. Thus, diagenetic phenomena might also explain these results. Measurements of the Sr content of the meat of local shellfish would help to resolve this, as would examination of the Archaic bones by newer techniques such as solubility profiles. The second implication of our data is that Sr/Ca is of limited value in reconstructing complex human diets, at least in the southwestern Cape. Reduction in Sr/Ca at higher trophic levels in seen to occur only in specific prey-predator pairs, rather than across broad trophic categories. Moreover, there is significant variability among species within trophic levels. Since prehistoric human diets in this region were complex, incorporating a wide range of marine and terrestrial species, dietary shifts both within and between trophic levels may affect skeletal Sr/Ca.
436
J. C. SEALY
AND A. SILLEN
These observations raise the question of whether Sr/Ca can be used to infer meat intake elsewhere in the world where dietary composition is likely to have been similarly complex. The answer may be “no”. It seems that Sr/Ca may only be used where large and distinct differences in diet are suspected, and where the Sr/Ca of the food species are also measured. Sr/Ca measurements can be used to detect dietary shifts, but not necessarily to ascribe them to changes in the intake of any particular food item. Our results suggest an explanation for a number of anomalies in previous studies of modern and fossil fauna which do not fit a broad trophic level model. For example, Schoeninger (pers. comm.) has shown that lions from Koobi Fora have Sr/Ca values higher than those recorded for other carnivores in the area. The likely explanation is that lions in the region are consuming herbivores or other foods which themselves contain high Sr/Ca (when compared to the foods of other carnivores). In a recent study of the biogenic Sr/Ca of fossil mineral from the Omo Shungura Formation, it was noted that Homotherium sp. (true sabre-toothed cats) also had elevated values when compared to other carnivores (Sillen, 1986). One simple explanation for this, which was not considered at the time, is that these animals specialized in consuming herbivore species with high Sr/ Ca (when compared to other herbivores). The explanations offered here can be further tested by additional analyses of modern and fossil fauna. Many applications to the hominid and associated fossil record remain. For example, significant adaptive differences among species of fossil hominids might be observable. Moreover, Sr/Ca may be helpful in the resolution of hypothesized prey-predator relationships involving hominids and certain specific carnivorous species. Thus, the method may ultimately have more palaeontological than archaeological utility. Sr/Ca is not a suitable technique for addressing dietary questions which concern relatively small changes in the role of meat as part of a varied mixed diet.
Acknowledgements
We are grateful to the large number of people who helped us to obtain the specimens. Jacqueline Portuesi assisted in preparing them for analysis. Cedric Poggenpoel and John Lanham helped prepare the figures. Comments by Susan Limbrey and an anonymous referee improved the manuscript. We acknowledge financial support from the Foundation for Research Development of the C.S.I.R. and the University of Cape Town.
References Bisel, S. L. C. (1980). A pilot study in aspects of human nutrition in the ancient Eastern Mediterranean, with particular attention to trace minerals in severalpopulationsfrom d@erent timeperiods. Ph.D. Dissertation, University of Minnesota, Minneapolis. Black, W. A. P. & Mitchell, R. L. (1952). Trace elements in the common brown algae and in sea water. Journal of the Marine Biological Association of the United Kingdom 30,515-584. Brown, A. (1973). Bone strontium content as a dietary indicator in human skeletalpopulations. Ph.D. Dissertation, University of Michigan, Ann Arbor. Buchanan, W. F. (1986). Sea shells ashore. Ph.D. Dissertation, University of Cape Town, Cape Town. Burnett, M. W., Settle, D. M. & Patterson, C. C. (1978). Comparative distributions of alkaline earths and Pb among tissues of marine plants and animals. Advances in Mass Spectrometry 7B, 1697-1701. Elias, R. W., Hirao, Y. &Patterson, C. C. (1982). The circumvention of the natural biopurification of calcium along nutrient pathways by atmospheric inputs of industrial lead. Geochimica et Cosmochimica Acta 46,2561-2580.
Sr AND
SriCa IN MARINE
AND
TERRESTRIAL
FOODWEBS
437
Fornaciari, G., Trevisani, E. M. & Ceccanti, B. (1984). Indagini paleonutrizionali e determinazione de1 Piombo osseo mediante spettroscopia ad assorbimento atomic0 sui resti scheletrici di epoca tardo-romana (IV secolo d.C >) della “Villa dei Gordiani” (Roma). Archivioper l’rlntropologia e la Etnologia 114, 149-176. Hendey, Q. B. (1983). Palaeontology and palaeoecology of the Fynbos region: an introduction. In (H. J. Deacon, Q. B. Hendey &J. N. Lambrechts, Eds) Fynbos Palaeoecology: A Preliminary Synthesis. South African National Scientific Programmes Report No. 75. Pretoria: Council for Scientific and Industrial Research, pp. 183-204. Katzenberg, M. A. (1983). Chemical analysis ofprehistoric human bonefromjive temporally distinctpopulations in southern Ontario. Ph.D. Dissertation, University of Toronto, Toronto. Kinsman, D. J. J. (1969). Interpretation of Sr*+ concentrations in carbonate minerals and rocks. Journal of Sedimentary Petrology 39,486508. Kostial, K., Gruden, N. & Durakovic, A. (1969). Intestinal absorption of calcium-47 and strontium-85 in lactating rats. CalciJied Tissue Research 4, 13-19. Kyle, J. H. (1986). Effect of post-burial contamination on the concentrations of major and minor elements in human bones and teeth-the implications for palaeodietary research. Journal of Archaeological Science 13,403-416. Ophel, I. L. (1963). The fate of radiostrontium in a freshwater community. In (V. Schultz &A. W. Klement, Eds) Radioecology. London: Chapman & Hall, pp. 213-216. Parkington, J. E. (1976). Follow the San: an analysis of seasonality in theprehistory of the southwestern Cape, South Africa. Ph.D. Dissertation, University of Cambridge, Cambridge. Parkington, J. E. (1984). Changing views of the Later Stone Age of South Africa. In (F. Wendorf & A. Close, Eds) Advances in World Archaeology 3,90-142. Price, T. D. (1985). Late Archaic subsistence in the midwestern United States. Journal of Human Evolution 14,449459. Price, T. D., Connor, M. & Parsen, J. D. (1985). Bone chemistry and the reconstruction of diet: strontium discrimination in white-tailed deer. Journal of Archaeological Science 12,419-442. Robertshaw, P. T. (1977). Excavations.at Paternoster, south-western Cape. South African Archaeological Bulletin 32,74-76. Robertshaw, P. T. (1979). Excavations at Duiker Eiland, Vredenburg District, Cape Province. Annals of the Cape Provincial Museums (Human Sciences) 1, l-26. Runia, L. T. (1987). Strontium and calcium distribution in plants: effect on palaeodietary studies. Journal of Archaeological Science 14,599-608. Schoeninger, M. J. (1979). Diet and status at Chalcatzingo: some empirical and technical aspects of strontium analysis. American Journal of Physical Anthropology 51,295-310. Schoeninger, M. J. (198 1). The agricultural “revolution”: its effect on human diet in prehistoric Iran and Israel. Paleorient 7,73-92. Schoeninger, M. J. (1982). Diet and the evolution of modern human form in the Middle East. American Journal of Physical Anthropology S&37-52. Schoeninger; M. J. & Peebles, C. S. (1981). Effect of mollusc eating on human bone strontium levels. Journal of Archaeological Science 8,391-397. Sealy, J. C. (1986). Stable Carbon Isotopes and Prehistoric Diets in the South- Western Cape Province, South Africa. Cambridge Monographs in African Archaeology 15, BAR International Series 293, British Archaeological Reports, Oxford. Sealy, J. C. & van der Merwe, N. J. (1986). Isotope assessment and the seasonal mobility hypothesis in the south-western Cape of South Africa. Current Anthropology 27,135-150. Segar, D. A., Collins, J. D. & Riley, J. P. (1971). The distribution of the major and some minor elements in marine animals. Part II. Molluscs. Journal of the Marine Biological Association of the UnitedKingdom51,131-136. Shearman, D. J. & Shirmohammadi, N. H. (1969). Distribution of strontium in dedolomites from the French Jura. Nature 223,606-608. Sillen, A. (1981a). Strontium and diet at Hayonim Cave. American Journal of Physical Anthropology56, 131-137. Sillen, A. (1981b). Strontium anddiet at Hayonim Cave, Israel; an evaluation of the strontium/ i calcium techniquefor investigating prehistoric diets. Ph.D. Dissertation, University of ‘2 Pennsylvania, Philadelphia.
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Sillen, A. (1986). Biogenic and diagenetic Sr/Ca in Plio-Pleistocene fossils of the Omo Shungura formation. Paleobiology 12,31 l-323. Sillen, A. (1988). Elemental and isotopic analyses of mammalian fauna from southern Africa and their implications for paleodietary research. American Journal of Physical Anthropology. (In press.) Smithers, R. H. N. (1983). Mammals ofthe Southern African Subregion. Pretoria: University of Pretoria Press. Spencer, H., Li, M., Samachson, J. & Laszlo, D. (1960). Metabolism of strontium-85 and calcium 45 in man. Metabolism 9,916925. Spencer, H., Warren, J. M., Kramer, L. & Samachson, J. (1973). Passage of calcium and strontium across the intestine in man. Clinical Orthopaedics and Related Research 91,225-234. Stuart, C. T. (1981). Notes on the mammalian carnivores of the Cape Province, South Africa. Bontebok No. 1, pp. l-58. Cape Department of Nature and Environmental Conservation. Toots, H. & Voorhies, M. R. (1965). Strontium in fossil bones and the reconstruction of food chains. Science 149,854855. Vinogradov, A. P. (1953). The Elementary Chemical Composition of Marine Organisms. Sears Foundation for Marine Research Memoir 2, Yale University, 647 pp. Walser, M. & Robinson, B. (1963). Renal excretion and tubular reabsorption of calcium and strontium. In (R. H. Wasserman, Ed.) The Transfer of Calcium and Strontium across Biological Membranes, New York: Academic Press, pp. 305-326.
Sr and Sr/Ca i&nMarine and Terrestrial Foodwebs in the Southwestern Cape, South Africa Judith C. Sealy” and Andrew Sillen” (Received 20 September 1987, revisedmanuscript
accepted 3 February 1988)
Strontium and calcium have been measured in a range of plants and animals (both marine and terrestrial) from the southwestern Cape of South Africa as part of an investigation of modern and prehistoric foodwebs in the region. First, the meat of marine molluscs and crustaceans are shown to have Sr and Sr/Ca values comparable to those of terrestrial plants. Thus, the consumption of these marine foods in this region cannot produce the markedly elevated Sr levels seen in archaeological human skeletons from coastal sites; such levels are shown to be a diagenetic phenomenon. Second,
reduction in Sr/Ca in higher trophic levels is seen only when predators are compared with their prey. However, individual herbivore or carnivore species cannot be taken to represent other animals in their respective trophic level. These data imply that Sr/Ca
is inappropriate for determining meat intake in complex prehistoric human diets in this region. The technique may be more useful in examining specific prey-predator relationships, including those in the early hominid fossil record. Keywords: STRONTIUM, DIAGENESIS.
CALCIUM,
DIET, BONE, FOODWEB, AFRICA,
Introduction The Sr concentrations of human bone (and/or the strontium-calcium ratio, SrjCa) have been used to study the relative proportions of meat and plant foods in the diets of prehistoric populations (Brown, 1973; Schoeninger, 1979, 1981, 1982; Katzenberg, 1983; Fornaciari et al., 1984; Price, 1985; Price et al., 1985; Sillen, 1986). The method stems from the demonstrated discrimination against Sr (in favour of Ca) in mammalian digestive tracts (Spencer et al., 1960, 1973; Kostial et al., 1969). In addition, mammalian kidneys excrete Sr more rapidly than Ca (Spencer et al., 1960; Walser & Robinson, 1963). Thus, meat foods are expected to contain much less Sr and have lower Sr/Ca than plant foods. Skeletons of individuals ingesting large quantities of meat are expected to contain relatively low levels of Sr. This discrimination against Sr, or biopurification of Ca, has been demonstrated in at least one natural foodweb, a remote sub-alpine ecosystem in the High Sierras incorporating plants, herbivores and carnivores (Elias et al., 1982). The trophic reduction in Sr/Ca seen in this study has been assumed to be universal and thus apply to other ecological settings. “Department Africa.
of Archaeology,
University
of Cape l’own,
Rondebosch
7700, South,
425 03054403/88/040425
+ 14 $03.00/O
0 1988 Academic
Press Limited
426
J. C. SEALY
AND
Figure 1. Geological geological substrates
A. SILLEN
map of the southwestern Cape showing from which animals were collected (Table
the four 1).
principal
Archaeological studies have used Sr and Ca measurements of the skeletons of animals from different trophic levels as reference points for herbivore and carnivore diets (Sillen, 1981a, b; Price et al., 1985); however, actual foodwebs have not been considered. For example, gazelles and foxes have been chosen to represent herbivores and carnivores, respectively, in Near Eastern sites (Sillen, 1981a, b, 1986), but foxes certainly do not consume gazelles. In this study, we examine the assumption that Sr/Ca is relatively invariant within trophic levels. This project is part of a larger programme of investigation into the diets of Late Stone Age hunter-gatherers and pastoralists in the southwestern Cape of South Africa using a number of archaeometric techniques, including Sr/Ca. The geology of this area is complicated, with a land surface composed of a mosaic of different rock types (Fig. 1). Possible differences in soil Sr/Ca between these areas constitute another potential complication in this undertaking (Toots & Voorhies, 1965). In addition, local prehistoric populations are known to have eaten seafoods, including shellfish. It has been suggested that mollusc eating produces elevated bone Sr levels (Bisel, 1980; Schoeninger & Peebles, 1981; Kyle, 1986) thus obscuring variations in bone Sr/Ca due to differential intake of meat and plant foods. To evaluate Sr/Ca as atrophic level indicator in this environment, we have conducted a survey of Sr/Ca in indigenous foodstuffs which were consumed by prehistoric people. In
Sr AND Sr/Ca IN MARINE AND TERRESTRIAL FOODWEBS 427
addition, we have sampled carnivores to make possible the examination of the distribution of these elements in the foodweb as a whole. Samples of terrestrial animals were collected from the different geological substrates, together with a range of edible plants and marine organisms, and their Sr/Ca measured. The results, presented in Part I of this article, lead us to question the belief that consumption of marine foods produces elevated bone Sr/Ca. In Part II, we further examine this issue and present data showing that elevated Sr/Ca in bone from marine shell middens is a diagenetic, not a dietary, phenomenon. Part I
Archaeological and ethnographic research in the area has provided a fairly detailed picture of the kinds of plants and animals that made up Late Stone Age diets (Parkington, 1976, 1984; Buchanan, 1986). However, the relative importance of the various items remains uncertain. Plant foods included a number of varieties of underground bulbs and corms, especially those of the family Iridaceae. Thesehave durable fibrous corm casings that constitute a large proportion of the identifiable archaeological plant food residues. The genera Watsonia and Moraea were particularly important. Seeds and berries were also consumed, as were certain soft fleshy leaves, stems and flowers, although these are probably severely under-represented in the archaeological record. Meat food, at least in the Late Holocene, was most often obtained from small animals such as rock hyrax, tortoises or small antelope such as duiker, steenbok and grysbok. At the coast, fish, stranded seals and whales, marine birds and shellfish were important items of diet. These organisms persist in the wild in the research area. One of us (J.S.) has already made an extensive collection of the foods eaten by prehistoric people for stable carbon isotope analysis (Sealy, 1986; Sealy & van der Merwe, 1986). We have drawn on this collection for the study described here, supplementing it with additional specimens where necessary. More than 80 specimens of plants and animals important in prehistoric diets in the southwestern Cape are analysed here, together with some carnivores. Carnivores were not food items, but their inclusion in the sample allows us to examine the distribution of Sr/Ca in the foodweb as a whole. The aim of this article is to provide an overview of the foodweb, rather than to examine Sr/Ca variability within species (see Sillen, 1988). The number of observations for each species is small, so the results should be regarded as cautionary rather than conclusive. However, our conclusions rest upon the demonstration of overlapping Sr/Ca values for different kinds of foods. We do not believe that these patterns would be substantially altered by larger sample sizes. Methods
Meat, plant and shellfish samples were cleaned of surface contaminants, freeze-dried and then ashed in a muffle furnace at 600 “C. Bone and shell samples were not ashed. Three to five milligrams of the resulting ash (or bone or shell chips) were weighed with a PerkinElmer AD-4 microbalance and digested in 100 ul concentrated HNO, (Aristar) in borosilicate glass culture tubes at 1.50“C. The samples were brought to dryness and redissolved in 1 ml of 0.2 N HNO,. These solutions provided stocks from which aliquot samples could be removed for analysis. The digestion procedure was performed in duplicate for each specimen. If the difference in Sr/Ca between duplicates was greater than lo%, repeat digestions of the sample, again in duplicate, were analysed. Concentrations of Ca in digestion stocks were measured with flame atomic absorption spectrophotometry employing a Varian model AA6 AAS, following methods described elsewhere (Sillen, 1981a, b, 1986). Concentrations of Sr were measured with a PerkinElmer model 5000 AAS fitted with an HGA graphite furnace. Recovery was found to be 104% when 500 ng Sr and 1000 ng Sr were added to samples before digestion (Sillen,
J. C. SEALY AND A. SILLEN
428
Table 1. SrjCa andSr of bones of terrestrial of the southwestern Cape
Cenozoic marine sands
Animal Porcupine (Hystrix Hyrax (Procavia
Malmesbury shales 1.7 (347)
ajiiicae-australis) capensis)
Hare (Lepus capensis) Tortoise (Chersina angulata)
Duiker (Sylvicapra grimmia) Steenbok/grysbok (Raphicerus sp.) Bat-eared fox (Otocyon megalotis)
3.6 (839) 52(1172) 3.0 (628) 1.4 (337) 6.1 4.3 5.9 54 1.5
d&erent
Table Mountain sandstones 3.0 (577) 3.8 (867) 4.1 (1037)
geological
regions
Witteberg/ Bokkeveld shales and quartzites 3.0 (642)
1.5 (388)
(1277) (999) (1320) (1161) (2.54)
4.7 (1054) 3.6 (840)
3.5 (813)
I.1 (267)
0.8 (179) 1.3 (285)
1.1 (266) 2.3(581) 2.5 (414)
The initial value is the Sr/Ca ratio; bone in ppm. Each pair of numbers
animalsfrom
2.3 (405)
the number in brackets represents the analysis
is the Sr content of one animal.
of the
1986). Some samples from this study were also analysed by an independent method, isotope dilution, and the Sr contents as measured by atomic absorption were found to be 104-105% of the values obtained by isotope dilution, thus confirming the recovery data. Ratios were calculated on the basis of ng Sr/ug Ca. Results
Bone Sr/Ca of modern herbivores and insectivores collected from the four principal geological substrates of the southwestern Cape are presented in Table 1 (also see Figure 1). Samples were collected in areas of undisturbed natural vegetation. The animals are all species known to have been eaten by Late Stone Age populations, except bat-eared foxes which are recent immigrants to the region (Hendey, 1983). (These are included because they are one of only two species we were able to obtain from a Malmesbury shale area. Such shales weather to produce fertile soil and, as a result, are today intensively cultivated.) Hyrax and tortoise are the two animals most frequently found in archaeological sites. From Table 1, it is clear that geological variation is not an important source of Sr/Ca variability in this region: the values for each species are broadly similar on all rock types. However, some species have consistently higher Sr and Sr/Ca than others: all the specimens of hyrax and tortoise measured yield Sr/Ca values between 3 and 6, whereas the ratios for hares and small bovids (duiker and steenbok/grysbok) are between 0.8 and 2. Hyrax, tortoise, duiker and grysbok are browsers. Hares and steenbok include a certain amount of grass in their diets. At least one early paper in this field suggested that the browsing/grazing status of an animal might affect its Sr/Ca (Toots & Voorhies, 1965). A recent study of northwestern Zimbabwe fauna has shown that’specialized grazers have lower Sr/Ca than other animals, although more generalized grazers do not (Sillen, 1988).
Sr AND Sr/Ca IN MARINE AND TERRESTRIAL
FOODWEBS 429
However, browsing/grazing habits do not fully explain the patterning in Sr/Ca amongst the herbivores in this study: hyrax and tortoise are browsers and have high Sr and Sr/Ca, while duiker, also a browser, has relatively low Sr and Sr/Ca. Table 2 contains the results of analyses of meat and bone from the same animal. In all cases, meat contains far less Sr than bone. It also contains correspondingly less Ca. Thus, absolute levels of Sr and Ca in these two tissues vary far more than Sr/Ca. Table 3 documents Sr/Ca of a number of terrestrial carnivores. UCT 729,730 and 1722 were collected in an area of coastal marine sands; the other animals all come from sandstone regions. They are therefore directly comparable to the herbivores in Table 1. Contrary to expectations, the Sr contents and Sr/Ca of the carnivores overlap values at the lower end of the herbivore range (especially those for small bovids). An explanation may lie in closer examination of the carnivore diets. Black-backed jackal and Cape fox are not consistent carnivores, since in addition to meat they eat considerable quantities of insects, arachnids, birds’ eggs, vegetable matter, etc. (Smithers, 1983). These could account for the relatively high Sr/Ca of two of the black-backed jackals (UCT 1718,729):’ Leopards, caracal and African wild cat prey principally on mammals, although the two smaller cats consume some reptiles and insects (Smithers, 1983). The values for the caracal and wild cat are at the lower end of the carnivore range, consistent with a largely meat diet. At first glance, the readings for leopards seem anomalous. We know that these animals consume almost nothing but meat, yet their Sr/Ca varies from 0.8 to 1.6, at the upper end of the carnivore range in this region even when compared with such mixed feeders as black-backed jackal. This anomaly is resolved by recognizing that the principal prey of leopards in the mountains of the western Cape is hyrax (Stuart, 1981). As demonstrated above, hyrax (and tortoise) have elevated Sr/Ca compared with other herbivores (Table 1). Thus, a reduction in Sr/Ca occurs in this circumscribed trophic relationship but is not apparent in generalized comparisons of herbivores with carnivores. These relationships are summarized in Figure 2. It is clearly essential to consider actual prey-predator relationships in order to observe reduced Sr/Ca in high trophic level organisms. In the sample of herbivores and carnivores from the southwestern Cape analysed here, there is no clear reduction in carnivore Sr or Sr/Ca compared with the values for many of the herbivores. However, if one compares hyrax with leopard, or fish with seals and cormorants (actual prey-predator pairs), then the reduction in Sr/Ca in the higher trophic level becomes apparent. Table4contains the Sr and Sr/Ca ofsome important indigenous food plants and a sample of wild honey, a favourite food of protohistoric local hunter-gatherers. Strontiumcalcium values for the plants vary from 4.1 to 10.8. These figures are rather higher, and the range is larger than that reported for plants from an experimental field in the Netherlands (Sr/Ca from 2.95 to 6.25) (Runia, 1987). The plants analysed here, however, include a greater range of plant parts and species (including a succulent and two aquatic species). The range of Sr/Ca values of the plants in Table 4 is approximately the same as that of the meat samples in Table 2, although the absolute abundances of both Sr and Ca are generally much higher in plants than in meat. Honey in this region has rather low Sr/Ca (Table 4). The Sr/Ca for edible tissue of the marine organisms listed in Table 5 varies from 1.Oto 15.8. Seafoods do not contain markedly more Sr than terrestrial foods; the Sr contents of all the marine foods measured fall within approximately the same range as that of the edible terrestrial plants. The consumption of marine foods would not appear to lead to elevated dietary Sr. Available studies on marine shellfish elemental composition report low concentrations of both Sr and Ca in the bodies of the animals, when compared to corresponding shells
769
766
767 '
741
730
879
878
771
137
728
Laboratory number
Hare (Lepus capensis) Eland (Taurotragus oryx) Tortoise (Chersina angulata) Hyrax (Procavia cape&s) Hyrax (Procavia capensis) Caracal (Felis caracal) Seal (Arctocephaluspusillus) Cormorant (Phalacrocorax capensisj Steenbras fish (Lithognathus lithognathus) Hottentot fish (Pachymetapon blochii)
Animal
Table 2. Sr/Ca
analyses
978
1179
186
132
82
839
1172
1054
79
337
Sr (bone) (Ppm)
of meat and bone from
219
223
221
193
234
234
225
225
291
240
Ca (bone) (PPt)
the same animal
4.5
5.3
0.8
0.7
a.3
3.6
5.2
4.1
0.3
1.4
Sr/Ca ( x 1000)
16
105
17
9
1
20
36
44
31
78
Sr (meat) @pm)
8
15
4
1
1
4
6
7
7
7
Ca (meat) @pt)
2.0
7.0
4.2
9.0
I.0
5.0
6.0
6.3
4.4
11.1
Sr/Ca (x 1000)
0.1
5.1
1.4
4.1
-
4.2
3.7
4.9
36
4.2
Meat Ash %
Sr
AND
Sr/Ca
Table 3. SrjCa Laboratory number 729 1717 1718 1716 1722 730 1887 1888 1889 1893
156 157 758 886 887 890 1045 888 & 1885
MARINE
of carnivore
AND TERRESTRIAL
bonesfrom
the southwestern
FOODWEBS
43 1
Cape Sr/Ca (x 1000)
Animal Black-backed jackal (Canis mesomelas) Black-backed jackal (Canis mesomelas) Black-backed jackal (Canis mesomelas) Cape fox (Vulpes chama) African wild cat (Felis lybica) Caracal (Felis caracal) Leopard (Pantherapardus) Leopard (Pantherapardus) Leopard (Pantherapardus) Leopard (Pantherapardus)
Table 4. SrjCa Laboratory number
IN
ofplantfoodsfrom
272
279
1.0
151
200
0.8
299
183
1.6
120
192
0.6
110
207
0.5
82
234
0.3
310
194
1.6
167
174
0.9
251
167
1.5
143
171
0.8
the southwestern
Cape Sr/Ca (x 1000)
Plant Moraea fugax corms Grielum humzjiisum roots Prionium serratum stems Wild onion bulbs Aponogeton distyachos flowers Hoodia sp. stem Oxalis spp. corms Honey
Ash %
226 1562 122 451 316
38 186 30 54 44
5.9 8.4 4.1 8.3 7.2
10.8 3.6 9.2 1.9 5.4
555 76 72
87 7 28
6.4 10.8 2.6
11.1 2.3 0.2
(Vinogradov, 1953; Burnett et al., 1978) (see also Table 6). Since both Sr and Ca are low in muscle, the Sr/Ca of this tissue may resemble that of the shell. However, species are highly variable in their shell Sr content. This is related to the proportion of calcite to aragonite in their skeletons (Vinogradov, 1953). For example, 10 species of marine mollusc from the Irish Sea had Sr contents in shell varying between 14 and 1300 ppm; bodies were far less variable. [Comparable values were obtained for a freshwater species (Segar et al., 1971).] Strontium concentrations in the meat of a wide variety of shellfish species are reported to be some two to IO-fold that of seawater (Black & Mitchell, 1952; Burnett et al., 1978), although in one instance a concentration factor of 140 has been reported (Kyle, 1986). In a study of radiostrontium distribution in a freshwater lake, “Sr levelsin the soft tissues of clams represented a concentration factor 730 times that of the lake water; values for the
432
J. C. SEALY
m---m
AND
Carnivore
A. SILLEN
bones
.
-
Herbivore
bones
-
Plants
SrKa
x 1000
Figure 2. The ranges of Sr/Ca in terrestrial plants compared herbivores and carnivores from the southwestern Cape. Note carnivore readings with the lower end of the herbivore range.
Table 5. Sr/Ca edible tissue) Laboratory number 741 740 761 786 818 766 769 1020 2320 1021 2293 2464 830
of marine foods from
Animal Seal (Arctocephaluspusillus) Penguin (Spheniscus demersus) Cape cormorant (Phalacrocorax capensis) Crayfish (Jasus lalandii) Crayfish (Jasus lalandii) Steenbras fish (Lithognathus lithognathus) Hottentot fish (Pachymetapon blochii) Limpet (Patella granatina) Limpet (Patella granatina) Limpet (Patella granularis) Limpet (Patella granularis) Black mussel (Choromytilus meridionalis) Seaweed (Ulva sp.)
the southwestern
Cape.
with the bones of the overlap of the
(AN readingsfor
Ash %
meat/
Sr/Ca (x 1000)
9
1
9.0
8
8
1.0
17
4
4.3
7.6
547
46
11.9
8.6
536
34
15.8
5.1
105
15
7.0
0.1
16
8
2.0
8.2
357
58
6.2
9.5
428
66
6.5
8.5
277
65
4.3
11.0
241
65
3.7
14.0
261
17
15-3
7.8
703
79
8.9
2464
2320
2293
Laboratory number
14.0
9.5 261
428
241
11.0
meat andshells Sr meat (PPm)
of shellfish
Ash % (meat)
of Sr and Ca contents
17
66
65
Ca meat tPPt)
The shellfish in this table are the three most common species in the southwestern refuse in middens for which data are available (Parkington, 1976; Robertshaw,
Limpet (Patella granularis) Limpet (Patella granatina) Mussel (Choromytilus meridionalis)
Shellfish
Table 6. Comparison
409 387
1226 1093
of the shell
2.8
3.0
2.8
Sr/Ca shell ( x 1000)
up over 70%
440
Ca shell (Ppt) 1219
Sr shell (mm)
Cape middens. Together, they make 1977, 1979; Buchanan, 1986).
15.3
6.5
3.7
Sr/Ca meat ( x 1000)
434
J. C. SEALY
AND
A. SILLEN
clam shells were not reported (Ophel, 1963). These data, concerning the radioactive nuclide “Sr rather than common Sr, are clearly anomalous when compared with literature on the common Sr contents of molluscs. They should not be used to infer common Sr contents of marine foods in reconstructing palaeodiets. Part II ( Elevated Sr levels have been reported in human skeletons recovered from both freshwater and marine shell middens (Schoeninger & Peebles, 1981; Kyle, 1986). Skeletons from middens in the southwestern Cape also contain large amounts of Sr. Since these values are not readily explicable in terms of the Sr/Ca of marine foods reported in Part I or in the literature, we believe they are likely to result from a post-depositional effect. To test this hypothesis, we examined bones from two archaeological human skeletons recovered from shell middens in the southwestern Cape by means of the solubility profile method (Sillen, 1986). Methods Solubility profiles rely on the differential solubility of diagenetic, when compared with biogenic apatite (Sillen, 1986). Bone specimens are freezer-milled and 50 mg of the resulting powder placed in an Eppendorf microcentrifuge tube. One millilitre of 100 mM acetic acid/sodium acetate buffer adjusted to pH 4.5 is added to the powder, and the preparation is placed in an ultrasonic bath for exactly 1 min, then centrifuged in an Eppendorf microcentrifuge. After centrifugation for 10 s, the supernatant is decanted and saved for elemental analysis (as described above). New buffer is added to the residue, and the procedure repeated at least 20 times. The series of supernatants thus represents a profile of soluble mineral: the first washes contain the most soluble mineral, washes 5-10 less soluble mineral, washes lo-15 even less soluble mineral, and so on. After 20 washes, the least soluble mineral is present as a residue in the centrifuge tube. Results The results of this procedure are presented in Figure 3. In both samples, Sr/Ca decreases rapidly in the first washes. After wash ~$11, however, Sr/Ca remains constant. In UCT 1683, the curve flattens to a value roughly 25% of the initial ratio (that of the first wash); for UCT 1738, the curve flattens to approximately 30% of the Sr/Ca in the initial wash. For comparison, the solubility profile of fresh bone is presented in this figure. In contrast to fresh bone, the archaeological specimens release considerable Sr from a highly soluble, diagenetic mineral compartment. Unlike whole specimen analyses, the Sr/Ca of washes 1l-20, and of the residues, fall within the range of biologically explainable values. Such diagenetic accumulation of Sr in bone results from high levels of Sr circulating in shell middens (see also Kyle, 1986). It bears restating that shells contain high levels of both Sr and Ca, in contrast to shellfish meat (Table 6). Moreover, the Sr/Ca of shells may not reflect the levels of free Sr and Ca in shell middens available for diagenetic uptake by the interred bone. In the geologically young matrices of interest to archaeologists, relatively more Sr (compared with Ca) may be available. This is because, in marine carbonates, Sr is more mobile than Ca, as demonstrated by the gradual reduction in matrix Sr over geological time (Kinsman, 1969; Shearman & Shirmohammadi, 1969). Discussion The data presented above have implications not only for the application of Sr/Ca to southwestern Cape archaeology, but also for the use of Sr/Ca’ relationships in other archaeological contexts.
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6
0
4
8
IO Wash
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number
Figure 3. Solubility profiles of two archaeological human bones and 1738) and a modern American sheep bone. The sheep bone is demonstrate the flat curve characteristic of biogenic Sr/Ca. Absolute and Ca are not comparable to those from the southwestern Cape. 0, + , UCT 1683; +, modern sheep.
(UCT 1683 included to values of Sr UCT 1738;
First, the data indicate that the consumption of seafood is unlikely to cause especially elevated skeletal Sr/Ca. This observation, coupled with the demonstration of highly soluble Sr in skeletons derived from marine shell middens, suggests that diagenesis is responsible for elevated Sr seen in these skeletons. Although our data do not directly address freshwater middens, we suspect a similar process may occur in such contexts. In at least one study, freshwater molluscs have been shown to be comparable in Sr/Ca to marine molluscs (Segar et al., 1971). This observation may require re-evaluation of some archaeological conclusions made previously. In an analysis of six individuals from the site of Lu”2.5 in northern Alabama, elevated Sr was found in Archaic when compared with Mississippian skeletons (Schoeninger & Peebles, 1981). This was thought to be due to higher Sr content of the Archaic diet, which (unlike the Mississippian) included freshwater molluscs. However, the Archaic skeletons were also buried within a shell-midden, whereas the agricultural Mississippians presumably were not. Thus, diagenetic phenomena might also explain these results. Measurements of the Sr content of the meat of local shellfish would help to resolve this, as would examination of the Archaic bones by newer techniques such as solubility profiles. The second implication of our data is that Sr/Ca is of limited value in reconstructing complex human diets, at least in the southwestern Cape. Reduction in Sr/Ca at higher trophic levels in seen to occur only in specific prey-predator pairs, rather than across broad trophic categories. Moreover, there is significant variability among species within trophic levels. Since prehistoric human diets in this region were complex, incorporating a wide range of marine and terrestrial species, dietary shifts both within and between trophic levels may affect skeletal Sr/Ca.
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These observations raise the question of whether Sr/Ca can be used to infer meat intake elsewhere in the world where dietary composition is likely to have been similarly complex. The answer may be “no”. It seems that Sr/Ca may only be used where large and distinct differences in diet are suspected, and where the Sr/Ca of the food species are also measured. Sr/Ca measurements can be used to detect dietary shifts, but not necessarily to ascribe them to changes in the intake of any particular food item. Our results suggest an explanation for a number of anomalies in previous studies of modern and fossil fauna which do not fit a broad trophic level model. For example, Schoeninger (pers. comm.) has shown that lions from Koobi Fora have Sr/Ca values higher than those recorded for other carnivores in the area. The likely explanation is that lions in the region are consuming herbivores or other foods which themselves contain high Sr/Ca (when compared to the foods of other carnivores). In a recent study of the biogenic Sr/Ca of fossil mineral from the Omo Shungura Formation, it was noted that Homotherium sp. (true sabre-toothed cats) also had elevated values when compared to other carnivores (Sillen, 1986). One simple explanation for this, which was not considered at the time, is that these animals specialized in consuming herbivore species with high Sr/ Ca (when compared to other herbivores). The explanations offered here can be further tested by additional analyses of modern and fossil fauna. Many applications to the hominid and associated fossil record remain. For example, significant adaptive differences among species of fossil hominids might be observable. Moreover, Sr/Ca may be helpful in the resolution of hypothesized prey-predator relationships involving hominids and certain specific carnivorous species. Thus, the method may ultimately have more palaeontological than archaeological utility. Sr/Ca is not a suitable technique for addressing dietary questions which concern relatively small changes in the role of meat as part of a varied mixed diet.
Acknowledgements
We are grateful to the large number of people who helped us to obtain the specimens. Jacqueline Portuesi assisted in preparing them for analysis. Cedric Poggenpoel and John Lanham helped prepare the figures. Comments by Susan Limbrey and an anonymous referee improved the manuscript. We acknowledge financial support from the Foundation for Research Development of the C.S.I.R. and the University of Cape Town.
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