Strontium and paleodietary research: A review

The measurement of skeletal strontium/calcium (Sr/Ca) ratios can provide information on the proportion of meat and vegetable foods in the diets of prehistoric peoples. This information is based in the well-documented reduction of Sr/Ca ratios in terrestrial food chains. The reduction, and therefore the paleodietary technique, is complicated by (a) differences in Sr/Ca ratios entering food chains, and (b) metabolic considerations such as age, pregnancy, etc. Changes in Sr/Ca ratios during interment may also obscure biological Sr/Ca levels. The theoretical basis of the technique, its complications, and practical use by anthropologists are reviewed, in an attempt to define the quality of information currently and potentially available from Sr/Ca analyses.     

Paleodietary analysis on the prehistoric population of El Hierro (Canary Islands)

In order to deepen our knowledge of the dietary habits of the prehispanic inhabitants of El Hierro, we have determined bone strontium (Sr), manganese (Mn), magnesium (Mg), copper (Cu), zinc (Zn), and calcium (Ca) in 52 human tibiae (23 belonging to males and 20 to females individuals) buried in a single burial cave; in 21 modern individuals who served as controls; and in 11 bones of herbivores found at archeological sites of the Canary Islands. Results suggest that females consumed a more vegetarian diet, although site-corrected Sr/Ca ratio of both males and females speaks for a mixed-diet consumption.     

Changes in Sr/Ca, Ba/Ca and 87Sr/86Sr ratios between trophic levels in two forest ecosystems in the northeastern U.S.A.

The variability and biologicalfractionation of Sr/Ca, Ba/Ca and ⁸⁷Sr/⁸⁶Srratios were studied in a soil–plant–invertebrate–bird food chain in two forested ecosystems withcontrasting calcium availability in the northeasternU.S.A. Chemical measurements were made of the soilexchange pool, leaves, caterpillars, snails, and boththe femurs and eggshells of breeding insectivorousmigratory songbirds. ⁸⁷Sr/⁸⁶Sr values weretransferred up the food chain from the soil exchangepool to leaves, caterpillars, snails and eggshellswithout modification. Adult birds were the oneexception; their ⁸⁷Sr/⁸⁶Sr values generallyreflected those of lower trophic levels at each site,but were lower and more variable, probably becausetheir strontium was derived in part from foods intropical winter habitats where lower⁸⁷Sr/⁸⁶Sr ratios are likely to predominate. Sr/Ca and Ba/Ca ratios decreased at each successive trophiclevel, supporting previous suggestions that Sr/Ca andBa/Ca ratios can be used to identify the trophic levelat which an organism is primarily feeding. The changesin Sr/Ca and Ba/Ca ratios we measured for vegetationand insects were comparable to similar measurementsmade previously (but based on single samples of eachorganism) in an alpine ecosystem. Changes in Sr/Ca andBa/Ca ratios between birds and their food have notpreviously been measured, but the values we obtainedwere similar to those for herbivorous and carnivorousmammals at similar trophic levels. Our results provideevidence that supports the use of Sr/Ca ratios in thedetermination of human paleodiets and suggests thatSr/Ca ratios may also provide a useful tool in studiesof modern food webs. Furthermore, our findings suggestthat ⁹⁰Sr from nuclear fallout will notbioaccumulate in forests and that changes in Sr/Caratios between trophic levels will need to beconsidered in some cases when using⁸⁷Sr/⁸⁶Sr as a tracer of calciumbiogeochemistry.     

Paleodietary analysis on the prehistoric population of El Hierro (Canary Islands)

In order to deepen our knowledge of the dietary habits of the prehispanic inhabitants of El Hierro, we have determined bone strontium (Sr), manganese (Mn), magnesium (Mg), copper (Cu), zinc (Zn), and calcium (Ca) in 52 human tibiae (23 belonging to male and 20 to female individuals) buried in a single burial cave; in 21 modern individuals who served as controls; and in 11 bones of herbivores found at archeological sites of the Canary Islands. Results suggest that females consumed a more vegetarian diet, although site-corrected Sr/Ca ratio of both males and females speaks for a mixed-diet consumption.     

Strontium and diet at Hayonim Cave

Faunal and human bones from the Natufian and Aurignacian levels of Hayonim Cave, Israel, were analyzed for calcium, strontium, and phosphate, in order to investigate the efficacy of strontium measurements for determining the proportion of meat in human diets. This site in the western Galilee was appropriate for a test of the technique since a)herbivore and carnivore fauna were present in numbers from two different time periods, b) well-characterized human skeletons were also present in at least one of these levels, and c)the diets of the individuals examined were basically well understood. On the basis of Sr/Ca values, a large difference was observed between Natufian herbivore bones and carnivore bones in the manner predicted by the diets of these species. Sr/Ca values for the adult humans from the same level fell midway between the herbivore and carnivore ranges. However, a different pattern was observed for Aurignacian fauna; no difference could be found between Sr/Ca ratios of herbivore and carnivore bones. The findings suggest that, in certain circumstances, the technique may provide important new paleodietary information. However, at any given site or level, both herbivore and carnivore fauna should be analyzed before conclusions about human diets are drawn from it.     

Age-related changes in strontium to calcium ratios in rat tissues

The Sr/Ca ratios in plasma, urine, bone, and soft tissues for various ages after weaning in male and female rats were determined to examine the effects of aging on the discrimination between strontium (Sr) and calcium (Ca) under physiological conditions. Age-related changes in the Sr/Ca ratios were similar in all tissues; the Sr/Ca ratios decreased rapidly until about 25-wk-old and then slowly, from that period on, reaching much lower values than in the diet. When the logarithm of the Sr/Ca ratio in each tissue was plotted against the logarithm of age, a linear relationship was observed with statistically significant (p less than 0.05) regression lines. The higher levels of Sr/Ca ratios in all tissues of the younger rats could be explained by the high efficiency of Sr absorption by the small intestine early in life. Parameters for the equations between age and Sr/Ca ratio differed with tissues, suggesting the existence of specific discrimination mechanisms in each tissue.     

Effects of reproduction on sexual dimorphisms in rat bone mechanics

Osteoporosis most commonly affects postmenopausal women. Although men are also affected, women over 65 are 6 times more likely to develop osteoporosis than men of the same age. This is largely due to accelerated bone remodeling after menopause; however, the peak bone mass attained during young adulthood also plays an important role in osteoporosis risk. Multiple studies have demonstrated sexual dimorphisms in peak bone mass, and additionally, the female skeleton is significantly altered during pregnancy/lactation. Although clinical studies suggest that a reproductive history does not increase the risk of developing postmenopausal osteoporosis, reproduction has been shown to induce long-lasting alterations in maternal bone structure and mechanics, and the effects of pregnancy and lactation on maternal peak bone quality are not well understood. This study compared the structural and mechanical properties of male, virgin female, and post-reproductive female rat bone at multiple skeletal sites and at three different ages. We found that virgin females had a larger quantity of trabecular bone with greater trabecular number and more plate-like morphology, and, relative to their body weight, had a greater cortical bone size and greater bone strength than males. Post-reproductive females had altered trabecular microarchitecture relative to virgins, which was highly similar to that of male rats, and showed similar cortical bone size and bone mechanics to virgin females. This suggests that, to compensate for future reproductive bone losses, females may start off with more trabecular bone than is mechanically necessary, which may explain the paradox that reproduction induces long-lasting changes in maternal bone without increasing postmenopausal fracture risk.     

Étude de la discrimination strontium-calcium chez le Haricot en fonction du rapport et des teneurs respectives en Sr et Ca de la solution nutritive

The discrimination between strontium and calcium is not only conditioned by the Sr/Ca ratio of the nutrient solution but also by the total dose (Ca + Sr) of the medium. The variation of the observed ratio in relation to the total dose is closely related with the Sr/Ca ratio of the solution. Sr is preferentially fixed by the roots when the Sr/Ca ratio of the solution is low; such a result is observed in the shoots for higher ratios. For all the treatments, the OR of the leaves is lower than unity (discrimination against strontium). When the stable strontium concentration of the solution is not detectable, an increase in the calcium concentration reduces the radiostrontium retention by the roots.     

The Uptake of Calcium and Strontium by Plants from Soils and Nutrient Solutions

Plants have been grown in soils and nutrient solutions containingdifferent Ca/Sr ratios to find out whether there is any biologicaldiscrimination between calcium and strontium. When tomato plants are grown in nutrient solutions containingmolar Ca/Sr ratios from 2/1 to 4,000/1, the shoots do not distinguishbetween the elements, but the roots absorb strontium preferentiallyat low concentrations of this element. Since ion exchange resins show the same phenomenon, though toa less marked extent, this is believed to be a physico-chemicalrather than a biological effect. No chemical reagent has been found which can extract calciumand strontium from soils in the same ratio as plants. Ammoniumacetate extraction may give misleading results for the plant-availableCa/Sr ratios in the soil. Data for the Ca/Sr ratios in barleyand lucerne grown on soils containing ratios from 3/1 to 650/1emphasize this point.     

Determining the sources of calcium for migratory songbirds using stable strontium isotopes

We investigated natural variations in the stable isotopic composition of strontium (a surrogate for calcium) in the bones of a single species of breeding migratory songbird, as well as in their eggshells, egg contents, and food sources. We use this information to determine the sources of calcium to these migratory songbirds and their offspring. Samples were collected from two locations in the northeastern USA (Hubbard Brook, NH, and Downer Forest, VT.) that differed in soil geochemistry. The mean ⁸⁷Sr/⁸⁶Sr ratios of food items (caterpillars and snails), eggshells, and egg contents were indistinguishable within each site, but significantly different between the two sites. Mean ⁸⁷Sr/⁸⁶Sr ratios for the bones of adult females were significantly different between the two sites, but values were significantly lower than those of food items and eggshells at each site. Two of four adult individuals studied at each site had ⁸⁷Sr/⁸⁶Sr ratios lower than the entire range of values for local food sources. Mixing calculations indicate that up to 60% of skeletal strontium and calcium was derived from foods consumed in the winter grounds where lower ⁸⁷Sr/⁸⁶Sr ratios predominate. At each study site, the ⁸⁷Sr/⁸⁶Sr ratio of eggshells differed significantly between clutches, but the mean clutch ⁸⁷Sr/⁸⁶Sr ratios were unrelated to the skeletal ⁸⁷Sr/⁸⁶Sr ratio of the laying adult. These findings suggest that strontium (and hence calcium) for eggshell production in this species is derived predominantly from local food sources in breeding areas. Thus, reductions in available calcium in northern temperate ecosystems due to the influences of acid deposition could be potentially harmful to this and other species of migratory bird.     

Biological discrimination between calcium and strontium in kidneys and bone of young and adult rats

Effects of aging on the biological discrimination between calcium (Ca) and strontium (Sr) by the kidneys and bone were studied in male and female rats of 5 to 50 wk of age by examining Sr/Ca ratios in the plasma, urine, and bone. The Ca-Sr discrimination at the reabsorption process in the kidneys was not affected by aging in male or female rats. On the other hand, discrimination between the two elements was shown to be age-related at the absorption process in the digestive tract, and became more strict with age. The reverse situation was observed in the discrimination of Ca and Sr in the femur; younger rats discriminated the two elements more strictly than older animals.     

The effect of ethinyl oestradiol on calcium and bone metabolism in peri- and postmenopausal women

The effects of ethinyl oestradiol on plasma and urinary calcium and other indices of bone turnover have been compared in peri- and postmenopausal women. In postmenopausal women ethinyl oestradiol caused significant decreases in the fasting plasma total and ionised calcium and phosphate concentrations, in alkaline phosphatase activity and in the fasting urinary Ca/Cr and OHPr/Cr ratios. Similar, but less marked, effects were observed in peri-menopausal women, with significant decreases in the fasting plasma phosphate and urinary Ca/Cr ratio. Small decreases in the plasma total and ionised calcium concentrations, in alkaline phosphatase activity and in the urinary OHPr/Cr ratio also occurred but were not statistically significant in our sample. Ethinyl oestradiol therefore appears to reduce bone loss in peri- as well as post-menopausal women, at least in the short term, but the effect is less pronounced in the former group.     

Strontium:cakium concentration ratios in otoliths of herring larvae as indicators of environmental histories

Synopsis Elemental analyses, using wave-length dispersive electron microprobe techniques on otoliths from reared Atlantic herring larvae, Clupea harengus, showed trace quantities of strontium relative to that of calcium, and an inverse relationship between Sr/Ca concentration ratios and rearing temperature. These data are consistent with those for coral aragonite, in that there appears to be an inverse temperature effect on physiological incorporation of strontium in the otolith aragonite. Our determinations of Sr/Ca concentration ratios of lab-reared herring larvae showed that the deposition of strontium relative to calcium and the rearing temperature were related, where: T (° C) = −2.955 [Sr/Ca] × 1000 ± 19.172. This principle thus makes it possible to use Sr/Ca concentration ratios in fish otoliths to delineate past temperatures experienced by an individual. Further, combining electron microprobe analyses with scanning electron microscope (SEM) examinations of daily increments in the same otolith makes it possible to reconstruct the temperature history for an individual fish on a time scale of days. An example of the application of the technique to an approximately six-month-old field-caught herring larva is given, and the limitations of the technique are discussed.     

Nonlinearity in the relationship between bone Sr/Ca and diet: Paleodietary implications

Strontium in archaeological human bones is widely, almost paradigmatically, used as a measure of the relative dietary abundances of plants and meat. Quantitative modeling reveals, however, that there is not a simple proportional relationship between bone strontium and the dietary plant/meat ratio. While knowledge of specific foods and their compositions may permit accurate calculation of average bone strontium levels, knowledge of bone strontium does not inversely allow accurate calculation of specific foods. Although bone strontium quantitatively reflects the average dietary Sr/Ca ratio, it is disproportionately sensitive to high-calcium foods and can be easily affected by minor dietary constituents and culinary practices. Bone strontium, and by analogy, barium, should be seen as a reflection of the high-mineral dietary components rather than a quantitative index of trophic position. © 1995 Wiley-Liss, Inc.     

青海喇家遗址人类遗骸的锶同位素比值分析

喇家遗址位于青海省民和县,主体为齐家文化的遗存,距今大约3900~4300年。喇家遗址古代人类迁移活动的研究,对于探索黄河上游古代文明,推动该地区齐家文化研究,有着积极的学术意义。本文首先采用电感耦合等离子体质谱技术对喇家遗址出土的22个个体的44份牙釉质和骨骼样品进行了元素分析,根据检测结果排除了受到污染的2份骨骼样品。其次,通过热电离质谱分析方法对喇家遗址出土22个个体的42份牙釉质和骨骼样品及8个猪牙釉质样品的锶同位素比值进行了测定。检测数据表明,猪牙釉质的锶同位素比值的标准偏差远小于其他动物,可以推断猪由当地饲养的可能性是最大的,因此其牙釉质锶同位素比值可以代表当地的锶同位素水平。经过计算得到8个猪牙釉质的锶同位素比值的平均值为0.710930,将该平均值加或减2倍标准偏差可以得到喇家遗址当地的锶同位素比值范围在0.711058~0.710802之间。以此为前提可以推测,本次检测的22个个体中,有17个个体牙釉质的锶同位素比值在遗址当地的锶同位素比值范围内,可能为本地出生。还有2个个体牙釉质在当地的锶同位素比值范围上下限附近,姑且存疑;仅有3个个体牙釉质的锶同位素比值在遗址当地的锶同位素比值范围以外,表明这些个体可能在其他地区出生,去世后埋葬在遗址中。此外,数据分析还显示,本地出生人群牙釉质的锶同位素比值平均值与骨骼的锶同位素比值平均值比较接近,暗示喇家遗址当地古人类生活方式可能比较统一。     

Migratory history of the spotted flagtail, <Emphasis Type="Italic">Kuhlia marginata</Emphasis>

The spotted flagtail, Kuhlia marginata (Kuhliidae) is commonly found in streams throughout the Ryukyu Archipelago, including Okinawa Island, Japan. Although it has been suggested that they spawn at sea, little is known about their migratory history. The aim of the study was to clarify their migration history based on otolith microchemistry analysis, size composition and the gonadal development of fish collected from the Genka River on Okinawa Island. All strontium/calcium (Sr/Ca) ratios of otoliths remained high around their cores, and then dropped rapidly at some distance from the core. The estimated standard length corresponding to decreases correlated with the size of new recruits collected in the river (ca. 20 mm in standard length: SL). Multiple increases, the sign for migrations into the high saline area, were recognized intermittently around the otolith margin. As the estimated SL at these increases in the Sr/Ca ratio occurred above the minimum maturation size (female: 95.5 mm SL; male: 83.5 mm SL), such multiple increases in Sr/Ca ratios were caused by spawning migration. Therefore, we conclude that the catadromous pattern of K. marginata is as follows; this species grows in the sea during the early life stage until ca. 20 mm SL and then grow in the freshwater area until maturation, before they migrate again to the sea for spawning. Sr/Ca ratio profiles also suggested that a large number of males did not return to the freshwater area after spawning, whereas females might spawn several times during their lives around the inshore area.     

Discrimination between strontium and calcium in suckling rats

Discrimination between strontium (Sr) and calcium (Ca) was examined in suckling rats and compared with that in older rats after weaning. Concentrations of Sr and Ca and the Sr/Ca ratios in serum and femur of 10-d old and 21-d old rats were determined. The Sr concentrations and Sr/Ca ratios in the serum and femur of 10-d old rats were lower than those of 21-d old rats, that could be explained by the fact that 10-d old rats ingested only maternal milk in which the Sr/Ca ratio was much lower than the laboratory diet. The relative ratios of Sr/Ca in serum and femur to that in the diet were found to be higher in 10-d old rats compared with those in 21-d old rats, and also higher than those in the older rats after weaning, as described in our previous publication. This result may reflect that discrimination between Sr and Ca during intestinal absorption is lacking in very young animals before weaning and develops after this age. Renal discrimination between Sr and Ca in the suckling rats at 10 d of age was evaluated by determining the relationship between the relative clearances of Ca and Sr. The mathematical model proposed by Walser and Robinson was applied on these results and the parameter for the equation, that is, the discrimination constant, was shown to be higher in 10-d old rats compared to those in young (7 wk of age) and adult (25 wk of age) rats. This result suggests that the discrimination of Sr in favor of Ca during the tubular reabsorptive process may not be fully developed in the very young rats before weaning.     

Identifying calcium sources at an acid deposition-impacted spruce forest: a strontium isotope,alkaline earth element multi-tracer approach

Depletion of calcium from forest soils has important implications for forest productivity and health. Ca is available to fine feeder roots from a number of soil organic and mineral sources, but identifying the primary source or changes of sources in response to environmental change is problematic. We used strontium isotope and alkaline earth element concentration ratios of trees and soils to discern the record of Ca sources for red spruce at a base-poor, acid deposition-impacted watershed. We measured ⁸⁷Sr/⁸⁶Sr and chemical compositions of cross-sectional stemwood cores of red spruce, other spruce tissues and sequential extracts of co-located soil samples. ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios together provide a tracer of alkaline earth element sources that distinguishes the plant-available fraction of the shallow organic soils from those of deeper organic and mineral soils. Ca/Sr ratios proved less diagnostic, due to within-tree processes that fractionate these elements from each other. Over the growth period from 1870 to 1960, ⁸⁷Sr/⁸⁶Sr and Sr/Ba ratios of stemwood samples became progressively more variable and on average trended toward values that considered together are characteristic of the uppermost forest floor. In detail the stemwood chemistry revealed an episode of simultaneous enhanced uptake of all alkaline earth elements during the growth period from 1930 to 1960, coincident with reported local and regional increases in atmospheric inputs of inorganic acidity. We attribute the temporal trends in stemwood chemistry to progressive shallowing of the effective depth of alkaline earth element uptake by fine roots over this growth period, due to preferential concentration of fine roots in the upper forest floor coupled with reduced nutrient uptake by roots in the lower organic and upper mineral soils in response to acid-induced aluminum toxicity. Although both increased atmospheric deposition and selective weathering of Ca-rich minerals such as apatite provide possible alternative explanations of aspects of the observed trends, the chemical buffering capacity of the forest floor-biomass pool limits their effectiveness as causal mechanisms.     

Tracking dietary transitions in weanling baboons (Papio hamadryas anubis) using strontium/calcium ratios in enamel

Strontium and calcium are incorporated into developing teeth in a manner that reflects changing physiological concentrations in the body. A new model predicts changes in strontium/calcium (Sr/Ca) ratios in response to dietary transitions experienced at birth and during the weaning period. Microsampling of longitudinal thin sections of tooth enamel using laser ablation inductively coupled plasma mass spectrometry provides a basis for the systematic evaluation of variation in Sr/Ca ratios within the tooth crown. Incremental growth markers in enamel are used to determine the age of onset of enamel mineralization at each sampling point. Thin sections of 5 teeth from 2 wild-caught baboons (Papio hamadryas anubis) were systematically analysed using this technique. Intra- and intertooth analyses of Sr/Ca ratios reveal a pattern of dietary development during the period of enamel formation that is consistent with observational data on the timing of weaning behaviour in anubis baboons.     

Sr/Ca and early hominin diets revisited: new data from modern and fossil tooth enamel

A previous study of strontium/calcium (Sr/Ca) ratios in Paranthropus suggested that it consumed more animal foods than was previously believed. However, that study looked at Sr/Ca in fossil bone, which is known to be highly susceptible to diagenesis. Enamel, in contrast, is resistant to post-mortem alteration making it a more appropriate material for Sr/Ca analysis of Plio-Pleistocene fossils. Yet, we know virtually nothing about Sr/Ca in the enamel of modern African mammals, much less fossil taxa. To address this gap, we studied Sr/Ca in tooth enamel from modern mammals in the greater Kruger National Park, South Africa, as well as fossil fauna from the Sterkfontein Valley. Grazing herbivores have the highest Sr/Ca, followed by browsers and carnivores in both modern and fossil fauna. This similarity in ecological Sr/Ca patterning between modern and fossil fauna shows that diagenesis has not obscured the primary dietary signals. Australopithecus has significantly higher Sr/Ca than Paranthropus, and higher Sr/Ca than fossil papionins, browsers, and carnivores. Paranthropus has lower Sr/Ca than grazers, but its Sr/Ca is higher or equal to that of fossil papionins, browsers, and carnivores. Thus, Sr/Ca for both hominins is relatively high, and provides no direct evidence for omnivory in either taxon. The consumption of underground resources or insects are among the possible explanations for the highly elevated Sr/Ca in Australopithecus.