Judith Sealy

Stable carbon isotope ratio differences between bone collagen and bone apatite, and their relationship to diet

A large set of δ13C values for both the organic (collagen) and inorganic (apatitic) components of bone and tooth is presented for southern African fauna. Equations are obtained to describe variation in the relationship between these two isotopic values (the “apatite-collagen spacing”) with trophic level, for herbivores, carnivores and omnivores respectively. The empirically derived equations are slightly different from earlier predictions, and the implications are discussed. Differences between apatite and collagen δ13C values in a sample of prehistoric human skeletons from the southwestern Cape coast are considered in the light of relationships derived from faunal data. Unexpectedly small isotopic differences seen in many of these skeletons may be attributable to the particular mix of marine and terrestrial components of their diets.

Nitrogen isotopic ecology in southern Africa: Implications for environmental and dietary tracing

Abstract In order to establish baseline nitrogen isotope data for certain African ecosystems, we have measured the 15 N 14 N of some 300 marine and terrestrial organisms. The majority of these specimens come from the southwestern Cape, and were chosen to represent a cross-section of the foods important in prehistoric diets in the region. δ 15 N analyses of 78 Holocene human skeletons from the same area are interpreted in the light of these results. Additional terrestrial animal samples were collected from the northern and eastern Cape and from Botswana and Malawi. They represent a wide range of climatic and environmental zones, from semi-desert to sub-tropical swamps. The patterning in the values for marine organisms is consistent with previously published data; that for terrestrial organisms, however, is more complex than recent studies have indicated. Our data confirm the proposal that animal δ 15 N values vary with rainfall: high δ 15 N values for herbivores occur in areas receiving less than 400 mm of rain per annum. We critically examine a recently proposed model explaining this phenomenon, and suggest some additional mechanisms which should be considered. In such arid areas, nitrogen isotope ratios cannot be used as marine terrestrial indicators, but may provide some indication of the trophic level of the food consumed. Dietary studies on human populations can only be undertaken with a thorough appreciation of the isotopic ecology of the relevant foodweb.

Beyond lifetime averages: tracing life histories through isotopic analysis of different calcified tissues from archaeological human skeletons

Stable-isotopic analyses of human bone, now an established aid to dietary reconstruction in archaeology, represent the diet as averaged over many years. Separate analysis of different skeletal components enables changes in diet and place of residence to be tracked, giving a fuller life-history for long-dead individuals.

87Sr86Sr as a dietary indicator in modern and archaeological bone

Strontium isotope ratios (87Sr86Sr) in human and animal bone are inherited, via foodstuffs, from rocks in the areas in which an individual lived. If such an area includes more than one rock type with differing 87Sr86Sr, or is coastal region with terrestrial 87Sr86Sr different from that of the sea, then bone strontium isotope ratio measurements provide a measure of the relative importance of foods from each isotopic zone. In the southwestern Cape of South Africa, marine and coastal terrestrial areas (covered by marine sands) should yield foods with 87Sr86Sr close to the marine average, while the Cambrian and pre-Cambrian shales and sandstones of the hinterland ought to be enriched in 87Sr (from radioactive decay of 87Rb during the considerable time elapsed since they were laid down). Analyses of modern animal bones show that these patterns do prevail, furnishing us with a new means of reconstructing prehistoric diets. Preliminary measurements on archaeological human material recovered from coastal shell-midden sites apparently reflect heavy reliance on coastal foods. However, these bones have been shown to be contaminated with diagenetic marine strontium from the shells in the middens. Experiments on faunal specimens show that diagenetic strontium can be removed by repeated washing in acetic acid/sodium acetate buffer. Mineral with 87Sr86Sr similar to the burial environment dissolves first, while subsequent washes remove strontium with different 87Sr86Sr. For two coastal human skeletons, however, the marine/coastal terrestrial nature of the isotopic signature remains. Further experimental work is required in order to establish the limits of the technique; however, a combination of the solubility profile method and strontium isotope analysis promises to provide a powerful tool for reconstructing palaeodiets.

Isotope Assessment and the Seasonal- Mobility Hypothesis in the Southwestern Cape of South Africa1

This paper consists of a stable carbon isotopic assessment of the diets of the Holocene human inhabitants of the southwestern Cape, South Africa. Samples of the foods these people ate were collected from each of the four major physiographic zones in the area and their 13C/12C rations measured. A total of more than 200 such analyses enabled the estimation of the average 13C values of prehistoric human diets in each zone. This information is used to interpret 13C measurements on a series of archaeological human skeletons. The results are consistent with a model of prehistoric subsistence behaviour in which a number of people living at the coast made intensive use of marine food resources throughout the Holocene, consuming such a large proportion of these foods that they must have spent much if not all of their time at the coast. Most inland skeletons reflect an almost entirely terrestrial diet. These results contradict hypotheses about seasonal population movements between the coast and the interior generated from excavated archaeological material. Considerable changes in many of our current views of the Late Stone Age of the southwestern Cape will have to be made in order to accommodate these data.

Social, spatial and chronological patterning in marine food use as determined by ä 13 C measurements of Holocene human skeletons from the south‐western Cape, South Africa

Abstract Radiocarbon dates, a13C measurements and observations on the sex and dentition of a series of prehistoric human skeletons from the south‐western Cape coast reveal changes in the utilisation of marine foods during the Holocene. Only a few skeletons date from the early and middle Holocene, but a substantial number date from 3000 to 2000 BP. During life, these people were heavily reliant on marine foods. People whose skeletons post‐date 2000 BP ate more mixed diets with a larger proportion of terrestrial foods.This pattern mirrors that seen in food waste from sites excavated at or near Elands Bay. It seems, however, to be true of a much larger area of the Cape, extending as far south as the Cape Peninsula. Changes in the composition of diet through the Holocene were more marked for men than for women. Tooth wear and the occurrence of dental caries vary according to sex, and to the degree of reliance on marine foods.

The chronology of the introduction of pastoralism to the Cape, South Africa

A careful survey of reports of early sheep in southernmost Africa combines with new radiocarbon dates to revise our knowledge of early pastoralism in the Cape. The new chronology shows the keeping of domestic stock and the making of pottery are not simultaneous and intertwined but separate events in a more complex history.

A record of rapid Holocene climate change preserved in hyrax middens from southwestern Africa

The discovery of sensitive paleoenvironmental proxies contained within fossilized rock hyrax middens from the margin of the central Namib Desert, Africa, is providing unprecedented insight into the region’s environmental history. High-resolution stable carbon and nitrogen isotope records spanning 0–11,700 cal (calibrated) yr B.P. indicate phases of relatively humid conditions from 8700–7500, 6900–6700, 5600–4900, and 4200–3500 cal yr B.P., with a period of marked aridity occurring from 3500 until ca. 300 cal yr B.P. Transitions between these phases appear to have occurred very rapidly, often within <200 years. Of particular importance are: (1) the observed relationship between regional aridifi cation and the decline in Northern Hemisphere insolation across the Holocene, and (2) the signifi cance of suborbital scale variations in climate that covary strongly with fl uctuations in solar forcing. Together, these elements call for a fundamental reexamination of the role of orbital forcing on tropical African systems, and a reconsideration of what factors drive climate change in the region. The quality and resolution of these data far surpass any other evidence available from the region, and the continued development of this unique archive promises to revolutionize paleoenvironmental studies in southern Africa.

Investigating Identity and Life Histories: Isotopic Analysis and Historical Documentation of Slave Skeletons Found on the Cape Town Foreshore, South Africa

Isotopic analysis of skeletons excavated during the 1950s has confirmed that they are the remains of shipwreck victims: slaves on board the Portuguese slaving brig Pacquet Real when it sank on 18 May 1818. Twenty-five slaves drowned and the remaining 133 became “Prize Negroes” at the Cape. The isotopic signatures are consistent with values expected for people living in an African village eating a terrestrially based diet. Analyses of different skeletal elements, i.e., teeth, long bone, and rib, are shown to be a valuable tool in tracing change or consistency in diet during a persons life, because different skeletal elements form at different stages of life and, subsequently, remodel at different rates. A comparison of isotope ratios from different skeletal elements indicates a change in diet in all these individuals, probably coincident with their enslavement. Variation between individuals in the isotopic composition of diets eaten early in life is sufficiently large to deduce heterogeneous origins for the group.

Chemistry and Paleodietry Research: No More Easy Answers

While isotopic and elemental analyses of prehistoric skeletons have made an important contribution to paleodietary research over the last 10 years, certain problems in the application of these techniques only now are emerging. These problems, affecting both isotopic and trace-element studies, mainly are due to the peculiar interdisciplinary nature of the field, rather than to any technological barrier. With minor exceptions, techniques developed largely in other sciences have been grafted on to archaeological problems. This no longer suffices because gaps remain in the scientific grounding of these techniques that need to be addressed before more complicated archaeological questions can be resolved. While the necessary studies may seem to be of little immediate anthropological interest, they are vital if continued progress in paleodietary research is to characterize the years ahead.