Student Research Project
Barium and strontium in modern food webs

Students
Quynh Diem Dui
Major: Biology

Cynthia Hallock
Major: Geology and Classic Studies

Professor
James H. Burton, Professor, Laboratory for Archaeological Chemistry, University of Wisconsin, Madison

One of the key agendas of the Laboratory for Archaeological Chemistry is the assessment of prehistoric diets through the analysis of human bone. We focus upon barium (Ba) and strontium (Sr) because they are alkaline-earth elements that substitute for calcium in hydroxyapatite, the mineral component of bone. These two elements enter bone in proportion to their dietary levels, reflecting their contributions to the dietary pool of mineral-forming cations: Ca+Ba+Sr (conveniently expressed as the dietary Ba/Ca and Sr/Ca ratios). Thus, bone analyses have the potential to reveal prehistoric diets.

In an additional phenomenon, known as biopurification, organisms physiologically discriminate against the assimilation of strontium and barium in preference for calcium. Herbivores, which retain less strontium and barium, have lower Ba/Ca and Sr/Ca ratios than the plants in their diets. Carnivores, eating meat of herbivores or other animals whose Ba/Ca and Sr/Ca ratios already reflect some degree of biopurification, have even lower Ba/Ca and Sr/Ca ratios than herbivores. Therefore, analyses of Ba and Sr may reveal patterns of consumption such as the relative importance of gathering (i.e., plants, producing higher bone Ba/Ca and Sr/Ca ratios) versus hunting (i.e., meat, producing lower ratios). Understanding these patterns of consumption is key to a number of highly significant anthropological issues such as the evolution of agriculture.

In the Laboratory for Archaeological Chemistry, undergraduates and others are exploring the distributions of these elements in various food webs from a variety of environments. Undergraduate students obtain sets of animals or animal bones, and sometimes plants and soils, from a specific ecosystem or environment. Specimens are typically provided by taxidermists and government biologists (Fish and Wildlife Service and departments of natural resources). Students chemically analyze long bones such as femurs because they contain dense cortical tissue that reflects a long-term dietary average. Bone is converted in a kiln to bone ash, which is then dissolved in strong acid and analyzed by inductively coupled plasma emission spectrometry (ICPS) for calcium, strontium, barium, and other elements that may yield additional information.

Student projects have verified biopurification in a typical temperate environment and have also revealed that Ba/Ca and Sr/Ca ratios decrease with ascending position in nonmammalian food chains such as in marine and lacustrine ecosystems. Student projects have also shown strong environmental patterns such as strontium enrichment in desert environments and strontium depletion in tropical rain forests. This research is expanding the kinds of dietary questions that archaeologists can address and also suggesting the possibility of assessing paleoenvironments through chemical analysis of fossil bone.

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