The chemical analysis of osteological remains has played an important role in the reconstruction of subsistence patterns in past human populations. Conventional approaches used plant and animal, as well as utensil, remains. The interaction between these elements is complex due to difficulties in preservation of food remains. Certain aspects of the chemical composition of human bone are determined by the types of food consumed, providing a more direct and objective means to generate human subsistence patterns.
Bone tissue is a composite of two kinds of component: the organic component is mainly a large protein molecule known as collagen (90%), and the inorganic component is hydroxyapatite, in the form of calcium phosphate, which impregnates the collagen matrix.
The principal chemical analyses for dietary reconstruction are those of carbon and nitrogen stable isotopes (performed mainly on the organic portion of the bone) and those of trace elements (strontium, barium, etc.) (on the inorganic portion).
Stable isotope analysis There are two stable isotopes of carbon: 12C and 13C. Ratios of these isotopes in mammalian bones reflect the consumption of different foods. There are two types of plants according to the photosynthetic pathways they use: the so-called C3 plants, which discriminate against the heavier isotope of carbon (13C) and are enriched in 12C, and the C4 plants (generally speaking, tropical gramineae such as maize, sorghum, sugarcane, millet, and soya), which do not discriminate the 13C isotope. On the other hand, high levels of 13C occur in certain kinds of marine animals exploited by humans. Since the carbon in bone collagen comes from food consumed, the isotopic ratios allow for differentiating between foods of marine or terrestrial provenance, and in turn, the latter can be separated according to the proportion of C3 and C4 plants ingested.
Organisms eating more C3 plants will show higher 12C/13C ratios in their bones. The isotope ratio is expressed: d 13C% = [(13C/12C sample / 13C/12C standard) - 1] x 1000. The standard for carbon is a marine carbonate fossil from the Peedee formation in South Carolina (PDB, PeeDee Belemnite).
Existing data indicate that subsistence based entirely on C3 plants provides an isotopic ratio of between -18% and -22%, whereas one based on C4 plants has a value of between -4% and -7%.
Like carbon, nitrogen occurs in two isotopic states: 14N and 15N. The heavier (15N) concentrates as it travels up through the food chain. Marine plants have higher concentrations of this isotope than land plants. As a consequence, people feeding on marine food sources are expected to have higher 15N/14N ratios than those subsisting on terrestrial food sources.
Isotope analysis is made from a tiny sample of bone tissue (1 g), from which collagen is extracted to measure the isotope values with a mass spectrometer.
Trace element analysis Trace elements are chemical elements that are found in small amounts in the body. The main source of trace elements is diet, with the bone being the main depositary in the organism. The variations that occur in the concentration of these throughout the trophic chain are the reason for their use in the study of the diet.
The strontium (Sr) enters the land-based food chain from the soil and ground water via roots of plants. The amount of strontium gradually decreases moving up the food chain (from herbivores to omnivores and carnivores). The more appropriate method of analysis consists in comparing Sr levels in the different elements of the trophic chain represented in a site. This Sr measurement indicates the relative proportion of vegetables in the human diet. Elements such as barium (Ba) and magnesium (Mg) have a similar behavior to Sr. Other elements, such as copper (Cu) and zinc (Zn), reflect meat intake, being deposited mainly in the muscular tissue and, to a lesser extent, in the bone. Accordingly, carnivores will feature the highest levels of these elements, with reduced amounts in omnivores and herbivores.
Assessing the results for trace elements is complex due to the possible existence of diagenetic processes that, following burial, may alter the biogenic proportions of the trace elements present in the bone. This makes it essential, therefore, to perform examinations of the taphonomy of soil-buried bones.
There are numerous applications for chemical analyses in bioarchaeology, with major ones involving studies dealing with the introduction of C4 plants such as maize, those referring to intra - and inter-populational social differences, the assessment of marine versus terrestrial food webs, and the change of diet in the Mesolithic-Neolithic transition.
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