User:Jmariemueller/Bioarchaeology

Overview
Stable isotope biogeochemistry is a powerful tool that utilizes variations in isotopic signatures and relates them to biogeochemical processes. The science is based on the preferential fractionation of lighter or heavier isotopes, which results in enriched and depleted isotopic signatures compared to a standard value. Essential elements for life such as carbon, nitrogen, oxygen, hydrogen, and sulfur are the primary stable isotope systems used to interrogate archeological discoveries. Isotopic signatures from multiple systems are typically used in tandem to create a comprehensive understanding of the analyzed material. These systems are most commonly used to trace the geographic origin of archaeological remains and investigate the paleodiets, mobility, and cultural practices of ancient humans. Over the past few decades, the use of isotope geochemistry in the context of archaeology has dramatically increased.

Carbon
Stable isotope analysis of carbon in human bone collagen allows bioarchaeologists to carry out dietary reconstruction and to make nutritional inferences. These chemical signatures reflect long-term dietary patterns, rather than a single meal or feast. Isotope ratios in food, especially plant food, are directly and predictably reflected in bone chemistry, allowing researchers to partially reconstruct recent diet using stable isotopes as tracers. Stable isotope analysis monitors the ratio of carbon 13 to carbon 12 (13C/12C), which is expressed as parts per mil (per thousand) using delta notation (δ13C). The 13C and 12C ratio is either depleted (more negative) or enriched (more positive) relative to an international standard. The original standard used in carbon stable isotope analysis is Pee Dee Belemnite (PDB), though this material has since been exhausted and replaced. 12C and 13C occur in a ratio of approximately 98.9 to 1.1. The ratio of carbon isotopes in consumers varies according to the types of plants digested with different photosynthesis pathways. The three photosynthesis pathways are C3 carbon fixation, C4 carbon fixation and Crassulacean acid metabolism. C4 plants are mainly grasses from tropical and subtropical regions, and are adapted to higher levels of radiation than C3 plants. Corn, millet and sugar cane are some well-known C4 domesticates, while all trees and shrubs use the C3 pathway. C4 carbon fixation is more efficient when temperatures are high and atmospheric CO2 concentrations are low. C3 plants are more common and numerous than C4 plants as C3 carbon fixation is more efficient in a wider range of temperatures and atmospheric CO2 concentrations.

The different photosynthesis pathways used by C3 and C4 plants cause them to discriminate differently towards 13C leading to distinctly different ranges of δ13C. C4 plants range between -9 and -16‰, and C3 plants range between -22 to -34‰. The isotopic signature of consumer collagen is close the the δ13C of dietary plants, while apatite, a mineral component of bones and teeth, has an ~14‰ offset from dietary plants due fractionation associated with mineral formation. Stable carbon isotopes have been used as tracers of C4 plants in paleodiets. For example, the rapid and dramatic increase in 13C in human collagen after the adoption of maize agriculture in North America documents the transition from a C3 to a C4 (native plants to corn) diet by 1300 CE.

Skeletons excavated from the Coburn Street Burial Ground (1750 to 1827 CE) in Cape Town, South Africa, were analyzed using stable isotope data in order to determine geographical histories and life histories of the interred. The people buried in this cemetery were assumed to be slaves and members of the underclass based on the informal nature of the cemetery; biomechanical stress analysis and stable isotope analysis, combined with other archaeological data, seem to support this supposition.

Based on stable isotope levels, eight Cobern Street Burial Ground individuals consumed a diet based on C4 (tropical) plants in childhood, then consumed more C3 plants, which were more common at the Cape later in their lives. Six of these individuals had dental modifications similar to those carried out by peoples inhabiting tropical areas known to be targeted by slavers who brought enslaved individuals from other parts of Africa to the colony. Based on this evidence, it was argued that these individuals represent enslaved persons from areas of Africa where C4 plants are consumed and who were brought to the Cape as laborers. These individuals were not assigned to a specific ethnicity, but it is pointed out that similar dental modifications are carried out by the Makua, Yao, and Marav peoples. Four individuals were buried with no grave goods, in accordance with Muslim tradition, facing Signal Hill, which is a point of significance for local Muslims. Their isotopic signatures indicate that they grew up in a temperate environment consuming mostly C3 plants, but some C4 plants. Many of the isotopic signatures of interred individuals indicate that they Cox et al. argue that these individuals were from the Indian Ocean area. They also suggest that these individuals were Muslims. It was argued that stable isotopic analysis of burials, combined with historical and archaeological data can be an effective way in of investigating the worldwide migrations forced by the African Slave Trade, as well as the emergence of the underclass and working class in the colonial Old World.

Nitrogen
The nitrogen stable isotope system is based on the relative enrichment or depletion of 15N in comparison to 14N in an analyzed material (δ15N). Carbon and nitrogen stable isotope analyses are complimentary in paleodiet studies. Nitrogen isotopes in bone collagen are ultimately derived from dietary protein, while carbon can be contributed by protein, carbohydrate, or fat in the diet. δ13C values help distinguish between dietary protein and plant sources while systematic increases in δ15N values as you move up in trophic level helps determine the position of protein sources in the food web. 15N increases about 3-4% with each trophic step upward. It has also been suggested that the relative difference between human δ15N values and animal protein values scales with the proportion of that animal protein in the consumer's diet, though this interpretation has been questioned due to contradictory views on the impact of nitrogen intake through protein consumption and nitrogen loss through waste release on 15N enrichment in the body.

When interpreting δ15N values of human remains, variations in nitrogen values within the same trophic level are also considered. Nitrogen variations in plants, for example, can be caused by plant-specific reliance on nitrogen gas which causes the plant to mirror atmospheric nitrogen isotopic values. Enriched or higher δ15N values can be achieved in plants that grew in soil fertilized by animal waste. Nitrogen isotopes have been used to estimate the relative contributions of legumes verses nonlegumes, as well as terrestrial versus marine resources to the diet. While other plants have δ15N values that range from 2 to 6‰, legumes have lower 14N/15N ratios (close to 0‰, i.e. atmospheric N2) because they can fix molecular nitrogen, rather than having to rely on nitrates and nitrites in the soil. Therefore, one potential explanation for lower δ15N values in human remains is an increased consumption of legumes or animals that eat them. 15N values increase with meat consumption, and decrease with legume consumption. The 14N/15N ratio could be used to gauge the contribution of meat and legumes to the diet.

Oxygen
The oxygen stable isotope system is based on the 18O/16O (δ18O) in a given material, which is either enriched or depleted relative to a standard. The field typically normalizes to both Vienna Standard Mean Ocean Water (VSMOW) and Standard Light Antarctic Precipitation (SLAP). This system is famous for its use in paleoclimatic studies but it also a prominent source of information in bioarchaeology.

Variations in δ18O values in skeletal remains are directly related to the isotopic composition of the consumer's body water. The isotopic composition of mammalian body water is primarily controlled by consumed water. δ18O values of freshwater drinking sources vary due to mass fractionations related to mechanisms of the global water cycle. Evaporated water vapor will be more enriched in 16O (isotopically lighter; more negative delta value) compared to the body of water left behind which is now depleted in 16O (isotopically heavier; more positive delta value). An accepted first-order approximation for the isotopic composition of animal drinking water is local precipitation, though this is complicated to varying degrees by confounding water sources like natural springs or lakes. The baseline δ18O used in archaeological studies is modified depending on the relevant environmental and historical context of surrounding water sources.

δ18O values of bioapatite in human skeletal remains are assumed to have formed in equilibrium with body water, thus providing a species-specific relationship to oxygen isotopic composition of body water. The same cannot be said for human bone collage, as δ18O values in collagen seem to be impacted by drinking water, food water, and a combination of metabolic and physiological processes. While δ18O values from bone minerals are essentially an averaged isotopic signature throughout the entire life of the individual, dental enamel reflects isotopic signatures specific to early life since enamel is not biologically remodeled.

While carbon and nitrogen are used primarily to investigate the diets of ancient humans, oxygen isotopes offer insight into body water at different stages in a consumer's life. δ18O values are used to understand drinking behaviors, animal husbandry , and track mobility. 97 burials from the ancient Maya citadel of Tikal were studied using oxygen isotopes. Results from tooth enamel identified statistically different individuals, interpreted to be individuals from Maya lowlands, Guatemala, and potentially Mexico. Historical context combined with the isotopic data from the burials is used to argue that migrant individuals were a part of lower and higher social classes within Tikal. It is further suggested that the female migrants who arrived in Tikal during Early Classic period could have been the brides of Maya elite.

Sulfur
The sulfur stable isotope system is based on small, mass-dependent fractionations of sulfur isotopes in an analyzed material. These fractionations are then reported relative to Canyon Diablo Troilite (V-CDT), the agreed upon standard for the field. The ratio of the most abundant sulfur isotope, 32S, compared to rarer isotopes such as, 33S, 34S, and 36S, is used to characterize biological signatures and geological reservoirs. The fractionation of 34S (δ34S) is particularly useful since it is the most abundant of the rare sulfur isotopes, allowing the fractionations to be biogeochemically meaningful and analytically resolvable. This system is less commonly used on its own and typically acts as a secondary source of information that compliments isotopic values of carbon and nitrogen. In bioarchaeology, the sulfur system has been used to investigate consumer paleodiets and spatial behaviors through the analysis of hair and bone collagen. Dietary proteins incorporated into living organisms tend to determine the stable isotope values of their organic tissues. Methionine and cysteine are the two canonical sulfur-containing amino acids. Of the two, δ34S values of methionine are considered to better reflect isotopic compositions of dietary sulfur, since cysteine values are impacted by diet and internal cycling. While other stable isotope systems have significant trophic shifts, there is only a small shift (~0.5‰) observed between the δ34S values. Consumers yield isotopic signatures that reflect the sulfur reservoir(s) of the dietary protein source. These characteristic values are determined by the isotopic nature of sulfate in the environment. Animal proteins sourced from marine ecosystems tend to have δ34S values between +16 and +17‰,, terrestrial plants range from -7‰ to +8‰, and proteins from freshwater and terrestrial ecosystems are highly variable. The sulfate content of the modern ocean is very well-mixed with a δ34S of approximately +21‰, while riverine water is heavily influenced by the sulfur-bearing minerals in surrounding bedrock and terrestrial plants are influenced by the sulfur content of local soils. Estuarian ecosystems have increased complexity due to seawater and river inputs. The extreme range of δ34S values for freshwater ecosystems often interferes with terrestrial signals, making it difficult to use the sulfur system as the sole tool in paleodiet studies.

Various studies have analyzed the isotopic ratios of sulfur in mummified hair. Hair is a good candidate for sulfur studies as it typically contains at least 5% elemental sulfur. One study incorporated sulfur isotope ratios into their paleodietary investigation of four mummified child victims of Incan sacrificial practices. δ34S values helped them determine that the children had not been eating marine protein before their death. Historical insight coupled with consistent sulfur signatures for three of the children suggests that they were living in the same location 6 months prior to the sacrificial ceremony. Studies have also measured δ34S values of bone collagen, though the interpretation of these values was not reliable until quality criteria for the analysis was published in 2009. Though bone collagen is abundant in skeletal remains, less than 1% of the tissue is made of sulfur, making it imperative that these studies carefully assess the meaning of bone collagen δ34S values.