Stable Isotope Analysis The Significance of Stable Isotopes
Stable Isotope Analysis
The Significance of Stable Isotopes • Isotopes are atoms of an element that vary by mass, meaning greater or fewer neutrons. • The isotopes of interest to archaeologists are those that are lighter and which figure in organic compounds, principally those of strontium, carbon, nitrogen, and sulfur.
Isotopic Chemistry and the Human Body: Strontium • Soils of different regions will exhibit differing levels of the isotope strontium 87 ( 87 Sr) reflecting the ratio of calcium and potassium in the underlying bedrock. 87 Sr is the decay product of rubidium 87 ( 87 rb), an unstable isotope, and rubidium is found in association with potassium.
• Humans take in strontium as they eat plants grown in the soil, and it becomes fixed into their bones, teeth, and hair as strontium ions are similar to those of calcium. • The exchange of strontium in bones with diet is ongoing, so bone isotopic ratios will tell you where a dead person most recently lived. • Strontium levels are locked into teeth when they form in childhood, so they will inform as to where a person was as a child.
Plant Isotopic Categories • Plants vary in their isotopic ratios of carbon and nitrogen depending on the way they obtain (“fix”) these elements through photosynthesis. • C 3 Plants: Photosynthesis results in 3 -carbon molecule products. These are plants of temperate climates and regions with plentiful groundwater. Examples: wheat, rice, barley, oats, & legumes.
• C 4 Plants: Photosynthesis results in 4 -carbon molecule products. These are plants of the tropics. Examples: corn and sugarcane. • There also CAM (Crassulacean acid metabolism) plants. These are plants in arid environments that shut their stomata (pores) during the day to avoid water loss. Examples: prickly pear, agave, mesquite and pineapples.
Dietary Signatures • A diet high in C 3 plants leaves behind low levels of 13 C. • A diet high in C 4 plants and most CAM plants leaves high levels of 13 C in bone collagen. • A diet high in marine foods products results in high levels of 13 C, 15 N, and 34 S.
Coxcatlán Cave, Tehuacán Valley
Richard “Scotty” Mac. Neish Tehuacán Valley, Puebla
The Tehuacán Archaeological Botanical Project 1961 -4 • Scotty Mac. Neish had an abiding interest in examination the origins of the domestication of various plants. • He reasoned that surviving evidence for the earliest domesticated plants in the New World would come from dry caves. • He therefore excavated a number of caves in the high and arid Tehuacán Valley in Mexico.
• The cave deposits were found to be thick and span 10, 000 years. • Nomadic bands occupied the cave during the rainy season. • The remains of three cultigens were found there: maize, beans, and bottle gourd. • The maize was thought to date to 5, 000 BC, the beginning of the Coxcatlán Phase. The date has since been revised to 3, 540 BC by direct AMS dating of cobs (Smith 2005).
A re-evaluation of the isotopic and archaeological reconstructions of diet in the Tehuacan Valley Paul Farnsworth, James Brady, Michael De. Niro, and Richard Mac. Neish American Antiquity 1985. Mac. Neish in 1967 estimated the relative contribution of various types of plants to the diet by looking at relative quantities of remains in the strata he examined. This study looked at the skeletal remains and coprolites of people living in the Tehuacán Valley.
Conclusions “The isotopic data suggest that the ancient Tehuacanos switched from a hunting and gathering economy to one dependent upon the heavy exploitation of tropical grasses (such as maize or Setaria) or CAM plants (such as cacti or maguey) sometime between the El Riego and Coxcatlan phases. By at least 4500 BC the Tehuacano diet consisted of at about 90% C 4 or CAM plants or of meat from animals whose diet had a similar make-up. ”
The St. Brice’s Day Massacre • King Æthlred “Unræd” (Ill-advised) of Wessex ordered that all Danes in England be killed by the Anglo-Saxon population. This mass killing was timed to happen on Nov. 13 th 1002 AD. • The Danish men of Oxford were said to have been killed after they had taken refuge in St. Frideswides’ Church. The church was set of fire to drive them out.
In 2008 the remains of 38 males were found to have been dumped in the ditch of a Neolithic henge by archaeologists excavating on the campus of St. John’s College, Oxford University. They had been killed by being stabbed multiple times. Some skeletons show charring.
Radiocarbon Dates Averaging the dates yielded a span of 893 -978 AD, too early for the massacre
Stable Isotope Analysis • A team from Oxford undertook stable isotope analysis (δ 1 3 C and δ 1 5 N) on the collagen from the bones of some of the individuals, and also measured δ 1 3 C and δ 1 8 O in the dental enamel carbonate, as well as 8 7 Sr/ 8 6 Sr in the dental enamel. Bone collagen δ 1 3 C and δ 1 5 N • All the data were closely clustered, and the values (average δ 1 3 C =-19. 63‰, s. d. = 0. 32‰; average δ 1 5 N = 12. 01‰, s. d. = 0. 74‰) are typical of a diet based primarily on a C 3 -photosynthetic system (which encompasses most terrestrial plants apart from C 4 tropical grasses). The δ 1 5 N values are, however, slightly towards the higher end of what is typical of a purely terrestrial ecosystem.
Dental Enamel • The levels of strontium isotopes in dental enamel of two individuals were distinct from a southern UK population. The other samples were only slightly different, reflecting the fact that the strontium levels found in the soil of Denmark and northern Germany are not very dissimilar to those from S. E. England.
Conclusions • The team brought up the possibility that the older than expected radiocarbon date range could be from reservoir effect from the potential seafood diet. • The team thought it plausible that the massacre victims were Vikings, but they offered the hypothesis that they were members of a raiding party executed before the massacre occurred, based upon a comparison with another mass grave at Weymouth, Dorset.
A subsequent bone pit discovered in 2009 in Weymouth, Dorset.
Stable Isotope Comparisons • The remains dated to 980 -1030 AD. Weymoth’s and St. John’s Dental Enamel Southern England values
Weymouth’s and St. John’s Dentine and Bone Collagen
Conclusions • The Weymouth Ridgeway skeletons represent a group of young men from a variety of places in the Scandinavian countries, since the δ 18 O p is outside the expected range for the UK, and consistent with a colder climate, and the 87 Sr/ 86 Sr values are not local to the south of England. They conclude that the burial represents the result of the execution of a raiding party. Given the isotopic similarities between the two datasets, it is possible that the mass burial in St John’s may also represent a mixed group of Scandinavians.
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