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Isotope Geochemistry in Archaeology

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Isotope Geochemistry in Archaeology I 4106 Isotope Geochemistry in Archaeology SIMANTJUTAK, H.T. 2011. Available at: http://ogan-com- specific problems depending on the mechanism munity.blogspot.fr/2012/04/goa-harimau-part-1-gua- by which one nuclide becomes naturally enriched harimau.html or depleted. The basis of the application of some SOUZA, C. & W.G. SOLHEIM II. 1976. New area of rock paintings in Irian Jaya, Indonesian New Guinea, stable isotopes is that the variation in their atomic in K.K. Chakravarty (ed.) Rock art of India: 182-95. mass leads them to behave differently during New Delhi: Arnold-Heinemann. processes such as evaporation, precipitation, SUKKHAM,A.,P.S.C.TACON & N.H. TAN. 2011. Rock art of Phrayanaga (Viking) Cave, southern Thailand: freezing, photosynthesis, and incorporation into the illustration of ancient vessels. Available at: the body. Another approach is based on taking http://www.themua.org/collections/items/show/1214. advantage of the varying abundance of some stable isotopes within different geological units to allow proveniencing of various materials. This can only be achieved if the isotope ratios remain Isotope Geochemistry in unchanged after incorporation into the sample. Archaeology Unstable isotopes are most commonly used as a geochronological tool for establishing the Ian Moffat age of materials such as organics, calcium Department of Archaeology, Flinders University, carbonates, and igneous rocks. Unstable nuclides Adelaide, SA, Australia that have either too many or too few neutrons Research School of Earth Sciences, The spontaneously transform by beta decay, alpha Australian National University, Acton, ACT, decay, or spontaneous nuclear fission. The Australia decay speed can be calibrated to time and is usually expressed as a “half-life.” On the basis of a known decay rate and original abundance Introduction ratio, the comparison of the relative abundance of a stable nuclide to an unstable nuclide can Isotope geochemistry is an important scientific provide an age estimation of the material studied. technique that has made a significant contribu- In the case of radiocarbon dating, the abundance tion to archaeological research. Isotope tech- of 14C in the sample is compared to modern levels niques measure the relative abundance of a of 14C. number of nuclides of the same (or derivative) Isotope analysis can be performed directly on element as a means of investigating a variety of archaeological materials or on geological mate- natural processes. Both stable (H, O, C, N, Ca, rials to provide a context for archaeological sites. Sr,Cu,Pb,S)andunstable(U,Th,K,Ar)isotope A particular advantage of using isotopic methods systems are analyzed as part of archaeological in archaeological investigations is that it provides investigations. quantifiable information that can be compared to Isotopes (often referred to as a nuclide in the the material culture record. singular) are variants of a particular element, which share the same number of protons but have varying numbers of neutrons. Isotopes are Definition referred to as stable or unstable, depending on whether they undergo radioactive decay. Some Isotope geochemistry is the measurement of nuclides are primordial, meaning they have the relative abundance of different species existed since the beginning of the universe, of the same element that have the same number while others are the product of the decay of of protons but varying numbers of neutrons. other elements. Isotopes of the same element A wide range of elements and materials can be generally share the same chemical behavior. analyzed to provide insights into age, diet, Stable isotopes can be applied to studying mobility, climate, and provenience with impor- a variety of processes with their applicability to tant implications for archaeology. Isotope Geochemistry in Archaeology 4107 I Key Issues/Current Debates/Future as it is considered more resistant to post-burial Directions/Examples diagenesis. The oxygen isotope analysis of marine A wide (and increasing) number of isotope sys- carbonates or ice cores allows the global ice tems can be applied to archaeological questions. volume to be estimated through time. This This entry introduces each isotope system that led the Quaternary to be divided into Marine has been applied in archaeological research and Isotope Stages, which provide a broad framework discusses the rationale for its use. for climate variation between glacial and inter- Hydrogen has two stable isotopes: 1H and 2H glacial periods. These changes are driven by (often known as deuterium). Isotope results for Milankovitch cyclicity, in which the eccentricity, this element are usually reported as dD that obliquity, and precession of the Earth’s orbit vary represents 2H/1H compared to the Vienna Stan- on a regular timescale. The Marine Isotope Stage dard Mean Ocean Water standard. This method, changes have had a significant effect on plant, which is suitable to the analysis of collagen, has animal, and hominin biogeography. The oxygen been used to determine paleoclimate (Leyden isotope composition of mammal tooth and bone et al. 2006) and diet (Reynard & Hedges 2008) reflects the d18O of ingested water and macronu- for archaeological samples. This technique works trients in food (Sponheimer & Lee-Thorp 1999). I on the basis that hydrogen isotope ratios of Ingested water has a composition similar to mete- precipitation vary according to climate and oric water, which is sensitive both to temperature geography. Hydrogen in plant tissues, particu- and precipitation. The water in shallow-rooted larly from shallow-rooted plants, is generally plant stems and roots reflects meteoric water, sourced from leaf water derived from growing although the value from leaves is enriched in season precipitation. There is no fractionation of 18O. The effect of mammalian diet and physiol- hydrogen during water uptake by terrestrial ogy on isotope composition is poorly understood, plants and the dD composition of herbivore tissue although the relative proportion of oxygen correlates to that of their diet, providing a time- derived from food or water may be significant. averaged composition over their lifetime. dD Overall, oxygen isotope analysis is a very values are fractionated with trophic level, commonly applied technique in archaeological with an increase of 30–50 ‰ from herbivores to research that has frequently been used as omnivores and 10–20 ‰ from omnivores to her- a proxy for climate or diet. bivores, and are not influenced by other parame- Carbon isotopes, reviewed by Lee-Thorp ters, such as those which influence d15N (Reynard (2002), can be applied to archaeological biogenic & Hedges 2008). As a result, this method can be minerals to determine the nature of vegetation used for sample provenience (using existing included in diet. The method works on the basis maps of dD), to examine changes in precipitation that 13C is strongly discriminated against regime or to measure trophic level. during photosynthesis; however, the degree to Oxygen has three stable isotopes: 16O, 17O, which this occurs varies between different and 18O. Results are commonly reported as d18O photosynthetic pathways. The C3, C4, and which reflects the ratio of 18Oto16Ocompared CAM photosynthetic pathways have distinctly to a known isotope standard such as Standard different levels of 13C, with C4 plants ranging Mean Ocean Water. Oxygen isotopes are frac- between 9 ‰ and À16 ‰ and C3 plants ranging tionated by a variety of processes with the between À22 ‰ and À34 ‰. C4 plants are heavier 18O being preferentially precipitated principally grasses which are predominant in or frozen and the lighter 16O being preferentially areas with higher levels of solar radiation. The evaporated. Oxygen isotope analysis of tooth carbon isotope composition of biominerals such enamel can be focused on the oxygen atoms as bone, teeth, and shell will reflect the plant from either the phosphate or carbonate portion, material in diet. Carbon isotopes have also been although phosphate is traditionally favored applied to determine the proveniencing of I 4108 Isotope Geochemistry in Archaeology archaeological marble. Carbon isotope composi- and level of dairy consumption. Calcium isotope tion may also reflect the degree of marine results are usually reported as d44/42Ca or d44/40C. resources within an individual’s diet, as there is There appears to be a significant fractionation a significant difference in the isotopic composi- between diet and bone but little fractionation tion of dissolved ocean bicarbonate and between diet and soft tissues. atmospheric carbon dioxide which is reflected in The application of strontium isotopes as plant values (Richards & Trinkaus 2009). a provenience tool in archaeology is summarized Radiocarbon dating, a derivative of carbon by Bentley (2006). Strontium has four naturally isotope analysis, measures the amount of 14C occurring isotopes including 84Sr, 86Sr, 87Sr, and (which has a half-life of 5730 years) in a sample 88Sr of which the relative abundance of these and compares this value to the level in modern isotopes is invariant and they are essentially systems. The dating technique, which is suitable stable on archaeological time scales. The analysis for organic materials, is ubiquitous in archaeol- of the strontium isotope composition of archaeo- ogy due to its relatively low cost and accurate logical materials can provide important informa- and precise results. More effective sample tion about the mobility of a range of mammals, pretreatment methodologies
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