Isotope analysis in archaeology

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Isotope analysis has many applications in archaeology, from dating sites and artefacts, determination of past diets and migration patterns and for environmental reconstruction. [1]

Contents

Information is determined by assessing the ratio of different isotopes of a particular element in a sample. The most widely studied and used isotopes in archaeology are carbon, oxygen, nitrogen, strontium and calcium. [2]

An isotope is an atom of an element with an abnormal number of neutrons, changing their atomic mass. [2] Isotopes can be subdivided into stable and unstable or radioactive. Unstable isotopes decay at a predictable rate over time. [2] The first stable isotope was discovered in 1913, and most were identified by the 1930s. [2] Archaeology was relatively slow to adopt the study of isotopes. Whereas chemistry, biology and physics, saw a rapid uptake in applications of isotope analysis in the 1950s and 1960s, following the commercialisation of the mass spectrometer. [2] It wasn't until the 1970s, with the publication of works by Vogel and Van Der Merwe (1977) and DeNiro and Epstein (1978; 1981) that isotopic analysis became a mainstay of archaeological study. [3] [4] [5]

Isotopes

Carbon

The decay rate of the radioactive C isotope, used in radiocarbon dating artefacts (public domain image). Radioactive decay of Carbon-14.png
The decay rate of the radioactive C isotope, used in radiocarbon dating artefacts (public domain image).

Carbon is present in all biological material including skeletal remains, charcoal and food residues and plays an integral role in the dating of materials, through radiocarbon dating. [6] The ratio of different carbon isotopes naturally fluctuates over time, and, by analysing the composition of carbon dioxide (CO2) in ancient air bubbles trapped in ice cores, a chronological record of these fluctuations can be constructed. [7] Primary producers (such as grasses) absorb and sequester CO2 during photosynthesis, these plants are then eaten by consumers (such as cows, and later humans) which inherit this same CO2 signature. Therefore, by matching the carbon isotope ratios from a sample to ratios from the ice core record, the sample can be assigned to a broad period. [6] [1] After death, an organism no longer absorbs CO2, 14C's instability causes its concentration to decrease over time [8] The predictable rate at which this occurs is known as an element's decay rate.

Oxygen and nitrogen

Oxygen and nitrogen occur in the form of different isotopes which vary in their proportions geospatially and climatically. [9] [10] Oxygen is absorbed into the body in the form of H2O and is used in the growth of tissues. As with carbon, oxygen isotopic ratio variances can be attributed to specific locations and the proportion of O isotopes can therefore contribute to the reconstruction of past climates, understanding of diets and water consumption, seasonality, mobility patterns, life history and elements of culture. [9] [10]

Strontium

Strontium is naturally deposited in hydroxyapatite, the mineral component of bones and teeth, following its consumption in food and water. [11] Each locale has a unique Sr isotope ratio and, therefore, the ratio found in a bone or enamel sample can be cross referenced against a record of environmental Sr ratios and assigned to a region. [11] Dental enamel forms in childhood, therefore, Sr extracted from dental enamel reflects the environment in which an individual lived during infancy and childhood. Bone, however, is constantly being renewed and can therefore be used to infer the adult diet and location of the individual. [11] As such, if the Sr ratios are the analogous in the bones and teeth, it can be inferred that an individual remained in the same general region throughout their life. [2] If the ratios differ, the individual's birthplace and death place can be mapped, allowing inference of their movements. [1] This has been applied to determine the functionality and significance of Stonehenge, finding that both the visitors and cattle used in feasting travelled great distances, with Sr ratios attributed to both Scotland and Wales. [12] [13]

Calcium

Alongside strontium, dietary calcium is deposited in bones teeth, however Ca is more readily deposited than Sr in humans and animals who consume primarily or exclusively plants. [1] Therefore, the greater the Ca:Sr ratio in sample, the more herbivorous the animal was likely to be.

Methodology

Isolation

Before the isotopes can be separated and a ratio can be determined, the desired component of the tissue must be isolated. Such components include collagen, carbonate and apatite. [1] Each component requires different means of isolation, and methods must be further specialised to account for the varied levels of decay and contamination which may occur as a result of taphonomy. [2]

In the case of collagen, there are three main modes of isolation:

The latter is most effective in the instance of very poorly preserved bone, although it also faces an increased risk of contamination by other organic matter. [2] Consequently, the supposedly isolated sample should be analysed and only tested if the readings fall within an acceptable range; most mass spectrometers now include a gas analyser as well as a combustion chamber to streamline this process. [2] [20]

Mass spectrometry

Mass spectrometer, used to separate and measure elemental ions (public domain image). Mass spectrometer schematics.png
Mass spectrometer, used to separate and measure elemental ions (public domain image).

Mass spectrometry is used to separate and measure distinct isotopes present in a sample. Archaeologists typically employ isotope ratio mass spectrometers or IRMSs, consisting of an inlet system, ion source, mass analyser and multiple ion detectors. [2]

The sample is usually introduced into the mass spectrometer as a gas, with oxygen and carbon being introduced as carbon dioxide. [2]

Strontium is too unstable to be easily handled in gas form, instead, it is evaporated and ionised in a vacuum. This use of a solid source is referred to as thermal ionisation mass spectrometry or TIMS. [2] More recently, strontium isotopes have been at the centre of discussion and investigation into the use of laser ablation inductively coupled mass spectrometry (ICP-MS), which is also of interest due to its less invasive nature. [2]

Electron bombardment ionises the gas, allowing the molecules to be focused into a beam which is then split by mass into smaller beams - forming a "mass spectrum". [2] The relative intensities of the different beams is then measured in the ion collector and relayed as isotope ratios. [2]

Application and examples

Paranthropus dietary reconstruction

Plants can be characterised by the ratio of carbon isotopes they sequester, due to alterations in the evolution of photosynthetic biochemical pathways [21] . So-called C3 plants fix CO2 into a 3-carbon molecule and have a greater proportion of 12C, whereas C4 plants fix it into a 4-carbon molecule, and have a carbon isotope signature with higher 13C. [1] This signature translates across trophic levels and can be used to determine the diets of people and animals. Isotopic analysis has been used to illuminate the diets of the different species of the Paranthropus genus. It was determined that P. boisei had a reduced ratio of C3:C4, meaning they likely consumed a greater proportion of grasses and sedges than trees, shrubs and temperature grasses. [22] [1] P. aethiopicus showed a similar trend, [23] whereas P. robustus was a generalist, with a broader dietary niche. [24] Furthermore, carbon isotope analysis shows that around 2.37 million years ago, hominins displayed a widespread shift to favour C4 plants. [24]

"Ötzi the Iceman" and reconstructing Neolithic lifeways

Ötzi is a Neolithic man who, in 1991, was found in an Alpine glacier between Austria and Italy. [25] [26] Ötzi is exceptionally well preserved since his body was dehydrated and encapsulated in glacial ice. [27] Radiocarbon dating gave an age of approximately 5,200 years old. TIMS, ICP-MS and gas mass spectrometry have all been applied to the strontium, lead, and oxygen isotopes [28] in Ötzi's bones and teeth. His teeth indicated a likely birth and early childhood near to where the Eisack and Rienz rivers confluence. [27] In his adulthood, however, Ötzi's bones suggest that he moved to the lower Vinschgau and Etsch valley. [27] More recent isotopic data, gathered from his gut contents, provides yet another timescale and hint that Ötzi's movement could be attributable to seasonal migration. [29]

White Sands trackway and peopling of North America

The earliest compelling evidence for human habitation of the Americas comes from the Clovis complex, between 11,050 and 10,800 14C yr B.P. [30] However, a series of human tracks were identified at White Sands National Park, New Mexico, which have been dated contentiously dated to between 23,000 and 21,000 years ago - during the Last Glacial Maximum. [31] [32] Alongside anatomically modern humans, the trackway shows impressions created by a Columbian mammoth and a giant ground sloth. [31] The upper biostratigraphic limit for when the impressions were made could therefore be determined by consideration of the extinction dates of mammoths and ground sloths. [31] More precise dates were able to be gained via radiocarbon dating of ditch grass (ruppia cirrhosa) embedded in the prints. [32] These seeds produced a date of 23,000-21,000 years ago. [32]

However, 14C dates are not infallible, and this remains a topic of debate. A recent counterproposal posits that the trackways were, in fact, created by the Clovis culture and the pre-existing proposed dates of first habitation should not be moved. [33] False dates may have been produced as older strata containing the seeds could have been eroded and displaced onto the damp clay, before being impressed in by footsteps. [33] Alternatively, aquatic plants like ditch grass reflect the 14C levels in their environment when living, if 14C was deficient in the habitat, this could imply a false date. [33]

Related Research Articles

<span class="mw-page-title-main">Radiocarbon dating</span> Method of determining the age of objects

Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.

Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of Earth itself, and can also be used to date a wide range of natural and man-made materials.

<span class="mw-page-title-main">Carbon-14</span> Isotope of carbon

Carbon-14, C-14, 14C or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic matter is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, California. Its existence had been suggested by Franz Kurie in 1934.

<span class="mw-page-title-main">Isotope analysis</span> Analytical technique used to study isotopes

Isotope analysis is the identification of isotopic signature, abundance of certain stable isotopes of chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food web, to reconstruct past environmental and climatic conditions, to investigate human and animal diets, for food authentification, and a variety of other physical, geological, palaeontological and chemical processes. Stable isotope ratios are measured using mass spectrometry, which separates the different isotopes of an element on the basis of their mass-to-charge ratio.

<span class="mw-page-title-main">Magnesite</span> Type of mineral

Magnesite is a mineral with the chemical formula MgCO
3. Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts.

<span class="mw-page-title-main">Accelerator mass spectrometry</span> Accelerator that accelerates ions to high speeds before analysis

Accelerator mass spectrometry (AMS) is a form of mass spectrometry that accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the different methods of mass spectrometry is its ability to separate a rare isotope from an abundant neighboring mass. The method suppresses molecular isobars completely and in many cases can also separate atomic isobars. This makes possible the detection of naturally occurring, long-lived radio-isotopes such as 10Be, 36Cl, 26Al and 14C.

Isotope geochemistry is an aspect of geology based upon the study of natural variations in the relative abundances of isotopes of various elements. Variations in isotopic abundance are measured by isotope-ratio mass spectrometry, and can reveal information about the ages and origins of rock, air or water bodies, or processes of mixing between them.

Bioarchaeology in Europe describes the study of biological remains from archaeological sites. In the United States it is the scientific study of human remains from archaeological sites.

<span class="mw-page-title-main">Hydroxyapatite</span> Naturally occurring mineral form of calcium apatite

Hydroxyapatite is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), often written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride or chloride, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.

An isotopic signature is a ratio of non-radiogenic 'stable isotopes', stable radiogenic isotopes, or unstable radioactive isotopes of particular elements in an investigated material. The ratios of isotopes in a sample material are measured by isotope-ratio mass spectrometry against an isotopic reference material. This process is called isotope analysis.

<span class="mw-page-title-main">Oxygen isotope ratio cycle</span> Cyclical variations in the ratio of the abundance of oxygen

Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to ancient ocean temperature which in turn reflects ancient climate. Cycles in the ratio mirror climate changes in the geological history of Earth.

The environmental isotopes are a subset of isotopes, both stable and radioactive, which are the object of isotope geochemistry. They are primarily used as tracers to see how things move around within the ocean-atmosphere system, within terrestrial biomes, within the Earth's surface, and between these broad domains.

<span class="mw-page-title-main">Cholestane</span> Chemical compound

Cholestane is a saturated tetracyclic triterpene. This 27-carbon biomarker is produced by diagenesis of cholesterol and is one of the most abundant biomarkers in the rock record. Presence of cholestane, its derivatives and related chemical compounds in environmental samples is commonly interpreted as an indicator of animal life and/or traces of O2, as animals are known for exclusively producing cholesterol, and thus has been used to draw evolutionary relationships between ancient organisms of unknown phylogenetic origin and modern metazoan taxa. Cholesterol is made in low abundance by other organisms (e.g., rhodophytes, land plants), but because these other organisms produce a variety of sterols it cannot be used as a conclusive indicator of any one taxon. It is often found in analysis of organic compounds in petroleum.

γ-Carotene (gamma-carotene) is a carotenoid, and is a biosynthetic intermediate for cyclized carotenoid synthesis in plants. It is formed from cyclization of lycopene by lycopene cyclase epsilon. Along with several other carotenoids, γ-carotene is a vitamer of vitamin A in herbivores and omnivores. Carotenoids with a cyclized, beta-ionone ring can be converted to vitamin A, also known as retinol, by the enzyme beta-carotene 15,15'-dioxygenase; however, the bioconversion of γ-carotene to retinol has not been well-characterized. γ-Carotene has tentatively been identified as a biomarker for green and purple sulfur bacteria in a sample from the 1.640 ± 0.003-Gyr-old Barney Creek Formation in Northern Australia which comprises marine sediments. Tentative discovery of γ-carotene in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is significant for reconstructing past oceanic conditions, but so far γ-carotene has only been potentially identified in the one measured sample.

The Suess effect is a change in the ratio of the atmospheric concentrations of heavy isotopes of carbon (13C and 14C) by the admixture of large amounts of fossil-fuel derived CO2, which contains no 14CO2 and is depleted in 13CO2 relative to CO2 in the atmosphere and carbon in the upper ocean and the terrestrial biosphere. It was discovered by and is named for the Austrian chemist Hans Suess, who noted the influence of this effect on the accuracy of radiocarbon dating. More recently, the Suess effect has been used in studies of climate change. The term originally referred only to dilution of atmospheric 14CO2 relative to 12CO2. The concept was later extended to dilution of 13CO2 and to other reservoirs of carbon such as the oceans and soils, again relative to 12C.

<span class="mw-page-title-main">Bomb pulse</span> Sudden increase of carbon-14 in the Earths atmosphere due to nuclear bomb tests

The bomb pulse is the sudden increase of carbon-14 (14C) in the Earth's atmosphere due to the hundreds of above-ground nuclear bombs tests that started in 1945 and intensified after 1950 until 1963, when the Limited Test Ban Treaty was signed by the United States, the Soviet Union and the United Kingdom. These hundreds of blasts were followed by a doubling of the relative concentration of 14C in the atmosphere.

<span class="mw-page-title-main">Medieval bioarchaeology</span> Study of human remains recovered from medieval archaeological sites

Medieval bioarchaeology is the study of human remains recovered from medieval archaeological sites. Bioarchaeology aims to understand populations through the analysis of human skeletal remains and this application of bioarchaeology specifically aims to understand medieval populations. There is an interest in the Medieval Period when it comes to bioarchaeology, because of how differently people lived back then as opposed to now, in regards to not only their everyday life, but during times of war and famine as well. The biology and behavior of those that lived in the Medieval Period can be analyzed by understanding their health and lifestyle choices.

Robert Norman Clayton was a Canadian-American chemist and academic. He was the Enrico Fermi Distinguished Service Professor Emeritus of Chemistry at the University of Chicago. Clayton studied cosmochemistry and held a joint appointment in the university's geophysical sciences department. He was a member of the National Academy of Sciences and was named a fellow of several academic societies, including the Royal Society.

<span class="mw-page-title-main">Fractionation of carbon isotopes in oxygenic photosynthesis</span>

Photosynthesis converts carbon dioxide to carbohydrates via several metabolic pathways that provide energy to an organism and preferentially react with certain stable isotopes of carbon. The selective enrichment of one stable isotope over another creates distinct isotopic fractionations that can be measured and correlated among oxygenic phototrophs. The degree of carbon isotope fractionation is influenced by several factors, including the metabolism, anatomy, growth rate, and environmental conditions of the organism. Understanding these variations in carbon fractionation across species is useful for biogeochemical studies, including the reconstruction of paleoecology, plant evolution, and the characterization of food chains.

<span class="mw-page-title-main">Silicon isotope biogeochemistry</span> The study of environmental processes using the relative abundance of Si isotopes

Silicon isotope biogeochemistry is the study of environmental processes using the relative abundance of Si isotopes. As the relative abundance of Si stable isotopes varies among different natural materials, the differences in abundance can be used to trace the source of Si, and to study biological, geological, and chemical processes. The study of stable isotope biogeochemistry of Si aims to quantify the different Si fluxes in the global biogeochemical silicon cycle, to understand the role of biogenic silica within the global Si cycle, and to investigate the applications and limitations of the sedimentary Si record as an environmental and palaeoceanographic proxy.

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