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Radiocarbon Dating

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Radiocarbon Dating Zenobi Group Department of Chemistry and Applied Biosciences https://zenobi.ethz.ch/ 12/11/2020 Radiocarbon dating Dr. Stamatios Giannoukos 1 Why carbon isotope analysis? Carbon isotope analysis is relevant to multiple disciplines ….. which need affordable analytical tools Environmental monitoring Archaeology (dating, diet, migration) (isotopes from fossil fuels) Medicine (isotopes as metabolic tracers) Geology (dating, diet, metabolism) 2 Constraints • There is no single ideal method of dating that can produce accurate results for every kind of sample, in every context, for every chronology. • Each method of dating has constraints around its use and effectiveness. • Sometimes, it is not possible to use a particular dating method. Examples of some of the constraints of using a dating method include: the size of the sample the type of sample contamination of the sample the type of associated geo-archaeological materials the need to minimize destructive testing the chronologies involved 3 Isotope detectives 12C/13C metabolism; environment e.g. marine reservoir effect diet e.g. C3 or C4 plants 14N/15N trophic level and diet 16O/18O ancient temperature 87Sr/86Sr food origins migration 40Ar/39Ar geochronology 4 Isotope detectives – example 13C All life on Earth is based on carbon. Carbon has two stable isotopes. 12C, which makes up 98.93% of carbon and 13C, which makes up 1.07%. Bone structure The inside of bone contains a complex arrangement of tubes of mineralised collagen. Collagen is a fibrous protein with a long triple helix structure. This helix structure is protected by many layers of mineralised bone. This multiple layer protection means that collagen survives in the archaeological record. Carbon 13 levels across history Representation of how carbon 13 levels have varied over time. The y axis shows the ratio of 13C to 12C. A high ratio indicates a high level of fish consumption. 5 Isotope detectives – example - δ13C & δ15N http://www.schoolscience.co.uk/zooarchpage6 δ13C and δ15N detailed information about diet and so social status; also (indirectly) migration 6 Isotope detectives – example - 16O/18O The dominant oxygen isotope is 16O, meaning it has 8 protons and 8 neutrons, but 18O, an isotope with 10 neutrons, also exists. δ18O changes directly as a result of temperature fluctuations, so it provides a very good record of the climate. δ18O values that are high represent cold climates, while lower values indicate a warm climate this trend occurs because of the effects of precipitation and evaporation since it is lighter than 18O, 16O evaporates first, so in warm, tropical areas, the ocean is high in 18O. Additionally, as water vapor condenses to form rain, water droplets rich in 18O precipitate first because it is heavier than 16O. Thus, the cold, polar regions are depleted in 18O as it all precipitates out in the lower latitudes, but they are high in 16O. On the other hand, the tropics possess a large amount of 18O but have little 16O. 7 Isotope detectives – example 87Sr/86Sr Strontium has four stable isotopes. Two of these are 86Sr and 87Sr. The ratio of 87Sr to 86Sr in the environment depends on the age of the rocks beneath the soil. Traveling Cows in 2500 BC - Sr 87/86 ratios for cow tooth enamel found at Durrington Walls Strontium isotope evidence from about 15 cows’ teeth found at Durrington Walls, a Neolithic village near Stonehenge, show that they may have been raised in Scotland. 8 Carbon dating – how does it work ? • When something dies it stops 14 eating C • 14C (radiocarbon) decays without being replenished • 14C half life is 5730 years • Can be checked against known dates • Works on organic material e.g. wood, bone etc. •Not applicable to metals 14 N 9 The carbon cycle 10 Carbon dating -λt A=A0e λ= (ln2)/t1/2 Only 1 in a trillion C atoms is 14C 11 2 problems Extreme sensitivity (<<one in a trillion…) 10-14 to detect differences of 100 years Extreme resolution (background/sample isobars) 45K just for sample isobars (analysing CO2) Δ 12 Carbon dating Age is expressed in radiocarbon years C before present (BP) Percent modern Carbon (pmC) pmC =100*0.5 C /5730 Measured using a mass spectrometer e.g. an isotope ratio mass spectrometer, an Accelerator Mass Spectrometer (AMS) 13 Radiometric Dating - Liquid scintillation counting Liquid scintillation counting (LSC) is a standard laboratory method to quantify the radioactivity of low energy radioisotopes, mostly beta-emitting and alpha-emitting isotopes. The LSC detection method requires specific cocktails to absorb the energy into detectable light pulses. To efficiently transfer the emitted energy into light, LSC cocktails must consist of two basic components: •The aromatic, organic solvent •The scintillator(s) or fluors 14 Isotope Ratio Mass Spectrometer Measures the relative abundance of isotopes in a given sample – stable isotopes, radiogenic isotopes 15 Accelerator Mass Spectrometer Acceleration of the ions to extraordinarily high kinetic energies mass analysis • The more efficient way to measure radiocarbon content of a sample • The carbon 14 content is directly measured relative to the carbon 12 and carbon 13 present • The method does not count beta particles but the number of carbon atoms present in the sample and the proportion of the isotopes 16 Accelerator Mass Spectrometer Advantages − Small sample size - 20 - 500 mg, whereas conventional methods need at least 10 gr (wood and charcoal) - 100 gr (bones and sediments). − Sensitivity - carbon dating small particles like blood particles, a grain, or a seed have been made possible. − Analysis time – AMS takes less time to analyze samples for C-14 content compared to radiometric dating methods that can take up to 1 or 2 days. An AMS has a run time of a few hours per sample. − High precision and low backgrounds. Disadvantages − High cost – millions of dollars − Contamination - Rigorous pretreatment is needed to make sure contaminants have been eliminated and will not lead to substantial errors during the carbon dating process. 17 Isotopic Fractionation Isotopic fractionation of carbon isotopes 12C, 13C and 14C involves alterations in the equilibrium distribution (ratios) of isotopic species as a function of their atomic mass as a result of natural biochemical processes. Examples: • Photosynthesis favors one isotope over another. After photosynthesis, the isotope 13C is depleted by 1.8% in comparison to its natural ratios in the atmosphere. • The inorganic carbon dissolved in the oceans is generally 0.7% enriched in 13C relative to atmospheric carbon dioxide. 18 Isotopic Fractionation Fractionation also describes variations in the isotopic ratios of carbon brought about by non-natural causes samples fractionation in the laboratory through a variety of means; incomplete conversion of the sample from one stage to another or from one part of the laboratory to another. The transfer of gases in a vacuum system may involve fractionation error if the sample gas is not allowed to equilibrate throughout the total volume Atoms of larger or smaller mass may be favored in such a situation. CORRECTION for “isotopic fractionation” using the stable isotopes 13C and 12C. This correction factors out error introduced from metabolic and respiratory pathway differences between the modern reference standard material and the sample material. The unit of measure is termed “δ13C” 19 AMS or Radiometric Dating? Choosing the best method for C14 dating depends on: • the quantity of the available sample • value of the sample (how much of it you can afford to be destroyed..?) - AMS dating, for example, involves burning a sample to convert it to graphite. Advantages of AMS Radiocarbon Dating over Radiometric Analysis (a) small sample size needed (as little as 20 mg) it is recommended for radiocarbon dating of blood particles, grains, seeds, small artifacts, or very expensive or rare materials (b) takes less time than radiometric method (less than 24 hours) (c) higher precision than radiometric techniques 20.
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