The radiocarbon 14C dating method has been used for decades to accurately determine the age of a wide range of artefacts. But our relentless use of fossil fuels has pumped a type of carbon into the atmosphere that is starting to confuse the dating technique. By 2050, scientists warn, new fabrics could have the same radiocarbon date as items 1,000 years old!
According to new research published in the journal Proceedings of the National Academy of Sciences, growing emissions from the burning of fossil fuels are threatening the effectiveness of the radiocarbon dating method.
The decay of carbon isotopes within living organisms can be used to date organic materials, such as charcoal, or shell and bone, and some inorganic materials, excluding metals.
Radiocarbon dating was developed in the late 1940s by Willard Libby, as a method to measure radioactivity.
The method measures carbon-14, a radioactive form of the element carbon, produced in Earth’s atmosphere, then absorbed by plants through photosynthesis. Animals that go on to eat the plants ingest the carbon-14. Scientists are then able to work out the age of almost anything organic by comparing the level of carbon-14 to non-radioactive carbon in the sample.
Carbon-14 or radiocarbon is a weakly radioactive isotope of carbon. It is an isotopic chronometer.
Carbon-14 is produced at a constant rate in the atmosphere and is found in a fixed ratio compared to Carbon-12 in living plants and animals. Ratio of Carbon-14 to Carbon-12 in organic material decreases by half every 5,730 years. Thus, the radioactive isotope Carbon-14 decays relatively quickly in geological terms, the radiocarbon dating method is only really useful for archaeological or sub-fossil materials – like wood, leather, fabric, antlers or bones. As such, radiocarbon dating is of little use in geological dating.
There are three principal techniques used to measure carbon-14 content of any given sample:
- gas proportional counting,
- liquid scintillation counting, and
- accelerator mass spectrometry.
Gas proportional counting is a conventional radiometric dating technique that counts the beta particles emitted by a given sample.
Beta particles are products of radiocarbon decay – electrons and positrons.
This method is destructive as the carbon sample is first converted to carbon dioxide gas, before measurement in gas proportional counters can be carried out.
Liquid scintillation counting (LSC) is another radiocarbon dating technique, popular in the 1960s. In this method, the sample is in liquid form and a scintillator is added.
A scintillator produces a flash of light when it interacts with a beta particle. A vial with a sample is passed between two photomultipliers, and it is only when both devices register the flash of light that a count is made. Liquid scintillation counting is the standard laboratory method to quantify the radioactivity of low energy radioisotopes, mostly beta-emitting and alpha-emitting isotopes. The sensitive LSC detection method requires specific cocktails to absorb the energy into detectable light pulses.
Accelerator mass spectrometry (AMS) is a modern radiocarbon dating method that is considered to be the more efficient way to measure radiocarbon content of a sample. In this method, the carbon-14 content is directly measured, relative to the carbon-12 and carbon-13 also present in the sample. The method does not count beta particles, but rather the number of carbon atoms present in the sample and the proportion of the isotopes.
Radiocarbon dating labs use Oxalic Acid I and Oxalic Acid II as modern standards.
Radiocarbon measurements are reported as Conventional Radiocarbon Age.
Accelerator Mass Spectrometry
The development of the AMS technique in the 1980s enabled 14C dating of samples containing as little as a few milligrams of carbon, which is around 1000 times less than what could be done in previous conventional techniques. The relative numbers of the atoms of different carbon isotopes in the sample are directly measured and the radiocarbon age is determined.
A system for the preparation of samples for AMS dating was developed at the Gliwice Radiocarbon Laboratory in 1999. As yet, the system has been used to produce graphite targets from plant macro-fossils, charcoal, peat, bones, shells and pollen extracts.
Due to the very small sample amount, considerable effort is put into avoiding contamination with either modern or inactive carbon during the sample preparation.
The purpose of the preparation of samples before radiocarbon measurement is the extraction of material that contains indigenous carbon in a quantity sufficient to measure the 14C content, the removal of contaminating substances which usually give different ages, and the production of a medium for appropriate measurement technique (e.g. CO2 for gas proportional counters or graphite for AMS).
Usually, the samples need physical cleaning or separation under microscope.
- Organic samples, like charcoal or organic remains, are most commonly retrieving using the AAA (Acid-Alkali-Acid) method;
- Shells are cleaned and the outer part is dissolved in a weak acid;
- For bones, the collagen is extracted according to the modified Longin’s method.
After the chemical pre-treatment, the sample material in a quantity corresponding to about 1 mg of carbon is placed into a quartz tube with copper dioxide (the source of oxygen needed for combustion) and silver wool (for the removal of gaseous sulphur and chlorine compounds). The tube is then maintained for several hours at a constant 900°C temperature.
The tube with the CO2 derived from the sample is incised, then placed into an arm of a vacuum line with a ball joint. After overnight pumping to a high 10-4 mbar vacuum, the arm is cut off with a cock, the tube is cracked and the gas is released. Water vapour is frozen in a trap cooled down to around -70°C with a mixture of dry ice and alcohol. The CO2 is collected in a sealed glass vial. The amount of obtained CO2 is measured, and the excess of gas removed or stored in a separate vial.
The CO2 is reduced to graphite during the reaction with hydrogen at the temperature of 600-630°C (dependent on the reduction rate of the sample) in the presence of iron as a catalyst. The graphite is deposited on the iron powder introduced into a small quartz tube. The reactor and iron powder are previously heated overnight at 90°C under continuous pumping.
Prior to graphitisation, the iron is oxidised and reduced in order to increase its catalytic properties. Then CO2 and H2 are introduced in stechiometric amounts, with around 20% excess of H2. During the reduction, H2O (water) is produced and continuously removed by freezing out in a mixture of dry ice and alcohol. The progress of the reaction is monitored by measuring the pressure.
After 3 to 4 hours, the reaction is complete.
Fe-C powder is pressed into a tablet which is used as a target in the sputter ion source of the accelerator. The target is stored in argon atmosphere until the measurement.
Prepared graphite targets are then sent to an AMS laboratory for the measurement – Poznań Radiocarbon Laboratory, in Poland or Leibniz Laboratory for Radiometric Dating and Isotope Research in Kiel, Germany. Each batch of samples is accompanied by at least two modern standard (Oxalic Acid) and two background (coal or marble, containing no radioactive carbon) samples, prepared in the same way as samples of unknown age which are used for the age calculation.
In the accelerator mass spectrometry (AMS) technique, the graphite targets are loaded in the sputter ion source which produces a beam of negative carbon ions. Subsequently, low-energy mass analysis is performed with the use of magnets and sometimes also electrostatic analysers. The negative ions of the isotope of interest are accelerated to the terminal of the accelerator at a potential of at least 0.5 up to several million volts (MV). There, the negative ions are converted to positive ions by the removal of several electrons during a stripping reaction with a gas (usually Argon) or carbon foils, and accelerated further to ground potential. After subsequent magnetic and electrostatic analysis, the ions are identified in an ion detector.
Since its creation, the 14C method has been used to accurately determine the ages of thousands of artefacts and to uncover cases of art fraud. Perhaps the most famous case where radiocarbon dating was used, was in the investigation of the Shroud of Turin, which scientists in 1988 determined had actually originated in the 13th century – over 1,200 years after the death of Christ – whose image it was assumed to represent.
The paper written by Ray Rogers established that the C-14 sample area was contaminated by cotton and that the sample gave a positive test for vanillin which shows that the material was much younger relative to the rest of the shroud which does not test positive for vanillin.
However, the use of carbon based fossil fuels, such as coal and oil, since the industrial revolution, have increased the amount of non-radioactive carbon in the atmosphere.
As emissions grow, so does the diluting effect on carbon-14. The accuracy of the ageing technique is gradually lost.
As carbon-14 decays over time, the fraction decreases and that is how it is used for dating. But if we change this ratio of radioactive carbon to total carbon, by unwittingly adding up non-radioactive carbon, which is what is happening with fossil fuels. The useful effect is diluted.
The study looked at the likely carbon emissions pathways over the next century, and suggested that by 2020, the increases in non-radioactive carbon could start hindering the dating technique. It concluded that any current measurements on new products will likely end up having the same fraction of radiocarbon to total carbon as something that’s lost it over time due to decay. So, measuring the fraction, they will look like they have the same age for radiocarbon dating.
According to the research, at current rates of emissions increase, a new piece of clothing in 2050 would have the same carbon date as a robe worn by William the Conqueror 1,000 years earlier! This will really depend on how much emissions increase or decrease over the next century, in terms of how strong this dilution effect gets.
Rapidly reducing emissions might mean we stay around a carbon age of 100 years in the atmosphere. Strongly increasing emissions would mean an age of 1,000 years by 2050, and around 2,000 years by 2100.