Skip to 0 minutes and 7 secondsFor this segment, I'd like to introduce Dr. Stewart Fallon, who is the head of the Radiocarbon Dating Laboratory at the Australian National University.

Skip to 0 minutes and 22 secondsThe first step is to drill the bone and collect the powder. We then do tests that verify the bone is suitable for dating.

Skip to 0 minutes and 33 secondsIn the chemistry laboratory, we dissolve the bones with the dilute acid so we can isolate the collagen. The collagen is the organic protein in bone that is the most robust and most suitable for radiocarbon dating.

Skip to 0 minutes and 50 secondsThe next step is we take the cleaned collagen sample, and we freeze dry it to remove all of the water from the sample. After freeze drying, we end up with nice, pure, clean, fluffy collagen. The next step is we have to convert the collagen to carbon dioxide. And to do this, we weigh out 2 milligrams of collagen, and we load it into a quartz tube.

Skip to 1 minute and 29 secondsInside the quartz tube, we add copper oxide and silver. Copper oxide provides oxygen to generate the carbon dioxide. Now, we load the sample on a vacuum line, where we evacuate all of the air out of the quartz tube. We then use a flame torch to seal the tube with our pure collagen sample inside. We next put the samples into the oven at 900 degrees for six hours. This is to make CO2 inside the quartz tubes.

Skip to 2 minutes and 5 secondsThe first step is to load the quartz tube onto the vacuum line. We evacuate, or remove, all of the air from around the tube, and then we crack the sample. We isolate the carbon dioxide using liquid nitrogen, which is cold enough to freeze carbon dioxide into a solid. We transfer the carbon dioxide into an individual reactor. We then add hydrogen to the carbon dioxide. We heat it at 560 degrees, and after two hours, the carbon dioxide has turned into elemental carbon or graphite. We then take the graphite, and we press it into our small, aluminium sample holders.

Skip to 3 minutes and 22 secondsNow, we will load the samples into the accelerator mass spectrometer. Our sample wheel holds 39 samples. Inside we have a mixture of standards, secondary standards, blank samples, and our unknown samples.

Skip to 3 minutes and 43 secondsLet me show you how the accelerator mass spectrometer works. Our graphite samples are inside our ion source. In the ion source, we heat up cesium, which is a metal. The positive cesium ions interact with the graphite, and generate negative carbon ions. These ions are accelerated to 44,000 volts. The ions are then separated in a magnetic field, or a mass spectrometer, where we can separate out carbon-12, carbon-13, and carbon-14. The ions and molecules, which are also in the beam, are further accelerated to 245,000 volts. The ions and molecules then go through a cloud of helium gas, where all the molecules are broken up.

Skip to 4 minutes and 36 secondsThe beam then goes through another magnetic field, or the second mass spectrometer, in the accelerator mass spectrometer, where we can measure carbon-12 and carbon-13. Carbon-14 is further filtered by an energy filtre, and we measure each carbon-14 atom as it hits a silicon detector.

The Carbon 14 (C-14) dating method

The Carbon 14, or radiocarbon dating method is one of the best-known methods of dating human fossils, and has been around since the late 1940s.

The Carbon 14 (C-14) dating method is a radiometric dating method. A radiometric dating uses the known rate of decay of radioactive isotopes to date an object. Each radioactive isotope has a known, fixed rate of decay, which we call a half-life. The half-life is the amount of time it takes for a quantity to fall to half of the value that it started with.

This means that if we know the isotope and its rate of decay, then we can calculate how old the substance is.

So now we need a little science lesson about carbon.

The element carbon occurs in nature in three isotopic forms. Carbon 12 (12-C) is stable and represents 98.9% of the carbon in the atmosphere. The rest (1.1%) is mostly made up of Carbon 13 (13-C), which is also stable, and Carbon 14 (C-14) which is unstable – or radioactive. In our atmosphere, only about one in a trillion carbon atoms is C-14.

Most of the C-14 in our atmosphere is produced in the upper atmosphere by the action of cosmic rays on nitrogen (N-14) to produce C-14. Once C-14 is produced, it starts to decay back to nitrogen. The atmosphere has constant levels of C-14 – the production of new C-14 in the atmosphere and the decay of C-14 balance each other in a steady state equilibrium.

These three different forms of carbon are oxidised and dispersed through our atmosphere. The oxidised carbon is absorbed by plants through photosynthesis. Once it is in the plants, it enters the food chain. When the C-14 enters the plant or animal, it remains in equilibrium with the atmosphere. However – once the tissue dies (i.e. the animal dies or a tree’s sapwood converts into hardwood) then the tissue is no longer being replaced and the level of C-14 continuously decreases through radioactive decay.

C-14 entering the food chain

C-14 entering the food chain. © Fiona Grün



If we know how much C-14 was in the living tissue, we can measure the amount of C-14 in the dead plant or animal and then compare these to assess how long it has been dead. We can do this because we know the decay rate of C-14 (it has a half-life of 5,730 years). The result is the radiocarbon age of the sample.

But how do we know?

The next question, of course, is – how do we know the amount of C-14 that was in the living tissue when it died? Well, this isn’t simple to determine. As it turns out, the production of C-14 in the atmosphere has not been constant over time, because of fluctuations in the amount of cosmic rays reaching the earth. We’ve been able to confirm this by measuring tree rings and comparing their known age with the radiocarbon result. This allowed us to create carbon calibration curves. However, our tree ring chronology only goes back 12,000 years (look up dendrochronology if you are especially interested in this aspect of dating). So, we’ve had to use other materials such as corals, speleothems (remember the “cave popcorn”?), lake sediments and ice-cores to create calibration curves for radiocarbon dating. We are working hard to extend these calibration curves as much as possible – currently we are at around 55,000 years.

This means that radiocarbon dates are given as a calibrated date – which is our best estimate based upon our current calibration curves, but introduces a margin for error. As research continues to update our calibration curves, we need to go back to the original radiocarbon dates and recalibrate them.

That’s one challenge.

Remember that we said earlier that the naturally occurring amount of C-14 in the atmosphere is about one in one trillion carbon atoms? That’s another challenge. In such small amounts, C-14 is very difficult to measure and it is also very sensitive to contamination. When we first started using radiocarbon dating, huge samples were required. Today we use (very expensive) accelerator mass spectrometers (AMS) to count C-14 atoms in a sample. And we are also constantly researching new methods of sample preparation to help reduce the likelihood of contamination.

All of this combines to produce dates that can be difficult to assess as correct. Certainly, a single radiocarbon dating of a human fossil should be treated with caution.

In the laboratory video we show you how we extract bone collagen from a sample to give us original, unaltered bone material to test in the AMS. You’ll see that it’s quite a process.

Your task

What did you think of the process shown in our video?

Do you think radiocarbon dating is a valid method of dating human fossils?

Select the comments link below and share your thoughts.

References

Aitken, M.J. (1990). Science-based dating in Archaeology. Longman Inc., New York.

Wood, R. (2012). Explainer: what is radiocarbon dating and how does it work? The Conversation.

Grün, R. (2006). “Direct Dating of Human Fossils”. Yearbook of Physical Anthropology, 49: 2-48

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This video is from the free online course:

A Question of Time: How We Date Human Evolution

Griffith University