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Radiocarbon Dating

October 24, 2019

 

 

 

 

 

Radiocarbon or carbon dating is an amazing scientific technique developed by a chemist called Willard Libby back in the late 1940s. People were so impressed by him that he also received the Nobel Prize for it in 1960. What Libby wanted to do was use the known properties of radioactive isotopes, which decay at a constant rate in order to measure time.

 

                                                                                                      Willard Libby

 

Now you’re probably thinking, what is an isotope?! The simplest way to put it is that elements have a certain number of electrons on rings orbiting the nucleus, which contain the protons, and neutrons. The protons and the electrons define the element, each element has a specific number of these, and you can’t really change that unless you’re causing a chemical reaction because then it would change the chemical properties of the element. BUT the number of neutrons within the nucleus of the atom can change, which then increases or decreases its atomic weight,.  These final products with the additions or subtractions of neutrons are called isotopes. Different elements have a different number of possible isotopes, but because we’re focussing on carbon dating, we're only talking about carbon today. 

 

 

Carbon normally has 6 protons, 6 neutrons and 6 electrons. It’s a very stable element, everything organic on the planet is made out of carbon. Carbon has three isotopes: Carbon 12, the original carbon, Carbon 13 with one extra neutron and Carbon 14. Now the first two are very stable, but Carbon 14, it doesn't like that you have two extra neutrons in there. It’s unstable, meaning it’s RADIOACTIVE. All three of these isotopes exist naturally in the world, but in varying quantities. 99% of all carbon is Carbon 12, the OG Carbon. Then 1% is Carbon 13 and only tiny tiny trace amounts of Carbon 14 exist. Actually it’s such a small amount we have to use exponents just to tell you about it! Only about 1- 1.5 atoms for every 10^12 Carbons in the world. For every 10^12 atoms of carbon 12 and 13, we have between 1 and 2 atoms of carbon 14.  Now you may look at this number and think that it’s impossible to use such a miniscule amount of atoms, but a person weighing 70kg has (7* 10^27) 7 Billion BILLION carbon atoms inside of them, so you can see that there would be enough carbon 14 to find in the body, or in anything organic, really.

 

 

Carbon 14 atoms are unstable because they’re essentially having an identity crisis. They’re created from Nitrogen atoms that were chilling in the upper part of the earth’s atmosphere. Nitrogen makes up 78% of the earth’s atmosphere, so it is the most abundant element. Cosmic subatomic particles from the sun come bumping around, eventually forming neutrons. Then once in a while, one of those neutrons will bump into a nitrogen atom at just the right angle to bump out one of the protons from the nitrogen’s nucleus. Now the weight of Nitrogen, 14, doesn’t change because the proton has been replaced, not just shooed away. BUT now there are only 6 neutrons inside instead of 7. And you can’t have nitrogen with 6 protons, you have carbon 14!

 

This is constantly happening inside our atmosphere. The nitrogen loses an electron and one of the protons in its nucleus turns into a neutron. These carbon 14 atoms are then floating around in the atmosphere, and they eventually bond with oxygen, creating molecules we all know like carbon monoxide, and dioxide. So every so often, one of these molecules that contains a carbon 14 isotope, will be absorbed by plants because plants breathe CO. Then of course animals eat the plants, and then some other animals eat the plant eating animals, etc etc, until it makes its way into the bloodstream, the bones, every part of them! Once the carbon 14 has found a nice home, they then decay and turn back into a nitrogen atom by turning their extra neutron into a proton. This is a process called beta decay. 

 

Beta Decay

 Image: windows2universe.org

 

All of this is constantly happening over the life of a living organism. We are constantly absorbing, decaying, and expelling carbon 14. It’s a whole cycle. We are constantly renewing our levels of C14 while we are eating, breathing and just living our lives… until we die that is. And that’s when the archaeological stuff comes in! When we die, there is no new carbon coming in, and the carbon that is already in our bones, continues to decay. And they decay at a constant rate.

 

We can think about this concept as either a timeline or a clock, whichever you prefer. If you think of food at the grocery store, it all has something we call a “shelf life”, a time period in which it will remain fresh and juicy. After the shelf life, it begins to decay. And different foods have different shelf lives. Carbon 14 doesn’t have a shelf-life per-se, where we know exactly what day it goes off, but it does have something called a half life, and a very specific one at that. 5730 years. So that means, at the end of every 5730 years, the radiocarbon atoms in a dead organism will be half of what they were at the beginning of those 5730 years. So if we have 100 carbons atoms at time of death, then in 5730 years, we’ll have 50, and after 11460, we’ll have half of that, so 25, and so on and so on into minuscule decimals numbers are too small for us to measure.

 

 

Image: hyperphysics.phy-astr.gsu.edu

 

 

As an archaeologist, when you dig up an animal bone and you need to get it dated, in the lab, they’ll analyse a sample of it by using an Accelerator Mass Spectrometer (AMS). This machine accelerates the carbon isotopes and then counts the number of Carbon 12 and Carbon 14 atoms.  Carbon 12 doesn’t decay, so it’s a perfect benchmark to measure the carbon 14 in a living thing. What you can then do is compare this ratio to the carbon 14 amounts that you would expect to find in a currently living thing, and based on that you can get an age of that bone.

 

 

Here’s the thing with radiocarbon dating, it heavily relies on two major assumptions:

 

One: that Carbon 14 Decay occurs at a constant rate. And two: that the ratio of C12 and C14 atoms in the atmosphere has remained constant over time. Now the first assumption, that one is pretty accurate. Science has already tested this over and over again for us, so we don’t have to worry about that. But the second one, the belief that carbon 14 levels have remained constant in the atmosphere over time, that one can get a bit tricky.

 

The truth is no, it isn’t perfectly constant, because remember these reactions with the nitrogen atoms in the atmosphere are not a constant thing. They happen all the time but not at a regular, measurable rate. For example, if you look at tree rings, you’ll see that the atmosphere had a lot more Carbon 14 before 1000 BC than today. This is why it’s very important to use other dating techniques

 

Raw radiocarbon dates are reported in year “BP” – before present. “Present” itself is set at 1950 CE because the present is always… the present and that gets confusing for dating. This way we have one point in time to calibrate everything off of.  This date was set because that was around the time Willy Libby developed this technique, but it is also because after this date, there was a lot of atomic bomb testing, which altered the carbon 12 and carbon 14 levels.

 

 

Nuclear testing had a huge influence

on the carbon levels in the atmosphere 

 

 

But that doesn’t mean carbon ratios were perfect before nuclear testing either. Levels have fluctuated forever. Sunspots for example could have an effect, but also we’ve been burning crazy tons of fossil fuels as early as about 300 years ago.

 

So radiocarbon dating on its own is not the most reliable, BUT we’re able to use other forms of dating techniques depending on what material we are dating, to help adjust and calibrate this method. One of the more popular techniques for landscape ages is dendrochronology. Within the tree rings, we’re able to measure the carbon 12 and 14 ratios for each year, going back thousands and thousands of years.

 

With this information, radiocarbon dating gets a lot easier! The second way is to use speleothems, which are mineral deposits in caves that are made up of calcium carbonate, meaning they have measurable carbon in them! We can use this up to like 50,000 years. Carbon dating itself  after 50/60,000 years, it’s not as useful as it could be because the amounts of carbon aren’t very measurable.

 

 

 Speleothems

Image: Wolfgang Glock [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]

 

 

C14 on dating bone is majorly dependent on the preservation of collagen in the bones (collagen being that protein in your bones made up of amino-acids, built of carbon, oxygen, and hydrogen). This method is the more popular one people resort to when discussing sampling skeletons, however, it needs to be clarified that considering the error rates of C14, it is one of many methods used to date bone, in narrowing down a time period rather than determining an absolute one. Nowadays, we use it as one method with several other more effective DNA and isotope sampling strategies.

 

With all of this data, we of course can then use databases for people to stick in their BP dates, and get a date range for their tested object. And it’s a date range, not an exact date, because of all of the limitations we have just talked about. Usually labs will give you a calibrate date range after testing and it’ll look something like this:

 

2400 +/- 100 BP

 

So the BP is Before Present, 1950. We know about that, and the plus minus thing is the range, so the dating is somewhere between 100 years before 2400 BP and 100 years after 2400 BP

 

We put this date range on a graph, alongside the carbon measurements over time that we’ve gotten from tree rings or speleothems. One axis has the BP dates, and the other has the calendar years, so the ones we use for everyday dating. Next we draw two horizontal lines where the date + and – are until they meet the carbon. Right when they meet, we then draw vertical lines down. The space between these two vertical lines is our calibrated date range for calendar years. This is a pretty wide date range, but it does then have a 95% probability that the date falls within it. If we want to narrow it down a bit, of course the probability decreases to about 68%.

 

It’s a complicated process, and isn’t completely accurate, but it’s one of the most valuable things we have for dating organic materials. Think of human and animal remains, but also items made out of tortoise shells like purse handles, or leather goods, even though leather can be such a rare find. We can also do it on wood, and charcoal! We can even use these organic finds to help date inorganic materials, all thanks to an archaeologist’s favourite thing: context. If a carved stone, or some piece of pottery is found together with an organic object, the dates of both objects are probably going to be the same if we correlate the carbon 14 with other forms of dating for inorganic materials.

 

There’s just one other thing we need to look out for with radiocarbon dating, and that’s what we call the “old wood effect”. Wood is a great example of an organic material that can be used and repurposed over long periods of time. If you use wood to build a house, you’re usually using the inner wood from the log, which is, if you’ve seen the dendrochronology video, older than when you’re actually building the house. The house could then stand for hundreds of years. It could be a long time until it burns down, collapses, or gets repurposed before it falls into the archaeological record.

 

The same goes for family heirlooms and antiques. Objects could be passed down for generations before entering the record. This is why we can’t just look at objects individually. We need to look at assemblages, all of the objects in relation to the other, in order to come up with a proper date and idea of the finds.

 

Carbon dating has provided the most useful way of answering the “WHEN” question when it comes to archaeology. While it has its flaws, the method can be used all over the world, as it only relies on organic, uncontaminated material. And, it can also date back to 50,000 years! The method has been used for cultures that don’t have historical records or timelines, to help put things into context; it’s been used on cave paintings, torch marks, and even some more controversial artefacts like the Shroud of Turin. It’s crazy to think that you can get so much information out of a sample the size of a grain of wheat, but there you have it.  

 

 

Have any more questions about archaeology or history? Shoot me an email: digitwithraven@gmail.com

 

 

Looking to Find Out More?

 

Kamen, Martin D. (1963). "Early History of Carbon-14: Discovery of this supremely important tracer was expected in the physical sense but not in the chemical sense". Science. 140 (3567): 584–90. DOI:10.1126/science.140.3567.584

 

Carbon 14: Age Calculation http://www.c14dating.com/agecalc.html

 

How Does Radiocarbon Dating Work? https://www.radiocarbon.com/about-carbon-dating.htm

 

Blakemore, Erin. Radiocarbon helps date ancient objects—but it's not perfect https://www.nationalgeographic.com/culture/archaeology/radiocarbon-dating-explained/

 

Beta Decay - Tyler DeWitt: https://www.youtube.com/watch?v=uqAA_D9Mi_I

 

Jefferson Lab, Beta Decay - https://education.jlab.org/glossary/betadecay.html

 

 

 

 

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