Carbon 14 Dating: Complications and Mitigations

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

Carbon 14 dating depends very heavily on knowing the original concentration of carbon 14 compared to carbon 12. But, is the ratio of carbon 14 to carbon 12 perfectly constant everywhere on Earth? If not, then what are the factors contributing to these variations and how are these factors mitigated?

Carbon 14 is formed from cosmic rays hitting the Earth’s atmosphere. The carbon starts out by combining with oxygen in the atmosphere to first become carbon monoxide and finally carbon dioxide. It does not take all that long for carbon dioxide to become dispersed and mixed throughout the atmosphere—in times like weeks or a small number of months. Given the half-life of 5,730 years, this is small potatoes.

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The First Complication: Unequal Dispersion of Carbon 14 in the Water Bodies

The carbon then disperses into living things via plants and the food chain. However, to get to a sea creature like a lobster, carbon dioxide has to get into the ocean, and that takes a longer amount of time.

The carbon dioxide with carbon 14 in it has to make it into the water, get absorbed by the algae, then eaten by a clam or a starfish or something, and then a lobster.

Carbon 14 is radioactive and it decays. That means that while this process of cosmic rays becoming part of a lobster is happening, carbon 14 is decaying. And that fact can cause changes in how the age of something is figured out.

Summarizing the Difference in Ratios

The ratio of carbon 14 to carbon 12 is about one part per trillion in the atmosphere. That categorization is a little unwieldy: one part per trillion; so, instead, this is called the expected ratio in the atmosphere and is assigned a number of 100%. If there is some other place on Earth where the concentration is half that, say one part per two trillion, the ratio can be said to be 50%. This is just an easier way to categorize it.

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The Way Around the First Complication

The percentage ratio in the surface waters of the world’s oceans is about 95%, which is to say that the amount of carbon 14 to carbon 12 is lower than in the atmosphere. And in the deep ocean, it is even less than 95%. This has serious consequences when comparing the age of things that lived on the surface of the Earth to something that lives in the ocean.

So, one has to be mindful of these variations in the oceans before calculating the age of a sea creature such as lobster.

The Second Complication

Of course, this effect due to the different ratio of carbon 14 to 12 is not the only complication that has to be taken into consideration. For instance, the Southern Hemisphere has a much larger fraction of water surface area, while the Northern Hemisphere has more landmass. This means that there is more diffusion of carbon dioxide into the ocean in the south.

The Way Around the Second Complication

This means that the atmosphere of the Southern Hemisphere is slightly depleted of carbon 14; and therefore, objects dated on the surface of the Earth in the Southern Hemisphere appear to be a little older than the same object in the Northern Hemisphere. But this effect again can be mitigated with the careful calculation of these differentials.

The Third Complication

Here is another complication. Carbon 14 dating is based on the idea that the carbon 14 to carbon 12 ratio in the atmosphere is constant over time. This also turns out to not be true. But, this is not a fatal problem. This does not kill the idea of carbon dating entirely.

The Way Around the Third Complication

The variation of the carbon ratio can be studied by using tree rings. When trees absorb carbon, they absorb it on the outside. The core of the tree does not get new wood. Essentially, the center of the tree is the old part, and the outer part of the tree is new. So, if a tree can be found that is, for example, 200 years old, a coring can be taken and the carbon ratio for each year calculated.

Now 200 years is not all that old, so another tree can be found which died 150 years ago, but was 200 years old when it died. This particular tree germinated 350 years ago. And the time the two trees overlapped was 50 years. Since the thickness of tree rings depends on the weather that year, one can match the two trees pretty well if they grew not too far apart. By doing this over and over again for many trees, scientists have been able to come up with a basic set of tree rings going back to about 12,000 years ago.

By doing this, it is possible to figure out the carbon 14 to 12 ratio for all of these years. And, once it is known how it changed, it is possible to take that into account. It is not easy, but it is doable.

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The Other Factors That Change the Carbon Ratio

So, what are some of the things that can change the ratio of the two different kinds of carbon?

Complication 1: Nuclear Tests Change the Carbon Ratio

The most obvious one revolves around the fact that carbon 14 is radioactive. From 1945 to 1963, America, the Soviet Union, China, Great Britain, and France tested nuclear weapons above ground. These explosions put a significant amount of carbon 14 into the air.

It started small with the standard uranium and plutonium bombs but escalated as hydrogen bomb testing began. The size of these bombs grew over the years and several tons of carbon 14 were thrown into the atmosphere. In fact, by 1963, the ratio of carbon 14 to carbon 12 in the atmosphere almost doubled.

Mitigation of Complication 1

Since the partial test ban treaty of 1963, which banned testing by America and the Soviets on the Earth’s surface, under the water, or in space, the ratio has moved back to normal. It is not quite there yet, but the extra carbon 14 was absorbed in the ocean and other places and things are getting back to normal. It has taken over 50 years, and it is not finished, but it is getting there.

Complication 2: Burning of Coal Changes the Carbon Ratio

There is another way in which mankind has changed the carbon ratio in the atmosphere that is actually kind of interesting. Now this mechanism of making carbon 14, which is absorbed in the biosphere and then slowly dies away, is not new. It has been going on for billions of years, and this includes the Carboniferous period in the history of the Earth, which ran from about 360 million years ago until 300 million years ago.

That was the period of time in which there were enormous swamps, populated by giant insects and primitive amphibians. Oh, yes, and trees, lots and lots of trees. Those trees died and fell in swamps and were buried. And, from that, the Earth now has coal. When those trees fell in the swamps, they had a carbon 14 to 12 ratio that was appropriate for the air around them. But the half-life of carbon 14 is 5,730 years and that was over 300 million years ago.

All of the carbon 14 has decayed away and this means that coal consists of only carbon 12 and 13, but no carbon 14 at all. And, of course, since the start of the Industrial Revolution, people have been mining and burning coal and releasing the exhaust gases in the form of carbon dioxide into the atmosphere. This has the effect of reducing the ratio of carbon 14 to carbon 12 in the atmosphere. It is a small effect, about 3%, but it amounts to a 250-year shift in the determined date if it is not taken into account.

Mitigation of Complication 2

So, this is how mankind can change the carbon 14 to 12 ratio, what can Mother Nature do? Remember that the dominant mechanism for making carbon 14 is when cosmic rays from outer space hit the atmosphere. Cosmic rays are typically protons moving at high speeds. When a charged particle like a proton moves in a magnetic field like that generated by the Earth or the Sun, the path of the particle is changed. And because of the sunspot cycle, the magnetic field of the Sun changes. And, over long time scales, the magnetic field of the Earth changes, too. Because of these changes in the magnetic field of the Sun and Earth, the rate at which carbon 14 is made also changes. Now, this can be measured from the trick with tree rings, but it is something that has to be taken into account.

Volcanic Emissions Change the Ratio

There is another way in which the atmospheric ratio of carbon can be changed over small geographical areas. This can occur near volcanoes, which constantly emit carbon dioxide. Since carbon dioxide has been trapped underground for millions of years, it is emitted without any carbon 14. This changes the carbon 14 to 12 ratio in the vicinity of the volcano.

Mitigation of All the Complications

Notwithstanding all the complications, carbon 14 dating remains a powerful technique and is indispensable for archaeologists doing studies of biological samples that are under 60,000 years old or so. Archaeologists would be lost without the approach. But doing this is hard. It takes real effort to compensate for the variables.

The bottom line is that making precise scientific measurements is a job for professionals. The scientific method and robust criticism from other scientists give people confidence that carbon dating is effective and accurate.

Common Questions about Carbon 14 Dating: Complications and Mitigations

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