Fossil fuels and carbon dating
(appeared in July 2015)

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Emissions from fossil fuels are found to be pumping older carbon into the atmosphere, says S.Ananthanarayanan.

The carbon content of the atmosphere has a tiny part that is radioactive. As processes that add radioactive carbon to the atmosphere are in balance with the rate of its depletion by radioactivity, the percentage of carbon that is radioactive stays constant. During the last century, and more so lately, industry has added huge quantities of carbon from coal and petroleum, which are fossil fuels, to the atmosphere. This carbon is of great antiquity and almost all its radioactive material it contains has decayed. The proportions of normal and radioactive carbon in the atmosphere are therefore being altered by human activity.

Heather D Graven at Imperial College, London, in a paper in the journal, Proceedings of the Academy of Sciences of the USA(PNAS), describes a study of how the composition, apart from sheer quantity, of atmospheric carbon is changing, and its implications.

Constant levels

The processes by which radioactive carbon enters and leaves the atmosphere make sure that the net quantity of radioactive carbon, under normal circumstances, stays constant. Creation of radioactive carbon comes about by the action of cosmic rays and the effects on nitrogen atoms in the upper atmosphere. The nitrogen atom has a nucleus with fourteen particles, seven positively charged protons, and seven neutrons and is denoted as 14N. As a result of cosmic ray action, one of the protons is exchanged for a neutron, leaving a nucleus with only six protons, which makes it a nucleus of carbon, and eight neutrons. The normal carbon nucleus has six protons and only six neutrons, with a total of twelve particles. Although there are now two more neutrons, this does not change chemical properties, and the nucleus is still one of carbon, although with mass 14, in place of 12. But the extra load of neutrons creates instability and this form of carbon, denoted as 14C, ultimately decays, going right back to 14N, where it came from.

The average time that 14C takes to decay is 8,251 years, which works out to saying that the number of 14C atoms, which are there in billions, in a sample would drop to half that number in 5,730 years. When a sufficient number of radioactive atoms accumulate, as a result of the production by the action of cosmic rays, the number that decay becomes equal to the number that are produced at any time. As the earth has been around for millions of years,there is now a balance in the number produced in the upper atmosphere and the number that decay, and the total number is almost constant.

Carbon Dating

The tissue of living things contains a great deal of carbon, and this carbon content, which is regularly cycled through the atmosphere, by respiration and nutrition, also has the same ratio of 14C and normal C as the atmosphere. But when the animal dies, exchange of carbon with the environment stops and the 14C content only decays, without addition of fresh 14C. The level of radioactivity in an organism that has died would hence gradually drop, to half in 5,730 years and so on.

This declining rate of radioactivity in non-living organic matter has thus become a handy way to estimate how long it has been since a once living thing has died. If the level of radioactivity is one fourth of that of the atmosphere, for instance, we can say that it died 11,460 years ago, or 22,920 years ago if it is one eighth, and so on. This is the method now perfected for dating archeological remains, specimens of art, making use of the carbon in bones, fossils, canvas or papyrus, or even vegetable dyes used for cave drawings, etc.

But the basis of the method, it will be noticed, is the presumption that the object, at the time when its material was living, had the same level of 14C content as the atmosphere today. As the level of 14C in the atmosphere is taken as having been constant for a long time, the presumption is generally valid and so has been the method of dating by radioactive carbon. This presumption, however, has started breaking down since the last century or so, when human action has started disrupting the environment.

The method of carbon dating itself was discovered only in 1949 and since then, it has made a great difference to our view of antiquity. Radiocarbon dating has been verified and possible variations have been calibrated with the help of other standards, like known historical periods, changes of the thickness of layers in the bark of ancient trees during known climate extremes. Extensive studies have provided a ‘correction’ chart with the help of which estimates made by direct carbon dating can be adjusted to take into account variations in the level of radioactive content of the atmosphere at different times, etc., and the method has been generally reliable

Climate change

The first large variations in the level of 14C in the atmosphere arose when nations began mid air nuclear testing, in the 1950s and 1960s. When large scale nuclear explosions are detonated in the air, as opposed to underground or in the sea, there is a huge generation of 14C and there is a spike in its levels in the atmosphere. The carbon content of the earth is actually largely in its vegetative cover and very largely in the oceans. But in the years immediately after the epoch of mid-air testing, the newly created 14C had not reached these carbon reservoirs and was concentrated in the atmosphere. The percentage, hence, almost doubled at the time, although it has greatly dropped since then, as carbon spreads into the surface and then the deep sea.

The much larger effect on 14C content, however, is the consequence, over the last century and very much in recent decades, of the carbon emission from the use of fossil fuels. Coal and petroleum represent vegetable matter of millions of years ago and the carbon they contain started off with the traces of 14C which were present in the atmosphere, and hence in plants and trees, at that time. But once buried and compressed, on the way to turn into coal or oil, they stopped communicating with the atmosphere and hence to replenish their content of 14C, which has been decaying since millions of years. We can readily imagine that the radioactive carbon that remains after such a long time is negligible.

When coal or mineral oil is burnt in power plants, the CO2 that is released hence contains very little radioactive carbon, and since the last century, these depleted emissions have been reducing the relative 14C content of the atmosphere. The research that Dr Graven has carried out is to make use of available records and extrapolations, with the help of IPCC (Intergovernmental Panel on Climate Change), estimates of CO2 emission during the rest of the century, to study how projected emissions during climate change would affect the percentage of 14C in the atmosphere.

The simulations carried out by Dr Graven indicate serious reduction in 14C content, with depletion equal to an aging by 1000 years, compared to pre-industrial levels, by 2050 and by 2000 years by 2100, if the present rate of emission of carbon from fossil fuels continues. This is to say that a specimen that belongs to the year 2100 would show the 14C content of a specimen of the year 100 AD. Such distortion of the basis for carbon dating would affect a number of applications of the technique, with samples of recent origin simply not permitting dating, as they would display very low 14C levels. The technique has been used for detecting forgeries, where the recent origin of a fake being passed off as an ancient originalis revealed, for instance. This would no longer be available as the recent material of the fake would also show low 14C levels, as it were older, Dr Graven says

Another area which would be affected is the use of radiocarbon measurement for the purpose of estimating the volume of fossil based carbon emissions in urban areas. The present comparison of global 14C levels with the level in the urban area gives a fair measure. But with falling global levels, much more precision measurements would be necessary to attain equally reliable results, Dr Graven says.

Interconnectedness

Ordinary carbon itself, like all other elements, did not arise on the earth, but in exploding stars, that gave rise to the cloud that led to the solar system. It is the radioactive 14C that is generated every instant on the earth and fortunately, not so fast that it builds up to make life impossible because of radioactivity. The active reservoirs of carbon, apart from fossil fuels, are the atmosphere, the surface ocean and the deep ocean.The atmosphere, holds only 1.9% of this and the ocean surface another 2.4%, while the deep ocean has more than 90%.

Radioactive carbon which arises in the atmosphere combines with oxygen as carbon dioxide and takes some years to mix with the surface ocean. The carbon in the deep sea is over 1000 years old and this also mixes with the surface to some extent. The carbon content of fish in shallow waters is thus a little depleted in 14C and much more so deeper down.

The most depleted reservoirs, of course are coal and oil deposits which are millions of years old. But even if their release into the atmosphere were to wreck the value of carbon dating for archeology, detecting forgeries or even estimating emissions in cities, etc., the loss would be negligible compared to other effects, of global warming, sea level rise, crop disruption…. But the drop levels of radioactive carbon is an instance of how subtle and widespread, not even connected with the climate, are effects of marginal warming and CO2 rise. And the study helps learn about the how inter-related are the processes of the earth.

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