Carbon 14 formation and decay

It has become my custom here at Interesting Thing of the Day to choose topics that I think will be unfamiliar to most readers—a sort of implicit “I’ll-bet-you’ve-never-heard-of-this” test. I think it’s fair to say that any educated person over the age of 10 or so has probably heard of carbon dating. But I realized as I was pondering this topic that even as an adult with a fair amount of scientific knowledge, I could not articulate exactly how or why carbon dating works. So I did a bit of research to fill in the gaps in my understanding, and not surprisingly I found the details to be quite interesting. What did surprise me was the huge number of websites and books vigorously attacking the legitimacy of what I had thought was a fairly straightforward, uncontroversial test. Apparently carbon dating is right up there with evolution in terms of the disdain it evokes from certain religious groups. As is often the case, the controversy over this topic is at least as interesting as the topic itself.

Carbon Copies

Carbon dating begins, logically enough, with carbon. High in the atmosphere, cosmic rays strike nitrogen atoms, producing a radioactive carbon isotope known as carbon-14 (or 14C); this is why it’s technically known as radiocarbon dating or, sometimes, carbon-14 dating. Carbon-14, along with the more common, stable (nonradioactive) carbon isotopes carbon-12 and carbon-13, combine with oxygen to produce carbon dioxide. In the process of photosynthesis, plants “breathe” this carbon dioxide, convert the carbon into carbohydrates for fuel, and then release the oxygen into the atmosphere as a byproduct. So some of the residual carbon in plants is carbon-14. Animals, in turn, eat the plants (or eat other animals that have eaten the plants), and thus the carbon-14 atoms propagate throughout the food chain. The result is that everything that is alive, or once was, contains some number of carbon-14 atoms. Although the number of carbon-14 atoms varies from one organism to another, the proportion of carbon-14 atoms to carbon-12 atoms is basically constant—and roughly the same as the proportion found in the atmosphere.

Carbon-14 decays (loses its radioactivity, converting back to nitrogen-14) at a known rate; its half-life, or the time it takes for half a given number of carbon-14 atoms to decay, is about 5,730 years. When an organism dies, it stops acquiring new carbon-14 atoms. Given that the ratio of carbon-14 atoms to carbon-12 atoms in a living thing is a constant, one can determine the number of each in a sample of organic matter (using sensitive equipment to detect the amount of radiation remaining), and then do a little bit of math to determine how long it’s been since the organism expired. For example, if a sample emits radiation indicating the presence of 10 carbon-14 atoms and we know from its mass that it originally must have contained 20, that means the plant or animal from which the sample was taken died about 5,730 years ago.

Eventually, however, all the carbon-14 atoms will decay—or at least enough of them will that the amount of radiation they emit can no longer be distinguished from ordinary background radiation. So for all practical purposes, carbon dating is useful only for samples up to about 50,000 years old (though this depends somewhat on the mass of the sample—and some advanced techniques can reliably measure carbon-14 levels low enough to indicate an age of 100,000 years). Thus, carbon dating would not be useful in, for example, assessing dinosaur bones.

Constants and Change

There are other limitations in carbon dating too. For example, with a few exceptions, the technique can only be applied to once-living items such as bone, leather, wood, and cloth—not, say, rocks or metal. More importantly, though, the accuracy of carbon dating rests on several crucial assumptions. For one thing, the rate of carbon-14 production in the atmosphere (and thus the level of cosmic ray activity) must have been pretty much constant for the past several dozen millennia. Likewise, the proportion of carbon-14 to carbon-12 in the environment must have remained fairly constant. And in any given sample, one must be certain that contaminants from other time periods are not present—a sometimes-tricky issue.

As a matter of fact, increased hydrocarbon emissions over the past century have greatly increased the amount of carbon-12 in the atmosphere, while nuclear detonations during the past 50 years or so have increased the amount of carbon-14. And at other points in history, climatic changes and other large-scale global events have altered the picture in other ways. So scientists performing carbon dating routinely calibrate their findings to adjust for these known issues, using other dating techniques (such as counting the rings on old trees) to corroborate their findings and help them fine-tune the scale.

But these and other seeming sources of uncertainty have been seized upon by some very vocal groups of creationists as loopholes, allowing them to challenge the validity of carbon dating. According to some people, a literal reading of the Bible (taking into account all the genealogies and so forth) yields a creation date of around 7,000 years ago. Thus, any scientific finding that seemingly assigns an older date to any object must be mistaken—and carbon dating, critics suggest, if it is to be believed at all, should be calibrated to the Bible. Creationist arguments against carbon dating (and every other scientific statement that suggests an older Earth) are extensive, passionate, and very impressive-sounding—though they are most often made by people without actual scientific credentials, and therefore dismissed (with varying degrees of refutation) by the scientific community.

As an outsider to this debate, I find it all rather amusing, in a sad sort of way. Discussions between creationists and mainstream scientists typically have an apples-and-oranges character, much like discussions between “pro-choice” and “pro-life” advocates or politically “liberal” and “conservative” partisans. Each side has unshakeable beliefs and therefore insists on bending any available evidence to support them, so very little real discussion takes place. Speaking as a layperson, rather than a professional in science or theology, I find the evidence supporting the reliability of carbon dating vastly more compelling than the evidence supporting a 7,000-year-old Earth. But then, I don’t have an axe to grind, and even I did, carbon dating couldn’t tell us how old it was.

Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on October 26, 2004.

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