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 the other day 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 Web sites 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 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. [Article Continues…]