C14 Dating

At school, you’re told about C14 dating and how it is used to date ancient humans and their tools. So it came as surprise to learn about all the questions and criticism that surround the accuracy of this dating technique.

But first, one needs to understand a bit more about the background of C14 than what’s usually taught. And the first step in that path is to learn how C14 is produced in the first place.

The short version is that cosmic rays bombard the atmosphere with enough force to produce C14. And this is also where the first criticism of C14 dating starts: the earth’s magnetic field influences how many cosmic rays are deflected away from earth. Which in turn influences the amount of C14 that is produced. And here’s the kicker: the earth’s magnetic field changes over time! In other words, the amount of C14 at different periods of time varies. So how do you know the amount of C14 that was there in a sample that’s 20,000 years old v/s 3,000 years old?

When an item is alive, it absorbs this C14. But after death, it doesn’t absorb anymore C14. And as the C14 decays, it’s quantity decreases. So in theory, if you knew the ratio of C14 to C12 (regular carbon) in a sample at the time of death, then you can measure the ratio today, and the change in ratio gives you the age. And that’s the problem, say the critics: you can’t know the ratio at the time of death: you can only guess. To make matters worse, there’s no way to be absolutely sure that the sample did not get contaminated with other carbon after it’s death.

C14 is also very rare: only one in a trillion carbon atoms is C14. Which makes counting the number of C14 atoms very difficult. Worse, with that level of rarity, an error in counting even by a few atoms changes the ratio quite a bit.

Lastly, the composition of the earth’s atmosphere has changes over time. Some of the constituents, like water vapour, affect the absorption of C14 by organisms on the surface. And, similar to the earth’s magnetic field, the amount of water vapour has varied in the atmosphere over time.

As you can see, most of these criticisms are not about the technological limitations (except the one about counting C14 atoms) of this dating technique. Rather, they’re about the change in the “environmental variables” over time. Which makes it difficult, if not impossible, to refute those criticisms.

Why then does C14 dating still survive? Mostly because any date claim based on "scientific" techniques brings a degree of respectability and hence, believability. Many scientists feel that C14 based dates should be taken seriously only if other, independent evidence produces a date close to the one found via C14. In other words, use C14 for confirmation, not determination.

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