The Truth About Carbon Dating

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Have you ever wondered how scientists calculate the age of dead plants and animals? This is a topic that has fascinated me for many years, because while it may sound very simple and straightforward on the surface, as you will see, it is really a rabbit hole that leads into all sorts of unexpected places. Come with me as we explore the strange, bizarre world of Carbon-14 dating…

Let me start with an explanation of what Carbon-14 dating is. Like all empirical testing methods, Carbon-14 dating is based on a few assumptions (you have to start somewhere, right?) Almost all land-dwelling plants and animals absorb two elements from the environment: Carbon-12 and Carbon-14, a radioactive element formed in the upper atmosphere. The ratio of Carbon-12 to Carbon-14 is assumed to be constant. When a plant or animal dies, the Carbon-12 remains constant, but the Carbon-14, since it is radioactive, continues to decay (into Nitrogen-14). Since the Carbon-14 is not being replenished, the total amount of it reduces linearly over time.

The original idea behind Carbon dating was to deduce the age of an organism by looking at the amount of Carbon-14 left in it and calculating its age based on the time it should have taken for the missing Carbon-14 to decay. Pretty logical and straightforward, right?

But there is a problem. The assumptions are wrong.

The ratio of Carbon-12 to Carbon-14 in the atmosphere has not remained constant. The amount of Carbon-14 produced in the upper atmosphere is dependent upon the amount of radiation coming from the Sun, which we know has changed significantly. On top of that, the introduction of nuclear energy and nuclear weapons have affected the ratio of radioisotopes in the atmosphere.

When Carbon dating was originally introduced, scientists attempted to validate it by Carbon dating dead trees and comparing the calculated age with the known age of the trees based on their rings. What they found was that Carbon dating yielded wildly inaccurate ages for the trees.

In order to overcome the obviously false assumption that the ratio of Carbon-12 to Carbon-14 is constant, scientists attempted to recalibrate the function by adding a second variable: the ratio of C-12 to C-14 at any given point in time. But how could they know what the past ratios were?

The answer? They couldn’t, at least not without a time machine or some very old documents from an obscure scientist who was measuring radioisotopes way back when. They had two choices: Throw the entire Carbon dating system out, or make another assumption.

They opted for another assumption, the only possible assumption left: the age of the organism itself. In the case of dating a tree of relatively known age (based on its rings), they would have recalibrated the dating function with a C-12 to C-14 ratio that would have yielded the known correct age. And, as you would imagine, when calibrated using tree rings, Carbon dating yields fairly consistent results for anything wood or made from wood, provided it's not more than 4,000 years old. For it’s around this time that dates yielded by carbon dating and tree ring dating begin to diverge. Why this is so is another interesting topic, which I’ll touch on later.

But we’re not out of the rabbit hole just yet.

Carbon dating absolutely cannot be used to accurately date aquatic plants or animals because the amount of carbon in the ocean is vastly different than the amount in the atmosphere. This also makes Carbon dating useless for animals that eat seafood. Now here’s a fun question: What happens to the ratio of C-12 to C-12 if a living animal drowns in the ocean? Will Carbon dating be able to produce an accurate age? Probably not. The nature of animals is that they have multiple pathways to absorb elements from the surrounding environment (stomach, lungs, membranes). According to the Argonne National Laboratory,

Most carbon-14 is almost completely absorbed upon ingestion, moving quickly from the gastrointestinal tract to the bloodstream.

This would mean that Carbon dating would not be very useful in an area that was victimized by a flooding sea.

What about a land-dwelling plant that perished in such a flood? Would Carbon dating be able to yield an accurate date for it? Since plants get their Carbon-14 from photosynthesis alone, I think it's possible for Carbon dating to yield an accurate date for some of the larger plants, as thy would not quickly decay over the next several hundred years, whereas smaller plants would.

While Carbon dating can be a useful and valid method of dating a limited number of organisms, it lacks the robustness one would expect from alternative dating methods, such as tree rings or archaeology. There are other radiometric dating methods available, but they are also rife with similar problems. The bottom line: take such dates with a grain of salt, with the understanding that, while they could be correct, they could even more likely be completely wrong.

Read more about how carbon dating works and about Carbon-14 and dinosaur bones.