explainer – Clamsplaining

This is the second part in a series how isotopes work and how they are scientifically fascinating. Part I here.

It turns out a horse is not just a horse, of course. The horse is a collection of atoms, and each of those atoms has a particular isotopic “flavor”, and the collection of isotope types in the horse tells a story. At the end of the day, scientists are simply interested in reading and telling stories about our world. The tail….er, tale of the horse is written by myriad interacting processes in the universe which influence the horse’s stable isotope ratios.

As I mentioned last time, carbon-12 is much, much more common than carbon-13 is on our planet, due to nuclear fusion of helium-4 in the sun. there are nearly 99 carbon-12’s on earth for every carbon-13. But that’s the base ratio if you took our whole planet, put it in a blender and mixed it all up. If you measured a particular object, such as a horse, it likely does not follow that measure exactly. It has become differentiated from the global average by numerous factors which have altered the isotope ratio.

In isotopic chemistry, fractionation is our name for any process which creates a preference for a certain isotope. If chemical reactions had no bias toward any particular isotope, that 99 to 1 ratio of carbon-12 to carbon-13 would be present in literally everything including you and me. But it turns out that the biochemical dice are loaded- to make the ratio even more biased!

The enormous Rubisco enzyme. No one said photosynthesis was simple. Source: Wikipedia

Photosynthesis is the process by which plants take carbon dioxide gas in the atmosphere and “fix” it to make sugars, which they then use for food. The core enzyme responsible for this carbon fixation is called Rubisco (short for Ribulose-1,5-bisphosphate carboxylase/oxygenase). This enormous molecule is likely the most abundant enzyme on earth. And it turns out that it has a favorite flavor when it comes to the carbon it fixes into sugar.

In fact, the entire plant is discriminating against carbon-13 in several of the processes of photosynthesis. Carbon dioxide molecules diffuse more quickly into the plant’s leaves if they include the lighter carbon-12 rather than carbon-13. “Light” CO₂ also dissolves more easily in the plant’s fluids. But the biggest fractionation happens when the Rubisco molecule gets hold of CO₂ and breaks it. At each of these steps, the light carbon-12 is more likely to be used by the plant than its heavier siblings. There are various thermodynamic reasons for why this is the case, but the plant is essentially a sieve removing more of those heavy carbons at every step. At the end of the process, the plant is left isotopically “lighter” than the CO₂ gas surrounding it that it breathes in.

Because you are what you eat, this means that you are suspiciously carbon-light, and there’s nothing you can do about it. Should have thought of that before you decided to be dependent on plants as the factory for your carbon-based molecules. Next time, we’ll talk about how we measure this, and the kinds of science that can happen once you have a nice consistent measurement to use to compare isotopic ratios between samples.

As you may well know, every element is defined by its number of protons contained the nuclei of its atoms. Hydrogen has one. Carbon has six. This is non-negotiable. But every element can be found in multiple “flavors” known as isotopes. This flavor depends on the isotope’s atomic mass, which is determined by the number of neutrons present in the nucleus of that atom. Neutrons are kind of like atomic ballast. Unlike protons, which have a positive charge, they are neutral, but they do have a mass. Different isotopes have different numbers of neutrons, determining their atomic mass but preserving its particular elemental identity (which would only change if you changed the number of protons present).

Let’s focus on carbon, an element which I think about daily, though every element has isotopes and I could pick many other examples. Hope you’re OK with that, but if not it’s my blog so deal with it. So carbon has been found or created in up to 15 flavors. A whopping 98.9% of all the carbon on Earth occurs as carbon-12 (written as ¹²C), which has six protons and six neutrons, adding up to an atomic mass of about 12 atomic mass units (amu). It’s the most common because it’s the product of three helium-4 isotopes fusing together, each weighing 4 amu + 4 amu + 4 amu adding to make a single carbon-12. This is a very common reaction in stars, and because you are stardust, it is also the most common flavor of carbon in you.

But we make other flavors by adding neutrons. You can make carbon-13 with six protons and seven neutrons. This is a rare flavor, accounting for almost all of the remaining 1.1% of carbon found on earth. It is also the only other stable form of carbon. I note that it’s stable because all the other 13 known flavors of carbon are unstable, and many are only known from the laboratory because they are too short-lived to be found in the environment.

It turns out that if an element’s atomic nucleus is too light, or too heavy, that element will become radioactive and decay with time, continuously firing off pieces of itself out of frustration. Carbon-14 is the most famous and common of these radioactive isotopes of carbon, and it still only makes up 1 in every million million atoms of carbon on earth. Carbon-14 fires off particles and decays into nitrogen-14 because it is more stable orientation for the protons and neutrons to be in, for physics reasons I won’t get into here.

Carbon-14 does this in a very predictable, methodical pattern. It’s difficult to predict when an individual carbon-14 atom will do this, but if you take any object you have just created, like a piece of pottery, for example, you can be pretty much certain that in 5,730 years, only 1/2 of the carbon-14’s will still be present. The rest decided they’d rather be nitrogen-14. This is non-negotiable and you’d best learn to accept it. But it means that we can sniff out the age of a lot of interesting mysterious objects if we know the amount of carbon-14 present in the environment (which we often do) and measure the amount present in the object today. You have some restrictions. For example, for objects that are too old, too little of the carbon-14 would be left for you to measure accurately.

Carbon-14 dating, often just called radiocarbon dating, is very useful in figuring out the ages of stuff, but I’m mostly interested in the stable isotopes of carbon. Next week I’ll talk about why that is, and what kind of questions I can answer by looking at amounts of different stable carbon isotopes in a sample. See you then!