As we describe this project, we refer to the “chemistry” of tooth enamel. Chemistry can mean a lot of things, so what does it mean in this context?
Let’s start with tooth enamel.
Tooth enamel is composed of a mineral called apatite. I spelled that correctely. Apatite is the mineral – a geology thing – whereas appetite is the gee-I-want-to-eat thing that we’re more familiar with.
Apatite is a phosphate mineral, meaning that it has in its matrix the phosphate unit, PO<sub>4</sub>. Its chemical formula is written as: Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH,F,Cl)<sub>2</sub>
Apatite is really a mineral group that always contains calcium (Ca) and phosphate, and may also contain the hydroxyl group (OH), fluorine (F), and chlorine (Cl). To keep ourselves sane, I just refer to the mineral in teeth (and in bones) as bioapatite.
The chemistry we are looking at is the phosphate unit, and things that substitute in and replace the phosphate unit, especially carbonate, CO<sub>3</sub>
Oxygen, in both the carbonate and the phosphate components of enamel comes mostly from the water that the animal ingested. That water reflects the environment (temperature, rainfall, humidity) in which the animal lived. Carbon, found only in carbonate, comes from the foods that the animal ate.
Because tooth enamel is solid mineral, once its formed, the chemical signature of the oxygen and carbon are fused in and remain unchanged through the life of the animal and usually even after its fossilized. Bones have bioapatite in them, but also have lots of organic components, which is why they can grow and heal. This also means that the chemical signature of oxygen and carbon in bones are usually altered over the course of the animal’s life and during fossilization.
The way we get information about water and diet from oxygen and carbon is by measuring the relative amounts of the different kinds of oxygen and carbon in the apatite.
Whoa turbo. Did you say different kinds of carbon and oxygen?
Yes, that’s exactly what I said. And you probably know this already, just didn’t know you knew.
There’s this word out there that strikes fear into many. The word is “isotope.”
No, you don’t need lead shielding. Every atom of oxygen and carbon or any other element is an isotope. It’s just that some are radioactive and some aren’t. You probably know about carbon-14. It’s radioactive and used for ‘carbon dating.’
What you probably don’t know is that there is carbon-12 and carbon-13 as well. These are stable isotopes. These aren’t radioactive and compose more than 99% of the carbon in the universe.
Oxygen also has three isotopes: oxygen-16, oxygen-17, and oxygen-18. They’re all stable.
In our analyses, we measure the relative amounts of oxygen-16 to oxygen-18 (or carbon-12 to carbon-13). The relative amounts of the isotopes of oxygen tell us about the evaporation of water, which relates to temperature. The water that’s evaporating is the water in ponds, lakes, and streams from which the animal was drinking.
We can measure the oxygen amounts from both the phosphate and the carbonate, but the carbonate is tons easier and safer and gives us information about the relative amounts of carbon-12 and carbon-13.
The carbon in tooth enamel comes from the diet of the animal and ultimately from the plants that are eaten. The relative amounts of carbon-12 and carbon-13 in plants are mostly dictated by the plant’s metabolism, and by both aridity and tree cover.
When we analyse both carbon and oxygen together, we can learn a lot about an enviroment.
And now we’re back to the point of this project. The ‘best’ tooth to use is the third molar, but we aren’t sure if other teeth in the jaw can provide an adequate representation of the animal’s environment. By analyzing third molars and incisors (and other teeth) from a single jaw, we can be confident that all the teeth grew in the same environment, so arguably, they should contain the same amounts of carbon and oxygen.
But there are reasons why different teeth might differ in their chemistry. That feels like another post..