Hair Chemistry and Geography Project

First of all, a big thank you to everyone who has sent hair my way for analysis. The project is starting to wind down and enough data have been gathered for me to share some information with you.

Let’s get some background out of the way first. I’m looking at a strontium (Sr) isotopes in hair. Animals don’t have much use for Sr. When it shows up it’s usually because it was mistakenly grabbed instead of a calcium atom (notice how Sr sits right under calcium on the periodic table. As you might imagine, our hair doesn’t have a lot of calcium in it and it has even less strontium. It is sometimes easy for people who are not used to looking at isotope data to confuse concentrations of an element with isotopic composition of an element. I’m not presenting any information about Sr concentrations in hair in this post. I’m presenting information about the amount of 87Sr to 86Sr (87Sr having an extra neutron compared to 86Sr). Instead of just a ratio, which might be more intuitive I’m presenting things in a format called E87Sr. I’m not going to define it here, but just know that low numbers mean less 87Sr relative to 86Sr and high number mean more 87Sr relative to 86 Sr.

Then why would I look at Sr? Why not look at an element that hair has plenty of, like nitrogen or oxygen? Good question. What element you should look at depends on what question you want to ask. If your question is, was this animal an herbivore or a carnivore, then nitrogen isotopes are a pretty useful thing to look at. If you want to know where the mammal was when it grew the hair, nitrogen isotopes are not particularly useful. Nitrogen isotopes don’t vary in any systematic way over large geographic spaces.

It turns out that oxygen is a pretty decent element to look at if you what to know the provenance of hair. But there has already been a lot of excellent work done on oxygen isotopes in hair. In order to help refine the ability to trace a mammal to a more precise location, it would help to have another isotope system. Preferably one that didn’t change based on precipitation patterns (which is what drives the isotopic differences of oxygen isotopes in hair).

Sr is a well-known isotope system that doesn’t fractionate based on rainfall patterns. The hypothesis that’s out there is that differences in the geology underlying particular places will cause differences in Sr isotopes values of hair from animals in those different regions (Figure 1 from Beard and Johnson, 2000). That’s basically what this study is testing.

Figure 1

Question 1. Where does Sr in hair come from?
Basically there are 3 possible answers. Food, water, dust. For now, let’s ignore dust. Is there a lot of Sr in water? Not really, in some karst aquifers (limestone areas) there might be a fair amount of Sr in the water, but it pales in comparison to how much Sr is in food. There is the possibility that maybe an animal’s body preferentially absorbs the strontium dissolved in water over that in food. It would be nice if animals got most of there strontium from water because typically people drink water from local sources, making it easy to tie their Sr isotope values from hair to where the hair was grown. But the reality is that most of the Sr available to animals probably comes from food. For humans, food might be a problem because we are known for moving food around all over the place. It might be that everybody’s hair all looks the same (in terms of Sr isotopes) because the food supply is so homogenized.

Figure 2

Question 2. Does hair from different places have different Sr isotope values?
If food homogenization is a big problem, hair from all areas will look the same regardless of the underlying geology. Is hair from people in different areas different? Yes. See Figure 2. These data are all from the United States and Canada. They might not as different as we would like, but hair E87Sr is varying as we move from west to east.

Question 3. Is there a local signal present? There looks to be at least some local signal present. Compare figure 1 and 2. There are some similarities, note the generally low values in the west and the generally high values in the east. Besides, if not a local signal what else would be driving differences in hair E87Sr?

Question 4. Does the Beard and Johnson model work? Not very well. You see some general correlations, as mentioned before, but when I pinpoint hair samples on their model and plot them versus the actual values measure, I don’t see a very good correlation (Figure 3). The slope of the fit line is about 0.44 and the R2 value is ~0.3.

Figure 3

Question 5. Can you do better? Probably, if I create a new model based just off of the data I’ve collected then presumably it would be better. An example of that model is in Figure 4. Underlying geology seems to play a role but some other factors, possibly involving food distribution must be at work.

Figure 4