Vuelva a Ushuaia

Ushuaia Bay upon landing

Hello once again!

I figure it's a good time to start writing again since it's been several months since I last updated the blog. Quite a bit has happened in the intervening months. I've ran the South American fur seal samples I collected and they've provided some puzzling results. I measured the carbon and nitrogen stable isotope from the bits of processed seal bits (collagen if you recall from my previous posts). Stable isotopic values of various elements will show you varying characteristics of the world based on how they cycle through biogeochemical systems (and an animal's biochemistry). Carbon and nitrogen are particularly useful for measuring ecological characteristics, like finding out one's trophic level (if you are a vegan, vegetarian, or a meat eater.)

When interpreting the results of these isotopic values, there isn't much of a basis for comparison if you have nothing to compare against. In classical studies, you need to know what the values are at the base of the food chain as well as potential dietary components of your study animal. Even without this info, there are some inferences we can make. There are some noticeable differences between male and female fur seals from 6400 BP, but age did not seem to have a large effect. Most of the fur seals seem to cluster together toward a lighter carbon isotopic ratio.

Pulled from the latest version of my proposal

Based on literature values of fur seal dietary components from modern collections, I can see if diet corrected values of my ancient seals fall within expected dietary boundaries of modern fur seals. Essentially, you connect the dots of the most extreme dietary values, creating a diet polygon, and should see your animal's diet corrected values fall within the polygon. From there you can create an isotopic mixing model to see which dietary components contribute the most to the animals diet. For example, I should be able to see if the seals are eating mostly squid or mostly fish.

Alas, this is not what happened when I compared modern diet to ancient animal. There has clearly been some kind of shift over time as none of my seals fell within the diet polygon. Modern dietary components are much lighter in carbon than the more recent seals. Something has changed in the carbon. One might argue "Hey wait a sec, what about the Suess effect?"

The Suess effect is a change in the ratio of the atmospheric concentrations of heavy isotopes of carbon (¹³C and ¹⁴C) by the admixture of large amounts of fossil-fuel derived CO₂, which is depleted in ¹³CO₂ and contains no ¹⁴CO₂.

However, I had already applied a Suess affect correction based on literature values, and not including this correction would have actually made some of the individuals fall within the diet polygon. In any case, I have more work to do grappling with this issue and doing some ANOVAs.

Analysis of variance (ANOVA) is a collection of statistical models used to analyze the differences among group means and their associated procedures (such as "variation" among and between groups), developed by statistician and evolutionary biologist Ronald Fisher.

Ok enough Wikipedia citations. My guess at the moment is that there's been a shift in the base of the food web over the years and seals are probably eating roughly the same things that were 6400 years ago. I won't be able to find out whether that's true or not until I find some way to measure the base of the food web, either by looking directly at animals near the base of the food web (preserved in the archaeological record) or by looking doing compound specific amino acid stable isotope analysis of seals over time. The idea is that certain amino acids do not change in as they move up the food web as they undergo minimal chemical reactions (aka source amino acids). I just got to try out this technique before leaving Merced, and while it's certainly time intensive I plan to do a lot more of it when I return.

Also I will be leaving for fieldwork (ie. doing Indiana Jones style excavation of sites) in two days at a place called Moat Bay for almost a month. I'll be taking lots of pictures, but I won't be able to post them until I return. Expect some lots of photos in the future!