ácidos y bases

Ushuaia from the oven room window

Today was my Argentine lab-mate Daniella's birthday (Feliz Cumple!) and so the various members of different labs and offices nearby congregated in the kitchen to exchange gifts, eat delicious cake, and pass around bottomless cups of coffee and mate. This is the third birthday that's happened since I've arrived here, and I'm beginning to think that these aren't really birthdays. People just like to have cake every few days. One of the professors joked that I must be thinking along these lines, apparently there's just a high concentration of birthdays this time of year. He continued to say their new hiring criteria here is that your birthday isn't in July, presumable to prevent endless cake eating. In any case it can be nice to have a break every once in awhile from working in the lab and reading papers.

A bone sample in acid solution

Over the last week and a half I've been checking the samples everyday. To measure trustworthy dietary information from bones, there are a few steps that need to take place. Like I mentioned previously, I'm interested in the bone collagen since collagen exclusively records diet. In order to get at that collagen we need to first dissolve the bone mineral around it (bioapatite for short), in a process known as demineralization or acidification. For smaller pieces of bone, like the ones I'm demineralizing, you use a fairly weak concentration of acid, just 0.5 molar (M) Hydrochloric acid (HCl). (For comparison, the concentration of HCl in your stomach is around 0.1 M.) This slowly dissolves the bioapatite so you can prevent it from dissolving the collagen, too. After 24 hours the HCl has completely reacted with bioapatite, releasing carbon dioxide gas, so a new acid solution must be applied until nothing but collagen is left. (Don't screw the caps on completely, that gas will eventually escape catastrophically, blowing those caps off in the middle of the night). Others will use a weaker concentration of HCl, 0.2 M, though this will take longer. I could dissolve the bones in a few minutes with the raw 12 M HCl, but it would likely take the collagen along with it. It took two days with the 0.5 M HCl for my bone samples to be reduced to collagen, which retains the shape of the original hard bone, but is now soft, spongy and rubbery.

Humic material extracted from old seal collagen

Most people tend to wash their hands if they put them in the dirt after a few minutes. What do you do if they've been sitting in dirt for several thousand years? As the soil scientists at UC Merced can attest to, soils are home to quite possibly the most complex and unknown ecosystems on Earth, hosting vastly diverse organisms and an incredible number of species. These microbial communities often vary considerably from place to place, seasonally, and even diurnally (thoughout the day). The most significant byproduct of soil microbes, or the byproducts that can create untrustworthy stable isotope values in any case, are humic acids, a general term that characterizes the extremely complex (and acidic) organic compounds that are excreted by these microbes. Apply a weak base for several days though and you rectify several thousand years of microbial waste. After some rinsing of the remaining collagen with water and application of 0.1 M Sodium hydroxide (NaOH) for several days, your collagen bits are as good as new! Each individual bone varies in how dirty it is, so those with very little humic acid will be ready after 24 hours, others might take up to five days.

Samples about to be dried in the oven

The next step in the process is to dry the collagen samples. Now that the collagen has been sitting in liquids for the past week and half or so, they're quite waterlogged. What better way to dry off than to be cooked at 55⁰ C (135⁰ F) for 24 hours. Afterward, the samples are ready to weighed to account for the collagen percent yield, which tells us how well preserved the bones are (which hopefully will be around 25% for every sample). Then I will weigh a tiny amount (1 mg, from original pieces of 150-250 mg) into 3x5 mm tin boats, ready to be analyzed by the element analyzer and mass spectrometers in the Fogel lab back at UC Merced.
 
So far I've been limited to 82 samples, since that's how many plastic vials I have. Now that those plastic vials are mostly free again, I'll start this process again with the remaining unprocessed 78 seal bones. Hooray!

When not working in the lab, I've been discussing background material and general ecological and archaeological information related to my project with Pancho, and possible directions where I can fill in gaps in our understanding of the evolution of this ecosystem over the last several thousand years. In a future post I plan to talk about the ecology of Tierra del Fuego and some ideas I'm playing around with. Stay tuned!