The Zen of Isotopes

I am master of the world’s smallest spoon. To be spoon master, one’s hand must be steady. My little spoon with its few grains of silt and sand must be placed into what seems like the world’s smallest capsule of foil. Once the sediment is inside, that capsule gets transferred to the balance. If it doesn’t weigh 15 milligrams, sediment grains get carefully added or removed. The master must not knock the spoon or the capsule at any point in the process.

Remember the board game called operation that you played as a kid? If your jittery hand went slightly astray while you were working on the ‘patient’, the buzzer went off and you lost?  It’s like that.

This life of measuring and weighing in the lab is necessary for anyone who wants to use stable isotopes to help get information about creatures or ecosystems. By taking only a minute sample of our material of interest, we can answer big questions about organisms or ecological processes. At Raincoast, we have successfully used this technique to learn about diet in bears and wolves, and I am now using it to learn about historic salmon abundance.

While it’s easy to get bogged down in interpreting and analyzing what stable isotopes are actually telling you, the reason for their widespread use is fairly straightforward. Lets take Nitrogen.

99% of the world’s nitrogen atoms have 7 protons and 7 neutrons. This is the common 14N isotope. However, 1% of nitrogen has 7 protons and 8 neutrons (15N isotope), making it heavier and generally harder for organisms and processes to use. The preference for the lighter isotope leads to ‘enrichment’ of the heavier one.

Marine environments, and therefore salmon, tend to be more enriched in 15N than their terrestrial counterparts. When looking for past signs of salmon in the environment, this allows us to hunt for the heavy nitrogen signal left behind (in this case) from their decayed bodies. I’m tracking 15N in the centuries of sediment layers at the bottom of a lake. Ideally, the amount of this heavier isotope should be proportional to the abundance of salmon. This gives us a tiny window into what historic salmon abundance was really like.

Back in the lab, once the little foil capsule has the correct amount of sediment, it gets carefully rolled into a little ball. From there, it joins hundreds of other little balls of foil and they head to the mass spectrometer. Inside, combustion occurs, and I get readings that tell me about carbon and nitrogen “signatures” of the sample.

Mostly all of this requires patience. No dreaming of the field, and no cups of coffee. The spoon master works on.

Misty MacDuffee
April 2006
From a lab at the University of Victoria

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