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Carbon 13 - the fishy isotope - answers page


1. If you were trying to verify the isotope ratio data in Fig. 5.1, what questions might you ask?

I would want to know where the bones were found; in coastal or inland sites.

2. What might cause a drop in fish eating during neolithic and Saxon times?

One theory is that the rise in agriculture meant that communities did not need to travel to hunt for marine fish. If they had all the fresh protein they needed, there was no need to risk traveling to hunt wild food. Another theory is that cultural taboos grew up about fish eating. The arguments continue.

3. Why do you think bone collagen survives intact for so long?

Collagen in bone is protected by being enclosed in mineralised bone. This protects it from the environment. Samples can be demineralised in the laboratory to leave just the collagen protein molecules.

4. What is the mass of a CO2 molecule where the carbon atom is 12C?

12 + (2 x 16) = 44

5 .What is the mass of a CO2 molecule where the carbon atom is 13C?

13 + (2 x 16) = 45

6. Measuring the mass/charge ratio of CO2 will show up the difference of one neutron mass between the two isotopes of carbon. What is the ratio of masses?


7. Can you think of something that might mess up the results? (Keep thinking about isotopes)

Oxygen also has stable isotopes  16O and 18O.  So the ratios for the CO2 molecules could be 44/45 or 44/47 or 44/49 or various other combinations.  Most oxygen is 16O, but analysts need to be aware of the problem.

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Fig. 5.3

Fig. 5.4

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