Carbon 13 - the fishy isotope

All life on Earth is based on carbon.
Carbon has two stable isotopes. ¹²C, which makes up 98.93% of carbon and  ¹³C, which makes up 1.07%.

Note: ¹²C is pronounced "C twelve"

Sea water contains more ¹³C than fresh water. The higher ¹³C ratio passes through the marine food chain so that fish have more ¹³C  than say pigs or chickens. Humans who eat a lot of fish end up with more ¹³C in their bones than those who don't.
 
When this was discovered, archaeologists went back to hundreds of bone samples and tested for ¹³C levels.



Fig. 5.1 Carbon 13  levels across history

Fig. 5.1 shows how carbon 13 levels have varies over time. The y axis shows the ratio of ¹³C to ¹²C. A high ratio indicates a high level of fish consumption.

NOTE: The δ¹³C notation on the y axis is used for this type of data. The quantities of C-13 are very small. It is easier for scientists to compare them to an internationally agreed standard ratio of C-13 to C-12. So if δ¹³C = -21 it is 21 parts per thousand less than the international standard. Most δ¹³C values are negative. This looks odd, but all it means is that most C-13 levels are below the standard.

This data shows that the consumption of fish went up and down through history. It looks like the Romans liked their fish, but the Saxons did not. Was it because agricultural communities did not need fish and were happy to stay at home and use farmed animals for protein? Was it a taboo about eating fish? All these questions need to be considered by looking at other historical evidence, but without isotope analysis it is true that these questions might never have been asked.

How do you measure isotope ratios when the difference between two isotopes is ONE NEUTRON?

The answer is Isotope Ratio Mass Spectrometry.
Collagen from bone is burnt in a furnace to produce CO₂. The CO₂ is ionised to give it a charge, then accelerated in a vacuum through a magnetic field. The magnetic field bends the path of the CO₂. ¹³C is one neutron heavier than ¹²C so it curves a tiny bit less.
The device is measuring the mass/charge ratio. The more mass the less it will bend. The more charge, the more it will bend.




Fig. 5.2 Isotope Ratio Mass Spectrometer http://en.wikipedia.org/wiki/Isotope-ratio_mass_spectrometry

Questions

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

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

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

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

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

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)

 

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