Nitrogen 15 - you are what you eat

We have seen how ¹³C / ¹²C isotope ratios (called δ¹³C) tell us about how much marine food was in human diets.

Nitrogen also has a main isotope ¹⁴N (99.636%) and a small amount of a stable isotope ¹⁵N (0.364 %)

The level of the ¹⁵N isotope in bone collagen is linked to the accumulation of ¹⁵N from eating protein. Protein contains amino acids, which are the source of the nitrogen. As you go up in the food chain the nitrogen in amino acids is passed upwards, leading to high ¹⁵N levels accumulating in your own protein, including collagen in your bones. The level up the food chain is called the trophic level.

Only a few milligrams of collagen are need to do this test. The instrument used is an Isotope Ratio Mass Spectrometer. It can resolve the mass of a compound with an accuracy of 0.5 parts per million!

Plotting δ¹³C and  δ¹⁵N  on a scatter diagram gives us information about how high up the trophic level and how marine it is.

NOTE: The ¹⁵N/¹⁴N ratio is called δ¹⁵N "delta 15 N"
The delta notation is exactly the same as for δ¹³C. It means the difference between the ratio ¹⁵N/¹⁴N in the sample and an internationally agreed reference. The δ¹⁵N values in the chart below are positive. That means they are above the international standard, while the δ¹³C values are negative, which means they are below the international standard.





Fig.6.1    ¹⁵N levels show how high up the food chain you are (the trophic level)


Collagen in bone is often well preserved even from as far back as Neanderthal times (up to 250,000 years).



Fig 6.2 Neanderthal vs modern human δ¹³C vs  δ¹⁵N.

Fig. 6.2 shows that we eat a higher proportion of animal protein with high trophic levels and also a higher proportion of marine protein than Neanderthals.

This diagram comes from an American scientific paper. It is aimed at university level scientists, but it is useful to see what a science paper looks like. Check out how many references are used!
http://www.pnas.org/content/106/38/16034.full

Questions

1. What can Fig. 5.4 tell you about Neanderthal life?

2. Are these groups of Neanderthals and modern humans completely distinct?





 

CLICK FOR ANSWERS



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