Carbon-12

And you can look at it in two ways, but I'll do it from the basic principle of photosynthesis first.

When it comes to photosynthesis, algae and plants prefer to use carbon 12.

Carbon 12 is much lighter and they use it to make sugar.

That's the bottom of the food chain.

They can't really use carbon-13, but it is in the atmosphere.

These are atoms of the same element with different masses because their nuclei contain different numbers of neutrons.

Carbon-12 and carbon-13 are both carbon, but carbon-13 is slightly heavier.

Hydrogen also has a heavier stable isotope called deuterium.

Methane samples can therefore be examined for variations in both their carbon and their hydrogen.

And all those data sets are recording values of Delta 13C, which simply put, it's an isotopic signature.

It basically tells you what the ratio is between 13C and 12C in the atmosphere.

Okay, so now just a little bit about isotopic signatures of various sources.

Fossil fuel carbon carries the lightest isotopic signature.