It is relatively easy for us to work out how much oxygen is found in the air available on the planet today but determining what the air was like millions of years ago is considerably more difficult. Researchers have traditionally looked at the surfaces of iron-bearing rocks and studied how much the iron has rusted. While this works, it is not ideal and the search has been on for a better method. Now a team is revealing that they have found one in the form of salt.
When ocean water evaporates, beautiful cubic salt crystals get left behind and drops of water are sometimes trapped inside them. The researchers behind the new work speculated that tiny traces of ancient air might get trapped with the water inside these crystals and that analysis of this air might allow them to identify how much oxygen, nitrogen and carbon dioxide was present in the atmosphere long ago. They tested this idea out in the lab with salt crystals that they collected from numerous sites all around the world and the results are simply sublime. You can read more in The Economist article that I wrote on this here.