A proxy is an indicator for something. For example, the density of  a sediment core could be a proxy for porosity. Isotopes are atoms of the same element with differing number of neutrons. There are three isotopes of oxygen. ¹⁶O and ¹⁸O are present in the air and the oceans.

The ratio of ¹⁸O to ¹⁶O gives us an estimate of what the ocean water was like through time. Organisms that build their tests from ions in ocean water record the amount of ¹⁸O and ¹⁶O that was present during their life cycle. The ratio of ¹⁸O to ¹⁶O in the tests of ocean organisms acts as a proxy for ice volume. How? ¹⁶O preferentially evaporates because it is the lighter of the two isotopes. Since it’s easier to evaporate the smaller isotopes, rain/snow are high in ¹⁶O. This would make glaciers ¹⁶O – rich too. Given the aforementioned facts, ocean water is higher in ¹⁸O during glacial periods, and one would also expect ice volume to be greater when ¹⁸O values are higher in ocean water and ocean organisms that build their tests from the water. The ratio in the critters also acts as a proxy for ice volume.

On an unrelated note, sunset was stunning this evening. I could tell the ship had gained some latitude because sunrise was after 21:00 tonight!  I walked out on the deck at the perfect time to see the sunset, a gorgeous glitter path AND crepuscular rays! Some of my shipmates beat me to it. I think that everyone got a chance to look.