From the Bookshelf of The Transition Movement…

➤  An isotope is an atom — an elementary particle — with varying numbers of neutrons. Even the “normal” variation is an isotope; it’s just going to be the most common variation. Thus hydrogen comes naturally in three varieties (with ¹H being that “normal” version), and four additional human-made varieties. Since neutrons aren’t charged, this affects the element’s chemistry only in subtle ways.

➤  One important isotope is that of oxygen, because it can start off in H₂O, and have subsequent effects seen in CaCO₃ and CO₂. “Normal” oxygen has eight neutrons, and since it also has eight protons, it is known as O-16 (or “¹⁶O”). The heavier O-17 (one extra neutron) and O-18 (two extra neutrons) isotopes are also stable.

➤  The extra neutron(s) also makes any water containing the heavier isotope heavier, which has the critical result that it evaporates with a little more difficulty than “normal” water (for more information, see the Wikipedia article on kinetic fractionation).

➤  This is crucial: because heavy water is more reluctant to evaporate, seaborne clouds will have fewer of the heavier oxygen isotopes, while the remaining seawater has relatively more.

➤  Since all the precipitation that eventually ends up on land originally comes from those clouds, that precipitation is isotopically lighter. Which means that snow has that relatively “lighter” attribute, too, and so do glaciers, and those huge ice sheets created during ice ages.

➤  So the amount of the planet’s water that ends up “stored” as ice or snow on land is therefore directly correlated with the varying ratio of oxygen isotopes left in the ocean. Woo-hoo!

➤  How do scientists discover that difference? That (slightly isotopically heavier) oxygen is taken up by billions and billions of microscopic sea creatures (the Foraminifera) to create their shells, commonly out of calcium carbonate. That’s CaCO₃, with three oxygen atoms and thus a few dozen chances of having one of those slightly-more-likely-to-be-heavy oxygen isotopes.

➤  As those microorganisms die, their shells remain and accumulate. When the layer of sediment they accumulate in is compressed into rock over geological time, we end up with limestone, such as the stuff the pyramids in Egypt are made out of. But for our purposes —

➤  When scientist dig up core samples of the sediments deep in the ocean, they can analyze the variation in the isotopic ratio of oxygen, and thus determine the varying ratio of water still in the oceans versus ice that was present in that era on land.

➤  The correlation can be validated by comparing the results to ice that has been stuck in ice sheets or glaciers for hundreds of thousands of years. See the Wikipedia page on ice cores for more information. Both the water (H₂O) and air (O₂ and CO₂) in tiny bubbles in the ice has oxygen than can be compared, as well as dust, pollen, etc., that lets us determine age in multiple ways, which are examined for consistency.

➤  Once the scientific community has debated and refined that methodology, it has been validated. Then results from the sediment samples which are far older than any of the extant ice cores can be considered used in further analysis.