First snowfall last night. Only a slushy half-inch on the ground this morning, just enough to set the Bahamas meter ticking. A beautiful walk, however. The sky just fading from black to a pale indigo. The leaves crisp under a frosty rime. I'm not the first one on the path; someone with a dog has tracked before me.
Here's a little snowy reflection for you.
The ocean are are full of water, H2O. Most of the water molecules contain an atom of oxygen with 8 protons and 8 neutrons, oxygen-16. A tiny but precisely measurable percentage of the water molecules contain an oxygen-18 atom, an isotope with two extra neutrons.
When sea water evaporates, the slightly lighter O-16 water molecules are preferentially lifted into the atmosphere, leaving the ocean slightly richer in O-18 water.
But the water in the atmosphere soon falls back into the sea as rain. If it falls as rain on the land, it runs back to the sea. In either case, the equilibrium isotopic ratio of O-16 and O-18 is restored.
But if, as during an ice age, the water in the atmosphere precipitates as snow and stays on the continents as ice, then the oxygen isotopic ratio of sea water is slightly tipped toward O-18 -- with more of the lighter O-16 water molecules trapped on the land.
Microorganisms that live in the sea build their bodies with sea water, and the oxygen isotopic ratio in their tiny skeletons is the same as the water in which they live. When they die, they fall to the ocean floor and build deep beds of sediments.
Along comes my daughter (and her scientific colleagues) and pulls up long cores (cylinders) of the sediments, representing millions of years of microorganisms living and dying. She picks the skeletons out of the muck, and determines their oxygen isotopic ratio with an instrument called a mass spectrometer. The ratio varies as she goes down the core and back in time, as the glaciers come and go.
Plot the data, and one has a record of continental glaciation going back millions of years -- buried on the bottom the the sea!