Tuesday, June 14, 2011

A matter of scale


Remember that "sweet little story" I told back on June 4 about how Mo uses the ratio of oxygen isotopes O16 and O18 in microscopic fossil organisms from ocean cores as a proxy for the Earth's average temperature? I was thinking about that again as we sailed across the Caribbean Sea to Curacao.

Water, water everywhere, as far as the eye can see, in every direction. Three-quarters of the planet's surface covered in water three or four kilometers deep. A seemingly endless vastness of water. H2O.

Most of those water molecules have an O16 oxygen atom, because those are the kind most commonly cooked up in stars. About one water molecule out of 400 has an O18 atom with two extra neutrons.

Now pause here and go back and read again that "sweet little story."

OK, here's what I was thinking about, and a little back-of-the-envelope calculation.

How many water molecules in a thimbleful of water? Well, it's a big number, but here's a way to think about it. There are roughly as many water molecules in a thimbleful of water as there are thimblefuls of water in all of the oceans of the Earth.

Molecules are almost unimaginably tiny, oceans almost inconceivably large. Standing on the deck of the JOIDES Resolution I tried to imagine the unimaginable number of thimblefuls of water that reached to the far horizons and beyond. And the delicacy of parsing the atomic constituents of microfossils.

What a difference in scale!

A mass spectrometer counts the O16s and O18s in a vaporized microfossil retrieved from ocean sediments, a ratio that is pretty much the same for all of the Earth's oceans at the same moment of geologic time. A ratio that stands as a satisfying proxy for the average temperature of the Earth.