Saturday, June 04, 2011

A sweet little story of science at its best

How can one tell anything about the Earth's past climate from the compacted muck of the ocean floor?

Here’s one way.

Oxygen atoms come in basically two kinds -- isotopes -- oxygen 16, with 8 protons and 8 neutrons, and oxygen 18, with 8 protons and 10 neutrons. About 99.7 percent of naturally occurring oxygen is O16 (I should use superscripts for the 16 and 18, but I won't).

O16 is lighter than O18, by a bit more than 10 percent.

Water, H2O, is a molecule with one oxygen atom and two hydrogen atoms. A water molecule with an O16 atom is lighter than a water molecule with an O18 atom.

The water vapor in the atmosphere comes mostly from the sea, by evaporation. In effect, the energy of sunlight lifts molecules out of the sea into the air, the same way water in an open pan evaporates in a warm room.

O16 based molecules, being lighter, evaporate a bit easier, and are less likely to fall immediately back into the sea. So the ratio of O16 to O18 molecules in the air is slightly more in favor of O16 than in the water, where the heavier molecules are more likely to be left behind.

The water in the air eventually falls as rain or snow onto sea or land. If on land, it flows in rivers back to the sea. So the original mix of O16 to O18 in the sea is maintained.

But! If the snow that falls on the land doesn't melt, because the climate is colder, and compacts into ice -- glaciers -- it doesn't make its way back to the sea and instead piles up on the continent. Which means the prevalence of the heavier isotope O18 in the sea increases slightly but measurably.

Planktonic organisms in the sea build their skeletons out of the atoms in the sea, so the ratio of O16 to O18 in their bodies is the same as in the sea at the time they are alive. They die and their skeletons fall to the sea floor, eventually, over tens of millions of years, building up hundreds of meters of sediment.

The JOIDES Resolution drills into these deep compacted sediments and brings up long "cores," representing millions of years of Earth history. Paleoclimatologists (like Mo her grad students), painstakingly "pick" the microscopic skeletons out of the columns of sediment and use an instrument called a mass spectrometer to vaporize the fossil skeletons and separate their atoms by weight -- including O16 and O18, the ratio of which reflects the volume of continental ice at the time the creatures lived. An exquisite continuous record of ice ages of the past!

But how does one determine the age of the sediments? Ah, now that's another story. More later on.

Here is a pic of the ship's superbly equipped core lab.