Originally published 7 July 1986
In recent years, geologists have made spectacular progress discovering the Earth’s past climate. In particular, they have established a reliable chronology for the ice ages.
Only a few years ago we were taught that four great ice ages affected the Earth over the past few million years. We now know that there have perhaps been 20 episodes of continental glaciation, occurring at regular intervals of 100,000 years.
Researchers use a variety of clues to reconstruct past climates. One of the cleverest and most useful is the method of oxygen isotopes, invented by Cesare Emiliani in the mid-1950s. The full potential of that invention is only now being exploited. It is hard to pick up a paper on climatology these days that does not make reference to the method. It is a fascinating story, and it perfectly demonstrates the challenge and rewards of scientific research.
But the story has several twists and turns, so bear with me. The story begins with isotopes.
There are two kinds of oxygen atoms. The most abundant form is oxygen-16, with eight protons and eight neutrons in the nucleus of the atom. A tiny fraction of oxygen atoms — two atoms out of 1000 — have two extra neutrons in the nucleus (oxygen-18). The extra neutrons change the weight of the atoms, but not their chemical properties. Atoms that differ only in the number of neutrons are called isotopes. Oxygen-18 is the heavy isotope of oxygen.
Emiliani’s clue
Every molecule of water contains one oxygen atom. The water molecules that contain oxygen-18 are slightly heavier than the others. When sunlight evaporates water from the surface of the sea, there is a tendency for the heavier water molecules to be left behind. And if the heavier molecules are evaporated, they tend to fall more quickly back into the sea. So as water is evaporated from the sea, the sea becomes enriched in oxygen-18.
Much of the water that is evaporated from the oceans is carried by winds onto the continents where it falls as rain or snow. If the water makes its way back to the sea, then the balance of isotopes in the oceans is restored. But if snow becomes locked up on the land in the form of ice, the oceans remain enriched in heavy oxygen.
So the relative abundances of the two oxygen isotopes in sea water depends upon the amount of ice that is stored on the continents. More oxygen-18 means more ice. Less oxygen-18 means less ice. This was Emiliani’s clue to the ice ages.
Unfortunately, samples of sea water from the remote past are unavailable for analysis. But microscopic marine organisms build their skeletons with oxygen, calcium, and carbon taken from the water. The abundances of oxygen isotopes in the skeletons will be the same as in the water. When the organisms die, they fall to the floor of the sea and the skeletons accumulate as fossil sediments. The isotopic compositions of the fossils reflect conditions in the sea at the time the organisms were alive.
Geologists have invented techniques for recovering sediments from the deep ocean floor. A device called a mass spectrometer can be used to assay the relative number of oxygen-18 and oxygen-16 atoms in fossils from the sediments. The age of the sediments can be determined by using radioactivity, magnetism, and the fossils themselves. Combining the isotope abundances and the age of the sediments, a continuous calendar of the ice ages can be constructed. So millions of years of glacial history have been recorded in sediments on the bottom of the sea.
Making connections
The most controversial part of this story is the assumption that marine organisms incorporate oxygen-16 and oxygen-18 into their bodies in the same proportions as those atoms exist in sea water. There are several reasons why this might not be strictly true. But most geologists believe the oxygen isotopes tell a convincing story of the ice ages.
James Burke called his TV series on science and technology Connections. The title is appropriate. The best science is always a matter of making unexpected connections. Who would have guessed that there is a connection between the thickness of arctic ice and the skeletons of microscopic creatures that live three miles down on the floors of tropic oceans? But the connection is there, for the researchers who were clever enough to find it and exploit it.
I recently had the opportunity to look through a microscope at the fossils of those little planktonic creatures from long ago, dragged up from the floor of the deep ocean. In the delicacy and variety of their skeletal architecture they are remarkably beautiful. A census of their atoms is an equally delicate and beautiful indicator of the Earth’s past climates. In death the organisms became a record of their environment.
