In some regions, strong ocean currents sweep nutrients up from the bottom to feed a thriving population.
Called upwelling, the phenomenon drives plant and animal populations up until the nutrients are all used, and the microscopic plants and animals die.
The ratio of these different types of oxygen in the shells can reveal how cold the ocean was and how much ice existed at the time the shell formed.
In general, the shells contain more heavy oxygen when ocean waters are cold and ice covers the Earth.
The most valuable fossils found in sediment cores are from tiny animals with a calcium carbonate shell, called foraminifera.
One species of foraminifera lives in the icy waters of the Arctic above Iceland and near Antarctica.
When Mc Manus and other scientists began to uncover a large number of fossils of polar foraminifera in cores collected off the coast of Great Britain as part of an ongoing research project, they knew that the waters there had once been much colder.