Lots of coastal sand in a layer of sediment far from shore means that the surface of the sea must have been very cold when the sediment was deposited. Only icebergs and sea ice carry this type of sand to the middle of the ocean. When forams die, their shells drift to the ocean floor and are preserved in the sediment. Because different types thrive in different water temperatures, when scientists find foram deposits they can estimate the temperature when the animals lived.
Forams take up two kinds, or isotopes, of oxygen to build their shells.
When the water is warm, they use more of one type than the other. Measuring the relative levels of these oxygen isotopes in the shells provides another way of estimating past water temperatures. Different plants thrive at different levels of temperature and precipitation. By identifying leaves, seeds, bits of wood and pollen in lake cores, scientists can estimate climate conditions at the time when those kinds of plants were growing. During cold periods, the plant cover is sparse, resulting in more soil erosion.