Ocean Sediments Record Past Climate

By measuring the ratio of different oxygen isotopes (oxygen-16 and oxygen-18) in deep sea sediments, an indirect measure of ice sheet accumulation can be obtained. This is because the continental ice sheets were formed from water that evaporated from the ocean and became part of the ice sheet after falling out of the atmosphere as snow. Since oxygen-16 is lighter than oxygen-18, it evaporates more readily from the ocean than oxygen-18 and becomes incorporated into the ice sheet. When there is a lot of glacial ice on the continents, the ocean water is enriched with oxygen-18. The concentrations of oxygen-16 and oxygen-18 are recorded in the fossil skeletons of foraminifera, which are unicellular animals that build their skeletons out of calcium carbonate, incorporating oxygen from the seawater. By measuring the amounts of oxygen-16 and oxygen-18 in the skeletons of foraminifera from different levels of sediment from the ocean floor, scientists can construct graphs that show fluctuations in glacial ice through