The organic component of sea sediment includes the remnants of sea-dwelling microscopic plankton, which provide a record of past climate and oceanic circulation. For example, by studying the chemical composition of plankton shells, we can reveal information about past seawater temperatures, salinity (saltiness), and nutrient availability. Indeed, such techniques have been used to reconstruct ocean temperatures over the last 100 million years, and have confirmed continental drift theories of climate change that a long term global cooling has taken place since the extinction of the dinosaurs.