Chemical oceanography is the study of the behaviour of the chemical elements within the Earth's oceans. The ocean is unique in that it contains _ in greater or lesser quantities _ nearly every element in the periodic table. Thus much of chemical oceanography describes the cycling of these elements both within the ocean and with the other spheres of the Earth system (see biogeochemical cycle). These cycles are usually characterised as quantitative fluxes between constituent reservoirs defined within the ocean system. Of particular global and climatic significance are the cycles of the biologically active elements such as carbon, nitrogen, and phosphorus as well as those of some important trace elements such as iron. Another important area of study in chemical oceanography is the behaviour of isotopes (see isotope geochemistry) and how they can be used as tracers of past and present oceanographic and climatic processes. For example, the incidence of ¹⁸O (the heavy isotope of oxygen) can be used as an indicator of polar ice sheet extent.