There are biogeochemical cycles for the chemical elements calcium, carbon, hydrogen, mercury, nitrogen, oxygen, phosphorus, selenium, and sulfur; molecular cycles for water and silica; macroscopic cycles like the rock cycle; also as human-induced cycles for synthetic compounds like polychlorinated biphenyl (PCB). In some cycles, there are reservoirs where a substance remains for an extended period of your time. Ecological systems (ecosystems) have many biogeochemical cycles operating as a neighborhood of the system, for instance, the water cycle, the carbon cycle, the organic process, etc. In ecology and natural science, a biogeochemical cycle or substance turnover or cycling of drugs may be a pathway by which a chemical substance moves through biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth.