OXYGEN CYCLE

The biogeochemical cycle that describes the movement of oxygen within and between its three main reservoirs: (1) the atmosphere, (2) the biosphere and (3) the lithosphere. The interaction of biological, chemical and physical processes on and beneath the Earth’s surface determines the concentration of oxygen and oxygen variations through time. oxygen gas is one of the major compounds found in the atmosphere and is a component of coupled biogeochemical cycles of many other elements (see carbon cycle, iron cycle, nitrogen cycle, phosphorus cycle, silicon cycle, sulfur cycle).

Almost all living things need oxygen as it is used in the process of creating energy in living cells. photosynthesis by land plants and phytoplankton in lakes and oceans drives the oxygen cycle by producing oxygen and sugars from carbon dioxide and water. Photosynthesis accounts for most of the atmospheric oxygen but a small amount of it is produced from the breakdown of atmospheric water and nitrous oxide by ultraviolet radiation in a process called phytolysis.

Oxygen is removed from the atmosphere by aerobic respiration and decay as organisms consume oxygen and release CO2. Oxidation of sulfide minerals, the formation of iron oxides and the oxidation of reduced volcanic gases also consume atmospheric oxygen. The lithosphere is the main reservoir or sink of oxygen, with the oxygen held in a form that is unavailable for use by organisms within the silicate minerals and oxide minerals. limestone, for example, is rich in oxygen as it is composed of the calcium carbonate shells of marine organisms. weathering processes release the oxygen from the lithosphere.

Waterbodies such as oceans, lakes and rivers contain large amounts of dissolved oxygen as oxygen from the atmosphere can transfer into water very quickly due to the movement of the water. Gas solubility is temperature dependent and so oxygen concentrations are greater in colder, high-latitude surface waters than in waters near the Equator. Aquatic organisms breathe the dissolved oxygen by filtering it out of solution and the dissolved oxygen is replenished by the process of reaeration. As organisms are constantly using up the oxygen in the water and oxygen is constantly re-entering the water from the air, the amount of dissolved oxygen remains relatively constant. However, if consumption exceeds supply, such as during an algal bloom, anoxia can develop and the oxygen cycle fails. Small changes in oxygen production and consumption, resulting in large fluxes of oxygen to and from the atmosphere, have the potential to generate large shifts in atmospheric oxygen concentration over geologically short periods of time. Although oxygen levels have varied over geological time due to an imbalance of oxygen production and consumption, evidence suggests that oxygen levels were stable over wide spans of geological time; the reasons for this are still poorly understood. oxygen isotopes derived from palaeo records (ice cores, lacustrine sediments, marine sediments, etc.) can be used to infer changes in the oxygen cycle and to reconstruct past climates.