The OMZ, or Oxygen Minimum Zone, is a mid-depth oceanic volume with very low oxygen content. Its oxygen values are equal to or less than 1 micromole of O2 per kilogramme of water.
These zones cover 9% of the global ocean and are found in all regions. One of the most extensive, intense and shallowest OMZs is located in the southeast tropical Pacific Ocean (often called the ESP, East South Pacific).
The particular local characteristics of oceanic circulation and meteorology partially explain the existence of these zones, which can extend laterally over great distances, measuring tens or even hundreds of kilometres. Their expansion and their position vary over time, on a scale of a year or even over much longer periods.
These zones also seem to constitute reserves of dissolved carbon dioxide.