Largest dead zone

The largest Oxygen Minimum Zone (OMZ; also known as a "dead zone") is located in the Gulf of Oman, between Oman and Iran. At its peak extent, which occurs between June and August, midwater (depths of 200–800 m) oxygen concentrations drop below 6 μmol/kg (the threshold at which seawater is considered to be suboxic) across the the entire 181,000-km² (70,000-sq-mi) Gulf of Oman. There has always been an OMZ in this region, where the waters often become hypoxic (oxygen levels of less than 60 μmol/kg), but a study published in Geophysical Research Letters on 27 Apr 2018 revealed that the extent, depth and intensity of this OMZ has increased significantly since the 1990s.

An Oxygen Minimum Zone is an area in which the oxygen levels in the water have fallen significantly below the levels required to support many forms of life. The concentration of dissolved oxygen in seawater varies depending on location, depth of measurement and the time of year, but midwater concentrations are typically at least 120 μmol/kg, which is the level required to sustain larger fish. Seawater is considered to be "hypoxic" if oxygen concentrations drop below 60 μmol/kg.

The cause of the intensification of the Gulf of Oman OMZ is not fully understood, but it is has been hypothesized that it is being driven by rising global temperatures. Warmer water holds less dissolved oxygen, so rising ocean temperatures play a part, but a larger role might be played by rising air temperatures. The region around the Gulf of Oman is home to some of the hottest places in the world, and in recent years several temperature records have been broken there. Extreme temperatures heat the surface layer of seawater, which reduces its density and causes it to sit on top of the colder water almost like a layer of oil. This disrupts the currents which usually mix the oxygenated upper-level water with the deoxygenated water below.

The extreme conditions of the Gulf of Oman OMZ have a number of dire environmental consequences. While ocean life can mostly avoid the dead zone, occasional disruptions – such as from storms – can push the deoxygenated water up to the surface or down to the seabed, killing everything that gets caught up in it. Furthermore, within the dead zone certain types of phytoplankton that are able to extract oxygen from nitrates (NO₃) grow explosively (a phenomenon called an algal bloom). These blooms clog desalination plants on the coast, and can also become toxic to living things that consume them. Also, the process of extracting oxygen from nitrates converts the NO₃ into various other nitrogen containing compounds, including N₂O (nitrous oxide) – a potent greenhouse gas.

The Gulf of Oman OMZ has been known to science for a long time, but has been little studied in recent decades due to the volatility of the region around it. It has been an active theatre of war on several occasions since the 1980s and pirate activity in the neighbouring seas makes it off-limits to crewed research vessels. The study that revealed these changes was conducted by Dr Bastien Queste of the University of East Anglia, UK, and collaborators from Sultan Qaboos University in Muscat, Oman. The researchers used torpedo-like drones called "water-gliders" to obtain readings from across the Gulf of Oman at various different depths in 2015 and 2016.