Euxinia

Euxinic: (term derived from "Euxin Bridge", the old name of the Black Sea) corresponds to the appearance of hydrogen sulfide in a closed aquatic system (Lake Tanganyka, Caspian Sea...) or confined (Black Sea, Baltic Sea...) because of the non-renewal of deep waters and the disappearance of dissolved oxygen in depth (anoxia).

Generally, below a depth of 200 meters, a chemocline separates the more oxygenated surface waters (often subject to stronger currents and tidal effects) from the anoxic deep waters (due to almost non-existent currents) where anaerobic bacteria develop, generating hydrogen sulfide (possibly in gaseous form, but which dissolves rapidly and acidifies the water).

These deep waters are cooler (except for hot spots in trenches or around some of the deepest fractures), but are significantly more saline, dense, acidic, turbid, and darker. They are not conducive to the survival of upper water animal species or the plant species and chlorophyllous algae that can still be seen at shallower depths near the coast. In some of the deeper areas, they may also contain high concentrations of dissolved hydrocarbons by diffusion, resulting from the decomposition of surface marine species and their progressive deposition on the peaty bottom. This decomposition is favored by the presence of anaerobic bacteria. The last inorganic elements (calcites, phosphates, silicates and various metallic oxides) are also precipitated and lead with time to the formation of hydrated rocks under the peat, in which hydrocarbons will continue to form and concentrate, under the action of extremophilic bacteria.