text: The basic requirements for the formation of euxinic conditions are the absence of oxygen (O), and the presence of sulfate ions (SO), organic matter (CHO), and bacteria capable of reducing sulfate to hydrogen sulfide (HS). The bacteria utilize the redox potential of sulfate as an oxidant and organic matter as a reductant to generate chemical energy through cellular respiration. The chemical species of interest can be represented via the reaction:
  2CHO + SO → HS + 2HCO
  In the reaction above, the sulfur has been reduced to form the byproduct hydrogen sulfide, the characteristic compound present in water under euxinic conditions. Although sulfate reduction occurs in waters throughout the world, most modern-day aquatic habitats are oxygenated due to photosynthetic production of oxygen and gas exchange between the atmosphere and surface water. Sulfate reduction in these environments is often limited to occurring in seabed sediments that have a strong redox gradient and become anoxic at some depth below the sediment-water interface. In the ocean the rate of these reactions is not [https://online.science.psu.edu/biol011_active002/node/4343 limited]  by sulfate, which has been present in large quantities throughout the oceans for the past 2.1 billion years. The Great Oxygenation Event increased atmospheric oxygen concentrations such that oxidative weathering of sulfides became a major source of sulfate to the ocean. Despite plentiful sulfate ions being present in solution, they are not preferentially used by most bacteria. The reduction of sulfate does not give as much energy to an organism as reduction of oxygen or nitrate, so the concentrations of these other elements must be nearly zero for sulfate-reducing bacteria to out-compete aerobic and denitrifying bacteria. In most modern settings these conditions only occur in a small portion of sediments, resulting in insufficient concentrations of hydrogen sulfide to form euxinic waters.
  Conditions required for the formation of persistent euxinia include anoxic waters, high nutrient levels, and a stratified water column. These conditions are not all-inclusive and are based largely on modern observations of euxinia. Conditions leading up to and triggering large-scale euxinic events, such as the Canfield ocean, are likely the result of multiple interlinking factors, many of which have been inferred through studies of the geologic record at relevant locations. The formation of stratified anoxic waters with high nutrient levels is influenced by a variety of global and local-scale phenomena such as the presence of nutrient traps and a warming climate.