Euxinia
Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen and a raised level of free hydrogen sulfide. Euxinic bodies of water are frequently strongly stratified; have an oxic, highly productive, thin surface layer; and have anoxic, sulfidic bottom water. The word "euxinia" is derived from the Greek name for the Black Sea which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during part of the Proterozoic eon proposed by Donald Canfield, an American geologist, in 1998. There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.
Background
Euxinia most frequently occurred in the Earth's ancient oceans, but its distribution and frequency of occurrence are still under debate. The original model was that it was quite constant for approximately a billion years. Some meta-analyses have questioned how persistent euxinic conditions were based on relatively small black shale deposits in a period when the ocean should have theoretically been preserving more organic matter.Before the Great Oxygenation Event happened approximately 2.3 billion years ago, there was little free oxygen in either the atmosphere or the ocean. It was originally thought that the ocean accumulated oxygen soon after the atmosphere did, but this idea was challenged by Canfield in 1998 when he proposed that instead of the deep ocean becoming oxidizing, it became sulfidic. This hypothesis is partially based on the disappearance of banded iron formations from the geological records 1.8 billion years ago. Canfield argued that although enough oxygen entered the atmosphere to erode sulfides in continental rocks, there was not enough oxygen to mix into the deep ocean. This would result in an anoxic deep ocean with an increased flux of sulfur from the continents. The sulfur would strip iron ions from the sea water, resulting in iron sulfide, a portion of which was eventually buried. When sulfide became the major oceanic reductant instead of iron, the deep water became euxinic. This has become what is known as the Canfield ocean, a model backed by the increase in presence of δ34S in sedimentary pyrite and the discovery of evidence of the first sulfate evaporites.
Anoxia and sulfidic conditions often occur together. In anoxic conditions anaerobic, sulfate reducing bacteria convert sulfate into sulfide, creating sulfidic conditions. The emergence of this metabolic pathway was very important in the pre-oxygenated oceans because adaptations to otherwise inhabitable or "toxic" environments like this may have played a role in the diversification of early eukaryotes and protozoa in the pre-Phanerozoic.
Euxinia still occurs occasionally today, mostly in meromictic lakes and silled basins such as the Black Sea and some fjords. It is rare in modern times; less than 0.5% of today's sea floor is euxinic.
Causes
The basic requirements for the formation of euxinic conditions are the absence of oxygen, and the presence of sulfate ions, organic matter, and bacteria capable of reducing sulfate to hydrogen sulfide. 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:2CH2O + SO42− → H2S + 2HCO3−
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 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.
Nutrient traps
In order for euxinic conditions to persist, a positive feedback loop must perpetuate organic matter export to bottom waters and reduction of sulfate under anoxic conditions. Organic matter export is driven by high levels of primary production in the photic zone, supported by a continual supply of nutrients to the oxic surface waters. A natural source of nutrients, such as phosphate, comes from weathering of rocks and subsequent transport of these dissolved nutrients via rivers. In a nutrient trap, increased input of phosphate from rivers, high rates of recycling of phosphate from sediments, and slow vertical mixing in the water column allow for euxinic conditions to persist.Geography
The arrangement of the continents has changed over time due to plate tectonics, resulting in the bathymetry of ocean basins also changing over time. The shape and size of the basins influences the circulation patterns and concentration of nutrients within them. Numerical models simulating past arrangements of continents have shown that nutrient traps can form in certain scenarios, increasing local concentrations of phosphate and setting up potential euxinic conditions. On a smaller scale, silled basins often act as nutrient traps due to their estuarine circulation. Estuarine circulation occurs where surface water is replenished from river input and precipitation, causing an outflow of surface waters from the basin, while deep water flows into the basin over the sill. This type of circulation allows for anoxic, high nutrient bottom water to develop within the basin.Stratification
Stratified waters, in combination with slow vertical mixing, are essential to maintaining euxinic conditions. Stratification occurs when two or more water masses with different densities occupy the same basin. While the less dense surface water can exchange gas with the oxygen-rich atmosphere, the denser bottom waters maintain low oxygen content. In the modern oceans, thermohaline circulation and upwelling prevent the oceans from maintaining anoxic bottom waters. In a silled basin, the stable stratified layers only allow surface water to flow out of the basin while the deep water remains anoxic and relatively unmixed. During an intrusion of dense saltwater however, the nutrient-rich bottom water upwells, causing increased productivity in the surface, further enhancing the nutrient trap due to biological pumping. Rising sea level can exacerbate this process by increasing the amount of deep water entering a silled basin and enhancing estuarine circulation.Warming climate
A warming climate increases surface temperatures of waters which affects multiple aspects of euxinic water formation. As waters warm, the solubility of oxygen decreases, allowing for deep anoxic waters to form more readily. Additionally, the warmer water causes increased respiration of organic matter leading to further oxygen depletion. Higher temperatures enhance the hydrologic cycle, increasing evaporation from bodies of water, resulting in increased precipitation. This causes higher rates of weathering of rocks and therefore higher nutrient concentrations in river outflows. The nutrients allow for more productivity resulting in more marine snow and subsequently lower oxygen in deep waters due to increased respiration.Volcanism has also been proposed as a factor in creating euxinic conditions. The carbon dioxide released during volcanic outgassing causes global warming which has cascading effects on the formation of euxinic conditions.