Euxinia
[2] 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.
[4] 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.
[1] The bacteria utilize the redox potential of sulfate as an oxidant and organic matter as a reductant to generate chemical energy through cellular respiration.
In the reaction above, the sulfur has been reduced to form the byproduct hydrogen sulfide, the characteristic compound present in water under euxinic conditions.
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 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 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.
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.
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.
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.
The nutrients allow for more productivity resulting in more marine snow and subsequently lower oxygen in deep waters due to increased respiration.
The carbon dioxide (CO2) released during volcanic outgassing causes global warming which has cascading effects on the formation of euxinic conditions.
Other evidence for anoxic burial of black shale includes the lack of bioturbation, meaning that there were no organisms burrowing into the sediment because there was no oxygen for respiration.
Since ancient oceans cannot be directly observed, scientists use geology and chemistry to find evidence in sedimentary rock created under euxinic conditions.
Using stoichiometry and knowledge of redox pathways, paleogeologists can use isotopes ratios of elements to determine the chemical composition of the water and sediments when burial occurred.
[4] Under euxinic conditions, some trace elements such as Mo, U, V, Cd, Cu, Tl, Ni, Sb, and Zn, become insoluble.
[2] Supporting Canfield's original hypothesis, 1.84 billion year old sedimentary records have been found in the Animike group in Canada that exhibit close to full pyritization on top of the last of the banded iron formations, showing evidence of a transition to euxinic conditions in that basin.
Bottom waters may have been more widely suboxic than anoxic, and there could have been negative feedback between euxinia and the high levels of surface primary production needed to sustain euxinic conditions.
[31] Further work has suggested that from 700 million years ago (late Proterozoic) and onward, the deep oceans may have actually been anoxic and iron rich with conditions similar to those during the formation of BIFs.
[1] The periodic presence of euxinic conditions in the Lower Cambrian has been supported by evidence found on the Yangtze platform in South China.
[34] Geological records from the paleozoic in the Selwyn Basin in Northern Canada have also shown evidence for episodic stratification and mixing, where, using δ34S, it was determined that hydrogen sulfide was more prevalent than sulfate.
[35] Although this was not originally attributed to euxinia, further studies found that seawater in that time likely had low concentrations of sulfate, meaning that the sulfur in the water was primarily in the form of sulfide.
[39] Presence of a biomarker for anaerobic photosynthesis by green sulfur bacteria has been found spanning from the Permian to early Triassic in sedimentary rock in both Australia and China, meaning that euxinic conditions extended up quite shallow in the water column, contributing to the extinctions and perhaps even slowed the recovery.
[1] Small surface area to depth ratios allow multiple stable layers to form while limiting wind-driven overturning and thermohaline circulation.
[1] Within the chemocline, highly specialized organisms such as green sulfur bacteria take advantage of the strong redox potential gradient and minimal sunlight.
[42] Black Sea sediment contains redox reactions to depths of tens of meters, compared to single centimeters in the open ocean.
[47] This can result in the release of immense concentrations of stored toxic gasses from the anoxic bottom waters, such as CO2[46] and H2S, especially from euxinic meromictic lakes.
[4] Sediments export during these events increased the concentrations of dissolved phosphates, inorganic bioavailable nitrogen, and other nutrients, resulting in a harmful algal bloom.
[53] An increase in productivity coincident with post glacial nutrient loading probably caused a transition from oxic to anoxic and subsequently euxinic conditions around 14.5 thousand years ago.
