Sulfur cycle definition, steps, examples, significance, human impact, and human influence on the sulfur cycle are included in this article.

The sulfur cycle is a collection of processes by which sulfur moves.Many minerals are affected by biogeochemical cycles.Sulfur is an essential element for life because it is a component of many genes and cofactors, and sulfur compounds can be used as oxidants or reductants.The global sulfur cycle involves the transformation of sulfur species through different oxidation states, which play an important role in both geological and biological processes.

Sulfur has four main oxidation states, which are -2, +2, +4, and +6.The sulfur species of each oxidation state is listed.

Sulfur can be found in SO42 to -2 in sulfides.Depending on its environment, sulfur can give or receive electrons.Most sulfur was present in minerals on the anoxic early Earth.The amount of mobile sulfur increased through volcanic activity as well as weathering of the crust in an oxygenated atmosphere.The oceans SO42 is the main oxidizer in the sulfur sink.[4]

SO42 is reduced and converted to organic sulfur, which is an essential component of the human body.The majority of sulfur is found in rocks that are salty, such as rich shales, evaporite rocks, and calcium and magnesium carbonates.There is carbonate-associated sulfate.Three major processes control the amount of sulfate in the ocean.

Sea spray or wind blown sulfur rich dust are the primary natural sources of sulfur to the atmosphere.A large amount of SO2 has been added to the air from the burning of coal and other fossil fuels.The release of sulfur to the atmosphere has been caused by large scale burning of coal measures in the past.The climate system has been disrupted by this, and is one of the reasons for the extinction event.There is a citation needed.

The major biogenic gas emitted from the sea is produced by the decomposition of dimethylsulfoniopropionate from dying phytoplankton cells in the ocean's photic zone.DMS is the largest natural source of sulfur gas, but it only has a residence time of one day in the atmosphere, and most of it is deposited into the ocean.It is involved in the formation of clouds.

Sulfate can be reduced through the dissimilatory sulfate reduction pathway.The reduction of sulfate by organic compounds leads to the production of hydrogen sulfide.

The main products and reactants of sulfate reduction are very similar.The reactants are organic compounds and dissolved sulfate, and the products are carbonates, sulfur and metal sulfides.Because of the mutually exclusive temperature regimes, the reactive organic compounds differ for BSR and TSR.The main organic reactants are organic acids and branched/n-alkanes.The products of the reaction are H2S(HS) and HCO3.[7]

Sulfate is reduced in two different thermal regimes in high and low temperature environments.BSR occurs at lower temperatures from 0-80 C, while TSR happens at much higher temperatures.The lowest confirmed temperature for TSR is 127 C and the highest temperature is around 160-180 ­C.There are two different regimes because sulfate-reducing microbes can no longer metabolize at higher temperatures.BSR can happen at temperatures up to 120 C.[9]

BSR and TSR occur at different depths.In shallow settings such as oil and gas fields, BSR takes place.In modern marine environments, BSR can take place because of the high concentration of dissolved sulfate in the water.The high amounts of hydrogen sulfide found in oil in gas fields is thought to arise from the oxidation of petroleum hydrocarbons by sulfate.It is generally accepted that TSR is responsible for most of the reactions, especially in deep or hot reservoirs.The temperatures are much higher in deep reservoirs.TSR occurs at rates in the order of hundreds of thousands of years, while BSR is geologically instantaneous.TSR appears to be a geologically fairly fast process, even though it is much slower than BSR.

There are two key processes in the sulfur cycle.Roughly 10% of the total gas is produced in BSR settings, whereas the majority of it is in TSR settings.It is assumed that TSR has taken over if there is more than a few percent of H2S.The thermal cracking of hydrocarbons doesn't provide much in the way of H2S.The availability of organic reactants and sulfate is one of the factors that affects the amount of H2S.[15]

Oxygen is used to oxidize hydrogen sulfide in order to produce sulfur or sulfate.The chemical reactions are listed below.

The primary sulfur oxidizingbacteria in modern oceans are Thiomicrospira, Halothiobacillus, and Beggiatoa.The symbiont produces organic carbon for sustaining the metabolism of the host.The produced sulfate usually combines with the calcium ion to form gypsum, which can form widespread deposits near mid-ocean spreading centers.[18]

There are 25 known isotopes of sulfur, but only four are stable.Two of the four comprise almost 100 percent of S on Earth.Only 4.21% of S occurs in 34S.Our solar system has a fixed ratio of these two isotopes.The ratio of sulfur in the bulk Earth is thought to be the same as that of the meteorite.The international standard for that ratio is 0.00.The 34S is a ratio in per mill.Positive values correlate to increased levels of 34S, while negative values do the same in a sample.

Formation of sulfur minerals through non-biogenic processes does not substantially differentiate between the light and heavy isotopes, therefore sulfur isotope ratios in gypsum or barite should be the same as the overall isotope ratio in the water column at their time of precipitation.The more rapid enzymic reaction with 32S is what distinguishes the two isotopes.Sulfate metabolism results in an isotopic depletion of -18, and repeated cycles of oxidation and reduction can result in values up to -50.On the order of +21, the average present day seawater values are 34S.

The sulfur cycle has coevolved with the biosphere becoming more negative with increases in biologically driven sulfate reduction, but also show substantial positive excursion.Positive excursions in the sulfur isotopes mean that there is an excess of deposition rather than oxidation of sulfide minerals on land.[19]

The sulfur cycle in marine environments has been studied using sulfur isotope systematics.The global oceans have sulfur storage of 1.3 1021 g, mainly occurring as sulfate with a value of +21.The overall input flux is 1.0 1014 g/year with the sulfur isotope composition.Riverine sulfate derived from the weathering of sulfide minerals is the primary input of sulfur to the oceans.Other sources include volcanic degassing, which releases reduced sulfur species.The oceans produce two major outputs of sulfur.Sulfate is buried either as marine evaporites or carbonate-associated sulfate, which accounts for 6 1013 g/year.The second sulfur sink is buried in the shelf or deep seafloor.The total marine sulfur output flux is 1.0 1014 g/year, which matches the input fluxes, implying that the modern sulfur budget is at steady state.The residence time of sulfur is 13,000,000 years.[23]

Primary information on the evolution of the sulfur cycle can be found in the composition of sedimentary sulfides.

The total inventory of sulfur compounds on the surface of the Earth is the total outgassing through geologic time.Rocks analyzed for sulfur content are usually organic-rich and controlled by biogenic sulfur reduction.Since they do not discriminate between the heavy and light sulfur isotopes, average seawater curves should mimic the ocean composition at the time of deposition.

The Earth had a theoretical 34S value of 0.There would be no fractionation since there was no activity on early Earth.During volcanic eruptions, all sulfur in the atmosphere would be released.The atmosphere was swept clean of sulfur gases when the oceans were small.Most systems in the Archean appeared to be sulfate-limited.Some small Archean evaporite deposits need at least locally elevated concentrations of sulfate in order for them to be supersaturated and precipitate out of solution.[25]

The age of the oldest rocks on Earth is 3.8 Ga.The biosphere was not developed enough to fractionate sulfur, so the rocks have a 0 isotopic value.[26]

The global ocean has a weak source of sulfate due to the low concentrations of 34S.The rock record shows the first evidence for minimal fractionation in evaporitic sulfate in association with magmatically derived sulfides at 3.4 Ga.There is evidence for anoxygenic phototrophicbacteria in this fractionation.

It is the first evidence of oxygen production.Oxygen is needed in the atmosphere to oxidize sulfur.This shows the evolution of the oxygen and sulfur cycles.

The first evidence for sulfate reduction can be found in the age of 2.5 Ga, which has a 34S.[25]

2.3 Ga sulfate increases to more than 1 mM; this increase in sulfate is related to the "Great Oxygenation Event", when the redox conditions on Earth's surface are thought to have shifted from reducing to oxidizing.The shift would have led to an increase in sulfate in the oceans.For the first time, large isotopic fractionations have been produced.At this time, there was a rise in seawater sulfate, but it was still less than 15% of present-day levels.[28]

Banded iron formations are common in the Archean and Paleoproterozoic, but their disappearance marks a shift in chemistry of ocean water.There are alternating layers of iron oxides and chert.BIFs are only formed if the water is allowed to supersaturate in dissolved iron and there is no free oxygen or sulfur in the column.The water must become oxygenated in order for ferric rich bands to form, otherwise it will be sulfur poor.BIFs are thought to have formed during the initial evolution of organisms that had phases of population growth.Due to this over production they would poison themselves causing a mass die off, which would cut off the source of oxygen and produce a large amount of CO2 through the decomposition of their bodies, allowing for anotherbacterial bloom.After 1.8 Ga sulfate concentrations, the rate of sulfate reduction was greater than the delivery of iron to the oceans.[25]

Along with the disappearance of BIF, the end of the Paleoproterozoic also marks the first large scale exhalative deposits showing a link between the two.The sulfate in seawater was higher in the Paleoproterozoic than it is today, but still lower than today's values.Sulfate levels in the Proterozoic are a proxy for atmospheric oxygen because sulfate is produced mostly through weathering of the continents.The low levels in the Proterozoic suggest that atmospheric oxygen fell between the abundances of the Phanerozoic and the deficient Archean.

A renewed deposition of BIF marks a significant change in ocean chemistry.The oceans were cut off from oxygenation due to snowball earth episodes where the entire globe was covered in ice.In the late Neoproterozoic high carbon burial rates increased the atmospheric oxygen level to 10% of its present-day value.An oxic deep ocean may have been allowed for the appearance of multicellular life after another major oxidizing event occurred on Earth's surface.[28]

Over the last 600 million years, the average value of SO4 has ranged from a low of +30 to a high of +10.During extinction and climate events, changes in seawater 34S occurred.[29][30][31][32][33][34]

Changes in the sulfur cycle can be observed over a ten million year period.When the sulfate is reduced again, Oxygen is incorporated into the sulfur cycle through sulfate oxidation.Oxygen may be able to be used to trace the sulfur cycle because of the different sulfate sources in the ocean.For the same reason that lighter sulfur isotopes are preferred, biological sulfate reduction preferentially selects lighter oxygen isotopes.It was possible to better constrain the sulfur concentrations in sea water through the last 10 million years by studying oxygen isotopes.The sulfur processing was disrupted because of the sea level changes caused by the Pliocene and Pleistocene glaciers.As compared to preglacial times before 2 million years ago, this was a drastic change.

The Great Oxygenation event is characterized by the disappearance of sulfur isotope mass-independent fractionation in the records.According to the mass dependent fractionation law, the MIF of sulfur is determined by the deviation of the measured 33S value from the inferred value.A transition of global sulfur cycles was represented by the Great Oxidation event.The sulfur cycle was influenced by the UV radiation and the associated photochemical reactions.The atmospheric O2 needs to be less than 105 of present atmospheric level to preserve sulfur isotope mass-independent fractionation signals.After the Great Oxygenation event, atmospheric pO2 exceeded 105 present atmospheric level.Oxygen played an important role in the global sulfur cycles after the Great Oxygenation event.The free O2 in Earth's surface environment is caused by the burial of pyrite.[39]

Sulfur is involved in production of fossil fuels and a majority of metal deposits because of its ability to act as an oxidizing or reducing agent.The majority of the major mineral deposits on Earth contain a substantial amount of sulfur.If the respective transition or base metals are present or transported to a sulfate reduction site, iron sulfides, galena and sphalerite will form as by-products of hydrogen sulfide generation.Sulfur deposits may form if the system runs out of reactive hydrocarbons.Sulfur is a reducing agent for natural gas and it has a close relationship with ancient hydrocarbon seeps.[28]

Local country rocks, sea water, and marine evaporites are some of the important sources of sulfur in Ore deposits.The concentration of precious metals and their precipitation from solution are limited by the presence or absence of sulfur.sulfides can be determined by the temperature, pH, and redox states.Until they reach reducing conditions, most sulfide brines will remain in concentration.

Ore fluids are linked to metal rich waters that have been heated within a basin under elevated thermal conditions.The basin's redox conditions exert an important control on the redox state of the metal-transporting fluids and deposits.A substantial portion of the sulfide must be supplied from another source if metal-rich fluids are to be by necessity comparatively sulfide deficient.The euxinic water column is a necessary source of the sulfide.Baryte formation is suggested by the 34S values of barite being consistent with a seawater sulfate source.[28]

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