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Lead section
The global sulfur cycle involves the transformations of sulfur species through different oxidation states, which play an important role in both geological and biological processes.

Sulfur oxidation state
Sulfur has six oxidation states in nature, which are -2, 0, +2, +4, and +6. The common sulfur species of each oxidation state are listed as follows:

S2-: H2S, FeS, FeS2, CuS

S0: native sulfur

S4+: SO2, sulfite (SO32-)

S6+: SO42- (H2SO4, CaSO4), SF6

Marine sulfur cycle
The sulfur cycle in marine environments has been well-studied via the tool of sulfur isotope systematics expressed as δ34S. The modern global oceans have sulfur storage of 1.3 × 1021 g, mainly occurring as sulfate with the δ34S value of +21‰. The overall input flux is 1.0 × 1014 g/year with the sulfur isotope composition of ~3‰. Riverine sulfate derived from the terrestrial weathering of sulfide minerals (δ34S = +6‰) is the primary input of sulfur to the oceans. Other sources are metamorphic and volcanic degassing and hydrothermal activity (δ34S = 0‰), which release reduced sulfur species (e.g., H2S and S0). There are two major outputs of sulfur from the oceans. The first sink is the burial of sulfate either as marine evaporites (e.g., gypsum) or carbonate-associated sulfate (CAS), which accounts for 6 × 1013 g/year (δ34S = +21‰). The second sulfur sink is pyrite burial in shelf sediments or deep seafloor sediments (4 × 1013 g/year; δ34S = -20‰). The total marine sulfur output flux is 1.0 × 1014 g/year which matches the input fluxes, implying the modern marine sulfur budget is at steady state. The residence time of sulfur in modern global oceans is 13,000,000 year.

The Great Oxidation Event and sulfur isotope mass-independent fractionation
The Great Oxygenation Event (GOE) is characterized by the disappearance of sulfur isotope mass-independent fractionation (MIF) in the sedimentary records at around 2.45 billion years ago (Ga). The GOE represented a massive transition of global sulfur cycles. Before the GOE, the sulfur cycle was heavily influenced by the ultra violet (UV) radiation and the associated photochemical reactions, which induced the sulfur isotope MIF (Δ33S ≠ 0). The preservation of sulfur isotope MIF signals requires the atmospheric O2 lower than 10-5 of present atmospheric level (PAL). The disappearance of sulfur isotope MIF at ~2.45 Ga indicates that atmospheric pO2 exceeded 10-5 PAL after the GOE. Oxygen played an essential role in the global sulfur cycles after the GOE, such as oxidative weathering of sulfides. The burial of pyrite in sediments in turn contributes to the accumulation of free O2 in Earth’s surface environment.

Sulfur-oxidizing bacteria in hydrothermal vents
Hydrothermal vents emit hydrogen sulfide that support the carbon fixation of chemolithotrophic bacteria that oxidize hydrogen sulfide with oxygen to produce elemental sulfur or sulfate. The chemical reactions are as follows:

CO2 + 4H2S + O2 -> CH2O + 4S0 + 3H2O

CO2 + H2S + O2 + H2O -> CH2O + SO42- + 2H+

In modern oceans, Thiomicrospira, Halothiobacillus, and Beggiatoa are primary sulfur oxidizing bacteria, and form chemosynthetic symbioses with animal hosts. The host provides metabolic substrates (e.g., CO2, O2, H2O) to the symbiont while the symbiont generates organic carbon for sustaining the metabolic activities of the host. The produced sulfate usually combines with the leached calcium ions to form gypsum, which can form widespread deposits on near mid-ocean spreading centers.

Article resources
1. Brimblecombe, Peter (2014). The global sulfur cycle. In Holland, Heinrich D.; Turekian, Karl K. (Eds). Treatise on Geochemistry (Vol. 10, pp. 559-591). Elsevier, Amsterdam. https://doi.org/10.1016/B978-0-08-095975-7.00814-7

2. Fike, David A.; Bradley, Alexander S.; Rose, Catherine V. (2015). Rethinking the ancient sulfur cycle. Annual Review of Earth and Planetary Sciences. 43: 593-622. https://doi.org/10.1146/annurev-earth-060313-054802

3. Canfield, Donald E. (2004). The evolution of the earth surface sulfur reservoir. American Journal of Science. 304 (10): 839-861. https://doi.org/10.2475/ajs.304.10.839

4. Kah, Linda C.; Lyons, Timothy W.; Frank, Tracy D. (2004). Low marine sulphate and protracted oxygenation of the Proterozoic biosphere. Nature. 431: 834-838. https://doi.org/10.1038/nature02974

5. Farquhar, James; Bao, Huiming; Thiemens, Mark (2000). Atmospheric influence of Earth’s earliest sulfur cycle. Science. 289 (5480): 756-758. https://doi.org/10.1126/science.289.5480.756

6. Johnston, David T. (2011). Multiple sulfur isotopes and the evolution of Earth’s surface sulfur cycle. Earth-Science Reviews. 106 (1-2): 161-183. https://doi.org/10.1016/j.earscirev.2011.02.003

7. Konhauser, Kurt O.; Lalonde, Stefan V.; Planavsky, Noah J.; Ernesto, Pecoits; Lyons, Timothy W.; Mojzsis, Stephen J.; Rouxel, Olivier J.; Barley, Mark E.; Rosìere, Carlos; Fralick, Phillip W.; Kump, Lee R.; Bekker, Andrey (2011). Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event. Nature. 478: 369-373. https://doi.org/10.1038/nature10511

8. Berner, Robert A.; Raiswell, Robert (1983). Burial of organic carbon and pyrite sulfur in sediments over phanerozoic time: a new theory. Geochimica et Cosmochimica Acta. 47 (5): 855-862. https://doi.org/10.1016/0016-7037(83)90151-5

9. Sievert, Stefan M.; Hügler, Michael; Taylor, Craig D.; Wirsen, Carl O. (2008). Sulfur Oxidation at Deep-Sea Hydrothermal Vents. In: Dahl, Christiane; Friedrich, Cornelius G. (Eds). Microbial Sulfur Metabolism (pp. 238-258). Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-72682-1_19

10. Klotz, Martin G.; Bryant, Donald A.; Hanson, Thomas E. (2011). The microbial sulfur cycle. Frontiers in Microbiology. 2: 241. https://doi.org/10.3389/fmicb.2011.00241

11. Pedersen, Rolf B.; Rapp, Hans T.; Thorseth, Ingunn H.; Lilley, Marvin D.; Barriga, Fernando J. A. S.; Baumberger, Tamara; Flesland, Kristin; Fonseca, Rita; Früh-Green, Gretchen L.; Jorgensen, Steffen L. (2010). Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge. Nature Communications. 1: 26. https://doi.org/10.1038/ncomms1124

Content
The article provides a good overview of the sulfur cycle, including not only the cycling processes among the primary geologic reservoirs on a global scale but also biogeochemical mechanisms on a microbial scale. All the subsections in the article are relevant to the topic of the S cycle. The most recommended part is the detailed description of the sulfur cycle evolution in geologic history which provides a thorough background to understand the S cycling pattern in modern Earth.

Generally, the information in the article is still valid and is expressed appropriately. However, some additions or corrections could be made to make it better. The figure showing the global sulfur cycle does not clearly depict each reservoir, and the flux values are missing. Despite the fact that the sulfate reduction is well introduced, the reverse process which is sulfur oxidation (especially the chemolithoautotroph metabolism by S-oxidizing bacteria in hydrothermal vent systems) hasn't been assigned enough space to illustrate. In terms of expression of sulfur isotope section, the “fractionation factor” would be a better term to replace “depletion” which is vague and imprecise. Also, mass-independent fractionation (MIF) of sulfur isotope, which plays a critical role in the discovery of Great Oxygenation Event (GOE), is not included in the article.

In the article, there are links to other related articles, such as sulfur assimilation, sulfate reducers, dimethylsulfide, gypsum, hydrogen sulfide, sulfuric acid, etc.

Tone
Overall, the article is presented in a neutral position, without any biased points of a specific topic.

Talk page
The present conversations on the talk page of the article include the revisions of minor concepts or the suggestions of spreading the article topic. Also, an editor modified the article in an external link and inviting the other editors to give feedback. The article of “Sulfur cycle” is part of the WikiProject Ecology and WikiProject Soil. In both projects, the article is rated as Start-Class on the quality scale and High-Importance on the importance scale. Currently, this article is also the subject of a Wiki Education Foundation-supported course assignment.