View source for Δ34S

{{Short description|Standardized measurement method}}
{{DISPLAYTITLE:δ<sup>34</sup>S}}
The '''δ<sup>34</sup>S''' (pronounced ''delta 34 S'') value is a standardized method for reporting measurements of the ratio of two stable [[isotopes of sulfur]], <sup>34</sup>S:<sup>32</sup>S, in a sample against the equivalent ratio in a known reference standard. The most commonly used standard is Vienna-Canyon Diablo Troilite (VCDT). Results are reported as variations from the standard ratio in parts per thousand, [[per mil]] or ''per mille'', using the ‰ symbol. Heavy and light sulfur isotopes [[fractionation|fractionate]] at different rates and the resulting δ<sup>34</sup>S values, recorded in marine sulfate or sedimentary [[sulfide]]s, have been studied and interpreted as records of the changing [[sulfur cycle]] throughout the earth's history.

==Calculation==
Of the 25 known [[isotopes of sulfur]], four are [[stable isotope|stable]].<ref>{{cite journal| vauthors = Audi G, Bersillon O, Blachot J, Wapstra AH |title=The NUBASE evaluation of nuclear and decay properties |journal=[[Nuclear Physics A]] |date=December 2003 |volume=729 |issue=1 |pages=3–128 |doi=10.1016/j.nuclphysa.2003.11.001 |bibcode=2003NuPhA.729....3A|citeseerx=10.1.1.692.8504 }} {{closed access}}</ref> In order of their abundance, those isotopes are <sup>32</sup>S (94.93%), <sup>34</sup>S (4.29%), <sup>33</sup>S (0.76%), and <sup>36</sup>S (0.02%).{{sfn|Hoefs|2009|p=71}} The δ<sup>34</sup>S value refers to a measure of the ratio of the two most common stable sulfur isotopes, <sup>34</sup>S:<sup>32</sup>S, as measured in a sample against that same ratio as measured in a known reference standard. The [[Delta (letter)#Lower case|lowercase delta character]] is used by convention, to be consistent with use in other areas of [[Isotope geochemistry#Stable isotope geochemistry|stable isotope chemistry]].<ref>{{cite book |title=Environmental Tracers in Subsurface Hydrology |date=2000 |publisher=Springer Science+Business Media |location=New York City |isbn=978-1-4615-4557-6 |page=511|doi=10.1007/978-1-4615-4557-6| veditors = Cook PG, Herczeg AL |url=https://link.springer.com/content/pdf/bbm%3A978-1-4615-4557-6%2F1.pdf}}</ref> That value can be calculated in [[per mil]] (‰, parts per thousand) as:<ref name="reviews">{{cite journal| vauthors = Canfield DE |title=Biogeochemistry of Sulfur Isotopes|journal=[[Reviews in Mineralogy and Geochemistry]]|date=2001|volume=43|issue=1|pages=607–636|doi=10.2138/gsrmg.43.1.607|bibcode=2001RvMG...43..607C}} {{closed access}}</ref>

:<math chem>\delta \ce{^{34}S} = \left( \frac{\left( \frac\ce{^{34}S}\ce{^{32}S} \right)_\mathrm{sample}}{\left( \frac\ce{^{34}S}\ce{^{32}S} \right)_\mathrm{standard}} - 1 \right) \times 1000</math> ‰
Less commonly, if the appropriate isotope abundances are measured, similar formulae can be used to quantify ratio variations between <sup>33</sup>S and <sup>32</sup>S, and <sup>36</sup>S and <sup>32</sup>S, reported as δ<sup>33</sup>S and δ<sup>36</sup>S, respectively.<ref>{{cite journal| vauthors = Whitehouse MJ |title=Multiple Sulfur Isotope Determination by SIMS: Evaluation of Reference Sulfides for Δ<sup>33</sup>S with Observations and a Case Study on the Determination of Δ<sup>36</sup>S|journal=[[Geostandards and Geoanalytical Research]]|date=March 2013|volume=37|issue=1|pages=19–33|doi=10.1111/j.1751-908X.2012.00188.x|s2cid=95283077 }} {{closed access}}</ref>

===Reference standard===
[[File:Canyon Diablo meteorite by Matthew Bisanz.JPG|thumb|right|Troilite from the Canyon Diablo meteorite was the first reference standard for δ<sup>34</sup>S.|alt=A worn brown-red-gold space rock covered in smoothed pock-marks sits mounted in a museum.]]
Sulfur from meteorites was determined in the early 1950s to be an adequate reference standard because it exhibited a small variability in isotopic ratios.<ref name="inaccurate">{{cite journal| vauthors = Beaudoin G, Taylor BE, Rumble III D, Thiemens M |title=Variations in the sulfur isotope composition of troilite from the Cañon Diablo iron meteorite|journal=[[Geochimica et Cosmochimica Acta]]|date=October 1994|volume=58|issue=19|pages=4253–4255|doi=10.1016/0016-7037(94)90277-1|bibcode=1994GeCoA..58.4253B}} {{closed access}}</ref> It was also believed that because of their extraterrestrial provenances, meteors represented primordial terrestrial isotopic conditions.<ref name="sealreview">{{cite journal| vauthors = Seal II RR |title=Sulfur Isotope Geochemistry of Sulfide Minerals|journal=[[Reviews in Mineralogy and Geochemistry]]|date=2006|volume=61|issue=1|pages=633–677|doi=10.2138/rmg.2006.61.12|bibcode=2006RvMG...61..633S|url=https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1354&context=usgsstaffpub}} {{closed access}}</ref>  During a meeting of the [[National Science Foundation]] in April 1962, [[troilite]] from the [[Canyon Diablo (meteorite)|Canyon Diablo meteorite]] found in Arizona, US, was established as the standard with which δ<sup>34</sup>S values (and other sulfur stable isotopic ratios) could be calculated.<ref name="inaccurate" />{{sfn|Hoefs|2009|p=72}} Known as Canyon Diablo Troilite (CDT), the standard was established as having a <sup>32</sup>S:<sup>34</sup>S ratio of 22.220 and was used for around three decades.<ref name="inaccurate" /> In 1993, the [[International Atomic Energy Agency]] (IAEA) established a new standard, Vienna-CDT (VCDT), based on artificially prepared [[silver sulfide]] (IAEA-S-1) that was defined to have a δ<sup>34</sup>S<sub>VCDT</sub> value of −0.3‰.{{sfn|Hoefs|2009|p=72}} In 1994, the original CDT material was found not to be isotopically homogeneous, with internal variations as great as 0.4‰, confirming its unsuitability as a reference standard.<ref name="inaccurate" />

==Causes of variations==
[[File:Terrestrial δ34S values.jpg|thumb|right|δ<sup>34</sup>S<sub>VCDT</sub> values for several geologic reservoirs|alt=A graph with δ34S values from &minus;50 to 40‰, with meteorites around 0‰, igneous rocks &minus;5 to 15‰, petroleum and coal &minus;10 to 20‰, modern seawater sulfate around 20‰, ancient marine evaporites at 10 to 35‰, and modern and ancient sedimentary pyrite at &minus;50 to 15‰.]]
Two mechanisms of [[fractionation]] occur that alter sulfur stable isotope ratios: kinetic effects, especially due to the metabolism of [[sulfate-reducing bacteria]], and isotope exchange reactions that occur between sulfide phases based on temperature.{{sfn|Hoefs|2009|pp=73, 77}} With VCDT as the reference standard, natural δ<sup>34</sup>S value variations have been recorded between &minus;72‰ and +147‰.<ref>{{cite journal | vauthors = Lever MA, Rouxel O, Alt JC, Shimizu N, Ono S, Coggon RM, Shanks WC, Lapham L, Elvert M, Prieto-Mollar X, Hinrichs KU, Inagaki F, Teske A | display-authors = 6 | title = Evidence for microbial carbon and sulfur cycling in deeply buried ridge flank basalt | journal = Science | volume = 339 | issue = 6125 | pages = 1305–1308 | date = March 2013 | pmid = 23493710 | doi = 10.1126/science.1229240 | s2cid = 10728606 | bibcode = 2013Sci...339.1305L | url = https://archimer.ifremer.fr/doc/00176/28767/ | author6-link = Rosalind Coggon }} {{closed access}}</ref><ref>{{Cite journal| vauthors = Drake H, Roberts NM, Reinhardt M, Whitehouse M, Ivarsson M, Karlsson A, Kooijman E, Kielman-Schmitt M | display-authors = 6 |date=June 2021|title=Biosignatures of ancient microbial life are present across the igneous crust of the Fennoscandian shield|url=|journal=Communications Earth & Environment|language=en|volume=2|issue=1|pages=1–13|doi=10.1038/s43247-021-00170-2|doi-access=free}}</ref>

The presence of sulfate-reducing bacteria, which [[redox reaction|reduce]] [[sulfate]] ({{chem|SO|4|2-}}) to [[hydrogen sulfide]] (H<sub>2</sub>S), has played a significant role in the oceanic δ<sup>34</sup>S value throughout the earth's history. Sulfate-reducing bacteria metabolize <sup>32</sup>S more readily than <sup>34</sup>S, resulting in an increase in the value of the δ<sup>34</sup>S in the remaining sulfate in the seawater.<ref name="sealreview" /> [[Archean]] [[pyrite]] found in [[barite]] in the [[Warrawoona Group]], Western Australia, with sulfur fractionations as great as 21.1‰ hint at the presence of sulfate-reducers as early as {{Ma|3470}}.<ref>{{cite journal | vauthors = Shen Y, Buick R, Canfield DE | title = Isotopic evidence for microbial sulphate reduction in the early Archaean era | journal = Nature | volume = 410 | issue = 6824 | pages = 77–81 | date = March 2001 | pmid = 11242044 | doi = 10.1038/35065071 | s2cid = 25375808 | bibcode = 2001Natur.410...77S }} {{closed access}}</ref>

It is now better known that the degree of isotope fractionation during microbial sulfate reduction depends on the cell-specific sulfate reduction rate of the sulfate-reducing microorganism.<ref>{{cite journal | vauthors = Wing BA, Halevy I | title = Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 111 | issue = 51 | pages = 18116–18125 | date = December 2014 | pmid = 25362045 | pmc = 4280625 | doi = 10.1073/pnas.1407502111 | bibcode = 2014PNAS..11118116W | doi-access = free }}</ref><ref>{{cite journal | vauthors = Wenk CB, Wing BA, Halevy I | title = Electron carriers in microbial sulfate reduction inferred from experimental and environmental sulfur isotope fractionations | journal = The ISME Journal | volume = 12 | issue = 2 | pages = 495–507 | date = October 2017 | pmid = 29087380 | doi = 10.1038/ismej.2017.185 | pmc = 5776465 }}</ref> The relative extent of sulfur isotope fractionating activities, including sulfate reduction, sulfide reoxidation and disproportionation, determines the isotopic compositions of the minerals or fluid measured.<ref name="Tsang">{{Cite journal | vauthors = Tsang MY, Wortmann UG |date=July 2022 |title=Sulfur isotope fractionation derived from reaction-transport modelling in the Eastern Equatorial Pacific |journal=Journal of the Geological Society |volume=179 |issue=5 |doi=10.1144/jgs2021-068 |bibcode=2022JGSoc.179...68T |s2cid=248647580 |issn=0016-7649}}</ref> Other than microbial activities and environmental conditions, isotopic compositions also change due to diffusion, accumulation and mixing after burial.<ref>{{Cite journal | vauthors = Jørgensen BB|date=January 1978 |title=A comparison of methods for the quantification of bacterial sulfate reduction in coastal marine sediments: II. Calculation from mathematical models |url=http://www.tandfonline.com/doi/abs/10.1080/01490457809377722 |journal=Geomicrobiology Journal |language=en |volume=1 |issue=1 |pages=29–47 |doi=10.1080/01490457809377722 |issn=0149-0451}}</ref><ref>{{Cite book | vauthors = Berner RA |url=http://worldcat.org/oclc/1164642477 |title=Early Diagenesis: A Theoretical Approach |date=2020 |isbn=978-0-691-20940-1 |doi=10.1515/9780691209401|location=Princeton, NJ|publisher=Princeton University Press|oclc=1164642477 }}</ref><ref name="Tsang"/>

The δ<sup>34</sup>S value, recorded by sulfate in marine [[evaporite]]s, can be used to chart the [[sulfur cycle]] throughout earth's history.<ref name="sealreview" /><ref name="reviews" /> The [[Great Oxygenation Event]] around {{Ma|2400}} altered the sulfur cycle radically, as increased atmospheric oxygen permitted an increase in the mechanisms that could fractionate sulfur isotopes, leading to an increase in the δ<sup>34</sup>S value from ~0‰ pre-oxygenation. Approximately {{Ma|700}}, the δ<sup>34</sup>S values in seawater sulfates began to vary more and those in sedimentary sulfates grew more negative. Researchers have interpreted this excursion as indicative of an increase in [[water column]] oxygenation with continued periods of [[Anoxic waters|anoxia]] in the deepest waters. Modern seawater sulfate δ<sup>34</sup>S values are consistently 21.0 ± 0.2‰ across the world's oceans, while sedimentary sulfides vary widely. Seawater sulfate δ<sup>34</sup>S and [[δ18O|δ<sup>18</sup>O]] values exhibit similar trends not seen in sedimentary sulfide minerals.<ref name="sealreview" />

== See also ==
* {{delta|13|C|link}}
* {{delta|15|N|link}}
* [[Isotopic signature]]
* [[Isotope analysis]]
* [[Isotope geochemistry]]

== References ==
{{reflist|30em}}

== Sources ==
* {{cite book| vauthors = Hoefs J |title=Stable Isotope Geochemistry|date=2009|publisher=Springer-Verlag|location=Berlin|isbn=978-3-540-70703-5|edition=6th|doi=10.1007/978-3-540-70708-0}}

{{DEFAULTSORT:Delta34S}}
[[Category:Isotopes of sulfur]]
[[Category:Environmental isotopes]]
[[Category:Geochemistry]]
[[Category:Bioindicators]]