{"refrec":{"BRefID":209777,"RR":"<b>Dale, A. W. ; Brüchert, V.; Alperin, M.; Regnier, P.</b> (2009). An integrated sulfur isotope model for Namibian shelf sediments. <i>Geochim. Cosmochim. Acta 73(7)</i>: 1924-1944. <a href=\"http://dx.doi.org/10.1016/j.gca.2008.12.015\" target=\"_blank\">dx.doi.org/10.1016/j.gca.2008.12.015</a>","BEntID":201749,"PublicFlag":1,"CheckedFlag":1,"wosflag":1,"vabbflag":1,"RefStringPartII":". <i>Geochim. Cosmochim. Acta 73(7)</i>: 1924-1944. <a href=\"https://dx.doi.org/10.1016/j.gca.2008.12.015\" target=\"_blank\">https://dx.doi.org/10.1016/j.gca.2008.12.015</a>","DocTypID":8,"DocType":"Journal article","MarineFlag":1,"FreshFlag":0,"BrackishFlag":0,"TerrestrialFlag":0,"Authorstring":"Dale, A. W. ; Brüchert, V.; Alperin, M.; Regnier, P.","OrigTitleTranslFlag":0,"Authorstringtrunc":"Dale, A. W.  <i>et al.</i>","Englishabstract":"In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (<sup>32</sup>S, <sup>34</sup>S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria <i>Thiomargarita namibiensis</i> which oxidize sulfide (H<sub>2</sub>S) to sulfate (SO<sub>4</sub><sup>2-</sup>) using sea water nitrate (NO<sub>3</sub><sup>-</sup>) as the terminal electron acceptor. Microbial sulfide oxidation (SO<sub>x</sub>) is highly efficient, and the model predicts intense cycling between SO<sub>4</sub><sup>2-</sup> and H<sub>2</sub>S driven by coupled SR and SO<sub>x</sub> at rates exceeding 6.0 mol S m<sup>-2</sup> y<sup>-1</sup>. More than 96% of the SR is supported by SO<sub>x</sub>, and only 2–3% of the SO<sub>4</sub><sup>2-</sup> pool diffuses directly into the sediment from the sea water. A fraction of the SO<sub>4</sub><sup>2-</sup> produced by <i>Thiomargarita</i> is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H<sub>2</sub>S production is trapped as sedimentary sulfide, mainly pyrite (FeS<sub>2</sub>) and organic sulfur (S<sub>org</sub>) (~0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS<sub>2</sub> and S<sub>org</sub> show an isotope composition of ~5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H<sub>2</sub>S of <sup>34</sup>S towards the sediment-water interface suggests that <i>Thiomargarita</i> preferentially remove H<sub>2</sub><sup>32</sup>S from the pore water. A fractionation of 20–30‰ was estimated for SO<sub>x</sub> (eSOx) with the model, along with a maximum fractionation for SR (eSR–max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by <i>Thiomargarita</i> could, in principle, allow the measured isotope data to be simulated using much lower fractionations for eSOx (5‰) and eSR (78‰).","AbstractOtherLang":null,"BibLvlCode":"AS","StandardTitle":"An integrated sulfur isotope model for Namibian shelf sediments","OrigTitleLangCode":"en","OrigTitleLangCodeExtended":"eng","OrigTitleLangID":15,"DateLastModified":{"date":"2024-12-10 01:32:52.928458","timezone_type":1,"timezone":"+01:00"},"UserAccessRight":null,"UserAccID":null,"AuthorKeywords":null,"OtherDescriptors":null,"Notes":null,"AnaPub":2009,"MonPub":null,"DateUpdate":"2014-10-23","DateCreate":"2011-11-22","SecASFANote":null,"ConfID":null,"PeerRev":1,"VlizCoreFlag":1,"WoScode":"WOS:000264518500010","VABBcode":null,"OpenAcc":0,"DOI":"10.1016/j.gca.2008.12.015"},"refs":null,"anarec":{"AnaID":209777,"PubliDate":2009,"Pagination":"1924-1944","XtraPublOfAnaID":null,"ISBN":null,"Volume":"73","Issue":"7","BRefMon":null,"BRefMonRR":null,"BRefXtra":null,"BRefXtraRR":null,"SerBRefID":42901,"SerRR":"Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc..  ISSN 0016-7037; e-ISSN 1872-9533","StandardTitleSer":"Geochimica et Cosmochimica Acta","ISSN":"0016-7037","AbbrevSer":"Geochim. Cosmochim. Acta","StandardTitleMon":null,"StartPage":1924,"Pages":21,"ToPubliDate":null,"BRefBibLvlCode":"S","SerNotes":null},"monrec":null,"serrec":null,"relations":null,"relationsRev":null,"addrec":null,"othpubs":null,"ownerships":null,"authors":[{"AutName":"Dale","Firstname":"Andrew","Initials":"A. 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