An integrated sulfur isotope model for Namibian shelf sediments
Dale, A. W. ; Brüchert, V.; Alperin, M.; Regnier, P. (2009). An integrated sulfur isotope model for Namibian shelf sediments. Geochim. Cosmochim. Acta 73(7): 1924-1944. dx.doi.org/10.1016/j.gca.2008.12.015
In: Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc.. ISSN 0016-7037; e-ISSN 1872-9533, meer
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 (32S, 34S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargarita namibiensis which oxidize sulfide (H2S) to sulfate (SO42-) using sea water nitrate (NO3-) as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between SO42- and H2S driven by coupled SR and SOx at rates exceeding 6.0 mol S m-2 y-1. More than 96% of the SR is supported by SOx, and only 2–3% of the SO42- pool diffuses directly into the sediment from the sea water. A fraction of the SO42- produced by Thiomargarita 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 H2S production is trapped as sedimentary sulfide, mainly pyrite (FeS2) and organic sulfur (Sorg) (~0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS2 and Sorg 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 H2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H232S from the pore water. A fractionation of 20–30‰ was estimated for SOx (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 Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for eSOx (5‰) and eSR (78‰).
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