An integrated sulfur isotope model for Namibian shelf sediments

Abstract

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 (H 2S) to sulfate ( SO 4 2 - ) using sea water nitrate ( NO 3 - ) as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between SO 4 2 - and H 2S 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 SO 4 2 - pool diffuses directly into the sediment from the sea water. A fraction of the SO 4 2 - 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 H 2S production is trapped as sedimentary sulfide, mainly pyrite (FeS 2) and organic sulfur (S org) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS 2 and S org 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 2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H 2 32S from the pore water. A fractionation of 20–30‰ was estimated for SOx (ε SOx) with the model, along with a maximum fractionation for SR (ε SR–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 ε SOx (5‰) and ε SR (78‰).