Sulfate reduction and hydrogen sulfide oxidation rates in marine sediments with cryptic sulfur cycling

Sedimentary systems affected by high fluxes of reactive iron develop a so-called ‘cryptic’ sulfur cycle in which hydrogen sulfide is nearly fully reoxidized and its concentrations of hydrogen sulfide in the porewaters are in the submicromolar range. The sediments of the Gulf of Aqaba represent a classic example of such a system. The goal of this work was to provide quantitative constraints on hydrogen sulfide concentrations in the sediments of the Gulf of Aqaba. Sulfate reduction rates in the sediments of the Gulf of Aqaba were found to be lower than in marine sediments that had not been affected by high fluxes of the reactive iron, while the rate constants of hydrogen sulfide oxidation were found to be higher than in the highly reactive iron-rich sediments of the Svalbard fjord. A combination of slow rates of sulfate reduction and fast rates of hydrogen sulfide oxidation results in concentrations of hydrogen sulfide in the porewater, both measured and calculated, that are below 100 nmol l⁻¹. We suggest that a similar cycling of sulfur species may occur in organic-matter-poor marine systems situated in dry environments with highly reactive iron mineral delivery, such as the Red Sea and the Atlantic Ocean in the vicinity of the Sahara.