Quantitative constraints on the cryptic sulfur cycling in marine sediments affected by high fluxes of reactive iron from terrigenous sources

In marine sediments with high content of reactive iron oxides, nearly all hydrogen sulfide, which is produced by microbial sulfate reduction, is reoxidized to sulfide oxidation intermediates (sulfur, thiosulfate, and sulfite) and, eventually, to the terminal oxidation product, sulfate. Such sulfur cycling is called “cryptic” and is found in a wide variety of marine and limnic systems. Cryptic sulfur cycling may be fast but leaves scarce geochemical evidence. One of the marine systems, which is characterized by a cryptic sulfur cycle in the sediments is the Gulf of Aqaba, Red Sea. The gulf is strongly affected by high fluxes of aeolian dust deposition from the adjacent deserts, especially from Sahara, which is rich in reactive Fe(III) and Mn(IV) phases. Multiple lines of evidence, including presence of trace amounts of hydrogen sulfide, sulfide oxidation intermediates and pyrite in the sediments, isotopic composition of sulfate as well as direct measurements of microbial sulfate reduction rates prove the presence of cryptic sulfur cycling in the sediments of the Gulf of Aqaba. Quantification of the rates of microbial sulfate and iron reduction as well as of abiotic oxidation of hydrogen sulfide in the sediments allows us to provide for the first time quantitative constraints on the cryptic sulfur cycling in marine sediments.