New thoughts on deep sedimentary sulfur cycling ( the deep " cryptic S cycle " )

The sulfur cycle in marine sediments exerts a major control on the redox state of the ocean and atmosphere. The overall driver in the sulfur cycle is the microbial mediated sulfate reduction to sulfide (SR). We broadly divide sedimentary SR into two categories: organoclastic sulfate reduction (OSR), which acts as the terminal electron acceptor during the degradation of buried organic matter; and methanotrophic sulfate reduction (MSR), better known as the sulfatedependent anaerobic oxidation of methane (AOM). When integrated over depth, the vast majority of sulfate reducing activity occurs within the surface sediments (e.g. 0 – 30 cm below surface), whereas low rates of OSR and a distinct a distinct layer of MSR (where the sulfate and methane gradients overlap) characterize deeper sedimentary sulfur cycling. In near-surface sediments a vigorous oxidative sulfur cycle operates, and only a small fraction of the sulfide produced via OSR becomes permanently buried as pyrite (FeS2).