Vertical distribution of dissolved sulphur species in coastal marine sediments

Few field data are yet available to identify factors which control the speciation of dissolved sulphur in marine sedimentary environments, particularly in H2S-poor porewaters of deep coastal sediments. The vertical distribution of major sulphur species in the Saguenay Fjord (Canada) sediment porewaters was determined and the influence of bioturbation is discussed. Concentrations of dissolved sulphides and polysulphides (ΣH2S + ΣSn2−) in the top 40 cm of sediment were usually very low (< 1 μM). This feature is attributed to the abundance of reactive iron minerals (up to 45 mg g−1 dry wt) which precipitate dissolved sulphides as FeS and FeS2. Under the non steady-state sedimentation conditions encountered near the head of the Fjord, where an organic-poor clay layer was deposited following a landslide 30 years ago, several sulphur species generated by active anaerobic oxidation of organic matter in the indigenous sediments were detected along with their oxidation products. The concentration of these soluble oxidation products of S(−II), including thiosulphate and sulphite (S2O32− + SO2−3), elemental sulphur (S80), and organic sulphides (ΣRSR) peaked just above the allochtonous clay layer. This peculiar geochemical behaviour is attributed to the sustained bioturbational transport of oxidized particles and molecular oxygen to a zone of active SO42− reduction. These field results provide fresh insights into our general understanding of the speciation of dissolved sulphur in sedimentary marine environments.