The isotopic mass balance of sulphur in oceanic sediments (the Pacific Ocean as an example)

Dissimilatory sulphate reduction is the main biogeochemical reaction responsible for the cycle of sulphur and its isotopic composition in modern oceanic sediments. The maximal intensity of this process, measured with the help of radioactive isotopes, is typical of sediments in the peripheral regions of the ocean to a depth of 1000 m and makes up 10–60 μg S/kg wet silt/day.The minimal enrichment with ³²S of the buried, reduced sulphur and the maximal accumulation of heavy isotopes in sulphate sulphur of pore waters are observed under conditions of the most intensive biogeochemical processes. Further away from continents the intensity of bacterial sulphate reduction diminishes to a fraction of 1 μg S/kg wet silt/day, the reduced sulphur becomes enriched in ³²S (up to ? 50.0‰), and the isotope composition of sulphate sulphur in pore waters approximates that in oceanic water and the solid phases of sediments.The amount of reduced sulphur in the total sulphur buried in modern sediments of the Pacific Ocean is twice the amount of sulphate sulphur. The δ³⁴S value of the total sulphur buried and removed from the cycle makes up ? 17.9‰. This value is comparable to that of the average isotope composition of total sulphur in clay rocks of the lithosphere.