A study has been made of the sulphur isotope ratios of different groups of sulphur compounds in recent marine sediments from the Pedernales field of northeastern Venezuela at depths of 20, 80, and 160 feet. Data are presented for these ratios for sulphate sulphur, elemental sulphur, pyrite sulphur, organic sulphur, and those sulphides soluble in hydrochloric acid.

It was found that all the sulphur including the organic materials in sediment samples obtained at the various depths are depleted in S³⁴ within a range of 10-20^pmil as compared with the original sea-water sulphate isotope ratio. The results indicate further that bacterial reduction of the marine sulphate is almost complete at depths of 12-20 feet and that sulphates found at greater depths are probably formed by oxidation of pyrite in situ without any further isotope fractionation.

It is suggested that over very long periods of time, a partial exchange of sulphur isotopes between sulphate and pyrite in close contact takes place, which could account for the high S³⁴ depletion found for pyrite and sulpbate of ancient sedimentary rocks.