Isotopic analyses of pyritic, organic, and other sulfur compounds present in cores of modern marine sediments from the Pedernales oil-field area showed a depletion in S 34 of 10-20 parts per mil relative to sea-water sulfate, which is assumed to be the source of most of the sulfur in the sediments. This isotope fractionation is attributed to bacterial reduction of marine sulfate which takes place near the surface, with reduction essentially complete at depths of 12-20 feet. Sulfate is formed at greater depths mainly by oxidation of pyrite, involving little or no fractionation.

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