Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the gulf of California |
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| Authors: | MB Goldhaber IR Kaplan |
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| Institution: | Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024 U.S.A. |
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| Abstract: | Sediment cores were taken from the Gulf of California, and pore waters recovered by mechanical squeezing. The chemistry and isotopic abundance of sulfur in these pore fluids were compared with coexisting solid phases to deduce the mechanisms involved in pyrite formation. The results suggest that burrowing activities of benthonic organisms supply sulfate sulfur to sediments to depths of approximately 0.5 m from the surface. This is inferred from essentially constant pore water concentration profiles of dissolved ions in horizons where sulfate reduction is demonstrated by the presence of iron sulfides.For a core from Pescadero Basin, it is estimated that beneath the mixed zone, diffusion adds 0.4% sulfur by dry weight of sediment, whereas burial of sulfate adds less than 0.1% sulfur. It is shown that diffusion can add isotopically light sulfur to sediments, due to more rapid relative addition of 32SO42? compared to 34SO42? down a concentration gradient maintained by bacterial processes. The overall net isotopic value of the sulfate so added is . The depth distribution of S-isotope in sulfur is controlled by the balance between a bacterial kinetic isotope effect preferentially removing 32S relative to 34S, and the supply of sulfate by diffusion. The isotopic fractionation factor, α, calculated by a mathematical formulation which takes diffusion into account, is larger (1.060±0.010) than when sulfate reduction is assumed to occur in a closed system (1.035). The larger value is supported by the sulfur isotope distribution in metastable iron sulfide. Essentially, the same open-system α was calculated for a core from Carmen Basin. |
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