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Title

Isotopic composition of authigenic sulfide minerals at the base of the sulfate-reduction zone: Implications for diagenesis at the sulfate-methane interface

Document Type

Conference Presentation

Publication Date

12-2000

Abstract

Pore-water sulfate concentration and sulfur isotopic composition within marine sediments are controlled by microbially-mediated sulfate reduction processes. Oxidation of sedimentary organic matter typically drives interstitial sulfate depletion in marine sediments but anaerobic methane oxidation (AMO) is an important sulfate reduction process in deep-water, methane-rich sediments. In modem, non-euxinic, deep-water marine sediments, the isotopic composition of sulfide minerals (native sulfur, iron monosulfides, and pyrite) within the sulfate reduction zone (SRZ) is highly depleted in 34S (d34S = -25 to -50o/oo CDT). This depletion is attributed to sulfur disproportionation and other sulfur cycling processes in the upper SRZ. However, the isotopic composition of sulfur residing in authigenic sulfide minerals formed near the sulfate-methane interface (SMI), at the base of the sulfate reduction zone, is typically enriched in 34S. Interstitial sulfate from Blake Ridge sediments (piston core 11-8, and ODP sites 994,995) have d34S values ranging from +29.1 to +51.6o/oo CDT. The average sulfur isotopic composition of bulk sulfide minerals within the SRZ is -38.8 + 5.3o/oo, whereas d34S values average -12.1 + 12.7o/oo at and below the SMI. Maximum 34S enrichments at the 3 sites are +23.6, -12.7, and -12.2o/oo, respectively, showing 34S enrichments of 62.4, 26.1, and 26.6o/oo relative to the average value for sulfide minerals in the overlying sulfate-containing sediments.

Sulfate depletion by AMO at the SMI localizes production of interstitial dissolved sulfide, resulting in horizon-restricted formation of authigenic sulfide minerals. Because natural microbially-mediated sulfate reduction has a large isotopic fractionation (a =1.029-1.059), sulfate, dissolved sulfide, and sulfide minerals formed at the SMI contain distinctive sulfur isotopic compositions enriched in 34S. The occurrence of 34S-enriched sulfide minerals marks the location of active sulfide mineralization at the base of the sulfate reduction zone. When the stratigraphic position of the SMI remains static, authigenic sulfide mineral phases become concentrated over time, and their bulk d34S values show intensified 34S enrichment. We speculate that these 34S enrichments in sulfide minerals will persist in the geologic record and identify the stratigraphic locations of former SMIs.

Conference Name

American Geophysical Union

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