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ABSTRACT: A Model for Bacterially Controlled Deep-Water Anaerobic Mineralization of Glauconite and Phosphorite

Joel B. Thompson, Terry J. Beveridge, William S. Fyfe

A working model for the authigenesis of glauconite and phosphorite on continental margins associated with upwelling will be presented. This model is based on data from modern and ancient glauconitic and phosphatic deposits and existing bacteriological and geological literature. Glauconitic and phosphatic facies are commonly associated with oxygen-minimum zones (OMZs). Modern OMZs can consist of one or more redox interfaces depending on the abundance of nitrogen, iron, and sulfur. Multiple interfaces are derived from a series of bacterial and chemical redox reactions that occur at various depths within the OMZ and sediment. Diffusion of nitrogen, iron, and sulfur between the redox interfaces creates a recycling mechanism for the long-term maintenance of zones of denitrific tion, iron reduction, and sulfate reduction at either the sediment/water interface or within the sediment. The location of these geochemical facies is controlled in part by anaerobic bacterial respiration and sedimentation. The bacteriological and geological evidence indicates that glauconitization occurs within the denitrification zone and that phosphatization occurs within the iron reduction zone. Large regional microenvironments of glauconitization and phosphatization are created by microbially mediated redox reactions involving nitrogen and iron cycling under low-oxygen conditions. The numerous bacteria that create these geochemical facies may also be important in the nucleation of these minerals and as a potential hydrocarbon source in these organic-rich rocks.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990