Abstract

Dolomite precipitation from seawater is normally inhibited by the presence of sulfate, the low activity of the carbonate ion, and the high enthalpy of hydration of the magnesium ion. However, in modern microbial sediments, anoxic degradation of cyanobacterial material by sulfate-reducing bacteria can significantly alter ambient water chemistry: sulfate is removed, and enzymatic breakdown of proteins releases ammonia, thereby increasing pH and carbonate alkalinity to levels necessary for dolomite formation. Magnesium may be supplied from complexed ion pairs in solution, and from the breakdown of cyanobacterial sheaths and mucilage. In the ancient past, where benthic microbial communities dominated the depositional environment, it is postulated that large-scale, shallow subsurface modification of ambient waters by sulfate-reducing bacteria created conditions favorable to widespread dolomite formation. Early diagenetic cherts from the Cambrian Eilean Dubh Formation of northwestern Scotland preserve successive stages of the taphonomic history of cyanobacterial biofabrics in sediments of a continuously aggrading shelf. Depth-related sequential degradation of cyanobacterial mat is accompanied by the appearance and increase in abundance of authigenic dolomite, culminating in a crystalline dolomitic fabric with only residual organic clots. Silicified oolitic grainstone fabrics preserve various ooid morphologies. These form a diagenetically modified sequence in which progressive degradation of component microbial material, together with leaching of the ooid shell, is accompanied by the appearance and increase in abundance of intragranular authigenic dolomite. Total dissolution of ooids resulted in the release of a package of crystals as sediment; similar sequences are observed in peloids and intraclasts. Ensuing dolomitic rock fabrics bear no clue to their origin, except where grains are preserved in early-silicified "windows". These relationships can be explained by biochemical modification of porewaters during organic diagenesis, and provide evidence for widespread, authigenic dolomite formation in the shelf sediments. This organic degradational model may serve as a basis for understanding the origin of thick platformal dolostone sequences of the Proterozoic.

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