MICROBIAL MINERALIZATION OF ORGANIC-MATTER - MECHANISMS OF SELF-ORGANIZATION AND INFERRED RATES OF PRECIPITATION OF DIAGENETIC MINERALS

Carbonate concretions attract study because, unlike intergranular ceme nts, they form conspicuous spheroidal or laterally extensive bodies. H owever, they pose a fundamental challenge to uniformitarianism because no concretions identical to geologically preserved ones are forming t oday. Nevertheless, understanding their origin can be accomplished by simulation of geological processes, using present-day processes and po re-water compositions. The successive reactions (mainly microbial) deg rading organic-matter during sediment burial produce inorganic species which may form carbonate and sulphide minerals and can be characteriz ed by stable isotope and chemical compositions. Pyrite-rimmed, spheroi dal carbonate carbonate concretions (Jurassic) resulted from outward d iffusion of microbially produced sulphide which reacted with inwardly diffusing iron. Extensive, bedded siderite concretions (Coal Measures) were formed by microbial reduction of Fe(III) which could only procee d because the reaction was buffered by precipitation of carbonate prod uced by methanogens degrading more deeply buried organic matter. By-pr oducts of the reactions may either inhibit or promote initiation of si milar precipitation reactions locally. The former case leads to situat ions where initial random localization of reaction sites causes' self- organized reaction within the sediment (applicable to the Jurassic exa mple). Simulations of the Jurassic concretions' growth process, using present day pore-water solute concentrations of calcium, sulphide and iron, give results which correspond with the spatial distribution of m ineral precipitates observed in geological samples. Calculated rates o f mineral precipitation give minimum durations 7400 to 52000 years, mu ch shorter than previous estimates. These results suggest that low rat es of microbial sulphate reduction, relative to present day measured v alues, were needed and accord with the inferred depth of formation and pore-water sulphate concentrations.