Microbial mineralization of organic matter: mechanisms of self-organization and inferred rates of precipitation of diagenetic minerals

Abstract

Carbonate concretions attract study because, unlike intergranular cements, they form conspicuous spheroidal or laterally extensive bodies. However, they pose a fundamental challenge to uniformitarianism because no concretions identical to geologically preserved ones are forming today. Nevertheless, understanding their origin can be accomplished by simulation of geological processes, using present-day processes and pore-water compositions. The successive reactions (mainly microbial) degrading organic-matter during sediment burial produce inorganic species which may form carbonate and sulphide minerals and can be characterized by stable isotope and chemical compositions. Pyrite-rimmed, spheroidal carbonate carbonate concretions (Jurassic) resulted from outward diffusion 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 proceed because the reaction was buffered by precipitation of carbonate produced by methanogens degrading more deeply buried organic matter. Byproducts of the reactions may either inhibit or promote initiation of similar precipitation reactions locally. The former case leads to situations where initial random localization of reaction sites causes self-organized reaction within the sediment (applicable to the Jurassic example). 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 mineral precipitates observed in geological samples. Calculated rates of mineral precipitation give minimum durations 7400 to 52000 years, much shorter than previous estimates. These results suggest that low rates of microbial sulphate reduction, relative to present day measured values, were needed and accord with the inferred depth of formation and pore-water sulphate concentrations.