Non-steady state microbiological diagenesis and the origin of concretions and nodular limestones

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Abstract
Summary: Morphological, chemical, isotopic and textural studies of pyritiferous carbonate concretions from the Jet Rock (Lower Jurassic, England) are all consistent in indicating an origin by anaerobic methane oxidation. Concretionary growth occurred in a stratigraphically thin, sub-surface zone of uncompacted sediment, where methane (diffusing from below) was consumed to stimulate a late phase of locally intense sulphate reduction, which caused the precipitation of CaCO 3 and isotopically heavy FeS 2 . The carbon isotope composition of the concretionary carbonate was indistinguishable from that of sulphate reduction (using sedimentary organic matter), due to the simultaneous diffusion of isotopically light CH 4 and isotopically heavy dissolved carbonate from the underlying methanogenic zone. The alkalinity generated by anaerobic methane oxidation resulted in precipitation being restricted to a thin, vertically confined zone beneath the sediment/water interface. A pause in deposition then allowed continued precipitation in the form of concretionary growth. Different stages in the evolution of isolated concretions into nodular limestones depend on the magnitude of carbonate supersaturation in the zone of anaerobic methane oxidation and the duration of the pause in sedimentation. Vertically confined zones of carbonate precipitation may also be developed deeper in the sediment due to iron reduction, a further pause in deposition can then produce isotopically heavy carbonate concretions in the zone of methanogenesis.