Abstract

Chemical and isotopic studies of Jurassic carbonate concretions indicate an origin by anaerobic methane oxidation. Concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late, rejuvenated phase of sulfate reduction. Similar chemical and isotopic characteristics are shown by diagenetic carbonates in several limestone-shale sequences, and an origin by anaerobic methane oxidation is proposed for these sequences also. The evolution of isolated concretions into diagenetic limestones depends on the extent of, and depth variations in, carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus. Diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive δ34S values than the adjacent shales.

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