Significance of stable carbon isotope trends in carbonate concretions formed in association with anaerobic oxidation of methane (AOM), Middle and Upper Devonian shale succession, western New York State, USA

Abstract

Carbonate concretions are concentrated along discrete stratigraphic horizons within Middle and Upper Devonian carbonaceous black and organic-deficient gray shale of western New York State. Textural characteristics preserved throughout studied concretions, including a clay grain microfabric typical of flocculated clay and spherical algal cysts, are consistent with a model entailing the formation of low-density, compaction-resistant calcium carbonate masses at shallow burial depth, within the zone of bacterial sulfate reduction. Burial of nascent concretions to the sulfate methane transition zone (SMTZ) was accompanied by the nearly complete infilling of porosity, the diagenetic consequence of the anaerobic oxidation of methane (AOM). Each concretionary horizon, then, defines the position of the SMTZ stabilized during an episode of reduced sedimentation rate. Most analyzed concretions (75%) of the Middle Devonian Marcellus Formation into the Upper Devonian Gowanda Formation display center-to-edge profiles of increasing bulk δ13C; far fewer concretions are characterized by radial profiles of decreasing δ13C. The common radial profiles of increasing δ13C can be attributed to AOM-related kinetic carbon isotope fractionation under diagenetic conditions of limited methane, which favors the production of dissolved inorganic carbon of increasing δ13C. Less common concretions displaying center-to-edge profiles of diminishing δ13C may record the anaerobic oxidation of more strongly 13C-depleted methane produced by carbon dioxide reduction, methanogenesis related to the degradation of organic matter within and near the SMTZ, or AOM back flux, an enzyme-mediated equilibrium isotope effect. These processes are favored by sulfate-limited diagenetic conditions more typical of the lower SMTZ and immediately underlying upper methanogenic zone (MEZ). Contradictory isotope profiles of texturally similar concretions can be accounted for by diagenesis associated with an extended SMTZ (ESMTZ) comprising the methane-limited SMTZ and the underlying sulfate-deficient upper MEZ. Vertical shifts of the diagenetic horizons of the ESMTZ induced by variable methane flux would have juxtaposed concretions displaying seemingly incompatible isotopic histories.