Concretions represent localized precipitation of minerals; yet, why is precipitation localized, rather than disseminated? To explore this question, Bojanowski and Clarkson examine petrographic and geochemical characteristics of a series of siderite nodules from Mississippian shale of Scotland. These nodules include a homogenous microstructure, but elevated δ13C and distinct isotopic shifts from center to edge, interpreted to reflect early precipitation favored by methanogenesis in microenvironments created by localized organic detritus. These results suggest that geochemical gradients preserved in concretions may result from geochemical gradients around organic matter, rather than changing composition of water during growth.
Origin of siderite concretions in microenvironments of methanogenesis developed in a sulfate reduction zone: an exception or a rule? By Maciej J. Bojanowski and Euan N.K. Clarkson