Mudrock‐hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition

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Abstract
Existing interpretations of cement textures and isotopic compositions may significantly under‐estimate the depth and duration of concretionary growth. Minus‐cement porosities can commonly under‐estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non‐cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over‐estimate rates of porosity loss at shallow (13 C‐depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local‐equilibrium model), or created where external fluids are introduced (the fluid‐mixing model).

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