Siderite concretions from freshwater and from marine and brackish-water shales of Pennsylvanian age have average delta C 13 contents (and standard deviations) of -4.75 (1.13) and +1.53 per mil (1.76), respectively, the isotopic values clearly discriminating between the ancient environments. Siderite concretions from brackish-water shales have an average delta C 13 content of +0.54 per mil (1.98); they thus are discriminated from the concretions from freshwater shales but not from concretions from marine shales.

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