Compilation of stable isotope data (delta 18 O and delta 13 C) for marine calcite, siderite, and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends. Although many of the dolomites have delta 18 O values that are compatible with precipitation from unaltered seawater at low temperatures, most of the calcites and siderites are far too depleted in 12 O for such an origin. This is surprising, as most of the concretions appear to have formed very early in the diagenetic history of the sediments, often within several meters of the sediment/water interface. The most likely cause of the anomalously depleted oxygen values is a decrease in pore-water delta 18 O values during early diagenesis--a result, perhaps, of a significant influx of meteoric water into the marine shelf environment, or water-mineral interaction (e.g., precipitation of 18 O-enriched minerals). The carbon isotopic field for dolomite is quite different from that for calcite and siderite. Highly positive delta 13 C values are fairly common only for dolomites. This indicates that, while dolomite concretions often form in sediments where methanogenesis is rapid, precipitation of calcite and siderite concretions is restricted to sediments where either no methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid delta 13 C values. This variation in carbon isotopic composition among the different minerals suggests that the rates of organic-carbon oxidation and sedimentation may control the mineralogy of carbonate concretions. High organic-carbon oxidation rates favor dolomite, and lower oxidation rates favor calcite or siderite.