Mineralogical and carbon and oxygen stable isotope compositions in siderite-rich concretions from a marsh within the Mississippi River Deltaic Plain are highly variable. The concretions have sideritic interiors and become more calcitic and dolomitic (ankeritic) near their margins. delta 13 C values of these concretions range from - 10.0 to + 7.4 per thousand (PDB) and delta (super 1*) O values range from -2.4 to - 8.9 per thousand (PDB). The stable isotope compositions exhibit two general trends. One is a growth trend that shows delta 13 C and delta 18 O depletion from the centers to the margins of the nodules and bands. The other is a progressive burial trend that shows 13 C enrichments and 18 O depletions at the centers of the concretions with depth. A zonal model for carbonate precipitation with emphasis on an early fresh water, non-sulfidic, methanic phase, followed by a sulfidic stage associated with marine inundation of the marsh, explains much of the observed carbon isotopic variations. Mixing of fluids plus precipitation in a partially closed chemical microsystem accounts for most of the oxygen isotope fractionations documented between growth layers and casts doubt on the utility of change in delta 18 O shifts as an indicator of depth or relative temperature of concretion formation for concretions that form in coastal environments.