Diagenetic siderite and other ferroan carbonates in a modern subsiding marsh sequence Journal Article uri icon

DCO ID 11121/1466-3571-6681-9711-CC

in language

  • eng

year of publication

  • 1992


  • 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-C-13 values of these concretions range from -10.0 to + 7.4 parts per thousand (PDB) and delta-O-18 values range from -2.4 to -8.9 parts per thousand (PDB). The stable isotope compositions exhibit two general trends. One is a growth trend that shows C-13 and O-18 depletion from the centers to the margins of the nodules and bands. The other is a progressive burial trend that shows C-13 enrichments and O-18 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-O-18 shifts as an indicator of depth or relative temperature of concretion formation for concretions that form in coastal environments.


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