THE MIGRATION OF IRON AND SULFUR DURING THE EARLY DIAGENESIS OF MARINE SEDIMENTS (PYRITE, CONNECTICUT, GEORGIA)

Authigenic pyrite is a common constituent of modern and ancient marine sediments. A facet of pyrite formation that has not received intensive investigation is the formation of localized concentrations of reduced iron and sulfur such as pyrite concretions and pyritized fossils. In this study the conditions, mechanisms, and controls of a modern example of early diagenetic pyrite aggregate formation is examined.;Examination of the features associated with the pyritization of mollusc shells shows that precipitation of pyrite on the shells involves transformation from a previously existing acid-volatile sulfide phase. Shells undergoing pyritization appear to act as passive templates for pyrite precipitation; evidence of a corrosive (towards CaCO3) replacement process is not observed. Independent of pyrite precipitation, mollusc shells from both the Connecticut and Georgia sediments exhibit a black pigmentation associated with alteration of the organic matrix of the shell. Laboratory syntheses of blackened shells and chemical analyses of the pigment indicate that this substance is an organo-sulfur compound formed by the reaction of the organic matrix of the shell with diagenetically produced dissolved sulfide.;Through the use of diagenetic models and laboratory-determined rates of sulfate reduction, it was found that variation in the reactivity (rather than concentration) of decomposable sedimentary organic matter is the primary cause of the observed differences in sediment chemistry and associated patterns of iron and sulfur migration.;The variation in organic matter reactivity between the Connecticut and Georgia sediments is also associated with differences in the degree of early diagenetic calcium carbonate dissolution present in these sediments. In sediments containing labile organic matter, the presence or absence of early diagenetic calcium carbonate dissolution is apparently a good indicator of the stability of environmental conditions at the time of sediment deposition.;Based on the distribution of pyritized shells, two similar sediment sampling localities were chosen for study: Sapelo Island, Ga. (pyritized shells present) and Indian Neck, Ct. (pyritized shells absent). The sediments from Connecticut contain high concentrations of both total dissolved sulfide and solid phase ferrous sulfides, while the sediments from Georgia are, in general, less sulfidic. The sediments from Georgia are also characterized by the presence of an interval of sediment extending down to 6-8 centimeters below the sediment-water interface in which post-oxic anoxic-nonsulfidic conditions are present. The presence of this interval of sediment appears to be critical to the pyritization of mollusc shells as the chemical conditions present within it are conducive to the migration and localized concentration of both iron and sulfur.;Finally, variation in the sources of decomposable organic matter supplied to the Connecticut sediments is shown to exert an important control on the value of the C/S (organic carbon-to-pyrite sulfur) ratio present in these sediments. (Abstract shortened with permission of author.)...