Iron sulfide formation and the in vivo pyritization of Mercenaria mercenaria (Linnaeus) in the salt marshes of St. Catherines Island, Georgia

Pyritization of the shells of living hard clams, Mercenaria mercenaria (L) and Atlantic ribbed mussels, Geukensia demissa (L) has been observed in coastal environments along the eastern U.S., from Virginia to Georgia. This phenomenon is especially prevalent in the salt marshes of St. Catherines Island, Georgia.;Hydrogen sulfide is produced by the action of sulfate reducing bacteria. Sulfide reacts with dissolved iron, precipitating mackinawite (FeS). Further reactions produce other Acid Volatile Sulfide (AVS) phases. The ultimate product is pyrite.;Mercenaria mercenaria inhabits tight anoxic burrows in tidal creek and low marsh sediments. Metabolic products from the clams support locally high concentrations of sulfate reducing bacteria. Mackinawite precipitates directly onto the clam shells. This is eventually converted to pyrite.;Three populations of living pyritized M. mercenaria were studied. Two of these inhabited tidal creek sediments, and the third existed in a low marsh area. Relict pyritized clams were collected from an erosionally exhumed 300 to 1000 year old marsh deposit.;Two hundred fifty-seven clams were emplaced into tidal creek sediments, and sub-sampled over two years. One hundred twenty-two (47%) were recovered. Of the 109 recovered from reducing environments, 52 (48%) were blackened by AVS coatings, and another 34 (31%) were pyritized. Visible pyrite was generated in less than 9 months. Only living clams were blackened or pyritized.;X-Ray Diffraction analyses confirmed that in-vivo pyritization initiates in less than 1 year, and suggested the presence of several AVS phases. Scanning Electron Microscope analyses indicated: (1) Pyrite on living bivalve shells forms through the conversion of AVS phases, such as mackinawite, greigite, troilite, and pyrrhotite. (2) Iron sulfide on shells exists as aggregates of tiny, nanometer-scale, anhedral crystals. (3) Pyrite nucleates in many small growth centers. These eventually coalesce into sheets. New growth centers form upon these sheets, imparting thickness and stability over time. (4) Iron sulfide penetrates shells along cracks and voids. Shell carbonate is not replaced. (5) In early diagenesis, shell aragonite begins to break down, and new mineral phases grow. (6) In-vivo pyritization is probably associated with a subset of pyritized fossils.