INFLUENCE OF BACTERIAL SURFACES AND HALOS IN THE PYRITIZATION OF FOSSILS

Fossils representing a spectrum of taphonomic states from Ordovician and Devonian pyrite-bearing beds suggest that at least three major factors control whether the nonmineralized anatomy of organisms is preserved by pyritization. Factors include: 1, burial in a dysoxic to anoxic environment or microenvironment; 2, the ratio of sulfide ions to dissolved reactive iron in sediment pore water; and 3, the presence of a reactive bacterial surface associated with decaying organic material. Under anoxic or dysoxic conditions, breakdown of organics during bacterial dissimilation processes produces sulfide ions through sulfate reduction, and the sulfide : reactive iron ratio determines the distance to which sulfide will diffuse from an organic mass until it is precipitated as an FeS intermediate or directly as pyrite.

Nonmineralized tissues are usually preserved in pyrite by thin crusts, whereas biomineralized parts alone are more commonly observed to be associated with pyrite concretions. The extent of pyrite precipitation, and the type(s) of tissue preserved through it are inferred to be related to the development of reactive bacterial surfaces on or surrounding decaying organisms. Pyrite encrustation of nonmineralized tissues suggests that bacterial surfaces were in direct contact with the tissues. However, enclosure of biomineralized parts in pyrite concretions suggests development of a bacterial halo around a decaying organism, similar to that observed in taphonomic experiments on modern arthropods. A bacterial halo may negatively affect the preservation of nonmineralized parts (precluding preservation of nonmineralized tissue) by adding metabolizable material to a decaying organic mass or allowing for greater diffusion of sulfide through pore waters, thus causing precipitation of pyrite well away from the center of a decaying mass.