Turning Pyrite Concretions Outside-In: Role of Biofilms in Pyritization of Fossils

Abstract

Studies integrating sedimentary geochemistry, diagenesis, and taphonomic experimentation provide new understanding about the development of pyrite concretions around organisms and the exceptional preservation of some nonmineralized tissues by pyrite crusts.As now interpreted, at least three factors influence the preservation of organisms by pyrite: 1) burial in a low oxygen environment or microenvironment; 2) ratio of sulfide ions to dissolved reactive iron in sediment pore waters; and 3) presence of reactive biofilms (microbial assemblages) associated with decaying organic material. Under low oxygen conditions, breakdown of organics allows for the release of sulfide ions into sediment pore waters, where they combine with reactive iron ions to form iron sulfides. Pyrite often preserves biomineralized structures (primarily shells) through concretionary overgrowths, whereas non-biomineralized tissues (such as internal soft parts) are usually preserved by thin pyrite crusts.The extent of pyrite precipitation and the type(s) of organically produced material(s) preserved by FeS2, seem to be related to the development of either reactive bacterial coatings that were in direct contact with decaying organic tissues or microbial assemblages (including bacteria and probably fungi) that formed halos around decaying organic tissues. Precipitation of pyrite to form a concretion apparently begins at multiple sites within a microbial halo, not just on the surface of the decaying mass.