EXPERIMENTAL CONDITIONS CONDUCIVE TO FOSSIL PYRITIZATION AND PATTERNS OF OCCURRENCE

Sedimentary pyrite is the product of organic carbon decay by sulfate reducing bacteria under anoxic conditions. The same processes cause soft tissues of decaying organisms to be replicated by pyrite. Numerous famous fossil lagerstätten, including the Chengjiang, Fezouata, and Beecher's Trilobite Bed are characterized by an abundance of fossil-associated pyrite, and it is a common component in Burgess Shale-type preservation. Since pyritization occurs on a reaction front between bacterially produced sulfides and the dissolved reduced iron in porewaters, diffusion-limitation is a requirement for maintaining the reaction front in close proximity to the surface of the organism, so fine-grained sediments are preferred. The limited rate of diffusion, however, could also limit the availability of sulfate that the bacteria require for their metabolism, and dissolved iron, which is usually present only in low concentrations as-is.

In order to elucidate the conditions necessary for fossil pyritization, a series of actualistic taphonomy experiments were conducted by inoculating deceased arthropods with sulfate-reducing bacteria under anoxic conditions in artificial seawater and sediment. Some samples were decayed under batch conditions, others in steady-state conditions, where some seawater is replaced daily to prevent ionic constituents from becoming dilute. Sediment size was varied to test the effects of differing diffusion rates. Sediment composition (quartz, kaolinite, calcite) and seawater iron concentrations and pH values were also varied. Sediment size was varied to test diffusion limitations. pH, sulfate concentrations, and iron concentrations were tested regularly to monitor the rate of decay and mineral precipitation. The resulting “fossils” and sediment samples were analyzed via variable pressure scanning electron microscopy and energy dispersive X-ray spectrometry to determine the locations, compositions, and crystal forms of associated mineral phases. The occurrences of pyrite and other iron sulfides on the surface of the organisms has ramifications for our current understanding of fossil pyritization.