RATE LIMITATIONS ON FOSSIL PYRITIZATION UNDER EXPERIMENTAL CONDITIONS

Fossil pyritization is commonly considered a product of bacterial sulfate reduction of organic tissues. The sulfate reducing microbes produce hydrogen sulfide, which, in anoxic conditions, remains available for pyrite formation in the presence of reduced iron. Decaying organisms are usually thought to accrue pyrite while buried under fine-grained sediment in quiescent, deep-water settings. This poses no problem for fossils with a small quantity of associated pyrite, but for pervasively pyritized fossils, it causes an interesting conundrum. Organic carbon, sulfate and iron II are all necessary for fossil pyritization, but sulfate and iron II originate in seawater, and would be limited by diffusion in fine-grained sediment and static water, potentially limiting the extent of pyritization. In such diffusion-limited conditions, the local sulfate supply would become exhausted with progression of microbial degradation, a phenomenon that is reflected in the pyrite sulfur isotopes of the fossil materials (Schiffbauer et al., 2014, Nature Communications). The continued availability of sulfate and iron is dependent on diffusion rates through fine sediment.

In this series of actualistic taphonomy experiments, arthropods were decayed by the sulfur reducing bacteria Desulfovibrio salexigens under anoxic conditions in artificial seawater, buried under quartz sediment. Grain size of the sediment was varied between replicate groups to examine the taphonomic effect of porosity. Further, experiments were conducted under batch conditions, and under steady-state conditions provided by a chemostat. Seawater chemistry was tested throughout the experimental period, to determine availability of limiting factors iron and sulfate. The resulting “fossils” were evaluated based on iron sulfide, organic carbon, and other mineralogical associations and texture, taphonomic fidelity, and preferential morphological association of minerals. This approach may help to refine the current understanding of the pyritization system, and the limits imposed by diffusion.