FOSSIL MINERALIZATION IN THE EDIACARAN GAOJIASHAN BIOTA: A CASE STUDY OF SOFT-BODY PRESERVATION THROUGH PYRITIZATION

Previous taphonomic investigations of soft-body preservation have focused on processes and conditions that delay degradation or destruction, including anoxia, lessened bioturbation, inhibition of autolytic decay, and early diagenetic sealing of sediments by carbonate cements. These processes facilitate, but do not guarantee, soft-body preservation. Constructive processes, serving to stabilize or replicate organic tissues through mineralization, are additionally necessary for soft-body preservation. In this case study, we investigate how authigenic pyritization has contributed to the preservation of the soft-bodied tubular organism Conotubus hemiannulatus in the Ediacaran Gaojiashan biota. Conotubus hemiannulatus is typically preserved in calcareous siltstone or microbial carbonate. Its organic tube walls provided a substrate for pyrite nucleation, with pyrite crystals growing away from tube walls. Within the tube, pyrite crystals grow centripetally, often leaving a central lumen that either remains open or is filled with later diagenetic calcite and then zoned calcite-dolomite crystals. These authigenic and diagenetic minerals were examined using SIMS pyrite sulfur isotope analysis, carbonate carbon isotope analysis, EDS elemental mapping, single-crystal XRD, and UV light microscopy. Our data indicate that initial pyrite precipitation was fueled by rapid and localized degradation of Conotubus hemiannulatus tube walls through bacterial sulfate reduction, which slowed as degradable organic matter within the tube was exhausted. Thus, pyritization often terminated before the tube was completely filled with pyrite. The remaining space within the tube allowed compaction of the fossil or was filled with generations of carbonate minerals with carbon and oxygen isotope signatures distinct from the surrounding sediments.