Framboidal pyrite shroud confirms the 'death mask' model for moldic preservation of Ediacaran soft-bodied organisms

The mechanisms by which soft-bodied organisms were preserved in late Ediacaran deep-marine environments are revealed by petrographic and geochemical investigation of fossil-bearing surfaces from the Conception and St. John’s groups (Newfoundland, Canada). Framboidal pyrite veneers are documented on fossil-bearing horizons at multiple localities. The pyrite is interpreted to have formed via microbial processes in the hours to weeks following burial of benthic communities. This finding extends the ‘death mask’ model for Ediacaran soft-tissue preservation (cf. Gehling, 1999) to deep marine settings. Remineralization and oxidation of pyrite to iron oxides and oxyhydroxides is recognized to result from recent oxidation by meteoric fluids in the shallow subsurface. Consideration of other global Ediacaran macrofossil occurrences reveals that pyrite has now been found in association with Ediacaran macrofossils preserved in all four previously described styles of moldic preservation (Flinders-, Conception-, Fermeuse- and Nama-type). This suggests that replication of external morphology by framboidal pyrite was a widespread mechanism by which soft-bodied organisms and associated organic surfaces were preserved in multiple facies and depositional environments 580–541 million years ago. The extensive global burial of pyrite in medium- to coarse-grained clastics and carbonates is a previously unrecognized yet potentially significant sink of iron and sulfur, and may have contributed to rising atmospheric and ocean oxygen concentrations across the late Ediacaran interval.