THE ROLE OF FRAMBOIDAL PYRITE AND SULFUR CYCLING IN EDIACARAN TAPHONOMY

Macrofossils from the late Ediacaran Period (~579-541 Ma) document the initial evolution and radiation of large and complex soft-bodied multicellular organisms. Such fossils occur globally as molds and casts in facies including deep-marine turbiditic regimes (e.g. Newfoundland), and shallow-marine shoreface and prodelta settings (e.g. South Australia, White Sea). The profuse preservation of soft-bodied organisms in diverse lithologies contrasts markedly with Phanerozoic deposits, where preservation of soft tissues is often temporally and spatially restricted.

Preservation of Ediacaran macrofossils in coarse sandstones has been explained by the “death mask” hypothesis (Gehling, 1999), whereby early diagenetic (microbially-induced) pyrite mineralization cast the exterior morphology of organisms prior to soft-tissue decay and lithification of the sediment. However, a paucity of evidence for pyrite or its oxidation products in other facies has led to alternative hypotheses positing multiple different taphonomic styles (Narbonne, 2005). In recent years, the “death mask” model has gained experimental and analytical support, including from localities where it was not previously considered to operate. New petrological and compositional data are here presented from bedding planes in Newfoundland, Canada, demonstrating for the first time that framboidal pyrite played an integral role in macrofossil preservation in Ediacaran deep-marine settings. Pyrite framboids formed laterally extensive sub-millimeter-thick surface veneers on all fossil-bearing bedding planes, and occur in association with a variety of smothering substrates. Pyrite oxidation, veneer thickness, and crystal size are noted to influence preservational quality.

In conjunction with recognition of an association between pyrite and its oxidation products with Ediacaran macrofossils at other global localities, this study suggests that the “death mask” model offers a universal explanation for moldic Ediacaran macrofossil preservation. The longevity of the Ediacaran taphonomic window may thus have been controlled by global marine sulfur cycling and the activity of sulfate reducers. Extensive pyrite burial throughout the ~40 Myr late Ediacaran interval may have significantly contributed to marine oxygenation.