A MICROBIAL ROLE IN ASH-BASED CONCEPTION-STYLE EDIACARAN LAGERSTäTTE

The soft-bodied Ediacara biota from the Mistaken Point Ecological Reserve in Newfoundland, Canada, includes the oldest complex macroscopic eukaryotes and represents the oldest Ediacara-type Lagerstätte yet discovered. Hundreds of thousands of specimens occur as either positive or negative impressions on the top of large bedding plane surfaces that crop out along the coast. Bedding plane exposure is facilitated by the recessive nature of the hundreds of volcanic ash beds which occur throughout this deep-water turbidite sequence. Several of the ash beds have yielded zircons, providing precise U-Pb age constraints (578-565 Ma). Volcanic ash has long been assumed to affect the preservation of fossils at Mistaken Point, but to date no detailed studies have characterized the properties of the ashes or the potential influence of microbial systems on early lithification and fossilization. We employed environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA) to characterize the chemistry and sedimentology of the volcanic ash and underlying turbidites within the Drook, Briscal, and Mistaken Point formations. Distinct chemical signatures at the ash-sediment interface suggest that microbial processes (likely bacterial sulphur metabolism) were responsible, at least in part, for early lithification through the precipitation of authigenic minerals. A complex interplay between the fallout ash and sulfate-reducing bacterial mat communities on the seafloor accounts for the exceptional preservation of the Ediacara biota at Mistaken Point.