Grant: Discovery Grants Program - Individual

The evolution of the Ediacara biota and its transition into the metazoan biotas of the Phanerozoic remains as a significant challenge to modern paleontology. Significant progress has been made in documenting the Ediacara biota and characterizing the associated ecosphere change. There has however been little consideration of organism-sediment-porewater interactions during this interval and it is these that that would have a impacted the, largely sessile, soft-bodied Ediacarans . Ediacaran siliciclastic sediments are rich in evidence of microbial mat grounds that, in the absence of large bioturbators and surface-sediment feeders, are likely to have significantly affected the rheology, biogeochemistry and productivity of near-seafloor siliciclastic sediments throughout the Ediacaran. The same microbial films are also implicated in the exceptional preservation of the soft-bodied Ediacara biota. One of the defining characters of the Ediacara biota is that, with few contentious late Ediacaran exceptions, they were dominantly non-motile epibenthos. This immobility is likely to have posed a significant challenge to the organisms themselves since hydrogen sulfide tends to build up under any object at the sediment-water interface that cuts the sedimentary microbiota off from oxygenated seawater. Hydrogen sulfide is a challenge to most modern macroscopic organisms because of their reliance upon cytochrome-c oxidase for metabolism within their cells. Sulfide is the most efficacious inhibitor to cytochrome-c oxidase, and as such animals require adaptations to cope with this common metabolite of the microbiota of marine sediments or suffer damage to their mitochondria and ability to produce energy. I consider that coping with sulfide would have been one of the primary drivers of the evolution of early macroscopic animal life. Modern organisms cope with the potential for sulfidic porewater buildup in siliciclastic sediments in a number of different ways: 1) biomineralization; 2) larval settling on sediments with low organic matter content; 3) mucus production; 4) larval settling on an impermeable substrate; 5) locomotion; 6) Sulfur detoxification by pumping of seawater across the organism-sediment interface; 7) oxidation of the near-organism porewaters by diffusion of oxygen from a circulatory system. The proposed research initiative takes a multi-pronged approach to the issue of sulfide toxicity in association with the non-mineralized Ediacara biota, employing a combination of approaches: 1) palaeoenvironmental analysis of sedimentary rock associated with Ediacaran impressions; 2) detailed petrography to search for evidence of microbial products (sulfides) and fossils; 3) stable isotopes of carbon and sulfur in association with fossil Ediacaran fossils using ICPMS; 4) observation of biogeochemical signatures of modern organisms in low sulfur sediments (in marine aquaria).