A VIEW OF MICROBIAL ECOSYSTEMS AND OXYGEN FROM THE MID-NEOPROTEROZOIC (780–730 MA) CHUAR GROUP, GRAND CANYON SUPERGROUP, ARIZONA (Invited Presentation)

Eukaryotes diversified in oceans that were spatially patchy with respect to oxygen. Linked paleontological and geochemical records from the mid-Neoproterozoic Chuar Group provide an opportunity to see how early protists inhabited these patchy environments. Lower Chuar rocks preserve 22 species of organic-walled microfossils, including protistan cysts, multicellular algae, and possible scale-bearing forms. Circular drill holes in several specimens indicate these organisms were preyed on by perforating predators similar to modern vampyrellid amoebae. The fossils occur in shales that record frequently anoxic bottom waters, even in the shallowest parts of the basin (Fe speciation data on the same samples: Johnston et al., 2010); there is no correlation between bottom water oxygen levels and the presence of microfossil assemblages, their diversity, or the distribution of individual species. Thus these species were probably planktonic, living in persistently oxic surface waters and sinking upon death or encystment into episodically anoxic sediments below. In contrast, drill holes have been found only in fossils associated with oxic bottom waters. Though preliminary, this suggests that the predators may have been benthic and restricted to oxic bottom waters, preying at least in part on resting cysts that fell from the surface.

In the upper Chuar Group, tests of amoebae are abundant in mudstones and silicified microbial mats, and densely concentrated in carbonate nodules formed in organic-rich shales recording euxinic bottom waters. Circular and half-moon holes in some tests suggest they were victims of predation as well, though their much larger size suggests a different set of predators. Like their modern counterparts that live on the surface of living moss in peat bogs, the fossil testate amoebae were probably restricted to the oxic surface layers of microbial mats, their empty tests transported to euxinic habitats.

Taken together, the Chuar Group fossils indicate a thriving microbial eukaryotic ecosystem, including a diversity of predators. Preliminary evidence suggests that these ecosystems may have been restricted to oxic settings, supporting the view that oxygenation of Proterozoic oceans could have promoted more complex microbial food webs, and in turn protistan diversification.