CRYOGENIAN REEF COMPLEXES AS PROXIES FOR PALEOCEANOGRAPHIC CONDITIONS AT THE DAWN OF ANIMAL LIFE

Neoproterozoic reef complexes provide a window into marine ecosystems and seawater conditions during the early evolution of metazoans. Interglacial Cryogenian reef complexes from Australia developed significant margins over ~1km of steep platform relief from the seafloor, and contain a variety of frameworks including stromatolites, microbialites and enigmatic chambered boundstones. These deep-water, chambered boundstones are composed of macroscopic, encrusting and branching chambered structures with well-defined walls and distinct morphologies. Chambered structures appear to represent a unique reef-dwelling, non-photosynthetic fauna restricted to deep-water Cryogenian settings.

Reef facies are composed of dolomite and are well cemented by primary dolomite marine cements. These direct marine precipitates, as well as other depositional components, have been analysed using redox-sensitive proxies to determine seawater chemistry. Geochemical analysis reveals significant differences in uranium isotope composition between carbonate components and considerable variation in trace metal chemistry with depth. Marine cements have lower contamination element concentrations (e.g. Al, Zr, Th) than micrites and microbialites, and represent the best archive of ancient ocean conditions. Cements have high levels of iron in deep-water boundstone frameworks (e.g. 2-3wt% Fe), but only Fe-oxide inclusions in peritidal cements. This distribution suggests that these unusual Cryogenian reef frameworks developed in ferruginous (Fe-rich and anoxic) oceans under a surficial chemocline. Uranium isotopes from marine cements have relatively heavy values compared to modern seawater (median = -0.22 δ²³⁸U). These values appear to be a result of near quantitative, low-T reduction of uranium by abundant soluble iron in this ferruginous seawater. This combined geochemistry and sedimentology suggests a fundamental link between ocean chemistry and reef ecosystems during the terminal Precambrian. We interpret the development of enigmatic chambered frameworks at depth in these ferruginous oceans as a first attempt at the evolution of reef-dwelling metazoans just prior to the Neoproterozoic Oxygenation Event.