DID UNSTABLE REDOX STRUCTURE EDIACARAN METAZOAN COMMUNITIES?: THE USE OF EARLY MARINE CARBONATE CEMENTS (Invited Presentation)

The terminal Ediacaran oceanic redox landscape was highly heterogeneous, where many basins had a shallow and highly dynamic chemocline. How the benthos responded to this unstable redox is fundamental to understanding the ecological dynamics of the first complex communities, but most geochemical techniques provide only averaged signals over geological, rather than ecological, timescales. Here we show how the distribution of redox sensitive elements in early carbonate cements offers evidence for highly fluctuating redox over ecological timescales, which may have structured the ecology of the oldest metazoan communities.

Large, often essentially monospecific, Cloudina reefs (<20 m) form in shallow, high-energy mid-ramp settings in the Nama Group, Namibia (~547 Ma), and are composed of multiple, successive assemblages. Reefs cements show a parasequence from large, pseudomorphed aragonitic botryoids (up to 50 mm in radius) with mean Fe and Mn concentrations below detection limits (BDL), followed by dolomitised geopetal sediment infill with elevated Fe and low Mn with two generations of fibrous dolomite isopachous crusts with elevated Fe and variably preserved CL zonation. Cloudina skeletal material may also be dolomitised. This is followed by two calcite cements: brightly luminescing thin crust of calcite cement with elevated Mn and slightly elevated Fe, and finally a dull luminescent calcite cement with mean Fe and Mn BDL. These cements are interpreted to represent a succession of synsedimentary to early marine marine cements precipitated in oxic (botryoidal pseudomorphed aragonite), then ferruginous anoxic and Mg-rich (dolomite cements), then back to dysoxc/oxic conditions (two generations of calcite). Dolomite cement increases in volume from the base to top of each assemblage, which also terminates with the deposition of thin layers of dolomitised sediment and dolomite cement. We propose that these reefs grew close to the oxic chemocline, and are composed of successive communities whose growth was terminated by the cyclic influx of sediment and dolomitising, anoxic waters that shoaled during short-term, transgressive cycles. Such a simple ecological structure of repeated community re-growth is in marked contrast to the more persistent, biodiverse, reef-communities typical of the Phanerozoic.