REDOX STABILIZATION IN THE TERMINAL EDIACARAN: THE RECORD OF THE NAMA GROUP, NAMIBIA

Multi-proxy assessment of palaeoredox evolution has been the focus of many scientific investigations of Ediacaran stratigraphic sections worldwide. Most evidence points towards highly location-specific trends in water column oxygenation. Whilst the deeper water Mistaken Point sections of Newfoundland indicate early onset oxygenation coincident with abundant macroscopic soft-bodied Metazoa at ~580Ma, other younger sections such as those of the terminal Ediacaran Nama Group, Namibia, contain shallow water fossiliferous horizons which record fleeting oxic intervals punctuating largely anoxic ferruginous water column conditions. The redox evolution of the Nama Group as a whole, however, has largely remained unexamined even though this was deposited during the critical final ~10 million years of the Ediacaran.

Here we describe the first evidence for progressive oxygenation at a basin-scale derived from iron speciation analyses of multiple interbedded carbonate and shale sections. Data from Lower Nama Group (~550-547 Ma) outer-ramp sections indicate persistent ferruginous anoxia, whilst time-equivalent inner-ramp sections show intermittently oxic conditions, even though mid-ramp sections record persistent oxic conditions. Disruption of oxic conditions within inner-ramp settings occurs during periods of high stand and is inferred to be a consequence of incursion by deeper anoxic waters within a redox stratified water column. By contrast, upper Nama Group (~547-541 Ma) sections document a protracted basin-wide redox stabilisation, with more stable oxia found in both inner and particularly mid-ramp settings. The latter environments are characterised by larger and more ecologically complex calcifying metazoan communities. The trend towards sustained oxygenation of outer-ramp settings approaching the end of the Ediacaran indicates a deepening of the chemocline consistent with fewer incursions of anoxia into shallow waters.