Abstract
The late Ediacaran witnessed an increase in metazoan diversity and ecological complexity, marking the inception of the Cambrian Explosion. To constrain the drivers of this diversification, we combine redox and nutrient data for two shelf transects, with an inventory of biotic diversity and distribution from the Nama Group, Namibia (~550 to ~538 Million years ago; Ma). Unstable marine redox conditions characterised all water depths in inner to outer ramp settings from ~550 to 547 Ma, when the first skeletal metazoans appeared. However, a marked deepening of the redoxcline and a reduced frequency of anoxic incursions onto the inner to mid-ramp is recorded from ~547 Ma onwards, with full ventilation of the outer ramp by ~542 Ma. Phosphorus speciation data show that, whilst anoxic ferruginous conditions were initially conducive to the drawdown of bioavailable phosphorus, they also permitted a limited degree of phosphorus recycling back to the water column. A long-term decrease in nutrient delivery from continental weathering, coupled with a possible decrease in upwelling, led to the gradual ventilation of the Nama Group basins. This, in turn, further decreased anoxic recycling of bioavailable phosphorus to the water column, promoting the development of stable oxic conditions and the radiation of new mobile taxa.
Highlights
The earliest candidate metazoan body fossils appear in the fossil record in the Ediacaran at ~571 Ma1, with the first uncontroversial surface traces created by motile organisms known from ~560 Ma2, followed by skeletal hardparts at ~550 Ma3
The late Ediacaran (c. 580–541 Ma) oceans were likely characterised by low oxygen levels[7], and individual basins of the Ediacaran and Cambrian that were openly connected to the global ocean exhibit continued regional redox heterogeneity on million year timescales[5,8,9]
The observed variability in the local redox of late Ediacaran and early Cambrian marine environments may correspond to the relative distance from, and spatial extent of, oxygen minimum zones (OMZs) that developed in response to local productivity and resultant organic matter remineralization[10]
Summary
The earliest candidate metazoan body fossils appear in the fossil record in the Ediacaran at ~571 Ma1, with the first uncontroversial surface traces created by motile organisms known from ~560 Ma2, followed by skeletal hardparts at ~550 Ma3. The well-defined sequence stratigraphic framework of the Nama Group enables analysis of down-dip shales deposited time-equivalent to shallow, fossiliferous sandstone layers, and construction of variable-depth geochemical transects. This enables the relative position of the redoxcline to be tracked through time, as well as quantification of the incursion frequency of anoxic waters onto the shallow, inner ramp. We combine this approach with a recent compilation of biotic diversity and distribution throughout the Nama Group in order to clarify the relationship between local redox stability and benthic colonisation. A gradual, basin-wide transition from negative to positive carbonate carbon isotope values may record the local expression of recovery from the presumed-global Shuram excursion ( this remains uncertain), but overlying strata are not known to record (or may predate) the basal Cambrian negative carbon isotope excursion (BACE)[5,16]
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