A nutrient control on fluctuating oceanic redox conditions during the Early Cambrian radiation of animals

The Early Cambrian witnessed an increase in metazoan ecosystem complexity, likely linked to enhanced oxygen and nutrient availability. Ocean redox chemistry exerts a strong control on the biogeochemical cycling of the limiting nutrient phosphorus, but the response of phosphorus cycling to redox dynamics in the Early Cambrian ocean remains poorly resolved. Here, we report phosphorus phase association data for three sections documenting a bathymetric marine transect through terminal Ediacaran to Early Cambrian Stage 2 (post−538−521 Ma) on the Tarim Block, Northwest China. During the terminal Ediacaran to middle Fortunian, a fluctuating ferruginous-dysoxic oxygen minimum zone (OMZ) developed, with ferruginous conditions promoting phosphorus recycling in deeper water parts of the OMZ. Combined with enhanced upwelling, this stimulated high primary production and organic matter burial, resulting in the development of better oxygenated conditions during the late Fortunian to early Stage 2. The development of oxic-dysoxic conditions at this time promoted phosphorus burial in association with iron minerals, and combined with waning upwelling, the reduced supply of nutrients helped maintain these conditions. This likely promoted diversification of small shelly fauna in the late Fortunian. However, efficient phosphorus burial also suppressed oxygenic photosynthesis. Thus, the overall expanded oxygenation was punctuated by episodes of subsequent deoxygenation during Cambrian Stage 2, which were driven by relatively enhanced upwelling and an increased supply of bioavailable phosphorus from continental weathering during pulsed eustatic transgression. Enhanced phosphorus burial from early Stage 2 onward contributed to more stable and longer-lived oxic episodes, helping to facilitate the development of more complex ecosystem structures.