A Stratified Redox Model for the Ediacaran Ocean

Although the fossil record shows a clear explosion of animal diversity in the oceans at the beginning of the Cambrian period (∼542 million years ago), the evolutionary pressures driving this increase in diversity remain unclear. The likely scenario involves drastic changes in the distribution of oxygen in ocean basins, but global glaciations and poor preservation of rocks from this time often prevent a clear picture of global ecology before the dawn of animal life. Li et al. (p. 80 , published online 11 February; see the Perspective by Narbonne ) characterized the geochemical makeup of sedimentary rocks from south China that indicate alternating layers of sulfide- and iron-rich (i.e., sulfate-limited) anoxic waters in the several million years leading up to the Cambrian. These conditions may have set the stage for an eventual increase in productivity of photosynthetic life that oxygenated the oceans and allowed for rapid animal evolution.