Ediacaran reorganization of the marine phosphorus cycle

Significance

The evolution of macroscopic animals in the latest Proterozoic Eon is associated with many changes in the geochemical environment, but the sequence of cause and effect remains a topic of intense research and debate. In this study, we use two apparently paradoxical observations—that massively phosphorus-rich rocks first appear at this time, and that the median P content of rocks does not change—to argue for a change in internal marine P cycling associated with rising sulfate levels. We argue that this change was self-sustaining, setting in motion a cascade of biogeochemical transformations that led to conditions favorable for major ecological and evolutionary change.

Abstract

The Ediacaran Period (635 to 541 Ma) marks the global transition to a more productive biosphere, evidenced by increased availability of food and oxidants, the appearance of macroscopic animals, significant populations of eukaryotic phytoplankton, and the onset of massive phosphorite deposition. We propose this entire suite of changes results from an increase in the size of the deep-water marine phosphorus reservoir, associated with rising sulfate concentrations and increased remineralization of organic P by sulfate-reducing bacteria. Simple mass balance calculations, constrained by modern anoxic basins, suggest that deep-water phosphate concentrations may have increased by an order of magnitude without any increase in the rate of P input from the continents. Strikingly, despite a major shift in phosphorite deposition, a new compilation of the phosphorus content of Neoproterozoic and early Paleozoic shows little secular change in median values, supporting the view that changes in remineralization and not erosional P fluxes were the principal drivers of observed shifts in phosphorite accumulation. The trigger for these changes may have been transient Neoproterozoic weathering events whose biogeochemical consequences were sustained by a set of positive feedbacks, mediated by the oxygen and sulfur cycles, that led to permanent state change in biogeochemical cycling, primary production, and biological diversity by the end of the Ediacaran Period.