Ediacaran reorganization of the marine phosphorus cycle

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.

Keywords: Ediacaran; biosphere; phopshorus; phosphorite; sulfate.

Fig. 1.

Neoproterozoic to Cambrian secular variations in biogeochemical and paleontological records. These include sedimentary phosphorite resources, evaporite deposits, and Sr isotopic abundances in limestones, and within this context the rise of eukaryotic phytoplankton to ecological prominence and the appearance of macroscopic animals. Sources of data: evaporites (35), phosphorites (27, 74), strontium isotopes (43), phytoplankton (14), animal fossils (4). Note that resource estimation for pre-Ediacaran phosphorites can be challenging as most deposits are small and so attract limited industry interest.

Fig. 2.

The phosphorus content of Neoproterozoic and lower Paleozoic sedimentary rocks, including new P measurements as well as published compilations (45). The data are plotted as a box and whisker plot: Center lines indicate medians, boxes the inner quartiles, and whiskers 1.5 times the interquartile range. Outliers falling beyond the whiskers are shown as red dots; these include phosphorites as defined in ref. (wt % P₂O₅ >18%). The mean value is also included for each bin and denoted as a blue diamond. A full analysis of all geological time is available in SI Appendix. Pairwise comparison of time bins demonstrates that most time bins are significantly different—not unsurprising given the large number of samples. This gives confidence that the small effect sizes seen are an accurate reflection of the geologic record and not a sampling effect. Dev: Devonian; Sil.: Silurian; Ord.: Ordovician; Cam.: Cambrian; l. Ed.: late Ediacaran; e. Ed.: early Ediacaran; Cryo.: Cryogenian; Ton.: Tonian.

Fig. 3.

Deep-sea phosphorus concentration versus the efficiency of organic carbon burial. [P] has been normalized to our estimate for a low-sulfate, ferruginous ocean of the early Neoproterozoic Era. ε_C is the burial efficiency of organic carbon; r is the C:P ratio of organic matter deposited on the seafloor; ρ is the C_org:P_total ratio in sediments after authigenesis. Calculated values of [P] have been normalized to the result for ε_C = 0.9, r = ρ.