Archean phosphorus liberation induced by iron redox geochemistry

Abstract

The element phosphorus (P) is central to ecosystem growth and is proposed to be a limiting nutrient for life. The Archean ocean may have been strongly phosphorus-limited due to the selective binding of phosphate to iron oxyhydroxide. Here we report a new route to solubilizing phosphorus in the ancient oceans: reduction of phosphate to phosphite by iron(II) at low (<200 °C) diagenetic temperatures. Reduction of phosphate to phosphite was likely widespread in the Archean, as the reaction occurs rapidly and is demonstrated from thermochemical modeling, experimental analogs, and detection of phosphite in early Archean rocks. We further demonstrate that the higher solubility of phosphite compared to phosphate results in the liberation of phosphorus from ferruginous sediments. This phosphite is relatively stable after its formation, allowing its accumulation in the early oceans. As such, phosphorus, not as phosphate but as phosphite, could have been a major nutrient in early pre-oxygenated oceans.

Fig. 1

Chromatogram of phosphorus speciation of sample AKILIA-AK98. m/z + 47 (PO) was observed by ICP–MS. The peak at 6 min matches phosphite and the peak at 9.5 min matches the retention time of phosphate (Methods)

Fig. 2

NMR spectrum of heated Fe²⁺ and phosphate solution. Proton-coupled ³¹P NMR of a solution of FeCl₂ and Na₂HPO₄ heated to dryness (180 °C), under flowing N₂. All compounds are references to an external standard of 85% H₃PO₄ (0 ppm, as a frequency spectrum referenced to H₃PO₄ at 161.9 MHz). Phosphate is at 5.8 ppm, and pyrophosphate (HP₂O₇³⁻) is at −4.2 ppm. The small doublet at 4.95 and 1.46 ppm corresponds to phosphite (4% of total area), identified by its large H–P J-coupling constant of 565 Hz

Fig. 3

Model results of the extraction of phosphite and its predicted steady-state abundance in an anoxic ocean. a Water-rock ratios required to completely extract phosphite from ferruginous sediments (0.3% P, Fe²⁺/Fe³⁺ of 1/1). These calculations assume a total P content of 0.3% by weight, and that the P is in vivianite [Fe₃(PO₄)₂·8H₂O] or ferric phosphite—Fe₂(HPO₃)₃. Then, using the solubility data determined experimentally (ED), the quantity of water (pH 7.2) needed to completely dissolve phosphite was determined using the thermodynamic modeling program HSC Chemistry (Methods). Two scenarios were investigated: one where the water in contact with the rock had a starting Fe²⁺ concentration of 10⁻⁶ M, and other, with a concentration of 10⁻³ M. The amount of phosphite predicted to form from thermodynamics and shown to form from experimental reduction, also imply that lower quantities of phosphite produced at low temperatures (<160 °C) will also be easily extracted at low water-rock ratios. b Steady-state concentration of phosphite predicted within the early oceans, as a function of molarity of phosphite dissolved from rock after diagenesis, oxidative half-life of phosphite for modern day hydrothermal fluid fluxes, and the higher values that might be expected in the Archean. The blue square corresponds to the predicted phosphite flux from Fig. 3a, with the measured oxidative half-life of Fe(H₂PO₃)₂ under low O₂ conditions (Methods). The yellow circle corresponds to the current measurements of half-life oxidation by biology with the concentration of phosphite found within the ocean, which implies a very high flux from sediments or hydrothermal sources, or more likely indicating that oceanic phosphite is not currently steady state, with respect to abiotic sources. This biotic oxidation rate is likely overestimated (Methods)