Phosphorus controls on the formation of vivianite versus green rust under anoxic conditions

The formation of green rust (GR; a mixed ferric/ferrous hydroxide) and vivianite (ferrous phosphate) are likely to have exerted a major control on phosphorus (P) cycling in ancient anoxic oceans. However, the factors that influence the formation of these minerals under different chemical conditions are poorly constrained, which limits understanding of the pathways that ultimately result in P drawdown and retention in anoxic sediments. This, in turn, limits understanding of P cycling in anoxic oceans and hence potential productivity feedbacks. Here we explore the effect of dissolved P concentration on the formation of sulfate GR (FeIII2FeII3(OH)12SO4) versus vivianite (FeII3(PO4)2·8H2O) under anoxic conditions. Our results show that at low dissolved P concentrations and with P:Fe(II) molar ratios <1:30, P drawdown is effectively controlled by interlayer anion exchange and adsorption onto GR species, via the formation of amorphous Fe-P precursors. Such precursors may delay the precipitation of crystalline GR, but vivianite was not detected under these conditions. At higher dissolved P concentrations and P:Fe(II) ratios, GR also forms. However, the GR formed under these conditions rapidly dissolves, likely forming amorphous ferric hydroxides together with dissolved Fe(II) and phosphate, with the dissolved species subsequently reacting to form crystalline vivianite. Our observations agree with studies showing the water column formation of GR in modern oligotrophic, anoxic Fe-rich (ferruginous) settings, and provide support for a major role for GR in controlling P cycling in ancient oligotrophic ferruginous oceans. By contrast, in more productive ancient anoxic settings, enhanced redox-controlled P recycling and/or increased weathering inputs would have led to higher dissolved P concentrations in the water column and sediments. Our observations show that such conditions ultimately promote the formation of vivianite, which would have exerted a limiting control on the extent of P recycling in ancient, more productive settings, via the long-term fixation of P in the sediments.