The marine geochemistry of phosphorus links the burial rate of organic carbon in marine sediments to the oxygen content of the atmosphere and may serve as a major component of the system that controls atmospheric oxygen. The return flux of phosphate from marine sediments to seawater is an important part of the marine phosphorus cycle. This paper examines the relationship between the return flux of phosphate and the oxidation state of marine sediments, a necessary preliminary step in defining the efficacy of the oxygen control mechanism.

The diffusive return flux of phosphate from marine sediments to overlying bottom waters was calculated for 193 published pore water phosphate profiles that met a number of stringent criteria for sediment core and pore water recovery and processing. The phosphate return fluxes, scaled to carbon regeneration fluxes, are significantly greater from highly reduced sediments than from highly oxidized sediments. In highly reduced sediments the return phosphate flux from carbon regeneration is frequently augmented by a large phosphate flux released during the reductive dissolution of ferric (oxy)hydroxides. in highly oxidized sediments the return phosphate flux can be somewhat less than the flux to be expected from carbon regeneration. The missing phosphate is probably adsorbed on ferric (oxy)hydroxides in these sediments. The strong coupling between the oxidation state of marine sediments and the return phosphate flux to seawater suggests that the marine phosphate cycle is indeed an important part of the system that stabilizes atmospheric O₂.

The total preagricultural return flux of P from marine sediments was ca. 12 × 10¹¹mol P/yr. This rate is more than an order of magnitude larger than the riverine flux of total dissolved phosphorus to the oceans, ca. 0.3 × 10¹¹mol P/yr. We estimate that the total continental preagricultural flux of reactive P that ultimately appears dissolved in ocean water was ca. 3.5 × 10¹¹ mol P/yr. The large flux of continental P to seawater, via direct input of riverine dissolved inorganic and organic P and via the diagenetic return flux from reactive continental particulate P deposited in marine sediments, indicates that the marine residence time of phosphate with respect to terrigenous inputs is ca. 10,000 years. This figure depends quite heavily on the fraction of terrigenous, particulate-phase phosphate that is released to seawater during diagenesis. Variations in this fraction can significantly affect the marine residence time of phosphate and the relative proportion of detrital versus authigenic phosphate phases in marine sediments.