Phosphorus imbalance in the global ocean?

The phosphorus budget of the prehuman modern ocean is constrained applying the most recent estimates of the natural riverine, eolian, and ice‐rafted input fluxes; the phosphorus burial in marine sediments; and the hydrothermal removal of dissolved phosphate from the deep ocean. This review of current flux estimates indicates that the phosphorus budget of the ocean is unbalanced since the accumulation of phosphorus in marine sediments and altered oceanic crust exceeds the continental input of particulate and dissolved phosphorus. The phosphorus mass balance is further tested considering the dissolved phosphate distribution in the deep water column, the marine export production of particulate organic matter, rain rates of phosphorus to the seafloor, benthic dissolved phosphate fluxes, and the organic carbon to phosphorus ratios in marine particles. These independent data confirm that the phosphate and phosphorus budgets were not at steady state in the prehuman global ocean. The ocean is losing dissolved phosphate at a rate of ≥11.6 × 10 10 mol yr −1 corresponding to a decline in the phosphate inventory of ≥4.5% kyr −1 . Benthic data show that phosphate is preferentially retained in pelagic deep‐sea sediments where extended oxygen exposure times favor the degradation of particulate organic matter and the uptake of phosphate in manganese and iron oxides and hydroxides. Enhanced C: P regeneration ratios observed in the deep water column (>400 m water depth) probably reflect the preferential burial of phosphorus in pelagic sediments. Excess phosphate is released from continental margin sediments deposited in low‐oxygen environments. The dissolved oxygen threshold value for the enhanced release of dissolved phosphate is ∼20 μ M. Benthic phosphate fluxes increase drastically when oxygen concentrations fall below this value.