The oceanic phosphorus cycle describes how phosphorus moves through the ocean, accumulates with the sediments on the sea floor, and participates in biogeochemical reactions. We propose a new two-reservoir scenario of the glacial-interglacial phosphorus cycle. It relies on diagenesis in methane hydrate-bearing sediments to mobilize sedimentary phosphorus and transfer it to the ocean during times when falling sea level lowers the hydrostatic pressure on the sea floor and destabilizes methane hydrate. Throughout the cycle, primary production assimilates phosphorus and inorganic carbon into biomass which, upon settling and burial, returns phosphorus to the sedimentary reservoir. The impact of the two processes is not balanced: the former increases the oceanic phosphorus inventory whereas the latter decreases it. Primary production also lowers the partial pressure of COin the surface ocean, potentially drawing down COfrom the atmosphere. Concurrent with this slow ‘biological pump’, but operating in the opposite direction, a ‘physical pump’ brings metabolic COenriched waters from deep-ocean basins to the upper ocean. The two pumps compete, but the direction of the COflux at the air-sea interface depends on the nutrient content of the deep waters. Because of the transfer of reactive phosphorus to the sediment throughout a glaciation cycle, low phosphorus/ high COdeep waters reign in the beginning of the deglaciation, resulting in rapid transfer of COto the atmosphere. The new scenario provides another element to the suite of processes that may have contributed to the rapid glacial-interglacial climate transitions documented in paleo records.