Glacial-interglacial variations in marine phosphorus cycling: Implications for ocean productivity

Using a box model of organic carbon (C) and phosphorus (P) cycling in the global ocean, we assess the effects of changes in the continental supply of reactive P, oceanic mixing, and sea level on the marine P cycle on glacial-interglacial timescales. Our model results suggest that mixing is a dominant forcing during early glaciation, causing retention of P in the deep ocean, thereby lowering primary production and associated organic C and P burial. Sea level fall is the dominant forcing during late glaciation, when reduced coastal trapping of reactive P enhances its transfer to the open ocean, restoring primary production and P burial. During postglacial periods, changes in circulation and weathering dominate open ocean processes and oceanic primary production peaks. As primary production is reduced upon glaciation, changes in the marine P cycle are unlikely to drive enhanced primary production and CO ₂ drawdown during glaciation.