BENTHIC PHOSPHORUS REGENERATION, NET PRIMARY PRODUCTION, AND OCEAN ANOXIA - A MODEL OF THE COUPLED MARINE BIOGEOCHEMICAL CYCLES OF CARBON AND PHOSPHORUS

We examine the relationships between ocean ventilation, primary produc tion, water column anoxia, and benthic regeneration of phosphorus usin g a mass balance model of the coupled marine biogeochemical cycles of carbon (C) and phosphorus (P). The elemental cycles are coupled via th e Redfield C/P ratio of marine phytoplankton and the C/P ratio of orga nic matter preserved in marine sediments. The model assumes that on ge ologic timescales, net primary production in the oceans is limited by the upwelling of dissolved phosphorus to the photic zone. The model in corporates the dependence on bottom water oxygenation of the regenerat ion of nutrient phosphorus from particulate matter deposited at the wa ter-sediment interface. Evidence from marine and lacustrine settings, modem and ancient, demonstrates that sedimentary burial of phosphorus associated with organic matter and ferric oxyhydroxides decreases when bottom water anoxia-dysoxia expands. Steady state simulations show th at a reduction in the rate of thermohaline circulation, or a decrease of the oxygen content of downwelling water masses, intensifies water c olumn anoxia-dysoxia and at the same time increases surface water prod uctivity. The first effect reflects the declining supply of oxygen to the deeper parts of the ocean. The second effect is caused by the enha nced benthic regeneration of phosphorus from organic matter and ferric oxyhydroxides. Sedimentary burial of organic carbon and authigenic ca lcium phosphate mineral (francolite), on the other hand, is promoted b y reduced ocean ventilation. According to the model, global-scale anox ia-dysoxia leads to a more efficient recycling of reactive phosphorus within the ocean system. Consequently, higher rates of primary product ion and organic carbon burial can be achieved, even when the continent al supply of reactive phosphorus to the oceans remains unchanged.