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Abstract. Previous studies have suggested that enhanced weathering and benthicphosphorus (P) fluxes, triggered by climate warming, can increase theoceanic P inventory on millennial timescales, promoting ocean productivityand deoxygenation. In this study, we assessed the major uncertainties inprojected P inventories and their imprint on ocean deoxygenation using anEarth system model of intermediate complexity for the same business-as-usualcarbon dioxide ( CO2 ) emission scenario until the year 2300 andsubsequent linear decline to zero emissions until the year 3000. Our set ofmodel experiments under the same climate scenarios but differing in theirbiogeochemical P parameterizations suggest a large spread in the simulatedoceanic P inventory due to uncertainties in (1) assumptions for weatheringparameters, (2) the representation of bathymetry on slopes and shelves inthe model bathymetry, (3) the parametrization of benthic P fluxes and (4) the representation of sediment P inventories. Considering the weatheringparameters closest to the present day, a limited P reservoir and prescribedanthropogenic P fluxes, we find a +30  % increase in the total globalocean P inventory by the year 5000 relative to pre-industrial levels, causedby global warming. Weathering, benthic and anthropogenic fluxes of Pcontributed +25  %, +3  % and +2  %, respectively. The total rangeof oceanic P inventory changes across all model simulations varied between +2  % and +60  %. Suboxic volumes were up to 5 times larger than in amodel simulation with a constant oceanic P inventory. Considerably largeamounts of the additional P left the ocean surface unused by phytoplanktonvia physical transport processes as preformed P. In the model, nitrogenfixation was not able to adjust the oceanic nitrogen inventory to theincreasing P levels or to compensate for the nitrogen loss due to increaseddenitrification. This is because low temperatures and iron limitationinhibited the uptake of the extra P and growth by nitrogen fixers in polarand lower-latitude regions. We suggest that uncertainties in P weathering,nitrogen fixation and benthic P feedbacks need to be reduced to achieve morereliable projections of oceanic deoxygenation on millennial timescales.

Ocean phosphorus inventory: large uncertainties in future projections on millennial timescales and their consequences for ocean deoxygenation