Ocean carbon pumps: Analysis of relative strengths and efficiencies in ocean-driven atmospheric CO2 changes

An ocean carbon pump is defined as a process that depletes the ocean surface of ΣCO₂ relative to the deep-water ΣCO₂. Three pumps are recognized: a carbonate pump, a soft-tissue pump, and a solubility pump. The first two result from the biological flux of organic and CaCO₃ detritus from the ocean's surface. The third results from the increased CO₂ solubility in downwelling cold water and is demonstrated by a one-dimensional upwelling-diffusion model of an abiotic ocean. In the soft-tissue and solubility pumps, working strengths are defined in terms of the ΔΣCO₂ each creates between surface and deep-water. Efficiencies of each pump are quantified as a ratio of working strength to potential maximum strength. Using alkalinity, nitrate, and ΣCO₂ to remove the carbonate pump signal from ocean or model data, the individual working strengths of the soft-tissue and solubility pumps can be calculated by scaling the soft-tissue's ΔΣCO₂ to the surface-to-deep ΔPO₄. This technique is applied to a three-box ocean model known to demonstrate high-latitude control of atmospheric CO₂ through a variety of circulation and biological changes. Considering each pump separately reveals that the various changes which lower pCO₂atm in the model are caused primarily by an increased solubility pump. Analysis of global ocean data indicates a positive solubility pump signal, subject to uncertainties in the C:P Redfield ratio and in the preindustrial _pCO_2atm. If C:P = 105 and _pCO_2atm = 270 μatm, the efficiency of the solubility pump is about 0.5. We suggest that this type of analysis of relative carbon pump strengths will be an effective method for inter-model and intra-model comparison and diagnosis of underlying oceanic mechanisms for _pCO_2atm changes.