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 pCO2atm. If C:P = 105 and pCO2atm = 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 pCO2atm changes.