The surface ocean has less dissolved CO2 than the deep ocean primarily because of the temperature-solubility effect (cold water holds more dissolved gas) and the biological pump: primary production in the surface ocean draws down CO2 via photosynthesis, while the organic matter produced sinks and remineralizes at depth, releasing CO2 back into deep water where there is no primary production to consume it.
Two linked processes lower surface CO2 relative to deep water. First, gas solubility increases as temperature decreases, so the colder deep ocean can hold more...
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