 Dissolved CO2 is acquired in the surface ocean via air-sea exchange. This dissolved CO2 is used by plants during photosynthesis in the fixation of organic matter in the surface ocean. As this organic matter falls through the ocean waters, it is gradually oxidized, and therefore, dissolved CO2 is returned back to deep ocean waters. As deep ocean waters age, therefore, they acquire more dissolved CO2, and therefore, old water masses contain higher concentrations of CO2 than do relatively new ocean water masses. As the organic material is oxidized, it gradually uses up dissolved oxygen in the ocean, and therefore, old water masses contain much less dissolved oxygen than do relatively new ocean water masses.