Of what use is the APO concentration?

APO is useful for extracting signals of oceanic origin from combined O2/N2 and CO2 data.  The exchanges of O2 and CO2 with land biota are very tightly coupled by the chemistry of photosynthesis and respiration.  APO is defined in such a way that the effect of land plants on the δ(O2/N2) term largely cancels the effect on the XCO2 term, leaving APO unchanged.  APO is strongly influenced by air-sea exchanges of O2 and CO2, which are not typically coupled in tight proportions, and by burning of fossil-fuels, particularly petroleum and natural gas, due their higher O2:C combustion ratio.