Figure 15. Sea surface oxygen and total CO₂ data as a tracer for the atmospheric exposure time of surface waters on the cold side of the front. Data from the shaded region were from Survey II from 2^o N to the equator. Oxygen is plotted as Apparent Oxygen Utilization (AOU), the deviation from saturation at 1 atm total pressure, with positive AOU denoting undersaturation. Subsurface waters in this region covary in their oxygen : total CO₂ concentrations in a ratio of -0.85:1 (the deviation from Redfield is due to the export of CaCO₃ and increasing solubility of CO₂ in colder waters). Upon exposure to the atmosphere, both oxygen and CO₂ will begin to equilibrate toward atmospheric saturation values.