Mixing accounts for more than half of biogeochemical changes along mode water ventilation pathways

Mode waters are critical for ocean ventilation and carbon sequestration. Using observations, we trace their subduction pathways and biogeochemical evolution. Solving modified mixing equations that account for respiration reveals that less than 50% of the oxygen changes along mode water ventilation pathways are due to respiration within the water mass, the rest being due to mixing with oxygen-poorer surrounding waters. Consequently, measured changes in oxygen or Apparent Oxygen Utilization overestimate respiration by a factor of up to two, as do derived biogeochemical quantities such as remineralized carbon. Measured nitrate changes either overestimate or underestimate remineralization depending on surrounding concentrations. Mean true respiration rates in mode waters range from −0.1 to −0.4 μ mol. Applying a fixed stoichiometric ratio to this respiration, we find that the total carbon export is highest in Southern Ocean mode waters, while carbon remineralization rates are highest in subtropical mode waters.