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 waters 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 carbon export. 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 µmolkg−1yr−1. Southern Ocean mode waters export significantly more carbon (0.16 PgC) compared to other water masses (0.002-0.060 PgC), while subtropical mode waters exhibit the highest export rates, 1.5–9.5 mgCm-3yr-1, compared to less than 2.5 mgCm-3yr-1 in subpolar mode waters.