Fig. 4.

Spatial (increasing eastward) and temporal (increasing with time) trends in Δ(N₂/Ar) in the deep North Atlantic suggest a high rate of benthic denitrification, equal to or in excess of that suggested by a biogeochemical model (17) evaluated at the sample locations (dashed red line of best fit to model data, SI Appendix, Fig. S12). Circles represent TTO/NAS tank measurements from the deep North Atlantic (below 2.9 km) and diamond indicates the mean of deep BATS measurements (16, 25). Orange-shaded region shows global-mean rate estimates below 3 km (16, 50–52). Ideal mean ages reflect the mean of two inverse model estimates (26, 27) and X error bars indicate the spread between these estimates, while Y error bars indicate 95% CIs of Δ(N₂/Ar) based on analytical uncertainties (measurements and corrections). The second (Right) Y-axis shows apparent excess N₂ concentrations determined by comparing measurements to physics-only model Δ(N₂/Ar) in the deep North Atlantic, as described in the main text.