Estimates of the amount of carbon sequestered in the ocean interior per unit iron (Fe) supplied, as quantified by the sequestration efficiency (C-effx), vary widely. Such variability in C-effx has frequently been attributed to estimate uncertainty rather than intrinsic variability. Here we derive new estimates of C-effx for the subpolar North Atlantic, where Fe stressed conditions have recently been demonstrated. Derived values of C-effx from across the region, including areas subject to atypical external Fe fertilization events during the year of sample collection (2010), ranged from 17 to 19 kmol C (mol Fe-1). Comparing these estimates with values from other systems, considered in the context of variable bloom durations in the different oceanographic settings, we suggest that apparent variability in C-effx may be related to the mode of Fe delivery.