Prevalence of strong vertical CO₂ and O₂ variability in the top meters of the ocean

Abstract The gradient in the partial pressure of carbon dioxide ( p CO 2 ) across the air‐sea boundary layer is the main driving force for the air‐sea CO 2 flux. Global data bases for surface seawater p CO 2 are actually based on p CO 2 measurements from several meters below the sea surface, assuming a homogeneous distribution between the diffusive boundary layer and the upper top meters of the ocean. Compiling vertical profiles of p CO 2 , temperature, and dissolved oxygen in the upper 5–8 m of the ocean from different biogeographical areas, we detected a mean difference between the boundary layer and 5 m p CO 2 of 13 ± 1 µatm. Temperature gradients accounted for only 11% of this p CO 2 gradient in the top meters of the ocean; thus, pointing to a heterogeneous biological activity underneath the air‐sea boundary layer as the main factor controlling the top meters p CO 2 variability. Observations of p CO 2 just beneath the air‐sea boundary layer should be further investigated in order to estimate possible biases in calculating global air‐sea CO 2 fluxes.