Anthropogenic CO₂ accumulation rates in the North Atlantic Ocean from changes in the ¹³C/¹²C of dissolved inorganic carbon

Abstract

The anthropogenic CO 2 accumulation rate for the North Atlantic Ocean was estimated on the basis of the decrease in the δ 13 C of the dissolved inorganic carbon measured between cruises in 1981 (Transient Tracers in the North Atlantic), 1993 (OACES) and 2003 (Repeat Hydrography). A mean depth‐integrated δ 13 C change of −15.0 ± 3.8‰ m yr −1 was estimated by applying a multiple linear regression approach to determine the anthropogenic δ 13 C decrease at 22 stations where δ 13 C depth profiles were compared. The largest and deepest anthropogenic δ 13 C decreases occurred in the subpolar ocean and, in contrast, the smallest and shallowest decreases occurred in the tropical ocean. A mean anthropogenic CO 2 accumulation rate of 0.63 ± 0.16 mol C m −2 yr −1 (0.32 ± 0.08 Pg C yr −1 ) in the North Atlantic Ocean over the last 20 years was determined from the mean depth‐integrated δ 13 C change and a ratio of anthropogenic δ 13 C to DIC change of −0.024‰ ( μ mol kg −1 ) −1 . Only half of the accumulated anthropogenic CO 2 in the North Atlantic during the last 20 years was the result of air‐sea CO 2 uptake, based on a comparison of the air‐sea 13 CO 2 flux to the DIC 13 inventory change, with the other half likely a result of northward advective transport.