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

Abstract

The anthropogenic CO2 accumulation rate for the North Atlantic Ocean was estimated on the basis of the decrease in the δ13C 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 δ13C change of −15.0 ± 3.8‰ m yr−1 was estimated by applying a multiple linear regression approach to determine the anthropogenic δ13C decrease at 22 stations where δ13C depth profiles were compared. The largest and deepest anthropogenic δ13C decreases occurred in the subpolar ocean and, in contrast, the smallest and shallowest decreases occurred in the tropical ocean. A mean anthropogenic CO2 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 δ13C change and a ratio of anthropogenic δ13C to DIC change of −0.024‰ (μmol kg−1)−1. Only half of the accumulated anthropogenic CO2 in the North Atlantic during the last 20 years was the result of air‐sea CO2 uptake, based on a comparison of the air‐sea 13CO2 flux to the DIC13 inventory change, with the other half likely a result of northward advective transport.