Quantifying anthropogenic carbon inventory changes in the Pacific sector of the Southern Ocean

Abstract The Southern Ocean plays a major role in mediating the uptake, transport, and long-term storage of anthropogenic carbon dioxide (CO 2 ) into the deep ocean. Examining the magnitude and spatial distribution of this oceanic carbon uptake is critical to understanding how the earth's carbon system will react to continued increases in this greenhouse gas. Here, we use the extended multiple linear regression technique to quantify the total and anthropogenic change in dissolved inorganic carbon (DIC) along the S04P and P16S CLIVAR/U.S. Global Ocean Carbon and Repeat Hydrography Program lines south of 67°S in the Pacific sector of the Southern Ocean between 1992 and 2011 using discrete bottle measurements from repeat occupations. Along the S04P section, which is located in the seasonal sea ice zone south of the Antarctic Circumpolar Current in the Pacific, the anthropogenic component of the DIC increase from 1992 to 2011 is mostly found in the Antarctic Surface Water (AASW, upper 100 m), while the increase in DIC below the mixed layer in the Circumpolar Deep Water can be primarily attributed to either a slowdown in circulation or decreased ventilation of deeper, high CO 2 waters. In the AASW we calculate an anthropogenic increase in DIC of 12–18 μmol kg − 1 and an average storage rate of anthropogenic CO 2 of 0.10 ± 0.02 mol m − 2  yr − 1 for this region compared to a global average of 0.5 ± 0.2 mol m − 2  yr − 1 . In surface waters this anthropogenic CO 2 uptake results in an average pH decrease of 0.0022 ± 0.0004 pH units yr − 1 , a 0.47 ± 0.10% yr − 1 decrease in the saturation state of aragonite (Ω Aragonite ) and a 2.0 ± 0.7 m yr − 1 shoaling of the aragonite saturation horizons (calculated for the Ω Aragonite  = 1.3 contour).