Accumulation and uptake of anthropogenic CO2 in the Southern Ocean, south of Australia between 1968 and 1996

Abstract

Increases in the anthropogenic CO2 inventory for Southern Ocean waters to the south of Australia were estimated by comparing measurements made 28 years apart in 1968 and 1996. For this period, the deepest penetration of anthropogenic CO2 is consistent with the depth of 28 year old water from CFC‐11 age estimates. Significant accumulation of anthropogenic CO2 (13±10 μmol kg−1) was found in Antarctic Bottom Water (AABW) from 500–4500 m south of 58°S suggesting AABW to be an important water mass for the storage of anthropogenic CO2 in the deep ocean. Deep penetration of anthropogenic CO2 (<1900 m) was found in the sub‐Antarctic zone (SAZ) between 48°S and 51°S while little accumulation (<400 m) was observed south of 53°S. A simple one dimensional box model was used to investigate the anthropogenic CO2 uptake potential in the SAZ based on observed CFC‐11 concentrations and the calculated anthropogenic CO2 inventory. Formation and export of sub‐Antarctic Mode Water (SAMW) was estimated by reproducing the observed present day SAMW CFC‐11 concentration using time histories of atmospheric CFC‐11 concentrations. The anthropogenic CO2 uptake for the SAZ (45°–50°S) ranged from 0.73 to 0.86 μmol kg yr−1. The calculated range of SAMW formation and export rates only increased the anthropogenic CO2 uptake potential for the SAZ by as much as 18% (<0.13 μmol kg−1 yr−1). Extrapolating our estimate to the circumpolar SAZ gives an anthropogenic CO2 uptake of 0.07–0.08 GTC yr−1. Even though SAMW formation does not considerably enhance anthropogenic CO2 uptake in the SAZ, it provides an important mechanism for transporting anthropogenic CO2 into the ocean interior. SAMW therefore may be important for anthropogenic CO2 uptake on a larger regional scale.