Changes in the ¹³C/¹²C of dissolved inorganic carbon in the ocean as a tracer of anthropogenic CO₂ uptake

Abstract

Measurements of the δ 13 C of dissolved inorganic carbon primarily during World Ocean Circulation Experiment and the Ocean Atmosphere Carbon Exchange Study cruises in the 1990s are used to determine ocean‐wide changes in the δ 13 C that have occurred due to uptake of anthropogenic CO 2 . This new ocean‐wide δ 13 C data set (∼25,000 measurements) substantially improves the usefulness of δ 13 C as a tracer of the anthropogenic CO 2 perturbation. The global mean δ 13 C change in the surface ocean is estimated at −0.16 ± 0.02‰ per decade between the 1970s and 1990s with the greatest changes observed in the subtropics and the smallest changes in the polar and southern oceans. The global mean air‐sea δ 13 C disequilibrium in 1995 is estimated at 0.60 ± 0.10‰ with basin‐wide disequilibrium values of 0.73, 0.63, and 0.23‰ for the Pacific, Atlantic, and Indian oceans, respectively. The global mean depth‐integrated anthropogenic change in δ 13 C between the 1970s and 1990s was estimated at −65 ± 33‰ m per decade. These new estimates of air‐sea δ 13 C disequilibrium and depth‐integrated δ 13 C changes yield an oceanic CO 2 uptake rate of 1.5 ± 0.6 Gt C yr −1 between 1970 and 1990 based on the atmospheric CO 2 and 13 CO 2 budget approaches of Quay et al. [1992] and Tans et al. [1993] and the dynamic method of Heimann and Maier‐Reimer [1996] . Box‐diffusion model simulations of the oceanic uptake of anthropogenic CO 2 and its δ 13 C perturbation indicate that a CO 2 uptake rate of 1.9 ± 0.4 Gt C yr −1 (1970–1990) explains both the observed surface ocean and depth‐integrated δ 13 C changes. Constraining a box diffusion ocean model to match both the observed δ 13 C and bomb 14 C changes yields an oceanic CO 2 uptake rate of 1.7 ± 0.2 Gt C yr −1 (1970–1990). The oceanic CO 2 uptake rates derived from anthropogenic changes in ocean δ 13 C are similar to rates determined by atmospheric CO 2 and O 2 budgets [ Battle et al. , 2000 ], atmospheric δ 13 C and CO 2 measurements [ Ciais et al. , 1995 ], and GCM simulations [ Orr et al. , 2001 ].