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

Abstract

Measurements of the δ13C 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 δ13C that have occurred due to uptake of anthropogenic CO2. This new ocean‐wide δ13C data set (∼25,000 measurements) substantially improves the usefulness of δ13C as a tracer of the anthropogenic CO2 perturbation. The global mean δ13C 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 δ13C 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 δ13C between the 1970s and 1990s was estimated at −65 ± 33‰ m per decade. These new estimates of air‐sea δ13C disequilibrium and depth‐integrated δ13C changes yield an oceanic CO2 uptake rate of 1.5 ± 0.6 Gt C yr−1 between 1970 and 1990 based on the atmospheric CO2 and 13CO2 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 CO2 and its δ13C perturbation indicate that a CO2 uptake rate of 1.9 ± 0.4 Gt C yr−1 (1970–1990) explains both the observed surface ocean and depth‐integrated δ13C changes. Constraining a box diffusion ocean model to match both the observed δ13C and bomb 14C changes yields an oceanic CO2 uptake rate of 1.7 ± 0.2 Gt C yr−1 (1970–1990). The oceanic CO2 uptake rates derived from anthropogenic changes in ocean δ13C are similar to rates determined by atmospheric CO2 and O2 budgets [Battle et al., 2000], atmospheric δ13C and CO2 measurements [Ciais et al., 1995], and GCM simulations [Orr et al., 2001].