Reconstructing the oceanic ¹³C Suess Effect

The anthropogenic δ 13 C change for the time period 1968 to 1991 was determined based on calculations of the preformed 13 C/ 12 C of dissolved inorganic carbon (DIC) distributions on isopycnal surfaces in the main thermocline of the Pacific, North Atlantic and South Indian Oceans. The time rate of change of preformed δ 13 C (the 13 C Suess effect) along isopycnals was calculated using CFC‐derived water ages and yields a time history of the surface water δ 13 C change at the isopycnal outcrop location. The surface ocean Suess effect recorded on isopycnals decreased with increasing outcrop latitude from approximately −0.2‰ decade −1 within the subtropics to around −0.1‰ decade −1 in the subpolar oceans. In the Pacific Ocean these surface δ 13 C change rate reconstructions agree, both in magnitude and meridional trend, with direct observations of surface ocean δ 13 C changes reported from time series measurements and from comparisons of surface water δ 13 C of DIC measurements in 1970 and 1993. A global ocean average surface δ 13 C rate of change of −0.15 ± 0.04 ‰ decade −1 is determined, which is slightly smaller than a previous time series data and model‐based estimate (−0.171‰ decade −1 , [ Bacastow etal., 1996]). Depth integrations of the 13 C reconstructions in the Pacific, Indian, and Atlantic Oceans, when combined with these previous individual depth profile comparisons and Geochemical Ocean Sections Study bomb 14 C inventories [ Quay et al, 1992], imply a global depth‐integrated δ 13 C change rate of −9.7 ± 2.4‰ m yr −1 over the time period 1970–1990. These results imply a net oceanic CO 2 uptake rate of 1.9 ± 0.9 Gt C yr −1 over the time period 1970–1990 when applied to an atmospheric 13 CO 2 and 12 CO 2 budget.