Reconstructing the oceanic 13C Suess Effect

Abstract

The anthropogenic δ13C change for the time period 1968 to 1991 was determined based on calculations of the preformed 13C/12C 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 δ13C (the 13C Suess effect) along isopycnals was calculated using CFC‐derived water ages and yields a time history of the surface water δ13C 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 δ13C change rate reconstructions agree, both in magnitude and meridional trend, with direct observations of surface ocean δ13C changes reported from time series measurements and from comparisons of surface water δ13C of DIC measurements in 1970 and 1993. A global ocean average surface δ13C 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 13C reconstructions in the Pacific, Indian, and Atlantic Oceans, when combined with these previous individual depth profile comparisons and Geochemical Ocean Sections Study bomb 14C inventories [Quay et al, 1992], imply a global depth‐integrated δ13C change rate of −9.7 ± 2.4‰ m yr−1 over the time period 1970–1990. These results imply a net oceanic CO2 uptake rate of 1.9 ± 0.9 Gt C yr−1 over the time period 1970–1990 when applied to an atmospheric 13CO2 and 12CO2 budget.