Reconstructing the oceanic C-13 Suess effect

The anthropogenic delta(13)C change for the time period 1968 to 1991 was de termined based on calculations of the preformed C-13/C-12 of dissolved inor ganic carbon (DIC) distributions on isopycnal surfaces in the main thermocl ine of the Pacific, North Atlantic and South Indian Oceans. The time rate o f change of preformed delta(13)C (the C-13 Suess effect) along isopycnals w as calculated using CFC-derived water ages and yields a time history of the surface water delta(13)C change at the isopycnal outcrop location. The sur face ocean Suess effect recorded on isopycnals decreased with increasing ou tcrop latitude from approximately -0.2 parts per thousand decade(-1) within the subtropics to around -0.1 parts per thousand decade(-1) in the subpola r oceans. In the Pacific Ocean these surface delta(13)C change rate reconst ructions agree, both in magnitude and meridional trend, with direct observa tions of surface ocean delta(13)C changes reported from time series measure ments and from comparisons of surface water delta(13)C of DIC measurements in 1970 and 1993, A global ocean average surface delta(13)C rate of change of -0.15 +/- 0.04 parts per thousand decade(-1) is determined, which is sli ghtly smaller than a previous time series data and model-based estimate (-0 .171 parts per thousand decade(-1), [Bacastow et al., 1996]). Depth integra tions of the C-13 reconstructions in the Pacific, Indian, and Atlantic Ocea ns, when combined with these previous individual depth profile comparisons and Geochemical Ocean Sections Study bomb C-14 inventories [Quay et al., 19 92], imply a global depth-integrated delta(13)C change rate of -9.7 +/- 2.4 parts per thousand 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 tim e period 1970-1990 when applied to an atmospheric (CO2)-C-13 and (CO2)-C-12 budget.