A reconstruction of the prebomb Δ 14C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ 14C content are found distinctly south of the equator. Prebomb, low-Δ 14C surface water appears to owe its origin to the upwelling of ~15 ¿C water off the coast of Peru. The low-Δ14C water upwelling off Peru is shown to be derived from the ''13¿ Water'' thermostad (11¿--14 ¿C) of the Equatorial Undercurrent. Untritiated water in the lower part of the undercurrent had nearly the same Δ 14C content during the Geochemical Ocean Sections Study (GEOSECS) at the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ 14C levels were observed in 9¿--10 ¿C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low-Δ 14C water upwelling off Peru and the thermostad water in the undercurrent both originate as ~8 ¿C water in the subantarctic region of the southwest Pacific. This prescription points to the ''lighter variety'' of Subantarctic Mode Water (7¿--10 ¿C) as a possible source. Because prebomb Δ 14C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels of Δ 14C should be particularly unaffected by diapycnal mixing with warmer overlying water types.

We argue that successively less dense features of the South Pacific thermocline, like the Subantarctic Mode Water, the equatorial 13 ¿C Water, and the Peru upwelling, may be part of a single process of thermocline ventilation. Each evolves from the other by diapycnal alteration, while prebomb Δ 14C is nearly conserved. Detailed comparisons are made between the coral Δ 14C distribution and a model simulation of radiocarbon in Toggeweiler et al. (1989). While the Δ 14C data suggest a southern hemisphere thermocline origin for the equatorial Δ 14C minimum, the model produces its Δ 14C minimum by upwelling abyssal water to the surface via the equatorial divergence. In an appendix to the paper we present a new set of coral Δ 14C measurements produced over the last 10 years at Lamont-Doherty Geological Observatory and compile a post-1950 set of published coral Δ 14C measurements for use in model validation studies. ¿ American Geophysical Union 1991