The Peru Upwelling and the Ventilation of the South-Pacific Thermocline

A reconstruction of the prebomb DELTA-C-14 distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest DELTA-C-14 content are found distinctly south of the equator. Prebomb, low-DELTA-C-14 surface water appears to owe its origin to the upwelling of approximately 15-degrees-C water off the coast of Peru. The low-DELTA-C-14 water upwelling off Peru is shown to be derived from the "13-degrees Water" thermostad (11-degrees-14-degrees-C) of the Equatorial Undercurrent. Untritiated water in the lower part of the undercurrent had nearly the same DELTA-C-14 content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar DELTA-C-14 levels were observed in 9-degrees-10-degrees-C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low-DELTA-C-14 water upwelling off Peru and the thermostad water in the undercurrent both originate as approximately 8-degrees-C water in the subantarctic region of the southwest Pacific. This prescription points to the "lighter variety" of Subantarctic Mode Water (7-degrees-10-degrees-C) as a possible source. Because prebomb DELTA-C-14 is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels of DELTA-C-14 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-degrees-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 DELTA-C-14 is nearly conserved. Detailed comparisons are made between the coral DELTA-C-14 distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the DELTA-C-14 data suggest a southern hemisphere thermocline origin for the equatorial DELTA-C-14 minimum, the model produces its DELTA-C-14 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 DELTA-C-14 measurements produced over the last 10 years at Lamont-Doherty Geological Observatory and compile a post-1950 set of published coral DELTA-C-14 measurements for use in model validation studies.