Title: The Peru upwelling and the ventilation of the south Pacific thermocline
Abstract: A reconstruction of the prebomb Δ 14 C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ 14 C content are found distinctly south of the equator. Prebomb, low‐Δ 14 C surface water appears to owe its origin to the upwelling of ∼15°C water off the coast of Peru. The low‐Δ 14 C 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 14 C content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ 14 C levels were observed in 9°–10°C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low‐Δ 14 C 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 14 C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels Of Δ 14 C 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 14 C is nearly conserved. Detailed comparisons are made between the coral Δ 14 C distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the Δ 14 C data suggest a southern hemisphere thermocline origin for the equatorial Δ 14 C minimum, the model produces its Δ 14 C 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 14 C measurements produced over the last 10 years at Lamont‐Doherty Geological Observatory and compile a post‐1950 set of published coral Δ 14 C measurements for use in model validation studies.
Publication Year: 1991
Publication Date: 1991-11-15
Language: en
Type: article
Indexed In: ['crossref']
Access and Citation
Cited By Count: 420
AI Researcher Chatbot
Get quick answers to your questions about the article from our AI researcher chatbot