The Peru upwelling and the ventilation of the south Pacific thermocline

Abstract

A reconstruction of the prebomb Δ14C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ14C content are found distinctly south of the equator. Prebomb, low‐Δ14C 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 Δ14C content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ14C levels were observed in 9°–10°C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low‐Δ14C 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 Δ14C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels Of Δ14C 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 Δ14C is nearly conserved. Detailed comparisons are made between the coral Δ14C distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the Δ14C data suggest a southern hemisphere thermocline origin for the equatorial Δ14C minimum, the model produces its Δ14C 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 Δ14C measurements produced over the last 10 years at Lamont‐Doherty Geological Observatory and compile a post‐1950 set of published coral Δ14C measurements for use in model validation studies.

New views of tropical paleoclimates from corals

“…The penetration of bomb C into the water column has been measured by several investigators over the years as a component of water-sampling programs such as the Geochemical Ocean Sections Program (GEOSECS) and the World Ocean Circulation Experiment (WOCE) (Linick, 1980;O G stlund and Stuiver, 1980;Quay et al, 1983;Broecker et al, 1985). These C data have been important for critical tests in ocean circulation models (Toggweiler et al, 1991).…”

“…As an illustration of the ongoing e!orts, we consider records from the tropical and subtropical Paci"c from Dru!el (1987), Toggweiler et al (1991), , Guilderson and Schrag (1998), and Beck et al (submitted) (Fig. 10).…”

“…10. Post-bomb radiocarbon records from "ve locations in the tropical and subtropical Paci"c. Data are from French Frigate Shoals (Dru!el, 1987), Fiji (Toggweiler et al, 1991), Nauru , Christmas Island (Beck et al, submitted), and Galapagos Islands . variability in the tropical Paci"c re#ected variability in equatorial upwelling in the eastern Paci"c and lateral redistribution of radiocarbon by shifting surface currents.…”

Late Quaternary sea-level and tectonic changes in northeast Fiji

“…It is reasonable to suppose both that the processes of notch formation are less e¡ective in such lithologies and that the obliteration of erosional forms is more e¡ective. Toggweiler et al (1991), 2c range given.…”

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…This relatively greater depletion of silicic acid combined with preferential regeneration of N (relative to Si) from sinking organic debris [Nelson et al, 2002] results in the export of surface waters to the Subantarctic zone with low concentrations of Si relative to N [Sarmiento et al, 2004]. North of the APF these Si-depleted surface waters mix with deeper waters to form Sub-Antarctic Mode Water (SAMW) (Pacific basin) or South Atlantic Central Water (SACW) (Atlantic basin), which sink and fill the thermocline of the southern hemisphere oceans, eventually upwelling at the equator [Toggweiler et al, 1991;Sigman et al, 1999;Stramma and England, 1999;Brzezinski et al, 2002]. These mixtures feeding the thermocline have dissolved Si:N ratios of about 1:2 [Sarmiento et al, 2004].…”