CO 2 outgassing off central Chile (31–30°S) and northern Chile (24–23°S) during austral summer 1997: the effect of wind intensity on the upwelling and ventilation of CO 2-rich waters

Abstract

The distribution of pH and alkalinity has been used to calculate the distribution of total inorganic carbon (TC) and fugacity of carbon dioxide ( fCO 2) in the upper 200 m of the water column in coastal upwelling areas off northern Chile (23–24°S, near Antofagasta) and central Chile (30–31°S, near Coquimbo) during austral summer 1997. In these upwelling areas, colder surface waters were oxygen poor and strongly CO 2 supersaturated (100% near Antofagasta and 200% near Coquimbo), although below the pycnocline the CO 2 supersaturation invariably exceeded 200% in both areas. The larger surface CO 2 supersaturation and outgassing at 30°S were associated with stronger winds that promoted the upwelling of denser water (richer in CO 2) as well as a higher air–sea CO 2 transfer velocity. The consistent decrease in intensity of the southerly winds (as derived from NSCAT scatterometer data) from 30–31°S to 23–24°S suggests a corresponding decline in the intensity of the CO 2 outgassing due to upwelling. Additionally, we suggest here that the intensity of the local upwelling forcing (i.e. alongshore–equatorward winds) plays a role in determining the water mass composition and phytoplankton biomass of the coastal waters. Thus, while deep upwelling of salty and cold water resulted in high fCO 2 (up to 1000 μatm) and very low phytoplankton biomass (chlorophyll a concentration lower than 0.5 mg m −3), the shallow upwelling of less salty (e.g. salinity <34.5) and less CO 2-supersaturated water resulted in a higher phytoplankton biomass, which further reduced surface water fCO 2 by photosynthesis.