A seasonal tropical sink for atmospheric CO₂ in the Atlantic ocean: the role of the Amazon River discharge

Marine Chemistry, Volume 68, Issue 3, January 2000, Pages 183-201
J. F. Ternon, C. Oudot, A. Dessier and D. Diverres

Summary of Points:

· In the western equatorial Atlantic ocean, near-surface observations show that during summertime, the low-salinity oceanic water, arising from mixing with the Amazon River discharge at the equator, has low CO₂ fugacity levels.

· Near the coast of South America where the salinities are the lowest (S<20), the fugacity of oceanic CO₂ decreases down to 150 uatm and the shelf area acts as a significant sink for atmospheric CO₂.

· The dilution effect by low-salinity water only partly accounts for the decrease in CO₂, and the biological production in the Amazon Plume water enriched in nutrients lowers dissolved inorganic carbon and decreases the fCO₂ by nearly 30%.

· The low-salinity Amazon water tongue spreads northwestwards along the coast by the North Brazil Current (NBC) and is deflected eastwards north of 5°N in the NBC retroflection in summer.

· Consequently, the low-salinity and oceanic fCO₂ (below the atmospheric fCO₂ level) signatures may extend more than 2000 km eastwards. The impact of the river outflow on the air_¯sea CO₂ exchanges in the western region is demonstrated by using the climatologies of the sea surface salinity (SSS) to estimate the magnitude of the annual net CO₂ flux in the western part of the equatorial Atlantic.

· This is in contrast with the central and eastern parts that are sources for atmospheric CO₂.

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