Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0710279105 (2008)

Credit: ESA

Murky plumes of Amazonian fresh water are carried hundreds of kilometres out to sea (as pictured by satellite). Whether these waters suppress or enhance primary production in the Atlantic has been up for debate.

With this in mind, and with data from 82 oceanic field stations to hand, Ajit Subramaniam of Columbia University in New York and his co-workers have labelled the Amazon's discharge a carbon sink. They calculate that photosynthetic nitrogen-fixing organisms that assimilate iron and phosphorus in the plume — and then die and fall to the sea floor — sequester 20.4 million tonnes of carbon per year. A further 7.2 million tonnes of carbon is fixed annually by organisms that use nitrate delivered to the ocean by the river.