Amazon Powers Carbon Sink in Tropical Ocean

A new study from University of Southern California was just released, appearing in PNAS Early Edition the week of July 21, showing that run-off from the Amazon River powers a large carbon sink in the tropical North Atlantic ocean. New Scientist talks about it here.

According to the press realease, which I will quote at the end of the post, this overturns the previously held view that this area of the ocean was a net emmitter of carbon.

The river delivers iron and phosphorous to small organizims called diazotrophs which then fix nitrogen and carbon from the air and ultimately sink it to the bottm of the ocean.

The authors are pointing to implications for the study of “iron fertilization” of the oceans, a way to geoengineer carbon out of the atmosphere.

One other interesting, in a worrisome way, implication is that if the Amazon jungle were to die off and the river dry up considerably, a very likely consequence of anthropogenic climate disruption, the cessation of this process would be yet another positive feedback. Nice.

Here is the press release:

From University of Southern California Media Relations
Los Angeles, Calif.
Tel: (213) 740-2215 Fax: (213) 740-7600
Experts Directory:

CONTACT: Carl Marziali, (213) 740-4751




Nutrients from the Amazon River spread well beyond the continental shelf and
drive carbon capture in the deep ocean, according to the authors of a
multi-year study.

The finding does not change estimates of the oceans¹ total carbon uptake,
but it reveals the surprisingly large role of tropical oceans and major

The tropical North Atlantic had been considered a net emitter of carbon from
the respiration of ocean life. A 2007 study estimated that ocean¹s
contribution to the atmosphere at 30 million tons of carbon annually.

The new study, appearing in PNAS Early Edition the week of July 21, finds
that almost all the respiration is offset by organisms called diazotrophs,
which pull nitrogen and carbon from the air and use them to make organic
solids that sink to the ocean floor.

Diazotrophs ³fix² nitrogen from the air, enabling them to thrive in
nutrient-poor waters. They also require small amounts of phosphorus and
iron, which the Amazon River delivers far offshore.

That is all the diazotrophs need to pull carbon from the air and sink it in
the ocean.

The other great tropical rivers of the world also may contribute to carbon
capture, said senior author Doug Capone, professor in the USC Wrigley
Institute for Environmental Studies at the University of Southern
California, adding that studies on such rivers are in progress.

The study¹s results present new options for the controversial practice of
iron fertilization. Some biologists believe that seeding the oceans with
iron could increase production of carbon-fixing organisms and help mitigate
climate change.

Upwelling circulation in cooler waters makes them unlikely candidates for
long-term carbon capture, said Capone, who explained that a permanent carbon
sink instead may be more feasible in the warm oceans.

Capone said that iron fertilization would increase diazotroph activity and
that the stratified tropical waters should be able to keep captured carbon
solids from returning to the surface in the short term.

³The most appropriate places are probably not the high latitudes but rather
the low-latitude areas where nitrogen fixation is a predominant process,²
Capone said.

But Capone also noted the risks of iron fertilization, including increased
production of other greenhouse gases and unpredictable effects on the food

Nevertheless, he said, ³if we choose as a human society to fertilize areas
of the oceans, these are the places that probably would get a lot more bang
for the buck in terms of iron fertilization than we would at high

The other authors on the multi-year study were researchers from the
University of Georgia, Athens; San Francisco State University; the
University of Liverpool; the University of Hawaii, Honolulu; Rutgers
University; Georgia Institute of Technology; and UCLA.

Ajit Subramaniam of Columbia University was first author.

The National Science Foundation¹s Biocomplexity in the Environment program
provided most of the project¹s funding.

JULY 16, 2008

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