Amazon River Powers Tropical Ocean's Carbon Sink

ScienceDaily (July 22, 2008) — 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 oceans' total carbon

uptake, but it reveals the surprisingly large role of tropical oceans

and major rivers.

The tropical North Atlantic had been considered a net emitter of

carbon from the respiration of ocean life. A 2007 study estimated

that ocean's 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 "fix" 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 Amazon River is the largest river in the world by volume; it also

has the largest drainage basin on the planet, accounting for some one

fifth of Earth's total river flow. Because of its vast dimensions,

it's sometimes called "the river sea."

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 study's 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. "The most appropriate places are probably not the high

latitudes but rather the low-latitude areas where nitrogen fixation

is a predominant process," Capone said.

But Capone also noted the risks of iron fertilization, including

increased production of other greenhouse gases and unpredictable

effects on the food web.

Nevertheless, he said, "if 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 latitudes."

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 Foundation's Biocomplexity in the Environment

program provided most of the project's funding

posted to ClimateConcern