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Desert Dust Feeds Deep Ocean Life

Dust from the Sahara Desert provides most of the iron found in the Atlantic Ocean, according to research that also sheds light on how the oceans help sequester carbon dioxide
dust plume off Africa


Dust plume off the Sahara desert over the northeast Atlantic Ocean. 
Credit: NASA via Wikimedia Commons

LONDON − Marine scientists have a better answer for why the deep ocean is blue while coastal waters are usually green, as well as a better handle on the more complex questions about the ocean's role in the absorbing all the carbon dioxide we're pumping into the atmosphere.

The insight comes as scientists measure levels of iron dissolved in the Atlantic ocean and trace the metal back to its source.

And in the course of settling these points, they have also answered questions about North Africa's importance to the rest of the world. Turns out the Sahara keeps the world's oceans and the Amazon supplied with valuable dust.

Tim Conway and Seth John of the University of South Carolina report in the journal Nature that they devised a way to sample large volumes of seawater to identify the content of dissolved iron in the water, and then to distinguish the ratio between different isotopes of that iron.

An isotope is a natural variant of an element, and it often indicates a different source of origin. Coal from China, for instance, can be differentiated from Appalachian coal based on its isotopic signature.

Iron is a vital trace element: Without it, mammals cannot make hemoglobin to transport oxygen around the bloodstream, and plants cannot make chlorophyll to photosynthesize tissue from air and sunlight.

The deep oceans have everything needed for plant growth – sunlight, carbon, nitrogen and water – but they don't have iron. That is one reason why they tend to be blue while nutrient-rich coastal waters are green. Estuaries and deltas are rich in iron and other nutrients and good for algal growth. Because ocean phytoplankton – microscopic plants sustaining the marine food web – cannot get enough iron, there is a limit to the carbon dioxide they can absorb from the atmosphere. So iron is an element in the global carbon cycle. And it doesn't need to be available in huge quantities.

"I did a calculation once on a ton of sea water. The amount of iron in that ton of water would weigh about as much as a single eyelash," John said. "The key reason that everybody cares about iron is because it limits the growth of phytoplankton such as algae in maybe a fifth of the ocean."

The researchers collected 600 samples of sea water during a cruise across the North Atlantic on a research ship. Then they set to work trying to identify the origin of the few billionths of a gram of iron in every liter of the water collected.

A measurable proportion of oceanic iron seeped up from deep within the crust through hydrothermal vents along the mid-ocean ridge. Another fraction came from sediments on the African coast, and more than 10 percent came from oxygenated muds on the American coast.

But they also found that the answer had been blowing in the wind. Somewhere between 71 percent and 87 percent of the iron in their samples was delivered by dust storms from the Sahara desert. That is, life in the deep ocean depended on an annual delivery of fertilizer from one of the world's emptiest and most parched regions.

The play between dust and life has fascinated scientists for more than a decade. In 2006, Israeli researchers found that more than half the dust needed to fertilize the Brazilian rainforest blew in from just one desiccated valley in Chad.

Two years later a team in Liverpool in the UK confirmed the role of Saharan dust as a mineral source for the Atlantic ocean and in 2007 Swiss and German microbiologists analyzed dust samples collected by Charles Darwin.

They found that wind-blown dust could transport microbes from West Africa all the way to the Caribbean. An estimated 50 million tons of Saharan dust is blown across the Atlantic to the Amazon every year.

So the South Carolina research is just another example of science in action: A painstaking increment to human knowledge rather than a breakthrough. It adds quantifiable figures to a picture already taking shape. It is a reminder that intercontinental migration is as old as life itself. And it also helps explain a little bit more about the global climate machine.

Researchers have already theorized that airborne dust must play a role in cloud formation – and therefore in rainfall and drought – and even that dust storms may play a role in damping down hurricanes.

If more dust in the oceans and the forests means more carbon uptake from the atmosphere, then cycles of superstorms of dust could also help tweak the global thermostat. "It could help us understand past climate change, like glacial-interglacial cycles," John said.

"There would have been huge changes in dust fluxes to the ocean in glacial times, and so understanding how much iron comes from dust in the modern day helps us figure out whether that was an important driver of glacial-interglacial cycles."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media company.

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