Scientists prove dust from the Sahara desert provides most of the iron found in the Atlantic ocean.
Marine scientists report in the Journal Nature they have measured levels of iron dissolved in the Atlantic ocean, and worked out where it came from. The team headed by Seth John of the University of South Carolina used samples they gathered on their long sea voyage as well as looking back at the historical record including dust samples gathered in mid-atlantic by Charles Darwin during his Voyage on the research ship Beagle in 1831!
In the course of doing so, they have helped explain in more detail why the deep distant ocean away from land is mostly a blue desert while coastal waters are usually green, and at the same time helped answer more complex questions about the ocean’s role as the most powerful CO2 regulator on this blue planet.
They have also detailed the mechanism of North Africa’s importance to the rest of the world. It keeps the oceans and the Amazon supplied with vital mineral dust and the all important iron therein. The researchers following the course of legendary oceanographer the late John Martin they refined a way to sample large volumes of seawater to identify the infinitesimal content of dissolved iron in the water, and then to distinguish the ratio between different isotopes of that iron.
Iron is a vital trace element in nature: without it, red-blooded animals including humankind cannot make haemoglobin to transport oxygen around the bloodstream and plants cannot make effective use of chlorophyll to photosynthesize CO2 using air and sunlight to make more plant biomass.
The distant oceans far from land have everything needed for plant growth – sunlight, carbon, nitrogen and water – but they lack vital iron. That is the key reason why they tend to be blue deserts while nutrient-rich coastal waters are blue-green and green.
Estuaries and deltas are rich in iron and other nutrients and good for algal growth. Because ocean phytoplankton (microscopic plants which sustain 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 great 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,” says Dr John. “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, and set to work trying to identify the origin of the few billionths of a gram of iron in every litre of the water collected.
They found that a measurable proportion of oceanic iron seeped up from deep within the crust through hydrothermal vents along the mid-ocean ridge. A fraction came from sediments stirred up by currents on the African coast, and more than 10% came from oxygenated muds similarly stirred up by currents on the American coast.
They also found that the answer to vital iron in the ocean had been blowing in the wind. Somewhere between 71% and 87% was delivered by dust storms from the Sahara desert. That is, life in the deep open ocean far from shore depended on an annual delivery of fertiliser from one of the world’s emptiest and most parched regions.
The role of dust and ocean life has fascinated scientists for more than a century. In 1831 when Charles Darwin was the now famous Voyage of the Beagle he collected dust from the middle of the Atlantic Ocean that came from a dust storm that shrouded the sun. As the ships dutiful naturalist he speculated that dust must be nourishing ocean life. Later in the 1930’s ocean scientists again theorized on how dust in the wind was likley nourishing ocean plankton blooms. In 1989 the late great oceanographer John Martin released his epic work proving that iron in windblown dust had been defining the ice ages and warm periods between those ice ages. In 2006, Israeli researchers found that more than half the dust needed to fertilise 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 analysed dust samples collected by Charles Darwin. Just this month, February 2015, NASA researchers working with the CALIPSO satellite that was launched in 2006 have reported on their findings that Saharan dust is providing nearly 100% of the vital phosphorus that sustains the Amazon rainforest. If the dust were to stop the Amazon would begin dying within a year.
Explaining the past
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 theorised 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,” Dr John says.
“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.”