Windblown iron carried on dust particles from the Sahara travels long distances. The critical nutrient is ferried to plants in the Amazon and to phytoplankton in the Atlantic Ocean and elsewhere. But much of this iron is initially locked up in molecules that are not bioavailable—cells simply can’t use it.
New research published in Frontiers in Marine Science shows that the farther this iron travels on windblown dust, the more bioavailable it becomes because of chemical reactions it undergoes while in the atmosphere.
Iron is essential for some of life’s most basic processes, playing a key role in biomolecules responsible for photosynthesis, DNA repair, and more. It’s a key constraint on the growth of phytoplankton, the bedrock of ocean ecosystems and a major driver of the planet’s carbon cycle. Iron-rich dust from the Sahara can therefore have a tremendous impact on distant ecosystems.
“There are neat interconnections across huge scales of time and space,” said Vernon Morris, an atmospheric scientist at Arizona State University who was not involved with the study. Morris leads a project called AEROSE (Aerosols and Ocean Science Expeditions) that has been following Saharan dust storms across the North Atlantic since 2004.
Connecting Air and Ocean
Making the chemical link between what happens to iron-rich dust in the atmosphere and what happens to it in the oceans has been challenging. Researchers have measured total iron levels in ocean floor sediment cores, but this approach doesn’t consider whether that iron was in forms that organisms can use, said Timothy Lyons, a biogeochemist at the University of California, Riverside, and a coauthor of the new study.
“If it’s not soluble, it’s not available to life.”
“If it’s not soluble, it’s not available to life,” he said.
Lyons and his colleagues set out to connect the chemistry of iron in the ocean and in the atmosphere. “There’s a disconnect in characterizing this dust,” he said.
The researchers examined the chemistry of ocean sediment iron, carefully cataloging the different types of iron minerals present. They focused on sediment cores taken from four sites in the North Atlantic Ocean that are along the path traveled by Saharan dust plumes—two from near the west coast of Africa and two from nearer the East Coast of North America. The cores record 120,000 years’ worth of sediment that had settled slowly to the ocean floor.
To ensure that the iron they were examining was dust-borne, they chose study sites far from iron-rich ocean ridges. The ratio of iron to aluminum in the samples was consistent with that of rocks coming off a continent, suggesting the samples came from a dust source and giving the researchers added confidence.
The researchers analyzed minerals such as magnetite, pyrite, and forms of carbonate. They measured the ratio of bioavailable forms of iron to the total amount of iron within these minerals.
Lyons and his colleagues had to solve a biogeochemical puzzle. Bioavailable iron landing in the ocean would likely have been taken up by phytoplankton—so it would not be left in the sediment or it would be in a different form now than it was when the dust landed. “We looked for telltale signs of iron conversion and loss,” Lyons said. Some of the minerals they analyzed contain bioavailable forms of iron, whereas others contain iron in forms that likely used to be bioavailable but were transformed after being consumed by phytoplankton, settling to the seafloor, and reacting with sediments.
Their analysis showed that the samples taken from sites farther away from North Africa had less iron in bioavailable forms—presumably because it had been consumed by ocean organisms. This indicates that the longer the dust stayed in the air, the greater the amount of bioavailable iron it contained, Lyons said.
Atmospheric Reactions
The sediment study doesn’t tell researchers exactly how the dust-borne iron was transformed in the atmosphere, but research by atmospheric chemists provides some clues.
Saharan dust plumes are “like a chemical reactor.”
Saharan dust plumes don’t mix much with their surroundings. They’re “like a chemical reactor,” Morris said. The dust plumes scatter UV light and have elevated ozone levels. Reactions in the atmosphere likely lead to oxidation of iron minerals, and these reactions convert some of the minerals into acids, which are more soluble in the ocean, Lyons said.
Morris was excited to read the study’s results, which he said provide more detailed chemical information about ocean iron than previous studies. Morris added that there’s more work to be done to connect atmospheric chemistry to fertilizing life in the oceans. Now that researchers have data about iron dust in the atmosphere and in the sediment, he hopes further research will look at the concentration of different forms of iron in the water column along the Saharan dust plume study route.
Some studies have shown that Saharan dust storms are becoming more intense and more frequent, and the dust gets carried over longer distances, Morris said. It’s not clear whether this means more bioavailable iron is being delivered to the world’s oceans or what the impact on the planet’s carbon cycle will be, he explained.
—Katherine Bourzac, Science Writer
