Iron blown by the wind and carried by dust particles from the Sahara travels long distances. The essential nutrient is transported 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 cannot use it.

New research published in Frontiers of Marine Science shows that the further this iron travels on wind-blown dust, the more bioavailable it becomes due to the chemical reactions it undergoes while in the atmosphere.

Iron is essential to some of life’s most fundamental processes, playing a key role in the biomolecules responsible for photosynthesis, DNA repair, and more. This is a major constraint to the growth of phytoplankton, the foundation of ocean ecosystems and a major driver of the planet’s carbon cycle. Iron-rich dust from the Sahara can therefore have a considerable impact on distant ecosystems.

“There are interesting interconnections across immense scales of time and space,” said Vernon Morrisan atmospheric scientist at Arizona State University who was not involved in the study. Morris leads a project called AEROSE (Aerosols and Ocean Science Expeditions) that has been tracking Saharan dust storms across the North Atlantic since 2004.

Connecting air and ocean

Establishing a chemical link between what happens to iron-rich dust in the atmosphere and what happens to it in the oceans has been a challenge. The researchers measured total iron levels in seafloor sediment cores, but this approach does not take into account whether that iron was in forms that organisms can use, said Timothy Lyonsbiogeochemist at the University of California, Riverside and co-author of the new study.

“If it’s not soluble, it’s not available for life,” he said.

Lyons and his colleagues set out to connect the chemistry of iron in the ocean and the atmosphere. “There is a disconnect in the characterization of this dust,” he said.

The researchers examined the iron chemistry of ocean sediments, carefully cataloging the different types of iron minerals present. They focused on sediment cores taken from four sites in the North Atlantic Ocean located along the path traveled by the Saharan dust plumes: two near the west coast of Africa and two closer to the east coast. of North America. The cores record 120,000 years of sediment that slowly settled to the ocean floor.

To ensure that the iron they examined came from dust, they chose study sites far from iron-rich ocean ridges. The iron-to-aluminum ratio in the samples was consistent with that of rocks from a continent, suggesting that the samples came from a dust source and giving additional confidence to the researchers.

Researchers analyzed minerals such as magnetite, pyrite and forms of carbonate. They measured the ratio between bioavailable forms of iron and the total amount of iron contained in these minerals.

Lyons and his colleagues had to solve a biogeochemical puzzle. Bioavailable iron released into the ocean would likely have been absorbed by phytoplankton. So it wouldn’t be left in the sediment or it would be in a different form than it was when the dust landed. “We looked for telltale signs of iron conversion and loss,” Lyons said. Some of the minerals analyzed contain bioavailable forms of iron, while others contain iron in forms that were likely bioavailable but were transformed after being consumed by phytoplankton, deposited on the seafloor, and reacted with the sediments.

Their analysis showed that samples taken from sites further away in North Africa contained less iron in its bioavailable forms, likely because it had been consumed by ocean organisms. This indicates that the longer the dust stays in the air, the more bioavailable iron it contains, Lyons said.

Atmospheric reactions

Studying sediments doesn’t tell researchers exactly how the iron in the dust was transformed in the atmosphere, but research by atmospheric chemists provides some clues.

Saharan dust plumes do not mix much with their surroundings. They are “like a chemical reactor,” Morris said. Dust plumes scatter UV light and have high ozone levels. Reactions in the atmosphere likely lead to the oxidation of iron minerals, and these reactions convert some of these minerals into acids, which are more soluble in the ocean, Lyons said.

Morris was excited to read the study results, which he said provide more detailed chemical information about oceanic iron than previous studies. Morris added that there is still work to be done to connect atmospheric chemistry to fertilizing life in the oceans. Now that researchers have data on iron dust in the atmosphere and in sediments, he hopes that further research will look at the concentration of different forms of iron in the water column along the route of study of the Saharan dust plume.

Some studies have shown that dust storms in the Sahara are becoming more intense and more frequent, and that dust is being transported longer distances, Morris said. It’s not clear whether this means more bioavailable iron will be dumped into the world’s oceans or what the impact will be on the planet’s carbon cycle, he said.

—Katherine Bourzac, scientific editor

Quote: Bourzac, K. (2024), Saharan dust carries iron that nourishes life in the distant ocean, Eos, 105, https://doi.org/10.1029/2024EO240490. Published October 30, 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
Unless otherwise noted, images are subject to copyright. Any reuse without the express permission of the copyright owner is prohibited.

Related