A transatlantic flight could turn Saharan dust into a major ocean nutrient

As dust from the Sahara blows thousands of kilometers across the Atlantic Ocean, it becomes progressively more nutritious for marine microbes, a new study suggests.

Chemical reactions in the atmosphere grind iron minerals into dust, making them more soluble in water and creating an essential nutrient source for iron-starved seas, researchers report Sept. 20. Frontiers in Marine Science.

Dust clouds settling in the Atlantic can create phytoplankton blooms that support marine ecosystems, says Timothy Lyons, a biogeochemist at the University of California, Riverside. “Iron is incredibly important for life,” he says. Phytoplankton requires it to convert carbon dioxide into sugars during photosynthesis.

By further studying dust transport and chemical reactions in the atmosphere, scientists can better understand why parts of the ocean are biological hotspots for phytoplankton and fish.

Over 240 million metric tons of Saharan dust blows over the Atlantic Ocean each year. In Bermuda, the Bahamas and other islands, the earth turns red. But most of it settles in the ocean, providing a major source of iron in areas too far from land to get it from rivers.

Lyons and marine geologist Jeremy Owens, then also at UC Riverside, tried to answer another dust question: Have the types of dust settling in the Atlantic changed over the past 120,000 years? They analyzed the minerals that flowed from the dust in four cores ripped from the muddy sea floor – two in the eastern Atlantic near Africa and two from farther west near North America.

What they found prompted another line of inquiry.

In dust and soils around the world, approximately 40 percent of the iron is typically present within “reactive” minerals such as pyrite or carbonates. This type of iron can be decomposed by weak acids and can be used by life. In core samples from the bottom of the Atlantic, only about 9 percent of the iron in dust ores sampled farther west consisted of reactive iron ores, compared with about 18 percent in dust ores sampled from closer to Africa. That, Lyons says, was “the big surprise.”

He and Owens, now at Florida State University in Tallahassee, concluded that during the dust’s several-day flight across the Atlantic, more and more of its reactive iron was altered—attacked by acids and ultraviolet radiation, which separated the minerals.

“There are photochemical transformations that tend to make iron more soluble” in water, Lyons says. As the modified iron later settles into the ocean, it is dissolved—and devoured—by phytoplankton. The only reactive iron that reaches the bottom of the sea is the stuff that wasn’t altered during air transport and wasn’t ingested later. Their results suggest that the farther the desert dust flies, the less iron remains.

By spawning phytoplankton blooms, iron leaching from the dust can also feed small fish and other animals that graze on the plankton, as well as predators that eat the grazers. A recent study suggested that Atlantic tuna, an important commercial fish, is attracted to areas where the Saharan dust has settled.

The new results are plausible because previous studies have shown that iron minerals react in the atmosphere, says Natalie Mahowald, an atmospheric scientist who studies dust at Cornell University. Their conclusion “matches what I thought was going on,” she says.

But she points out that Saharan dust isn’t the only possible source of that iron: The samples came from far enough north in the Atlantic that some of their iron could have come from smoke from fires in North America over 120,000 years. the last one. she says.

Determining a dust source buried deep on the sea floor can be challenging. But Owens and Lyons tried to identify dust fingerprints by measuring the ratios of iron to aluminum and the ratio of light iron atoms to heavy iron atoms in their samples. Both measurements were roughly consistent with the type of dust coming from the Sahara, they found. It may be possible, in the future, to analyze sediment from more places in the Atlantic, providing a clearer picture of how dust has blown across the ocean and changed chemically.