Ocean life thrives on Saharan dust blown thousands of miles away

Iron is a micronutrient essential for life, enabling processes such as respiration, photosynthesis and DNA synthesis. Iron availability is often a limited resource in today’s oceans, meaning that increased iron flux into the oceans can increase the amount of carbon fixed by phytoplankton, with implications for global climate.

Iron is found in oceans and terrestrial ecosystems through rivers, glacial melt, hydrothermal activity, and especially wind. But not all of its chemical forms are “bioreactive,” meaning available for absorption by organisms in their environment.

“Here we show that iron bound to Saharan dust blown westward over the Atlantic has properties that change with distance traveled: the greater the distance, the more bioreactive the iron is,” said Dr. Jeremy Owens, associate professor at Florida State University and co-author of a new study in The Frontiers of Marine Science.

“This relationship suggests that chemical processes in the atmosphere convert less bioreactive iron into more accessible forms.”

The heart of the problem

Owens and his colleagues measured the amounts of bioreactive and total iron in drill cores from the Atlantic Ocean floor collected by the International Ocean Discovery Program (IODP) and its predecessors. The IODP aims to improve our understanding of changing climate and ocean conditions, geological processes, and the origin of life.

The researchers selected four cores based on their distance from the Sahara-Sahel dust corridor, which stretches from Mauritania to Chad and is known to be a major source of dust-bound iron for downwind areas.

The two cores closest to this corridor were taken about 200 and 500 km west of northwest Mauritania, a third in the mid-Atlantic, and the fourth about 500 km east of Florida. The authors studied the upper 60 to 200 meters of these cores, which reflect deposits from the last 120,000 years, the period since the previous interglacial.

They measured total iron concentrations along these cores, as well as iron isotope concentrations using a plasma mass spectrometer. These isotope data were consistent with those from Sahara dust.

They then used a series of chemical reactions to reveal the fractions of total iron present in the sediments as iron carbonate, goethite, hematite, magnetite and pyrite. The iron in these minerals, although not bioreactive, probably formed from more bioreactive forms through geochemical processes on the seafloor.

“Rather than focusing on total iron content as previous studies have done, we measured iron that can readily dissolve in the ocean and that marine organisms can access for their metabolic pathways,” Owens said.

“Only a fraction of the total iron in sediment is bioavailable, but this fraction could change during transport of iron away from its original source. We sought to explore these relationships.”

Blowing in the wind

The results showed that the proportion of bioreactive iron was lower in the westernmost cores than in the easternmost cores. This implies that a proportionally greater proportion of bioreactive iron had been lost to dust and had probably been used by organisms in the water column, so that it had never reached the bottom sediments.

“Our results suggest that during long-range atmospheric transport, the mineral properties of dust-bound iron, which was not originally bioreactive, change, making it more bioreactive. This iron is then taken up by phytoplankton before it can reach the bottom,” said Dr. Timothy Lyons, a professor at the University of California, Riverside, and the study’s final author.

“We conclude that dust reaching regions like the Amazon Basin and the Bahamas may contain iron that is particularly soluble and available for life, thanks to the great distance from North Africa, and therefore longer exposure to atmospheric chemical processes,” Lyons said.

“The transported iron appears to stimulate biological processes in the same way that iron fertilization can impact life in the oceans and on continents. This study is a proof of concept confirming that iron-bound dust can have a major impact on life at great distances from its source.”