Iron is an essential micronutrient for life, enabling processes such as respiration, photosynthesis, and DNA synthesis. Its availability is often a limiting resource in today’s oceans, which means that increasing its flux into the oceans could raise the amount of carbon fixed by phytoplankton, with consequences for global climate.
Iron reaches the oceans and terrestrial ecosystems through rivers, glacial melt, hydrothermal activity, and, above all, wind. But not all of its chemical forms are “bioreactive,” that is, available for organisms to absorb from their environment.
“Here we show that iron bound to Sahara dust blown westward across the Atlantic has properties that change with distance travelled: the farther the distance, the more bioreactive the iron,” says Jeremy Owens, associate professor at Florida State University and coauthor of a study in Frontiers in Marine Science.
“This relationship suggests that atmospheric chemical processes convert iron that is less bioreactive into forms that are more accessible,” he adds.
The heart of the matter
Owens and his colleagues measured the amounts of bioreactive iron and total iron in seabed drill cores from the Atlantic Ocean, collected by the International Ocean Discovery Program (IODP) and its predecessors.
The goal of IODP is to improve our understanding of climate change and ocean conditions, geological processes, and the origin of life.
Four cores were selected, based on their distance from the so-called Sahara-Sahel Dust Corridor. The latter extends from Mauritania to Chad and is known to be an important source of dusty iron for downwind regions.
The two cores closest to this corridor were collected about 200 km and 500 km to the west of the northwest of Mauritania, a third in the middle of the Atlantic, and the fourth about 500 km to the east of Florida.
The authors studied the upper 60 to 200 meters of these cores, reflecting deposits from the last 120,000 years – the time elapsed since the previous interglacial.
They measured total iron concentrations along these cores, as well as iron isotope concentrations with a plasma mass spectrometer. These isotope data were consistent with Saharan dust.
Next, they used a set of chemical reactions to reveal the fractions of total iron present in the sediments in the form of iron carbonate, goethite, hematite, magnetite, and pyrite.
Iron present in these minerals, while not bioreactive, likely formed from more bioreactive forms through geochemical processes on the seafloor.
“Rather than focusing on the total iron content, as previous studies did, we measured the iron that dissolves readily in the ocean and can be accessed by marine organisms for their metabolic pathways,” explains Owens.
“Only a fraction of the total iron in sediments is bioavailable, but that fraction can change during the transport of iron away from its original source. Our aim is to explore these relationships,” he adds.
Blowing in the wind
The results showed that the proportion of bioreactive iron was lower in the western cores than in the eastern ones. This implied that a correspondingly larger share of bioreactive iron had been lost from the dust and, presumably, used by organisms in the water column, never reaching the sediments on the bottom.
“Our results suggest that, during long-distance atmospheric transport, the mineral properties of iron that were originally bound to dust and are not bioreactive change, making it more bioreactive. This iron is then taken up by phytoplankton, before reaching the seafloor,” says Timothy Lyons, professor at the University of California, Riverside, and the senior author of the study.
“We conclude that dust reaching regions such as the Amazon basin and the Bahamas may contain particularly soluble and life-available iron, thanks to the great distance from North Africa and, consequently, longer exposure to atmospheric chemical processes,” he adds.
“The transported iron appears to be stimulating 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 that confirms that iron-bound dust can have a major impact on life far from its source,” he concludes.
