Iron reaches oceans through rivers, glaciers, hydrothermal activity, and especially wind. However, not all forms of iron are immediately usable by marine organisms.
"Here we show that iron bound to dust from the Sahara blown westward over the Atlantic has properties that change with the distance traveled: the greater this distance, the more bioreactive the iron," explained Dr. Jeremy Owens, associate professor at Florida State University and co-author of a new study published in 'Frontiers in Marine Science'. "This relationship suggests that chemical processes in the atmosphere convert less bioreactive iron to more accessible forms."
Studying Iron in Atlantic Ocean Sediments
Owens and colleagues examined iron levels in drill cores collected from the Atlantic Ocean's floor as part of the International Ocean Discovery Program (IODP), which aims to improve understanding of climate change, ocean conditions, and geology. Four cores were analyzed, chosen based on their proximity to the Sahara-Sahel Dust Corridor, a major source of dust-bound iron.
The cores ranged from 200 km west of Mauritania to 500 km east of Florida. By studying sediments from the last 120,000 years, the researchers measured total iron concentrations and isotopic compositions. The isotope data aligned with iron originating from the Sahara.
Using advanced chemical techniques, they identified various iron-containing minerals, such as iron carbonate and pyrite. These forms are not bioreactive but likely resulted from more bioreactive iron transforming on the ocean floor.
"Rather than focusing on total iron content as previous studies had done, we measured iron that can dissolve easily in the ocean, and which can be accessed by marine organisms for their metabolic pathways," Owens said.
Impact of Dust Transport
The study revealed that bioreactive iron levels were higher in cores closer to Africa and lower in those further west. This suggests that more bioreactive iron was consumed by marine organisms as the dust traveled, preventing it from settling on the ocean floor.
"Our results suggest that during long-distance atmospheric transport, the mineral properties of originally non-bioreactive dust-bound iron change, making it more bioreactive. This iron then gets taken up by phytoplankton, before it can reach the bottom," said Dr. Timothy Lyons, professor at the University of California at Riverside and the study's final author.
Lyons added that the transported dust reaching areas like the Amazon and the Bahamas likely contains highly soluble iron, promoting biological activity in distant ecosystems. This study supports the idea that iron-rich dust can profoundly impact oceanic and terrestrial life, even far from its origin.
Research Report:Long-range transport of dust enhances oceanic iron bioavailability
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