Massive, migrating dust clouds originating in the Sahara Desert are crucial to fostering life in the Atlantic Ocean as far away as the Amazonian basin, according to new evidence from an international team of researchers. This source of biodiversity, however, becomes particularly useful after traveling thousands of miles in the atmosphere.
Animal and plant life needs iron to thrive, and in much of the Atlantic Ocean, that micronutrient often originates in one of the world’s driest places, the Sahara Desert. But there are many variants of iron with different levels of bioreactivity—the higher the level, the more accessible it is for organisms like phytoplankton to use for photosynthesis and respiration. According to a study published on September 19 in Frontiers in Marine Science, dust plumes traveling from the Sahara Desert towards the US East Coast supply large amounts of iron across the Atlantic, but it’s the journey itself that makes the micronutrient suitable for the most distant organisms and plants.
[Related: The mysterious ‘star dune’ in the Sahara is on the move.]
“Rather than focusing on the 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,” Jeremy Owens, study co-author and a Florida State University associate professor, said in a statement. “Only a fraction of total iron in sediment is bioavailable, but that fraction could change during transport of the iron away from its original source. We aimed to explore those relationships.”
To investigate, researchers took drill core samples from the Atlantic seafloor obtained by the International Ocean Discovery Program (IODP) and measured their total iron levels. They then compared these to each sample’s distance from the Sahara-Sahel Dust Corridor. This region, located between Mauritania and Chad, is known for wind patterns that transport airborne particulates across thousands of miles in such large amounts that they are detectable by satellite imagery.
The team collected cores from 125 and 310 miles northwest of Mauritania, as well as about 310 miles from Florida’s east coast. Specifically, they focused on iron and mineral contents located 200-640 feet into each sample which correspond to the last 120,000 years. Much of the iron analyzed was found in different mineral compounds such as goethite, magnetite, pyrite, hematite, and iron carbonate—all of which are believed to develop from the seafloor’s bioreactive iron.
Researchers learned the levels of bioreactive iron are lower in samples closer to the Americas than those near Africa. This indicates the Saharan dust is heavier in bioreactive dust that gets used up by aquatic organisms before reaching the bottom of the ocean. At the same time, however, the remaining dust that continues along the atmospheric migration route undergoes prolonged exposure to various photochemical processes that increases their solubility through acid production.
“Our results suggest that during long-distance atmospheric transport, the mineral properties of originally non-bioreactive dust-bound iron change, making it more bioreactive,” Timothy Lyons, study co-author and a professor at the University of California Riverside, said in a statement. “This iron then gets taken up by phytoplankton, before it can reach the bottom.”
“The greater the distance, the more bioreactive the iron,” added Jeremy Owens, another study co-author and a Florida State University associate professor.
According to their findings, the team concludes that dust in areas including the Amazonian basin and the Bahamas may owe some of their life-sustaining iron contents to immense distances and atmospheric influences after arriving from North Africa.
