New evidence from an international team of researchers suggests that huge, moving dust clouds from the Sahara Desert are essential to supporting life in the Atlantic Ocean as far as the Amazon basin, but this source of biodiversity is especially useful after traveling thousands of miles through the atmosphere.
Animals and plants need iron to thrive, and in much of the Atlantic Ocean, this micronutrient often comes from the Sahara Desert, one of the driest places in the world. But there are different types of iron, which have different levels of bioreactivity. Higher levels make it more available to organisms like phytoplankton for photosynthesis and respiration. According to a study published Sept. 19, The forefront of marine scienceDust plumes moving from the Sahara Desert towards the east coast of the United States deliver huge amounts of iron across the Atlantic, but it is the journey itself that makes the micronutrient suitable for the most remote organisms and plants.
[Related: The mysterious ‘star dune’ in the Sahara is on the move.]
“Rather than focusing on total iron as previous studies have done, we measured the iron that is readily soluble in the ocean and available for marine organisms to use in their metabolic pathways,” study co-author Jeremy Owens, an associate professor at Florida State University, said in a statement. “Only a small fraction of iron in sediments is available to organisms, but that fraction can change as the iron is mobilized from its original source. We aimed to explore these relationships.”
To investigate, the researchers measured the total iron concentration in drill core samples taken by the International Ocean Discovery Program (IODP) from the Atlantic Ocean floor, then compared each sample’s distance from the Sahara-Sahel Dust Corridor. Located between Mauritania and Chad, this region is known for wind patterns that can carry atmospheric particles thousands of miles in quantities that can be detected by satellite imagery.
The team took cores from locations 125 and 310 miles northwest of Mauritania, and about 310 miles off the east coast of Florida. They looked specifically at iron and mineral content from the past 120,000 years at depths ranging from 200 to 640 feet in each sample. Much of the iron analyzed was contained in a variety of mineral compounds, including goethite, magnetite, pyrite, hematite and iron carbonate, all of which are believed to have originated from bioreactive iron on the seafloor.
The researchers found that samples closer to the Americas had lower levels of bioreactive iron than samples closer to Africa, suggesting that Saharan dust contains more bioreactive dust that is consumed by aquatic organisms before reaching the ocean floor. But at the same time, the remaining dust that continues to travel through the atmosphere is subject to prolonged exposure to various photochemical processes that increase its solubility through the production of acids.
“Our results suggest that long-distance atmospheric transport changes the mineralogy of iron bound to originally non-bioreactive dust, making it more bioreactive,” said study co-author Timothy Lyons, a professor at the University of California, Riverside. It said in a statement“This iron is then taken up by phytoplankton before it reaches the ocean floor.”
“The longer the distance, the more bioreactive the iron becomes,” added Jeremy Owens, another study co-author and an associate professor at Florida State University.
Based on their findings, the team concludes that some of the vital iron in dust from areas such as the Amazon basin and the Bahamas may have been the result of great distances traveled and atmospheric influences after arriving from North Africa.
