As iron-laden dust travels further from the Sahara Desert, atmospheric reactions make iron increasingly accessible to support life.
Iron is an essential micronutrient for life, playing a crucial role in processes such as respiration, photosynthesis, and DNA synthesis. In today’s oceans, iron is a limiting element. Therefore, enhancing the accessibility of iron could promote the carbon fixation process of phytoplankton, potentially impacting the global climate. This research highlights the significant role of the Sahara Desert in global climate change. Not only does it reflect substantial sunlight back into space, but the Sahara also serves as a nursery for oceanic microorganisms that fix carbon.
Iron enters oceans and terrestrial ecosystems through rivers, melting glaciers, hydrothermal activity, and particularly through wind. However, not all iron compounds are “biologically reactive,” meaning the state in which organisms can absorb iron from their environment.
Dr. Jeremy Owens, an associate professor at Florida State University and co-author of a new study in Frontiers in Marine Science, stated: “Here, we show that iron associated with dust from the Sahara Desert blowing westward to the Atlantic has properties that change with distance traveled: the farther it goes, the more biologically reactive the iron becomes.”
“This relationship indicates that chemical processes in the atmosphere transform less biologically reactive iron into more accessible forms.”
Dust storm in the Sahara.
The Earth’s Core Has a Voice Too
Owens and colleagues measured biologically reactive iron and total iron content in sediment cores from the bottom of the Atlantic Ocean, collected by the International Ocean Discovery Program (IODP) and previous expeditions. IODP aims to improve our understanding of climate change, ocean conditions, geological processes, and the origins of life. The researchers selected four cores based on their distance from the so-called Sahara-Sahel Dust Corridor, which stretches from Mauritania to Chad and is known as an important source of iron-bound dust for downwind regions.
The two cores closest to this corridor were collected approximately 200 km and 500 km west of northwestern Mauritania, the third core is located in the middle of the Atlantic, and the fourth is about 500 km east of Florida. The authors studied the upper 60 to 200 meters of these cores, reflecting sediments from the past 120,000 years – a time since the last glacial period.
They measured the overall iron concentration along these cores, as well as iron isotope concentrations using a plasma mass spectrometer. This isotope data matched the dust from the Sahara Desert.
Subsequently, they employed a range of chemical reactions to determine the composition of total iron found in sediments in the forms of iron carbonate, goethite, hematite, magnetite, and pyrite. Iron in these minerals, while biologically unreactive, has the potential to form from more biologically reactive forms through geochemical processes on the ocean floor.
Owens noted: “Instead of focusing on total iron as previous studies have done, we measured the iron that can be easily dissolved in the ocean and accessed by marine organisms for metabolic processes.”
“Only a portion of the total iron in the sediments is biologically reactive, but that portion can change during the transport of iron away from its original source.”
Blown by the Wind
The results indicate that the biologically reactive iron levels in the westernmost cores are lower compared to those in the easternmost cores. This implies that a greater proportion of biologically reactive iron has likely been lost from the dust and perhaps absorbed by organisms in the water during deposition. Consequently, it never reached the sediments at the bottom.
Dr. Timothy Lyons, a professor at the University of California, Riverside, and a lead author of the study, stated: “Our results suggest that during long-distance atmospheric transport, the mineral properties of iron associated with initially non-biologically reactive dust change, making it more biologically reactive. This iron is then absorbed by phytoplankton before it can reach the bottom.”
Lyons further explained: “We conclude that dust reaching areas like the Amazon River Basin and the Bahamas may contain particularly soluble iron that can support life, due to the long distance from North Africa and thus prolonged exposure to atmospheric chemical processes.”
“The transported iron appears to stimulate biological processes in the same way that iron absorption can affect life in the oceans and on continents. This study provides evidence for the concept that iron-bound dust can have a significant impact on life at great distances from its source.”
