Saharan dust is important for ecological protection and climate change

As iron-laden dust moved farther away from the Sahara, atmospheric reactions made iron increasingly accessible to support life.

Iron is an essential trace element for life, playing a key role in processes such as respiration, photosynthesis, and DNA synthesis. In today's oceans, iron is a limited element. Therefore, increasing the accessibility of iron could promote carbon fixation by phytoplankton, potentially affecting global climate. This study demonstrates the enormous role of the Sahara Desert in global climate change. In addition to reflecting significant sunlight back into space, the Sahara also serves as a breeding ground for carbon-fixing oceanic microorganisms.

Iron enters the oceans and terrestrial ecosystems through rivers, melting glaciers, hydrothermal activity, and especially wind. But not all iron compounds are 'bioavailable ,' meaning they are in a state where organisms can take up iron from their environment.

"Here, we show that iron bound to dust from the Sahara blowing westward toward the Atlantic has properties that change with distance traveled: the farther the distance, the more biologically reactive the iron," said Dr. Jeremy Owens, associate professor at Florida State University and co-author of a new study in Frontiers in Marine Science .

"This relationship suggests that atmospheric chemical processes convert less biologically reactive iron into more accessible forms ."

Dust storm in the Sahara.

The Earth's Core Can Talk

Owens and colleagues measured bio-reactive iron and total iron in cores drilled from the bottom of the Atlantic Ocean, collected by the International Ocean Discovery Program (IODP) and its predecessors. The IODP aims to improve our understanding of changing climate and 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 to be an important source of dust-bound iron to downwind regions.

The two cores closest to this corridor were collected about 200km and 500km west of northwestern Mauritania, the third core was in the mid-Atlantic, and the fourth core was about 500km east of Florida. The authors studied the top 60 to 200 metres of these cores, which reflect sediments from the last 120,000 years – the time since the last interglacial period.

They measured the overall iron concentration along these cores, as well as the iron isotope concentration using a plasma mass spectrometer. This isotope data matched dust from the Sahara Desert.

They then used a series of chemical reactions to determine the composition of the total iron present in the sediments as iron carbonate, goethite, hematite, magnetite, and pyrite. The iron in these minerals, although not biologically reactive, likely formed from more biologically reactive forms through geochemical processes on the seafloor.

'Instead of focusing on total iron as previous studies have done, we measured the amount of iron that is readily dissolved in the ocean and accessible to marine organisms for metabolism,' Owens said .

"Only a fraction of the total iron in sediments is biologically reactive, but that fraction may change during the transport of iron away from its original source."

Blown away by the wind

The results showed that the proportion of bioreactive iron in the westernmost cores was lower than that in the easternmost cores . This implies that a correspondingly larger proportion of bioreactive iron was lost from the dust and was probably taken up by aquatic organisms during sedimentation. Thus, it never reached the bottom sediments.

'Our results show that during long-distance atmospheric transport, the mineral properties of iron bound to dust that is initially unreactive change to make it more bioreactive. This iron is then taken up by phytoplankton before it can reach the bottom,' said Dr. Timothy Lyons, a professor at the University of California at Riverside and corresponding author of the study.

'We conclude that dust reaching regions such as the Amazon basin and the Bahamas may contain particularly soluble and life-usable iron, due to its long distance from North Africa and therefore longer exposure to atmospheric chemical processes,' Lyons added .

" The transported iron appears to be stimulating biological processes in the same way that iron uptake can impact life in the oceans and on continents. This study is proof of concept confirming that iron-bound dust can have a major impact on life at great distances from its origin."