Saharan dust supports ocean life far from the desert

Iron is a crucial micronutrient for life, playing an essential role in processes such as respiration, photosynthesis, and DNA synthesis.

In modern oceans, iron availability is often limited, meaning that increasing the supply of iron can boost carbon fixation by phytoplankton, which in turn influences the global climate.

Iron enters oceans and land ecosystems through rivers, melting glaciers, hydrothermal vents, and especially wind. However, not all iron forms are “bioreactive,” meaning not all forms are readily available for organisms to absorb.

Bioreactive iron in Saharan dust 

“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,” said co-author Jeremy Owens, an associate professor at Florida State University. 

“This relationship suggests that chemical processes in the atmosphere convert less bioreactive iron to more accessible forms.”

Studying cores from the Atlantic seafloor

Owens and his team analyzed bioreactive and total iron content from drill cores collected at the bottom of the Atlantic Ocean by the International Ocean Discovery Program (IODP) and its predecessors. 

The mission of this program is to deepen our understanding of climate change, oceanic conditions, geological processes, and the origin of life. 

The researchers selected four cores, varying in their proximity to the Sahara-Sahel Dust Corridor, which stretches from Mauritania to Chad and is a significant source of dust-bound iron.

The two closest cores were located approximately 200 km and 500 km west of northwestern Mauritania. The third was from the mid-Atlantic, while the fourth was around 500 km east of Florida. 

Iron levels over 120,000 years

The experts studied the upper 60 to 200 meters of these cores, which represent deposits spanning the last 120,000 years, covering the period since the previous interglacial.

Using a plasma-mass spectrometer, the team measured total iron concentrations and iron isotope levels in the cores. These isotope readings matched the dust originating from the Sahara.

Focus of the research

Additionally, the researchers used chemical reactions to identify iron present in the form of various minerals, such as iron carbonate, goethite, hematite, magnetite, and pyrite.

While these minerals are not bioreactive, they likely formed from more bioreactive iron through geochemical processes on the seafloor.

“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,” said Owens.

“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.”

Saharan dust and ocean life

The study’s results revealed that the proportion of bioreactive iron was lower in the westernmost cores compared to the easternmost ones. 

This indicated that a significant amount of bioreactive iron had been lost during transport, presumably consumed by organisms in the water before reaching the seafloor.

“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 senior author Timothy Lyons, a professor at the University of California at Riverside.

Dust-bound iron supports ocean life

According to Professor Lyons, the team concluded that Saharan dust which reaches regions like the Amazonian basin and the Bahamas may contain iron that is particularly soluble and available to ocean life

This is due to the great distance from North Africa, and thus a longer exposure to atmospheric chemical processes.

“The transported iron seems to be stimulating biological processes much in the same way that iron fertilization can impact life in the oceans and on continents,” said Professor Lyons.

“This study is a proof of concept confirming that dust-bound iron can have a major impact on life at vast distances from its source.”

The study is published in the journal Frontiers in Marine Science.

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