Saharan dust supports ocean life thousands of kilometres away

Scientists are unravelling the role of dust from the Sahara in supporting life as far away as the Amazon Basin and the Bahamas.

It’s not the dust but the iron it contains which helps life to thrive, and as the dust travels through the atmosphere, the iron components change.

The further dust-bound iron travels from the Sahara, the more bioavailable the iron is to marine life. This phenomenon, caused by chemical reactions in the atmosphere, has implications for the ocean ecosystem and carbon cycle predictions.

North Africa’s Sahara Desert is the largest hot desert on Earth and the largest source of atmospheric mineral dust. This dust can be carried by winds vast distances with Saharan dust regularly crossing the Atlantic Ocean.

Mineral dust supplies important nutrients to the ecosystems where it lands. In particular, iron is a micronutrient essential to life as it enables key functions such as respiration, photosynthesis and DNA synthesis.

But not all forms of iron can be taken up by living things – some forms of iron are more “bioavailable” than others.

To assess how the distance Saharan dust travelled impacted iron bioavailability, the US-based research team studied drill cores from four locations on the floor of the Atlantic Ocean. 

“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,” says Jeremy Owens, an associate professor at Florida State University and a co-author on the study.

Owen and colleagues measured concentrations of iron isotopes inside the drill cores with a plasma-mass spectrometer. The drill cores collected were deep enough to reflect 120,000 years of Saharan dust deposition.

Their results showed lower concentrations of iron in cores further way from the Sahara, implying that more iron was taken up by marine organisms at these locations.

“We conclude that dust that reaches regions like the Amazonian basin and the Bahamas may contain iron that is particularly soluble and available to life, thanks to the great distance from North Africa, and thus a longer exposure to atmospheric chemical processes,” says Timothy Lyons, a professor at the University of California at Riverside and the study’s senior author.

In ocean ecosystems, iron is often a limiting resource, which means that additional iron leads to population growth of organisms like phytoplankton. Understanding the population dynamics of phytoplankton will improve predictions of the marine ecosystems that depend on phytoplankton, sometimes referred to as “the grass of the sea”.

Phytoplankton also play an important role in carbon cycling. It forms an integral part of the “biological pump” which sequesters atmospheric CO₂ at the bottom of the ocean. Phytoplankton contribute to this pump by fixing atmospheric carbon into organic matter during life and then sinking to the ocean floor upon death.

The authors of the study conclude that improved understanding of dust-bound iron bioavailability will improve predictions of marine ecosystem productivity and atmospheric carbon modelling.

The research is published in Frontiers in Marine Science.