How Saharan dust provides a boost for oceanic life

Iron contained in Saharan dust, blown across the Atlantic, is beneficial to ocean life and becomes more beneficial, the further it travels, a new study has found.

Iron – in its soluble, bioreactive form – is an important micronutrient in the sea, playing a role in enabling photosynthesis in phytoplankton. This is significant, as the process of photosynthesis sees CO2 taken out of the atmosphere and converted into sugars. 

In fact, phytoplankton are responsible for 40% of the world’s carbon capture and storage.

The caveat mentioned above is that to be useful to sea life, iron needs to be in its bioreactive form, so it can be taken up by phytoplankton.

The new research has found that, as the iron contained in Saharan dust is carried through the air, it contains becomes increasingly more bioreactive.

Dr Jeremy Owens, an associate professor at Florida State University and a co-author on the study said: ‘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.

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

Drill cores from four sites along what is known as the Sahara-Sahel Dust Corridor were examined. The researchers focussed on the upper 60m to 200m of these cores, which contain deposits from the last 120,000 years.

Owens said: ‘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.

‘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.’

It was found that the proportion of bioreactive iron furthest away from Africa was smaller than from the cores nearest to it. Perhaps counter-intuitively, this is good news because it suggests that bioreactive iron had never reached the bottom of the ocean, having been taken up by phytoplankton before that could happen.

Dr Timothy Lyons, a professor at the University of California at Riverside and the study’s final author said: ‘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.

‘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.

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

The full research can be read here.