In the past, we have come up with the idea of fertilizing the oceans with shipwrecked iron to reduce carbon dioxide. The last experiment failed miserably. Meanwhile nature said, “hold my beer.” of Frontiers in Marine Science and Florida State University attended this press release.
Marine life thrives thanks to the Sahara dust blown in from thousands of kilometers away
The more dust-bound iron is blown from the Sahara, the more it becomes available to life through atmospheric reactions..
Iron is an essential micronutrient for life, enabling processes such as respiration, photosynthesis, and DNA synthesis. The availability of iron is often the limiting resource in the ocean today, which means that increasing the flow of iron into the ocean could increase the amount of carbon fixed by phytoplankton, with consequences for global climate.
Iron ends up in oceans and terrestrial ecosystems through rivers, melting glaciers, hydrothermal activity, and especially wind. But not all forms of these chemicals are ‘bioreactive’, that is, available for organisms to take up from their environment.
“Here we show that iron bound in dust from the Sahara blown west across the Atlantic has properties that change with the distance traveled: the greater the distance, the more bioreactive the iron,” said Dr Jeremy Owens, associate professor at Florida State University. and co-authors in a new study on Frontiers in Marine Science.
“This relationship suggests that chemical processes in the atmosphere convert less bioreactive iron into a more accessible form.”
The crux of the matter
Owens and colleagues measured the amount of bioreactive and total iron in drill cores from the bottom of the Atlantic Ocean, collected by the International Ocean Discovery Program (IODP) and earlier versions. The IODP aims to improve our understanding of climate change and ocean conditions, geological processes, and the origin of life. The researchers selected four cores, based on their distance from the so-called Sahara-Sahel Dust Corridor. The latter extends from Mauritania to Chad and is known as an important source of iron for dust for windy areas.
The two cores closest to this corridor were collected about 200km and 500km west of northwestern Mauritania, the third in the mid-Atlantic, and the fourth about 500km east of Florida. The author studied above 60 to 200 meters of this core, describing the deposits over the last 120,000 years – the time since the previous interglacial.
They measured the total concentration of iron along the core, as well as the concentration of iron isotopes with a plasma mass spectrometer. This isotopic data is consistent with dust from the Sahara.
He then used a series of chemical reactions to reveal the total fraction of iron present in the sediment in the form of iron carbonate, goethite, hematite, magnetite, and pyrite. The iron in these minerals, although not bioreactive, may be formed from more bioreactive forms through geochemical processes on the seafloor.
“Instead of focusing on total iron content as we’ve done before, we’re measuring iron that’s easily soluble in the ocean, and accessible to marine organisms for their metabolic pathways,” Owens said.
“Only a small fraction of the total iron in sediments is bioavailable, but that fraction can change during iron transport away from its original source. We aim to explore this relationship.
Blowing in the wind
The results show that the proportion of bioreactive iron is lower in the westernmost core than in the easternmost. This suggests that a greater proportion of bioreactive iron has been lost from the dust and may have been used by organisms in the water column, thus never reaching the bottom sediments.
“Our results show that during long-distance atmospheric transport, the mineral properties of non-bioreactive dust-bound iron change, becoming more bioreactive. This iron is then taken up by phytoplankton, before it can reach the bottom,” said Dr. Timothy Lyons, professor at the University California in Riverside and the last author of the study.
“We conclude that the dust that reaches areas like the Amazon basin and the Bahamas may contain iron that is dissolved and available for life, because of its distance from North Africa, and thus longer for atmospheric chemical processes,” said Lyons.
“The transported iron seems to stimulate biological processes in the same way that iron fertilization affects life in the oceans and on the continents. This study is a proof of concept that confirms that iron-bound dust can have a large impact on life at great distances from its source.
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