Iron is an essential micronutrient crucial for sustaining life, powering processes like respiration, photosynthesis, and DNA synthesis. In today’s oceans, the availability of iron often serves as a limiting factor. By increasing the influx of iron into these oceans, we have the potential to boost the amount of carbon fixed by phytoplankton, with far-reaching implications for the global climate.
The pathways through which iron finds its way into oceans and terrestrial ecosystems are diverse, including rivers, melting glaciers, hydrothermal activity, and wind. However, not all forms of iron are bioreactive, meaning they may not be readily accessible for uptake by organisms from their environment.
“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 Dr. Jeremy Owens, an associate professor at Florida State University and a co-author on a new study in Frontiers in Marine Science.
“This relationship suggests that chemical processes in the atmosphere convert less bioreactive iron to more accessible forms.”
Owens and colleagues conducted a groundbreaking study analyzing the levels of bioreactive and total iron in drill cores from the depths of the Atlantic Ocean collected by the prestigious International Ocean Discovery Program (IODP). The IODP has a noble mission of enhancing our comprehension of shifting climate patterns, oceanic phenomena, geological processes, and the genesis of life itself.
The team handpicked four cores, strategically chosen based on their proximity to the renowned Sahara-Sahel Dust Corridor, stretching from Mauritania to Chad, a crucial source of dust-bound iron for downwind regions.
The two cores closest to this corridor were extracted approximately 200km and 500km west of northwestern Mauritania, with a third core retrieved from the mid-Atlantic, and the fourth obtained approximately 500km to the east of Florida. The researchers delved into the upper 60 to 200 meters of these cores, offering insights into deposits spanning the last 120,000 years – a timeline reaching back to the previous interglacial period.
They meticulously analyzed the total iron concentrations in the sediment cores and examined iron isotopes using a plasma-mass spectrometer. The findings pointed to a compelling connection with dust originating from the Sahara.
Furthermore, they employed a series of chemical reactions to unveil the different forms of iron present in the sediments, including iron carbonate, goethite, hematite, magnetite, and pyrite. These minerals, although not bioreactive, are believed to have originated from more bioreactive forms through fascinating 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.”
The findings revealed a striking contrast between the westernmost and easternmost cores, with a noticeably lower proportion of bioreactive iron in the former. This suggests a compelling narrative of bioreactive iron being lost from the dust and actively utilized by organisms in the water column, thus never reaching the sediment at the bottom.
“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 Dr Timothy Lyons, a professor at the University of California at Riverside and the study’s final author.
“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,” said Lyons.
“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.”
Journal reference:
- Bridget Kenlee, Jeremy D. Owens, Robert Raiswell, Simon W. Poulton, Silke Severmann, Peter M. Sadler, Timothy W. Lyons. Long-range transport of dust enhances oceanic iron bioavailability. Frontiers in Marine Science, 2024; DOI: 10.3389/fmars.2024.1428621