Southern Ocean receives much more iron from anthropogenic sources | Polarjournal
You can’t tell by looking at it, but the Southern Ocean is highly productive thanks to a generally good availability of the micronutrient iron. Photo: Julia Hager

Primary production in the Southern Ocean depends critically on the concentration of the micronutrient iron. Only when sufficient iron is available algal blooms can develop in the summer months, which in turn benefit the zooplankton (krill) and subsequently penguins, seals, whales and seabirds. Until now, scientists have assumed that iron input comes almost exclusively from natural sources. Now, however, Japanese researchers have discovered in a recent study that not only about ten percent of the iron is of human origin, as assumed, but considerably more.

Iron is transported long distances as dust that enters the atmosphere over the continents and thus also reaches the Southern Ocean. But the dust contains iron not only from natural sediments, but also from anthropogenic sources, with the fossil fuel combustion releasing most of the iron into the atmosphere.

The study, led by Professor Hitoshi Matsui and Mingxu Liu of the Graduate School of Environmental Studies at Nagoya University, Japan, in collaboration with scientists at Cornell University and the University of Colorado, found that human-made iron inputs could be up to ten times higher than previously thought. Their study was published in the journal npj Climate and Atmospheric Science.

By combining data from aircraft measurements with an advanced global atmospheric model, the researchers found that human-made iron is a major contributor to fertilization of the Southern Ocean, accounting for up to 60 percent of the total. In earlier studies, the anthropogenic input was thus significantly underestimated at only ten percent.

Diatoms and other phytoplankton organisms require iron for photosynthesis. If there is too little of it, there is less phytoplankton that can bind CO2 from the atmosphere. Photo: Julia Hager

However, according to the model, the supply of iron in the Southern Ocean will decrease significantly in the future because of planned reductions in carbon emissions. This could have unexpected effects on the global climate, as phytoplankton would no longer thrive as well due to the lack of iron and would therefore bind less carbon from the atmosphere.

“Iron is a crucial micronutrient to sustain ocean phytoplankton growth and primary production in the Southern Ocean where it modulates atmospheric CO2 levels,” Dr. Matsui said. “A potential decline in iron availability, with the tightening controls on global fossil fuel emissions in the coming decades, may limit carbon storage in marine ecosystems and actually exacerbate global warming.”

The major goal of the global community in the coming decades is to drastically reduce global fossil fuel emissions and achieve carbon neutrality. However, the latest findings from the study require that future models fully consider the role of anthropogenic sources of iron in the Southern Ocean.

Julia Hager, PolarJournal

Link to the study: Mingxu Liu, Hitoshi Matsui, Douglas S. Hamilton et al. The underappreciated role of anthropogenic sources in atmospheric soluble iron flux to the Southern Ocean. npj Climate and Atmospheric Science, 2022; 5 (1) DOI: 10.1038/s41612-022-00250-w.

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