Dust in the Waters
Each spring, powerful dust storms in the deserts of Mongolia and northern China send thick clouds of particles into the atmosphere. Eastward winds sweep these particles as far as the Pacific, where dust ultimately settles in the open ocean. This desert dust contains, among other minerals, iron — an essential nutrient for hundreds of species of phytoplankton that make up the ocean’s food base.
Now scientists at MIT, Columbia University, and Florida State University have determined that once iron is deposited in the ocean, it has a very short residence time, spending only six months in surface waters before sinking into the deep ocean. This high turnover of iron signals that large seasonal changes in desert dust may have dramatic effects on surface phytoplankton that depend on iron.
“If there are changes to the sizes of deserts in Asia, or changes in the way people are using land, there could be a larger source of dust to the ocean,” says Chris Hayes, a postdoc in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS). “It’s difficult to predict how the whole ecosystem will change, but because the residence time [of iron] is very short, year-to-year changes in dust will definitely have an impact on phytoplankton.”
The team’s results are published in the journal Geochemica et Cosmochimica Acta. Co-authors include Ed Boyle, a professor of ocean geochemistry at MIT; David McGee, the Kerr-McGee Career Development Assistant Professor in EAPS; and former postdoc Jessica Fitzsimmons.
Dust to dust
Certain species of phytoplankton, such as cyanobacteria, require iron as a main nutrient to fuel nitrogen fixation and other growth-related processes. Hayes estimates that up to 40 percent of the ocean contains phytoplankton species whose growth is limited by the amount of iron available.
As desert dust is one of the only sources of oceanic iron, Hayes wanted to see to what extent changing levels of dust would have an effect on iron concentrations in seawater: Does iron stick around in surface waters for long periods, thereby making phytoplankton less sensitive to changes in incoming dust? Or does the mineral make a short appearance before sinking to inaccessible depths, making phytoplankton depend much more on seasonal dust?
Continue reading at MIT News.
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