So Saito and his colleagues were surprised by what their samples revealed when later studied in the lab. Once filtered and analyzed, some of the seawater showed unexpectedly high levels of iron and manganese. When Abigail Noble, then a WHOI graduate student, and Saito plotted the sites where the iron-rich samples were taken, they realized the samples formed a distinct plumea cloud of nutrients ranging in depth from 1,500 to 3,500 meters that spanned more than 1,000 km of the South Atlantic Ocean.
"We had never seen anything like it," Saito says. "We were sort of shockedthere's this huge bull's-eye right in the middle of the South Atlantic Ocean. We didn't quite know what to do with it, because it went contrary to a lot of our expectations."
The plume's ratio of iron to helium was 80-fold higher than ratios reported for faster-spreading ridges in the southeastern Pacific Ocean.
The serendipitous discovery casts doubt on the assumption that slow-spreading ridges are iron-poor, and it raises questions about the use of helium as an indicator for iron flux in hydrothermal vents, Saito says.
"We've assumed that low helium means low iron, and our study finds that that's not true," Saito says. "There's actually quite a lot of iron coming out of these slow-spreading regions in the Atlantic, where people thought there would be little to none."
And that has profound implications, because iron is a critical element for ocean life. Iron is known to spur the growth of phytoplankton in many marine habitats, especially those important in the ocean's carbon cycle, which, in turn, impacts atmospheric carbon dioxide levels and Earth's climate. Because more than half the world's seafloor ridges are slow-spreading, the team's discovery suggests there may be far more iron from the
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