August 20, 2013
Unexpected Ocean Plume Challenges Estimates Of Iron Abundance
April Flowers for redOrbit.com - Your Universe Online
A vast plume of iron and other micronutrients more than 620 miles long has been discovered billowing from hydrothermal vents in the South Atlantic Ocean. Teams of scientists from Woods Hole Oceanographic Institute (WHOI) and the University of Liverpool, UK say their findings, published in an upcoming issue of Nature Geoscience, call into question past estimates of iron abundances and may challenge researchers’ previous assumptions about iron sources in the world’s seas.
The team didn’t set out to find iron plumes when they set sail aboard the RN Knorr in 2007 as part of the Cobalt, Iron and Micro-organisms from the Upwelling zone to the Gyre (or CoFeMUG, pronounced “coffee mug”) expedition. CoFeMUG intended to map chemical composition and microbial life between Brazil and Namibia. The scientists sampled the seawater at frequent intervals and multiple depths along the way, and then stored the samples for in-depth analysis back on land.
The expedition’s route passed over a band of mountains and valleys running along the Atlantic Ocean floor from the Arctic to the Antarctic called the Mid-Atlantic Range. This region is where several of the Earth’s major tectonic plates are slowly spreading apart. Fissures in the Earth’s crust, known as hydrothermal vents, are found along the ridge. These vents have not been studied extensively, however, because slow-spreading ridges are thought to be less active than fast-spreading ones.
In previous studies, very little helium has been found coming from mid-Atlantic vents. Since helium is released from the Earth’s mantle through hydrothermal vents, scientists routinely use it as an indicator of vent activity. Because of this lack of helium, researchers also assumed the mid-Atlantic vents spew very little iron as well.
The team was surprised, therefore, by what their samples revealed when they were studied later in the lab. The seawater was filtered and analyzed, revealing unexpectedly high levels of iron and manganese. Saito worked with Abigail Noble, then a WHOI graduate student, to plot the sites where the iron-rich samples were taken. It was then they realized the samples formed a distinct plume - a cloud of nutrients ranging in depth from 1,500 to 3,500 meters that spanned more than 620 miles of the South Atlantic Ocean.
“We had never seen anything like it,” Saito says. “We were sort of shocked — there’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 iron-to-helium ratio of the plume was 80-fold higher than ratios reported for faster-spreading ridges in the southeastern Pacific Ocean, casting doubts on the assumption slow-spreading ridges are iron-poor. These findings also raise 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.”
That high iron ratio has profound implications. Iron is a critical element for ocean life and is known to spur the growth of phytoplankton in many marine habitats, especially those important in the ocean’s carbon cycle. This, in turn, impacts atmospheric carbon dioxide levels and thus Earth’s climate. Over half the planet’s seafloor ridges are slow-spreading, meaning the team’s discovery indicates there may be far more iron in these locations than previously estimated.
“We need to understand where iron is in the ocean and where it’s coming from to understand the role of iron in the marine carbon cycle with any confidence,” Saito says.
The researchers hope the exact shape and extent of the plume, and how much of its iron and other micronutrients persist and rise to the surface, can be revealed by further studies. Finding the answers to these lingering questions will help researchers truly understand how hydrothermal vents affect the ocean as a whole, according to Saito.