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Chemical Atlas Shows Where Seas Are Tainted—And Where They Can Bloom

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Science  07 Mar 2014:
Vol. 343, Issue 6175, pp. 1070
DOI: 10.1126/science.343.6175.1070
Tainted.

Red and yellow areas highlight areas in the Atlantic Ocean that have relatively high traces of lead from decades of pollution.

CREDITS: 3D GRAPHICS BY STEVEN VAN HEUVEN, COURTESY OF HEIN DE BAAR, ROB MIDDAG, ABIGAIL NOBLE, AND CHRISTIAN SCHLOSSER

HONOLULU—About 1000 meters down in a remote expanse of the Atlantic Ocean sits an unusual legacy of humanity's love affair with the automobile. It's a huge mass of seawater infused with minute traces of lead, once widely emitted by cars burning gasoline laced with the toxic metal. Decades ago, the United States and Europe banned leaded gas and many other uses of lead, but the pollutant's fingerprint lingers on—as shown by a remarkably detailed atlas of digital maps unveiled here last week at the 2014 Ocean Sciences Meeting.

The atlas, which includes 3D maps and rotating animations, is the first product of GEOTRACES (geotraces.org), a $300 million international collaboration to document the presence of key trace metals and other substances in the world's oceans. Researchers from dozens of nations gathered the initial release of data on about 30 cruises since 2010, collecting nearly 30,000 water samples at 787 study sites. Then, using painstaking techniques, which involved wearing "moon suits" and working in ultraclean laboratories to prevent contamination, they measured often minute quantities of more than 200 substances, both humanmade and natural.

The roster includes important "micronutrients" such as iron, which can fuel plankton blooms and influence how the ocean responds to climate change, and isotopes of carbon and thorium, which help scientists track the movements of seawater and the amount of dust that falls into the ocean. The breadth and accuracy of the data makes GEOTRACES "a huge improvement over what we were able to do in the past," says ocean chemist Hein de Baar of the Royal Netherlands Institute for Sea Research in Texel, a GEOTRACES participant.

Oceanographers began organizing GEOTRACES more than a decade ago. Existing studies were plagued by limited sampling and contamination of seawater samples from sources such as metal ship hulls and instrument cables. To overcome such problems, GEOTRACES recruited dozens of teams to take samples from numerous depths and developed strict sampling and analytical methods. "Everything has to be precise and squeaky clean," says chemical oceanographer Christian Schlosser of the University of Southampton in the United Kingdom.

The resulting maps are letting researchers "see things that we couldn't see before," says chemical oceanographer Abigail Noble of the Massachusetts Institute of Technology (MIT) in Cambridge.

The lead images, for example, tell a sobering story of past pollution—and continuing contamination. That mass of lead-tainted seawater deep in the central Atlantic started at the surface decades ago, where it collected airborne lead particles, Noble says. As the surface water slowly sank into the deep ocean, it became a time capsule recording "the incredible impact that we have had on the oceans in the past, and how it changes over time."

Although the elevated lead levels stand out as red blotches on the GEOTRACES maps, the concentrations pose little threat to humans or wildlife—at roughly 12 parts per trillion, they are far lower than the 15 parts per billion that the U.S. government considers to be of concern in drinking water. "You probably aren't going to see stupid fish or whales swimming around," says MIT ocean scientist Edward Boyle, alluding to the brain damage that can be caused by lead exposure. And lead levels in much of the Atlantic have dropped dramatically over the past few decades, mostly thanks to the lead phaseout in the United States and Europe.

Still, the maps show that lead contamination continues in some parts of the world. Off the southern tip of Africa, surface waters with relatively high traces of lead are flowing into the Atlantic from the Indian Ocean. That's probably due to the continuing use of leaded gasoline in parts of Africa and Asia. Another hot spot is where the Mediterranean Sea empties into the eastern Atlantic. The lead concentrations there "are some of the highest we saw anywhere" in the Atlantic, says chemical oceanographer Rob Middag of the University of Otago, Dunedin, in New Zealand. That may be because the Mediterranean is a relatively enclosed body of water with heavily settled shores and has collected pollution for centuries.

Other maps highlight the distribution of micronutrients such as zinc and cadmium, which can fertilize plant growth and will help determine how much planet-warming carbon dioxide the sea can soak up. And a few clearly reveal features such as the plumes of trace metals pumped into the deep sea by hydrothermal vents, or how dust blowing off Africa's deserts amplify surface levels of iron in nearby seas.

This detailed picture of ocean chemistry also raises new questions. Charts of isotopic data, for instance, suggest that researchers are underestimating the amount of dust that falls into certain parts of the ocean, potentially undermining computer models that use isotope data to help reconstruct ocean circulation patterns in the past. GEOTRACES is forcing researchers to find "lots of ways to politely say the old data was wrong," De Baar says. "Many old papers are now going to be primarily of historic interest." And this is just the beginning. As new cruises are completed in the coming years, particularly across the Pacific and Southern oceans, the GEOTRACES data trove will continue to grow.

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