Web edition: June 12, 2012
Print edition: July 14, 2012; Vol.182 #1 (p. 12)
SELFOSS, Iceland — Geoscientists have exposed another assault on Earth’s protective ozone layer — not by manufactured chemicals, but by gas ejected in the blasts of huge volcanic eruptions.
A new study shows that volcanic rocks in Nicaragua contain bromine, an element known for speeding ozone’s destruction in the upper atmosphere. When magma erupted to form those rocks, scientists say, it also released huge amounts of bromine into the air — enough to destroy large parts of the ozone layer for several years.
“We have to be aware of this,” says Kirstin Krüger, a meteorologist at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany. “Large-scale tropical eruptions have the potential to deplete ozone on a big scale.”
Krüger presented the work, led by GEOMAR volcanologist Steffen Kutterolf, on June 12 at an American Geophysical Union conference on volcanism and the atmosphere.
The scientists studied rocks formed during 13 big Central American eruptions over the past 70,000 years. Volcanoes at tropical latitudes are good at injecting the stuff they erupt into the stratosphere, some 16 kilometers up. When elements such as chlorine and bromine reach that high, they help trigger a series of reactions in which ozone’s three oxygen atoms break apart and recombine with other atoms.
Researchers have previously measured chlorine coming from volcanoes, such as 1991’s Mount Pinatubo eruption in the Philippines, and watched it destroy ozone overhead. But the new work is the first to pin down bromine in such detail. The element is 60 times as efficient as chlorine at destroying ozone, Krüger says.
Kutterolf and his colleagues collected thousands of rock samples both on and offshore, then analyzed bromine concentrations in tiny glass bubbles that formed within the rocks when the magma erupted out of the volcano. The scientists found enough bromine in the bubbles to suggest that 4,000 to 600,000 tons of bromine came out per eruption.
Enough bromine would have made it to the stratosphere to create at least double the ozone-destroying potential seen at the highest modern-day levels, Krüger says. It would have taken three to six years for the chemicals to clear out so that ozone could begin to recover.
It’s still not clear what makes a particular eruption rich in bromine, or whether the bromine would have destroyed ozone locally or globally once aloft.
About three-quarters of atmospheric bromine comes from human-made sources. One-quarter is natural, produced by the sea or by volcanoes. “Wherever it comes from, it will destroy the ozone,” Krüger says. Most human-made ozone-depleting chemicals were phased out by the 1987 Montreal Protocol.
The new study is an important step in better quantifying bromine from present-day eruptions, says Tamsin Mather, a volcanologist at the University of Oxford in England. “If we can apply this to other volcanoes,” she says, “we can really get a handle on how much bromine is coming out.”
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