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British scientists have solved an important mystery; how traces of the element mercury with volcanic signatures ends up in polar ice cores far away from any volcanoes.

"It has always been a mystery how trace metals, like mercury, with a volcanic signature find their way into polar ice in regions without nearby evidence of volcanic activity," said Dr David Pyle of Oxford University's Department of Earth Sciences who led the research team with colleague Dr Tamsin Mather. "These traces only appear as a faint 'background signal' in ice cores but up until now it has still been difficult to explain."

The team from Oxford and Cambridge universities tested volcano fumes in Mt Etna, Sicily and Masaya in Nicaragua and found them rich in mercury vapour and also tiny particles around 10-20 nanometres in size, according to an Oxford University press release.

They found the nanoparticles were small enough to be carried around the world and, though unsure of their makeup, believe them to be involved in the formation of clouds, the seeding of the oceans with nutrients and regulation of how much solar energy reaches the Earth.

The researchers were surprised just how much mercury they found in their measurements of the volcano vents. In just one part of the Masaya volcano, they found around seven tonnes of volcanic mercury escaping into the atmosphere.

"That one vent of one volcano can produce 7 tonnes of mercury a year is astounding," said Oxford's Dr Melanie Witt, "that's considerably more than total industrial emissions of mercury from the UK - recorded at about 5.5 tonnes in 2000."

"It confirms our suspicions that volcanoes are an important part of the global mercury cycle: what we need to understand next is where this mercury ends up and what effects it may have on the environment."

The team's findings more than solve a mystery, they also have important climate implications said Dr Rob Martin of the University of Cambridge.

"This is exciting and important since we didn't know that volcanoes were a natural source of particles as small as this," said Dr Martin.

"The existence of these particles is potentially very important for the climate system - they may control how clouds form, and how much solar energy reaches the Earth's surface. What we don't know yet, though, is what these nanoparticles are made of: whether they are tiny droplets of frozen magma, or salts that condense due to cooling of high-temperature volcanic fumes."

The study was funded through research grants and research fellowships from the Royal Society, the Natural Environment Research Council (NERC), and the Leverhulme Trust.

The findings were published in the Journal of Geophysical Research.