Melting ice in Antarctica is releasing dangerous amounts of mercury that have been trapped for hundreds of years, according to researchers from Peking University in Beijing.
The scientists found the thaw is triggering a "sharp increase" in the amount of the toxic metal building up in sediments around the Antarctic coastline, with "profound" consequences for the ecology of the polar environment, they said.
In a paper published in PNAS Environmental Sciences, the team studied sediment samples from 16 locations near the Antarctic Peninsula, the continent's northwestern arm. They found mercury is accumulating on the continental shelf there at a rate of 93 micrograms per square metre per year, roughly double the rate seen on other continental shelves.
That rate has risen 160 per cent since the start of industrialisation, the researchers said, pointing to human-caused climate change as a driver.
Mercury is a heavy metal naturally present at low levels throughout the ecosystem. Volcanoes and tectonic activity release mercury vapours into the atmosphere, which are gradually absorbed by phytoplankton, tiny ocean plants. Some of these plants are eaten by fish, passing mercury up the food chain, while others die and carry their trapped mercury to the seabed. Without human interference, this natural process generally stays within safe limits.
Human activities such as burning coal and processing metals also release mercury into the atmosphere in large quantities, the researchers said, and this has pushed global mercury levels up to the point where it poses a health risk to animals and humans.
But the rise in atmospheric mercury alone cannot explain the accumulation rates found around the Antarctic Peninsula, the researchers said. Instead, they found the rapid melting of the continent's ice shelf is responsible.
Two ways the ice releases mercury
As the ice shelf retreats, two processes are at work, according to the researchers. Melting ice exposes more open water to sunlight and air, giving phytoplankton more room to grow and draw mercury from the atmosphere. At the same time, the researchers said warming has "further promoted this enrichment by accelerating terrestrial input through ice melt, erosion and weathering."
Antarctica's ice sheets have slowly absorbed mercury from the atmosphere over thousands of years, keeping it safely trapped. Now the melting ice is releasing that stored mercury into the ocean.
Since the start of industrialisation, the amount of mercury released in meltwater has increased by 550 per cent, the researchers found, while the amount absorbed from the atmosphere has risen by 350 per cent. These effects are especially pronounced around the Antarctic Peninsula because of how fast its ice is melting, the team said. A separate figure cited alongside the study put the overall rise in mercury accumulation around Antarctica since industrialisation at 190 per cent.
Despite making up less than 5 per cent of Antarctica's total area, the Peninsula region accounts for 60 per cent of the total mass lost from ice shelves over the last 30 years, according to the researchers.
Nearly 90 per cent of glaciers in the region are now in retreat at a rate well above the limits of natural variation, the researchers said.
The warning comes as separate studies show mercury is already building up in the Antarctic ecosystem. Research conducted last year found that 62 per cent of Antarctic seabird species had mercury levels in their bodies above the "no adverse health effect" threshold. A quarter of all species studied were found to be at moderate to high risk of mercury toxicity.
