Melting glaciers release toxic chemical cocktail

Decades after most countries stopped spraying DDT, frozen stores of the insecticide are now trickling out of melting Antarctic glaciers. The change means Adélie penguins have recently been exposed to the chemical, according to a new study.

The trace levels found will not harm the birds, but the presence of the chemical could be an indication that other frozen pollutants will be released because of climate change, says Heidi Geisz, a marine biologist at Virginia Institute of Marine Science in Gloucester in the US. She led a team that sampled DDT levels in the penguins.

She worries that glaciers could release an alphabet soup of chemical pollutants into the ocean, including PCBs and PBDEs – industrial chemicals that have been linked to health problems in humans.

"DDT is not the only chemical that these birds are ingesting and it is certainly not the worst," Geisz says.

Chemists first synthesised DDT (dichloro-diphenyl-trichloroethane) in 1874, but the chemical wasn't used an insecticide until the 1940s. DDT spraying slashed malaria rates in many countries, but the chemical's environmental toll was starting to cause concern.

Rachel Carson's 1962 book, Silent Spring, brought these concerns to the general public, and described, amongst other things, how birds of prey exposed to high levels of DDT lay thin, easily cracked eggs.

In 1972, the US banned the pesticide, and the UK followed suit in 1984. Some countries still use DDT to fight mosquito-borne diseases such as malaria and dengue, but worldwide usage has plummeted – from 40,000 tonnes per year in 1980 to 1,000 tonnes per year now.

DDT latches onto small airborne particles then migrates toward the poles. Geisz, who has worked in Antarctica since 1999, sought to gauge long-term changes in pollutants found in the continent's seabirds.

A 1964 survey found modest amounts of the pesticide in Adélie penguins, and Geisz's team expected to see even less four decades later. Instead, her team found DDT levels unchanged in birds that live near the continent's western peninsula.

As DDT crawls up the food chain, from plankton to krill to penguins, it breaks down into a sister molecule called DDE. The more DDE in an animal, the longer the chemical has been around, Geisz says. But her team recorded low levels of DDE in the birds, suggesting a fresh source.

Geisz couldn't figure out where the DDT came from until she looked back at glacial records. In the 1950s and 60s, Antarctic glaciers swelled, potentially locking in chemicals like DDT.

However, average winter temperatures on the Antarctic Peninsula have warmed 6 °C the past 30 years, and glaciers now melt faster than they grow. A recent study noted elevated levels of DDT in glacial runoff.

As the continent's western ice sheet melts, the DDT drips back into the ecosystem at a rate of 1 to 4 kg per year, her team estimates.

Derek Muir, a researcher at Environment Canada in Burlington, Ontario, says Arctic glaciers ought to store even more of the pesticide, but Arctic animals seem to be shedding the pesticide.

"The declines in DDT in seals and seabirds in the Canadian Arctic and in polar bears in eastern Greenland suggest it is not having a large impact," he says.

Even so, researchers ought to look more closely for evidence that melting glaciers are pumping chemicals like DDT into the Arctic, Muir says.

To make that case stronger for Antarctica, Geisz plans to track the flow of other pollutants from glaciers to birds.

Journal reference: Environmental Science & Technology (DOI: 10.1021/es702919n)

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