Melting Antarctic ice is leaking stored mercury into the Southern Ocean

Antarctica is a long way from coal plants and mines, which is why it has spent two centuries as a cold trap for mercury. A new paper in the Proceedings of the National Academy of Sciences says that trap is now leaking.

Researchers led from Peking University analysed 16 sediment cores from the continental shelf off the Antarctic Peninsula, the fastest-warming part of the continent. Nearly all the cores showed a sharp rise in mercury since industrialisation. On average, accumulation is up 160 percent, with the climb steepening from the 1950s. About 56 percent of the mercury in those cores came from the atmosphere. The rest is tied to glacier melt, weathering and erosion.

Model runs of the global mercury budget point to a 550 percent increase in mercury released from Antarctic glaciers since industrialisation, Eos reported on 14 August. Co-author Maodian Liu wrote that climate change acts as an amplifier of mercury cycling and that the two problems are not independent. The paper's authors, including Chengzhen Zhou, put the rise in ice-melt-driven terrestrial release at 550 percent and air-sea exchange at 350 percent. Export of mercury from the peninsula into the open ocean is up about 400 percent.

Once mercury sits in cold sediment it can become methylmercury, the form that moves up the food web from plankton to krill to fish to seals and birds. The authors note that mercury in species such as Antarctic toothfish, marlin and sharks has already been recorded above earlier levels and, in some cases, above food-safety limits. Some of that fish is sold.

Charles Driscoll at Syracuse University, who was not on the paper, told Eos the work fits a wider pattern: thawing Arctic permafrost is also giving up stored mercury, and heavier storms in the northeastern United States are moving metals the same way. Liu's warning is blunt. Even if new emissions from coal and mining fall, mercury already locked in Antarctic ice will keep coming out as the ice melts. Burdens will not drop in step with cuts.

The open question is scale. The cores are from the peninsula, not the whole continent. Liu wants to know whether the same trend holds elsewhere around Antarctica. Driscoll wants a better read on how much of this mercury actually becomes methylmercury in very cold water, a conversion that usually runs faster when it is warmer. For policy, the paper is another argument that mercury rules and climate policy have to be read together.