People and animals living in the Arctic tend to have high amounts of mercury in their tissues, and now researchers have uncovered a growing source of the most toxic variety of the metal. The team reports that Arctic sea ice holds large amounts of methylmercury that may enter marine ecosystems at increasing rates as the ice melts due to climate change

The study is the first to measure methylmercury in so-called multiyear Arctic sea ice, a long-lasting form of the ice that sits atop the open ocean waters. Lars-Eric Heimbürger, a chemical oceanographer at the University of Bremen, in Germany, who was not involved in the study, says the work is important because it suggests that as more of this ice melts due to rising temperatures, more methylmercury will enter Arctic food webs.

Mercury ends up in the Arctic as emissions from sources such as coal power plants and wildfires travel through the atmosphere. Once in the Arctic, photochemical reactions oxidize the metal and deposit it onto the ice. Scientists think bacteria species in the ice convert this inorganic mercury into methylmercury, which marine microbes like plankton easily take up. Eventually, the methylmercury makes its way up the food chain, posing a risk not just to wildlife but also to indigenous peoples who eat animals at the top of the chain, such as seals. In fact, some studies have concluded that Inuit women have among the highest blood concentrations of total mercury in the world.

But researchers know little about how much mercury cycles through the Arctic and how it gets methylated there. Research in the Arctic is a challenge because it’s expensive to travel there and difficult to sample ice properly. Sarah A. Beattie of the University of Manitoba and her colleagues wanted to study multiyear sea ice as one possible input of methylmercury into the Arctic.

The team made two separate journeys to the Arctic in 2011 and 2013. While there, Beattie and coworkers had to brave more than just the elements. They also suffered heartbreak. Three people, including Beattie’s Manitoba colleague Klaus Hochheim, died in a helicopter accident in September 2013. The tragedy served as motivation for the scientists, Beattie says. “We really didn’t want this sacrifice to be in vain.”

Traveling on an icebreaker, the team went to two Canadian Arctic sites—one in the Beaufort Sea, another in the McClure Strait. There, the researchers extracted multi-meter-long ice cores. In a cleanroom lab aboard the ship, researchers analyzed the samples’ total mercury content with cold vapor atomic fluorescence spectroscopy (CVAFS). Back at the university, they paired gas chromatography with CVAFS to assess the methylmercury component.

The ice cores had mercury concentrations ranging from 0.65 to 60.8 pM. Concentrations of the methylated form peaked at 2.64 pM. Those values may seem small, Beattie explains, but methylmercury is highly potent, and its concentrations can increase by a factor of a million or more as it travels up the food chain. Based on these findings, Beattie’s team estimated that, at the current rate of ice melt, 42 kg of methylmercury enters the Arctic ecosystem each year.

To better understand how climate change will affect this source of methylmercury, Beattie and Bremen’s Heimbürger say researchers should collect ice at non-Canadian Arctic sites. That data would help determine mercury distribution across the Arctic.