Mercury, that heavy metal with the oh-so-alluring liquid shimmer, is extremely toxic to humans and the environment.
The persistent environmental pollutant is globally ubiquitous—mercury travels well through the atmosphere, eventually making its way into plants, glacial ice, and soils. As glaciers melt, ice-trapped mercury is released into the atmosphere as well as downstream ecosystems.
In a new study, scientists examined how glacial ecosystems responded to these increased mercury doses. They found a surprising result: The area of vegetation in front of a melting glacier was an effective storage area for escaping glacial mercury.
Researchers noted that although climate change led to the release of glacial mercury, warmer temperatures also encouraged biomass to thrive.
Sources and Sinks
The global cycling of mercury is filled with sources and sinks. Mercury pollution sources are easy to pinpoint, said Che-Jen Lin, an atmospheric scientist at Lamar University in Beaumont, Texas: Half of the mercury released to the atmosphere comes from burning coal and small-scale gold mining.
“It’s important to understand that mercury is a naturally occurring contaminant,” said Kevin Schaefer, a geochemist at the National Snow and Ice Data Center. “But in the last 150 years, we’ve been perturbing this natural balance by burning coal.”
Identifying mercury sinks has been more of a challenge. In a new paper in the Proceedings of the National Academy of Sciences of the United States of America, Lin and his colleagues tackle what role vegetation plays in the global mercury cycle.
Like any other nutrient, mercury is taken up by the plant and incorporated into its biomass. “What it does is it binds to certain locations in the organic material and then it becomes attached to the plants,” said Schaefer, who was not involved in the study. When the plant dies, the mercury gets recycled in the system, he explained. “It releases the mercury just like it releases nutrients like nitrogen.”
Lin said there were theories about how—and how much—mercury could be absorbed and stored in vegetation, but there wasn’t a reliable way to measure where the mercury was coming from. “Starting about 10–15 years ago, measurements of mercury stable isotopes became much more sensitive and a lot more advanced,” he explained. “That’s when we could start to compare the minute differences in isotopic composition of mercury to environmental samples.”
Lin’s team picked a site in the Himalaya-Tibetan Plateau to test how much atmospheric mercury was being absorbed by the biomass in front of a retreating glacier. “We saw the glaciers recessing, and we clearly saw that the density of the vegetation presented a gradient,” said Lin. The situation was perfect for testing mercury accumulation over time.
Using isotope analysis, the team could tease out subtle details of the mercury, including which vegetation incorporated the majority of its mercury from the atmosphere as opposed to geological sources.
They found that in three succession sites in front of the glacier, vegetation had accumulated 400 to 600 metric tons of mercury since about 1850. The team estimated that by the year 2100, vegetation will take up about 3 times more atmospheric mercury compared to mercury released by the melting ice (about 300 metric tons versus 95 metric tons).
“What’s interesting about this particular paper is it really highlights the role of the land biosphere plants in modulating and controlling the global mercury budget,” said Schaefer. He added that although this concept has been around for a while, the researchers’ data clearly show “the succession of plants from retreating glaciers can more than compensate the release of mercury from the glaciers itself.”
Schaefer added that the team finding evidence of plants modulating mercury cycle “totally fits exactly what we’re seeing in the permafrost.”
Climate and Mercury
In the past, scientists thought that melting ice meant a lot of substances would be released back into the atmosphere and remain unabsorbed, said Lin. But this study showed otherwise.
“We anticipate with global warming, temperature will rise, and more glaciers will melt,” said Lin. “That means that the vegetation density is going to become greater, and because of that, there will be more mercury sequestered in the vegetative ecosystem compared to what’s released from the melting ice.”
Lin cautioned that scientists have to be open to what the effects of climate change might bring. “We do not fully understand how that global changes will affect our understanding of global cycling of trace compounds such as mercury,” he said, adding that there can be many different processes going on. “If we do not measure [the changes], then we do not know what is really going on.”
Understanding how mercury cycles through Earth’s systems has rippling effects, not the least of which is risk to food and water supplies, said Schaefer.
“It’s pretty clear from like this paper that the carbon cycle is modulating the mercury cycle, and the biomass is acting as a huge reservoir of mercury,” said Schaefer. “We have to account for plants—plant growth and death in the global budget of sources and sinks of mercury.”
—Sarah Derouin (@Sarah_Derouin), Science Writer