Thawing permafrost soils release mercury | Polarjournal
The Arctic permafrost soils, here in the Yukon region of Alaska, contain twice as much mercury as all other soils, the oceans and the atmosphere combined. If it enters rivers as a result of thawing processes, the pollutant load on fish and other animals would increase. Photo: Trevor Porter

It has long been known that the thawing permafrost soils release huge quantities of the greenhouse gases carbon dioxide and methane into the atmosphere, further intensifying climate change. So far, however, little attention has been paid to the other elements that are still trapped in the permafrost soil and are released as the thawing process continues. In a new study published in the journal Nature Communications, scientists focused on the heavy metal mercury in the region around the Yukon River in Alaska.

According to a 2018 study, Arctic permafrost soils naturally contain about twice as much mercury as all other soils, the oceans and the atmosphere combined. Mercury in soil originates mainly from atmospheric deposition and is bound to organic plant material. In unfrozen soil, microorganisms decompose the organic material over time and mercury is released. In permafrost soils, on the other hand, the microbial decomposition processes are literally frozen and thus the mercury is also trapped. Due to the increasing warming and further thawing of the permafrost soils, the biological processes are reactivated and mercury is released into the environment. How much, where and when, however, remained unclear for a long time. The new study, which involved scientists from the National Snow and Ice Data Center of the University of Colorado Boulder, the University of South Florida, the Los Alamos National Laboratory, the U.S. Geological Survey and Harvard University, is now using computer modeling to provide the first more precise findings.

Schematic illustration of the mercury cycle with the main deposits in white (in gigagram mercury) and rivers (in gigagram mercury per year) in black. Arctic permafrost soils contain an estimated 1,656 gigagrams of mercury (active layer + permanently frozen soil). graph: Schuster et al. 2018, adapted and updated by Amos et al. 2013

The researchers focused their calculations on two scenarios of the Intergovernmental Panel on Climate Change (IPCC) regarding future greenhouse gas emissions: RCP4.5 (average increase in greenhouse gas concentration; emissions rise until 2040, then decline) and RCP8.5 (very strong increase in greenhouse gas concentration; emissions rise until the end of the century).

“Thaw-released mercury will work its way from the land to the river and ultimately, into the oceans, and thaw-released mercury in gaseous form will encircle the world.”

Kevin Schaefer, National Snow and Ice Data Center, University of Colorado Boulder
Mercury deposits in the permafrost zones of the northern hemisphere in four soil layers 0-30cm, 0-100cm, 0-300cm and permafrost in milligrams of mercury per square meter. Graphic: Schuster et al. 2018

If carbon dioxide emissions continue at current rates (RCP8.5) – which is what it looks like at present – the amount of mercury escaping from the thawing permafrost soil by 2050 will be so high that within a few decades the fish in the Yukon River will be heavily contaminated with the highly toxic heavy metal that they will no longer be safe for human consumption with regard to food safety guidelines.
Scientists expect mercury concentrations in the Yukon to double by the end of the century if carbon dioxide emissions continue at current rates.

Total mercury loss (in milligrams mercury per square meter; elemental Hg and Hg cations) considering the scenario RCP8.5, summed from 1901 to 2300. The extent of the Yukon River Basin is outlined in red. Graphic: Schaefer et al. 2020

However, if the global community succeeds in reducing emissions (RCP4.5), mercury concentrations will increase by only 14 percent by the end of the century. As a result, concentrations in water and fish tissues will remain within food safety guidelines.

“A lot will depend on what we do in terms of response to climate change,” said Kevin Schaefer of the National Snow and Ice Data Center in Colorado, the study’s lead author.

Total mercury loss under the RCP4.5 scenario is much lower, summed from 1901 to 2300. Graphic: Schaefer et al. 2020

According to Schaefer, the release of mercury from permafrost soils not only affects indigenous communities in Alaska and Canada, who depend on fish in the Yukon for income, food and culture. In fact, the more than 3,000 kilometer long river is “a canary-in-the-coal mine kind of thing, an indicator of what might happen over the whole Arctic,” Schaefer said. “Thaw-released mercury will work its way from the land to the river and ultimately, into the oceans, and thaw-released mercury in gaseous form will encircle the world. What happens in the Yukon is going to affect the entire globe, not just the people who live on or around the Yukon River.”

Julia Hager, PolarJournal

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