Burning wetlands unleash sequestered mercury in wake of climate change

Released into the atmosphere most prodigiously with the launching of the industrial age, the toxic element falls back onto Earth, and accumulates particularly in North American wetlands. A Michigan State

University researcher working closely with the U.S. Geological Survey finds wildfires, growing more frequent and intense, are unleashing this sequestered mercury at levels up to 15 times greater than originally calculated.

The report, “Wildfires threaten mercury stocks in northern soils,” appears this week in the online edition of Geophysical Research Letters.

“This study makes the point that while peat lands are typically viewed as very wet and stagnant places, they do burn in continental regions, especially late in the season when water tables are depressed,” said Merritt Turetsky, assistant professor of plant biology and fisheries and wildlife at MSU. “When peat lands burn, they can release a huge amount of mercury that overwhelms regional atmospheric emissions. Our study is new in that it looks to the soil record to tell us what happens when peat soil burns, soil that has been like a sponge for mercury for a long time.”

Normal atmospheric conditions naturally carry the mercury emitted from burning fossil fuel and other industry northward, where it eventually settles on land or water surfaces. The cold, wet soils of the boreal forest region in Alaska and northern Canada have been efficient resting places for mercury.

“When we walk across the surface of a peat land, we are standing on many thousands of years of peat accumulation,” Turetsky said. “This type of wetland is actually doing us a service. Peat lands have been storing mercury from the atmosphere since well before and during the Industrial Revolution, locking it in peat where it’s not causing any biological harm, away from the food web.”

In addition to industrial activity, climate change also appears to be disrupting mercury’s cycle. Increasingly, northern wetlands are drying out. Forest fires are burning more frequently, more intensely, and later in the season, which Turetsky believes will make peat lands more vulnerable to fire. In May, Turetsky co-wrote another Geophysical Research Letters paper that documented recent changes in North American fires and suggested that more frequent summer droughts and severe fire weather have increased burn areas.

“We are suggesting that environmental mercury is just like a thermometer. Levels will rise in the atmosphere with climate change, but due to increasing fire activity in the north and not solely due to warming,” said Jennifer Harden, soil scientist at the U.S. Geological Survey and co-author of the study.

In this month’s paper, Turetsky, with co-authors Harden and James Crock of the U.S. Geological Survey; Hans Friedli and Lawrence Radke of the National Center for Atmospheric Research; and Mike Flannigan and Nicholas Payne of the Canadian Forest Service, measured the amount of mercury stored in soils and vegetation of forests and peat lands, then used historical burn areas and emission models to estimate how much of that mercury is released to the atmosphere at a regional scale during fires.

The group has spent more than five years studying prescribed burns in addition to natural fires to measure the influence of burning on terrestrial mercury storage. They also have sampled smoke plumes to measure atmospheric mercury levels as fires blaze.

Their findings indicated that drier conditions in northern regions will cause soil to relinquish its hold on hundreds of years of mercury accumulation, sending that mercury back into the air at levels considerably higher than previously realized.

“We’re talking about mercury that has been relatively harmless, trapped in peat for hundreds of years, rapidly being spewed back into the air,” Turetsky said. “Some of it will fall back onto soils. Some will fall into lakes and streams where it could become toxic in food chains.

“Our findings show us that climate change is complex and will contribute to the pollution of food chains that are very far away from us, in remote regions of the north.”

The research was funded by the U.S. Geological Survey, the National Center of Atmospheric Research (supported by the National Science Foundation), and the Electric Power Research Institute. Turetsky’s May paper in Geophysical Research Letters was funded by NASA. Turetsky’s work also is supported by the MSU Michigan Agricultural Research Station.