Northern tree rings hold centuries-old air pollution records

While the North has a long history of mining, the industry’s emissions weren’t always formally monitored. A new study instead uses the record of pollution stored for hundreds of years in tree rings. 

Researchers from the University of Toronto Mississauga examined core samples from 15 trees to better understand annual mercury pollution in Bear Creek, Yukon. The former gold mining town outside Dawson City operated from 1905 to 1966.

Scientists found nearby trees held a record of 151 years of mercury levels. Changes in those levels precisely matched mining activities at the site. The findings were published in the scientific journal Environmental Pollution. 

“It seems the history of gold mining at Bear Creek has a really strong influence on local atmospheric mercury concentrations,” said paleo-climatologist Trevor Porter, an assistant professor at the university. 

“This was very clear when we looked at the mercury profiles in 15 different trees. They’re all really telling us a similar story.”

Mercury was used in placer mining – the mining of stream beds for deposits – as it binds to gold and helps separate the precious metal from sediment. Deposits were then heated, releasing mercury into the atmosphere where trees would absorb it.

Porter said mercury levels increased rapidly in tree rings from 1923 to 1930, when several major mining operations were consolidated at Bear Creek. The highest mercury concentrations were found in the 1930s, he said, during maximum gold production at the site. Finally, the biggest decline in mercury levels came in 1966, the same year the Bear Creek camp closed.

Researchers compared the Bear Creek samples to those taken from trees in the Northwest Territories’ Mackenzie Delta.

“The Mackenzie Delta represents one of the most northern treeline sites you can get to in Canada,” Porter said. “This was a place where we were able to look at how mercury changes naturally in the atmosphere.”

That comparison showed mercury concentrations did not return to normal at the Bear Creek site until the 1990s. The study concludes this was likely due to re-emission of mercury from contaminated soil. 

Porter said the Mackenzie Delta cores also show a long-term increase in mercury emissions starting in the late 1600s, a result of the industrial revolution, with a peak in the late 1970s.

The Mackenzie Delta cores, tree cores from another central Yukon site, and ice cores from Alaska all show atmospheric mercury levels have once again been slowly increasing over the past decade. Researchers believe that’s due to rising mercury emissions in Asia. 

“We know for sure that there are rising mercury emissions over certain parts of Asia, industrializing areas of Asia,” Porter said.  “This may explain why we see it over in northwestern Canada, Alaska, and not at other places that are further away from those sources.”

Porter said it’s important to learn about mercury in the environment as bacteria can react with mercury in water, soil and plants, transforming it into methyl mercury, which is poisonous. 

“Once this gets into the food chain, it can have all kinds of negative consequences for the health of animals and humans,” he said.

“We need to understand the mercury cycle and how it may change in the future.” 

Porter hopes to continue his research by examining long-term changes in the mercury cycle on a much broader geographic scale.