UTM prof looks to tree rings for clues about environmental contamination in the Yukon

A U of T Mississauga geography professor is mining the past for information about the long-reaching impacts of atmospheric mercury contamination.

As a paleoclimatologist and biogeographer, Trevor Porter examines the stable isotopes found in leaf waxes, permafrost and tree rings for information about the climate of a particular time. Often, that means digging through layers of semi-frozen muck in Alaska, searching for clues about carbon dioxide levels during the Pleistocene era, but a new project has Porter examining events closer to our own time.

Using dendrochronology—the study of tree rings—Porter is investigating mercury levels from the last days of the Klondike Gold Rush. He is hoping the tree ring samples will provide a natural calendar of mercury emissions at Bear Creek, a busy gold mining town that operated from 1905 to 1966 just outside of Dawson City, Yukon. During the smelting process, mercury was used to separate gold from other minerals before being burned off. The resulting emissions left the land around surrounding area contaminated with mercury.

Porter and his research team have been taking core samples of trees growing in the region to see if the rings can provide a year-by-year account of atmospheric mercury levels created by the gold operations. The technique is a new method of measuring mercury levels, but early results look promising.

“In northern regions, trees can live up to 600 years and are very sensitive to their environment, so they are really useful for figuring out what climate has done over time,” Porter says. “We want to use the same technique to understand how past levels of mercury behaved.”

“Some literature suggests that trees take in mercury, along with carbon dioxide, and lock it into the wood, but to date there hasn’t been an excellent control showing whether the mercury came through the atmosphere or the soil,” Porter says. “We wanted to test this idea with a site where we know the record of local mercury emissions and to see if those tree ring records reflect the record. “

“If the tree rings are truly a good archive for atmospheric mercury, we should see a direct correlation between the mercury in the tree rings and the historic record of mercury from the gold operations in Bear Creek.”

Porter is collaborating with mercury specialist and fellow UTM geographer Igor Lehnherr using new equipment installed in Porter’s lab in September. The DMA-80 Direct Mercury Analyzer vaporizes the tree ring samples and collects the mercury for analysis.

The resulting data will help Porter and Lehnherr establish long-term trends in atmospheric mercury concentrations. “We know that mercury has human health impacts,” Porter says. “Mercury from the atmosphere can get into aquatic environments where it is absorbed by increasing larger organisms, bioaccumulating by larger animals through the food chain, and could be consumed by humans who eat wild foods from oceans, rivers and lakes. It has major health implications for society. To model how these health impacts may play out in the future, we need to know how the atmospheric pool of mercury behaved in the past.”

The project is funded through UTM’s Research and Scholarly Activity Fund and a Discovery grant from the Natural Sciences and Engineering Research Council of Canada.

Watch a video of Trevor Porter's 2015 field research trip to collect tree and permafrost core samples in central and northern Yukon >