The tree-ring mercury record of Klondike gold mining at Bear Creek, central Yukon.

Use of elemental mercury (Hg) to enhance placer gold recovery is an effective method dating back centuries, but is associated with significant atmospheric Hg losses. This method was widely used in the Canadian Klondike region during most of the 20th century when the mining industry experienced rapid growth. While the health risks associated with Hg pollution are now well understood, few studies have assessed the environmental legacy of Hg use in the Klondike. We used an annually resolved Picea glauca tree-ring Hg record (1864-2015) to reconstruct and evaluate changes in local atmospheric Hg concentrations associated with gold production at the Bear Creek mining camp. Major temporal trends in the record are consistent with the scale of Bear Creek operations and are distinct from background trends at an unimpacted control site. Tree-ring Hg concentration increased most rapidly from 1923 to 1930, a period when several major mining operations were consolidated at Bear Creek. The highest Hg concentrations, ∼2.5× greater than pre-mining era, occurred in the 1930s, coinciding with maximum gold production at this site. Post-World War II economic factors adversely affected the industry, causing declining tree-ring Hg concentrations from 1939 to 1966. Closure of the Bear Creek camp in 1966 coincided with the strongest tree-ring Hg decline, although a return to background levels did not occur until the 1990s, likely due to re-emission of legacy Hg from contaminated soils. Finally, a robust increase was observed over the last decade, similar to other tree-ring Hg records in N.W. Canada, which is linked to rising Hg emissions in Asia. The Bear Creek tree-ring Hg record provides a unique opportunity to study the impact of Klondike gold mining on the local environment at annual resolution and demonstrates great potential to use Picea tree rings to study past changes in atmospheric Hg from local and global emissions. MAIN FINDINGS: A 151-year long, annually resolved tree-ring Hg record was developed at a historic Klondike gold-mining site to investigate the influence of mining-related Hg emissions on the local atmosphere and environment. Compared to a control site, the tree-ring Hg record documents highly elevated atmospheric Hg concentrations during the period mining activities were ongoing at this site.