Paleolimnological Assessment of Wildfire-Derived Atmospheric Deposition of Trace Metal(loid)s and Major Ions to Subarctic Lakes (Northwest Territories, Canada)

Wildfires release terrestrial elements to the atmosphere as aerosols, and these events are becoming more frequent and intense in the Arctic boreal forest as the climate is warming. We quantified the impact of atmospheric deposition of aerosols from local wildfires on metal(loid) fluxes using macroscopic charcoal accumulation rates, historical fire mapping, and element concentrations in Pb-210-dated lake sediment from five subarctic lakes with small catchments. Lake sediments showed small but significant increases in fluxes (median = 5-10%) for 22 trace metals, metalloids, or major ions following fire events. The impact of wildfire aerosols on element fluxes was mostly due to short-term (<= 2 years) increasing sedimentation rate (6 41% increase), whereas sediment element concentrations were not strongly impacted. Wildfire-associated deposition to lake sediments was mainly composed of Ca, Al, Fe, Mg, K, Mn, and Na, which are major constituents of ash from burned biomass, but changes in sediment flux were greatest for Sb, As, Ni, Ba, Mn, Mo, and Sr compared to pre-disturbance conditions. Compared to anthropogenic sources of pollution, wildfire-associated atmospheric fluxes of metal contaminants to the lakes (e.g., Hg, Pb, As, Sb, and Cd) were low. This study provides quantitative estimates of wildfire impacts on atmospheric geochemical fluxes to subarctic lakes, which can be used for modeling larger-scale impacts under changing fire regimes. Plain Language Summary The subarctic boreal forest is facing major changes in fire regimes in response to climate change, and it is predicted that wildfires will become increasingly frequent and severe in the near future. Wildfires release terrestrial elements to the atmosphere as aerosols, and the impact of ash fallout from local fires on metal(loid) deposition is not well characterized. Wildfire fallouts contain certain elements that can be essential to living organisms (e.g., Ca, Mg, Mn, and Fe) and others that can be toxic (e.g., Pb, Hg, Cd, and As). In this research, we used lake sediment to estimate the amount of material deposited by wildfires in lakes of the subarctic boreal forest over the last 150 years. We found that the input of major ions (e.g., Na, Mg, Ca, and K), metals (e.g., Pb, Hg, Al, and Fe), and metalloids (As and Sb) was elevated during wildfire periods but only by a small amount. The impact of atmospheric deposition was observable in sediment records, indicating that element accumulation in lake sediment can be influenced by wildfires occurring outside the catchment. This study suggests that large areas, including many lakes, will receive additional metal, metalloid, and major ion inputs from more frequent wildfire fallout. Key Points Wildfire fallout was associated with increased fluxes of 22 metal(loid)s and major ions in lake sediment layers Increases in element fluxes to the sediment were small, related to greater sedimentation rates, and lasted <2years Element fluxes to lakes increased by >= 30% during few fire events (5-30% of events, depending on the element)