Fire Aerosols Affect Southeastern Alaskan Glaciers

The most expensive and extensive wildfires in U.S. history have occurred during the last decade. This increase in fire activity encompasses areas that are often fire hotspots, such as boreal forests, as well as areas where wildfires are rare, such as the Alaskan tundra. Fire activity has also intensified in other areas of the world, including the 2019-2020 Australian bushfires and Siberian “zombie fires,” where winter snowfall does not extinguish smoldering, and fires reignite in the spring.

Fires emit smoke plumes and associated aerosols that can travel tens of thousands of kilometers in the atmosphere and can be deposited far from their source. These fire combustion products include sugars (levoglucosan, mannosan, and galactosan) that are only produced through burning vegetation. As these three sugars are specific to cellulose (the main material that makes up plant cells and fibers) combustion, they provide measurable proof that fires have influenced the material (ice, sediments, lakes) on which they are deposited. Ratios between these three sugars can also determine what type of vegetation (hardwoods, softwoods, or grasses) burned by the fire. This specificity is helpful when examining a mixture of aerosols from unknown sources.

The Juneau Icefields in southeastern Alaska are among the most rapidly retreating glaciers on Earth. Dark aerosols from wildfires and from fossil fuel burning that are deposited on the glacier surfaces can accelerate glacier melt. However, it is often difficult to identify the original sources of these dark aerosols.

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USGS researchers created a new analytical method to measure levoglucosan, mannosan, and galactosan in snow and ice to determine whether fire aerosols are affecting the Juneau Icefield. A transect of six cores extending ~8 m into glacial ice were drilled across the Juneau Icefield in 2016 and 2017. Each core incorporated snow deposited during the past ~3 years and provided material from areas on the icefield that receive different amounts of precipitation and wind scour. The snow and ice layers in the cores were analyzed at sub-seasonal resolution to see if and what smoke aerosols were present.

The results definitively demonstrate that wildfires deposit aerosols onto the Juneau Icefield. Using a combination of remote sensing products, known fire activity, and back trajectories, researchers determined that fires from central Alaska and Siberia deposited aerosols on the Juneau Icefield. The input from local household fires in the Juneau region seemed to have a negligent impact on the glaciers because if household fires were the main source, the highest concentrations of sugar aerosols would have been expected in the ice closest to the city and this was not the case. This deposition of fire aerosols adds to the dark aerosols from fossil fuel burning which together speed the melting of the Juneau Icefield. Additionally, the freshwater input from the Juneau Icefield glaciers can alter the downstream marine ecosystem, with potential implications for regional fisheries.

The paper, “Boreal blazes: Biomass burning and vegetation types archived in the Juneau Icefield,” was published in Environmental Research Letters.

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