Twentieth Century Black Carbon and Dust Deposition on South Cascade Glacier, Washington State, USA, as Reconstructed From a 158‐m‐Long Ice Core

Light absorbing particles (LAPs) include black carbon (BC) and mineral dust and are of interest due to their positive radiative forcing and contribution to albedo reductions and snow and glacier melt. This study documents historic BC and dust deposition as well as their effect on albedo on South Cascade Glacier (SCG) in Washington State (USA) through the analysis of a 158‐m (139.5‐m water equivalent [w.e.]) ice core extracted in 1994 and spanning the period 1840–1991. Peak BC deposition occurred between 1940 and 1960, when median BC concentrations were 16 times higher than background, likely dominated by domestic coal and forest fire emissions. Post 1960 BC concentrations decrease, followed by an increase from 1977 to 1991 due to melt consolidation and higher emissions. Differences between the SCG record and BC emission inventories, as well as ice core records from other regions, highlight regional differences in the timing of anthropogenic and biomass BC emissions. Dust deposition on SCG is dominated by local sources and is variable throughout the record. Albedo reductions from LAP are dominated by dust deposition, except during high BC deposition events from forest fires and during 1940–1960 when BC and dust similarly contribute to albedo reductions. This study furthers understanding of the factors contributing to historical snowmelt and glacier retreat in the Cascades and demonstrates that ice cores retrieved from temperate glaciers have the potential to provide valuable records of LAP deposition. Plain Language Summary Light absorbing particles (LAPs) include black carbon (BC, i.e., soot) produced by the incomplete combustion of fossil and biofuels and mineral dust. In the atmosphere, LAP can lead to atmospheric warming, while LAP deposited on snow and glaciers causes darkening, leading to increased solar energy absorption, warming, and faster melt. The role of LAP in climate change is a large source of uncertainty because LAP emissions and deposition are spatially and temporally heterogeneous. We used an ice core retrieved from South Cascade Glacier in Washington State (USA) to reconstruct BC and dust deposition. BC deposition between 1940–1960 is 16 times higher than during the preindustrial period, likely dominated by domestic coal and forest fire emissions. Differences between the SCG ice core and BC emission inventories, as well as ice cores from other regions, highlight regional differences in BC emissions from humans and forest fires. Dust is a