Seasonal snowpack and glaciers provide an important source of water in Washington State, but in recent decades they have undergone substantial decline. Warming temperatures are commonly identified as the dominant cause of this decline, but the deposition of light absorbing impurities (LAI) onto snow and glacier surfaces can be an even larger driver of melt. LAI include black carbon (BC) produced by the incomplete combustion of fossil and biofuels, and dust emissions from desert regions and land use change. When deposited on highly reflective snow and glacier ice, LAI cause darkening of the surface, resulting in greater absorption of solar energy, heating of the snow/ice, and accelerated snow and glacier melt. We analyzed an ice core from South Cascade Glacier in the North Cascades of Washington State to assess variations in LAI deposited on snow and glacier surfaces since 1750 AD, and the associated implications on melt and the availability of water resources. The BC record is 75% complete, and the preliminary record indicates low background BC concentrations in the early 20th century, followed by approximately a magnitude increase in peak and background concentrations, and a subsequent reduction in BC at the top of the record. Concentrated BC layers in excess of 100 ng/g likely resulted from BC deposition from forest fire events. Once the dust analyses and dating are complete, we will be able to: determine the timing of LAI deposition on the glacier; assess the relative absorption of solar energy from dust versus BC; and evaluate the role of LAI in reducing glacier albedo in the context of glacier melt and water resources.

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