20th Century Black Carbon and Dust Deposition on South Cascade Glacier, Washington Reconstructed from the South Cascade Ice Core

Abstract:

Glaciers and seasonal snowpack in Washington State have undergone significant decline over the past 50 years. While warming global temperatures are widely recognized as the cause of glacial decline, the deposition of light absorbing impurities (LAI) can also contribute to increased melt. The primary sources of LAI are dust and black carbon (BC). These particles are subject to atmospheric transport and undergo both wet and dry deposition. When LAI are deposited, the albedo of the glacial surface is lowered resulting in increased energy absorption and melt. We analyzed a 158 m long ice core collected from the South Cascade Glacier in the North Cascades of Washington State to reconstruct 20th century LAI deposition. The ice core was analyzed for BC using a Single Particle Soot Photometer (SP2), and for dust using gravimetric filtration and ICP-MS. Pb210 analysis indicates that the bottom age of the ice core is 1916 +/- 18 AD, and tritium analysis is being conducted to further constrain the depth-age scale. Background and peak BC concentrations increased with depth over the top third of the core, indicating higher atmospheric BC loading at the time of deposition. Several segments of the ice core contained visible black impurity layers and analysis determined these outlying samples contained BC concentrations exceeding 100ng/g (maximum = 558 ng/g). These visible impurity layers are distributed throughout the entire length of the core and may originate from regional forest fire activity, a major natural source of BC. Dust concentrations were variable throughout the entire record, with elevated concentrations occurring between 48 – 60 m and 96 – 102 m. The BC and Fe ICPMS data are used to estimate the relative absorption of BC and dust. The chronology of LAI deposition throughout the 20^th century will enable the modeling of historic albedo reductions on South Cascade Glacier, and aid in assessing the contribution of LAI to glacial melt.