Dark Glacier Surface of Greenland's Largest Floating Tongue Governed by High Local Deposition of Dust  
[PERSON]  
1 Alfred-Wegener-Institut Helmholtz-Zentrum fur Polar- und Meeresforschung,  
D-27560 Bremerhaven, Germany; [EMAIL_ADDRESS] (L.S. ); [EMAIL_ADDRESS] (N.N. ); [PERSON]DRESS] (V.H. ); [EMAIL_ADDRESS] (R.Z.) organic matter and fine debris, e.g., sedimentary rock particles from nearby mountains. Dust is also used to date ice sheet retreat and advance from ice cores [9]. While some impurities are directly connected to emissions from human activities, others may only be indirectly affected by anthropogenic effects, like changes in wind fields, that affect erosion and transport of debris from nearby mountains. [PERSON] et al. [10] conducted a UAV-borne survey over a prominent dark zone of in the western part of the Greenland Ice Sheet. They found that 73% of the variation in albedo can be attributed to three constituents of the surface: dust, black carbon and pigmented algae. The effect of cryoconite in this area is negligible as it only accounts for \(<\)1%. The optical behavior of the ice-sheet components affect strongly the evolution of the melting season since while snow and firn are capable of reflecting the short-wave infrared radiations, water and ice are absorbing terms in such a wavelength domain [11]. While the visible (VIS) albedo (\(\lambda<0.7~{}\upmu\)m) is above 0.9 and 0.7 for snow/firn and water/ice, respectively, the near infrared (NIR) albedo (\(\lambda>0.7~{}\upmu\)m) is, in fact, below 0.1 for water and ice [11]. The occurrence of those components on the surface, combined with the meteo-climatological conditions of the season, control the evolution of the melting season, that can be accelerated by the removal of the uppermost snow cover. The importance of this process is governed by the duration of the snow cover and the distance of the snow line from the ice margin [12]. Inter-annual variations are controlled by the screen effect produced by the snow cover that is more reflective than ice cover, which are more sensitive to light absorption and consequently to heating and melting. Furthermore, [PERSON] et al. [13] focused the attention on the different ice types outcropping during the melting season, evidencing how the presence of bare-ice covers are correlated to the air temperature, and consequently to the capacity of melting on reaching ice layers characterized by higher impurity contents. Several authors [11; 14; 15] parameterised the snow albedo as a function of snow physical parameters and solar illumination. The snow physical parameters that govern albedo are snow grain size and shape, snow impurity concentrations, liquid water content, and their layer structure in the snowpack, together with snow depth. When the snow depth is optically thin, the underlying surface albedo also affects the snow albedo. While the near-infrared albedo strongly depends on snow grain size [11], the visible albedo is more dependent on the concentrations of snow impurities such as black carbon (BC) and mineral dust [11]. The growth of grains in wet sintering is reducing the albedo dominantly in the NIR, as NIR reflectance is governed by grain size [16]. This is suspected to be a self enhancing effect with more absorption leading to more albedo reduction [4]. Black carbon is a major agent of climate forcing through its role in changing snow/ice albedo [17]. There are several processes that change the optical properties of aerosol BC after deposition in the snow and metamorphism within the snow pack [18]. The long-term transport of BC aerosol over the last decades has been document at several Arctic sites [19; 20; 21]. However, our understanding of the long term trend of BC transport and subsequent deposition across the Arctic appears fragmented. Recently, a study has found 10 year long trends to exhibit no statistically significant change for several areas including Svalbard (Ny-Alesund), Greenland (Summit) and a slight increasing trend at Alert [22].