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Articles, Mineral Dust in the Darkening of the Ablation Zone

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Articles, Mineral Dust in the Darkening of the Ablation Zone The Cryosphere, 11, 2393–2409, 2017 https://doi.org/10.5194/tc-11-2393-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Impact of impurities and cryoconite on the optical properties of the Morteratsch Glacier (Swiss Alps) Biagio Di Mauro1, Giovanni Baccolo2,3, Roberto Garzonio1, Claudia Giardino4, Dario Massabò5,6, Andrea Piazzalunga7, Micol Rossini1, and Roberto Colombo1 1Remote Sensing of Environmental Dynamics Laboratory, Earth and Environmental Sciences Department, University of Milano-Bicocca, 20126 Milan, Italy 2Earth and Environmental Sciences Department, University of Milano-Bicocca, 20126 Milan, Italy 3National Institute of Nuclear Physics (INFN), Section of Milano-Bicocca, Milan, Italy 4Institute for Electromagnetic Sensing of the Environment, National Research Council of Italy, Milan, Italy 5Department of Physics, University of Genoa, Genoa, Italy 6National Institute of Nuclear Physics (INFN), Genoa, Italy 7Water & Life Lab SRL, Entratico (BG), Italy Correspondence to: Biagio Di Mauro ([email protected]) Received: 11 April 2017 – Discussion started: 4 May 2017 Revised: 20 September 2017 – Accepted: 21 September 2017 – Published: 1 November 2017 Abstract. The amount of reflected energy by snow and ice glacier. The presence of EC and OC in cryoconite samples plays a fundamental role in their melting processes. Differ- suggests a relevant role of carbonaceous and organic material ent non-ice materials (carbonaceous particles, mineral dust in the darkening of the ablation zone. This darkening effect (MD), microorganisms, algae, etc.) can decrease the re- is added to that caused by fine debris from lateral moraines, flectance of snow and ice promoting the melt. The object of which is assumed to represent a large fraction of cryoconite. this paper is to assess the capability of field and satellite (EO- Possible input of anthropogenic activity cannot be excluded 1 Hyperion) hyperspectral data to characterize the impact of and further research is needed to assess the role of human light-absorbing impurities (LAIs) on the surface reflectance activities in the darkening process of glaciers observed in re- of ice and snow of the Vadret da Morteratsch, a large val- cent years. ley glacier in the Swiss Alps. The spatial distribution of both narrow-band and broad-band indices derived from Hyperion was analyzed in relation to ice and snow impurities. In situ and laboratory reflectance spectra were acquired to charac- 1 Introduction terize the optical properties of ice and cryoconite samples. The concentrations of elemental carbon (EC), organic carbon Mountain glaciers represent an important source of fresh wa- (OC) and levoglucosan were also determined to character- ter across the planet. These resources are seriously threat- ize the impurities found in cryoconite. Multi-wavelength ab- ened by global climate change (Immerzeel et al., 2010), and a sorbance spectra were measured to compare the optical prop- widespread reduction of glacier extension has been observed erties of cryoconite samples and local moraine sediments. in recent years (Oerlemans, 2005; Paul et al., 2004). Surface In situ reflectance spectra showed that the presence of im- processes that promote glacier melting are driven by both purities reduced ice reflectance in visible wavelengths by climate (i.e., temperature and precipitation) and changes in 80–90 %. Satellite data also showed the outcropping of dust albedo. The latter is mainly influenced by the metamorphism during the melting season in the upper parts of the glacier, of snow in the infrared part of the reflectance spectrum of revealing that seasonal input of atmospheric dust can de- snow and by the impurity content (such as dust, soot, ash, crease the reflectance also in the accumulation zone of the algae, etc.) in the visible domain (Painter et al., 2007). The studies on the impact of light-absorbing impurities (LAIs) on Published by Copernicus Publications on behalf of the European Geosciences Union. 2394 B. Di Mauro et al.: Impact of impurities and cryoconite on glaciers the cryosphere on a global (Flanner et al., 2007, 2009), re- most of the literature focused on Arctic, Antarctic and Asian gional (He et al., 2014; Lee et al., 2017; Painter et al., 2010; glaciers, while little information is available for the European Sterle et al., 2013) and local scale (Oerlemans et al., 2009) Alps (Edwards et al., 2013; Franzetti et al., 2016), despite suggest that LAI accumulation on snow and ice decreases their high sensitivity to environmental and climatic changes the albedo, with consequences on the radiative and mass bal- (Beniston, 2005). The Alps rise on the top of the Po river ances, both in mountain glaciers and ice sheets (Dumont et plain, one of the most industrialized and densely populated al., 2014; Gabbi et al., 2015; Wittmann et al., 2017). areas of Europe. As a consequence, the presence of anthro- Carbonaceous particles such as black carbon (BC), el- pogenic activities is expected to strongly influence the geo- emental carbon (EC) and organic carbon (OC) represent chemical and radiative characteristics of cryoconite (Baccolo an important class of LAIs because, in accordance to their et al., 2017; Cook et al., 2015; Hodson, 2014; Tieber et al., chemical structure, they absorb visible light with extreme 2009). efficiency (Andreae and Gelencsér, 2006; Hartmann et al., Within this study, we demonstrate the potential of field 2013). Their role in the climatic system has been largely and satellite hyperspectral data to characterize the spatial dis- acknowledged in the scientific literature (e.g., Bond et al., tribution of impurities and cryoconite on the surface of the 2013). Given their capability to absorb visible light, carbona- Vadret da Morteratsch Glacier (Swiss Alps) and to evaluate ceous particles are responsible of a positive radiative forc- their effect on snow and ice reflectance. Specific objectives ing when deposited on snow and ice (Doherty et al., 2010; consist in (i) the analysis of the variability of the Morteratsch Hadley and Kirchstetter, 2012; Meinander et al., 2014). Re- Glacier surface reflectance combining hyperspectral satel- cently, Painter et al. (2013) combined ice core data and cli- lite data and field spectroscopy data, (ii) the exploitation of mate simulations, suggesting that carbonaceous LAI deposi- different narrow-band and broad-band indices to determine tions may have played a crucial role in fostering the end of the impact of impurities and cryoconite on the glacier, and the Little Ice Age in the European Alps. (iii) the characterization of the optical and chemical proper- Recent studies showed that Alpine glaciers are undergo- ties of cryoconite in the study area. ing a darkening process, and this was ascribed to the impact of regional and/or global warming and to the deposition and accumulation of LAIs on the glacial surface (Azzoni et al., 2 Data and methods 2016; Gautam et al., 2013; Ming et al., 2012, 2015; Qu et al., 2.1 Study area 2014). Impurities such as BC are commonly present in the atmosphere in Alpine regions (Lavanchy et al., 1999; Méné- Vadret da Morteratsch (46◦2403400 N, 9◦5505400 E, hereafter goz et al., 2014; Nair et al., 2013; Sandrini et al., 2014). Their referred as Morteratsch) is a glacier located in the Bern- sources can be both natural (e.g., biomass burning, wildfires) ina Range in the European Alps (Fig. 1) (Oerlemans and and anthropogenic (e.g., fossil fuel and biofuel combustion). Klok, 2002). Morteratsch is a large valley glacier with an Instead, mineral dust (MD) originates from natural sources altitude that spans from 2030 to 3976 m a.s.l., with an area including areas surrounding the glaciers or distant arid re- of 15.81 km2. It is a typical north-facing Alpine glacier ex- gions (e.g., deserts). The determination of the composition periencing negative mass balances in the last century (Ne- of non-ice material is fundamental to attribute the prove- mec et al., 2009; Oerlemans et al., 2009; WGMS, 2017). The nance of these particles and to assess the actual role of an- glacier apparatus is constituted by the Morteratsch Glacier thropogenic and natural activity in decreasing the albedo of and its tributary, the Pers Glacier. A long record of measure- glaciers and accelerating the melt. ments shows that the Morteratsch Glacier front has retreated When impurities are deposited on snow and ice, they can 2.65 km since 1878 (VAW/ETHZ & EKK/SCNAT, 2016). aggregate on the surface of the glacier, forming a character- Modeling simulations based on emission scenarios predicted istic sediment defined as cryoconite (Nordenskiöld, 1883). an almost total deglaciation of Morteratsch at the end of the Cryoconite is constituted of dust and organic matter, so it century (Zekollari et al., 2014). decreases the albedo of the ice and promotes its melting (Takeuchi et al., 2001). Cryoconite can accumulate in typi- 2.2 Field campaign cal holes in the ablation areas of mountain and polar glaciers (i.e., “cryoconite holes”) or can be dispersed on their surface A field campaign was conducted on 30 July 2015 on the abla- (Stibal et al., 2012). Its geochemical and microbiological tion zone of the Morteratsch Glacier (see Fig. 1). Field spec- composition has been studied in polar and nonpolar glaciers tral measurements were collected to characterize the spec- (Aoki et al., 2014; Bøggild et al., 2010; Hodson et al., 2007; tral reflectance of the glacier surface characterized by dif- Nagatsuka et al., 2014; Takeuchi et al., 2014; Wientjes et al., ferent physical conditions and impurity content. Four main 2011) to evaluate its provenance and to determine the role classes were identified by visual inspection: clean ice, dirty of microorganisms (i.e., extremophiles) in sustaining life in ice, melt pond and cryoconite. A minimum of three differ- such harsh environments. Nevertheless, a lot of uncertainty ent points were measured for each class. Spectral reflectance still exists regarding its formation and evolution.
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