Osmium isotope and trace elements reveal melting of Chhota Shigri Glacier, western Himalaya, insensitive to anthropogenic emission residues

Abstract. The western Himalaya glaciers seasonally melt, in part, controlled by the presence of ice surface impurities in the form of dust, organic, and inorganic particles. The hitherto knowledge that dark-colored impurities on the ice surface are a mechanistic driver of heat absorption and thus enhancing ice mass wasting makes understanding the concentrations, origin, and pathways of emission residues on the glacier surface a global concern to conserve the Himalayan ice mass that provides water to more than one billion people. Yet, the source, origin, and pathways of metal impurities on the ice surface of Himalayan glaciers remain poorly constrained. Here, we present major and trace element geochemistry, rhenium-osmium (Re-Os) isotopes composition of cryoconite – a dark-colored aggregate of mineral and organic materials – on the ablation zone of the Chhota Shigri Glacier (CSG) considered as a benchmark glacier for process understanding in the western Himalaya. We find that the cryoconite possesses elemental ratios and crustal enrichment factor that reveal a predominant crustal source. Further, the ¹⁸⁷Os/¹⁸⁸Os composition in cryoconite varies from non-radiogenic (0.36) to radiogenic (1.31) compositions. Using a three-component isotope mixing model we show that the Os in cryoconite is dominantly derived from local rocks with negligible input from anthropogenic Os sources. Given that the CSG has limited debris cover (~ 3.4 %) and the near absence of anthropogenically derived particles; our results suggests that dark-colored surficial deposits of anthropogenic dust particles are not one of the significant drivers of glacier melting in the western Himalaya, as observed elsewhere.