Physical and chemical evolution of dissolved organic matter across the ablation season on a glacier in the central Tibetan Plateau

Abstract. The physical evolution (metamorphism) of snow is known to affect the chemical composition of dissolved organic matter (DOM) within it. Here, we present a comprehensive study on the Dongkemadi glacier in the central Tibetan Plateau by analyzing surface snow/ice samples collected from May to October 2015. Based on their physical descriptions, these samples were grouped into four categories, i.e., fresh snow, fine firn, coarse firn, and granular ice that represented the different stages of snowmelt. The concentrations of dissolved organic carbon (DOC) decreased from fresh snow (26.8 μmol L⁻¹) to fine firn (15.0 μmol L⁻¹) and then increased from fine firn to coarse firn (26.1 μmol L⁻¹) and granular ice (34.4 μmol L⁻¹). This reflected the dynamic variations in DOC observed during snowmelt. The use of excitation emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC) identified three protein-like components (C1, C2 and C4) and one microbial humic-like component (C3), which reflected the presence of significant amounts of microbially derived DOM in surface snow/ice. The molecular level compositions of DOM identified using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) also showed the presence of molecules that were newly produced during snowmelt. These results suggest that snowmelt not only induced a loss of DOM but also intensified the in situ microbial activities that enriched and modified it. These findings are important for understanding the evolution of the physical and chemical characteristics of DOM during the ablation season and can also shed some light on the nature of biogeochemical cycles in cryospheric regions.