Molecular characterization and transformation of dissolved organic matter in snow and meltwater from Baishui Glacier No. 1, Mt. Yulong

Temperate glaciers are highly sensitive to variations in climate and environmental conditions. Investigating the chemical composition of dissolved organic matter (DOM) in glacier snow is essential for understanding its characteristics, sources, and transformation processes within glacial systems. This study aims to elucidate the chemical composition and transformation of DOM in snow environment by analyzing samples collected from snowpits, surface snow, and snow meltwater at Baishui Glacier No. 1 on Mt. Yulong during May and June. The average concentrations of dissolved organic carbon (DOC) in snow meltwater collected in May (1.63 ± 0.63 mg L⁻¹) and June (1.54 ± 0.35 mg L⁻¹) were both significantly higher than those measured in snowpit samples from May (0.74 ± 0.10 mg L⁻¹) and June (0.54 ± 0.10 mg L⁻¹), as well as in surface snow samples from May (0.65 ± 0.31 mg L⁻¹) and June (0.69 ± 0.30 mg L⁻¹). However, the concentrations of DOC in samples from the same category did not show significant variation between May and June. Using excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor (PARAFAC) analysis, three protein-like components (C1, C2, and C3) and one humic-like component (C4) were identified. The protein-like components accounted for more than 75% of the total DOM in all snow samples, indicating that the fluorescent DOM originated from biological or microbial sources. Significant differences in the relative proportions of the four fluorescent components were observed between snowpit samples from May and June, whereas no significant variations were noted in the other sample types. Furthermore, a clear transformation from protein-like to humic-like components was observed during the transition from snowpits to snow meltwater. Further analysis using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that DOM in these snow samples was predominantly composed of aliphatic and peptide-like compounds (30.9%-50.9%), suggesting a substantial microbial contribution. FT-ICR MS data also demonstrated compositional shifts in DOM among snowpit, surface snow, and meltwater samples. Specifically, aliphatic and peptide-like compounds were progressively transformed into unsaturated compounds with high oxygen content, polyphenolic species, and condensed aromatic compounds during their transition from snowpit to meltwater. Therefore, the relative contribution of terrestrial-derived DOM increased during the transition from snowpit to snowmelt. Furthermore, an increase in heteroatom content in the DOM of meltwater samples indicated continuous chemical transformations likely driven by biological activity and/or photochemical processes during snowmelt and leaching.