Molecular Insights into Glacial Cryoconite Dissolved Organic Matter Evolution under Dark Conditions during the Ablation Season on the Tibetan Plateau

Cryoconite granules are dark-colored spherical aggregates and an important sink for organic matter accumulated in supraglacial environments. However, the variations in dissolved organic matter (DOM) concurrent with the leaching process during cryoconite evolution are poorly understood. This study characterized the evolution of DOM from cryoconite leaching throughout a 30 day in situ experiment in the dark on a mountain glacier of the Tibetan Plateau. The results show that the concentration of dissolved organic carbon (DOC) leached from cryoconite decreased initially from 13.41 +/- 2.71 to 7.10 +/- 2.86 mg C L-1 from day 0 to day 3 but then gradually increased to 53.96 +/- 4.31 mg C L-1 by day 30 with an average DOC leaching rate of 20.32 +/- 0.27% in one ablation season. The DOM absorbance (a(254) and a(365)) at each time point (not including day 0) was highly correlated with the DOC concentration (n = 4, r = 0.98), suggesting an enrichment of chromophoric DOM during the leaching process. The molecular composition of DOM analyzed through Fourier transform ion cyclotron resonance mass spectrometry determined that the initial cryoconite DOM from day 0 was enriched in aliphatic (26%) and peptide-like compounds (20%), suggesting high biolability. Between days 0 and 3, extensive loss of peptide-like and aliphatic compounds was exhibited. Then, from day 3 to 30, 43% of leachate DOM was primarily composed of unique unsaturated, polyphenolic, and condensed aromatic compounds, which were newly produced or leached from cryoconite. Finally, the leaching flux of DOC from cryoconite on the Tibetan Plateau in one ablation season and its potential radiative forcing are evaluated.