Accelerated melting of mountain glaciers due to global warming has a significant impact on downstream biogeochemical evolution because a large amount of labile dissolved organic matter (DOM) is released. However, the DOM evolution processes from glacier to downstream are not well understood. To investigate these processes, samples from the glacial surface and terminating runoff of a mountain glacier on the Tibetan Plateau were collected simultaneously throughout the melting season. The samples were analyzed to determine the dissolved organic carbon (DOC) contents and chemical compositions by means of a combination of fluorescence excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicate that the DOC concentrations were higher in the snow samples than in the glacial runoff samples, although a significantly higher concentration of inorganic ions was found in the glacial runoff samples, suggesting the dominant source of DOM in the glacial runoff was the glacier. The EEM-PARAFAC revealed four fluorescent components in both the snow and glacial runoff samples. However, significantly different ratios between the four components of these two categories of samples suggested chemical, physical and/or biological evolution of DOM during transport. Molecular chemical composition analyses by FT-ICR MS revealed that the DOM composition varied dramatically between the glacier and the glacial runoff. More than 50 % of the molecules were transformed from aliphatic and peptide-like compounds in the snow samples into highly unsaturated and phenolic-like compounds in the glacial run-off samples. The potential chemical transformation of DOM was likely related to biological and/or photolytic evolution during transport. Our results suggest that chemical evolution of glacial DOM could occur during the downstream transport, which is expected to be useful for further research exploring the fate of DOM and carbon cycling from the ciyaspheric environment and evaluating the biogeochemical effects.

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.

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.

Organic matter, upon dissolution into the aqueous state as dissolved organic matter (DOM), can undergo mineralization by microbes. There has been increasing effort to characterize DOM released from thawing permafrost because it may perpetuate a permafrost carbon feedback. Permafrost-derived DOM often has a composition that can be highly susceptible to mineralization by microbes, but most studies to date that characterize permafrost-derived DOM have been limited to select regions, and tend to focus on a single type of permafrost (sometimes unspecified) that reflects a particular deposit type. Importantly, diversity in the nature of the deposit, formation of permafrost, and thaw modification processes leads to spatial and stratigraphic variability in its properties, but our understanding of variation in the composition of DOM derived from differing permafrost types (end-members) is poor. Here, we used ultrahigh-resolution mass spectrometry to characterize DOM composition derived from a series of permafrost end-member types that are commonly found within the thaw-vulnerable western Canadian Arctic, including: tills (glacially deposited), diamicton (thawed and remobilized material of mixed origin), lacustrine (lake basin sediments into which permafrost has aggraded), peat (partially decomposed organic material), and Yedoma (syngenetic silty loess) deposits. We identified marked variation in DOM composition among permafrost end-member types. Tills were compositionally dissimilar to all other permafrost end-members. Compounds unique to Yedoma were predominantly aliphatic, while compounds unique to peat, lacustrine, and diamicton spanned saturation and oxygenation gradients. All permafrost leachates were generally higher in aliphatics, lower in aromatics, and less oxygenated than active layer leachates. Compositional differences appear to reflect variation in permafrost parent materials, and particularly strong effects from past modification processes while in the unfrozen or thawed state. Constraining DOM composition and assessing its stratigraphic variability will become more pressing as the spatial and stratigraphic extent of thaw increases with future warming.

Ongoing global temperature rise has caused significant thaw and degradation of permafrost soils on the Qinghai-Tibetan Plateau (QTP). Leaching of organic matter from permafrost soils to aquatic systems is highly complex and difficult to reproduce in a laboratory setting. We collected samples from natural seeps of active and permafrost layers in an alpine swamp meadow on the QTP to shed light on the composition of mobilized dissolved organic matter (DOM) by combining optical measurements, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry, radiocarbon (C-14), and solid-state C-13 nuclear magnetic resonance spectroscopy. Our results show that even though the active layer soils contain large amounts of proteins and carbohydrates, there is a selective release of aromatic components, whereas in the deep permafrost layer, carbohydrate and protein components are preferentially leached during the thawing process. Given these different chemical characteristics of mobilized DOM, we hypothesize that photomineralization contributes significantly to the loss of DOM that is leached from the seasonally thawed surface layer. However, with continued warming, biodegradation will become more important since biolabile materials such as protein and carbohydrate are preferentially released from deep-layer permafrost soils. This transition in DOM leachate source and associated chemical composition has ramifications for downstream fluvial networks on the QTP particularly in terms of processing of carbon and associated fluxes.