Spectral Characterization of Dissolved Organic Matter in Seawater and Sediment Pore Water from the Arctic Fjords (West Svalbard) in Summer

Contrasting the physical and chemical characteristics of dissolved organic matter between glacier and glacial runoff from a mountain glacier on the Tibetan Plateau

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 runoff 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 cryospheric environment and evaluating the biogeochemical effects.

Subsea permafrost as a potential major source of dissolved organic matter to the East Siberian Arctic Shelf

Arctic subsea permafrost contains more organic carbon than the terrestrial counterpart (~1400 Pg C vs. ~1000 Pg C) and is undergoing fast degradation (at rates of ~10 to 30 cm yr^-1 over the past 3 decades) in response to climate warming. Yet the flux of organic carbon sequestered in the sediments of subsea permafrost to overlying water column, which can trigger enormous positive carbon-climate feedbacks, remain unclear. In this study, we examined the dissolved organic matter (DOM) diffusion to bottom seawaters from East Siberian Sea (ESS) sediments, which was estimated at about 943-2240 g C m^-2 yr^-1 and 10-55 g C m^-2 yr^-1 at the continuous-discontinuous transition zone of subsea permafrost and the remainder shelf and slope sites, respectively. The released DOM is characterized by prevailing dominance (≥ 98%) of low molecular weight (M_n < 350 Da) fractions. A red-shifted (emission wavelength >500 nm) fluorescence fingerprint, a typical feature of sediment/soil DOM, accounts for 4-6% and 7-8% in the fluorescence distributions of seawaters and pore waters, respectively, on ESS shelf. Statistical analysis revealed that seawaters and pore waters possessed similar DOM composition. The estimated total benthic efflux of dissolved organic carbon (DOC) was ~0.7-1.0 Pg C yr^-1 when the estimate was scaled up to the entire Arctic shelf underlain with subsea permafrost assuming the width of continuous-discontinuous transition zone is 1 to 10 m. This estimation is consistent with the established ~10-30 cm yr^-1 degradation rates of subsea permafrost by estimating its thaw-out time. Compiled observation data suggested that subsea permafrost might be a major DOM source to the Arctic Ocean, which could release tremendous carbon upon remineralization via its degradation to CO₂ and CH₄ in the water column.