Particulate organic matter dynamics and degradation in Arctic fluvial systems

The Arctic is warming two to four times the rate of global average. The increase in air temperatures causes permafrost (i.e., perennially frozen ground) to thaw and release previously frozen organic carbon (OC) to the contemporary carbon cycle. Permafrost stores large amounts of organic carbon (~1300 ± 200 Pg), which equals up to half of the belowground OC globally. Re-mineralization of the released permafrost OC can add greenhouse gases (CO2, CH4) to the atmosphere enhancing climate warming. Gradual permafrost thaw happens when the active layer (i.e., the topmost layer of permafrost that thaws during summer months) deepens due to climate warming releasing largely dissolved organic carbon (DOC). On the contrary, in permafrost regions with high ground ice-content, permafrost thaw happens abruptly (i.e., thermokarst) as landscapes subside or collapse due to melting of ice. Abrupt permafrost thaw releases dominantly particulate organic carbon (POC). While degradation of DOC has been extensively studied in Arctic fluvial systems, degradation of POC is still poorly characterized. In this study, we investigate POC composition and degradation in two different areas: i) in the thaw streams draining abrupt permafrost thaw features, retrogressive thaw slumps (RTS), on the Canadian Peel Plateau, and ii) in the Kolyma River, which is one of the major Arctic rivers draining to the Arctic Ocean. We also study carbon dynamics and water chemistry parameters in lower order streams within the Kolyma watershed in two hydrologically distinct seasons: spring freshet and summer. We use macro(molecular) methods, pyrolysis – gas chromatography mass spectrometry and lipid biomarkers (n-alkanes, n-alkanoic acids), to analyse POC composition and degradation status. For further compositional analysis, we use carbon isotopes (δ13C, Δ14C). Additionally, we employ spatial data analysis and statistical modelling to characterize the watersheds and POC sources. Our results indicate that POC composition is seasonally dependent, and it defines biodegradability of POC. On the Peel Plateau, POC consists largely of aromatic moieties and includes petrogenic carbon that are not easily degradable. By contrast, Kolyma River POC degrades relatively fast during summer, when it is mostly of autochthonous sources. However, freshet POC, dominated by allochthonous POC, is not readily degradable. During freshet, DOC is susceptible to adsorption to particles and/or flocculation, potentially attenuating its climate impact. The lower order streams within the Kolyma River watershed react fast to increase in air temperatures during spring freshet with increased surface water temperatures and depletion in δ13C-POC, suggesting early onset of primary production. Changes in water temperature and δ13C-POC were not as pronounced in the Kolyma River. These results suggest that lower order streams may start primary production and POC degradation earlier in the season than the larger ones and thus, start emitting greenhouse gases earlier. The degraded POC is mostly autochthonous, and more studies are needed to investigate whether degradation of autochthonous POC may stimulate degradation of allochthonous or permafrost POC. These results highlight the heterogeneity of the Arctic fluvial networks and the differences in their response to climate warming.

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