Size matters: Aerobic methane oxidation in sediments of shallow thermokarst lakes.

Shallow thermokarst lakes are important sources of greenhouse gases (GHGs) such as methane (CH4 ) and carbon dioxide (CO2 ) resulting from continuous permafrost thawing due to global warming. Concentrations of GHGs dissolved in water typically increase with decreasing lake size due to coastal abrasion and organic matter delivery. We hypothesized that (i) CH4 oxidation depends on the natural oxygenation gradient in the lake water and sediments and increases with lake size because of stronger wind-induced water mixing; (ii) CO2 production increases with decreasing lake size, following the dissolved organic matter gradient; and (iii) both processes are more intensive in the upper than deeper sediments due to the in situ gradients of oxygen (O2 ) and bioavailable carbon. We estimated aerobic CH4 oxidation potentials and CO2 production based on the injection of 13 C-labeled CH4 in the 0-10 cm and 10-20 cm sediment depths of small (~300 m2 ), medium (~3000 m2 ), and large (~106 m2 ) shallow thermokarst lakes in the West Siberian Lowland. The CO2 production was 1.4-3.5 times stronger in the upper sediments than in the 10-20 cm depth and increased from large (158 ± 18 nmol CO2 g-1 sediment d.w. h-1 ) to medium and small (192 ± 17 nmol CO2 g-1 h-1 ) lakes. Methane oxidation in the upper sediments was similar in all lakes, while at depth, large lakes had 14- and 74-fold faster oxidation rates (5.1 ± 0.5 nmol CH4 -derived CO2 g-1 h-1 ) than small and medium lakes, respectively. This was attributed to the higher O2 concentration in large lakes due to the more intense wind-induced water turbulence and mixing than in smaller lakes. From a global perspective, the CH4 oxidation potential confirms the key role of thermokarst lakes as an important hotspot for GHG emissions, which increase with the decreasing lake size.

Environmental controllers for carbon emission and concentration patterns in Siberian rivers during different seasons.

Despite the importance of small and medium size rivers of Siberian boreal zone in greenhouse gases (GHG) emission, major knowledge gaps exist regarding its temporal variability and controlling mechanisms. Here we sampled 11 pristine rivers of the southern taiga biome (western Siberia Lowland, WSL), ranging in watershed area from 0.8 to 119,000 km2, to reveal temporal pattern and examine main environmental controllers of GHG emissions from the river water surfaces. Floating chamber measurements demonstrated that CO2 emissions from water surface decreased by 2 to 4-folds from spring to summer and autumn, were independent of the size of the watershed and stream order and did not exhibit sizable (>30 %, regardless of season) variations between day and night. The CH4 concentrations and fluxes increased in the order "spring ≤ summer < autumn" and ranged from 1 to 15 µmol L-1 and 5 to 100 mmol m-2 d-1, respectively. The CO2 concentrations and fluxes (range from 100 to 400 µmol L-1 and 1 to 4 g C m-2 d-1, respectively) were positively correlated with dissolved and particulate organic carbon, total nitrogen and bacterial number of the water column. The CH4 concentrations and fluxes were positively correlated with phosphate and ammonia concentrations. Of the landscape parameters, positive correlations were detected between riparian vegetation biomass and CO2 and CH4 concentrations. Over the six-month open-water period, areal emissions of C (>99.5 % CO2; <0.5 % CH4) from the watersheds of 11 rivers were equal to the total downstream C export in this part of the WSL. Based on correlations between environmental controllers (watershed land cover and the water column parameters), we hypothesize that the fluxes are largely driven by riverine mineralization of terrestrial dissolved and particulate OC, coupled with respiration at the river bottom and riparian sediments. It follows that, under climate warming scenario, most significant changes in GHG regimes of western Siberian rivers located in permafrost-free zone may occur due to changes in the riparian zone vegetation and water coverage of the floodplains.

Organic carbon, and major and trace elements reside in labile low-molecular form in the ground ice of permafrost peatlands: a case study of colloids in peat ice of Western Siberia.

The fate of organic carbon (OC), nutrients and metals accumulated in thawing permafrost ice is at the forefront of environmental studies in the Arctic. In contrast to a fairly good understanding of the chemical nature of dissolved OC (DOC) and metals in surface Arctic waters, the speciation and colloidal status of solutes accommodated in the dispersed ground ice remain virtually unknown. Here we used a size fractionation procedure (centrifugal ultrafiltration) to quantify the proportion of colloidal (3 kDa to 0.45 µm) and conventionally dissolved low molecular weight (LMW<3 kDa) fractions of DOC, and major and trace elements in the porewater and ice of 5 peat cores sampled along a 400 km permafrost and climate gradient in the largest peatland in the world, the Western Siberian Lowland (WSL). We discovered that the strong (a factor of 2 to 10) increase in the total dissolved (<0.45 µm) concentration of DOC and most major and trace elements in the peat ice relative to the peat porewater from the thawed layer was essentially linked to an increase in the LMW<3 kDa fraction. This increase in the potentially bioavailable fraction in the peat ice relative to the porewater was especially pronounced for DOC, P and many trace elements including metal micronutrients, and was observed throughout all permafrost zones. This contrasted with element distribution in the upper (thaw) layer, where the majority of these elements were present in the colloidal pool. Following previous experiments on permafrost peatland surface waters, we hypothesized that the freeze-thaw cycles of peat porewater were responsible for generation of the LMW fraction in the bottom part of the peat core. Results of this study demonstrate that carbon, and macro- and micro-nutrients as well as trace metals in ground ice of permafrost peatlands are essentially present in a low molecular weight (<3 kDa) and potentially bioavailable form that can strongly impact the riverine export fluxes of solutes during permafrost thaw.

Dispersed ground ice of permafrost peatlands: Potential unaccounted carbon, nutrient and metal sources.

The physical and chemical consequences of massive ground ice (wedges) melt upon permafrost thaw is one of the central issues of environmental research linked to climate warming in the Arctic. Little is known about the chemical properties of dispersed ground ice abundant throughout permafrost peatlands that can easily melt with increasing active layer thickness (ALT). This is especially pertinent in continental lowlands, that account for sizeable areas of the Arctic, and contain high amount of organic carbon in both solid (peat) and liquid (porewater) phases. Here we studied 8 peat cores (0-130 cm depth)-comprised of porewater from the active layer (0-45 cm) as well as ice dispersed in frozen peat (40-130 cm)-across a latitudinal profile of Western Siberia Lowland (WSL) extending from discontinuous into continuous permafrost zones. Dissolved Organic Carbon (DOC), alkali and alkaline-earth metals (Ca, Mg, Sr, Ba, Li, Rb, Cs), sulfate, phosphorus, some trace elements (Al, Fe, Mn, Zn, Ni, Co, V, As, Y, REE, Zr, Hf, U) were sizably [more than 3 times] enriched in peat ice compared to peat porewaters from the active layer. In most sampled cores, there was a local maximum of strong enrichment (up to factors between 14 and 58) in DOC, P, Ca, Mg, Mn, Fe, Sr, As located 30-50 cm below the active layer. This maximum likely occurred due to solute concentration during full freezing of the soil column during winter. There was a sizable correlation between DOC, Al, Fe and other major and trace element concentrations that suggests strong control of organic complexes and organo-mineral (Al, Fe) colloids on element migration throughout the peat profile. The pool of C, major cations and trace metals in peat ice (40-130 cm) was approximately 3-55 times higher than the pool of these elements in porewaters from the active layer (0-40 cm). A 1-m increase of the ALT over the next 100 years is capable of mobilizing 58 ± 38 Tg of DOC from soil ice into the rivers and lakes of the WSL latitudinal belt (63-67 °N). This fast lateral export of C (3.7 ± 2.7 t C km-2 y-1) may double current C yields in WSL rivers (3.4 ± 1.3 t C km-2 y-1). A strong increase (150-200%) in riverine export of Zn, P and Cs may also occur while other micronutrients (Fe, Ni, Co, Ba, Mo, Rb) and toxicants (Cd, As, Al) may be affected to a lesser degree (20-30% increase). We propose a global peat ice inventory in permafrost regions is essential for assessing the consequences of permafrost thaw on surface aquatic systems.

Enhanced particulate Hg export at the permafrost boundary, western Siberia.

Arctic permafrost soils contain large amounts of organic carbon and the pollutant mercury (Hg). Arctic warming and associated changes in hydrology, biogeochemistry and ecology risk mobilizing soil Hg to rivers and to the Arctic Ocean, yet little is known about the quantity, timing and mechanisms involved. Here we investigate seasonal particulate Hg (PHg) and organic carbon (POC) export in 32 small and medium rivers across a 1700 km latitudinal permafrost transect of the western Siberian Lowland. The PHg concentrations in suspended matter increased with decreasing watershed size. This underlines the significance of POC-rich small streams and wetlands in PHg export from watersheds. Maximum PHg concentrations and export fluxes were located in rivers at the beginning of permafrost zone (sporadic permafrost). We suggest this reflects enhanced Hg mobilization at the permafrost boundary, due to maximal depth of the thawed peat layer. Both the thickness of the active (unfrozen) peat layer and PHg run-off progressively move to the north during the summer and fall seasons, thus leading to maximal PHg export at the sporadic to discontinuous permafrost zone. The discharge-weighed PHg:POC ratio in western Siberian rivers (2.7 ±â€¯0.5 µg Hg: g C) extrapolated to the whole Ob River basin yields a PHg flux of 1.5 ±â€¯0.3 Mg y-1, consistent with previous estimates. For current climate warming and permafrost thaw scenarios in western Siberia, we predict that a northward shift of permafrost boundaries and increase of active layer depth may enhance the PHg export by small rivers to the Arctic Ocean by a factor of two over the next 10-50 years.