A snap-shot assessment of carbon emission and export in a pristine river draining permafrost peatlands (Taz River, Western Siberia)

Mobilization of dissolved organic carbon (DOC) and CO2 from the frozen peat to surface waters in the permafrost zone of high latitude regions is expected to enhance under on-going permafrost thaw and active layer thickness deepening. Here we explored one of the most remote, pristine, unregulated and yet environmentally important rivers in western Siberia (Taz). This subarctic river drains through forested and tundra peat bogs over a gradient of permafrost and climate and likely acts as an important conduit of CO2 to the atmosphere and carbon and nutrient exporter to the Arctic Ocean. In a snapshot study during end of spring flood–beginning of summer baseflow (July 2019), we monitored daytime CO2 and CH4 concentrations and measured CO2 emissions using floating chambers in the main stem (700 km from the upper reaches to the mouth) and 16 main tributaries and we also assessed day/night variations in the emissions. We further tested the impact of land cover parameters of the watershed and tributaries. Based on regular monitoring of the terminal (gauging) station, we quantified the C export to the Arctic Ocean during the study period. We revealed sizable CO2 emissions from the main stem and tributaries (1.0 ± 0.4 and 1.8 ± 0.6 g C-CO2 m−2 d−1, respectively). The CO2 concentrations positively correlated with dissolved organic carbon (DOC), whereas the CH4 concentrations could be partially controlled by dissolved nutrients (N, P) and proportion of light coniferous forest at the watershed. The overall C emission from the water surfaces (4,845 km2) of the Taz basin (150,000 km2) during open water period (6 months, May to October) was estimated as 0.92 Tg C (>99.5% C-CO2, <0.5% C-CH4) which is twice higher than the total dissolved C (organic and inorganic) riverine export flux during the same period. Applying a “substituting space for time” approach for northern and southern parts of the river basin, we suggest that the current riverine CO2 emission may increase 2 to 3 fold in the next decades due to on-going climate warming and permafrost thaw. When integrating the obtained results into global models of C and biogeochemical cycle in the Arctic and subarctic region, the use of the Taz River as a representative example of continental planes should help to estimate the consequences of frozen peatland thaw on CO2 cycle in the Arctic and subarctic regions

Testing landscape, climate and lithology impact on carbon, major and trace elements of the Lena river and its tributaries during a spring flood period

Transport of carbon, major and trace elements by rivers in permafrost-affected regions is one of the key factors in circumpolar aquatic ecosystem response to climate warming and permafrost thaw. A snap-shot study of major and trace element concentration in the Lena River basin during the peak of spring flood revealed a specific group of solutes according to their spatial pattern across the river main stem and tributaries and allowed the establishment of a link to certain landscape parameters. We demonstrate a systematic decrease of labile major and trace anion, alkali and alkaline-earth metal concentration downstream of the main stem of the Lena River, linked to change in dominant rocks from carbonate to silicate, and a northward decreasing influence of the groundwater. In contrast, dissolved organic carbon (DOC) and a number of low-soluble elements exhibited an increase in concentration from the SW to the NE part of the river. We tentatively link this to an increase in soil organic carbon stock and silicate rocks in the Lena River watershed in this direction. Among all the landscape parameters, the proportion of sporadic permafrost on the watershed strongly influenced concentrations of soluble highly mobile elements (Cl, B, DIC, Li, Na, K, Mg, Ca, Sr, Mo, As and U). Another important factor of element concentration control in the Lena River tributaries was the coverage of the watershed by light (for B, Cl, Na, K, U) and deciduous (for Fe, Ni, Zn, Ge, Rb, Zr, La, Th) needle-leaf forest (pine and larch). Our results also suggest a DOC-enhanced transport of low-soluble trace elements in the NW part of the basin. This part of the basin is dominated by silicate rocks and continuous permafrost, as compared to the carbonate rock-dominated and groundwater-affected SW part of the Lena River basin. Overall, the impact of rock lithology and permafrost on major and trace solutes of the Lena River basin during the peak of spring flood was mostly detected at the scale of the main stem. Such an impact for tributaries was much less pronounced, because of the dominance of surface flow and lower hydrological connectivity with deep groundwater in the latter. Future changes in the river water chemistry linked to climate warming and permafrost thaw at the scale of the whole river basin are likely to stem from changes in the spatial pattern of dominant vegetation as well as the permafrost regime. We argue that comparable studies of large, permafrost-affected rivers during contrasting seasons, including winter baseflow, should allow efficient prediction of future changes in riverine 'inorganic' hydrochemistry induced by permafrost thaw

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 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

Carbon emission and export from the Ket River, western Siberia

Despite recent progress in the understanding of the carbon (C) cycle of Siberian permafrost-affected rivers, spatial and seasonal dynamics of C export and emission from medium-sized rivers (50 000–300 000 km2 watershed area) remain poorly known. Here we studied one of the largest tributaries of the Ob River, the Ket River (watershed = 94 000 km2), which drains through pristine taiga forest of the boreal zone in the West Siberian Lowland (WSL). We combined continuous and discrete measurements of carbon dioxide (CO2) concentration using submersible CO2 sensor and floating chamber flux (FCO2), with methane (CH4), dissolved organic and inorganic C (DOC and DIC, respectively), particulate organic C and total bacterial concentrations over an 800 km transect of the Ket River main stem and its 26 tributaries during spring flood (May 2019) and 12 tributaries during summer baseflow (end of August–beginning of September 2019). The partial pressure of CO2 (pCO2) was lower and less variable in the main stem (2000 to 2500 µatm) compared to that in the tributaries (2000 to 5000 µatm). In the tributaries, the pCO2 was 40 % higher during baseflow compared to spring flood, whereas in the main stem, it did not vary significantly across the seasons. The methane concentration in the main stem and tributaries was a factor of 300 to 1900 (flood period) and 100 to 150 times lower than that of CO2 and ranged from 0.05 to 2.0 µmol L−1. The FCO2 ranged from 0.4 to 2.4 g C m−2 d−1 in the main channel and from 0.5 to 5.0 g C m−2 d−1 in the tributaries, being highest during August in the tributaries and weakly dependent on the season in the main channel. During summer baseflow, the DOC aromaticity, bacterial number, and needleleaf forest coverage of the watershed positively affected CO2 concentrations and fluxes. We hypothesize that relatively low spatial and seasonal variability in FCO2 of the Ket River is due to a flat homogeneous landscape (bogs and taiga forest) that results in long water residence times and stable input of allochthonous dissolved organic matter (DOM), which dominate the FCO2. The open water period (May to October) C emission from the fluvial network (main stem and tributaries) of the Ket River was estimated to 127 ± 11 Gg C yr−1, which is lower than the downstream dissolved and particulate C export during the same period. The estimated fluvial C emissions are highly conservative and contain uncertainties linked to ignoring hotspots and hot moments of emissions, notably in the floodplain zone. This stresses the need to improve the temporal resolution of FCO2 and water coverage across seasons and emphasizes the important role of WSL rivers in the release of CO2 into the atmosphere.</p

Hydrochemistry of medium-size pristine rivers in boreal and subarctic zone: Disentangling effect of landscape parameters across a permafrost, climate, and vegetation gradient

We studied two medium size pristine rivers (Taz and Ket) of boreal and subarctic zone, western Siberia, for a better understanding of the environmental factors controlling major and trace element transport in riverine systems. Our main objective was to test the impact of climate and land cover parameters (permafrost, vegetation, water coverage, soil organic carbon, and lithology) on carbon, major and trace element concentration in the main stem and tributaries of each river separately and when considering them together, across contrasting climate/permafrost zones. In the permafrost-bearing Taz River (main stem and 17 tributaries), sizable control of vegetation on element concentration was revealed. In particular, light coniferous and broadleaf mixed forest controlled DOC, and some nutrients (NO2, NO3, Mn, Fe, Mo, Cd, Ba), deciduous needle-leaf forest positively correlated with macronutrients (PO4, Ptot, Si, Mg, P, Ca) and Sr, and dark needle-leaf forest impacted Ntot, Al, and Rb. Organic C stock in the upper 30–100 cm soil positively correlated with Be, Mn, Co, Mo, Cd, Sb, and Bi. In the Ket River basin (large right tributary of the Ob River) and its 26 tributaries, we revealed a correlation between the phytomass stock at the watershed and alkaline-earth metals and U concentration in the river water. This control was weakly pronounced during high-water period (spring flood) and mostly occurred during summer low water period. Pairwise correlations between elements in both river systems demonstrated two group of solutes—(1) positively correlated with DIC (Si, alkalis (Li, Na), alkaline-earth metals (Mg, Ca, Sr, Ba), and U), this link originated from groundwater feeding of the river when the labile elements were leached from soluble minerals such as carbonates; and (2) elements positively correlated with DOC (trivalent, tetravalent, and other hydrolysates, Se and Cs). This group reflected mobilization from upper silicate mineral soil profile and plant litter, which was strongly facilitated by element colloidal status, notably for low-mobile geochemical tracers. The observed DOC vs DIC control on riverine transport of low-soluble and highly mobile elements, respectively, is also consistent with former observations in both river and lake waters of the WSL as well as in soil waters and permafrost ice. A principal component analysis demonstrated three main factors potentially controlling the major and TE concentrations. The first factor, responsible for 26% of overall variation, included aluminum and other low mobile trivalent and tetravalent hydrolysates, Be, Cr, Nb, and elements strongly complexed with DOM such as Cu and Se. This factor presumably reflected the presence of organo-mineral colloids, and it was positively affected by the proportion of forest and organic C in soils of the watershed. The second factor (14% variation) likely represented a combined effect of productive litter in larch forest growing on carbonate-rich rocks and groundwater feeding of the rivers and acted on labile Na, Mg, Si, Ca, P, and Fe(II), but also DOC, micronutrients (Zn, Rb, Ba), and phytomass at the watershed. Via applying a substituting space for time approach for south-north gradient of studied river basins, we predict that climate warming in northern rivers may double or triple the concentration of DIC, Ca, Sr, U, but also increase the concentration of DOC, POC, and nutrients

Atmospheric CO2 and CH4 fluctuations over the continent-sea interface in the Yenisei River sector of the Kara Sea

Observations of the atmospheric sources and sinks of carbon dioxide (CO2) and methane (CH4) in the pan-Arctic domain are extremely scarce, limiting our knowledge of carbon turnover in this climatically sensitive environment and the fate of the enormous carbon reservoirs conserved in the permafrost. Especially critical are the gaps in the high latitudes of Siberia, covered by the vast permafrost underlain tundra, where only several atmospheric monitoring sites are operational. This paper presents the first two years (September 2018–January 2021) of accurate continuous observations of atmospheric CO2 and CH4 dry mole fractions at the recently deployed tower-based measurement station “DIAMIS” (73.5068° N, 80.5198° E) located on the southwestern coast of the Taimyr Peninsula, Siberia, at the Gulf of the Yenisei River that opens to the Kara Sea (Arctic Ocean). In this paper, we summarized the scientific rationale of the site, examined the seasonal footprint of the station with an analysis of terrestrial vegetation and maritime sector contributing to the captured atmospheric signal, and illustrated temporal patterns of CO2 and CH4 for the daytime mixed atmospheric layer over the continent–sea interface. Along with the temporal variations reflecting a signal caused pan-Arctic and not very much influenced by the local processes, we analyzed the spatiotemporal distribution of the synoptic anomalies representing the atmospheric signatures of regional sources and sinks of CO2 and CH4 for the studied high-arctic Siberian domain of ~625 thousand km2, with nearly equal capturing the land surface (54%) and the ocean (46%) throughout the year. Both for CO2 and CH4, we have observed a sea–continent declining trend, presuming a larger depletion of trace gases in the maritime air masses compared to the continental domain. So far, over the Kara Sea, we have not detected any prominent signals of CH4 that might have indicated processes of subsea permafrost degradation and occurrence of cold seeps–still mainly observed in the eastern Arctic Seas—The Laptev Sea and the East-Siberian Se

The effect of permafrost, vegetation, and lithology on Mg and Si isotope composition of the Yenisey River and its tributaries at the end of the spring flood

This work focuses on the behavior of the stable Mg and Si isotope compositions of the largest Arctic river, the Yenisey River and 28 of its major and minor tributaries during the spring flood period. Samples were collected along a 1500 km latitudinal profile covering a wide range of permafrost, lithology, and vegetation. Despite significant contrasts in the main physico-geographical, climate, and lithological parameters of the watersheds, the isotope composition of both dissolved Mg and Si was found to be only weakly influenced by the degree of the permafrost coverage, type of vegetation (forest vs. tundra), and lithology (granites, basalts, carbonates or terrigenous rocks). This observation is generally consistent with the lack of chemical uptake of Mg and Si by soil mineral formation and vegetation during the early spring. The radiogenic Sr isotope composition of the Yenisey and its tributaries varied within a narrow range (0.708 ⩽ 87Sr/86Sr ⩽ 0.711) reflecting the dominance of Phanerozoic rock weathering and/or atmospheric deposition on these compositions. The Mg and Si isotopic compositions of riverine samples reflect two main processes with distinct isotopic signatures. First, isotopically heavier Mg (δ26Mg = −1.0 ± 0.2‰) and isotopically lighter Si (δ30Si = 1.0 ± 0.25‰) are added to the waters by river suspended matter dissolution and leaching from vegetation biomass/topsoil litter. Second, isotopically lighter Mg (δ26Mg = −1.5 to −1.75‰) and isotopically heavier Si (δ30Si = 1.75–2.0‰) are delivered to the Yenisey's tributaries from deep underground water feeding the rivers via taliks. This lighter Mg and heavier Si isotopic composition is interpreted to originate from Precambrian dolomite dissolution and aluminosilicate dissolution coupled with authigenic mineral precipitation, respectively, in deep underground water reservoirs. Taking account of the isotopic composition evolution over the course of the year established earlier on mono-lithological watersheds of the Yenisey basin, the average annual isotopic signatures of the Yenisey river arriving to the Arctic Ocean are estimated to be δ26Mg = −1.58 ± 0.30‰ and δ30Si = +1.60 ± 0.25‰. As the Yenisey is the largest river feeding the Arctic Ocean and as it samples a large variety of environments and lithologies, these values may be reasonable estimates for the average Mg and Si isotopic composition of the dissolved riverine flux to the Arctic Ocean