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Permafrost thawing as a result of climate change has major consequences locally and globally for the biosphere as well as for human activities. The quantification of its extent and dynamics under different climate scenarios is needed to design local adaptation and mitigation measures and to better understand permafrost climate feedbacks. To this end, numerical simulation can be used to explore the response of soil thermo-hydric regimes to changes in climatic conditions. Mechanistic approaches minimize modelling assumptions by relying on the numerical resolution of continuum mechanics equations, but involve significant computational effort. In this work, the permaFoam solver is used along with high-performance computing resources to assess the impact of four climate scenarios of the Coupled Model Intercomparison Project – Phase 6 (CMIP6) on permafrost dynamics within a pristine, forest-dominated watershed in the continuous permafrost zone. Using these century time-scale simulations, changes in soil temperature, soil moisture, active layer thickness and water fluxes are quantified, assuming no change in vegetation cover. The most severe scenario (SSP5-8.5) suggests a dramatic increase in both active layer thickness and annual evapotranspiration, with maximum values on the watershed reached in 2100 of +46 % and +29 % respectively. For the active layer thickness, in current climatic conditions it would correspond to a 560 km southward shift. Moreover, in this scenario thermal equilibrium of near-surface permafrost with the new climatic conditions would not be reached in 2100, suggesting a further thawing of permafrost even in case of halting the climate change. [ABSTRACT FROM AUTHOR]

The carbon (C) cycle in inland waters, including carbon concentrations in and carbon dioxide (CO2) emissions from water surfaces, are at the forefront of biogeochemical studies, especially in regions strongly impacted by ongoing climate change. Towards a better understanding of C storage, transport and emission in Central Asian mountain regions, an area of knowledge that has been extremely poorly studied until now, here, we carried out systematic measurements of dissolved C and CO2 emissions in rivers and lakes located along a macrotransect of various natural landscapes in the Sayan–Altai mountain region, from the high mountains of the Western Sayan in the northwest of Tyva to the arid (dry) steppes and semideserts in the intermountain basins in the southeast of Tyva on the border with Mongolia. New data on major hydrochemical parameters and CO2 fluxes (fCO2) gathered by floating chambers and dissolved organic and inorganic carbon (DOC and DIC, respectively) concentrations collected over the four main hydrological seasons allowed us to assess the current C biogeochemical status of these water bodies in order to judge possible future changes under climate warming. We further tested the impact of permafrost, river watershed size, lake area and climate parameters as well as 'internal' biogeochemical drivers (pH, mineralization, organic matter quality and bacterial population) on CO2 concentration and emissions in lakes and rivers of this region and compared them with available data from other subarctic and mountain settings. We found strong environmental control of the CO2 pattern in the studied water bodies, with thermokarst lakes being drastically different from other lakes. In freshwater lakes, pCO2 negatively correlated with O2, whereas the water temperature exerted a positive impact on pCO2 in large rivers. Overall, the large complexity of counteracting external and internal drivers of CO2 exchange between the water surfaces and the atmosphere (CO2-rich underground DIC influx and lateral soil and subsurface water; CO2 production in the water column due to dissolved and particulate OC biodegradation; CO2 uptake by aquatic biota) precluded establishing simple causalities between a single environmental parameter and the fCO2 of rivers and lakes. The season-averaged CO2 emission flux from the rivers of Tyva measured in this study was comparable, with some uncertainty, to the C uptake fluxes from terrestrial ecosystems of the region, which were assessed in other works. [ABSTRACT FROM AUTHOR]

This study demonstrates the dependence between the forest burning rates and abnormal decrease in Siberian river discharges under the conditions of the permafrost zone. Our study area is in Central Siberia and Eastern Siberia/Yakutia. Four rivers (Podkamennaya Tunguska, Lower Tunguska, Aldan, and Viluy) were selected for the study. We analyzed the long-term and seasonal variation of river discharges (archive of The Global Runoff Data Centre for 1939–2015) together with the forest burning dynamics within the river basins (archive of Sukachev Institute of Forest for 1996–2015). We compared the discharges per year with the 77-year average value. Abnormally low levels of discharge constituted 58–78% of the averaged annual rate. An analysis of available chronologies of extreme fire events and relative burned areas (RBAs) showed a high correlation with intra-seasonal data on the runoff minima. The most significant response of river discharges to the wildfire effect was shown for the late summer/autumn season after extreme wildfires during the summer period. The deficit of the runoff was not explained by a low precipitation. Late summer and autumn anomalies of discharge were typical (r = −0.57...−0.77, p < 0.05) for rivers of Central Siberia in seasons of extreme forest burning. The correlation was lower for rivers of Eastern Siberia/Yakutia. [ABSTRACT FROM AUTHOR]

The biogeochemical transfers in the catchment of the Kulingdakan stream, which is a small tributary of the Kochechum river in the Yenisei basin, have been studied for more than a decade. Measurements of the chemical composition of the waters in the stream and in the other compartments of the watershed, such as mineral soil or litter horizons for instance, have been done. Along with these hydrogeochemical observations, characterizations of the chemical compositions of the constituents of the compartments themselves have been performed. The vegetation cover properties, heterogeneity and elemental composition have also been carefully assessed. This knowledge (e.g.: [1], [2]) allowed to build a conceptual hydrological scheme of the water fluxes in this Kulingdakan catchment, characterized by a strong impact of active layer dynamics. The main feature of this hydrological scheme is the high variability along slopes aspect, as summarized in Orgogozo et al. (2014) [3]. This conceptual scheme has then been investigated by cryohydrogeological numerical modeling [4]. The numerical study confirms the transfer mechanisms inferred by the biogeochemical studies, and allows to point out more precisely the impact on the fluxes of matter of the vegetation cover and especially of the dynamics of transfer processes within the root layers. The impact of evapotranspiration on water fluxes is studied numerically, which highlights a strong sensitivity to the variability of rooting depth and corresponding evapotranspiration at the slopes of different aspects in the catchment. On the basis of these results the potential evolution of the fluxes of water and elements from the slopes to the stream in the Kulingdakan watershed under climate change is briefly discussed using a substituting space-for-time approach (e.g.: [5]).[1] Bagard M-L et al. Geochimica et Cosmochimica Acta. 2013;114:169-187. [2] Viers J et al. Biogeochemistry. 2013;113:435-449. [3] Orgogozo L et al. in 'Permafrost: Distribution, Composition and Impacts on Infrastructure and Ecosystems', ed. Pokrovksy OS, Nova Publishers. 2014:153-172. [4] Orgogozo L et al. Permafrost Periglac. Accepted. [5] Kirpotin SN et al. Int J Environ Stud. 2018;75(3): 385-394. [ABSTRACT FROM AUTHOR]

Abstract: Stable Ca and radiogenic Sr isotope compositions were measured in different compartments (stream water, soil solutions, rocks, soils and soil leachates and vegetation) of a small permafrost-dominated watershed in the Central Siberian Plateau. The Sr and Ca in the area are supplied by basalt weathering and atmospheric depositions, which significantly impact the Sr isotopic compositions. Only vegetation significantly fractionates the calcium isotopes within the watershed. These fractionations occur during Ca uptake by roots and along the transpiration stream within the larch trees and are hypothesised to be the result of chromatographic processes and Ca oxalate crystallisations during Ca circulation or storage within plant organs. Biomass degradation significantly influences the Ca isotopic compositions of soil solutions and soil leachates via the release of light Ca, and organic and organo-mineral colloids are thought to affect the Ca isotopic compositions of soil solutions by preferential scavenging of 40Ca. The imprint of organic matter degradation on the δ 44/40Ca of soil solutions is much more significant for the warmer south-facing slope of the watershed than for the shallow and cold soil active layer of the north-facing slope. As a result, the available stock of biomass and the decomposition rates appear to be critical parameters that regulate the impact of vegetation on the soil–water system in permafrost areas. Finally, the obtained δ 44/40Ca patterns contrast with those described for permafrost-free environments with a much lower δ 44/40Ca fractionation factor between soils and plants, suggesting specific features of organic matter decomposition in permafrost environments. The biologically induced Ca isotopic fractionation observed at the soil profile scale is not pronounced at the scale of the streams and large rivers in which the δ 44/40Ca signature may be controlled by the heterogeneity of lithological sources. [Copyright &y& Elsevier]

Abstract: Warming of the permafrost accompanied by the release of ancient soil organic carbon is one of the most significant environmental threats within the global climate change scenario. While the main sites of permafrost carbon processing and its release to the atmosphere are thermokarst (thaw) lakes and ponds, the main carriers of carbon and related major and trace elements from the land to the Arctic ocean are Russian subarctic rivers. The source of carbon in these rivers is atmospheric C consumed by chemical weathering of rocks and amplified by plant uptake and litter decomposition. This multidisciplinary study describes results of more than a decade of observations and measurements of elements fluxes, stocks and mechanisms in the Russian boreal and subarctic zone, from Karelia region to the Kamchatka peninsula, along the gradient of permafrost-free terrain to continuous permafrost settings, developed on various lithology and vegetation types. We offer a comprehensive, geochemically-based view on the functioning of aquatic boreal systems which quantifies the role of the following factors on riverine element fluxes: (1) the specificity of lithological substrate; (2) the importance of organic and organo-mineral colloidal forms, notably during the snowmelt season; (3) the phenomenon of lakes seasonal overturn; (4) the role of permafrost within the small and large watersheds; and (5) the governing role of terrestrial vegetation in element mobilization from rock substrate to the river. Care of such a multiple approach, a first order prediction of the evolution of element stocks and fluxes under scenario of progressive warming in high latitudes becomes possible. It follows the increase of frozen peat thawing in western Siberia will increase the stocks of elements in surface waters by a factor of 3 to 10 whereas the increase of the thickness of active layer, the biomass and the primary productivity all over permafrost-affected zone will bring about a short-term increase of elements stocks in labile reservoir (plant litter) and riverine fluxes by a factor of 2. The change of the plant productivity and community composition under climate warming in central Siberia will be the most important factor of major and trace element fluxes increase (probably a factor of 2) from the soil to the river and, finally, to the Arctic Ocean. [Copyright &y& Elsevier]

Abstract: In order to constrain the origin and fluxes of elements carried by rivers of high latitude permafrost-dominated areas, major and trace element concentrations as well as Sr and U isotopic ratios were analyzed in the dissolved load of two Siberian rivers (Kochechum and Nizhnyaya Tunguska) regularly sampled over two hydrological cycles (2005–2007). Large water volumes of both rivers were also collected in spring 2008 in order to perform size separation through dialysis experiments. This study was completed by spatial sampling of the Kochechum watershed carried out during summer and by a detailed analysis of the main hydrological compartments of a small watershed. From element concentration variations along the hydrological cycle, different periods can be marked out, matching hydrological periods. During winter baseflow period (October to May) there is a concentration increase for major soluble cations and anions by an order of magnitude. The spring flood period (end of May-beginning of June) is marked by a sharp concentration decrease for soluble elements whereas dissolved organic carbon and insoluble element concentrations strongly increase. When the spring flood discharge occurs, the significant increase of aluminum and iron concentrations is related to the presence of organo-mineral colloids that mobilize insoluble elements. The study of colloidal REE reveals the occurrence of two colloid sources successively involved over time: spring colloids mainly originate from the uppermost organic-rich part of soils whereas summer colloids rather come from the deep mineral horizons. Furthermore, U and Sr isotopic ratios together with soluble cation budgets in the Kochechum river impose for soluble elements the existence of three distinct fluxes over the year: (a) at the spring flood a surface flux coming from the leaching of shallow organic soil levels and containing a significant colloidal component (b) a subsurface flux predominant during summer and fall mainly controlled by water–rock interactions within mineral soils and (c) a deep groundwater flux predominant during winter which enters large rivers through unfrozen permafrost-paths. Detailed study of the Kochechum watershed suggests that the contribution of this deep flux strongly depends on the depth and continuous nature of the permafrost. [Copyright &y& Elsevier]