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Pokrovsky O.S.,French National Center for Scientific Research | Pokrovsky O.S.,Institute of Ecological Problems of the North | Reynolds B.C.,ETH Zurich | Prokushkin A.S.,Russian Academy of Sciences | And 2 more authors.
Chemical Geology | Year: 2013

This work is devoted to the characterization of natural mechanisms of silicon isotope fractionation within Siberian watersheds and predicting the climate warming effect on Si fluxes from the land to the Arctic Ocean. To unravel the different sources of silica generated by basalt weathering in Central Siberia under permafrost and larch deciduous forest conditions, we measured the Si isotopic composition of large and small rivers, surface flow, interstitial soil solutions, plant litter and soils. The average annual discharge-weighted δ30Si values of the second largest tributary of the Yenissei River, Nyzhnaya Tunguska and its main northern tributary (Kochechum) are equal to 1.08±0.10‰ and 1.67±0.15‰, respectively, while their average annual Si concentrations are very similar (3.46 and 3.50mg/L, respectively). During summer baseflow, the dissolved Si isotope composition of both large rivers and a small stream ranges between 1.5 and 2.5‰. This is much heavier compared to the source basaltic rocks but similar to the fresh litter of Larix gmelinii, the dominating tree species in this region. It could be consistent with litter degradation in the uppermost soil horizons being the dominant source of solutes annually exported by Central Siberian rivers. During spring flood, accounting for 60-80% of annual Si flux, the δ30Si of the large rivers' dissolved load decreases by 1-1.5‰, thus approaching the value of the bedrock and the silicate suspended matter of the rivers (RSM). This may reflect the dissolution of the silicate suspended load at high water/mineral ratio. The winter δ30Si values of the large river dissolved load range between 1.0 and 2.5‰. During this period, contributing to ≤10% of the annual Si chemical flux, the interaction between bedrock (porous tuffs) and deep ground waters occurs at a very high solid/solution ratio, leading to the precipitation of isotopically light secondary minerals and enrichment of 30Si in the fluids that feed the river through the unfrozen flowpaths. Results of this study imply that more than a half of the silica transported by Siberian rivers may transit through the biogenic pool and that, like in other stable basaltic regions, bedrock-water interactions account for a lesser fraction of the silica flux. As a result of projected future climate warming and weathering increases in boreal regions, the δ30Si isotopic composition of large Siberian rivers is likely to shift towards less positive values. © 2013 Elsevier B.V. Source

Pokrovsky O.S.,French National Center for Scientific Research | Manasypov R.M.,Tomsk State University | Loiko S.V.,Tomsk State University | Shirokova L.S.,French National Center for Scientific Research | Shirokova L.S.,Institute of Ecological Problems of the North
Geochimica et Cosmochimica Acta | Year: 2016

On-going permafrost thaw in discontinuous permafrost regions produces significant amounts of small permafrost subsidence and depressions, while large lakes are likely to drain into streams and rivers. The intensification of permafrost thaw may alter the size distribution and chemical composition of organo-Fe-Al colloids in lakes and rivers. We used a continuum of surface water bodies, from permafrost subsidence, small depressions and thaw ponds to large lakes and rivers that drain the Western Siberia Lowland (WSL), to assess OC, major and the trace element size distribution between the 20-μm, 5-μm, 1.2-μm, 0.45-μm, 0.22-μm, 0.025-μm and 1-kDa (~1.4 nm) size fractions. This approach allowed us to distinguish the organic and organo-ferric colloids that were responsible for the transport of trace elements in surface waters and address their evolution during possible physico-chemical and biological processes. Both conventionally dissolved (<0.22 μm) and low-molecular-weight (<1 kDa) fractions exhibited an order of magnitude decrease in DOC/Fe in the landscape continuum "depressions and permafrost subsidence → thaw ponds → thermokarst lakes → streams → rivers". Thermodynamic modeling and on-site size separation suggested that a number of trace elements (TEs), including alkaline earth elements and several micronutrients (Zn, Ba, Mn, and Ni), decreased the degree of their binding to DOM along the landscape continuum, whereas the majority of insoluble TEs (Al, Fe, Co, Cd, Cu, Pb, REEs, Th, and U) remained complexed with DOM in the LMW<1 kDa fraction. Two primary sites of colloid generation included (i) ground vegetation and peat leaching, which supplied DOM complexes of divalent metals and organo-Al entities to thaw ponds and lakes; and (ii) Fe2+ oxidation and TE co-precipitation with Fe hydroxides in the presence of surface DOM at groundwater discharge sites within the riparian/hyporheic zones of rivers. Under a warming climate scenario, an increase in the thickness of the thawing depth will intensify the input of inorganic components from deep mineral horizons and possibly underground waters thus producing the enrichment of large lakes in Fe-rich colloids and particles. The speciation of divalent metal micronutrients (Cu, Ni, and Co) and toxic metals (Al, Cd, Pb, and U) that are complexed within DOM will most likely remain conservative. Overall, the WSL's surface water colloidal composition may shift from DOM-rich and DOM-Al-rich to Fe-rich, and the export of low-soluble trivalent and tetravalent hydrolysates from the soil to rivers will increase. © 2016 Elsevier Ltd. Source

Viers J.,University Paul Sabatier | Prokushkin A.S.,Russian Academy of Sciences | Pokrovsky O.S.,University Paul Sabatier | Pokrovsky O.S.,Institute of Ecological Problems of the North | And 6 more authors.
Biogeochemistry | Year: 2013

We measured the seasonal dynamics of major and trace elements concentrations in foliage of larch, main conifer species of Siberia, and we analyzed cryogenic soils collected in typical permafrost-dominated habitats in the Central Siberia. This region offers a unique opportunity to study element fractionation between the soil and the plant because of (i) the homogeneous geological substratum, (ii) the monospecific stands (Larix gmelinii) and (iii) the contrasted habitats (North-facing slope, South-facing slope, and Sphagnum peatbog) in terms of soil temperature, moisture, thickness of the active layer, tree biomass and rooting depth. The variation of these parameters from one habitat to the other allowed us to test the effects of these parameters on the element concentration in larch foliage considered with high seasonal resolution. Statistical treatment of data on larch needles collected 4 times in 3 locations during entire growing season (June-September) demonstrated that : (1) there is a high similarity of foliar chemical composition of larch trees in various habitats suggesting intrinsically similar requirements of larch tree growth for nutrients, (2) the variation of elemental concentrations in larch needles is controlled by the period (within the growing season) and not by the geographical location (South-facing slope, North-facing slope or bog zone) and (3) there are three groups of elements according to their patterns of elements concentration in needles over the growing season from June to September can be identified: (1): nutrient elements [P, Cu, Rb, K, B, Na, Zn, Ni and Cd] showing a decrease of concentration from June to September similar to the behaviour of major nutrients such as N, P and K; (2): accumulating elements [Ca, Mg, Mo, Co, Sr, Mn, Pb and Cr] showing an increase of concentration from June-July to September; (3): indifferent elements [Al, Zr, Fe, Ba, Ti, REEs (Pr, Nd, Ce, La, Gd, Er, Dy, Tb, Lu, Yb, Tm, Sm, Ho, Eu), Y, Th and U] showing a decrease of concentration from June to July and then an increase of concentration to September. A number of micronutrients (e. g., Cu, Zn) demonstrate significant resorption at the end of growing season suggesting possible limitation by these elements. Although the intrinsic requirement seems to be similar among habitats, the total amount of element stored within the different habitats is drastically different due to the differences in standing tree biomass. The partitioning coefficients between soil and larch appear to be among the lowest compared to other environments with variable plants, soils and climates. Applying the "space for time" substitution scenario, it follows that under ongoing climate warming there will be an increase of the element stock following enhanced above-ground biomass accumulation, even considering zero modification of element ratios and their relative mobility. In this sense, the habitats like south-facing slopes can serve as resultant of climate warming effect on element cycling in larch ecosystems for the larger territory of Central Siberia. © 2012 Springer Science+Business Media B.V. Source

Parham L.M.,University of New Hampshire | Prokushkin A.S.,Russian Academy of Sciences | Pokrovsky O.S.,CNRS Geosciences Environnement Toulouse | Titov S.V.,Russian Academy of Sciences | And 3 more authors.
Biogeochemistry | Year: 2013

Stream chemistry in permafrost regions is regulated by a variety of drivers that affect hydrologic flowpaths and watershed carbon and nutrient dynamics. Here we examine the extent to which seasonal dynamics of soil active layer thickness and wildfires regulate solute concentration in streams of the continuous permafrost region of the Central Siberian Plateau. Samples were collected from 2006 to 2012 during the frost-free season (May-September) from sixteen watersheds with fire histories ranging from 3 to 120 years. The influence of permafrost was evident through significantly higher dissolved organic carbon (DOC) concentrations in the spring, when only the organic soil horizon was accessible to runoff. As the active layer deepened through the growing season, water was routed deeper through the underlying mineral horizon where DOC underwent adsorption and concentrations decreased. In contrast, mean concentrations of major cations (Ca2+ > Na+ > Mg2+ > K+) were significantly higher in the summer, when contact with mineral horizons in the active zone provided a source of cations. Wildfire caused significantly lower concentrations of DOC in more recently burned watersheds, due to removal of a source of DOC through combustion of the organic layer. An opposite trend was observed for dissolved inorganic carbon and major cations in more recently burned watersheds. There was also indication of talik presence in three of the larger watersheds evidenced by Cl- concentrations that were ten times higher than those of other watersheds. Because climate change affects both fire recurrence intervals as well as rates of permafrost degradation, delineating their combined effects on solute concentration allows forecasting of the evolution of biogeochemical cycles in this region in the future. © 2013 Springer Science+Business Media Dordrecht. Source

Bundeleva I.A.,CNRS Biogeosciences Laboratory | Shirokova L.S.,French National Center for Scientific Research | Shirokova L.S.,Institute of Ecological Problems of the North | Pokrovsky O.S.,French National Center for Scientific Research | And 5 more authors.
Chemical Geology | Year: 2014

The impact of cyanobacteria Gloeocapsa sp. on calcium carbonate precipitation has been examined by combining physico-chemical macroscopic and in-situ microscopic techniques. For this, Ca adsorption and assimilation and kinetic experiments were used to assess the existence of the metabolic process responsible for CaCO3 mineralization by Gloeocapsa sp. Experimental products were characterized by Scanning and Transmission Electron Microscopy (SEM and TEM) imaging, XRD analyses, coupled with Confocal Laser Scanning Microscopy (CLSM) and Raman micro-spectroscopy. Ca carbonate precipitation experiments were performed at an initial pH of 7.8 to 9.4 and 25°C in supersaturated solutions (Ωcalcite=1.5 to 150) in the presence of active cyanobacterial cells. During cyanobacterial photosynthesis, the solution pH increased up to 9.5-10.8 after the first 5-10days of growth, the Ca concentration decreased and the supersaturation index attained a maximum followed by a gradual decrease due to progressive CaCO3 precipitation. Ca adsorption at the surface of live and inactivated Gloeocapsa sp. cells and Ca intracellular assimilation during cell growth were measured as a function of pH and Ca concentration in solution. The contribution of surface adsorption and intracellular uptake to total Ca removal from solution due to biocalcification does not exceed 10%. The presence of calcium carbonate, identified as calcite using Raman spectroscopy, on active Gloeocapsa sp. surfaces and in the vicinity of bacterial cell surfaces was evidenced using SEM. TEM and CLSM demonstrated cyanobacterial cell encrustation by CaCO3 precipitated in the form of nano-spheres adjacent to the cell surface. In contrast to other previously investigated calcifying bacteria, no cellular protection mechanism against Ca2+ adsorption and subsequent carbonate precipitation has been demonstrated for Gloeocapsa sp. This is most likely linked to the specific cellular organization of this species, which involves several cells in one single capsule. As such, planktonic cultures of Gloeocapsa sp. exhibit significant calcifying potential, making them important CO2-fixing microorganisms for both paleo-environmental reconstructions and technological applications. © 2014 Elsevier B.V. Source

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