Magnesium isotopes in permafrost-dominated Central Siberian larch forest watersheds

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ScienceDirect Geochimica et Cosmochimica Acta 147 (2014) 76–89 www.elsevier.com/locate/gca

Magnesium isotopes in permafrost-dominated Central Siberian larch forest watersheds Vasileios Mavromatis a,⇑, Anatoly S. Prokushkin b, Oleg S. Pokrovsky a,c, Je´roˆme Viers a, Mikhail A. Korets b a

Ge´osciences Environnement Toulouse (GET), CNRS, UMR 5563, Observatoire Midi-Pyre´ne´es, 14 Av. E. Belin, 31400 Toulouse, France b V.N. Sukachev Institute of Forest, SB RAS, Akademgorodok 50/28, Krasnoyarsk 660036, Russia c BIO-GEO-CLIM Laboratory, Tomsk State University, Tomsk, Russia Received 17 May 2014; accepted in revised form 8 October 2014; Available online 18 October 2014

Abstract To unravel the Mg isotope fractionation pathways within the continuous permafrost zone in the larch deciduous forest of Central Siberia, we measured the Mg isotopic composition of two large Siberian rivers (Nizhnaya Tunguska and Kochechum, which flow into the Yenisey), a small forested stream, and the major fluid and solid sources of Mg in the watershed: atmospheric precipitates, surface suprapermafrost flow, interstitial soil solutions, plant biomass, litter and mineral soils. The obtained results indicate a significant seasonal variation in riverine water Mg isotope signatures. During the winter baseflow, the Mg isotope composition of large rivers is significantly lighter than the source basaltic rocks and the atmospheric depositions. These differences support the presence of fluids enriched in lighter Mg isotopes, such as those affected by the mineral precipitation of secondary silicates or fluids that dissolve sedimentary carbonate rocks. During the spring flood and in the summer and fall seasons, the river fluid d26Mg values increased by 0.2–0.3& and approached the Mg isotope composition of the ground vegetation (dwarf shrubs, mosses) and the soil organic horizon. Overall, the riverine waters were 0.3–0.7& lighter than the unaltered bedrock and the deep minerals soil horizons. The Mg isotopic compositions of Larix gmelinii organs (i.e., stem wood, roots and needles) exhibit a low variability. However, an enrichment of 0.2–0.3& in the d26Mg of larch needles in the course of the growing season, from June to September can be observed. This enrichment most likely demonstrates uptake of isotopically heavier Mg by the plant in addition to the progressive thawing of the mineral soil (deepening of the active layer of the soil). Overall, the Mg isotope approach indicates the important contribution of vegetation (larch needles, mosses and dwarf shrubs) to the riverine Mg isotope signature and helps to reveal the contribution of isotopically light carbonate rocks in the large rivers of the Central Siberian Plateau. Ó 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Magnesium in continental waters is provided by chemical weathering at the watersheds and is affected by a num-

⇑ Corresponding author at: Institute of Applied Geosciences, Graz University of Technology, Rechbauerstrasse 12, A-8010 Graz, Austria. E-mail address: [email protected] (V. Mavromatis).

http://dx.doi.org/10.1016/j.gca.2014.10.009 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved.

ber of parameters, such as lithology, tectonics, erosion rate, climate, hydrology and vegetation cover (White and Blum, 1995; Gislason et al., 1996; Gaillardet et al., 1999; Stefansson and Gislason, 2001; Millot et al., 2002; Jacobson et al., 2003, 2010; Moore et al., 2013; Opfergelt et al., 2014). The variability of these parameters is well reflected in the range of Mg concentrations with the d26Mg values observed in riverine waters spanning more than 2.5& (Tipper et al., 2006). Lithology most likely exerts

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a major effect on both the concentration and stable isotope composition of Mg in riverine waters because (i) the dissolution rates of silicate and carbonate vary over 3 orders of magnitude under similar conditions (Schott et al., 2009) and (ii) carbonate minerals are overall depleted in 26Mg whereas silicates exhibit a small, but measureable, enrichment in the same isotopomer (Beinlich et al., 2014). Riverine waters in silicate catchments generally exhibit lighter Mg isotope compositions than their host rock material, an effect that has been attributed to the preferential uptake of heavy Mg isotopes during secondary silicate mineral formation (Teng et al., 2010; Opfergelt et al., 2012, 2014; Beinlich et al., 2014). Vegetation also affects the Mg isotope signature on the Earth’s surface because it exerts a significant impact on the weathering rates (Gislason et al., 1996; Stefansson and Gislason, 2001). For example, in Iceland, the Mg fluxes are four times higher for vegetated terrains than for barren areas (Moulton et al., 2000). Boreal, and in particular permafrost-bearing environments, present unique natural sites for testing the effects of vegetation and lithology on riverine elemental fluxes using stable isotopes. Moreover, in these regions, the large variations in the water discharge between the high flow in the late spring and the base flow in summer and fall leads in seasonal variations of the chemical and isotopic composition of riverine waters (Zakharova et al., 2005, 2007; Bagard et al., 2011, 2013; Pokrovsky et al., 2013; Viers et al., 2014). The relative contributions of surface, organic layer and plant litter versus leaching of the deep mineral soil horizons on elemental transport to the watershed, is highly seasonal and follows the change in the active (seasonally thawed) layer thickness (Bagard et al., 2011). During the high flow in the spring flood, the dominant source of elements in riverine waters is leaching from the organic soil horizon and plant litter and isotopic exchange between the suspended and dissolved loads. During the summer baseflow however riverine waters should bear the chemical and isotopic signature of the deep soil horizons in addition to autochthonous biological uptake processes within the river channel or the interstitial soil solutions. Finally, during the winter baseflow, which is only pronounced in large (>10,000 km2 watershed) rivers with unfrozen taliks beneath the river beds, should be heavily affected by feeding from the deep groundwaters and the water/rock interaction at high mineral/fluid ratios. Although vegetation exerts an important role on Mg fluxes in boreal regions, no study so far assesses both the effect of lithology and vegetation of riverine Mg isotope signatures over the hydrological year. Vegetation preferential uptakes 26Mg during higher plant growth (Black et al., 2008; Bolou-Bi et al., 2010), indicating that during the decomposition of litter, a significant local source of Mg with an isotope composition significantly different from that of the soil and underlying rocks may be present. The degree to which seasonal permafrost thaw controls the stable (Ca, Si, Zn) and radiogenic (Sr, U) isotopes in riverine waters, vegetation and soils has been fairly well characterized for the Central Siberian Plateau, a pristine site of basalts and cryosoils covered by deciduous larch forest (Bagard et al., 2011, 2013; Pokrovsky et al., 2013; Viers

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et al., 2014). In the present study, we extend this concerted approach to stable Mg isotopes to describe chemical weathering in two Central Siberian rivers and a small watershed located within the continuous permafrost zone. We aim to distinguish, on the seasonal scale, the effects of permafrost thaw, plant uptake, soil mineral reactivity and underground water inputs on the isotopic signature of dissolved Mg in two large rivers and a small stream. To this end, we analyzed the Mg isotope composition of riverine waters sampled over two years in addition to atmospheric and soil waters, bulk soil and vegetation of a small seasonally frozen watershed. We show the importance of both mineral and organic pools in the delivery of Mg from the soil to the river and we predict the possible consequences of ongoing climate change on the Mg isotope signature of Central Siberian rivers. 2. AREA OF STUDY AND SAMPLING PROCEDURE 2.1. Sampling site The water, soil, vegetation and precipitation samples were collected in the Kulingdakan catchment (64°17– 190 N, 100°11–130 E) of Central Siberia within the drainage basin of the Kochechum River, a northern tributary of the Nizhnyaya Tungunska River, which is the second largest tributary of the Yenisei River (Fig. 1A). This catchment area (4100 ha) is located 5 km north of the town of Tura in Central Siberia, a region that extends to the east of the Yenisey River over more than 3,500,000 km2 at an elevation of 130–1200 m. The Kochechum River drains the basaltic rocks of the Putorana Plateau, which is a 248Ma-old flood basalt complex cropping out over approximately 340,000 km2 (Electronic Supplementary Material (ESM) Fig. 1; ESM Table 1). Permafrost with a depth of 200–400 m extends throughout the study area (Brown et al., 2002). Further details on the geology and soils of this region are available in Pokrovsky et al. (2005) and Bagard et al. (2011). The primary difference between the two largest rivers, Kochechum (KO) and Nizhnyaya Tunguska (NT) is the different lithology of the watersheds, as illustrated in the lithological map presented in ESM Table 1. Tholeitic basalts underlain by tuffs characterize the watershed of the Kochechum River whereas basalts, tuffs and in the upper reaches (approx. 30% of the watershed), Cambrian sedimentary series of terrigenous and carbonate rocks are present in the Nizhnyaya Tunguska (see also the lithological maps in Pokrovsky et al., 2005; Bagard et al., 2011). These sedimentary series may host highly concentrated brines originating from the leaching of evaporite beds (mostly sodic brines in the Nizhnyaya Tunguska watershed). Similar to all other Siberian rivers, the Nizhnyaya Tunguska and Kochechum rivers present a contrasted hydrological cycle (R-ArcticNet: http://www. r-arcticnet.sr.unh.edu) characterized by three major hydrological periods: a very low water period from October to May, a spring flood in May–June, and an intermediate high water period from June to the end of September (Prokushkin et al., 2011).

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Fig. 1. (A) Map of Siberian Plateau showing the sampling locations and major river systems: Kochechum (1) and N. Tunguska (2), (B) Schematic representation of soil horizons in the study area.

The vegetation in the study area is dominated by larch (Larix gmelinii), dwarf shrubs (Ledum palustre L., Vaccinium vitis-idaea L. and Vaccinium uliginosum L.), mosses (Pleurozium schreberi (Brid.) Mitt., Hylocomium splendens (Hedw.) B.S.G. and Aulocomnium palustre (Hedw.) Schwaeger), and patches of lichens (Prokushkin et al., 2007; Viers et al., 2013). For this study, we selected larch stands that developed over three contrasting habitats, namely a southfacing slope, a north-facing slope and a Sphagnum peat bog (ESM Fig. 2). These habitats differ in the following

respects: (1) soil temperature and active layer thickness (ALT), (2) hydrological pathways, and (3) vegetation net primary productivity NPP and biomass (Prokushkin et al., 2007). The major characteristics of the habitats are described below and are listed in ESM Table 2. South-facing slopes are characterized by a deep active layer and warm and well-drained soils with probable drought stress in mid-summer and an average ALT of approximately 120 cm. The total above-ground biomass (i.e., stem and crown) is 57 t/ha, and the maximum

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rooting depth in the soil mineral layer is 60 cm. Northfacing slopes have cold soils with a maximum soil temperature at 5 cm of <5 °C, a thin active layer with permanently moist soil from downslope suprapermafrost water flow. The maximum depth of the active layer in September is 40 cm. The total above-ground biomass is 28 t/ha and the maximum rooting depth in the soil mineral layer is 10 cm. Finally, the peat bog had the shallowest ALT in mineral soil (20 cm in September) under acidic and excessively moist conditions. The rooting zone is adjusted to the upper 15–40 cm of peat and does not reach the mineral layer. The total above-ground biomass is the lowest among the study habitats at 7 t/ha. The nutrients in this habitat are mainly of atmospheric origin (Prokushkin et al., 2010). 2.2. Sample collection The Nizhnyaya Tunguska and Kochechum rivers were both sampled regularly during two hydrological cycles (December 2005 to November 2007) upstream of their confluence near the Tura settlement. For major and trace elemental analyses, the river waters were sampled once a month in the winter (October–April), five to twelve times during the spring flood period (May–June) and about four times per month in the summer and fall seasons (July–September). Larch needle sampling was performed on north- and south-facing slopes and in Sphagnum peat bogs during the same growing season (June–September 2006). In each plot, needle samples were collected from 3 trees with similar life status (diameter, height and crown development). From each tree, we sampled the foliage of 3–5 mid-crown branches and composited them to make one sample per tree. Sampling was performed on the same trees four times during the growing season; sampling began with young needles that had attained their maximum length on June 7, continued with mature needles at their maximum photosynthetic activity (July 18) and senescent needles (August 28) and was completed at the colored phase as needles started shedding (September 10). The shrinkage of needles before shedding was negligible. After collection, the plant material was cleaned on site with ultrapure water to remove possible aerosol dust particles attached to the surface and was then stored in clean plastic bags (Markert, 1995). The Mg concentrations were measured in the 3 tree samples for each microclimate environment at each date. Each concentration value reported in the Table 2 corresponds to the average and the standard deviation obtained from three independent analyses (Viers et al., 2013, Supplementary Materials). The Mg isotopic compositions were measured in the composited sample obtained from the 3 samples of each microclimate environment for the four periods: June, July, August and September. In September, stem discs (3 cm thick) were taken from every tree at breast height (1.3 m) to analyze the bark, and the integral (sapwood and heartwood) wood samples originating from the south-facing slope and the Sphagnum peat bog environments. In addition, in the north-facing slope we collected columns (0–13 cm depth) of surface organic material divided into moss, P. schreberii, (separate live and dead portions), litter and the soil organic layer.

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Replicates were aggregated into one sample. The Mg concentrations and isotopic compositions were measured in the composited samples. Soil samples were collected beneath the organic horizons (Fig. 1B) of the south-facing slope and the north-facing slope in mid-August 2006 (at the period of maximum ALT). One mineral soil column profile was sampled on each slope. The peat bog is frozen before the mineral soil so in this environment, only organic part of the soil horizon was sampled. The mineral soil sampling was performed with a 100 cm3 cylinder throughout the entire active layer to the permafrost (0–120 cm). The soil samples were passed through a 2 mm sieve and were air-dried on site. The samples were subsequently dried at 80 °C for 24 h and were finely ground with an agate mixer mill (Retsch, Germany). Given the extremely high homogeneity of the soil profile and the similarity between the unaltered basalt and the deepest (C) soil horizon (i.e., Pokrovsky et al., 2005, 2006, 2013; Bagard et al., 2013; Viers et al., 2013), the mineral soil horizon sampled below the rooting depth, 60–100 cm at the south-facing slope or 20–50 cm at the north-facing slope, can be considered to be unaltered mineral substrate. Organic layer leachates were collected during the frostfree season with zero tension lysimeters installed beneath the litter, as described in detail elsewhere (Prokushkin et al., 2007; Pokrovsky et al., 2013). At the north- and south-facing slopes, the soil organic layer porewaters were collected with pre-vacuumed porous ceramic cups installed at depths of 20, 40 or 60 cm. 3. ANALYSES 3.1. Sample preparation and Mg concentration measurement To measure the elemental concentrations, all samples were digested in Teflon vials within individual polycarbonate compartments (A100) containing Teflon hot plates in a clean room (ISO3). Between 100 and 200 mg of plant and soil were reacted first with hydrogen peroxide (H2O2) for 24 h at ambient temperature, were further digested in HNO3 and HF for 36 h at 80 °C, digested in HCl for 36 h at 80 °C and were finally treated with HCl and HNO3 for 36 h at 80 °C. The Mg concentrations were measured by ICP-MS (Agilent 7500ce). Indium and rhenium were used as internal standards to correct for instrumental drift and eventual matrix effects. The international geostandards of basaltic rock BE-N (from CRPG, France), lichens BCR-CRM 482 (from BCR, Belgium), and Pine Needles SRM 1575a (from NIST, USA) were used to check the efficiency of both the acid digestion protocol and the analysis. The relative standard deviations between the certified or recommended values and our measurements were lower than 10%. 3.2. Mg separation and isotopic measurements The riverine water samples and the acid digestion products of the soil and vegetation samples used for the Mg concentrations were chemically purified prior to analysis of the Mg isotopes by cation exchange chromatography.

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Table 1 Date, discharge, Mg concentration and Mg isotopic composition of riverine samples from the N. Tunguska and Kochechum rivers and the Kulingdakan stream and reference material used in this study. The notation “n” refers to measurement replicates. Q (m3/s)

Sample

Date (d/m/y)

Kochechum Kochechum Kochechum Kochechum Kochechum Kochechum Kochechum Kochechum Kochechum Kochechum Kochechum

10/31/2007 1/31/2008 3/30/2008 5/20/2008 8/1/2008 8/22/2008 9/9/2008 3/10/2009 5/6/2009 6/6/2009 8/10/2009

203 25 25 3627 1231 5263 1183 110 122 8882 281

N. N. N. N. N. N. N. N. N. N. N. N. N.

10/31/2007 12/23/2007 1/30/2008 3/30/2008 5/22/2008 8/1/2008 8/22/2008 9/9/2008 3/10/2009 5/25/2009 6/5/2009 6/15/2009 8/10/2009

602 83 48 39 16178 3720 6524 3617 140 7847 14178 9199 682

Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska Tunguska

Kuligdakan stream Kuligdakan stream

pH

d25Mg

2r

d26Mg

2r

2973 3705 3854 614 1611 1880 1183 6607 3760 986 3518

6.45 6.39 6.28 5.30 6.83 7.20 7.43 6.67 6.31 5.45 7.26

0.29 0.38 0.37 0.31 0.28 0.28 0.27 0.38 0.36 0.30 0.30

0.02 0.07 0.00 0.01 0.06 0.00 0.00 0.05 0.01 0.03 0.08

0.54 0.67 0.67 0.57 0.50 0.51 0.54 0.69 0.67 0.55 0.58

0.03 0.02 0.03 0.02 0.02 0.05 0.04 0.05 0.02 0.01 0.02

3 3 3 3 3 3 3 3 3 3 3

10115 12090 10215 11304 1236 3385 3521 3875 16951 2151 2230 3105 7526

7.45 7.68 7.64 6.93 6.16 7.67 7.37 7.68 7.84 6.24 6.33 6.58 7.59

0.47 0.52 0.55 0.53 0.43 0.40 0.38 0.46 0.56 0.44 0.44 0.49 0.55

0.03 0.01 0.06 0.07 0.04 0.04 0.01 0.03 0.04 0.02 0.02 0.01 0.06

0.93 1.02 1.07 1.07 0.86 0.78 0.77 0.91 1.10 0.84 0.87 0.97 1.03

0.04 0.05 0.01 0.08 0.05 0.06 0.05 0.06 0.04 0.01 0.01 0.03 0.05

3 3 3 3 3 3 3 3 3 3 3 3

0.34 0.32

0.06 0.04

0.63 0.56

0.03 0.08

3 3

0.00 1.34 1.32 1.44 1.21 0.14

0.04 0.04 0.03 0.06 0.05 0.07

0.00 2.64 2.61 2.79 2.40 0.25

0.07 0.08 0.05 0.08 0.07 0.09

110 21 6 21 6 6

Mg (lg/L)

01/06/09 07/08/09

DSM3 CAM-1 CAM-1a OUMg JDo-1 BE-N a

n

Processed through column chemistry.

Separation of the Mg from the matrix elements for the fluid and vegetation samples followed the protocol described earlier by Mavromatis et al. (2013) using the AG50W-X12 resin eluted with 1.0 M HNO3. The Mg was separated from the soils using the same resin and elution with 2.0 M HNO3, similar to Pogge von Strandmann et al. (2011). The yields after chromatographic separation were better than 99.8% of the total Mg loaded in the columns. The separation procedure was repeated when necessary to make the cation/Mg ratio in the sample <0.05 and thereby avoid potential interference in the mass spectrometry analyses. The magnesium isotopic ratios were measured with a Thermo-Finnigan ‘Neptune’ Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) at the GET laboratory (Toulouse, France). All the solutions were prepared in 0.3 M HNO3 and were introduced into the Ar plasma with a standard spray chamber. The instrumental mass fractionation effects were corrected by sample-standard bracketing, and all results are presented in delta notation relative to the DSM3 reference material as: h i dx Mg ¼ ððx Mg=24 MgÞsample =ðx Mg=24 MgÞDSM3  1Þ  1000

where x is the Mg mass of interest. The obtained results are consistent with mass-dependent fractionation (see Tables 1 and 2). The reproducibility of the d26Mg analyses was assessed by replicate analyses of three international Mg reference standards (DSM-3, CAM-1 and OUMg) and was typically better than 0.08& (see Table 1); these measurements were also in agreement with the previously published values (e.g., Tipper et al., 2006; Pogge von Strandmann et al., 2008). Moreover, dolomite standard JDo-1, basalt standard BE-N and Mg standard CAM-1 were processed identically and gave compositions (Table 1) similar to those reported by Bolou-Bi et al. (2009), Wombacher et al. (2009), Mavromatis et al. (2013, 2014) and Beinlich et al. (2014). 4. RESULTS 4.1. Riverine water concentration and Mg isotope composition River discharges exhibited strong seasonal variations in both the observed rivers for 3 consecutive hydrologic years (2007–2009, Fig. 2). During the observation period, the river-specific annual runoff varied between 229 and

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Table 2 Mg isotope composition of rain, soil solutions, solids, minerals and organic samples. The notation “n” refers to measurement replicates. d25Mg

2r

d26Mg

2r

n

0.31 0.28 0.35

0.02 0.06 0.07

0.57 0.59 0.71

0.04 0.05 0.03

3 3 3

S-slope, 20 cm S-slope, 60 cm N-slope, 20 cm N-slope, 60 cm

0.31 0.35 0.21 0.19

0.03 0.03 0.02 0.01

0.55 0.64 0.40 0.38

0.04 0.03 0.05 0.07

3 3 3 3

Mineral soil Mineral soil Mineral soil Mineral soil Mineral soil Mineral soil Soil org. horizon Soil org. horizon Soil org. horizon Soil org. horizon

N-slope, 20–50 cm, B/BC N-slope, 0–10 cm, A S-slope, 0–10 cm, A S-slope, 70–100 cm, C S-slope, 0–5 cm, A S-slope, 20 cm, B N-slope, mound, Oea S-slope, trough, Oi S-slope, trough, Oea N-slope, trough, Oi

0.15 0.10 0.10 0.02 0.23 0.24 0.16 0.32 0.14 0.21

0.01 0.01 0.03 0.06 0.02 0.03 0.03 0.06 0.03 0.06

0.26 0.21 0.23 0.09 0.47 0.51 0.30 0.57 0.24 0.36

0.08 0.08 0.04 0.04 0.08 0.05 0.05 0.06 0.06 0.09

3 3 3 3 3 3 3 3 3 3

Sphagnum live (green) Sphagnum dead (brown) Sphagnum live (green) Sphagnum dead (brown) Bark L. gmelinii Bark L. gmelinii Roots Stem wood Bog Stem wood S-slope Branches Seeds Needles Needles Needles Needles

N-slope N-slope S-slope S-slope S slope Bog B2 Fine roots < 2 mm B3 2/3 Wood bulked S2 3/3 Wood bulked S-slope Larch twigs + bark S-slope Seeds K94 N Trough Needles 08.2008 N Mound Needles 08.2008 S Mound Needles 08.2008 S Trough Needles 08.2008

0.27 0.28 0.27 0.28 0.23 0.16 0.17 0.14 0.22 0.13 0.16 0.26 0.24 0.16 0.17

0.02 0.03 0.09 0.03 0.03 0.02 0.07 0.02 0.06 0.09 0.06 0.03 0.01 0.04 0.05

0.57 0.57 0.49 0.56 0.39 0.29 0.31 0.25 0.45 0.30 0.37 0.51 0.45 0.28 0.35

0.09 0.06 0.09 0.06 0.04 0.06 0.05 0.06 0.04 0.08 0.06 0.05 0.08 0.03 0.02

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

Dwarf shrubs Dwarf shrubs Dwarf shrubs Dwarf shrubs Lichens Lichens

V. uliginosum stems V. uliginosum leaves V. vitis-idaea stems Kassandra Cladonia islandica Cladonia stellaris

0.21 0.29 0.34 0.19 0.09 0.25

0.01 0.03 0.03 0.08 0.03 0.04

0.43 0.57 0.68 0.38 0.16 0.48

0.04 0.06 0.01 0.05 0.05 0.09

3 3 3 3 3 3

Sample

Sample location

Mg (mg/g)

Rain September Snow-1 Snow-2 Soil Soil Soil Soil

solution solution solution solution

872 951 441 371 595 282 644 781 2120 1884 3746 1638 918 2198 1159 573 553

N = N-facing slope; S = S-facing slope. A, B, C, Oi, and Oea are soil horizons as shown in Fig. 1B.

291 mm for NT and 348 and 495 mm for KO. The most pronounced inter-annual variation in river runoff occurred in frost-free seasons. The runoff was only 41 and 97 mm in the extremely dry year of 2009, and 158 and 225 mm in the wet year of 2008 for NT and KO, respectively. The Mg concentrations in the river waters have statistically significant negative correlations with the discharges of both rivers (r = 0.71, p < 0.05) and followed a power function dependence (Fig. 3A). Magnesium behaved as non-conservative element in the studied system because its concentrations varied by only one order of magnitude between the minimum flow and the peak flow (Fig. 2) compared to the three order of magnitude variation in the discharge. The Mg concentrations steadily increased over the winter period and sharply decreased during the spring flood. A similar pattern was also observed in the frost-free

season during stormflow discharges. This pattern indicates that dilution of river waters with fluids with lower Mg concentration has a strong effect on Mg concentration in riverine waters. The observed seasonal behavior is similar to that observed for other major cations in this region (Bagard et al., 2011). The measured Mg concentrations are two to three times higher in the N. Tunguska river than the Kochechum river and range between 1.2 and 20.3 mg/L and between 0.6 and 6.6 mg/L in the N. Tunguska river and the Kochechum river, respectively (Fig. 2; Table 1). The Mg isotope compositions of the riverine samples are listed in Table 1. During the winter (October–May), the isotopic compositions of the riverine waters of both the N. Tunguska and the Kochechum rivers exhibit enrichment in the lighter isotopes relative to their composition during

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Fig. 2. Discharge and evolution of Mg concentration and Mg isotope composition in (A) Kochechum and (B) N. Tunguska rivers between the hydrological years 2007 and 2009.

summer time (Fig. 2). The d26Mg values of the Kochechum river waters in the winter averaging at 0.7& whereas during the same period of time, the water samples for the N. Tunguska river exhibit significantly lighter values, on average 1.1&. The spring flood, occurring in early June, is characterized by a large increase in the water discharge and leads to an increase in the 26Mg content of the waters in both rivers. This increase is rather limited in the Kochechum river; the water samples characterized after the flooding event had average d26Mg values of 0.5&. However, the increase of 26Mg content in the N. Tunguska river waters is significantly higher; the d26Mg values are as high as 0.7&. The stream Mg concentrations demonstrated large intra-seasonal and inter-annual changes with a general steady increase from the freshet to the fall; this trend followed the thawing of the permafrost and the deepening of the origin of solutes. The precipitation-driven changes in the discharge during the frost-free season appeared to be a major control factor for the stream Mg concentrations with larger values (up to 3.6 mg/L) observed at close to zero discharge in the extremely dry mid-summer season of 2009. The Mg isotope composition did not show any variation between samples collected at the freshet (0.63&) and extreme low flow (0.56 &) in 2009, despite large differences in the Mg concentrations at the date of sampling (i.e., 0.88 and 3.3 mg/L, respectively; ESM Fig. 3). Note

Fig. 3. Dynamics of discharge, Q, of (A) Mg concentrations and (B) Mg isotope composition of the Nizhnyaya Tunguska and Kochechum rivers in 2007–2009.

that the Kulingdakam stream could not be sampled during the winter because this stream is completely frozen. 4.2. Vegetation and soil Larch is the dominant vegetation species and composes approximately 75% of the total aboveground biomass (7–60 t/ha) in the ecosystem of the study area. The trunk biomass is the primary fraction of the larch biomass; representing, together with the branches (i.e., wood tissues), 94 ± 4% of total standing stock, whereas the percentage of the needles is 6 ± 3%. Note here that the Mg concentration of vegetation and soil samples can be found in ESM Table 3. The analyzed woody tissues (trunk wood) have lower Mg concentrations (p < 0.01), with average 358 ± 133 lg/gdry weight, compared to the non-woody needles (range from 1443 to 3078 lg/gdry weight for individual trees) (Fig. 4A). The bulk wood Mg concentrations varied relatively little among the habitats: from 306 ± 109 in trees grown in peat bog terrain to 398 ± 167 lg/gdry weight in trees on the south-facing slope. Interestingly, much more significant variation (p < 0.05) was observed within the tree trunk between sapwood and heartwood tissues; in the latter case, the values were almost 2-fold higher. Non-woody larch needles demonstrated well-pronounced seasonal changes. The maximum values were in

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the juvenile needles (June 7), the lowest values were in the mid-season of growth (July 18, p < 0.01) and steadily increased until the needle abscission on September 12 (p < 0.01, Fig. 4A). The variation of d26Mg in the woody and non-woody tissues of the larch trees falls within the range from 0.19& to -0.45& (Fig. 4B). Lower d26Mg values are found in woody tissues of the larches from the south-facing slope (0.45&) and also in the juvenile needles (0.36& and 0.46& in the peat bog and the southfacing slope sites, respectively). However, on a seasonal scale, a steady enrichment of larch needles in heavier isotope for both compared habitats is evident. This increase in the d26Mg values, which reached ca. 0.2& at the abscission stage, occurs at the same time than an increase in of the active layer thickness (r = 0.98, p < 0.05) as shown in Fig. 5. The moss-lichen strata is the second largest component of ecosystem biomass (610 t/ha) and contains relatively uniform amounts of Mg; in the compared habitats the average Mg was 830 ± 124 lg/gdry weight (Fig. 2A). In comparison to larch, the Mg isotopic composition of feather mosses (P. schreberi) showed greater depletion of the heavy Mg isotope and ranged relatively little among the compared habitats: from 0.49& to 0.57& with an average value of 0.53 ± 0.06& (Fig. 4B). Ericoid dwarf shrubs are usually the third major component of northern taiga forests, contributing ca. 1.7 t/ha to

Fig. 4. Concentration of Mg (A) and range of d26Mg values (B) in major vegetation types and genetic soil horizons from different habitats of permafrost terrain: north-facing slope, south-facing slope and peat bog. Note that non-woody tissues of larch were taken in September as abscised needles. The notations (1) and (2) stand for woody and non-woody tissue, respectively.

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Fig. 5. Changes in the concentrations and isotopic composition of dissolved Mg in atmospheric input and in soil solutions collected in different soil horizons. The triangles represent changes in the Mg concentrations, and the squares are Mg isotopic compositions (open symbols, south-facing slopes, filled symbols, north-facing slopes).

the ecosystem biomass pool. The analyzed dwarf shrubs species (n = 6) demonstrated significant variation in the Mg concentrations and isotopic compositions among the species and their tissues. In general, similarly to larch, the woody tissues of dwarf shrubs demonstrated lower concentrations (Fig. 4A) and heavier Mg compared to the non-woody tissues (Fig. 4B). The stems of the deciduous forms of dwarf shrubs (e.g., V. uliginosum) is slightly enriched in the heavier Mg isotopes relative to the stem of the evergreen species (V. vitis-idaea). The upper organic horizon of “fresh litter” (Oi horizon) located just beneath the moss strata showed a twofold increase in Mg concentration compared to moss tissues. Isotopic analysis has distinguished different d26Mg values between the north- and the south-facing slopes in the Oi horizon with N-facing habitat being isotopically heavier by 0.2& in 26Mg. In contrast, in the deeper horizon (Oea) of the soil organic layer, the south-facing slope organic matter contained slightly heavier Mg compared to the north-facing slope samples. The concentrations of Mg in the Oea horizon demonstrated a further increase by a factor of 2.6–3.0 relative to the upper Oi horizon and reached 3340 ± 1950 and 5460 ± 2100 lg/gdry weight for the northand south-facing slopes, respectively. In the mineral soil horizons (A, B/BC, and C), we observed a general trend of increasing Mg content (from ca. 20 up to 25 mg/gdry weight in the C horizon) and an enrichment in 26Mg towards the deeper horizons; the C horizon had a d26Mg composition of ca. 0.2& (Fig. 4). Despite the small differences in the Mg concentrations in

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the compared habitats, there are slightly heavier values of d26Mg in the soil of the south-facing slope. 4.3. Atmospheric precipitation and soil solutions The concentrations of dissolved Mg in the atmospheric precipitates were measured in a total of 61 samples collected during 2007–2009 (i.e., 20 monthly snow samples and 41 rain event samples). Both types of atmospheric precipitates demonstrated significant variations in the Mg concentrations, from 16 to 426 lg/L (100 ± 97 lg/L) in snow and from 31 to 2033 lg/L (310 ± 220 lg/L) in rain. Despite this high variation, there is no statistically significant dependence of [Mg] on the amount of precipitation (at p  0.1). One rain and two composite snow water samples exhibit nearly uniform values (0.62 ± 0.08&, Table 2). Note here that because of the low concentration of Mg in winter atmospheric precipitates, the two reported values correspond to the bulk of six individual snow samples collected monthly in the period 10/2007–03/2008. After contact with the organic matter accumulated on the soil surface, the atmospheric waters become significantly enriched in Mg (Fig. 5). For both slopes, the waters percolated through the organic layers (moss and O horizon), and a 10-fold increase was observed in the Mg content of the collected waters relative to the atmospheric precipitates (p < 0.001). However, small changes in the Mg concentrations were observed in soil solutions collected in the A and B horizons of the south-facing slope whereas the Mg concentrations varied from 2.3 mg/L in the O layer to 2.7 mg/L in the B horizon. In contrast, the steady increase in the Mg in solution from 1.7 in the O layer to 3.1 mg/L in the B horizon was observed in the north-facing slope soil. Further enrichment of the soil waters in Mg occurred in the deeper soil horizons (BC and C), where the Mg concentrations reached 6–8 mg/L and had larger values for the north-facing slope. The organic layer leachates and interstitial solutions of the mineral soil horizons collected on the south-facing slope demonstrated very little variation in isotopic composition, and their d26Mg values were close to those of atmospheric waters and the solid matter of the moss and Oi layers (Figs. 4B and 5). The mean value of d26Mg in the samples collected beneath the O horizon and in the B and C horizons is 0.62 ± 0.05& (n = 3). The magnesium in the north-facing slope soil solutions collected in B and BC horizons also did not show significant variation with depth, but was approximately 0.2& heavier than in the soil solutions from the south-facing slope. Overall, the obtained values were in good concordance with the d26Mg values for solid matter from the Oea, A and B/BC horizons of the respective habitats. 5. DISCUSSION 5.1. Magnesium sources in larch forest ecosystem surface waters and soils There are two major sources of Mg in the riverine waters of the Tura ecosystem: (i) Mg derived from soil weathering and plant litter degradation, and (ii) Mg from atmospheric deposition in the form of rain and snow.

The Mg isotope composition of the C horizon soils (Table 2; Fig. 4) consisting of coarse basaltic sand, are the most enriched in 26Mg and have a d26Mg value similar to that of basaltic rocks (d26Mg = 0.23 ± 0.11&; e.g., Teng et al., 2007; Handler et al., 2009; Bourdon et al., 2010). Note here that weathering of Siberian basalts is extremely slow given that the majority of the bottom of soil profile and the mother rock itself is permanently frozen. Moreover these basalts were already altered by posteruptive hydrothermal alteration as demonstrated in detailed mineralogical studies over large territory of Siberian basalts (see Pokrovsky et al., 2005 and references therein). Pokrovsky et al. (2005) and more recently Viers et al. (2013) demonstrated a very weak if not almost negligible differentiation of major (including Mg) and trace elements within depth normalized to immobile elements of the soil profile, in both a large latitudal profile of Central Siberian basalts and on slopes of different exposition. On the Tura site, studied in greater details in this work, the C horizon consists of coarse basaltic sands and whole rock debris whose Mg isotopic composition is very similar to that of other basalts. Note that similar conclusion has been reached for Si isotopic composition of Tura soils and unaltered rocks (Pokrovsky et al., 2013). The upper layers of the soil horizon (BC, B, A and O) are characterized by progressive upward depletion of 26 Mg (Fig. 5); this depletion indicates the loss of 26Mg from the parent rock. This feature may be related to the mineralogy and the formation processes of clays in the area. Based on a detailed mineralogical study and previous data for the Central Siberian Plateau, Pokrovsky et al. (2005) argued that the clays in soils developed in Siberian basalts including the Tura site are dominated by tri-octahedric hydrothermally produced smectites, containing nonexchangeable Mg in their silicate network. Among the secondary phases produced during contemporary weathering, only amorphous Fe-Al-organic-rich compounds and allophanes not containing Mg were identified. Thus, the main Mg pool in the soils is exchangeable Mg, which is subject to adsorption–desorption processes that are likely to control the isotopic distribution of Mg in soils. Indeed, Huang et al. (2012) and Opfergelt et al. (2014) recently suggested that Mg adsorption on the exchangeable sites of secondary minerals may be the main mechanism responsible for the loss of heavier Mg in the soil profiles. The second main source of Mg in rivers is atmospheric precipitation with a contribution however that does not exceed the 10% of the total concentration (Pokrovsky et al., 2005). The measured Mg isotope compositions of the rain and snow precipitates exhibits very small variations (d26Mg = 0.64 ± 0.08&; Table 2) and is slightly enriched in 26Mg compared to seawater (d26Mg = 0.8&). Similar Mg isotope compositions for rain and snow waters were previously reported from Bolou-Bi et al. (2012), Tipper et al. (2010), Pogge von Strandmann et al. (2008), Riechelmann et al. (2012) for different localities. Note here that d26Mg values as low as 1.6& have been reported for rainwater (Tipper et al., 2012). However, the limited number of samples in this study does not allow this range to be tested for this Central Siberia site.

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5.2. Evolution of d26Mg in plants over the course of the frostfree period The Mg isotope composition of the various parts of the plants analyzed in this study is overall heavier than that of the soils and soil solutions corresponding to the rooting zone (Fig. 4; Table 2). This result is in good agreement with previous experimental and natural observations of Mg isotope composition in higher plants with earlier studies to report enrichment of up to 1.1& in 26Mg in the bulk plant tissue relative to the growth medium (Black et al., 2008; Bolou-Bi et al., 2010, 2012). Comparison of our results with those of previous studies however, is not a straightforward process, because the seasonal presence of ice in the soils significantly affects the depth of the Mg uptake by the plant roots. Seasonal thawing of the active layer can significantly complicate the interpretation of Mg isotope composition of the plants in the area and may act in conjunction with the kinetic isotope fractionation, which is caused by the different transport reaction characteristics of light and heavy isotopes during diffusion and/or transport across the boundary layer from soil solution to the plant roots, to alter the calculated fractionation factors (O’Neil, 1986). These two processes may partially explain the highly depleted values in the early growing. A further enrichment in 26Mg occurs as Mg becomes involved in the construction of cell walls (Bolou-Bi et al., 2010). Indeed, the Mg isotope composition of larch needles from the south-facing slope measured in June is lighter by 0.25& compared to that in September (Fig. 6). A similar enrichment in 26Mg over the duration of the summer period is observed for the peat bog. In this latter case however, the difference between the beginning and the end of the summer period is limited to 0.15&. The systematic enrichment of the larch biomass in 26 Mg over the course of the vegetative season most likely reflects changes in the Mg source with increased thawing depths. This explanation is based on the fact that the active layer thickness progressively increases with depth during the growing season; thus the source of solutes may follow this increase. Indeed the mineral soil (C horizon) is significantly heavier than the A–B horizon and organic layer (Fig. 4B), and thus an enrichment of the heavy isotope in larch needles may reflect the increase in the availability of heavy Mg from deeper horizons. Following the recent study of Viers et al. (2013) on major and trace elements in larch needles over the vegetation season, we infer that in the Tura forest site the uptake and resorption of the nutrients is rather fast, at the scale of less than a week. The timescale of clay crystallization is certainly much longer, at the scale of months. As such, the observed seasonal variations of Mg isotope composition are likely to reflect the change in element source in the soil (depth of thaw layer) and the fractionation during transport from root to leaves rather than complex physico-chemical processes in the soil system including contemporary secondary mineral formation. As shown in Fig. 6, the thawing depth is limited to 20 cm in the peat bog but it extends down to 120 cm in the south-facing slope. This difference may also explain why the enrichment in 26Mg in larch needles from the south-facing slope is significantly larger than that observed

Fig. 6. Seasonal Mg concentrations (A) and isotopic evolution of Mg in larch needles from the south-facing slope and peat bog (B) and the changes in active layer thickness.

in the peat bog. Interestingly, a similar mechanism for the effect of progressive soil thaw on Zn isotope fractionation in larch needles has been proposed for this permafrost-bearing environment (Viers et al., 2014). According to these authors, the increase in the depth of the rooting zone and the decrease in the dissolved organic carbon (DOC) concentration in the root uptake zone during the soil thaw could allow progressive enrichment of larch leaves in heavier isotopes in the course of the vegetative season. For Mg, however, the effect of complexation with the DOM of the soil solution should be very weak because (1) the Mg2+-humic and fulvic acid complexation constants are much weaker than those of Zn2+ (Martell et al., 2003) and (2) there is a lack of measurable Mg isotope fractionation between Mg2+(aq) and Mg complexes with organic and organomineral colloids (1 kDa – 0.22 lm; Ilina et al., 2013). Interestingly, the fractionation of d44/40Ca between soil solution and <2 mm roots of larch has been previously reported to be +0.2& (Bagard et al., 2013), a value which is significantly lower than that reported for plants from other temperate and tropical systems. For d26Mg in larch, this difference is 0.2& to 0.3&, i.e., the roots are slighter heavier than the soil solution. One of the reasons for this difference between Ca and Mg could be the lack of insoluble secondary Mg mineral phases within the plant organs. Because these minerals are known to preferentially uptake

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light isotopes, there will be no enrichment of the heavier Mg isotope in the remaining fluid. Although the degree of Mg isotope fractionation during the release of Mg from degrading plant litter (larch needles) is currently unknown, by analogy with Ca, we suggest that this fractionation does not exceed 0.1& (Bagard et al., 2013). Because the degradation of plant litter alone is capable providing 90% of the annual Mg flux in the Central Siberian rivers (Pokrovsky et al., 2006), the organic-rich soil of the Mg litter pool should be able to greatly contribute to the riverine Mg isotopic signature. This contribution will be most pronounced at the highest discharges in the autumn and the following spring and will be controlled by the degradation of larch litter produced in September, which has the heaviest isotopic composition. 5.3. Factors that control the Mg isotopic signatures in riverine waters Both the rivers examined in this study exhibit measurable seasonal variations in the Mg isotope composition of the aqueous phase (Fig. 2). The major difference in the Mg isotope distribution between the two rivers is the 0.4– 0.5& depleted d26Mg values in the N. Tunguska river water during the winter baseflow relative to that of the Kochechum River. This difference is limited to 0.2–0.3& during the summer baseflow (Fig. 2). Note also that the observed changes in Mg isotope composition are accompanied with large changes in the Mg content in the two rivers, as shown in Fig. 2 where the N. Tunguska is generally 2–3 times more concentrated with Mg than the Kochechum River. The Mg isotope variations of the N. Tunguska river however, cannot be explained by simple dilution from snow melt in the late spring/early summer period. A plot of d26Mg as a function of Mg/Na concentration ratio (Fig. 7) indicates the existence of two pools of Mg: isotopically light at high concentration of Mg, mostly pronounced during winter baseflow in the Nizhnaya Tunguska River, and isotopically heavier pool present at high water level in the Kochechum River. The observed seasonal variations in the Mg isotope composition of riverine waters stems from the combination of (i) surface weathering, plant uptake and release from plant litter, which is similar for both rivers; (ii) secondary mineral formation in the deep underground horizons of both river basins, and (iii) different contributions from the dissolution of various bedrocks. The extent of these parameters varies significantly between the two rivers. The processes controlling factor (i) are discussed in Section 5.2. Factor (ii) is tightly linked to the specificity of the watersheds comprising several hundred meters of frozen porous tuffs (N. Tunguska) and massive basalts (Kochechum). During the winter period, the residence time of water in the rocks is significantly higher and deep groundwaters have to follow longer pathways to reach the surface in the permafrost-affected basins of the large rivers. Although the reported Mg isotope fractionation between secondary mineral and remaining fluids is rather small (i.e., 0.1– 0.3&), the Mg incorporation into secondary silicates such as clays is known to preferentially uptake the heavier

Fig. 7. Mg/Na versus d26Mg values for riverine samples from the N. Tunguska and the Kochechum rivers.

isotopes from their formation fluid (Tipper et al., 2006; Teng et al., 2007, 2010; Pogge von Strandmann et al., 2008; Beinlich et al., 2014). Thus, the enrichment of dissolved Mg in 24Mg during the winter baseflow in the Kochechum River, may reflect the formation of secondary Mg-bearing clays in the base rock. The Kochechum river waters are depleted in 26Mg over the low baseflow during the winter when secondary clay mineral formation is likely to occur whereas at high flow period they have isotopic compositions similar to that of the Kuligdakan stream, which only flows during the frost-free season. Thus, it can be assumed that during the spring and summer, the isotopic composition of the dissolved fraction of the Kochechum river waters mirrors the Mg-isotope composition of the topsoil and the shallow host rock horizon as described in the previous section. This assumption is further supported by the fact that the isotopic composition of the N. Tunguska river is closer to that of the Kochechum River and the Kuligdakan stream during high-flow periods, when the majority of Mg originates from shallow surface and subsurface organic and mineral horizons and the secondary mineral precipitation is minimal. The systematic difference in the d26Mg isotopic signatures between the Kochechum and N. Tunguska rivers suggests contributions from an additional source of isotopically light Mg to the N. Tunguska river. This source is most likely to be related to lithology. Indeed, the N. Tunguska river drains partly from the sedimentary rocks of the Siberian platform, which contains Precambrian carbonates with light isotopic composition (d26Mg = up to 2.2&; Pokrovsky et al., 2011); thus, the dissolved Mg fraction of this river exhibits an overall enrichment in 24 Mg compared to the Kochechum River. Indeed the [Mg] vs. [Ca] riverine concentration cross-plot presented in Fig. 8, indicates two distinctive trends for Kochechum and N. Tunguska rivers. The waters of the N. Tunguska river exhibit significantly higher Mg/Ca molar ratios, as well as higher concentration of both Ca and Mg compared to the Kochechum River, indicating the Late Proterozoic – Cambrian dolomite dissolution in the former case. This observation is in agreement with the 87/86Sr composition

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of riverine waters, as it has earlier been presented by Bagard et al. (2011). The 87/86Sr ratio in the N. Tunguska river draining basalts and carbonates is higher (i.e., 0.7087– 0.7086) compared to that of the Kochechum River (i.e., 0.7078–0.7082) draining only Permo-Triassian basalts and tuffs. Note here that the measured dissolution rates of Mg-bearing carbonates are 3–5 orders of magnitude higher than that of Mg-bearing silicates (Schott et al., 2009). Therefore, isotopically light Mg derived from dolomite dissolution should strongly affect the bulk isotopic composition of the river water. Assuming that the surface source of Mg (d26Mgsurface), represented by soil solutions and the Kulingdakan stream water, is close to 0.6 ± 0.1&, the fraction of Mg originating from deep underground water (Xdeep) can be linked to the Mg isotopic composition in the large rivers following Eq. (1): X deep  d26 Mgcarbonates þ ð1  X deep Þ  d26 Mgsurface ¼ d26 MgRiver water

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values for Xdeep (0.1 6 Xdeep 6 0.25) are observed during the summer months whereas the higher values for the Xdeep parameter are present during the winter months. This observation is in good agreement with the presence/effect of permafrost, which immobilizes the active surface layer of the N. Tunguska river watershed during the winter, and making the effect of carbonate-derived Mg more prominent. This treatment yields a proportion of deep carbonate groundwaters ranging from 65% to 10% for the Kochechum River and from 55 ± 5% to 40 ± 5% for the Nizhnyaya Tunguska River. Note that these are the maximum values and reflect the influence of deep groundwater sources during the winter baseflow period. Further studies of the upper reaches of the Nizhnyaya Tunguska River and other tributaries of the Yenisey River during winter are necessary to quantify the relative contributions of carbonate rock dissolution and secondary silicate precipitation in the deep underground reservoirs on the large river isotopic composition at the annual scale.

ð1Þ

This equation neglects the contribution of atmospheric Mg sources to the river which, according to mass balance calculations, does not exceed 10% of the riverine flux in the region (Pokrovsky et al., 2005). This simplification is further justified by the fact that the isotopic signature of Mg atmospheric precipitates (in the form of rain and snow) is close to 0.6& and virtually coincides with that of the surface sources. Furthermore, the equation assumes that during winter low flow, when all the surface layers are completely frozen, the rivers are fed by deep underground water via unfrozen taliks along the river bed (Bagard et al., 2011; Pokrovsky et al., 2013). The value of d26Mgcarbonates can be assessed thanks to a recent study of Mg isotope systematics in Cambrian and Neo-Proterozoic dolomites and magnesites of the Siberian Platform (Pokrovsky et al., 2011). The most reasonable approximation of d26Mg in carbonate rocks of the upper reaches of N. Tunguska basin ranges from 1.5 to 2.0&. Solving Eq. (1) for d26Mgcarbonates = 1.75 ± 0.25& and d26Mgsurface = 0.6 ± 0.1&, the parameter Xdeep varies between 0.15 and 0.45. The lower

Fig. 8. Mg vs. Ca riverine concentration cross-plot for the time period 2007–2009.

6. CONCLUSIONS AND PERSPECTIVES FOR CLIMATE WARMING IN SIBERIA The systematic study of Mg isotopes in various solid and fluid phases of the Central Siberian larch forest permafrostdominated ecosystems developed on basalt rocks, revealed first-order features of Mg fractionation while being taken up by vegetation from soil, migrating to the river and feeding the river through surface and deep groundwater reservoirs, depending on the hydrological season. The Mg isotope composition of larch needles measured in June is lighter than that in August or September. The systematic enrichment of 26Mg in larch needles over the course of the vegetative season may result from (1) progressive changes in the Mg source because of the increasing thawing depths, (2) fractionation induced by changes in the Mg transfer mechanism, and (3) retranslocation processes at the end of the season. The d26Mg of the large rivers exhibits significant seasonal variations. In winter (October–May), depletion in 26 Mg can be observed whereas at the maximum discharges, during the spring flood (beginning of June), the river water is enriched in 26Mg. During spring flood, the isotopic composition of the dissolved fraction mirrors the Mg-isotope composition of the top soil organic and mineral layers, which are similar to that of the host rocks. The high discharge during this time of the year almost excludes the formation of secondary phases that alter the Mg isotope composition of the aqueous phase. Similar processes affect the isotopic composition of the dissolved fraction in the N. Tunguska River. The observed difference between the high and low flood season is likely the result of the longer residence time of deep underground waters within watershed. This longer residence time allows a higher degree of interaction between the reacting fluids and the surrounding sedimentary (carbonate) rocks that are depleted in 26Mg. This scenario is consistent with the seasonal behavior of both the large rivers examined in this study. Overall, the magnitude of both the Ca and Mg isotope fractionation between the soil, plants and surface waters

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in Central Siberian basalts covered by larch forest on continuous permafrost is much lower than that in other, non-permafrost bearing environments. Moreover, the biologically induced Mg isotope fractionation observed at the soil profile scale during the active season is not directly detectable in the river water. As such, both the long-term and short-term Mg isotope signatures of riverine fluxes in Central Siberia are not likely to be significantly affected by the change in soil temperature regime, vegetation productivity and permafrost thaw linked to the occurring climate change. On the scale of other large Siberian rivers, one may anticipate quite low d26Mg values, close to 1& (light Mg) in rivers of the Eastern Siberia, which drain Early Phanerozoic and Precambrian carbonate rocks, such as the Lena River, the rivers of the European Russian boreal zone such as Severnaya Dvina or Pechora, which draining sedimentary rocks. In contrast, rivers draining igneous and effusive rocks, including granites (Aldan, upper reaches of Anabar) and basalts (Kotyi, Kheta), are likely to bear less negative d26Mg. However, because the majority of the flux occurs during the spring high flow, the global value of d26Mg from Russian boreal rivers to the Arctic Ocean between -0.5 and -1.5 &, similar to Central Siberian rivers (Table 1, Fig. 7) seems to be a reasonable approximation. Climate warming is unlike to significantly impact the isotopic signature of Mg flux in Siberian rivers. ACKNOWLEDGMENTS We are grateful to Je´roˆme Chmeleff for his assistance with the Mg isotope measurements. This study was supported EU project CarbFix (FP7-ENERGY-2011-1-283148), the French-Siberian Center of Education and Research, and by the BIO-GEO-CLIM Grant No 14.B25.31.0001 of the Russian Ministry of Science and Education. The associate editor, M. Novak and three anonymous reviewers are thanked for their constructive comments on our manuscript.

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