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Geochimica et Cosmochimica Acta 73 (2009) 2215–2228

The impact of water–rock interaction and vegetation oncalcium isotope fractionation in soil- and stream waters

of a small, forested catchment (the Strengbach case)

B. Cenki-Tok a,b, F. Chabaux a,*, D. Lemarchand a, A.-D. Schmitt c, M.-C. Pierret a,D. Viville a, M.-L. Bagard a, P. Stille a,*

a Universite de Strasbourg et CNRS, Laboratoire d’Hydrologie et de Geochimie de Strasbourg, Ecole et Observatoire des Sciences de la Terre,

1, rue Blessig, 67084 Strasbourg Cedex, Franceb Universitat Bern, Institut fur Geologie, Baltzerstrasse 1+3, 3012 Bern, Switzerland

c Universite de Franche-Comte et CNRS (UMR 6249 Chrono-environnement), 16, Route de Gray, 25030 Besanc�on Cedex, France

Received 7 May 2008; accepted in revised form 8 January 2009; available online 4 February 2009

Abstract

This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially therespective role of vegetation and water–rock interactions on Ca isotope fractionation in a continental forested ecosystem.The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathwaythrough the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, perco-lating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in thenortheast of France in the Vosges mountains. The d44/40Ca values of springs, brooks and stream waters from the catchmentare comparable to those of continental rivers and fluctuate between 0.17 and 0.87&. Soil solutions, however, are significantlydepleted in lighter isotopes (d44/40Ca: 1.00–1.47&), whereas vegetation is strongly enriched (d44/40Ca: �0.48& to +0.19&).These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, withdepletion in ‘‘light” calcium in the soil solutions from deeper parts of the soil compartments due to preferential 40Ca uptakeby the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soilsolutions most probably associated with water–rock interactions. The stream waters are marked by a seasonal variation oftheir d44/40Ca, with low d44/40Ca in winter and high d44/40Ca in spring, summer and autumn. For some springs, nourishingthe streamlet, a decrease of the d44/40Ca value is observed when the discharge of the spring increases, with, in addition, a clearcovariation between the d44/40Ca and corresponding H4SiO4 concentrations: high d44/40Ca values and low H4SiO4 concen-trations at high discharge; low d44/40Ca values and high H4SiO4 concentrations at low discharge. These data imply that dur-ing dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals,whereas during wet periods and high flow rates admixture of significant quantities of 40Ca depleted waters (vegetationinduced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearlyemphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water–rock–vegetation interface in asmall forested catchment.� 2009 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2009.01.023

* Corresponding authors.E-mail addresses: fchabaux@illite.u-strasbg.fr (F. Chabaux),

pstille@illite.u-strasbg.fr (P. Stille).

1. INTRODUCTION

Understanding elemental fluxes at global scale is oneof the principal goals of current research in terrestrialbiogeochemical cycling. In order to shed some light on

2216 B. Cenki-Tok et al. / Geochimica et Cosmochimica Acta 73 (2009) 2215–2228

rock–water–soil–plant interactions and unravel transferprocesses the application of classical isotopic techniqueshas been reinforced by the development of new isotopetools in geochemistry. Advances in analytical techniqueshave opened doors to the use of a series of non-conven-tional stable isotope systems (e.g. Fe, Ca, Mg, Si; see reviewarticle of Johnson et al., 2004).

Calcium isotope fractionation in marine carbonate-phosphate systems is well documented (e.g. Fantle andDePaolo, 2007; Farkas et al., 2007; Lemarchand et al.,2004; Schmitt et al., 2003a; Nagler et al., 2000; Zhu andMacDougall, 1998; Skulan et al., 1997). These studiesmainly focused on the impact of river input on the oceanicCa cycle by analyzing large-scale rivers as they are part ofglobal geochemical cycles that control climate and phos-phate/carbonate sediments. However, only a few studiesare dealing with calcium isotope fractionation in continen-tal ecosystems (e.g. Ewing et al., 2008; Holmden and Bel-anger, 2006; Schmitt et al., 2003b; Tipper et al., 2006,2008) and on the impact of biological activity on the Caisotope fractionation at the soil–water–plant–atmosphereinterface (Page et al., 2008; Wiegand et al., 2005) althoughit has already been shown that biological Ca isotope frac-tionation is an important process within the biogeochemi-cal Ca cycle (Skulan et al., 1997) and might have aconsiderable impact also on the Ca isotopic compositionof soil, soil solutions and probably river waters (Pageet al., 2008; Wiegand et al., 2005). Until now, the biogeo-chemical cycle of calcium has widely been studied in differ-ent ecosystems, however, without consideration of the Caisotope system; as for instance the Hubbard Brook Exper-imental Forest which has been observed for 50 years tomonitor Ca depletion in soils (Likens et al., 1998). Thisdepletion was suggested to be the result of leaching dueto interactions between soil minerals and acid rain inputsand due to deforestation causing a Ca depletion in the bio-mass reservoir (Blum et al., 2002). In this case it would beof great interest to know in how far these processes havehad an impact on the Ca isotope fractionation not onlyin vegetation but also soils and soil solutions. Similarlythe behavior of Ca isotopes during weathering or duringadsorption/desorption processes at the water–mineralinterface is still a matter of discussion. It appears thatCa isotopes are reactive to weathering (Ewing et al.,2008; Tipper et al., 2006). For example Tipper et al.(2006) showed that Ca together with Mg isotopes fraction-ate during weathering due to dissolution and precipitationof secondary minerals.

The aim of this study was to follow changes in spaceand time of the calcium isotopic compositions along thepathway of Ca through the hydro-geochemical reservoirsfrom atmospheric deposits to the outlet of the watershedvia throughfalls, soil solutions and springs in order toidentify processes controlling the Ca transport and theCa isotopic fractionation at the small catchment scale.Of special interest was to determine the role of vegetationand alteration on Ca isotope fractionation. This is neces-sary in order to transpose later on the fractionating effectsof vegetation and alteration on a more global scale of Cacycling.

2. MATERIALS AND METHODS

2.1. Site and sample description

To investigate the calcium biogeochemical behavior incontinental ecosystems, we focused on the Strengbachcatchment, which is a small forested watershed under tem-perate climate, located in the northeast of France in theVosges mountains ca. 60 km south of Strasbourg (Fig. 1).Since 1986 this site is a hydro-geochemical observatory(Observatoire Hydro-Geochimique de l’Environnement –http://ohge.u-strasbg.fr). It has been originally set to mon-itor the influence of acid rain on the depletion in nutrientelements in soils (Probst et al., 1995). The basement ofthe watershed is composed of Paleozoic granitic and meta-morphic rocks on which nutrient-poor soils have developed(Probst et al., 1992). Ninety percentage of the watershedsurface is covered by forest in which 80% are spruce(mainly Piceas Abies L) and 20% are beech trees (Fagus

sylvatica).For the characterization of precipitation and atmo-

spheric deposits, two rainwater and snow samples (PA col-lected at the highest point of the watershed; Fig. 1) as wellas two throughfall samples (PLH collected at the beech plotusing gutters) were analyzed (Table 1). The Strengbachstream was sampled regularly at the outlet (RS) of the wa-tershed at different stages of the hydrological cycle during2004 and 2005. The main springs emerging within the wa-tershed (SH, SG, BH, RH) as well as waters from the satu-rated zone (RUZS) were sampled at two differenthydrological periods (see Fig. 1 and Table 1). In addition,soil solutions were collected at the two experimental plotsdominated by different tree types, the beech plot (BP), lo-cated on the southern slope of the watershed and the spruceplot (SP), located on the northern one (Fig. 1). Soil solu-tions were collected by zero-tension lysimetric plates at�10 and �70 cm in the BP and at �5, �10, �30, �60 cmin the SP at four different periods (See Table 1). We alsoanalyzed leaves/needles, branches and roots of a beech treeand a spruce tree collected at the BP and SP, respectively, infall 2005 as well as one litter sample from each of the twoplots. We were also able to collect a few ml of sap by suc-tion under vacuum from a beech branch. All samples weremeasured at the Center de Geochimie de la Surface (nownamed by LHyGeS) in Strasbourg for calcium isotopes aswell as for major cation (Ca, Na, K) and Sr concentrations.In addition some samples collected at the outlet (RS) wereanalyzed for Sr isotopes.

2.2. Analytical techniques

2.2.1. Determination of major ion concentrations in waters

Water samples were collected in acid cleaned polyethyl-ene (HDPE) Nalgene� bottles and filtered through 0.45 lmMillipore� filters. The aliquot used for trace element andCa and Sr isotope analyses were stored under acidic condi-tions using HNO3 and kept in dark cold room. A non-acid-ified aliquot of filtered water, stored in polyethylene bottle,was used for major element concentration measurements(Na, K, Ca) by flame atomic absorption spectrometry with

Fig. 1. Map of the Strengbach catchment with sampling sites.

Ca isotope fractionation in soil- and stream waters 2217

a Perkin Elmer 430 spectrometer. Strontium concentrationswere determined by ICP-MS.

Preparation of vegetation samples: Vegetation sampleswere washed with deionised water and dried in an ovenat 60 �C. The dried material was reduced to powder usingan agate mortar. Hundred to 150 mg were weighed andpoured in a savillexTM beaker. The sample was covered with15 ml of HNO3 and 1 ml H2O2 and left overnight. The nextday the sample has been put for 15 min in ultrasonic bathand then concentrated on the hotplate at 120 �C. In casesolid residues left, HF was added and the beaker was keptat 70 �C for 24 h. Once the whole sample was dissolved, itwas evaporated to dryness. Organic compounds were re-moved by adding a mixture of H2O2 and HNO3 to thedry residue. After a 15 min ultrasonic bath, the solutionwas left overnight (cold) and, the next morning, when allorganic matter was destroyed, the samples were evaporatedto dryness.

2.2.2. Ca purification and isotopic determination

Ca isotopic compositions were determined by PTIMS(Triton Finnigan) using the double-spike 43Ca/48Ca tech-nique. Before Ca purification 5 lg of Ca sample was mixedwith the double-spike; this allows the correction of possible

isotopic fractionations due to Ca loss during the chemicalprocedure. The sample/spike ratio is adjusted to48Caspike/

48Casample = 9 to minimize the error propagation(Heuser et al., 2002).

The Ca of the sample-spike mixture has been extractedby ion chromatography using a cation exchange resin (Bio-rad AG 50Wx8 200–400 mesh) with 1.5 N HCl as eluantonto 0.6 mm diameter quartz columns filled with 2 ml of re-sin (for more details, see Schmitt et al., 2003a,b). In order toeliminate isobaric interferences caused by the presence ofK, Mg and Sr, the elution curve was truncated, leading toa recovery yield of about 70% of Ca. The total procedureblank for Ca isotopic determination (for water as well asfor vegetation samples) never exceeded 25 ng and no correc-tion for contamination was therefore applied. The equiva-lent of about 500 ng of Ca dissolved in 1.5 N HCl wasloaded onto outgassed Re 99.98% mono filaments and cov-ered by 0.5 ll of activator (TaO5 and phosphoric acid) at0.7 A. Ca isotopes were measured in two sequences s1:masses 40, 41, 42, 43 and 44 using L3, L1, C, H1, H3 cups,respectively, and s2: masses 44 and 48 using L2 and H2,respectively. The 43Ca–48Ca spike solution was calibratedfollowing the method classically used in the literature (i.e.Lemarchand et al., 2004). Reduction of the data to achieve

Table 1Ca isotope compositions and major element concentrations of all samples.

Sample Description Date Ca Ca1 Na K K1 H4SiO4 d44/40Ca* 2r** n*** Sr 87Sr/86Sr(lm/L) (lm/L) (lm/L) lm/L) (lm/L) (lm/L) ref.

SRM915ppb

Water

RS Outlet 28. Sept. 2004 70 90 18 140 0.72 0.17 3 11.5 0.72479RH Source 28. Sept. 2004 106 97 30 138 0.68 0.17 3 15.39BH Source 28. Sept. 2004 95 100 29 154 0.40 0.17 3 10.69SG Source Gneiss 28. Sept.2004 125 103 23 141 0.17 0.17 3 14.35CR Source collector 28. Sept.2004 74 96 20 157 0.40 0.17 3 11.03RUZS Saturated zone 28. Sept. 2004 36 89 6 103 0.28 0.17 3 5.01SH Source 28. Sept. 2004 23 84 16 144 0.82 0.21 2 3.71RS Outlet 22. May 2006 64 82 17 115 0.4 0.17 3 10.44 0.725195BH Source 22. May 2006 75 84 16 115 0.87 0.15 3 9.46SG Source Gneiss 22. May 2006 111 90 21 124 0.72 0.21 2 13.42CR Source collector 22. May 2006 66 83 18 133 0.35 0.15 3 10.18RUZS Saturated zone 22. May 2006 41 63 5 40 0.47 0.17 3 6.34SH Source Beech 22. May 2006 24 76 18 117 0.64 0.21 2 4.12

Outlet (RS)discharge

(L/s)

11.5 31. May 2005 65 78 13 0.73 0.21 2 13.35 0.7251748.54 29. March

200568 79 20 0.40 0.21 2 12.0 0.72573

20.56 3. May 2005 63 76 15 0.48 0.17 3 10.03 0.725344.8 11. July 2005 63 80 20 0.79 0.15 3 9.68 0.724665.45 28. Sept. 2004 70 90 18 0.72 0.21 2 11.50 0.7247959.3 2. Nov. 2004 73 84 20 0.77 0.15 3 11.05 0.7253612.0 13. Dec. 2004 75 88 19 0.41 0.21 2 11.10 0.7251942.6 24. Jan. 2005 73 83 21 0.83 0.21 2 12.29 0.72517

Soil solutions – spruce

F-5 �5 cm 28. Sept. 2004 11 21 70 59 43 46 0.85 0.17 3 2.54F-5 �5 cm 7. April 2006 21 71 36 62 1.00 0.15 3 4.42F-10 �10 cm 7. April 2006 12 13 39 19 44 42 0.77 0.15 3 4.12F-30 �30 cm 7. April 2006 15 13 69 15 20 88 0.78 0.17 3 1.9F-60 �60 cm 7. April 2006 12 10 1010 10 11 71 0.89 0.15 3 1.09F-5 �5 cm 22. May 2006 17 46 18 43 0.69 0.17 3 4.24F-10 �10 cm 22. May 2006 15 49 21 64 0.76 0.17 3 4.76F-30 �30 cm 22. May 2006 8 31 12 56 0.86 0.17 3 1.61F-60 �60 cm 22. May 2006 7 117 12 68 0.75 0.17 3 1.55

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Soil solutions – beech

E4 �10 cm 22. May 2006 5 7 30 14 22 34 1.00 0.21 2 1.05E5 – 60 cm 22. May 2006 7 2 52 2 2 99 1.47 0.21 2 1.05E4 – 10 cm 3. May 2005 5 34 19 54 0.97 0.17 3 1.00E5 – 60 cm 3. May 2005 1 64 1 99 1.41 0.17 3 0.52

Precipitation

PA Rainwater 22. May 2006 5 12 29 0.57 0.21 2 1.05PA Snow 24. Jan. 2005 3 34 2 1.29 0.21 2 3.73PA Rainwater 22. August

20054 4 4 0.93 0.17 3 0.4

PLH Throughfall 22. August2005

15 12 52 0.29 0.17 3 1.42

PLH Throughfall 22. May 2006 5 12 138 0.8 0.21 2

Ca Na Kppm ppm ppm

Vegetation (beech)

LP37 Upper branches Ø

1 c21. Sept. 2005 2303 15 1729 -0.01 0.17 3 6875

0.09 0.12 1LP34 Roots Ø 2–4 mm 13. Sept. 2005 2965 14 2324 -0.48 0.09 1 10601LP36 Upper leaves 21. Sept. 2005 5460 196 11253 0.64 0.21 2 5816LP38 Roots Ø 1–1.5 cm 13. Sept. 2005 1229 15 2114 0.08 0.15 3 5382LP39 Finest roots 13. Sept. 2005 1419 234 2651 0.17 0.21 2 7956LP40 Lower leaves 13. Sept. 2005 6698 169 9521 0.24 0.15 3 9858LD Sap 1.08 0.2 12.3 0.16 0.15 1 9.1

Vegetation (spruce)

ELV2 Roots Ø 2–4 mm 23. May 2006 2647 93.5 3415 0.01 0.09 1 59000.07 0.18 1

ELV6 Wood 23. May 2006 594 11 338 0.42 0.15 1 6300ELV5 Young needles 15. May 2006 457 7.5 11293 0.28 0.15 1 400ELV3 Older needles 15. May 2006 1736 35.5 4080 0.8 0.12 1 1200

Litter

LVP (LP24) Spruce litter 3220 0.64 0.10 1LHP (LP29) Beech litter 5292 0.50 0.10 1

1 Sampling campaign 2005 (n = 5).* d44/40Ca = {(44Ca/40Ca)sample/(44Ca/40Ca)ref. – 1)*1000}.

** 2r mean when n > 1 and 2rerror when n = 1.*** Number of analyses.

Ca

isoto

pe

fraction

ation

inso

il-an

dstream

waters

2219

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.01 0.02 0.03 0.041/Ca

water (28/09/04)

water (22/05/06)

-1.0

-0.5

0

0.5

1.0

1.5

0 0.2 0.4 0.6 0.8 1.0 1.2

waters

outlet

soil solutions

precipitation

vegetation

δ 44/4

0 Ca

(‰)

1/Ca

SH

RUZS

RS

RS

BH

SGRH

BH

SG

CR

2σ

2σ

outlet range (Schmitt et al., 2003; this study)

snow

rainwaterthroughfallrainwater

throughfall

δ 44/4

0 Ca

(‰)

a

b

Fig. 2. d44/40Ca versus 1/Ca. (a) For stream waters, sources,brooks and soil solutions, rainwater and vegetation. Only theprecipitation samples define a well correlated trend. (b) Close upfor sources and brooks variations.

2220 B. Cenki-Tok et al. / Geochimica et Cosmochimica Acta 73 (2009) 2215–2228

d44/40Ca values has been performed using the Newton–Raphson iteration technique (Albarede and Beard, 2004).This technique has the advantages to be quite robust, inparticular in being not sensitive to initial guesses as re-ported for other techniques (Heuser et al., 2002), and torapidly converge to precise analytical results. Calculationsare performed using the optimization toolbox provided bythe commercial Matlab� software. Repeated analysis ofsamples during the period of study yielded an external pre-cision (2rext) of ±0.30&. Each sample was measured threetimes in average, allowing to improve the statistical signifi-cance of a single d44/40Ca analysis. During the course of thework this external reproducibility was improved by apply-ing a protocol adapted from Holmden (2005). Thus exter-nal reproducibility turned down to ±0.1& and thesamples were only measured once (Table 1). It was vali-dated by repeated analysis of the NIST SRM 915 solutionduring the period of the work.

2.2.3. Sr purification and isotopic determination

Sample preparation for Sr isotope analysis is describedin detail elsewhere (e.g. Lahd Geagea et al., 2008). For Srpurification Eichrom’s Sr Resin was used according toPin and Zalduegui (1996). The Sr isotopic compositionswere determined using a fully automatic VG Sector thermalionization mass spectrometer with a 5-cup multicollectionsystem. During the measuring period the NBS 987 Sr stan-dard yielded 87Sr/86Sr = 0.71027 ± 2 (2r, n = 14) (e.g. Dur-and et al., 2005).

3. RESULTS

All results are presented in Table 1, including Ca, K, Naconcentrations and Ca isotopic ratios for all samples as wellas Sr isotopic ratios for the outlet water samples. Ca isotopedata are expressed as the permil deviation from the NISTSRM 915a standard solution: d44/40Ca = {(44Ca/40Ca)sample/(

44

Ca/40Ca)NIST SRM915 � 1)*1000}.

3.1. Precipitations

The rainwater, snow and throughfall samples show alarge range of d44/40Ca values (from 0.29 to 1.29&) andCa concentrations (from 3 to15 lmol/L; Fig. 2a). These re-sults are in good agreement with earlier published valuesfor samples collected in the same watershed (Schmitt andStille, 2005). Ca concentrations and d44/40Ca values for pre-cipitation are anti-correlated, with the snow sample havingthe highest d44/40Ca and the lowest concentration and sum-mer throughfall sample having the lowest d44/40Ca valueand the highest concentration(Fig. 2a).

3.2. Stream and spring waters

The Ca isotopic compositions of stream and springwaters collected over a 2 years sampling period (2004–2006) range from 0.17 to 0.87&. They are comparable withvalues previously published for the outlet of the watershed(Schmitt et al., 2003b). The magnitude of the variation ofd44/40Ca in stream and spring waters does not exceed 1&,

which is similar to the range observed at the worlds riverscale (Schmitt et al., 2003b; Tipper et al., 2006; Zhu andMacDougall, 1998) and emphasizes the very small magni-tude of the d44/40Ca variations in surface waters. The Caconcentrations range from 20 to 125 lmol/L. However, itappears from one sampling location to another that theCa concentration remains rather constant during the watercycle, whereas Ca isotope ratios are comparatively morevariable and tend to follow seasonal cycles (Fig. 3). Duringspring, summer and autumn, the d44/40Ca signature at theoutlet is clearly depleted in lighter isotopes (d44/40Ca:0.72–0.83&). The stream water samples collected in winter-time, however, point to a significant light isotope enrich-ment in the waters (d44/40Ca: 0.4–0.48&). These d44/40Cavalues are close to 0.4& found in apatite from the samecatchment (Schmitt et al., 2003b). Only the sample collectedin the winter 2005 is outside of this trend. This sampleshows 40Ca depletion and a summer-like d44/40Ca value of0.83&. This sample was collected during massive snow

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

8/1 10/1 12/1 2/1 4/1 6/1 8/1

apatite

2σ

δ44/4

0 Ca

(‰)

2004 2005

Fig. 3. Seasonal variation of the d44/40Ca of the Strengbach watersat the outlet of the catchment (filled diamonds); open diamond:snow melt.

Ca isotope fractionation in soil- and stream waters 2221

melting and corresponds to an elevated water discharge(>50 L/s). The snow sample collected the same day is char-acterized by an even higher d44/40Ca (1.29&). We thereforeconclude that the high stream water d44/40Ca most likely re-flects a major contribution of the melting snow to thestream water discharge.

The streamlet at the outlet and the different springs showthe same range of variation in d44/40Ca (Fig. 2b; d44/40Ca0.18–0.9&). SH and RUZS can be distinguished by theirlower Ca concentration (41 and 24 lm/L, respectively).The amplitude of variation with time of the d44/40Ca atBH and SG, the main springs of the catchment, is compa-rable to that of RS (ca. 0.5&). Contrary to what proposedby Schmitt et al. (2003b) there is no clear relation betweendissolved d44Ca and the water discharge. By contrast, the87Sr/86Sr isotope ratios of the stream water samples varysimilarly to the observations of Aubert et al. (2002) in func-tion of the discharge at the outlet (Fig. 4). The results indi-cate that during low water flow periods (discharge <5 L/s)the waters have less radiogenic Sr isotope ratios (0.7247)than during high water flow periods (>10 L/s; >0.725).

Fig. 4. Relationships between Sr isotopic compositions and thedischarge of the stream water at the outlet.

3.3. Soil solutions, vegetation and litter

Soil solutions have highd44/40Ca values compared tostream water, springs and brook samples, and lower andmore scattered Ca concentrations (Fig. 2a). The Ca concen-trations of the soil solutions from the spruce plot are signif-icantly higher than those in the beech plot; but at bothplots, the Ca concentrations of the soil solutions, and Cafluxes decrease with depth (Fig. 5; Table 1; average valuesof 11 samples collected over a period of 4 years). Similaris the behavior of K, which shows concentrations of up to0.038 lmol/L at the surface and 0.014 lmol/L at 60 cmdepth at the spruce plot and 0.018 and 0.002 lmol/L,respectively, at the beech plot. By contrast Na concentra-tions and Na fluxes tend to increase in the deeper part ofthe both plots. d44/40Ca values in soil solutions are the high-est ones found in the basin and range between 0.69 and1.47&. Thed44/40Ca values of the soil solutions from thespruce plot collected at three different hydrologic periodsdo not show any variation with depth and with time exceptfor the surface sample (F-5) whosed44/40Ca values varyfrom 0.69 to 1& (Fig. 6). In contrast, the soil solutions col-lected at the beech plot show significantly different d44/40Cavalues at 10 and 60 cm depth (1.00 and 1.47&, respec-tively), indicating that the deeper soil solution #E5 is de-pleted in the lighter isotopes compared to the shallowersoil solution #E4.

Vegetation is the reservoir with the lowest d44/40Ca val-ues and highest Ca concentrations (Fig. 2a). Its d44/40Cavalues are largely scattered, but the range of variationsseems to be quite similar for both beech and spruce treesamples. In addition, for both trees, roots and branchesare enriched in 40Ca (d44/40Ca: �0.50 to 0.08&), whereasleaves appear to be the most enriched in 44Ca (d44/40Ca:+0.64 to +0.24&). The two litter samples show d44/40Cavalues very similar to those found for leaves and needles.A very thin root sample (LP 39) is also enriched in the hea-vy 44Ca (d44/40Ca: +0.17&) compared to the larger rootsand has higher Na/Ca ratios (0.16). It may still containtraces of primary granite-derived minerals or secondary soilminerals that are enriched in Na (e.g. albite). Therefore, inagreement with Page et al. (2008), the isotope data of thebeech and spruce samples suggest that Ca fractionationoccurs during the transfer of Ca from tree roots to itsleaves.

4. INTERPRETATIONS AND DISCUSSION

4.1. Atmospheric inputs of calcium

Overall, dissolved Ca in river water has two distinctsources; one is the atmosphere (Drouet et al., 2005; Likenset al., 1998; Schmitt and Stille, 2005) and the other is theweathered continental bedrock (Aubert et al., 2001; Dijk-stra and Smits, 2002; Dijkstra et al., 2002; Drouet et al.,2005). Previous mass balance calculations suggested thatabout 50% of the Ca released out of the Strengbach basinwould derive from weathering (Probst et al., 2000). Thus,the other 50% of the dissolved Ca in stream water wouldhave an atmospheric origin (both wet and dry deposits).

0 5 10 15 20 25flux Na (kg/ha/yr)

spruce

beech

0 1 2 3 4 5 6 7 8flux Ca (kg/ha/yr)

70

60

50

40

30

20

10

0

0 5 10 15 20

dept

h (c

m)

flux K (kg/ha/yr)

Fig. 5. Depth dependent variation of Ca, Na and K fluxes.

δ 44/40Ca(‰)-1 - 0.5 0 0.5 1 1.5 2

soil soln (beech)

soil soln (spruce)

beech tree

sap (beech)

spruce tree

A 0

-10

-30

-60

dept

h (c

m)

B

δ 44/40Ca0.5 1.0 1.5

Fig. 6. (A) d44/40Ca fractionations in beech and soil solutions from spruce and beech plot and corresponding trees. (B) d4/40Ca variation infunction of depth in soil solutions from beech and spruce plots.

2222 B. Cenki-Tok et al. / Geochimica et Cosmochimica Acta 73 (2009) 2215–2228

Comparison of Sr and Nd isotope data in stream waterswith those of throughfall, lichen, and apatite however sug-gests that the stream waters Sr and Nd is not atmospherebut mainly apatite derived (Aubert et al., 2002; Stilleet al., 2006). Thus, by analogy with Sr the atmospheric im-pact on the Ca budget of the stream water might be com-paratively small. A weak atmospheric contribution ofmax. 15% can also be deduced by comparing the mean an-nual rainwater Ca flux on the watershed with the Ca fluxexported from the watershed at its outlet (Table 2). How-ever, it is true that a more important atmospheric contribu-tion (20–40%) might be deduced from beech or sprucethroughfall. Indeed, throughfall contains a higher Caamount than rain water collected outside the vegetal cover.However, even if throughfall Ca includes wet and dry atmo-spheric Ca depositions and hence constitutes an externalsource for the Ca budget of the catchment surface waters,

it also contains significant amounts of recycled Ca from leafand needle exudation, which has not to be considered as anexternal Ca source. The real contributions of these two po-tential Ca pools are difficult to constrain. Ca data give nev-ertheless some indications:

All the precipitation samples analyzed for this studytend to be aligned in a d44/40Ca vs. 1/Ca diagram(Fig. 2a) where the low [Ca]/high d44/40Ca end-member cor-responds to the snow and high [Ca]/low d44/40Ca to one ofthe two throughfall samples with d44/40Ca values close tothose measured in leaves and needles; this 40Ca enrichedend-member might therefore reflect strong interaction ofrain water with the vegetal cover where leaves and needlesexcrete Ca, Na and K (Dambrine et al., 1998). Alterna-tively, the high [Ca]/low d44/40Ca end-member characteris-tics are similar to those of carbonate rocks (De La Rochaand DePaolo, 2000) and, therefore, might also point to

Table 2Ca, K, Na and water fluxes in the two soil profiles of the catchment.

Ca d44/40Ca K Na Water fluxeskg/ha/yr kg/ha/yr kg/ha/yr mm

Rain (average) 2.56 1247Througfalls under spruce tree 9.46 1024Troughfalls under beech tree 4.71 1254Soil sol. 5 cm/spruce/F-5 7.95 0.7–1 15.24 13.27 1024Soil sol. 10 cm/spruce/F-10 F1: 5.59 0.8 20.26 12.15 966Soil sol. 30 cm/spruce/F-30 4.73 9.83 11.59 904Soil sol. 60 cm/spruce/F-60 F2: 3.34 0.8 4.93 20.25 895Soil sol. 10 cm/beech/E4 F1: 3.21 1 8.83 12.73 1258Soil sol. 70 cm/beech//E5 F2: 0.95 1.5 1.08 16.33 1154Outlet (RS) 21.42 800

A daily water balance model developed by Granier et al. (1999) has been used to estimate the soil drainage at different depths. This modelsimulates the dynamics of soil water depletion and recharge, and predicts the main components of forest water balance.

Ca isotope fractionation in soil- and stream waters 2223

the presence of carbonate dust in the throughfall sample.The position of the data points in the d44/40Ca – K/Ca mix-ing diagram (Fig. 7a) strongly suggests, that both leafexcretion and leaching of dry atmospheric deposits con-trolled the Ca isotopic composition of throughfall. The po-sition of the data points in such a diagram would implyindeed the contribution of at least three different end-mem-bers: one close to a seawater value, another one close torainwater with a significant carbonate signature (Schmittand Stille, 2005), and the third one with d44/40Ca valuessimilar to those of leaves and needles and high K/Ca ratios.This third end-member is most probably dominated by leafexcretion. Its high K/Ca ratio is indeed consistent with thehigh K/Ca values found in young needles and leaves as wellas in tree sap (Table 1). The data confirm therefore previoussuggestions that at a catchment scale the atmospheric Cabudget can be strongly affected by biologic activity and veg-etation excretions (Schmitt and Stille, 2005; Chabaux et al.,2005), especially in spring and summer. During winter theimportance of the canopy in controlling the chemical andisotopic signatures of throughfall is less important andthe rainwater and snow might be less enriched in Ca and

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this study Schmitt and Stille, 2005

Schmitt and Stille, 2005this study

Needles (spruce) Leaves (Beech) Sap (Beech)

SW

CE Sap

δδ44/40Ca(‰)

a

Fig. 7. (a) d44/40Ca versus K/Ca. for throughfall, rainwater, needles, lesolutions (E5) data points and the precipitation sample data points in th

in the lighter Ca isotopes from leave and needle excretions.Thus, at a regional scale, exchange between throughfall andrainwater and admixture of plant derived Ca especially dur-ing spring and summer to rainwater causes an increase ofthe rainwater Ca concentration and an enrichment in itslight 40Ca isotope. Consequently, the determination of theatmospheric Ca contribution to catchment surface watersusing mass balance budget calculations based only on an-nual Ca fluxes of rainwater and/or throughfall annualfluxes has to be used with caution. It gives only upper lim-its; more accurate estimates require other approaches. Ourresults certainly confirm the real potential of coupled iso-tope systems including Ca isotope analysis for such esti-mates (see also Schmitt and Stille, 2005).

4.2. Calcium isotopic fractionation by vegetation recycling

The control of the vegetation on the Ca cycle in forestedecosystems is well established based on Ca flux studies andhas been discussed in detail by several authors for theStrengbach catchment (Fichter et al., 1998; Dambrineet al., 1998b; Poszwa, 2000; Aubert, 2001). As for other

δδ44/40Ca(‰)

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F5 Soil Solution

Sept 04Apr 06

May 06

b

aves and sap. (b) Comparison of the position of the surface soile d44/40Ca versus K/Ca diagram.

2224 B. Cenki-Tok et al. / Geochimica et Cosmochimica Acta 73 (2009) 2215–2228

nutrient elements whose geochemical cycle is regulated bythe development of life, trees quantitatively assimilate Cafrom surface horizons of the soil (mostly between 20 and70 cm depth) and store it in the different parts of their veg-etal system (Dambrine et al., 1992; Poszwa, 2000). Ca re-turns back to soil as the litter falls and decomposes. Theuppermost litter layer of the soil contains not only dead or-ganic matter but is also enriched in atmospheric derivedcomponents. Decomposition of the dead matter (leaf litter)contributes to the formation of humus and allows a Ca fluxback to the deeper soil horizons where it is again taken upby the trees root system. (Johnson et al., 2000). Platznerand Degani (1990) and Schmitt et al. (2003b) proposed intheir pioneer studies on Ca isotopes that calcium recyclingby vegetation fractionates calcium isotopes and has a majorimpact on the isotopic composition of waters percolatingthe ecosystem.

Regular analysis of the soil solutions collected between2004 and 2006 on the two experimental plots allows the cal-culation of the Ca, K and Na fluxes through the soil profiles(Table 2 and Fig. 2). As observed in many forested catch-ments, the increase of the Na fluxes and concentrations insoil solutions with depth (Fig. 5; Table 2) is typical of anelement whose chemical budget is essentially controlled bymineral alteration and dissolution. In contrast, the concen-trations and fluxes of the nutrient elements Ca and K de-crease with depth at both experimental plots (Fig. 5).Since such an important loss of Ca and K in soil solutionsoccurs at the level of the trees root system, below a Ca andK enriched surface soil horizon in the two experimentalplots, we suggest according to Jobbagy and Jackson(2001) that the decrease of Ca (and K) fluxes and concen-trations in the soil solutions with increasing depth is dueto plant uptake and cycling of the nutrient elements. Theoxygen isotope data on soil solutions and correspondingroot systems of conifers from the same forested catchmentconfirm quite convincingly, for instance, that spruce absorbwaters from surface horizons, mainly from 20 to 40 cmdepth (Ladouche, 1997).

Therefore, the fact that soil solutions and vegetationsamples have distinct d44/40Ca values, the latter being typi-cally enriched in lighter Ca isotopes (Fig. 6a), indicates thatvegetation takes up most of their nutrients in soil solutionswith a preference for the lighter isotopes as suggested be-fore (Page et al., 2008; Perakis et al., 2006; Wiegandet al., 2005; Platzner and Degani, 1990; Skulan and DePa-olo, 1999). This causes a shift toward heavier values in theremaining soil solution (Schmitt et al., 2003b). In additionas Beech trees are considered as real Ca-pumps with theirdeeper root system, whereas spruce has a flatter rooting sys-tem and mobilize Ca in the organic matter-rich top (Bergeret al., 2006; Dijkstra and Smits, 2002), fractionation by veg-etation could easily explain why soil solutions collected atthe beech plot are more depleted in 40Ca at 60 cm (1.5&

for E5; Fig. 6b) than at 10 cm (1.0& for E4) (Table 1).However, in case all the Ca loss between samples #E4

and #E5 at the beech plot results from plant uptake, a the-oretical E5 composition can be estimated by mass balancecalculation if no other Ca sink or source are active on ayears base within this soil–plant system:

FCaðF1Þ � d44=40CaF1 F1 ¼ FCaðF2Þ � d44=40CaF2

þ Ftree � e44=40Catree ð1Þ

where FCa(F1) is the input Ca flux at E4, FCa(F2) is the out-put of Ca at 60 cm depth (sample #E5), and Ftree is the Caflux absorbed by plants between E4 and E5 levels (hereFtree = FCa(F1) � FCa(F2)). Using for the vegetation fluxthe two extreme d44/40Catree values of 0.5& (leaves – littersamples) and �0.5& (roots) theoretical d44/40CaE5 valuesof 2.2 and 4.5&, respectively, can be calculated for theE5 soil solution. Such a range of theoretical d44/40Catree val-ues is significantly higher than the measured delta value of1.5&. In terms of mass budget, such a difference mightpoint to the occurrence of a supplementary Ca loss fromsoil solutions within the soil profile of the beech plot witha Ca isotope fractionation greater than that induced bythe vegetation. Alternatively, it might point to the presenceof an additional Ca flux into soil solution with a low d44/

40Ca value. Since the sampling depth of the soil solutionscorresponds to the zone of root uptake where the soil isthe most depleted in Ca (at 10–70 cm depth: 0.03 wt%CaO for spruce plot and 0.13 wt% CaO for beech plot)and where Ca bearing minerals are transformed or disap-pear (Aubert et al., 2001; Fichter et al., 1998), we favorthe second scenario of a supplementary Ca flux.

Perakis et al. (2006) and Page et al. (2008) have shown‘‘that lesser values measured within the upper soil horizonswere attributable to recycling of biomass derived Ca in theforest floor”. If the low d44/40Ca value of the soil solutionsfrom the uppermost part of the soil profile at the beech plotcan be derived from a return flux of ‘‘light” Ca due todecomposition of organic matter, it does however not ex-plain the higher d44/40Ca value at greater depth of the beechsoil profile. It is therefore proposed that the supplementaryCa flux required to balance the Ca mass budget of the soilsolution of the beech plot comes from the soil horizons andrepresents therefore an ‘‘alteration” flux from soil to soilwaters. Under these conditions, the Ca fluxes and the isoto-pic composition of soil solutions between #E4 and #E5obey to the two following mass balance equations:

FCaðF1Þ þ FCaðexÞ þ FCaðdissÞ ¼ FCaðF2Þ þ Ftree ð2ÞFCaðF1Þ � e44=40CaE4 þ FCaðaltÞ � e44=40Caalt

¼ FCaðF2Þ � e44=40CaE5 þ Ftree � e44=40Catree ð3Þ

where FCa(alt) is the ‘‘alteration” Ca flux brought from thesoil horizons to soil solutions and d44/40Caalt its correspond-ing isotopic composition (the other terms are the same asfor Eq. (1)).

The Ca fluxes given by Poszwa (2000) for the Ca uptakeby trees at the beech plot (�17.3 kg/ha/yr) and the Ca recy-cling flux by litter (14.3 kg/ha/yr) allow to deduce a net an-nual Ca uptake by vegetation of 3 kg/ha/yr. In the casewhere all the tree Ca uptake is assumed to occur between10 and 60 cm an ‘‘alteration” Ca flux of �0.75 kg/ha/yr(Fex) to the soil solution can be estimated with a d44/40Caex

ranging from �4.4 to �0.36& (for a d44/40

Catree of �0.5& to0.5& , respectively). Even if such a mass balance calcu-lation remains preliminary, the range of calculatedd44/40Catree values could indicate, that other processes than

Ca isotope fractionation in soil- and stream waters 2225

dissolution of Ca bearing primary minerals such as apatiteor plagioclase, which have a d44/40Ca of 0.4& (Schmidtet al., 2003), equilibrate the Ca budget of the soil solutions(except if dissolution processes significantly fractionate Caisotope ratios). A possible process for explaining thed44/40Ca of the supplementary or ‘‘alteration” flux broughtto soil solutions would be dissolution of mineral phaseshaving already lost a part of their 40Ca, that is to say havingrecorded previous stage(s) of weathering. Such an interpre-tation would therefore make play to the secondary mineralsof soils a role maybe much more important than what it isgenerally envisaged in the control of the chemical composi-tion of soil waters. The Ca supplementary flux could be alsolikened to a Ca desorption flux from the Ca exchangeablepool of soils, to the only condition that the Ca from the soilexchangeable pool is not anymore in isotopic and chemicalequilibrium with the soil solutions. This is rather realistic toenvisage for the Aubure watershed since it is observed thatthe Ca export at the Strengbach outlet decreases constantlyduring the past 20 years suggesting that the small catch-ment system actually reached a transient stage with, maybe,geochemical disequilibrium. The choice between these dif-ferent scenarios would require the detail analysis of themain mineralogical phases of the soil horizons as well asthe isotopic analysis of their exchangeable Ca, which isclearly beyond the scope of this work.

Similar to the beech plot, the Ca fluxes calculated for thedifferent soil solutions of the spruce plot point to a signifi-cant loss of Ca (Fig. 5), which must be linked to treeabsorption. In contrast to the beech plot the spruce plot soilsolutions do not show significant variations of the d44/40Cabetween 10 cm (F-10) and 60 cm (F-60) (0.8&). However,the d44/40Ca values of the uppermost soil solution sample(F-5) vary significantly for the three different sampling peri-ods (0.7–1&; Table 1). These variations might reflect a timedependent fluctuation of the d44/40Ca values in the precipi-tation input and in the Ca leaf excretion. The position ofthe surface soil solution data points in the d44/40Ca vsK/Ca mixing diagram (Fig. 7b) could indeed favor suchan interpretation. The lack of variation of the d44/40Ca val-ues in the deeper soil solutions is more difficult to constrain.Even if the number of vegetation samples analyzed for thisstudy is limited, the obtained data indicate that the d44/40Cavalues of spruces are similar to those of beech trees. Thissuggests that the Ca isotope fractionation induced by thetrees Ca uptake might be similar for the two studied exper-imental plots. Similarly, the fact that leaf and needle litterhave very similar d44/40Ca values implies that the Ca isotopefractionation during the litter degradation is more or lessthe same for the two experimental plots. Therefore, differ-ences in the parameters like the intensity of the variousCa fluxes or differences in the mineralogical compositionsof the soil of the two experimental plots might have causedthe different values and the different depth evolutions ofd44/40Ca of the soil solutions collected in the beech andspruce plots. Further studies are now necessary in orderto precisely quantify the role of the different parameterscontrolling the Ca cycle in a forested ecosystem. The firstdata presented in this study certainly outscores the real po-tential of the Ca isotopes for such an aim. They suggest, for

instance, that the Ca flux in soil solutions and hence thechemical dynamics of Ca in the surface horizons could besignificantly controlled by the presence of secondary miner-als whose role has not yet been elaborated (See also Godde-ris et al., 2006).

4.3. The calcium isotope fractionation in streamlet and

sources

A previous Sr isotope study indicates that the streamwaters (RS) 87Sr/86Sr ratios vary in function of the dis-charge and that they are significantly higher during the highwater flow period compared to the low flow period, suggest-ing that the isotope ratios depend on the hydrological andmoisture conditions (Fig. 4) (Aubert et al., 2002). These dif-ferences have been explained by contributions of watersfrom the deep soil profile during the recession stage but alsoby the relative importance of waters from distinct contrib-utive areas such as the opposite slopes and the saturatedarea (ZS; Fig. 1) of the catchment. Similarly, (234U/238U)activity ratios of the stream waters collected at the outletof the catchment (RS) indicate that a significant part ofthe dissolved U transported by the waters comes from bed-rock and deep horizons of the weathering profiles and that,during flood events, a significant part of U comes fromsuperficial horizons of the soils (Riotte and Chabaux,1999).

The fact that the Ca flux at the outlet of the catchment(RS) is significantly higher than the fluxes of the differentsoil solutions (Table 2) indicates, as for Sr and U, that mostof Ca in the streamlet originates from dissolution of Ca richminerals in bedrock and/or deeper seated soil horizons. Bycontrast to U, the Ca isotope ratios of these stream watersare not discharge dependent but appear to vary in functionof the year seasons with low d44/40Ca in winter (low vegeta-tion activity) and high d44/40Ca in spring, summer and au-tumn (high vegetation activity) (Fig. 3). The observationthat the d44/40Ca values of the streamlet waters during win-ter are similar to the d44/40Ca values of Ca rich minerals likeapatite, suggests that the outlet waters preserve a lithologi-cal signature dominated by dissolution of apatite and feld-spar during winter, whereas during other seasons, it ismarked by water fluxes depleted in light Ca.

As demonstrated previously with help of Sr isotopes(e.g. Aubert et al., 2002) a key for the better understandingof the fluctuating d44/40Ca values in the stream water are thesource waters. The main sources were analyzed at two dif-ferent hydrological periods (Table 1). Two sources (CR andSH) show no significant d44/40Ca variation between thesetwo sampling periods, whereas for the other sources theird44/40Ca systematically decreases from dry to humid peri-ods (Fig. 6). CR is the principal and deeper seated sourceof the streamlet RS with a residence time of the CR waterin the underground of about 3 years (Viville et al., 2006);this timespan is certainly long enough for allowing an in-tense interaction between the waters and Ca rich mineralslike apatite and plagioclase and hence allowing the watersto reach a d44/40Ca value close to the apatite value (0.4;Schmitt et al., 2003b). The SH source samples (near thebeech plot) have also similar d44/40Ca values for the two

0

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δ 44/40 Ca

δ 44/40 Ca

a

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ig. 8. H4SiO4 vs d44/40Ca in sources draining waters in theturated RUZS zone (a) and at the northern slope (BH, RH, SG)) during dry and wet periods.

2226 B. Cenki-Tok et al. / Geochimica et Cosmochimica Acta 73 (2009) 2215–2228

sampling periods but in this case with higher values (0.64and 0.82&). The d44/40Ca values of the corresponding soilwaters are even higher (0.97–1.47&; Fig. 5) and are, as dis-cussed before, controlled by two important Ca fluxes, theremoval by vegetation and a Ca flux related to water rockinteractions in the soil column. Since the SH source is situ-ated close to the surface, which is depleted in Ca rich min-erals, it is suggested that its isotopic signature is largelyinfluenced by the vegetation (Ca removal) and the water–rock interactions (Ca input).

Thus, these data indicate that depending on the depth ofthe water sources in deeper soil compartments or the bed-rock their signature will be largely different: the deeperwaters will have a d44/40Ca value mainly controlled by dis-solution of the primary minerals (apatite, plagioclase); thewaters closer to the surface will have d44/40Ca values muchmore influenced by vegetation induced Ca isotope fractio-nations and hence characterized by higher values.

When focusing on the other springs a variation of theird44/40Ca is observed between dry and humid periods. This isespecially recognizable for the water samples from thenorthern slope (sources SG, RH and BH; Fig. 1) but possi-bly also for the two samples from the saturated zone(RUZS), even though the variation of the d44/40Ca valuesremain close to the error bars. From hydrological and hy-dro-geochemical observations (Ladouche et al., 2001) ithas been concluded that the RUZS waters (Fig. 1) drainduring dry periods (normally end of summer) only the or-ganic free parts of the lower soil profile below the trees rootsystem. These waters are normally more enriched in cationsand in H4SiO4 (Aubert, 2001) than during humid periodwaters. During this period, the RUZS waters drain theupper parts of the soil profile containing an important partof the trees root system, and are less concentrated in cationsand H4SiO4. This explains why the water sample collectedat the saturated zone in september at low flow rate (5 L/s)are more concentrated in H4SiO4 (0.103 mmol/L) than thesample collected in may at a significantly higher flow rate(11 L/s) (Fig. 8a). The time and hydrological-dependentcovariation between d44/40Ca and H4SiO4 observed forspring waters draining the northern slope (BH, RH, SG)(Fig. 8b) could also be explained in a similar way, i.e. interms of water mixing between surface waters slightly min-eralized and affected by Ca isotope fractionation inducedby vegetation cycling and deeper waters whose d44/40Ca va-lue is marked by the dissolution of primary minerals andare more concentrated in H4SiO4. In other words, duringdry periods and low water flow rate the source waters carrya Ca isotopic signature from alteration of soil minerals,whereas during wet periods and high flow rates admixtureof 40Ca depleted waters (vegetation induced signal) fromuppermost soil horizons controls the isotopic compositionof the source waters. Admixture of 40Ca depleted watersprobably caused also the increase of the d44/40Ca in streamwater during spring, summer and autumn. Such an addi-tional flux could come from local soil waters, and/or fromvariable contributions of the different sources nourishingthe streamlet at its outlet, as some of them have high d44/40Cavalues or show significant seasonal variation of theird44/40Ca (Fig. 5).

Fsa(b

It is important to stress here that the two sampling peri-ods for the analysis of the spring waters are periods of sig-nificant vegetation activity (May and September). Thiscertainly explains why the mobilization of these two Cafluxes principally varies with the mean discharge of thespring. There is however no reason to expect that the trendof variations between d44/40Ca and the discharge is the sameor even exist during winter, that is when the trees Ca pump-ing is much lower.

5. SUMMARY AND CONCLUSIONS

This study clearly emphasizes the potential of Ca iso-topes as tracers of biogeochemical processes at the water–rock–vegetation interface in a small forested catchment.Most important is the finding that at both soil profile andwatershed scales, the Ca isotopic compositions of waters(soil solution, source and stream waters) are mainly con-trolled by two very different processes: alteration andwater–rock interaction at one hand and biological activityon the other hand.

At the soil profile scale, soil solutions are significantlydepleted in lighter isotopes (d44/40Ca: 1.00–1.47&), whereasvegetation is strongly enriched (d44/40Ca: �0.50 to+0.19&); this confirms that preferential 40Ca uptake by

Ca isotope fractionation in soil- and stream waters 2227

plants causes depletion in light calcium in the soil solutionsfrom deeper parts of the soil profile at the level of the treesroot system. However, mass balance calculations per-formed at the scale of soil profiles indicate that in additionto this biological Ca isotope fractionation Ca isotoperatios of soil solutions are also influenced by other 40Cadepleted weathering fluxes from e.g. secondary soil mineralphases.

At the watershed scale, the d44/40Ca values of springs,brooks and stream waters are comparable to those of con-tinental rivers and fluctuate between 0.17 and 0.87&. Thestream waters are marked by a seasonal variation of theird44/40Ca values with low d44/40Ca in winter and highd44/40Ca in spring, summer and autumn. For some springs,nourishing the streamlet, a decrease of the d44/40Ca valueis observed when the discharge of the spring increases,with, in addition, a clear covariation between thed44/40Ca and corresponding H4SiO4 concentrations: highd44/40Ca values and low H4SiO4 concentrations at highdischarge; low d44/40Ca values and high H4SiO4 concentra-tions at low discharge. All these variations can be inter-preted in terms of water mixing between deep waters,marked by alteration of bedrock minerals (including theCa bearing minerals, i.e. apatite and plagioclases) andmore surficial waters significantly influenced by the prefer-ential 40Ca uptake by vegetation. The water flux enrichedin the lighter 40Ca isotope by alteration processes certainlyoperates the whole year but becomes especially visible dur-ing dry periods and/or low biological activity (i.e. winter).This scenario easily explains why stream waters at the out-let are isotopically heavier during spring and summer thanduring the winter period. Such an interpretation impliesthat, to some extent, Ca isotopes can be used similar toSr and U as a hydrological tracer, which enables to distin-guish between deep-seated water reservoirs (signal ofweathering) and water reservoirs situated close to the soilsurface within the trees root system. In addition, since Caisotope ratios in stream and source waters carry systemat-ically a dual signal linked to vegetation activity andwater–rock interactions, their analysis might become a rel-evant and helpful approach to constrain, at a watershedscale, the interrelationships between vegetation and weath-ering processes.

ACKNOWLEDGMENTS

We thank S. Benarioumil, E. Lemarchand, J. Prunier and D.Cividini for sampling assistance, B. Kiefel and E. Pelt for technicalhelp at the Triton, T. Perrone for Sr isotope analysis and D. Mil-lion for major element analyses. Discussions with D. Cividini andJ. Prunier during the course of this work were appreciated. Themanuscript benefited from constructive reviews by Ed Tipperand two anonymous reviewers as well as by the associated Editor,S. Kraemer. This study has been financially supported by REA-LISE (REseau Alsace de Laboratoires en Ingenierie et Sciencespour l’Environnement), the region of the Alsace, and the FrenchCNRS program ‘EC2CO-Cytrix’. B. Cenki-Tock acknowledgesthe funding of a post-doctoral grant by ‘‘la Fondation Simone etCino del Duca de L’Institut de France”. This is an EOSTcontribution.

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