Chemical Geology 383 (2014) 147–154

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Chemical Geology

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Sulphur isotope composition of dissolved sulphate in theCambrian–Vendian aquifer system in the northern part of the BalticArtesian Basin

Valle Raidla a,⁎, Kalle Kirsimäe b, Jüri Ivask a, Enn Kaup a, Kay Knöller c, Andres Marandi b,Tõnu Martma a, Rein Vaikmäe a

a Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estoniab Department of Geology, University of Tartu, Ravila 14a, 50411 Tartu, Estoniac UFZ Helmholtz Centre for Environmental Research, Department of Catchment Hydrology, Theodor-Lieser-Str. 4, 06120 Halle, Germany

⁎ Corresponding author.E-mail address: valle.raidla@ttu.ee (V. Raidla).

http://dx.doi.org/10.1016/j.chemgeo.2014.06.0110009-2541/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 March 2014Received in revised form 13 June 2014Accepted 17 June 2014Available online 26 June 2014

Editor: David R. Hilton

Keywords:GroundwaterBacterial activitySulphide oxidationSulphate reduction

The groundwater in the Cambrian–Vendian aquifer system with its δ18Owater values of about −22‰ (VSMOW)and a low radiocarbon concentration is of glacial origin from the Last Ice Age. Earlier surveys have highlighteda negative co-variance of sulphate and bicarbonate content in the groundwater of the Cambrian–Vendian aquifersystem, whereas the most depleted dissolved inorganic carbon δ13C values have been measured mainly ingroundwater samples with the lowest sulphate concentrations. In this paper we studied the origin of sulphateand the factors controlling the sulphur and carbon isotope geochemistry in the aquifer system. Direct sourcesof sulphate were not found, but relying upon δ18OSO4

measurements we suggest that the sulphate originatesfrom oxidation of sulphideminerals whereas the δ34S of the dissolved SO4

2− in the groundwater ismore enrichedthan the δ34S of the surrounding rocks.We show that bacterial activitymay have caused the enrichment of δ34S ofsulphate.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The Cambrian–Vendian (Cm–V) aquifer system is a confined water-body in western and north-western parts of the East-European Plat-form. In shallowly buried marginal areas of the aquifer system, particu-larly in northern part of the Baltic Artesian Basin, the groundwater isfresh and widely used in public water supply. The fresh groundwaterat the northern margin of the basin has the lightest known oxygen iso-topic composition in Europe (δ18Owater values of around −22‰) and alow radiocarbon concentration suggestive of glacial origin of the water(e.g. Vaikmäe et al., 2001).

Raidla et al. (2009) showed that the groundwater from the Cm–Vaquifer system is a mixture of three end-members— fresh glacial melt-water, relict Na–Ca–Cl brine and recent meteoric water. The mass-balance model of the carbonate system coupled with the modelling ofradiocarbon age of the groundwater from the Cm–V aquifer system(Raidla et al., 2012) shows that the infiltration of the water occurrednot earlier than 14,000 to 27,000 radiocarbon years ago, which is coevalwith the advance andmaximum extent of theWeichselian Glaciation inthe area (Kalm, 2012).

In this paper we study the sulphur isotope composition in the ground-water of the Cm–V aquifer system at its northernmost margin inNorth-Estonia. Our goal is to reveal the origin of sulphate and the factorscontrolling the sulphur and carbon isotope geochemistry in the aquifersystem'swater and rockmatrix, and to test evidence of bacterial activityin the aquifer system's hydrogeochemistry. Raidla et al. (2012) put for-ward a hypothesis that, although the origin of the degradable organicmatter and sources of sulphate in the groundwater are virtually un-known, it is possible that the isotope system has been modified by bac-terial activity, most likely via sulphate reduction.

2. The study area

The Cambrian–Vendian aquifer system is the shallow northern partof the Baltic Artesian Basin (BAB), which completely underlies the BalticStates and partly the border areas of Russia, Poland and Belarus. A largepart of the BAB lies beneath the Baltic Sea. The Cm–V aquifer system en-compasses the thick (up to 90 m) sequence of Ediacaran and Cambriansandstones alternating with clays and silty clays (Fig. 1). The Ediacaranand Cambrian sedimentary rocks outcrop into the Gulf of Finland and tothe bottom of the Baltic Sea. The aquifer system is confined by the un-derlying crystalline basement of the Palaeoproterozoic age and theoverlying Lontova–Lükati aquitard, and the groundwater is under pres-sure. In the northeastern Estonia the aquifer system is divided by

a

b

c

Fig. 1.A geological scheme of the northern Baltic Artesian Basin in Estonia with the positions of the studiedwells (a), theWest–East cross-section of northern Baltic Artesian Basin (b) andthe North–South cross-section of the northern Baltic Artesian Basin (c).

148 V. Raidla et al. / Chemical Geology 383 (2014) 147–154

Ediacaran clays of the Kotlin age into two aquifers: the upper Voronka(V2vr) and the lower Gdov aquifer (V2gd) (Fig. 1). In northern Estoniathe conductivity of the water-bearing rock is 0.5 to 9.2 m d−1, withthe average of 5 to 6 m d−1. Transmissivity in northeastern Estonia is300 to 350 m2 d−1, decreasing in southerly and westerly directions(Perens and Vallner, 1997).

The Lontova–Lükati aquitard, overlying the aquifer system is com-posed of silty clays, siltstones and clays of the Lower Cambrian age.The Lower Cambrian clays are diagenetically immature and plasticwith water content of about 20 to 30% (Kirsimäe and Jørgensen, 2000;Raidla et al., 2006). The thickness of the clayey complex is 90–100 min North-Estonia, but decreases towards the south until disappearingin South-Estonia (Fig. 1). The aquitard has a strong isolation capacitywith vertical hydraulic conductivity of 10−7 to 10−5 m d−1 (Perensand Vallner, 1997). The Lontova–Lükati clays are gradually replaced byinterbedded clay and sandstone in western part of the area and their

vertical hydraulic conductivity is N10−5 m d−1 (Perens and Vallner,1997). At the North-Estonian coastline the aquitard is incised by deepvalleys filled with loamy till and glaciofluvial gravel (Tavast, 1997).These valleys serve as recharge areas where the water from the uppergroundwater horizons infiltrates to the Cm–V aquifer system (Fig. 1).In the most part of northern and central Estonia, the siliciclastic rocksof the Cm–V aquifer system are covered by up to 300 m thick layer ofOrdovician and Silurian marine carbonate rocks.

The groundwater in northern part of the aquifer system is of Cl–HCO3–Na–Ca and Cl–HCO3–Ca–Na type with the TDS content between0.4 and 1.0 g L−1 (Savitskaja and Viigand, 1994). In southern part ofthe aquifer system the groundwater is replaced by saline relict Na–Clwater with TDS values of up to 22 g L−1 (Karise, 1997). The most char-acteristic property of the groundwater from the Cm–V aquifer system isits stable isotope composition. δ18Owater values are between−18.5 and−22‰ VSMOW (Vaikmäe et al., 2001), whereas the isotope

Table 1Chemical composition of the groundwater in the northern part of the Cm–V aquifer system. Cm–V — Cambrian–Vendian aquifer; V2gd — Gdov aquifer; V2vr — Voronka aquifer; * — chemical results by Perens and Boldureva (2008).

Well no Well ID Location Aquifer Depth Date Tem El. cond. Eh pH δ34S δ18OSO4δ18Owater δD δ13C Ca2+ Mg2+ Na+ K+ Cl− SO4

2− HCO3−

m °C μS · cm−1 mV ‰ VCDT ‰ VSMOW ‰ VPDB mg L−1

1 3344 Haapsalu City* Cm–V 295 13–Mar–07 10.8 292 −108.0 8.6 8.3 −5.3 −20.2 −10.8 38.1 13.3 86.2 6 155.3 32.1 109.82 9997 Risti village* Cm–V 280 13–Mar–07 9.8 329 −110.0 8.6 10.1 2.1 −20.4 −12.0 38.3 9.0 73.8 6.0 139.3 28.0 109.83 14927 Murru prison Cm–V 200 1–Oct–10 9.0 551 −134.0 8.9 12.0 −20.4 −153.0 41.7 9.3 65.5 6.8 125.3 18.0 115.94 552 Keila City* Cm–V 214 19–Mar–07 9.3 519 −118.0 7.7 11.0 1.2 −21.3 −12.3 40.9 11.8 57.2 8.6 123.0 26.4 122.05 14146 Tallinn. Kopli port Cm1ln 60 22–Mar–07 9.1 306 −79.1 7.9 24.9 −2.4 −20.6 −13.2 53.7 20.2 30.0 10.8 118.1 b3.3 164.76 14784 Tallinn. Kopli port Cm–V 65.5 22–Mar–07 8.4 469 −108.0 7.8 28.8 −20.7 −13.9 48.9 19.6 31.8 9.4 92.5 10.0 183.07 8914 Muuga port* Cm–V 75 27–Mar–07 9.3 728 −107.8 8.0 17.5 −22.0 −17.6 33.3 10.7 21.7 6.7 42.2 b3.3 170.88 10703 Rakvere City V2vr 222 25–Sep–07 11.9 322 7.9 20.9 −8.2 −20.1 −18.3 34.0 16.0 98.0 8.5 150.0 0.5 201.3

Rakvere City V2vr 222 1–Nov–11 12.3 443 −20.0 −150.8 −18.7 72.5 20.2 81.0 7.4 81.0 1.8 161.79 10714 Arkna water intake Cm–V 235 1–Nov–11 11.1 349 −20.5 −153.8 −17.5 43.4 14.1 72.1 5.8 72.1 b0.1 176.910 2738 Rakvere City V2gd 268 1–Nov–11 11.8 586 −20.2 −151.7 −19.4 96.8 23.4 92.9 8.5 92.9 1.7 173.911 2775 Kunda City V2gd 205 29–Nov–11 10.6 275 −60.5 7.6 30.5 −21.4 −162.0 −14.3 73.0 25.0 140.0 10.0 316.3 b0.1 244.112 3323 Unukse village* V2gd 207 12–Nov–07 11.7 1027 −96.5 7.8 19.7 −10.8 −21.1 −17.1 71.3 21.4 116.7 10.5 239.3 b3.3 237.913 2388 Aseri City V2gd 185 16–Jan–08 10.9 548 −69.6 7.8 20.4 −9.6 −21.3 −21.5 66.5 17.7 110.0 8.5 212.0 5.8 250.214 2386 Aseri City V2gd 200 29–Nov–11 10.9 258 −65.7 7.7 −21.1 −158.6 −22.0 84.9 21.0 104.5 8.5 104.5 b0.1 247.115 2233 Purtse village V2vr 130 29–Nov–11 8.5 233 −87.6 8.0 28.3 −20.1 −151.6 −15.7 59.1 18.2 92.5 8.6 179.6 b0.1 201.416 2249 Kohtla-Nõmme City V2gd 255 8–Jun–07 14.0 547 −35.1 7.3 26.8 6.0 −19.7 −20.2 49.1 20.1 140.0 8.0 275.5 9.5 152.517 2198 Sillamäe City V2vr 150 1–Dec–11 9.9 360 −96.0 8.2 55.1 −18.1 −136.7 −17.8 49.9 22.3 259.2 8.6 483.0 16.7 219.718 2217 Sillamäe City* V2vr 124 7–Jun–07 10.5 496 −69.3 7.8 27.3 8.9 −19.4 −17.0 20.0 2.4 197.0 4.5 192.2 1.9 231.9

Sillamäe City V2vr 124 1–Dec–11 10.1 285 −97.2 8.1 −19.4 −146.2 −17.3 17.0 5.6 157.8 6.1 157.8 b0.1 244.119 2197 Sillamäe City V2vr 151 01–Dec–11 10.1 825 7.9 43.6 −18.7 −140.4 −18.1 30.4 11.1 205.0 8.0 318.4 2.7 219.720 2207 Sillamäe City* V2gd 220 07–Jun–07 12.4 1041 −156.0 7.5 23.6 0.0 −18.6 −21.7 18.6 6.6 388.9 7.0 588.9 b3.3 152.5

Sillamäe City V2gd 220 01–Dec–11 11.9 650 −79.3 7.9 −18.6 −140.0 −21.4 25.7 8.8 395.8 10.7 395.8 3.4 164.721 2470 Viivikonna mining V2vr 180 1–Dec–11 11.5 339 −87.2 8.1 20.5 −18.9 −143.2 −17.4 22.7 7.6 157.8 9.0 201.5 b0.1 216.622 2092 Narva-Jõesuu City V2vr 101 24–Sep–07 9.6 524 −118.8 8.5 25.7 9.7 −18.4 −20.9 10.8 4.1 206.3 4.0 225.1 12.8 195.2

Narva-Jõesuu City V2vr 101 3–Nov–11 9.1 410 8.2 −18.3 −138.4 −21.2 11.9 3.9 143.7 5.7 143.7 b0.1 201.323 2084 Narva-Jõesuu City V2gd 211 3–Nov–11 10.8 1614 −86.4 8.1 −18.4 −138.0 −30.9 54.2 20.3 461.7 13.8 461.7 b0.1 183.024 3434 Narva mining V2vr 180 3–Nov–11 11.4 410 −98.9 8.3 −19.0 −143.7 −19.5 10.2 2.2 145.4 5.7 145.4 b0.1 234.9

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composition of the modern precipitation is −8 to −11‰ VSMOW(Punning et al., 1987). Low 14C activities show that the water was lastexposed to atmosphere over 10,000 years ago. A sub-glacial rechargemechanism is the most probable explanation of how the depletedwater reached the Cm–V aquifer system. During the Pleistocene thecontinental ice sheet that covered the area would have increased hy-draulic pressure at the base of the glacier, which would have reversedthe regional groundwater flow. The inflow of diluted glacial meltwaterinto the aquifer occurred along the outcrop area at the basin's margin(Raidla et al., 2009).

3. Material and methods

Groundwater was sampled and analysed fromwater supply and ob-servation wells during two fieldwork campaigns in 2007 to 2008 and2010 to 2011 in North-Estonia (Fig. 1. Table 1).

The amount of water collected for sulphate isotope analyses rangedfrom 5 to 100 L, depending on the quantity of dissolved sulphate. Thesamples were pre-filtrated and acidified/stirred to remove carbonates.The dissolved sulphate was precipitated using BaCl2·2H2O. The precip-itated BaSO4 was collected by filtration through nitrocellulose mem-branes, washed to remove residual BaCl2 and dried at 50 °C.

In thirteen samples collected in 2007 and 2008, sulphur isotopiccompositions were measured after conversion of BaSO4 to SO2 usingan elemental analyser (continuous flow flash combustion tech-nique) coupled with an isotope ratio mass spectrometer (Delta S,ThermoFinnigan, Bremen, Germany) at the Stable Isotope Laborato-ry of the UFZ in Halle/Saale, Germany. Samples collected in 2011were analysed at the Isotope Science Lab in Calgary, Canada, byusing the same technique, but a different mass spectrometer/ele-mental analyser combination (Thermo Finnigan DeltaPLUS XL andFison NA1500). Sulphur isotope measurements were performedwith an analytical error of the measurement of better than ±0.3‰and results are reported in delta notation (δ34S) as part per thousand(‰) deviation relative to the Vienna Cañon Diablo Troilite (VCDT)standard.

Oxygen isotope analysis of sulphatewas performed only for 11 sam-ples for which high temperature pyrolysis at 1450 °C in a TC/EA con-nected to a delta plus XL mass spectrometer (ThermoFinnigan,Bremen, Germany) with an analytical precision of better than ±0.5‰was used. Results of oxygen isotope measurements are expressed indelta notation (δ18OSO4

) as part per thousand (‰) deviation relative toVienna Standard Mean Ocean Water (VSMOW). For normalizing theδ34S and δ18OSO4

data, the IAEA-distributed reference material NBS 127(BaSO4) was used. The assigned values were +20.3‰ (VCDT) δ34Sand +8.6‰ (VSMOW) for δ18OSO4

.Measurements of stable isotopes of δ18O and δD in the water sam-

ples and δ13C in precipitated carbonates were performed at the labora-tory of mass spectrometry of the Institute of Geology at TallinnUniversity of Technology using a Thermo Fisher Scientific Delta V Ad-vantage mass spectrometer and GasBench II. Reproducibility was betterthan±0.1‰ for δ18Owater and±0.5‰ for δ13C. The results are expressedin ‰ deviation relative to Vienna Standard Mean Ocean Water(VSMOW) and Vienna Peedee Belemnite (VPDB) for O, D and C, respec-tively. For normalizing the δ18Owater and δ13C data, the IAEA-distributedreference materials VSMOW, SLAP, NBS 19 and LSVEC were used.

The pH of δ13C sample was set to pH N 12 by adding the requiredamount of concentrated CO2-free NaOH solution. Analytical gradeBaCl2·2H2O in excess of expected DIC was added to precipitate DIC inthe form of BaCO3. The results are expressed in ‰ deviation relative toVienna Standard Mean Ocean Water (VSMOW), Vienna Peedee Belem-nite (VPDB) and the Cañon Diablo Troilite (VCDT) standard for O, C andS, respectively.

Major ions have been measured on Dionex ICS-1000 ion chromato-graph at Tartu University, except for bicarbonate, which was measuredin the Laboratory of Geological Survey of Estonia by the titration

method. In addition, some results from earlier studies were used(Perens and Boldureva, 2008).

4. Results and discussion

4.1. Distribution of dissolved sulphate

The sulphate content in the studied samples from the Cm–V aqui-fer system is generally low ranging from below detection limit(0.1 mg L−1) to 32.1 mg L−1 (Table 1). Raidla et al. (2012) noticedthat the sulphate concentration was the highest in Western-Estoniaand low or almost undetectable in the central and eastern parts ofthe study area. δ34S values of the dissolved sulphate in the ground-water of the Cm–V aquifer system vary between 8.6 and 55.1‰ (av-erage 24.0‰). δ18OSO4

values of the dissolved sulphate rangebetween −10.8‰ and 9.7‰ (average −0.2‰) while δ18Owater andδDwater values of the groundwater samples vary between −18.4‰and −21.4‰ and −136.7‰ and −162.0‰, respectively (Table 1).

4.2. Direct sulphate sources

No direct sources of SO42−, such as gypsum or mirabilite deposits,

occur in the aquifer system's rock matrix (Puura et al., 1983; Raidlaet al., 2006). In our opinion, there are two possible sources of sulphate:(a) relict basinal brine or (b) oxidation of sulphide minerals.

Chemical composition of the Cm–V aquifer system varies both later-ally and vertically. The shallow dilutedwater along the northernmarginof the Baltic Artesian Basin (BAB)with the TDS content between 0.4 and1.0 g L−1 (Vaikmäe et al., 2008) changes to Cl–Na, Cl–Na–Ca and Cl–Ca–Na type groundwater with TDS contents ranging from 1 to 22 g L−1 insouthern, deeper part of the Cm–V aquifer system (Karise, 1997).Deep brines in the central part of the BAB are typical sedimentary basin-al Cl–Ca–Na fluids, with TDS of 60 to 140 g L−1 (Mokrik, 1997), enrichedwith respect to Ca2+ and depleted in Mg2+ and SO4

2−compared to theevapo-/cryo-concentration trend of the modern seawater (e.g.Lowenstein et al., 2003). Also, Karro et al. (2004) suggested that somesulphate in the Cm–V aquifer system could have come to the systemfrom deeper, saltier parts of the fractured crystalline rock underlyingthe aquifer rocks.

Using themass balance of geochemically conservative tracer compo-nents (Cl− and δ18Owater), Raidla et al. (2009) showed that the ground-water from the Cm–V aquifer system is a mixture of three major end-member sources: (a) the basinal brine, (b) diluted (meteoric) water,and (c) glacial melt-water. The results of mixing calculations (Raidlaet al., 2009) suggest that in the shallow waters at the northern marginof the aquifer system the glacial component comprises N80% of thewater. The content of the glacial component decreases and, accordingly,the brine component increases gradually with depth, and at ~700m theglacial component has decreased to about 30 to 40% (Raidla et al., 2009).The brine component is enriched with respect to sulphate and in thebasin's deeper southern part, in Latvia, sulphate concentrations are upto 1 to 2 g L−1 (Kondratas, 1967). According to Raidla et al. (2009),the relict component composes amaximumof 1 to 2% of the groundwa-ter from the Cm–V aquifer system in northern Estonia, which, assumingthat the sulphate content in the original brine was 2 g L−1, accounts forapproximately 20 to 40mg L−1 in glacially diluted water that is compa-rable to the maximum sulphate values in the area (Table 1).

Moreover, δ34S composition of the groundwater from the Cm–Vaquifer system averaging at 24‰ seems to support the idea that the sul-phate originates from relict seawater. Modern marine sulphate has theδ34S content of +20.1‰, but it has varied during the Phanerozoicfrom about 30‰ in the Cambrian to about 10‰ at the beginning of theTriassic (Bottrell and Newton, 2006).

However, the sulphate in amarine reservoir has a homogeneous andwell-defined δ18OSO4

isotopic composition of about 9.5‰ (Longinelli,1989), while the δ18OSO4

in the studied samples varies largely from

151V. Raidla et al. / Chemical Geology 383 (2014) 147–154

−10.8 to 9.7‰ (average−0.2‰). This suggests that the sulphate foundin the groundwater of the Cm–V aquifer system in northern Estoniaoriginates from neither dissolution of marine sulphate minerals nor rel-ict basinal brine.

Interpretation of sulphate's oxygen isotope composition is difficultbecause the oxygen incorporated in the dissolved sulphate can originatefrom both atmospheric oxygen and/or a water molecule. Sulphate's ox-ygen isotope composition can be derived from and/or altered throughseveral abiotic and biochemical processes, including disproportionationof elemental sulphur (e.g. Van Stempvoort and Krouse, 1994; Balci et al.,2012), oxidation of reduced compounds (pyrite, sphalerite etc.) (e.g.Balci et al., 2007; Heidel et al., 2013; Müller et al., 2013) and dissimila-tory sulphate reduction (e.g. Böttcher et al., 2005; Brunner et al., 2005,2012; Heidel and Tichomirowa, 2011). On the other hand, once formed,δ18OSO4

is a conservative environmental tracer as long as no biochemicalprocesses such as bacterial sulphate reduction affect the sulphate pool.At ambient environmental temperatures the oxygen isotope exchangebetween sulphate and water is limited and occurs at a significant rateonly in geothermal waters and/or in very low pH conditions (Krouseand Mayer, 2000; Tichomirowa and Junghans, 2009). However, Halaset al. (1993) show that equilibriumbetweenwatermolecules and resid-ual sulphate in groundwater is possible at low temperatures and innear-neutral pH (T = 9.5 °C and pH= 8) conditions at long residencetimes (~27,000 yr). This would result in covariance between δ18OSO4

and δ18Owater values. Groundwater's comparatively long residencetime (N14,000 yr) in the Cm–V aquifer systemwould imply that partialoxygen isotope exchange might have occurred. Nevertheless, the mea-sured δ18OSO4

and δ18Owater values do not show a strong positive trend(the correlation coefficient is 0.65; R2 = 0.42; Fig. 2) and isotope ex-change at low temperatures has not yet been proven in the Cm–V aqui-fer system in Estonia.

Raidla et al. (2012) put forward a hypothesis that the sulphate in theCm–V aquifer system could originate from the oxidation of sulphideminerals that occurred during the intrusion of glacial meltwater. Mostof the supraglacial waters appear to be undersaturated (c. 30 to 90%)with respect to atmospheric O2 (Brown et al., 1994). Normally, duringinfiltration to the glacier's basal part dissolved oxygen content in themelt-water decreases and reaches anoxic conditions. Still, basal meltingof the glacier may receive a subglacial supply of O2 dependent on thecomposition and concentration of gas bubbles in the basal ice (Tranteret al., 2002). In addition, subglacial waters may be heavily enriched indissolved gases, in case of O2 up to 50 times higher than air-equilibrated water, through refreezing (McKay et al., 2003). This couldmean that the oxidative meltwater penetrating into the aquifer systemprovoked the initial oxidation of sulphide minerals that provided mostof the SO4

2− found in the groundwater from the Cm–V aquifer system.Sulphite minerals (mainly pyrite, but occasionally sphalerite andgalenite) occur in both Ediacaran and Cambrian sediments (e.g. Raidla

R² = 0.42

-23 -21 -19 -17 -15

δ18)

SO

4‰

O(

δ18O ( )water ‰

-15

-10

-5

0

5

10

15

-25

Fig. 2. δ18Owater and δ18OSO4variation in the Cambrian–Vendian aquifer system.

et al., 2006) and in Proterozoicmetamorphosed crystalline rocks confin-ing the aquifer system from below (Petersell et al., 1991).

Balci et al. (2007) show that water is the main oxygen source ofsulphate during anaerobic and aerobic oxidation of pyrite, whereas abi-otic/aerated experiments of pyrite oxidation at different pH levels showthat 20 to 30% of the sulphate's oxygen comes from atmospheric O2

(Kohl and Bao, 2011). Heidel et al. (2013) have experimentally shownthat during oxidation of sulphide mixtures the molecular oxygen isthemain oxygen source of sulphate during the initial stage of oxidation,while the amount of water-derived oxygen in the sulphate increasesN70% at later stages.

The groundwater of the Cm–V aquifer system δ18OSO4values vary

from −10.8 to 9.7‰, while its δ18O values stay rather uniformly at−18.5 to −23‰ (Raidla et al., 2009), and the δ18O composition of O2

in the air is 23.5‰ SMOW (Kroopnick and Craig, 1972).During abiotic oxidation of sulphides (e.g. pyrite) the 18O is prefer-

entially taken up into sulphate and the reported positive isotope offsetsbetween sulphate andwater in the absence ofmolecular O2 vary around0 to 8‰ (e.g. Lloyd, 1968; Taylor et al., 1984; Van Everdingen andKrouse, 1985; Balci et al., 2007, 2012; Heidel et al., 2011). On average,themeasured isotopic values of the sulphate oxygen in the groundwaterfrom the Cm–V aquifer system are at least 5 to 10‰ higher than theywould be, provided that thewater was the only source of oxygen. How-ever, in oxidation experiments usingmixtures containing pyrite, galenaand sphalerite, Heidel et al. (2013) showed high ε18OSO4–H2O valuesreaching 16.1 to 18.9‰, whereas the δ18OSO4–H2O values in their experi-ments depended on the initial δ18Owater values. These numbers weresignificantly higher (by 6.9 to 9.1‰) in waters with the initial δ18Ovalue of −17.4‰ compared to waters with the initial δ18O value of8.0‰. An isotopic effect of this amplitude (14 to 17.6‰) is tied to thesulphite to sulphate oxidation step, which is paramount to the sulphateoxygen's isotopic composition (Müller et al., 2013). Oxygen isotope off-set of this range andmagnitude could explain the measured values andthe variation in the δ18OSO4

values of the groundwater from the Cm–Vaquifer system. Variations in the δ18OSO4

values could possibly be ex-plained by differences in amounts of dissolved oxygen and/or ratios be-tween different sulphide minerals.

Alternatively, the oxidation of sulphides in abiogenic and anaerobicenvironments could be driven by Fe3+ as an oxidant (e.g. Heidel andTichomirowa, 2011). However, the isotopic effect in this case is signifi-cantly lower (ε18OSO4–H2O = 2.3‰, Heidel and Tichomirowa, 2011)and could not explain the observed δ18OSO4

values in the groundwaterof the Cm–V aquifer system.

Earlier studies (e.g. Taylor et al., 1984; Van Stempvoort and Krouse,1994; Brunner et al., 2005) suggest that the enriched δ18OSO4

valuescompared to those in the initial sulphate oxygen pool indicate that bio-logical reactions occurred during the S oxidation process. However,Balci et al. (2007) show that δ18OSO4

values cannot be used conclusivelyto distinguish between biological and abiotic oxidation of pyrite. More-over, a percentage of the sulphate that is brought into the bacterial cellsduring sulphate reduction is not reduced to sulphide completely. In-stead, it undergoes isotope exchange between oxygen and water,reoxidates to sulphate, and is released back to the ambient sulphatepool (Mizutani and Rafter, 1973; Fritz et al., 1989; Brunner et al., 2005,2012; Knöller et al., 2006; Mangalo et al., 2007, 2008; Wortmannet al., 2007; Farquhar et al., 2008; Turchyn et al., 2010). Nevertheless,explanations of this mechanism and conditions that control the processhave remained speculative (e.g. Antler et al., 2013).

4.3. Sulphate δ34S

In Estonia, the δ34S values of sulphides scatter ratherwidely between−30 and 22‰ in the rocks of the Cm–V aquifer system (Petersell et al.,1991). δ34S values of the rocks correspond to the Poisson distribution,the most frequent values ranging from 0 to −5‰ (Fig. 3). At the sametime δ34S values of dissolved sulphate in the groundwater of the Cm–

10 403020

60

40

20

0

10

0

30

50

SO (mg L )42- -1·

δ34S

()

SO

4‰

Fig. 5. δ34S and SO42− correlation in the Cambrian–Vendian aquifer system.

25

20

15

10

5

020 010 -10 -20 -30

δ34S ( )SO4 ‰

freq

uenc

y

-40

Fig. 3. The distribution of δ34S values of sulphides in the rocks of the Cambrian–Vendianaquifer system.

152 V. Raidla et al. / Chemical Geology 383 (2014) 147–154

V aquifer system vary between 8.6‰ and 55.1‰ (average 24‰), sug-gesting that the groundwater's sulphate isotope composition generallycorresponds to the sulphides' δ34S values, but on average is moreenriched compared to sulphide minerals.

During abiotic and/or biotic oxidation of sulphide mineral oxidationat low temperatures only a small isotope fractionation of δ34S occurs(close to 2‰) (Pisapia et al., 2007; Heidel et al., 2009). Consequently,the production of sulphate, via oxidation of sulphide minerals, yieldsδ34S values of sulphate that are similar to those of the source sulphides.However, bacterial reworking of the original sulphate has a specific sig-nature expressed in the enriched δ34S values of dissolved sulphate(Strebel et al., 1990; Aharon and Fu, 2000; Spence et al., 2001).

δ13C values of DIC varying from −10.8 to −24.3‰ also indicatebacteria's influence in groundwater from the Cm–V aquifer system(Raidla et al., 2012). In the studied samples sulphate is a minoranion and HCO3− the dominant anion with concentrations between100 and 244 mg L−1 (Table 1). There is a negative co-variance of sul-phate and bicarbonate, whereas the most depleted δ13C values aremainly found in groundwater samples with the lowest sulphate con-centrations (Fig. 4a), except in the abundant presence of isotopicallydepleted carbonate cement (δ13C = −9 to −13‰) (Petersell et al.,1991; Raidla et al., 2012). Depleted δ13C values of DIC in associationwith low dissolved sulphate concentrations suggest the biologicalorigin of dissolved carbon, which is derived from anaerobic microbi-al oxidation of organic materials by sulphate reducing bacteria usingSO4

2− as the terminal electron acceptor (e.g. Jørgensen and Postgate,1982; Wadham et al., 2004).

During sulphate reduction significant isotope discrimination of sul-phur isotopes in the residual sulphatemodified by bacterial dissimilato-ry sulphate reduction has been observed both in experiments (Lloyd,1968; Mizutani and Rafter, 1973; Fritz et al., 1989) and in natural

0 403020100

-5

-10

-15

-20

-35

SO (mg L )42- -1·

carbonatecement( C -10 )δ13 ‰

-30

-25

a

Fig. 4. δ13C correlationwith SO42− (a) and δ34S (b) in the Cambrian–Vendian aquifer system. Isot

line.

environments (Zak et al., 1980; Böttcher et al., 1998; Ku et al., 1999;Aharon and Fu, 2000). The enrichment of dissolved sulphate δ34S along-side the depletion of δ13C in the groundwater of the Cm–V aquifer sys-tem (Fig. 4b) provides strong evidence of oxidation of organic materialthrough sulphate reduction bybacterial activity. Consequently, sulphatereduction leads to a simultaneous decrease in SO4

2− content andsulphur's isotopic enrichment. Nevertheless, no clear trend derivesfrom our data (Fig. 5), although the most positive δ34S values arefound in samples with the lowest sulphate concentration.We speculatethat thismay be caused by the secondary sulphide oxidation. In additionto oxygen, pyrite may be oxidized by NO3− or Fe3+ (e.g. Bottrell andTranter, 2002). Anoxic conditions prevail in the Cm–V aquifer system,where nitrogen oxides are missing and iron is represented in the formof Fe2+. Alternatively, Lefticariu et al. (2010) showed that radiolysis ofwater caused by radiation dose coupled to oxidation of sulphidescould be a source of sulphate in geological environments. Indeed, theδ18OSO4

values are more similar to δ18Owater values in the Cm–V aquifersystem in the areas, where the highest radon and radium contents werefound. However, the effect of this phenomenon is unclear and deservesfurther study.

5. Conclusions

In the Cm–V aquifer system, northern Estonia, oxygen isotope com-position of dissolved sulphate suggests that the sulphate was derivedfrom oxidation of sulphide minerals when isotopically depleted water(δ18Owater=−18 to−23‰) andmolecular oxygen contained in glacialmeltwater, intruded into the aquifer system during the Last GlacialMaximum.

Dissolved sulphate δ34S values are enriched compared with δ34Svalues of sedimentary-diagenetic and possibly hydrothermal sulphides

302010 400

bacterial activity

50 60

δ34S ( )SO4 ‰

b

opically depleted carbonate cement in rocks (δ13C=−9 to−13‰) ismarkedwith dotted

153V. Raidla et al. / Chemical Geology 383 (2014) 147–154

in the aquifer system's rocks. Additionally, the enrichment of sulphateδ34S is accompanied by depleted δ13C values of the dissolved inorganiccarbon in the groundwater of the Cm–V aquifer system. These phenom-ena can be explained by bacterial sulphate reduction that causes δ34Sisotope fractionation in the residual sulphate.

Acknowledgements

The activities of the present study were supported by MOBILITASPostdoctoral Research Grant 2009 MJD17 (A.M.), Estonian ScienceFoundation Grants ETF9196 and IUT20-34 (K.K.) and ETF8948 (R.V.)and Doctoral School of Earth Sciences and Ecology (V.R.). The paper isa contribution to IUT19-22 and to the INQUA/UNESCO supported G@GPS Project. The manuscript was improved by constructive commentsfrom Prof. Simon Bottrell and Prof. Stanislaw Halas. We thank Dr.Stephan M. Weise for δ34SSO4

and δ18OSO4analyses and Mrs. Helle

Pohl-Raidla for the English revision.

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