Carbon isotope systematics of the Cambrian–Vendian aquifer system in the northern Baltic Basin: Implications to the age and evolution of groundwater

Applied Geochemistry 27 (2012) 2042–2052

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Carbon isotope systematics of the Cambrian–Vendian aquifer system in the northern Baltic Basin: Implications to the age and evolution of groundwater Valle Raidla a,⇑, Kalle Kirsimäe b, Rein Vaikmäe a, Enn Kaup a, Tõnu Martma a a b

Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Department of Geology, University of Tartu, Ravila 14a, 50411 Tartu, Estonia

a r t i c l e

i n f o

Article history: Received 6 June 2011 Accepted 12 June 2012 Available online 21 June 2012 Editorial handling by W.M. Edmunds

a b s t r a c t Groundwater in the Cambrian–Vendian aquifer system has a strongly depleted stable isotope composition (d18O values of about 22‰) and a low radiocarbon concentration, which suggests that the water is of glacial origin from the last Ice Age. The aim of this paper was to elucidate the timing of infiltration of glacial waters and to understand the geochemical evolution of this groundwater. The composition of the dissolved inorganic C (DIC) in Cambrian–Vendian groundwater is influenced by complex reactions and isotope exchange processes between water, organic materials and rock matrix. The d13C composition of dissolved inorganic C in Cambrian–Vendian water also indicates a bacterial modification of the isotope system. The corrected radiocarbon ages of groundwater are between 14,000 and 27,000 radiocarbon years, which is coeval with the advance of the Weichselian Glacier in the area. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The groundwater of the Cambrian–Vendian aquifer system found in the northern part of the Baltic Basin, northern Estonia has a depleted stable isotope composition with d18O values of about 22‰, which has been suggested to be due to a glacial meltwater intrusion under the hydrostatic head of the Scandinavian continental ice during the last Ice Age (e.g., Punning et al., 1987; Vaikmäe et al., 2001). Raidla et al. (2009) interpreted the variations in geochemistry and O isotope composition of the Cambrian–Vendian groundwater as resulting from a mixing of three end-members, glacial meltwater, relict saline brine of Na–Ca–Cl composition, and recent meteoric water. The residence time of the water and the time of the mixing event(s), however, are unknown. The low 14C activity in the range of 1.4–9.3 pmC of the Cambrian–Vendian groundwater suggests that the water has been in the aquifer for 19,000–35,000 conventional radiocarbon years. Although the onset of Late Weichselian glaciation has not been directly dated in Estonia, it is unlikely to have occurred before 20– 22 ka ago (Kalm, 2006), while the start of deglaciation has been dated to 12,500 radiocarbon years ago and continued over 2 ka (Raukas, 2009). If the timing of the glaciation in this area is adopted then the radiocarbon dating would suggest that the infiltration of Cambrian–Vendian waters to the aquifer system occurred before the advance of the glacier itself, probably in periglacial conditions. Pearson and Hanshaw (1970), Clark and Fritz (1997) and Geyh (2000) have shown that conventional 14C-age calculation methods ⇑ Corresponding author. Tel.: +372 6203038; fax: +372 6203011. E-mail address: [email protected] (V. Raidla). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.06.005

may not be appropriate for converting the 14C-activity of the water into a realistic age. This results from the complicated C cycle of groundwater, which is controlled by several processes, such as dissolution of carbonate minerals and isotope exchange between water and the rock matrix, that cause progressive mixing/dilution of radiocarbon and the old C of the aquifer. Indeed, Raidla et al. (2009) showed that the Ca2+ and Mg2+ activity in the Cambrian– Vendian groundwater is controlled by the equilibrium dissolution of sedimentary dolomite and calcite, which suggests that the water’s C reservoir is influenced by 14C free inorganic C. Additionally, degradation of organic C can add C to the groundwater influencing the age of the water, causing an apparently older age (Herczeg et al., 1991; Aravena et al., 2003; van Stempvoort et al., 2005). In this contribution the chemical and the C isotope composition of the groundwater of the Cambrian–Vendian aquifer at its northern margin in the Baltic Basin were studied. The goal was to reveal the factors controlling the C isotope chemistry of the aquifer’s water and to elaborate the mass-balance model of the carbonate system for modeling the realistic groundwater residence time in the Cambrian–Vendian aquifer system. 2. The study area Ediacaran and Cambrian sandstones alternating with clays and silty clays represent the Cambrian–Vendian aquifer system in the northern part of the Baltic Basin. The thickness of the aquifer decreases from 80 to 90 m in NE Estonia to only a few meters on the West Estonian islands, disappearing entirely in South Estonia (Fig. 1). The aquifer pinches out into the Gulf of Finland and to

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V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

Gulf of Finland

100 km

0.3 0.5 Bo C-V unda r sys aqui y of tem fer

N

E

W 1 0.5 n grou

Russia

Lithuania

of ry da un lay Bo tlin c Ko

Latvia

w

een Boundary betw a tov Voosi and Lon formations

Estonia

>2

er flo dwat

Finland

S

Devonian Silurian Ordovician Cambrian Ediacaran

Elevation (m, a.s.l.)

major faults deep valleys

300 W E Lontova aquitard 200 Voronka aquifer Voosi Formation 100 0 -100 -200 -300 -400 Cambrian-Vendian aquifer system Kotlin clays Gdov aquifer -500 0 20 40 60 80 100 km

300 200 100 0 -100 -200 -300 -400 -500

Section

2

line -1 g·l aquifer outcrop area Salinity,

LEGEND basement carbonate rock

claystone

Elevation (m, a.s.l.)

S 300 200 100 0 -100 -200 -300 -400 -500

N Quaternary ian

Silur

Devonian

n

vicia

Ordo

brian caran Edia

Cam

Lokno-Mõniste uplift

0

ent

asem

lline b

crysta

300 200 100 0 -100 -200 -300 -400 -500

sand- and siltstone overburden

fault

20 40 60 80 100 km

Fig. 1. Geology of Estonia and West–East and North–South cross-sections.

the bottom of the Baltic Sea. It is confined between the underlying crystalline basement of Palaeoproterozoic age and the overlying Lükati–Lontova aquitard. The crystalline basement comprises mainly gneisses and biotite gneisses (Koistinen et al., 1996), and its upper part is fractured and weathered. The aquitard is composed of silty clays, siltstones and clays of Lower-Cambrian age. The thickness of the clayey complex is 90–100 m in North Estonia, but decreases towards the south until disappearing at the LoknoMõniste structural uplift in South Estonia. The aquitard has a strong isolation capacity with vertical hydraulic conductivity of 107–105 m day1 (Perens and Vallner, 1997). Westwards, where the Lükati–Lontova clays are gradually replaced by the interbedded clay and sandstone of the Voosi Formation, the isolation capacity of the aquitard is smaller than that of Lükati–Lontova clays (vertical hydraulic conductivity >105 m day1) (Perens and Vallner, 1997). At its northernmost margin, the aquitard and the waterbearing bedrock formation are penetrated by a dense set of buried valleys filled with loamy till and glaciofluvial gravel in the lower

parts of the valleys (Tavast, 1997). The latter serve as recharge areas through which the water from the upper groundwater aquifers infiltrates to the Cambrian–Vendian aquifer system. The valleys are oriented approximately perpendicularly to the north Estonian coastline (Fig. 1). In northern and central Estonia, the siliciclastic rocks of the Cambrian–Vendian aquifer are in most parts of the area covered by up to 300 m of Ordovician and Silurian marine carbonate rocks. In the northeastern sector of the aquifer, the water bearing succession is divided by Kotlin clays (thickness up to 53 m) into two independent aquifers: the upper Voronka and the lower Gdov aquifer (Fig. 1). The Voronka aquifer consists of quartzose sand- and siltstone with a thickness of up to 45 m in northeastern Estonia. The Gdov aquifer is formed by up to 68 m thick mixed-grain sandand siltstone and directly overlies the Precambrian basement. The clays of the Kotlin Formation serve as an upper confining unit. In northern Estonia, the hydraulic conductivity of the water-bearing rock is 0.5–9.2 m day1, with an average of 5–6 m day1. Transmis-

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V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

sivity in northeastern Estonia is 300–350 m2 day1, decreasing in southerly and westerly directions (Perens and Vallner, 1997). The depth of the aquifer increases towards the south, being 450–600 m in South Estonia and more than 700 m in northern Latvia and Russia. The groundwater is under pressure. The aquifer recharges in the southern part of the basin and the water flows to the north and NW. During the Pleistocene the glacial ice sheet would have increased the hydraulic pressure at the base of the glacier, which then would have reversed the regional groundwater flow. The inflow of the glacial meltwater into the aquifer probably occurred along the outcrop area of the aquifer at the basin’s margin. The Cambrian–Vendian groundwater in northern Estonia is characterized by Cl–HCO3–Na–Ca and Cl–HCO3–Ca–Na type of waters with TDS content between 0.4 and 1.0 g L1 (Savitskaja and Viigand, 1994). In southern and central Estonia, the aquifer system contains relict saline groundwater with TDS values of up to 22 g L1. In this zone Cl and Na+ predominate over all other ions (Karise, 1997).

carbon and C stable isotope composition were measured. If a radiocarbon sample was not taken, the sample for d13C was collected separately in a 5 L vessel and DIC precipitated similarly to the earlier description. For d18O of water 20 mL samples were collected. Measurements of radiocarbon and stable isotope ratios of d18O and d13C in the water samples were performed at the Department of Isotope-Paleoclimatology of the Institute of Geology at Tallinn University of Technology. Radiocarbon activities in groundwater were measured with a two canals liquid scintillation b-spectrometer. The 14C results are reported as percentage of the modern standard (pmC) and as the Apparent Radiocarbon Age (BP). The measurement error of activity of radiocarbon is estimated at ±0.5 pmC. The stable isotope ratio of O in water (d18O) and C in dissolved inorganic C (d13CDIC) were determined on a Finnigan MAT Delta-E mass spectrometer in samples collected until June 2006 and on a Thermo Fisher Scientific Delta V Advantage mass spectrometer after that date. Reproducibility was better then ±0.1‰ for water d18O and ±0.5‰ for d13CDIC. The results are expressed in ‰ deviation relative to Vienna Standard Mean Ocean Water (VSMOW) and Vienna Peedee Belemnite (VPDB) for O and C, respectively.

3. Material and methods In 1996–2009 the groundwater was sampled and analyzed in operational private and/or municipal water supply and observation wells (Fig. 2, Tables 1 and 2). In addition, some results of earlier studies were used: Savitskaja and Viigand (1994), Vaikmäe et al. (2001), Marandi et al. (2004). Sampling was done using the production pumps in the water supply wells and submersible pumps in the observation wells. In the field Eh, pH, electrical conductivity and temperature parameters were measured (Table 1). Samples were collected and analyzed for radioactive (14C) and stable isotope ratios (d18O, d13C), as well as for anions and cations (Tables 1 and 2). The major anion and cation composition was determined at the Geochemistry Laboratory of the Estonian Geological Survey by standard analytical techniques. Saturation indices for minerals and charge balance (the condition of electroneutrality of the groundwater) have been calculated using the computer code PHREEQC. For radiocarbon measurements large volume water samples (200–300 L) were taken. The pH of each C isotopes sample was set to pH > 12 by adding the required amount of concentrated CO2-free NaOH solution. Analytical grade BaCl2 or Ba(OH)2 in excess of expected DIC was added to precipitate DIC in the form of BaCO3. The same deposit was used for samples where both radio-

4. Results and discussion The geochemical composition, isotope data and mineral saturation indices of the water samples from the Cambrian–Vendian aquifer system are reported in Tables 1 and 2. 4.1. The carbon system The concentration of HCO 3 , the main C carrier in the aquifer system, increases gradually eastwards from about 100 mg L1 in West Estonia to 250 mg L1 in NE Estonia (Fig. 2). In East Estonia, where the aquifer system is divided into the upper Voronka and the lower Gdov aquifers, the HCO 3 concentration in the former is 150–230 mg L1 and in the latter, 120–170 mg L1 (exceptionally 250 mg L1) (Table 1). In most parts of the research area the content of CO2 is low, varying from 0.001% to 0.003% of the gas amount (Pihlak et al., 2003). Also, the amounts of CO2 are insignificant. 3 Therefore, HCO 3 can be considered to represent the dissolved inorganic C (DIC). The d13CDIC composition of the Cambrian–Vendian aquifer system water shows a considerably depleted composition of C in the

25 8 4 5

11

9 7 6

10

14

12 17

1

15

24 13

16

19

26 27 18

30 21 31 29 22 20

23 33

28

3

Cm-V 2

V 2vr -16.4

-14.3 -14.4 -15.5

-13.8

-15.4

-14.1 -15.2 -11.5

-20.8 -17.1 -20.1 -21.5 -17.8 -24.3 -17.7 -18.3 -21.7 -18.8 -21.5 -17.8 -16.4 -19.2 -16.9 -17.1 -17.8 -20.2 -20.2 -19.4 -18.9

V 2gd

>200 170-200 140-170 <140

50 km -12.0 -11.7

-

HCO3 , mg·L

-1

13 Fig. 2. Variation of d13CDIC values and HCO 3 concentration in the Cambrian–Vendian aquifer system. The d CDIC values are indicated in ‰ next to the circles showing the degree of HCO 3 change in the area.

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V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

Table 1 Chemical composition of the Cambrium–Vendian groundwaters in the northern part of the aquifer. Cm–V – Cambrian–Vendian aquifer not specified; V2gd – Cambrian–Vendian aquifer, Gdov subaquifer; V2vr – Cambrian–Vendian aquifer, Voronka subaquifer. 1– Savitskaja and Viigand (1994); 2 – Marandi et al. (2004). No.

Well No.

Location

Aquifer

Depth

Date

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

2968 3344 9997 538 552 206 4448 14146 14784 4734 8914 1096 2898 10714 3348 2384 10703 2302 2260 4778 2217 2470 2092 2738 3323 2388 2308 2249 2269 2432 2207 2084

Dirhami port Haapsalu city Risti village Pakri peninsula Keila city Saku brewery Tallinn. Nõmme Tallinn. Kopli Tallinn. Kopli Muuga port Muuga port Kehra city Vinni village Rakvere city Tapa city Aseri city Rakvere city Kiviõli city2 Aa village Jõhvi city Sillamäe city Viivikonna mining Narva-Jõesuu city Rakvere city2 Unukse village2 Aseri city Kiviõli city Kohtla-Nõmme city Jõhvi city Toila spa Sillamäe city1 Narva-Jõesuu city1

Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V Cm1 Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V V2vr V2vr V2vr V2vr V2vr V2vr V2vr V2gd V2gd V2gd V2gd V2gd V2gd V2gd V2gd V2gd

184 295 280 210 214 200 195 60 66 100 75 230 312 235 318 194 222 180 125 180 124 180 101 268 207 185 254 255 270 230 220 211

El. cond.

Tem

lS cm1 °C

m 20-Jun-01 20-Jun-01 13-Mar-07 03-Nov-97 19-Mar-07 20-May-97 05-Apr-01 28-Feb-01 20-Feb-01 12-Aug-96 30-Aug-95 19-Apr-05 13-Jun-01 22-Aug-96 29-Aug-02 07-Jun-07 16-Jan-08 26-Aug-02 07-Jun-07 08-Jun-07 16-May-97 23-May-97 24-Sep-07 26-Aug-02 27-Aug-02 16-Jan-08 08-Jun-07 08-Jun-07 08-Jun-07 07-Jun-07 04-Jul-94 04-Jul-94

8.2 8.6 8.6 8.2 7.7 7.4 8.2 8.1 8.6 8.0 8.1 7.8 8.2 7.8 7.9 7.6 7.9 7.9 7.4 7.4 7.8 7.8 8.5 7.8 7.8 7.8 7.6 7.3 7.1 7.6 7.4 8.5

667 617 329 300 519 553 590 540 490 728 310 940 692 550 819 474 402 841 426 561 496 524 1372 1150 548 436 547 988 812 1041 536

range of 11‰ to 24‰ (Table 2). The isotopic composition of C is enriched in the wells of the western part of the study area compared to those in the east (Fig. 2). The highest d13CDIC values are found in West Estonian coastal areas (11‰ to 12‰). In the central part of the studied area the d13CDIC values are about 16‰ to 18‰. In the eastern part of the study area, where the aquifer system is divided into two aquifers (Gdov and Voronka), the d13CDIC values of the upper and the lower aquifer are distinctly different. d13CDIC values of the Voronka aquifer are relatively stable at about 17‰ (exceptionally 20.9‰), whereas the wells of the lower Gdov aquifer show a d13CDIC value variation from 17‰ to 24‰ (Table 2). At the same time the d13CDIC in the uppermost groundwater in an active exchange zone recharged by modern precipitation is characterized by quite uniform values of 10‰ (Vaikmäe et al., 2001). The latter are controlled by the equilibrium between CO2 derived from plant respiration and degradation of organic material in the soil (d13C in the range of 24‰ to 30‰, Vogel, 1993), and by dissolution of carbonate minerals in the overlying marine Palaeozoic carbonate rocks and Quaternary sediments (d13C values around 0‰, Kaljo et al., 2004). However, the outcrop area of the Cambrian–Vendian aquifer system in the bottom of the present day Gulf of Finland where the sub-glacial meltwater was introduced to the aquifer, is several kilometres north of the northernmost distribution line of Ordovician marine carbonates. Usually, d13CDIC values in water increase concurrently with the increase in DIC concentration through dissolution of carbonate (e.g., Clark and Fritz, 1997). However, an opposite trend with depletion of the d13CDIC composition with increasing HCO 3 concentration in the Cambrian–Vendian groundwater in northern Estonia is obvious (Fig. 3). Similar depletion in d13CDIC values while DIC increases has been noticed in groundwaters affected by input of organic C (Herczeg et al., 1991; van Stempvoort et al., 2005;

11.2 12.1 9.8 9.4 9.3 9.6 9.7 9.2 8.8 9.3 9.1 10.5 12.9 12.0 10.3 11.1 11.7 11.2 9.8 12.1 10.5 11.5 9.6 12.3 11.9 10.9 11.0 13.9 14.1 13.5 12.4 9.2

Eh

Ca2+

mV

mg L1

110 118

79 108 108 93

196 54 74 212 50 45 69 119 80 216 70 58 35 23 53 56 126

60.6 38.6 38.3 42.3 40.9 42.7 47.9 48.2 46.6 63.1 32.3 96.4 49.1 88.3 44.6 64.1 34.0 22.5 40.1 12.6 20.0 7.8 10.8 97.8 75.4 66.5 57.3 49.1 58.3 39.5 17.4 48.7

Mg2+

10.9 10.6 9.0 10.5 11.8 8.9 11.0 18.3 17.3 21.3 13.1 26.6 18.2 26.9 16.5 19.4 16.0 7.3 13.4 5.3 2.4 5.3 4.1 23.1 20.9 17.7 27.1 20.1 24.2 17.5 7.0 21.8

Na+

K+

HCO 3

SO2 4

Cl

SIdol

SIcal

79.6 76.8 73.8 43.7 57.2 64.4 65.1 31.4 32.2 43.4 26.5 108.2 112.0 143.0 97.3 106.6 98.0 140.0 113.3 161.1 197.0 177.8 206.3 120.0 120.0 110.0 236.5 140.0 300.0 135.6 350.0 684.5

8.4 7.4 6.0 7.4 8.6 7.9 8.2 9.6 9.5 7.3 8.8 8.7 9.5 12.3 9.6 8.6 8.5 6.9 7.5 4.3 4.5 4.5 6.0 9.9 9.2 8.0 10.2 8.0 11.3 9.1 6.0 10.0

115.9 97.6 109.8 109.8 122.0 128.1 152.5 161.7 161.7 146.4 183.1 155.6 167.8 183.0 167.8 244.5 201.3 190.0 158.6 189.1 231.9 201.3 195.2 180.0 240.0 250.2 164.7 152.5 122.0 164.7 158.6 170.8

36.0 38.8 28.0 26.7 26.4 21.6 10.6 0.3 5.6 6.6 7.0 0.5 0.2 0.6 0.2 2.0 0.5 0.2 6.0 2.0 1.9 1.0 12.8 0.2 0.2 5.8 1.9 9.5 0.5 0.6 1.7 3.2

161.0 131.0 139.3 94.5 123.0 128.7 122.0 99.1 92.7 185.8 46.8 421.7 207.0 345.0 173.0 198.5 150.0 180.0 195.7 184.7 192.2 195.0 225.1 330.0 250.0 212.0 644.5 275.5 588.9 283.2 488.9 1086.3

0.27 0.76 0.69 0.29 0.84 1.49 0.38 0.45 1.34 0.31 0.27 0.07 0.80 0.39 0.03 0.03 0.13 0.46 1.18 1.80 1.08 1.19 0.05 0.37 0.47 0.35 0.32 1.00 1.52 0.38 1.80 1.02

0.44 0.58 0.60 0.38 0.21 0.46 0.45 0.38 0.84 0.33 0.28 0.24 0.53 0.37 0.17 0.18 0.15 0.06 0.41 0.79 0.14 0.59 0.17 0.42 0.43 0.39 0.03 0.40 0.67 0.05 0.78 0.63

McIntosh and Walter, 2006), though the Ca2+ and Mg2+ activity ratio in Cambrian–Vendian groundwaters suggests equilibrium with sedimentary dolomite and calcite (Raidla et al., 2009). This implies that both dissolution and isotope exchange with carbonate minerals and oxidation of light, probably organic, C has influenced the isotopic composition of the Cambrian–Vendian groundwater. Siliciclastic rocks of the Cambrian–Vendian aquifer system contain carbonate cement, mainly Fe-rich dolomite (Raidla et al., 2006). The cemented zones are in most cases found at contacts with clay beds of Cambrian Lontova and Ediacaran Kotlin age. The d13C analysis of these carbonate cements shows a negative isotopic composition of about 10‰ to 12.4‰ in the central part of the study area (Kalle Kirsimäe, unpublished data, 2010). 4.2. The sulfate system The SO2 content of the Cambrian–Vendian aquifer system 4 shows a zonal variation, with a SO2 content of 10–39 mg L1 in 4 the western part of the aquifer, and only up to 0–12.8 mg L1in the eastern area (Table 1). The decrease in SO2 4 content is gradual over about 50 km in a west-to-east direction in the western part of the area and the SO2 4 concentration does not change remarkably in the central and eastern part of the research area. There are two possible sources of SO2 4 : relict evaporitic sulfate and oxidation of sulfide minerals (mainly pyrite). Direct sources of SO2 4 , such as gypsum deposits, occur neither in the Cambrian nor the Ediacaran rocks, whereas pyrite is common in both Ediacaran and Cambrian sediments (Raidla et al., 2006). Oxygen concentrations in pressurized glacial meltwater can be three to five times greater than normal at equilibrium with the atmosphere at 0 °C (Stauffer et al., 1985; Glynn et al., 1999; Guimerà et al., 1999). This would suggest that the O2-rich meltwater penetrating into the aquifer provoked

No.

Well No.

Location

Aquifer

Depth

Date

m

d18O

14

d13Cmeasured

‰ VSMOW

pmC

‰ VPDB

C

d13Ccalculated

DICrecharge

DICcal

DICdol

DICSO4

%

Uncorrected age

Corrected age by Coetsiers and Walraevens (2009) 14

2968 3344 9997 538 552 206 4448 14146 14784 4734 8914 1096

Dirhami port Haapsalu city Risti village Pakri peninsula Keila city 3 Saku brewery3 Tallinn, Nõmme Tallinn, Kopli Tallinn, Kopli Muuga port Muuga port Kehra city

Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V Cm–V Cm1 Cm–V Cm–V Cm–V Cm–V

184 295 280 210 214 200 195 60 66 100 75 230

20-Jun-01 20-Jun-01 13-Mar-07 3-Nov-97 27-Aug-97 20-May-97 5-Apr-01 15-May-97 20-Feb-01 12-Aug-96 12-Aug-96 26-Nov-09

22.1 20.0 20.6 21.3 21.1 21.1 21.6 20.6 20.7 22.3 20.3 20.7

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

2898 10714 3348 2384 10703 2302 2260 4778 2217 2470 2092 2738 3323 2388 2308

Vinni village Rakvere city3 Tapa city Aseri city Rakvere city Kiviõli city Aa village Jõhvi city Sillamäe city3 Viivikonna mining3 Narva-Jõesuu city Rakvere city Unukse village Aseri city Kiviõli city

Cm–V Cm–V Cm–V Cm–V V2vr V2vr V2vr V2vr V2vr V2vr V2vr V2gd V2gd V2gd V2gd

312 235 318 194 222 180 125 180 124 180 101 268 207 185 254

13-Jun-01 22-Aug-96 29-Aug-02 25-Apr-06 25-Sep-07 26-Apr-06 25-Apr-06 25-Apr-06 16-May-97 23-May-97 24-Sep-07 12-Nov-07 12-Nov-07 16-Jan-08 24-Nov-09

20.1 20.3 20.3 20.7 20.1 19.7 20.0 19.3 19.3 18.7 18.4 20.3 21.1 21.3 19.9

28 29 30

2249 2269 2432

Kohtla-Nõmme city Jõhvi city Toila spa

V2gd V2gd V2gd

255 270 230

8-Jun-07 25-Apr-06 24-Nov-09

19.7 18.8 19.4

31 32

2207 2084

Sillamäe city3 Narva-Jõesuu city

V2gd V2gd

220 211

23-May-97 24-Sep-07

18.5 18.4

6.8 5.3 9.3 2.3 3.3 3.0 3.0 2.2 1.8 2.9 4.3

1.5 3.0

1.4 1.9

5.1

2.7

11.5 11.7 12.0 13.8 14.1 15.2 15.5 14.4 14.3 15.4 16.4 16.9/17.5

11.8 11.3 14.0 13.9 14.3 15.0 15.8 16.6 16.2 15.2 16.9 15.9

0.4 0.6 0.5 0.5 0.4 0.4 0.4 0.3 0.3 0.3 0.4 0.2

51.9 36.9 37.0 35.1 29.2 38.4 32.8 16.2 17.4 22.2 13.1 27.7

43.7 60.8 46.7 48.8 52.8 40.1 39.9 53.9 54.7 55.5 52.8 46.1

4.1 1.7 15.8 15.6 17.6 21.0 27.0 29.6 27.5 22.0 33.7 26.1

22000 24000 19000 30000 28000 28000 28000 30000

23406 18419 23893 13219 16559 15573 17610 16755

3458 6856 4628 16127 14060 16390 18162 21472

7808 3013 3199 15417 14216 16639 16977 17227

32000 28000 25000

15917 16499 26622

20896 20216 14923

21211 20378 16310

19.4 18.8 18.9 20.1 18.3 16.4 17.8 17.1 17.7 17.8 20.9 19.2 17.1 21.5 20.2/20.0

18.0 16.1 18.7 17.0 18.1 18.3 18.0 18.2 16.8 17.8 16.3 16.1 16.9 16.8 16.9

0.2 0.2 0.3 0.2 0.2 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

20.2 24.1 16.4 21.0 5.8 8.1 14.7 2.9 1.1 0.0 2.9 32.1 25.6 25.5 18.9

42.9 48.6 41.2 47.3 56.7 53.3 48.4 58.9 68.1 63.7 68.7 40.9 43.0 43.5 49.7

36.7 27.1 42.2 31.5 37.3 38.4 36.6 37.9 30.6 36.1 28.2 26.8 31.2 30.8 31.2

34000 28000

19585 23406

23822 22001

27190 21834

35000 32000

15676 16935

25673 24540

26797 24415

24000

26475

15111

18651

20.2 17.8 24.3/23.7

17.4 17.2 19.0

0.3 0.2 0.3

12.5 12.1 9.2

54.3 55.2 47.4

32.9 32.5 43.1

21.7 20.8

16.5 14.6

0.2 0.1

3.5 5.1

67.3 75.7

29.0 19.0

29000

21687

19690

25134

Corg = 100 pmC

V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

1 2 3 4 5 6 7 8 9 10 11 12

Corrected age by d13C mixing model 14 Corg = 100 pmC

14

Corg = 0 pmC

2046

Table 2 The isotopic composition, the results of geochemical modeling and the radiocarbon data of the Cambrian–Vendian groundwater. Cm–V – Cambrian–Vendian aquifer not specified; V2gd – Cambrian–Vendian aquifer, Gdov subaquifer; V2vr – Cambrian–Vendian aquifer, Voronka subaquifer. d13CDIC samples precipitated with Ba(OH)2 are given in italics. 3 –Vaikmäe et al. (2001).

2047

-5

-10

-10

-15

-15

C, ‰

-5

13

13

C, ‰

V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

-20

-20

-25 -25 -30 0

100

200

HCO3,

mg·L

300

-30

0

-1

20

30 2-

SO4 , mg·L

Fig. 3. d13CDIC and HCO 3 correlation in the Cambrian–Vendian aquifer system.

SO2 4

an initial oxidation of pyrite, which provided most of the found in the Cambrian–Vendian aquifer system. On the other hand, Drever (1988), Bottrell and Tranter (2002), and Heidel and Tichomirowa (2011) have shown that sulfide oxidation can be sustained using the alternative oxidization agent Fe3+ under anoxic conditions (1): 2þ FeS2 þ 14Fe3þ þ 8H2 O $ 2SO2 þ 16Hþ 4 þ 15Fe

ð1Þ

3+

A possible source of Fe required in the process could be hematite (Fe2O3), which is common in the clayey rocks of the Cambrian– Vendian aquifer system (Raidla et al., 2006). 2 There is a negative co-variance between HCO 3 and SO4 in the  Cambrian–Vendian groundwater where higher HCO3 concentrations correspond to lower SO2 4 concentrations (Fig. 4). Moreover, the most negative d13CDIC values are measured in waters with the lowest SO2 4 concentrations (Fig. 5). This would indicate anaerobic oxidation of organics that uses SO2 4 as the terminal electron acceptor, whereas the reduction of SO2 4 is typically catalyzed by bacteria of the genus Desulfovibrio and others (e.g., Jørgensen, 1982) producing dissolved sulfide and mineralized C, according to the generalized reaction (2):  2CH2 O þ SO2 4 ! H2 S þ 2HCO3

ð2Þ SO2 4

Therefore, it is possible that co-existence of and organic materials could have influenced the C system of the aquifer water by adding isotopically depleted C to the groundwater DIC during eastern part of the aquifer western part of the aquifer

40

10

30

20

40

50

-1

Fig. 5. d13CDIC correlation with SO2 in groundwaters of the Cambrian–Vendian 4 aquifer system.

organic C oxidation, though the measured HS contents, as an expected indication of bacterial activity in Cambrian–Vendian groundwater, stayed typically under the detection limit. Nevertheless, the absence of HS could be explained by the reaction of HS produced in the reduction processes with dissolved Fe2+ or Fe3+ oxides and precipitation of authigenic Fe-sulfide minerals (Appelo and Postma, 1999). The organic C in the aquifer could originate from the decomposition of organics in the Cambrian and Ediacaran clays, which contain up to 2.5% of organic material (Raidla et al., 2006). Alternatively, CH4 oxidation could be considered as a possible source of the old C. The CH4 content in the Cambrian–Vendian groundwater varies typically between 0.01% and 3% of the total gas, except for the Gdov subaquifer in NE Estonia, where the CH4 content increases to nearly 20% of the total gas (Pihlak et al., 2003). The d13CCH4 composition of the Cambrian–Vendian groundwater varies from 74.6‰ to 92‰ (Vaikmäe et al., 2001; Voitov et al., 1982), which indicates a bioorganic origin of the CH4 (Clark and Fritz, 1997). The source of CH4 in the Cambrian–Vendian aquifer system is unknown. Generally, it is assumed that the CH4 originates from anaerobic decomposition of the sediments of Eemian interglacial age (110,000 BP) (Raukas and Kajak, 1997). However, though bacterial SO2 4 reduction by CH4 has been demonstrated in CH4 submarine seeps and gas hydrate marine sediments (e.g., DeLong, 2000), CH4 is generally considered as biologically inert in anaerobic environments (e.g., Higgins et al., 1981) and therefore CH4 oxidation was not considered here. On the other hand, the organic C could have been derived from organic sediments/soils rich in organic matter (peat) that occur in the Gulf of Finland along the northern coast of Estonia and were deposited in the periglacial environment during the Weichselian prior to glacier advance into the area. It is possible that the meltwater, which penetrated into the Cambrian and Ediacaran rocks through outcrop areas in the Baltic Sea and the Gulf of Finland, was enriched with this easily degradable organic matter.

10

4.3. 0 50

100

150

200 -

HCO3, mg·L

250

300

-1

2 Fig. 4. HCO correlation in groundwaters of the Cambrian–Vendian 3 and SO4 aquifer system.

14

C age of the Cambrian–Vendian groundwater

The depleted stable O isotope composition and the low radiocarbon activity of the groundwater in the Cambrian–Vendian aquifer system have been interpreted earlier as indicators of its glacial origin while the timing and mechanism(s) of the recharge have remained unclear. The 14C activity measurements of the Cambrian– Vendian groundwater suggest the average conventional age of

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V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

the groundwater as varying between 19,000 and 35,000 BP. However, the chemical and isotopic composition of the water suggest that additional (old) C has been added to the water through dissolution of rock matrix and degradation of organic materials. Thus, the true age of the Cambrian–Vendian groundwater cannot be determined directly from the 14C activity of the DIC. Therefore, a mass balance model for identifying the origin and contribution of C from different sources was developed for the correction of the radiocarbon activity measurements in the Cambrian–Vendian aquifer system. The mass-balance model is, first, based on the assumption that the dissolution of carbonates has occurred in a closed system, i.e., under conditions of limited CO2 exchange where the recharge water becomes isolated from the influence of a CO2 reservoir. This means that H2CO3 consumed by calcite or dolomite dissolution is not replaced by dissolving CO2 to maintain a constant equilibrium H2CO3 activity and for every mole of calcite or dolomite dissolved the DIC increases by one mole for calcite and two moles for dolomite. Raidla et al. (2006) have shown that the carbonate cement is mainly composed of dolomite, whereas calcite is generally rare, except in West Estonia. Therefore, dolomite dissolution (3) was considered as the main source of old C.

CaMgðCO3 Þ2 $ Ca2þ þ Mg2þ þ 2CO2 3 ;  þ 2CO2 3 þ 2H $ 2HCO3

ð3Þ 2+

In the western part of the aquifer, where Ca input to the Cambrian–Vendian aquifer is indicated by increased Ca2+ concentrations (Fig. 6) it is assumed that calcite provided additional Ca2+ according to the following reaction:

CaCO3 $ Ca2þ þ CO2 3 ;  þ CO2 3 þ H $ HCO3

ð4Þ

On the other hand, in the eastern part of the research area, where the aquifer system is divided into the lower Gdov and upper Voronka aquifers, significant fluctuations in the Cl/Na+ mole ratio were found in some wells. While in most wells the Cl/Na+ ratio remains between 1.23 and 1.28, in several wells the Cl/Na+ ratio falls below 1 (Fig. 7). The enrichment of Na+ is accompanied with a decrease in Ca2+, Mg2+ and K+. Such phenomena can be explained by cation exchange (Toran and Saunders, 1999; Coetsiers and Walraevens, 2009), where Mg2+, Ca2+ and K+ have replaced the exchangeable Na+ in clay minerals. Possible cation exchange and

1 increasing influence of calcite dissolution

Mg/Ca ratio

0.8

0.6

0.4

0.2

0 W

E

relative position of the wells in west-east direction Fig. 6. Influence of calcite dissolution on groundwaters of the Cambrian–Vendian aquifer system.

700 600 500 400 300 200 100 0 0

200

400

600 -

Cl , mg·L

800

1000

-1

Fig. 7. Cl versus Na+ in groundwaters of the Cambrian–Vendian aquifer system.

the amount of Ca2+ and Mg2+ adsorbed to the exchanger were accounted for by normalizing the Ca2+ and Mg2+ concentrations with respect to the Cl/Na+ ratio of 1.25, which is a typical value for the groundwaters of the Cambrian–Vendian aquifer system not influenced by cation exchange. Cation exchange capacity (CEC) experiments with Estonian Cambrian and Ediacaran clays (Kalle Kirsimäe, unpublished data, 2010) suggest a Ca2+ and Mg2+ mole ratio 1:1 of the exchanged cations. In addition to Ca–Na cation exchange the chemical composition of some of the deepest wells (particularly in the Gdov aquifer) indicates a simultaneous increase in Cl and Ca2+ and decrease in Na+ concentration that is also accompanied with an increase in 226Ra activity of the groundwater. Radium-226 is mostly derived from crystalline basement rocks as a decay product of U (Savitskaja and Viigand, 1994; Mokrik et al., 2009; Forte et al., 2010), and the additional Ca2+ would then indicate a crystalline source (e.g., dissolution of plagioclase). The crystalline rocks underlying the Cambrian–Vendian aquifer system are composed of metamorphosed mafic rocks and alumino-gneisses with a CaO content of 2–7% (Puura et al., 1983; Kirs et al., 2009), and the groundwater in the fractured and weathered crystalline basement is characterized by high TDS (>2000 mg L1) and Cl– Na–Ca or Cl–Ca–Na composition (Karise, 1997). These elevated Ca2+ and Cl concentrations in the Cambrian–Vendian groundwater indicate that in some of the deepest wells, in addition to Ca– Na cation exchange, the mixing with deep saline waters can occur (Fig. 8). The oxidation of organic matter is assumed to be by SO2 4 reduction according to Eq. (2). The amount of the dissolved inorganic C resulting from SO2 reduction was determined by the difference 4 2 in the initial SO2 4 content and the remaining SO4 in the groundwater (e.g., Coetsiers and Walraevens, 2009). The initial SO2 4 content of groundwater in the western part of the aquifer was set to 40 mg L1, corresponding to the highest SO2 concentration 4 (38.8 mg L1) in the measured samples that also have the least depleted 13CDIC composition. In the eastern part of the aquifer, however, the initial SO2 must have been higher. There is a negative 4 correlation between SO2 and HCO 4 3 in the groundwater in the western part of the aquifer (Fig. 4). However, samples in the east1 ern part (with the lowest SO2 ) 4 concentration, typically <5 mg L  have higher HCO3 concentration than would be predicted from the  SO2 4 –HCO3 relationship in the western part of the aquifer. Therefore, assuming that bacterial SO2 reduction is a source of this 4 2 additional HCO 3 , a higher initial SO4 content is supposed in the eastern region, which was set to 60 mg L1. It is possible that extra SO2 was obtained by Fe3+ reduction according to reaction 1. 4

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V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

2.5

-5 Cm-V V2vr

2

-10

Cmeasured, ‰

V2gd

mixing with Ca-Na-Cl composition crystalline basement groundwater

1.5

13

1

-15

-20

mixing with Na-Ca-Cl composition crystalline basement groundwater

0.5

wells with Cl->200 mg·L-1 -25

0 0

500

1000 -

-25

1500

Cl , mg·L 

Fig. 8. The Cl and main cation ratios in groundwaters of the Cambrian–Vendian aquifer system on a plot and map. The Cl and Ca2+ concentrations in the Cambrian–Vendian aquifer increase gradually from west to east and decline abruptly in the eastern part.

2+

However, the Fe concentration in the Cambrian–Vendian groundwater is <0.7 mg L1 (Karro and Marandi, 2003), which means that 3+ the amount of HCO reduction would be less 3 originating from Fe 1 than 0.2 mg L . Moreover, one must consider that Fe2+ is not a reliable measure of Fe3+ reduction as it can be precipitated by subsequently formed HS as a pyrite phase. Estimating C isotope discrimination during degradation of organic material by bacterial activity is difficult because different bacterial types have different fractionation factors (Meckenstock et al., 2004). However, the breakdown of solid phase organic material does not discriminate between 13C and 12C (Boehme et al., 1996). In this case the effect of fractionation is low and d13Corg can be considered equal to 30‰, corresponding to the isotopic composition of d13Corg in Cambrian and Ediacaran sediments in the northern part of the Baltic sedimentary basin (Raidla et al., 2006). The cold climate grasses also have d13C values of about 25‰ to 32‰ (Cerling and Quade, 1993). The recharge water was assumed to represent an undersaturated glacial meltwater that was equilibrated with atmospheric pCO2 and d13Catm of the glacial period, 103.70 atm and 6.9‰, respectively (Leuenberger et al., 1992). It was assumed that the atmospheric CO2 pool had been used for dolomite and calcite dissolution and the isotopic fractionation effect was minimal. The theoretical d13CDIC was calculated through the following mass balance equation: d13 Ccalc ¼

DICrecharge  d13 Crecharge þ DICdol  d13 Cdol þ DICcal  d13 Ccal þ DICSO4  d13 CSO4 DICcalc ð5Þ

where DICrecharge – the amount of CO2 in the infiltrating meltwater was in equilibrium with atmospheric pCO2. It was taken to be 0.5 mg L1; d13Crecharge – d13C of atmospheric CO2, 6.9‰; DICdol, DICcal – calculated HCO 3 content from dissolution of dolomite and calcite according to Eqs. (3) and (4) where 1 mole of Mg2+ would signify the contribution of 2 moles of DIC and excess of Ca2+ (mCa2+–mMg2+) would signify the contribution of 1 mole of DIC; d13Cdol, d13Ccal – d13C of carbonate minerals, 11‰; DICSO4 – calculated HCO 3 content from oxidation of organic matter under bacterial SO4 reduction according to reaction (2) where the disappearance of 1 mole of SO2 4 would signify the contribution of 2 moles of DIC; d13 CSO4 – d13C of the oxidized organic material, 30‰.

-20

-15 13

-1

-10

-5

Ccalculated, ‰

Fig. 9. Comparison of measured d13CDIC values and calculated (modeled) d13C. Samples with elevated salinity (Cl > 200 mg L1) are shown with gray infill.

The compiled mass balance model yields a good correlation between the calculated d13CDIC and the measured d13CDIC (Fig. 9 and Table 2). It appears, however, that the samples with Cl content >200 mg L1 deviate most from the trend, which suggests involvement of additional Ca2+ from relict crystalline basement water that is not accounted for in the mass-balance model. Based on the mass balance equation, it is suggested that the predominant processes controlling the C pool in the Cambrian– Vendian groundwater, are dissolution of carbonate minerals and oxidation of organic matter by bacteria using SO4 reduction. Furthermore, in calculations it is essential to consider the locally occurring cation exchange and the influence of the dissolution of rocks in the crystalline basement. The proportions of dissolved C found by the mass-balance model were used for obtaining the initial amount of radioactive C according to the approach suggested by Coetsiers and Walraevens (2009). If it is assumed that the only source for 14C was atmospheric CO2 (with the initial activity 100 pmC), and C originating from dolomite and calcite dissolution as well as the organic matter oxidized by SO4 reduction is fossil (0 pmC) then, using Eq. (6), strongly negative groundwater radiocarbon (residence) ages are calculated (Table 2):

T¼

5730 q  A0  ln ln2 Ameasured

ð6Þ

where T = time, A0 = modern 14C activity (100 pmC), Ameasured = measured 14C activity, and q = correction value for the initial radiocarbon activity, fraction of the 14C containing C in the dissolved inorganic C pool. Coetsiers and Walraevens (2009) suggest that negative groundwater ages (i.e. the calculated initial activity is lower than the measured 14C activity) can indicate that at least some part of the C resulting from organic material degradation is not fossil but entered the aquifer from the recharge area and hence has the initial activity of 100 pmC. Dissolved organic matter (DOC) in the groundwater could originate from (bacterial) degradation of the soil organic matter, or it could be derived from the oxidation of organic matter in the sedimentary matrix (Routh et al., 2001). Unfortunately the dissolved or total organic C (TOC) was not measured in the studied samples. Also DOC and TOC (either chemical or biological O2 demand) were generally not analyzed in the Cambrian–Vendian groundwater studies and to the authors’ knowledge there is no published data. However, a few earlier unpublished reports with Cambrian–Vendian groundwater

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V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

analyses show the DOC content varying between 0.1 and 2.8 mg L1 (Rein Vaikmäe pers. comm., 2012), which indicates the presence of dissolved organic matter in the groundwater. As discussed above there are two possible sources for the organic matter. Firstly, the Cambrian and Ediacaran clays confining the aquifer contain up to 2.5% of organic material (Raidla et al., 2006). However, this more than 500 Ma old fossil organic matter has been diagenetically altered and is not easily biodegradable. Secondly, the organic sediments/soils rich in organic matter (peat) of approximately Middle Weichselian age (26,800–43,200 BP, Kalm, 2006) that occur in the Gulf of Finland along the northern coast of Estonia in the area where the meltwater most probably penetrated into the Cambrian and Ediacaran rocks. Consequently, if the radiocarbon activity of organic C is estimated to be 100 pmC then the presumed initial activity is higher than the measured 14C activity and realistic ages are obtained using the model proposed by Coetsiers and Walraevens (2009) (Table 2), except for samples from the westernmost wells (Dirhami and Haapsalu, Table 2), which have the lowest portion of oxidized 14C active C, but the highest SO2 concentration (Table 1). Most probably the 4 initial SO2 4 concentration of these samples is underestimated. Positive ages are obtained if an initial SO2 4 concentration of at least 43 mg L1 is presumed in these wells. In this case the groundwater radiocarbon ages for Dirhami (Sample No. 1) and Haapsalu (Sample No. 2) are 597 and 1544 radiocarbon years, respectively. Nearly the same ages (Table 2) are found by the d13C mixing model of Pearson and Hanshaw (1970), except for the westernmost wells, which show negative ages. The model (Eq. (7)) is based on variations in d13C abundance and on the assumption that carbonate dissolution occurs under closed system conditions.

q¼

d13 CDIC  d13 Ccarb d13 Corg  d13 Ccarb

ð7Þ

The radiocarbon ages in both the combined d13C-mixing and mass-balance model (Coetsiers and Walraevens, 2009) and the d13C-mixing model (Pearson and Hanshaw, 1970) vary mostly between 14,000 and 27,000 radiocarbon years (Table 2) and have a relatively good agreement with each other. As anticipated, notable differences between the calculated ages emerge in deeper wells. This phenomenon is most probably caused by the cation exchange and mixing between the salty water from the crystalline basement and the fresh Cambrian–Vendian groundwater, which, in turn, resulted in the increased contents of Ca2+ and Mg2+. However, as long the proportion of dissolved inorganic C derived from fossil and 14C active organic matter cannot be measured, the real ages of the groundwater vary between modern and the modeled age (Coetsiers and Walraevens, 2009). In addition, it is important to note that the obtained ages do not correspond to the groundwater infiltration time, but reflect the moment when the organic material died and exited from the active C cycle. If it is assumed that the dissolved organic matter in Cambrian–Vendian groundwater was derived from organic-rich sediments in the depression of the Gulf of Finland, the age of the sediments is related to the Middle Weichselian Warm Substage dated in Estonia at 26,800–43,200 BP (Kalm, 2006). The Scandinavian Ice Sheet models (e.g., Arnold and Sharp, 2002; Näslund et al., 2003; Siegert and Dowdeswell, 2004) suggest that the glacier’s advance to Estonia may have occurred around 20,000–22,000 BP. This conclusion is supported by Ukkonen et al. (1999) and Lunkka et al. (2001) who suggested that the southern part of Finland was ice-free from 35,000 to 25,000 BP. Recent hydrodynamical modeling of glacial meltwater penetration into the Michigan Basin under the Laurentide Ice Sheet (McIntosh et al., 2011) suggests that the pore water hydraulic head values increase under advancing glacier reached its peak at the

maximum glacial extent, during a full modeled advance – retreat glacial cycle (17 ka). Their model simulations clearly show infiltration of freshwaters into the basin during ice sheet advance, which in their case invaded at approximately 60–80 km lateral distance from the outcrop area (McIntosh et al., 2011). This means that the Cambrian–Vendian groundwater infiltration time could probably be somewhat younger than the oxidized organic matter which gives the radiocarbon signal for the Cambrian–Vendian groundwater, because the organic material under the glacier was deposited before the glacier advanced into the area. It is interesting to note, that the calculated radiocarbon ages from the Cambrian–Vendian aquifer system groundwater are coeval with the onset of the Weichselian Glacier that covered the Estonian territory ca 22,000–11,000 radiocarbon years ago (Kalm, 2006; Raukas, 2009). The obtained corrected radiocarbon dates from the Cambrian–Vendian groundwater suggest that the approximate time when active vegetation was interrupted in North-Estonia was between 14,000 and 27,000 radiocarbon years ago (Table 2). Nevertheless, important differences in residence times appear between samples from the western and eastern part of the research area. The calculated residence times of the groundwater in the western part of the area vary from 14,000 to 21,000 radiocarbon years, whereas in the eastern part of the aquifer system ages scatter between 20,000 and 27,000 radiocarbon years (Table 2). Older ages in the eastern part of the area might indicate that the advance of the ice stream (Peipsi–Pskov ice stream, Raukas, 2009) there started earlier or alternatively the infiltration took place before the advance of the ice sheet. However, the last assumption cannot be considered because the hydraulic head pressure required to reverse the groundwater flow in Cambrian–Vendian aquifer and for intrusion of fresh glacial meltwater is too high. In the western zone the radiocarbon ages from the Cambrian– Vendian groundwater correspond to the glacier regression period (10,000–14,000 radiocarbon years ago, Punning et al., 1981) when production of organic material was inconceivable. This is probably due to different degrees of aquifer openness. The Lontova–Lükati aquitard above the aquifer is composed of massive clays in the central and eastern part of the area with vertical hydraulic conductivity of 107–105 m day1. However, in the west the massive clays are gradually replaced by the interbedded clay and sandstone with vertical hydraulic conductivity >105 m day1 (Perens and Vallner, 1997). This means that water exchange in the western part of the aquifer was more enhanced and, possibly, this groundwater infiltration took place during the entire existence of the ice sheet. In the eastern part, where the aquifer lies between two massive clay layers, the water exchange was limited and residence times are older. This indicates that the intrusion of glacial meltwater in the eastern part of the aquifer system occurred during the glacier advance. This might indicate that a hydraulic head, high enough to reverse the groundwater flow in the well- isolated eastern part of the Cambrian–Vendian aquifer system, was created only during periods of the glacier’s active advance, and when the maximum thickness of about 1000–2500 m (Denton and Huges, 1981; Fjeldsskaar, 1989) in northern Estonia was attained.

5. Conclusions The formation of DIC in the Cambrian–Vendian groundwater has been influenced by the dissolution of carbonate mineral cement in the water-bearing siliceous rocks, and bacterial SO4 reduction, during which isotopically depleted C was added to DIC. The corrected radiocarbon age of the Cambrian–Vendian groundwater suggests that infiltration occurred not earlier than 14,000–27,000 radiocarbon years ago, which is coeval with the advance and maximum extent of the Weichselian Glaciation in the

V. Raidla et al. / Applied Geochemistry 27 (2012) 2042–2052

area. The estimated age could be overestimated because the dissolved organic matter in Cambrian–Vendian groundwater was derived from organic rich sediments in the depression of the Gulf of Finland, which predate the ice sheet advance into the area. Radiocarbon age as well as the chemical and isotopic composition of the groundwater in the Cambrian–Vendian aquifer system suggests that the western part of the aquifer were under glacial conditions which were hydrodynamically more open than the eastern part of the study area. In the eastern part of the research area the infiltration of subglacial meltwater occurred predominantly during the transgression period of the glacier. In the western part of the study area, where the Cambrian clay bed acting as an aquitard is thinner and sandier, meltwater infiltration occurred most likely throughout the existence of the ice sheet. Acknowledgements We thank Mrs. Helle Pohl-Raidla for improvement of the English. Financial support by the Estonian Science Foundation (Grant No. 9196 to K.K., Grant No. 8948 to R.V.) is gratefully acknowledged. This paper is a contribution to the Research Projects No. SF0180069s08 at the Department of Geology, UT and No. SF0320080s07 at the Institute of Geology, TUT. The manuscript was significantly improved by the constructive and most helpful comments of an anonymous reviewer and Prof. Kristine Walraevens to whom we address our special thanks. References Appelo, C., Postma, D., 1999. Geochemistry, Groundwater and Pollution. Rotterdam, Netherlands, Balkema. Aravena, R., Harrison, S.M., Barker, J.F., Abercrombie, H., Rudolph, D., 2003. Origin of methane in the Elk Valley coalfield, southeastern British Columbia, Canada. Chem. Geol. 195, 219–227. Arnold, N., Sharp, M., 2002. Flow variability in the Scandinavian ice sheet: modelling the coupling between ice sheet flow and hydrology. Quatern. Sci. Rev. 21, 485–502. Boehme, S.E., Blair, N.E., Chanton, J.P., Martens, C.S., 1996. A mass balance of 13C and 12 C in an organic-rich methane-producing marine sediment. Geochim. Cosmochim. Acta 60, 3835–3848. Bottrell, S.H., Tranter, M., 2002. Sulphide oxidation under partially anoxic conditions at the bed of the Haut Glacier d’Arolla, Switzerland. Hydrol. Process. 16, 2363–2368. Cerling, T.E., Quade, J., 1993. Stable Carbon and Oxygen Isotopes in Soil Carbonates. Am. Geophysical Union, Geophysical Monograph, p. 78. Clark, I., Fritz, P., 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers, New York. Coetsiers, M., Walraevens, K., 2009. A new correction model for 14C ages in aquifers with complex geochemistry. Application to the Neogene Aquifer, Belgium. Appl. Geochem. 24, 768–776. DeLong, E.F., 2000. Resolving a methane mystery. Nature 407, 577–579. Denton, G.H., Huges, T.J., 1981. The Last Great Ice Sheets. John Wiley & Sons, New York, p. 484. Drever, J.I., 1988. The Geochemistry of Natural Waters, second ed. Prentice Hall, Englewood Cliffs, NJ, p. 423. Fjeldsskaar, W., 1989. Rapid Eustatic Changes – Never Globally Uniform. Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society, pp. 13–19. Forte, M., Bagnato, L., Caldognetto, E., Risica, S., Trotti, F., Rusconi, R., 2010. Radium isotopes in Estonian groundwater: measurements, analytical correlations, population dose and a proposal for a monitoring strategy. J. Radiol. Protect. 30, 761–780. Geyh, M.A., 2000. An overview of 14C analysis in the study of groundwater. Radiocarbon 42, 99–114. Glynn, P.D., Voss, C.I., Provost, A.M., 1999. Deep penetration of oxygenated meltwaters from warm based icesheets into the Fennoscandian Shield. In: Use of Hydrogeochemical Information in Testing Groundwater Flow Models. Workshop Proc., Borgholm, Sweden, 1–3 September 1997. OECD/NEA, Paris, pp. 201–241. Guimerà, J., Duro, L., Jordana, S., Bruno, J., 1999. Effects of Ice Melting and Redox Front Migration in Fractured Rocks of Low Permeability. Report No. TR-99-19. Svensk Kärnbränslehantering AB, p. 86. Heidel, C., Tichomirowa, M., 2011. The isotopic composition of sulfate from anaerobic and low oxygen pyrite oxidation experiments with ferric iron – new insights into oxidation mechanisms. Chem. Geol. 281, 305–316. Herczeg, A.L., Torgersen, T., Chivas, A.R., Habermehl, M.A., 1991. Geochemistry of ground waters from the Great Artesian Basin, Australia. J. Hydrol. 126, 225–245.

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