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 Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia b Department of Geology, University of Tartu, Ravila 14a, 50411 Tartu, Estonia article info Article history: Received 6 June 2011 Accepted 12 June 2012 Available online 21 June 2012 Editorial handling by W.M. Edmunds abstract Groundwater in the Cambrian–Vendian aquifer system has a strongly depleted stable isotope composi- tion (d 18 O 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 d 13 C 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 d 18 O values of about 22‰, which has been suggested to be due to a glacial melt- water 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–Ven- dian groundwater as resulting from a mixing of three end-mem- bers, 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 un- known. The low 14 C 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 di- rectly dated in Estonia, it is unlikely to have occurred before 20– 22 ka ago (Kalm, 2006), while the start of deglaciation has been da- ted to 12,500 radiocarbon years ago and continued over 2 ka (Rau- kas, 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 14 C-age calculation methods may not be appropriate for converting the 14 C-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 dis- solution 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 Ca 2+ and Mg 2+ 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 14 C free inorganic C. Addition- ally, degradation of organic C can add C to the groundwater influ- encing 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 north- ern 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 de- creases 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 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.06.005 ⇑ Corresponding author. Tel.: +372 6203038; fax: +372 6203011. E-mail address: Valle.Raidla@gi.ee (V. Raidla). Applied Geochemistry 27 (2012) 2042–2052 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem