Boron isotope fractionation in groundwaters as an indicator of past permafrost conditions in the fractured crystalline bedrock of the fennoscandian shield

The Fennoscandian Shield has been subjected to several glaciations over the past million years, the last of which (Weichselian Ice Age) ended only at about 10 Ka. Here we used boron isotopes and B contents to (a) establish the degree of water-rock interaction (WRI) and (b) clarify freezing processes within groundwaters from the Äspö site in Sweden and from various sites in Finland. The high δ11B values recorded by all groundwaters (up to 51.9 ‰) including diluted, boron-poor, inland groundwaters suggest selective uptake of 10B into ice related to freezing processes under permafrost conditions. According to co-existing ice and residual brines in a Canadian frozen mine, this fractionation process, enhanced by Rayleigh fractionation, can generate a natural field of isotopic variation around 60‰ and provides a new application of B isotope that makes possible to easily characterise groundwaters that underwent past permafrost conditions.