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

摘要

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 Aespoe site in Sweden and from various sites in Finland. The high δ~(11)B values recorded by all groundwaters (up to 51.9 per thousand) including diluted, boron-poor, inland groundwaters suggest selective uptake of ~(10)B 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 ofisotopic variation around 60 per thousand and provides a new application of B isotope that makes possible to easily characterise groundwaters that underwent past permafrost conditions.