Ra isotope perspective on the hydrology and continuity of permafrost in the high Arctic

The continuous permafrost in the valleys of Svalbard is dotted by pingos, which are small hills formed by the near surface freezing of ascending groundwater. In this study, we used ³H and Ra isotopes to inquire into the sub-surface residence time of groundwater discharging at these pingos. While its low ³H suggests that the pingo-associated groundwater is basically not modern (i.e. older than 60 years), Ra isotopes imply that most water has an underground residence time of several hundred years. This is deduced from the lower than equilibrium ratios (activity ratios<21.7) of the long-lived to short-lived ²²⁶Ra/²²³Ra. Since the freezing age of the main body of permafrost in this area is >4000 years, the presence of younger water at depth suggests that the aquifer has been recharged after permafrost formation, which could take place via faults or through the non-frozen base of wet glaciers. This active hydrology suggests that permafrost in the valleys of Svalbard was at least locally discontinuous during the Late Holocene, with likely further implications to the release of greenhouse gases during the pre-industrial period.