The history of ground ice formation and intra-permafrost fluid flow as documented by Ra and Th isotopes

Abstract. While permafrost is considered a permanently frozen soil, it often demonstrates evidence for internal processes, including fluid migration. Here, we present data of the chemical composition, Ra, Th, and Ac isotopes of saline permafrost from three closely-retrieved cores drilled at Adventdalen, a fjord Valley in central Svalbard, which provides evidence for a fingering style intra-permafrost fluid flow. Ground ice of the different cores differs markedly in their salinity and composition. In one core, which has a composition similar to seawater, the long to short-lived isotope ratios, (²²⁶Ra/²²³Ra)_AR and (²²⁶Ra/²²⁴Ra)_AR, are relatively low, being similar to parent isotope activity ratios (²³⁰Th/²²⁷Ac and ²³⁰Th/²²⁸Th, respectively) on grain surfaces (CEC fraction). Ground ice of the two other cores, which are less saline and have Na/Cl and SO₄/Cl ratios higher than seawater, demonstrates much higher Ra isotope ratios, closer to parent ratios in the bulk sediment. It is suggested that the different isotope ratios are due to different residence times, and that the parameter controlling the isotope ratios is radium diffusion from inside the grains. While ground ice in the less saline cores was formed during permafrost formation (10–9 ka), ground ice average residence time in the more saline core is shorter, <<2,000 years, which did not allow a significant diffusion of the long-lived ²²⁶Ra from inside the grains. The latter is probably the result of a Late Holocene intrusion of saline fluids, arriving from a low-Th or high water:rock ratio basement rock. This highlights the internal dynamics of saline permafrost, which may affect its resilience to the ongoing global warming.