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

While permafrost is considered a permanently frozen soil, it often demonstrates evidence for internal processes, including fluid migration. Here, we present data on the chemistry, 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 recent fluid flow. Ground ice of the different cores differs markedly in its salinity and composition. In one core, which has a relatively high salinity (up to > 20 000 mgCl L −1 ), and a composition similar to seawater, the long to short-lived isotope activity ratios (AR), e.g., ( 226 Ra / 223 Ra) AR and ( 228 Ra / 223 Ra) AR , are relatively low, being similar to parent isotope activity ratios ( 230 Th / 227 Ac and 232 Th / 227 Ac, respectively) on grain surfaces (exchangeable fraction). Ground ice of another core, which is less saline and has Na/Cl and SO 4 / Cl ratios higher than seawater, demonstrates much higher Ra isotope ratios, closer to parent ratios in the bulk sediment. Ground ice in a third core, with chemical composition similar to the latter, shows high ( 226 Ra / 223 Ra) AR, albeit low ( 228 Ra / 223 Ra) AR. It is suggested that the different isotope ratios are due to different residence times, and that the parameter controlling the longer-lived ( 226 Ra and 228 Ra) activities (hence, long to short isotope ratios) is radium diffusion from inside the grains via partly liquidized nano-pores. While ground ice in the less saline cores could have been formed during permafrost formation (10–9 ka), ground ice in the more saline core went recently (years to decades) through reset of its Ra clock, which did not allow a significant diffusion of the long-lived 226 Ra and 228 Ra from inside the grains. This reset is probably the result of a Late Holocene intrusion of saline fluids, which arrived from a low-Th or high water : rock ratio basement aquifer, and mixed with the original, less saline ground ice. Recent salinization is also supported by the presence of high salinity all the way up into the syngenetic permafrost, which has been deposited during the Late Holocene. The above highlights the internal dynamics of saline permafrost, which may affect its resilience to the ongoing global warming.