Brine flow in permafrost, time constraints by Ra isotopes

The archipelago of Svalbard, at 76-80^oN, is located at the heart of the continuous permafrost zone, which means that all exposed areas (glacier-free) are covered by 100-400m thick permafrost. Permafrost in valleys is often saline due to Early Holocene seawater ingression, followed by regression and permafrost aggradation. We studied the chemistry and Ra isotopes of ground ice from saline permafrost at the Adventdalen valley, central Svalbard. Cores recovered from two adjacent drillholes, 10 km from the sea, within the Early Holocene seawater ingression, showed significantly different compositions. While one core, with up to seawater salinity, showed similar-to-seawater composition, ground ice in two less saline cores exhibited clear evidence for water-rock interaction. The cores also showed substantial differences in their radium isotopes. Ratios of long to short-lived isotopes (e.g. ²²⁶Ra/²²³Ra) in the ground ice of the less saline cores were much higher than in the more saline one (activity ratios of <<20 and >20, respectively). Notably, Ra isotope ratios in the latter were similar to (i.e. in secular equilibrium with) the ratios of their radioactive parents (e.g. ²³⁰Th/²²⁷Ac) on sediment surfaces (CEC fraction), while in the less saline cores, ratios were closer to parent ratios in the bulk sediments.  

Another drillhole, 5 km from the sea, intruded a cryopeg (permafrost with overcooled brines),  with hypersaline ground ice from 20m to 4m below surface and brine (50,000mg Cl l^-1) that flowed into the borehole at a depth of 11m. Composition of both brine and the ground ice was indistinguishable from that of seawater, indicative of freezing-associated solute rejection with no crystallization or water-rock interaction involved. Importantly, (²²⁶Ra/²²³Ra) activity ratios in brine and the ground ice were significantly lower than the equilibrium ratios (mostly <<10).  

It is suggested that the high (²²⁶Ra/²²³Ra) ground ice of the less saline cores represent the original Early Holocene sediment fluids, which had interacted with sediments, diluted and froze upon exposure to the atmosphere. On the other hand, the low (²²⁶Ra/²²³Ra) ground ice of the more saline cores is evidently much younger (no time for diffusion of the long-lived ²²⁶Ra from inside the grains), probably produced by Late Holocene brine infiltration from the underlying basement, which is evident in the brine found in the second site.

These observations demonstrate the complex history of permafrost and its liability to fluid migration. This further highlights another aspect in permafrost’s vulnerability and sensitivity to the ongoing climate change and warming.