Cryopegs are layers of unfrozen ground or of overcooled brines in the permafrost. A borehole drilled 20m into the permafrost at the Adventdalen fjord valley, 5 km from the sea, intruded a cryopeg, starting at 4m below surface. Section at the site is composed of valley-filling deltaic, fluviatile and aeolian sediments, underlain by Eocene to Cretaceous basement. Permafrost is ≥100 m deep, which was formed during Early to mid-Holocene, after exposure to the atmosphere due to elastic rebound.
Brine water, which seeped into the borehole at 11m, contained 50,000 mgCl-/l, while the partially frozen ground ice along the section carried between 7,600-27,700 mgCl-/l. Both brine and the ground ice showed seawater composition, with no evidence for WRI, implying derivation by freezing and saline water rejection.
As in groundwater, Ra isotope activities increased with ground ice salinity, but activities in the seeping brine were lower, probably due to a less effective water-rock interaction in the latter. This could suggest brine’s arrival from the basement.
Long-lived to short-lived 226Ra/223Ra activity ratios are relatively low (compared with old groundwater, e.g. Weinstein et al. 2021), mostly between 2.5-13, with the brine showing a ratio of 9.7. These are in secular equilibrium with parent isotope ratios (230Th/227Ac) on grain surfaces, while significantly lower than parent ratios in bulk sediments (<15 and 60-300, respectively). In another borehole, 5 km up-valley, Ra isotope ratios were much closer to parent ratios in the bulk sediment. We interpret these differences as a diffusion effect. This is to say that while in the up-valley permafrost, 226Ra had enough time to diffuse (partly?) out of the grains, this is not the case in the studied cryopeg, resulting in equilibrium with surface parent ratios. With the up-valley permafrost being younger than 10 ka, the cryopeg should probably be of Late Holocene age.