Radioactive Clocks in Arctic Ice Reveal Hidden Water Flow Inside Permafrost

Buried beneath the windswept valley of Adventdalen in central Svalbard, some of the ground ice has been hiding a secret: it is far younger than the frozen soil around it. A new study published in The Cryosphere by Dotan Rotem of Bar-Ilan University and colleagues shows that radioactive radium and thorium isotopes can act as minute-scale clocks inside permafrost, and those clocks reveal that saline fluids have recently surged through what was assumed to be permanently frozen ground. In one core drilled from a river terrace, the ice appears to have been reset within the last few decades, evidence of active fluid migration inside a permafrost landscape that climate models often treat as an impermeable, static block of frozen earth.

Permafrost is conventionally defined as soil or rock that has remained at or below 0 degrees Celsius for at least two consecutive years, and it blankets more than 20 percent of the northern hemisphere’s land area. Yet the definition conceals a subtlety that has fascinated cryospheric scientists for decades: permafrost is not necessarily completely frozen. Salts dissolved in pore water depress the freezing point, and capillary forces plus adsorption onto fine-grained mineral surfaces allow thin films of liquid water to persist at temperatures well below zero. Those unfrozen films can become highways for ions and fluids, permitting redistribution and migration of water within the cryotic pore space. Near-surface saline permafrost, in which soluble salts are abundant enough to significantly lower the freezing point, may cover as much as 35 percent of the continuous permafrost zone of the circumpolar north.

The research team drilled three cores at the ADE site in Adventdalen, a U-shaped fjord valley carved by glaciers and later flooded by seawater during the Early Holocene. Two boreholes spaced half a meter apart were drilled in March 2017 and treated as a single core, ADE-17, while two more, ADE-1 and ADE-2, were drilled in March 2022 within roughly 30 meters of the first. The valley fill at the site consists of fine aeolian deposits over fluviatile gravel and roughly twelve meters of deltaic sediments laid down when the sea reached this far up the valley, all resting on Lower Cretaceous shales. After the sea retreated following rapid glacial rebound, the exposed marine sediments froze from the top down, while later fluviatile and aeolian layers froze in place as they accumulated, creating a shallow syngenetic cap above the deeper epigenetic permafrost.

What the cores contained stunned the researchers. Ground ice chemistry differed dramatically between boreholes separated by no more than a few tens of meters. In ADE-17 and ADE-1, the shallow syngenetic ice was essentially fresh water, while the deeper epigenetic sections were brackish to saline, with chloride concentrations between 450 and 3230 milligrams per liter. ADE-2 was another story entirely: salinity reached 7960 milligrams of chloride per liter, with one sample exceeding 23,000 milligrams per liter and total dissolved solids above 40,000 milligrams per liter, saltier than seawater. Even the syngenetic section of ADE-2, deposited only two to four thousand years ago, carried total dissolved solids above 6000 milligrams per liter, a clue that something had pushed saline fluid upward into sediments that had never known the sea.

The ionic ratios told a story of two different waters. In ADE-17 and ADE-1, sodium-to-chloride and sulfate-to-chloride ratios far exceeded those of seawater, indicating that porewater had undergone substantial water-rock interaction, dissolving minerals or exchanging ions during long residence in the valley-fill sediments. In ADE-2, by contrast, the ratios closely matched seawater, with sodium-to-chloride values between 0.84 and 1.02, suggesting a marine-derived saline component that had experienced little chemical modification on its journey. Because freezing and salt expulsion would have produced sodium-to-chloride ratios lower than seawater rather than higher, the team ruled out brine formation by freezing, pointing instead to an intrusion of saline fluid that mixed with the original, fresher ground ice.

The decisive evidence came from the decay chains of uranium and thorium. Radium has four naturally occurring isotopes with half-lives spanning an extraordinary range: radium-223 and radium-224 decay within days, radium-228 within 5.75 years, and radium-226 within 1601 years. All are produced by the alpha decay of thorium isotopes, which bind tightly to mineral grains, while radium is comparatively mobile and tends to enter the dissolved phase. Alpha recoil during decay ejects radium from the mineral lattice into pore space. Because short-lived isotopes reach steady-state activities within weeks, the ratio of a long-lived isotope such as radium-226 to a short-lived one such as radium-223 depends almost entirely on how long the ice or fluid has sat in place, growing toward equilibrium with its parent over five to six half-lives, or roughly eight to ten thousand years for radium-226.

In ADE-17 and ADE-1, the long-to-short-lived radium ratios were high, mostly between 20 and 70, close to or above the secular equilibrium value of 21.7 and approaching the parent ratios measured in the bulk sediment. This implies that the ground ice in these cores is ancient, potentially as old as the Early Holocene permafrost that formed roughly 9000 to 9500 years ago after the sediments emerged from the sea. The team attributes the elevated long-lived radium activities to slow diffusion out of the mineral grains themselves, likely through partially liquidized nano-pores, where thin films of unfrozen water along grain boundaries allow ions to escape over years to decades. Simulations using an effective diffusion coefficient of 10 to the minus 13 square meters per second showed that radium could escape silt-sized grains within years by this pathway, enriching the pore space in radium-226 and radium-228 without requiring any contribution from the short-lived isotopes.

ADE-2 defied this pattern. Its long-to-short-lived radium ratios were strikingly low, between 4 and 14, and matched the parent isotope ratios found on grain surfaces in the exchangeable fraction of the sediment rather than in the bulk grains. In other words, the radium clock in this core had been reset recently, before the longer-lived isotopes had any chance to diffuse out from inside the grains or accumulate significantly from recoil. Modeling the buildup of the radium-226 to radium-223 ratio showed that with no adsorption, reaching equilibrium would take several thousand years, but with adsorption coefficients typical of seawater salinity, the timescale collapses to centuries or decades. The team concludes that the saline ground ice in ADE-2 is younger than 100 years, and possibly only years to decades old, the product of a Late Holocene, and likely very recent, intrusion of saline fluids that mixed with the original ground ice in a fingering style.

Where did those fluids come from? A direct connection to Adventfjorden roughly ten kilometers away seems implausible given the low hydraulic gradient and shale-dominated geology, which would produce residence times of thousands of years. The researchers instead point to a local sandstone aquifer beneath the permafrost, most likely the Helvetiafjellet Formation more than 70 meters below the surface, or possibly the deeper fractured Geerdalen Formation. Fractured sandstone typically carries a high water-to-rock ratio and low thorium content, which would explain both the seawater-like chemistry of the intruding fluid and its capacity to reset the radium clock through enhanced desorption. Artesian discharge through pingo structures nearby demonstrates that water does ascend from the sub-permafrost zone in this region, driven by hydraulic heads from surrounding mountains or by basal permafrost aggradation, and the same pressure may have forced saline fluids upward into the shallow permafrost at the ADE site.

The findings carry weight well beyond one Arctic valley. They demonstrate that saline permafrost is heterogeneous, dynamic, and permeable to fluid migration on human timescales, not the inert deep-frozen archive it is often assumed to be. As Arctic air temperatures continue to climb, thawing of saline permafrost may mobilize subsurface brines, threatening infrastructure built on frozen ground and potentially accelerating the release of stored carbon to the atmosphere. The same radioactive clocks that revealed a decades-old intrusion in Adventdalen now offer scientists a tool to detect such hidden flow elsewhere in the circumpolar north, turning the quiet chemistry of decaying atoms into an early-warning system for a warming Arctic.

Subject of Research: Ground ice residence times and intra-permafrost fluid flow in Svalbard permafrost traced with radium and thorium isotopes

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

Article References: Rotem, D., Weinstein, Y., Harlavan, Y., Torfstein, A., & Christiansen, H. H. (2026). The history of ground ice formation and intra-permafrost fluid flow as documented by Ra and Th isotopes. The Cryosphere, 20(9), 5303-5326. https://doi.org/10.5194/tc-20-5303-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5303-2026

Keywords: permafrost, ground ice, radium isotopes, thorium isotopes, Svalbard, Adventdalen, saline permafrost, fluid migration, geochronology, cryosphere, Arctic warming, hydrogeology