| 25 Mar 2026
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.
Rotem et al. present major ion and radium isotope measurements from three permafrost cores and discuss the mechanisms controlling radium isotope ratios within the ice. Based on differences in the radium isotope ratios between cores, they infer that the ice at one site must have had a shorter apparent residence time. We have very limited data on radioisotopes in permafrost and cryogenic environments in general, so I think this paper represents a valuable addition to the existing data. However, I have some major concerns with the authors’ interpretation of what is driving the radium isotope signatures of the permafrost, and these must be resolved before publication. I have listed my main concerns with the interpretation below, followed by more minor line-by-line suggestions for each section.
Minor comments:
Introduction:
Line 55: Kipp et al. (2025) also used Ra & Th isotopes to provide evidence of recent ice segregation and this citation could be added here.
Line 70: Change “has” to “had”
Lines 86-87: Th (thorium) should not be super script.
Line 89: “it will always be on the solids” – it would be better to say that thorium prefers the solid phase, or that the “majority” will be adsorbed. The way it is currently written makes it seem as if there will never be dissolved thorium, which is not the case.
Line 90: change “228Ra has 5.8 years” to “228Ra has a half life of 5.8 years”
Section 1.1: I recommend adding a summary of Ra isotopes in cryogenic environments to the end of this section, to provide a brief overview of what is already known (and/or unknown).
Methods:
Line 137: Could microwaving the samples change the adsorption/desorption of radium isotopes by increasing the temperature? This would not impact the Ra isotope ratios but could be mentioned as a possible sample processing artifact (similar to the addition of Ra-free water).
Line 144-145: “…both salts and adsorbed ions are assumed part of the pore space chemical composition.” – It’s not clear to me how this statement is related to your interpretation of the results. Are you saying that you consider both dissolved and surface adsorbed ions in the same category? Would this influence the interpretation of the low 224Ra/228Ra ratios, which assumes that most of the 224Ra stays adsorbed on grain surfaces?
Line 153: The salinities in this study are below that of seawater (with perhaps one exception), thus high salinity would not be a reason for low Ra adsorption efficiencies on fibers. This method has been shown to quantitatively extract Ra from seawater salinities (e.g. Moore and Ried, 1973, Journal of Geophysical Research). I recommend revising this sentence to focus only on the low pH/reducing conditions as the cause for low extraction efficiencies.
Line 160: Are 223Ra and 224Ra activities reported in excess of their parent isotopes, or are they reported as total activities? If excess, that calculation should be introduced somewhere in the methods and the activities should be listed as “excess” in Table C2.
Results:
Line 205: Since the ice is referred to as “saline” and “brackish”, it would be helpful to list the salinities in PSU in this section in addition to providing the chloride concentration. This would more easily allow the reader to compare with typical seawater salinities.
Line 276-278: There are some 224Ra activities <1 and several 223Ra activities >1 so I recommend rephrasing this sentence instead of saying that 223Ra activities were an order of magnitude lower than the other isotopes.
Line 281: I disagree with the statement that there is no 226Ra trend with depth- Figure 6 shows generally higher activities in the epigenetic section. I think this is also true to some extent for the short-lived isotopes, though I agree it is a weaker trend.
Line 285: I don’t think the word “Nevertheless” is appropriate here, because this sentence supports the previous statement (that the ratios do not show a coherent trend).
Line 295: This should reference table C2, not C3.
Line 297: An outlier of 1.0 is listed for ADE-17 but I do not see this value on Figure 9 or in Table C2.
Line 310: How does high 227Ac on grain surface explain the bulk sediment 230Th/227Ac ratios that are significantly higher than secular equilibrium? Or is there another explanation for the difference?
Discussion
Line 338: …”although probably also contains” should be “although it probably also contains”
Line 349: What kinds of processes would introduce saline water into the permafrost at this location? It is mentioned that it could be from intrusion or intra-permafrost segregation, but what would be the driver of these mechanisms? i.e. would the region need to be inundated with seawater or receive a pulse of groundwater?
Line 350: The comma between “results” and “support” is not needed.
Line 376: “triplicate” does not make sense here- should this be “triple”?
Line 376: This is not solely a result of recoil, but also suggests that 226Ra is staying adsorbed onto mineral grains rather than desorbing into the porewaters. This is consistent with previous studies that have shown low 226Ra desorption in permafrost (e.g. Kipp et al., 2025 estimated ~1-3% of 226Ra desorbed from frozen sediments in samples with low salinities- these numbers agree with the values the authors have calculated).
Line 386: Should this only reference Figure 8? Parent isotopes are not shown in Figure 7.
Line 397: The argument that the ratios depend solely on 226Ra is not consistent with the data; the short-lived isotopes are what changes between the cores (by orders of magnitude!), while 226Ra is similar in all samples.
Line 416: Or this suggests that the short-lived isotopes are not being released into the pore space.
Line 427: Remove “as a matter of fact” from the beginning of the sentence; it is not necessary
Line 430: Where did the estimate of 50% emanation come from? This seems quite high, especially given that Figure 11 shows high 226Ra adsorption on sediment grains. Or does the 50% emanation describe release from recoil? In that case I understand the 50% estimate, but another term needs to be added to describe how much is released from sediments into the pore space via desorption.
Lines 429-431: I am unable to replicate the result of 100,000 dpm/L from this calculation; I get a range of 10,000 – 40,000 dpm/L when I use the provided values.
Line 439: Rama is spelled with only one m
Line 458: Is there a typo in the equation? What does “erfc” mean? (this is not defined in the sentence)
Line 467-468: It is not internally consistent to argue that frozen/cold nanopore spaces would prevent advection but not diffusion.
Line 533: This reference does not mention Ra isotopes; should this be a different citation?
Section 5.4: Here the authors argue that the influence of adsorption is necessary to produce the observed ratios, which is inconsistent with the argument that adsorption does not influence 226Ra because of the cold temperatures.
Conclusion:
Line 555: “…which affects similary both radium isotope activities and the time of reaching secular equilibrium” – I do not understand this statement.
Line 565: The final statement about permafrost vulnerability in a changing climate does not seem related to the rest of the study- is the connection to the previous sentence that there will be more fluid migration as the climate warms? If so, this should be explained more clearly (I recommend adding another sentence or two explaining this hypothesized impact.)
Figures/Tables:
Please add error bars on data figures and in data tables for Ra & Th isotopes and isotope ratios.
Figure 2: arrows are missing between 231Pa, 227Ac, and 227Th.
Figure 6: missing key
Figure 7: What are the smaller inset plots showing? These data do not match the data in the larger figure panels (e.g. the 226Ra inset shows ADE-2 with higher Ra activities than all but one sample from ADE-17, while in the larger panel the activities are comparable between the cores). Also, in the 226Ra inset, the y-axis scale is 0-2 while the scale of the larger plot is 0-20; the 0-2 scale does not seem correct based on the reported sample activities.
Figure 7: Caption incorrectly lists the isotopes for (a) and (b)- they should be swapped.