the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 May 2026
Permafrost siderite reveals a hidden climate-sensitive inorganic carbon reservoir
Abstract. We report direct evidence of an inorganic carbon pool that is sensitive to permafrost thaw–siderite. Notably, siderite was absent in the seasonally thawed active layer above permafrost, implying this inorganic carbon reservoir may be lost upon thaw. Assuming siderite weathers quickly once permafrost thaws, we estimate siderite weathering could release carbon equivalent to about 10% of permafrost organic carbon losses over the next half-century. However, studies are needed to understand how widespread siderite is and to quantify its actual weathering rate. This study is submitted as a LESSONS Report because it documents a surprise result that opens up opportunities for new science.
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Status: open (until 01 Aug 2026)
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RC1: 'Comment on egusphere-2026-2641', Anonymous Referee #1, 03 Jul 2026
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AC1: 'Reply on RC1', Fernando Montaño López, 11 Jul 2026
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Reviewer 1.1: General Comments: The manuscript presents the results of Moessbauer spectroscopy of four permafrost samples all from the Arctic showing the presence of siderite (FeCO3). This is indeed an interesting finding that merits further investigation. However, in my opinion there is not yet enough data to be published as a LESSONS report and might fit better as a LETTERS post. This is because it is based on the results of single Moessbauer measurements of four samples from one region. The Moessbauer spectra are well resolved but there is still some noise and there is likely some error involved in the fitting process when the siderite component is as small as 6.5%. The authors provide an estimate that based on the four samples from Alaska, siderite may be a significant pool of mobilizable carbon in thawing permafrost. However, without further data to confirm this it is largely conjecture.
Answer 1.1: We thank you for your time and thoughtful comments regarding the scope of the manuscript, and for recognizing the novelty of the identification of siderite in Arctic permafrost.
We respectfully believe that our manuscript is better suited as a LESSONS Report rather than a LESSONS Post. According to the journal's aims, LESSONS Reports are intended to communicate "any well-substantiated finding that is not a classic positive result, describing the limitations, errors, surprises, shortcomings and opportunities for new science emerging from the scientific process, including non-confirmatory and null results. A LESSONS Report needs to offer a substantial insight and have wider relevance beyond the author’s immediate research”.
Although our conclusions are based on four chemically distinct permafrost samples from Alaska, this limitation primarily reflects the considerable instrument and analytical time required for Mössbauer spectroscopy. To strengthen our interpretation, we also analyzed four active layer samples, in which siderite was not detected. In addition, two of the permafrost samples were reanalyzed following iron carbonate extraction; one showed a marked reduction in the siderite signal, while the other exhibited complete dissolution of the siderite component. These complementary analyses provide independent support for our mineralogical interpretation beyond the initial spectral fitting.
Based on the fitting procedure and repeated evaluation of the spectra, the estimated uncertainty in the reported phase abundances is generally within 1–2 percentage points. Thus, even for the sample containing 6.5% siderite, the identified siderite component remains above the estimated fitting uncertainty and is supported by parameters consistent with those reported for siderite.
We agree that our estimate of the potential siderite carbon pool is preliminary, and we have intentionally presented it as a thought experiment rather than a definitive quantification. The primary objective of the manuscript is not to establish the magnitude of this carbon reservoir, but to highlight the previously overlooked occurrence of siderite in permafrost and its potential implications, thereby motivating further investigation.
Finally, while the direct Mössbauer evidence presented here is limited to Alaskan samples, multiple independent studies have reported geochemical and mineralogical indicators consistent with siderite occurrence in geographically distinct permafrost regions, including Iceland, Svalbard, Greenland, and Siberia (discussed further in our response below). This broader context suggests that the finding may have relevance beyond our study area. In light of this, we will revise the title to better reflect the exploratory yet robust nature of our study. The new title will emphasize that this work documents the occurrence of siderite in the investigated samples without implying a broader regional assessment, while still highlighting the significance of this first observation and its implications for future research. For these reasons, we believe the manuscript meets the criteria for a LESSONS Report by presenting a well-substantiated, broadly relevant finding that identifies an important knowledge gap and opens new directions for permafrost carbon research.
Detailed Comments:
R 1.2: Line 34: I understand that an exhaustive list is not appropriate here (it is missing some important environments like salt marsh sediments e.g. SEM analysis of siderite cements in intertidal marsh sediments, Norfolk, England - ScienceDirect) so the text could be altered to include ‘ for example’
A1.2: We agree that the examples provided are not intended to be exhaustive, and we will revise the sentence to include "for example" to make this clear. We also appreciate the additional reference to salt marsh sediments, which we will add and acknowledge that siderite occurs in a broader range of environments than those explicitly listed in the manuscript.
R1.3: Line 38: I think it would be helpful to include a research question or statement of motivation alongside the statement describing the results.
A1.3: We will edit the final sentence of the paragraph to introduce the research question and clarify why identifying siderite in Arctic permafrost is important:
Line 38 - However, despite the widespread occurrence of the geochemical conditions required for siderite formation in permafrost, its presence has not been directly confirmed. Establishing whether siderite occurs in Arctic permafrost is important because it represents a previously overlooked pool of inorganic carbon and provides insight into iron and carbon cycling under frozen conditions. Here, we investigate whether siderite occurs in Arctic permafrost and present, to our knowledge, the first direct evidence of siderite in Arctic permafrost.
R1.4: Line 40: It would be helpful to add where in the permafrost profile these soils were taken from (i.e. how close to thawing are they?)
A1.4: We agree that the depth and position of the samples within the permafrost profile provide important context for interpreting the results. We will revise the manuscript to specify the sampling depths and the location of the samples within the permafrost profile, including their relationship to the active layer and thaw front.
Line 41: “The analyzed samples were collected from perennially frozen soil at depths ranging from 11 to 54 cm below the bottom of the active layer.”
R1.5: Line 50: Could you also state what fraction of the inorganic/total C this would be? In Table 1 inorganic carbon is stated as a trace, is that also true if one assumes that all of the inorganic C is in the siderite? I see now you discuss this later. I think more should be made of the potential mismatch between the quantified siderite and the ‘trace’ amount of inorganic carbon measured, which is then extrapolated in the thought experiment later.
A1.5: We thank you for raising this point. We will clarify the relationship between the Mössbauer-derived siderite concentrations and the inorganic carbon values reported in Table 1. The "trace" designation was not based on a positive detection by the thermal mass loss analysis. Rather, the thermal analysis did not detect inorganic carbon in the first three samples, and we subsequently designated these values as "trace" because the Mössbauer analyses demonstrated the presence of siderite and, therefore, a small amount of inorganic carbon. Assuming that all of the carbonate associated with the identified siderite is present as FeCO₃, the corresponding siderite-derived carbon contents are approximately 0.11%, 0.07%, 0.04%, and 0.10% of the soil mass for the four samples, respectively. In the fourth sample, thermal mass loss detected 1.21% inorganic carbon, which is consistent with the presence of siderite together with other potential carbonate phases. We will revise the text to clarify these points and to make the assumptions underlying the later extrapolative thought experiment more explicit.
R1.6: Figure 1: At room temperature, is it really possible to unambiguously identify goethite and epidote?
A1.6: Our phase assignments are based on the Mössbauer parameters together with published reference data. The assignment of goethite is based on the presence of a distinct, well-resolved sextet whose parameters are consistent with those reported for goethite in room-temperature Mössbauer spectra (Dong et al., 2000; Fredrickson et al., 2003; Jaisi et al., 2005; Thompson et al., 2006; Campbell et al., 2012). Although Al substitution and limited crystallinity can influence the parameters, similar room-temperature spectra have been reported previously for Al-goethite, and we will add the appropriate references to the manuscript. We assigned the goethite-2 feature based on 77 K and 12 K spectra, that are shown in our previous study (Montaño-López et al., 2026). The second Fe(III) doublet was originally assigned to epidote because Mössbauer features with similar parameters have been identified as epidote in previous studies (Grodzicki et al., 2001; Zachara et al., 2003; Nagashima & Akasaka, 2010). However, we agree that this assignment cannot be considered definitive. To better reflect this uncertainty, we will revise the manuscript to refer to this component as an epidote-like Fe(III) feature rather than unequivocally assigning it to epidote.
Goethite references: Dong, H., Fredrickson, J. K., Kennedy, D. W., Zachara, J. M., Kukkadapu, R. K., & Onstott, T. C. (2000). Mineral transformations associated with the microbial reduction of magnetite. Chemical Geology, 169(3-4), 299-318. https://doi.org/10.1016/S0009-2541(00)00210-2
Fredrickson, J.K., Kota, S., Kukkadapu, R.K. et al. Influence of Electron Donor/Acceptor Concentrations on Hydrous Ferric Oxide (HFO) Bioreduction. (2003). Biodegradation 14, 91–103. https://doi.org/10.1023/A:1024001207574
Jaisi, D. P., Kukkadapu, R. K., Eberl, D. D., & Dong, H. (2005). Control of Fe (III) site occupancy on the rate and extent of microbial reduction of Fe (III) in nontronite. Geochimica et Cosmochimica Acta, 69(23), 5429-5440. https://doi.org/10.1016/j.gca.2005.07.008
Campbell, K. M., Kukkadapu, R. K., Qafoku, N. P., Peacock, A. D., Lesher, E., Williams, K. H., ... & Long, P. E. (2012). Geochemical, mineralogical and microbiological characteristics of sediment from a naturally reduced zone in a uranium-contaminated aquifer. Applied Geochemistry, 27(8), 1499-1511. https://doi.org/10.1016/j.apgeochem.2012.04.013
Montaño-López, F., Landis, J. D., Wilkins, S. D., Kukkadapu, R., Schaefer, S., von Fromm, S., Grandy, S., Ernakovich, J. G. and Hicks Pries, C. 2026. Organo-mineral interactions in active layer and permafrost soils along aging Arctic landscapes. Catena. https://doi.org/10.1016/j.catena.2026.110377
Epidote references: Grodzicki, M., Heuss-Assbichler, S. & Amthauer, G. Mössbauer investigations and molecular orbital calculations on epidote. (2001). Phys Chem Min 28, 675–681. https://doi.org/10.1007/s002690100150
Zachara, J. M., Kukkadapu, R. K., Gassman, P. L., Dohnalkova, A., Fredrickson, J. K., & Anderson, T. (2004). Biogeochemical transformation of Fe minerals in a petroleum-contaminated aquifer. Geochimica et Cosmochimica Acta, 68(8), 1791-1805. https://doi.org/10.1016/j.gca.2003.09.022
Nagashima, M., Akasaka, M. (2010). X-ray Rietveld and 57Fe Mössbauer studies of epidote and piemontite on the join Ca2Al2Fe3+Si3O12(OH)–Ca2Al2Mn3+Si3O12(OH) formed by hydrothermal synthesis. American Mineralogist; 95 (8-9): 1237–1246. https://doi.org/10.2138/am.2010.3418
R1.7: Line 84-92: I think this section needs some rewording. The first sentence read to me actually as arguments for why siderite would not form authigenically for the reasons given in the second sentence (When I read parent material I thought of the geology from which the soil was derived). Without porewater data to support the presence of higher pH or alkalinity, it is difficult to make a strong conclusion here.
A1.7: We will revise this section to clarify that bulk soil pH does not necessarily reflect the geochemical microenvironments in which siderite precipitates. Previous studies have shown that siderite can form under acidic or strictly anaerobic conditions in iron-rich porewaters and that microbial activity can create localized microenvironments that promote iron carbonate precipitation, even when the surrounding soil is acidic (Grengs et al., 2024; Xiong et al., 2017; Sánchez-Román et al., 2014, cited in the manuscript). We will state that our observations are consistent with, rather than conclusive evidence of, authigenic siderite formation and acknowledge that porewater measurements would be needed to further constrain the precipitation environment. To our knowledge, there is currently no direct evidence describing the mechanisms or environmental controls governing siderite formation in permafrost soils. This knowledge gap underscores the importance of our study to provide a foundation for future investigations combining geochemical measurements to better constrain its formation pathways and potential role in permafrost carbon cycling.
R1.8: Line 96: ‘carbon dioxide and carbonates’ to me is describing the gas and solid phases, I suggest adding a term for the aqueous phase.
A1.8: We thank the reviewer for this helpful suggestion. We agree that the original wording did not adequately represent the aqueous inorganic carbon species involved in siderite formation. We will revise the text to explicitly refer to dissolved inorganic carbon, including aqueous carbon dioxide and carbonate species.
Line 97: The formation of siderite generally relies on three key factors: 1) a supply of Fe, 2) high concentrations of carbon dioxide or carbonate ions (including aqueous CO₂, HCO₃⁻, and CO₃²⁻), and 3) reducing conditions (Fredrickson et al., 1998).
R1.9: Line 97 and then 99/100: is reducing conditions a more appropriate term than anaerobic conditions?
A1.9: We agree that “reducing conditions” is a more appropriate and general term than “anaerobic conditions” in this geochemical context. We will edit the manuscript accordingly.
R1.10: Line 111: is carbonate accurate – I thought CO2 and H2O would be more likely. Or bicarbonate. As again I see you discuss later.
A1.10: We thank the reviewer for this comment. We agree that “carbonate” was not sufficiently precise in this context. We will revise the text to more accurately reflect aqueous inorganic carbon species (CO₂(aq) and bicarbonate) as products of siderite weathering, consistent with established geochemical terminology.
Line 111: The products of siderite weathering include dissolved inorganic carbon species (CO₂(aq) or HCO₃⁻) and Fe oxyhydroxides...
R1.11: Line 117 – 132: I appreciate the thought behind this but it really is conjecture and I think the caveats given at the end should be presented at the start of the paragraph and note that it is based only on four soil samples all from Alaska. I am not sure quoting 10% as a value in the abstract is justifiable.
A1.11: We will move the last sentence up so it will be the second sentence in the paragraph.
We will change the abstract wording to better indicate the uncertainty: “A thought experiment indicates siderite weathering is potentially important to consider as a permafrost thaw-climate feedback. Based on an initial estimate that assumes siderite weathers quickly upon thaw, siderite weathering could release carbon equivalent to about 10% of permafrost organic carbon losses over the next half-century.”
R1.12: Line 135: what is a siderite indicator?
A1.12: Thank you for pointing this out. We agree that the term "siderite indicators" was imprecise. The studies we cited did not all directly identify siderite using the same analytical approach. For example, Jessen et al. (2014) inferred conditions favorable for siderite formation based on porewater analyses and geochemical modeling (PHREEQC saturation indices), Jones (2020) used iron carbonate dissolution extraction methods that extract siderite but also other iron carbonate phases, and Chen (2025) identified siderite by XRD, although XRD generally requires relatively higher mineral abundances and the study focused on mafic parent material where siderite concentrations can be expected to be higher. Additional evidence consistent with siderite occurrence in permafrost has been found in Alaska by Lipson et al. (2010, based on iron carbonate dissolution extraction), Joss et al.(2022, unresolved Mossbauer spectra), and Sowers et al. (2020, suggested by Fe EXAFS). Additionally, we will cite a report of siderite identified by electron microscopy in West Siberian permafrost (Kurchatova et al., 2016), which further supports that siderite occurs in geographically distinct permafrost environments. We will change the text to more accurately describe the available evidence and avoid overstating the occurrence of siderite across permafrost regions.
Kurchatova, A.N., Melnikov, V.P., Rogov, V.V. et al. 2016. Authigenic mineral formation in fluid permeability zones in the West Siberia Permafrost. Dokl. Earth Sc. 468, 571–573. https://doi.org/10.1134/S1028334X16060131
Citation: https://doi.org/10.5194/egusphere-2026-2641-AC1
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AC1: 'Reply on RC1', Fernando Montaño López, 11 Jul 2026
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General Comments:
The manuscript presents the results of Moessbauer spectroscopy of four permafrost samples all from the Arctic showing the presence of siderite (FeCO3). This is indeed an interesting finding that merits further investigation.
However, in my opinion there is not yet enough data to be published as a LESSONS report and might fit better as a LETTERS post. This is because it is based on the results of single Moessbauer measurements of four samples from one region. The Moessbauer spectra are well resolved but there is still some noise and there is likely some error involved in the fitting process when the siderite component is as small as 6.5%. The authors provide an estimate that based on the four samples from Alaska, siderite may be a significant pool of mobilizable carbon in thawing permafrost. However, without further data to confirm this it is largely conjecture.
Detailed Comments:
Line 34: I understand that an exhaustive list is not appropriate here (it is missing some important environments like salt marsh sediments e.g. SEM analysis of siderite cements in intertidal marsh sediments, Norfolk, England - ScienceDirect) so the text could be altered to include ‘ for example’
Line 38: I think it would be helpful to include a research question or statement of motivation alongside the statement describing the results.
Line 40: It would be helpful to add where in the permafrost profile these soils were taken from (i.e. how close to thawing are they?)
Line 50: Could you also state what fraction of the inorganic/total C this would be? In Table 1 inorganic carbon is stated as a trace, is that also true if one assumes that all of the inorganic C is in the siderite? I see now you discuss this later. I think more should be made of the potential mismatch between the quantified siderite and the ‘trace’ amount of inorganic carbon measured, which is then extrapolated in the thought experiment later.
Figure 1: At room temperature, is it really possible to unambiguously identify goethite and epidote?
Line 84-92: I think this section needs some rewording. The first sentence read to me actually as arguments for why siderite would not form authigenically for the reasons given in the second sentence (When I read parent material I thought of the geology from which the soil was derived). Without porewater data to support the presence of higher pH or alkalinity, it is difficult to make a strong conclusion here.
Line 96: ‘carbon dioxide and carbonates’ to me is describing the gas and solid phases, I suggest adding a term for the aqueous phase.
Line 97 and then 99/100: is reducing conditions a more appropriate term than anaerobic conditions?
Line 111: is carbonate accurate – I thought CO2 and H2O would be more likely. Or bicarbonate. As again I see you discuss later.
Line 117 – 132: I appreciate the thought behind this but it really is conjecture and I think the caveats given at the end should be presented at the start of the paragraph and note that it is based only on four soil samples all from Alaska. I am not sure quoting 10% as a value in the abstract is justifiable.
Line 135: what is a siderite indicator?