the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Aug 2026
Glacial controls on iron and manganese concentration and lability in an Arctic fjord
Abstract. Retreating glaciers are altering sediment and micronutrient fluxes to coastal waters, yet the magnitude and environmental impacts of change remain unclear. Here we examine how contrasting glacier systems in an Arctic fjord control the export and speciation of dissolved and particulate iron (Fe) and manganese (Mn) by comparing locations influenced by marine-terminating and land-terminating glaciers. Data was collected from two glacierized bays in Hornsund, a southern Svalbard fjord, during research cruises conducted in 2022 and 2023. Highly reactive ascorbate‑extractable particulate Fe was a major component of the Fe pool in both systems, exceeding dissolved Fe even beyond the inner coastal zone, indicating that reactive particulate Fe is transported farther offshore than previously assumed. The marine-terminating glacier influenced bay exhibited substantially higher concentrations of reactive particulate Fe and Mn, elevated dissolved Mn, and sedimentation rates one to two orders of magnitude greater than in the bay with a land‑terminating glacier. These differences might be a result of higher subglacial discharge, a larger glacier cover with enhanced chemical weathering, and a higher abundance of freshly weathered mineral surfaces in the marine-terminating glacier influenced bay. In contrast, the input of suspended particulate matter and reactive micronutrient phases from the land‑terminating system was lower, as advanced glacial retreat has led to stabilisation of proglacial sediments and the glacier is smaller. Based on this dataset, coastal micronutrient inputs are likely to decline as glaciers continue to retreat, due to falling glacier discharge and supply of reactive sediment enriched in reactive micronutrients.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4466', Anonymous Referee #1, 24 Sep 2026
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RC2: 'Comment on egusphere-2026-4466', Anonymous Referee #2, 25 Sep 2026
Review of Stachnik et al., “Glacial controls on iron and manganese concentration and lability in an Arctic fjord”
In the manuscript by Stachnik et al., data is presented on dissolved and particle speciation of iron (Fe) and manganese (Mn) from waters adjacent to two distinct, but proximally close, Arctic glaciers. These glaciers are distinguished by their setting, where one is marine-terminating and the other land-terminating. The goal of this study was to identify how these differences in glacial setting affect the export, phases, and seasonal dynamics of dissolved and reactive trace metals. To this end, seawater, freshwater, and suspended marine particles were collected and treated using established geochemical treatments. The authors conclude that highly reactive particulate Fe extends further from the glacier source than dissolved Fe, that the marine-terminating glacier produces higher fluxes of highly reactive particulate Fe and Mn which scales with sediment flux and sedimentation (from sediment traps), and that bioavailable fractions of trace metals (micronutrients) input is likely to decline under future glacier retreat.
Overall, this is an important study and one that builds on recent literature, which suggests that the bioavailable flux from glaciers into fjord settings is dominated by reactive particle fractions. In even more detail, the authors include flux and particle speciation that provides important information about the minerology of these fractions, which have been tied to bioavailability in previous studies. For the most part, their conclusions are supported by the data, but in some cases are not, and so it is recommended the authors do not try to overstate what the data show. One major criticism is the incorporation of macronutrient and other data was not done sufficiently to provide any evidence in support of their conclusions, and therefore should be removed (see comments by line). The manuscript could benefit from a judicious culling of detail and figures that do not support the final conclusions and to help the reader, being concise in its presentation. This includes limiting the amount of discussion in the presentation of results, and vis versa, do not present new data in the discussion without systematically presenting it in the results.
There are a few instances where I disagree with the authors interpretation of the data. For example, one major scientific concern is that higher sedimentation fluxes are a result of greater amounts of erosion beneath a given glacier, but this is only partly true because there was no size classification of the sediments, given that coarse-grained sediments sink out proximal to the glacier, compared to small sediments which remain suspended into distal locations. The comments about water mass retention are also not supported because without size characterization one can not determine how quickly they will sink, rather than demonstrating that sediments are retained due to bathymetric constraints. No discussion was included about potential other sources of trace metals, such as icebergs and shallow marine sediments.
The authors also make statements about dissolved-particle exchange and active cycling that is not necessarily supported in their data. In these environments, sediment surfaces are strong competitors for binding of metals, and so scavenging of colloidal oxide phases is particularly strong unless there is sufficient time to equilibrate with dissolved organic carbon ligands. For Fe, overwhelming numbers of competitive binding sites on particle surfaces and in particle-rich environments represent important sinks for dFe, rather than a sources.
A good path forward for eventual publication is to focus the presentation of their data to continuously highlight the main discussion about impacts of glacial setting on particle supply and speciation for two important micronutrients. The beginning is strong, but the reader is lost in between the results and discussion (with more results). Finally, please add nuance as to the limitations of not considering important aspects of trace metal sources and particle size dynamics.
My specific comments by line are below:
31: “reactive” used twice, might use something else
46: “comprising both” “and/or” does not make too much sense to me when combined
57: I think you mean, “seasonally” not “seasonality”
Fig. 1: It is not apparent what the PDD line represents from the caption. Later in the text it is explained that the PDD is integrated positive degree days over a 6 day moving interval. Please make clearer in caption when data is presented.
190: It is not clear what the process for this leach is as written. Please explain how each fraction is handled, including if volume is removed between the sequential leaches.
200: How is LOD determined?
198-204: In the text the units are mentioned to be nmol/kg, but then values are reported as µg/L. Please be consistent, or explain why there are differences in units. I believe you mean that seawater concentrations of dissolved and particle fractions are expressed in molar units per weight seawater.
234: “morphologies” spelling
255: This is very interesting! Wouldn’t the G glacier take longer from the production of sedments to reach the ocean by your hypothesis? Please clarify.
272: “highly reactive”?
Fig. S2: Why is one log scale? How do you interpret this in the context of the different sized particles sampled? What is the contribution of particles between 0.45-0.7 µm?
Fig. 4: There is no commentary in the manuscript as to why 2024 is much higher in terms of SPM. Please mention.
Fig. 5: Move down after the figure is called and referred to in the text.
322: Is the freshwater endmember interpreted as representative for both fjords? That this is the glacio-fluvial input from the subglacial drainage? Please clarify.
Section 4.2.1: Is this section discussing the entire water column, or just the surface? upper 30 m? Related, is Table S3 just surface? Table 1? In the captions, please make sure to indicate which samples are being presented.
369: What is meant by “Mn indices converged”?
Section 4.2.3: This section is beginning to sound like discussion. Please limit the amount of discussion and move to later sections.
379: Why would it be associated with mixing? Please clarify.
380: You are comparing bays and then seasons? This is difficult to understand when reading. What about summer had a stronger correlation? Summer is not being correlated.
Table 5.5: Please remove redundant correlations across the main diagonal of self-correlation. This will make it easier to analyze.
386: Consistently with what? Season?
394: Please write out the meaning, “correlation between dMn and salinity was…”
367-368: I don’t fully agree with this statement and the caveat that an additional source besides freshwaters could be present in this biogeochemically dynamic setting.
410: Please choose a different word here (e.g., integrated sediment accumulation between June and August).
Section 4.2.4: What about MnD? This was not included in the systematic discussion.
445: These variables were not presented in the results section.
446: delete “high”
450: Why would snowpack have low dFe? Please include a citation here.
459: There is no digestion of this data from the list to the conclusion. Please remove.
464: “differences between glacier weathering environments”
484: Do you mean TdFe? So a leach involving dilute HCl for a certain amount of time? How long were the samples stored acidified prior to filtration and analysis?
488: Please be more concise here.
492: “more sensitively” than what?
494: I disagree. How does the Berger leach obscure the role of pFe export? The authors are lacking evidence to make this statement, especially since these show similar differences between fjords, since we assume FeA is a component of the Berger leach pFe.
497: Remove “appears” from everywhere. A good alternative is “shows” or “demonstrates”.
498-499: Attenuating more slowly than dFe does not demonstrate near-conservative behavior. It would be good to have more information for how slowly, such as through a decay fit equation parameter comparison.
501-505: This sentence is difficult to understand as is. I do not agree this is evidence to support dissolved particle exchange.
507: “more than an…”
513-515: Need to separate out results from discussion as much as you can, otherwise risk diluted the clear messages with interpretations of data.
521: The is a novel framework, and I think should be elevated. This is a very excellent example of clear presentation.
Section 5.1.3: This paragraph does not add anything to the conclusions and there is limited interpretation. This could be a different story, but I will suggest of tying back in something about bioavailability. So far this was not done since the introduction. I recommend removing this section entirely.
572: There is no size data which could conflict with the interpretation. This could be due to coarse-sized particles contributing more and therefore sinking closer and quicker to the point source. We know that extensive sorting occurs in fluvial environments, and depending on the distance transported, the effect of particle sorting could be extreme.
580: What about icebergs and their ability to maintain SPM for longer through melting and grounding events? Currently there is no discussion about their role for the budget of meltwater and SPM, and this is a major oversight for a marine-terminating glacier system.
598: How does quantitative indicators support “glacier-ocean connectivity”? Please explain.
633: “Bioavailable Fe is…” The authors did not measure the detrital fraction associated with refractory lithogenics through a complete digest.
Citation: https://doi.org/10.5194/egusphere-2026-4466-RC2
Data sets
Biogeochemical properties of seawater and suspended particulate matter in an Arctic fjord under contrasting glacier influence (Hornsund, Svalbard, 2022–2023) Stachnik et al. https://zenodo.org/records/21371463?token=eyJhbGciOiJIUzUxMiJ9.eyJpZCI6Ijc5MTM4MmM2LWUwZWMtNGUwNy1iYzg5LWFhNGFkNmQ4YTQ3MSIsImRhdGEiOnt9LCJyYW5kb20iOiI2ZmVjNzJkMWIyZGEwNWFhNDM0ZWI2ODMyOGNiZTU5OCJ9.PgLbHyTRDRzp85qdBGSnsxo_uerwN3P_CGWjHefumvR_IlgWrrUGaJOiM4KMIzVO6EOOK9POMQRiLmVLsM5Vfw
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- 1
Manuscript review: Glacial controls on iron and manganese concentration and lability in an Artic fjord
In this study, the authors perform a suite of measurements of dissolved iron (Fe) and manganese (Mn) and suspended particles that were collected across two glacier systems with varying properties. Specifically, the authors investigated systems containing a large marine-terminating glacier (Hansbreen) and a smaller land-terminating glacier (Gåsbreen). A major fraction of the dataset from both systems (aside from CTD) included dissolved Fe and Mn concentrations, as well as measurements of Fe and Mn derived from chemical extractions of the SPM using ascorbic acid (FeA and MnA) and dithionite (FeD and MnD). Other parts of the dataset included some X-ray diffraction patterns and macronutrient measurements. The authors plot the Fe and Mn measurements in a number of ways, including via several “geochemical indices”, such as FeA/FeD, and FeA/dFe. Using the large amount of data presented in the manuscript, the authors find considerable differences in Fe and Mn behavior across the different glacier systems. The major trends in the dataset pointed to the system containing the larger marine-terminating glacier having substantially higher inputs of reactive particulate Fe and Mn, as well as dMn. This same system was also characterized by a significantly higher sedimentation rate than the land-terminating glacier. These results are placed generally in the context of the transition from marine-terminating to land-terminating glaciers, which would “progressively reduce glaciofluvial connectivity, erosive and weathering potential of the ice covered part of the catchment, and direct sediment export to the ocean”.
This study has several strengths. The introduction is organized and clear. The methods section is very detailed, with the sample collection protocols and analytical measurements appearing to be performed with great care. The results are also organized sufficiently and the figures are simple and clear. The discussion is also sufficient, with particularly good contextualization of specific measurements with previous investigations of similar types glacier systems. In general, the manuscript is written well and has few errors in spelling or syntax, though some mistakes show up more at the end of the manuscript. I also could not personally detect any obvious use of LLMs in the writing, which I have come to really appreciate in the current state of scientific writing.
The major issue I have relates mostly to the structural analysis of the suspended particulate matter (SPM), which I thought was weak. First, the XRD and SEM analysis in the results section reads largely as inconsequential and is poorly integrated in the manuscript. This part is short, is presented in a kind of off-handed way and has almost no bearing on how the manuscript is otherwise presented. I think this part can be removed without any impact to the remaining manuscript. Similarly, the section on macronutrients and organic carbon, which is actually given in the discussion even though it reads as a result in my view, is poorly integrated in the manuscript, has little consequence on the conclusions of this work as-written, and can also be removed without changing almost anything else about the manuscript. This is not to say that the XRD and macronutrient information is without benefits, rather, these data are simply not integrated in a meaningful way in the rest of manuscript, which leads the reader to ask why the data were collected and presented. This aspect of the manuscript should be improved by reframing some of the work or by handling these data in a better way.
Second, I think there is only limited benefits of applying chemical extractions of SPM to create operationally defined pools of FeA, FeD, MnA and MnD. I realize that extraction protocols have been useful in previous decades to operationally differentiate solid-phase Fe and Mn species. However, I think there has been enough evidence to show that results and conclusions based on chemical extractions alone can be confusing and prone to error and misinterpretations. Considering that the authors also mention the challenges of using extractions to identify Fe phases in the manuscript, I think the authors are also aware of these issues. I also think that the broader biogeochemical field is starting to move away from deriving conclusions based solely on extractions, particularly given that synchrotron methods are much more effective at measuring (directly) SPM speciation and are becoming more and more accessible, with a growing number of beam lines offering mail-in service for standard samples, such as these. I have the impression that the dataset is already quite complete and that performing an additional set of measurements to directly identify Fe and Mn speciation in the SPM is likely beyond the scope of the study. In that case, the authors must be more careful with their language to avoid overinterpreting the extraction results. The following statement is a good example, “The greater contribution of highly reactive particulate Fe to the Fe pool is not confined to the immediate vicinity of the glacier but extended across the bay and persisted beyond the sill, indicating substantial lateral transport…” In this statement, the authors use the operationally defined FeA fraction too loosely, almost as though “highly reactive particulate Fe” was a directly measured and unambiguously defined mineral. However, it is still unclear to me what is meant by “highly reactive particulate Fe” because, in my thinking, the structure of a highly reactive solid phase is not likely to be preserved in the environment over long times and distances. Here in this example, I think the authors specifically mean FeA, but I think the behavior of this material in the investigated glacier systems is at odds with the operational definition of a highly reactive Fe phase. Perhaps an ascorbic acid-soluble phase is encapsulated in an aggregate that preserves the solid from transforming over long distances, but then I would not describe this Fe species as highly reactive. In any case, I think this is a problem that the discipline has, and it is not necessarily the author’s job in this work to solve it, indeed I do not have a good answer about how to interpret the extraction results through the lens of assigning an actual Fe mineral phase myself. However, I at least think the authors need to revise the manuscript to avoid over-interpreting what the extraction results actually yield.
I have some smaller comments of a technical nature below:
Line 93: Missing a space between and2020
Line 135-136: A bit unclear why depths of 5 m is listed twice
Line 310: I perhaps missed this detail, but it would be nice to have a short explanation about why 2024 seemed to produce more SPM in the two systems compared to other years.
Line 439-440: This statement is written very broadly without some qualification. If the authors are referring to the systems investigated in the current work, I would make that clear, otherwise I would include some references here.
Line 456: “A” potential process
Line 467-468: This sentence reads as if FeA is a distinct mineral phase to me, which I think is incorrect. At the very least I would replace “is formed” with “can be formed”
Line 470: “the” Berger leach
First paragraph of section 5.1.2: Probably a writing style issue, but there are many missing parenthetical brackets in this paragraph “(110-130 nM sedimentary substratum (Yang et al. 2022)” needs a “)” at the end.
Line 507: more than “an” order
Line 518: Missing a space between Arctic and (Bhatia
Line 567: two .. at the end of the sentence.
Line 573: should probably be “promotes” instead of “promoting”
Line 582: Missing a space after the .
Line 588: Missing a space between “salinity” and “and”