This manuscript presents an important and timely contribution to our understanding of atmospheric iron deposition and its potential role in regulating marine productivity during the Pleistocene–Holocene transition. By generating the first continuous records of FeICP and operationally defined dissolved iron (DFe) from the EGRIP ice core, the authors provide a high-resolution perspective on how iron speciation, rather than total iron flux alone, varied across a major climatic transition. The study is carefully executed, clearly written, and well situated within the long-standing debate surrounding the “iron hypothesis” and its regional expression in HNLC systems. In particular, the finding that dissolved iron increased only modestly during the Younger Dryas, despite a large enhancement in total iron, represents a valuable constraint on the effectiveness of aeolian iron fertilization in the North Pacific region.

Overall, this study represents a significant methodological and conceptual advance. By shifting the focus from total iron flux to iron solubility and chemical form, the authors provide a more nuanced framework for evaluating the climatic impact of atmospheric iron deposition. With minor clarifications regarding bioavailability and broader oceanographic implications, this manuscript will be of high interest to the paleoclimate, biogeochemistry, and Earth system science communities.

1. Age model and chronological constraints

• How sensitive are the observed millennial- to centennial-scale variations in FeICP and DFe to uncertainties in the EGRIP age model?
• Could short-term depositional variability or age-model smoothing influence the alignment between iron speciation, acidity, and climatic transitions (e.g., the Younger Dryas)?

2. Analytical methods and proxy interpretation

• How robust is the operational definition of CFA-derived DFe as a “labile” iron fraction across different aerosol sources and depositional conditions?
• Are there potential analytical or post-depositional processes in snow and firn that could alter iron solubility or speciation prior to measurement?
• Can the authors further clarify how volcanic versus dust-derived iron is distinguished analytically, and whether their dissolution behavior differs under the CFA protocol?

3. Bioavailable iron and global implications

• How do the authors evaluate the extent to which CFA-DFe represents iron that is truly bioavailable to marine phytoplankton, rather than merely chemically soluble?
• What additional constraints—such as iron speciation analyses, ligand-binding considerations, or experimental dissolution and incubation studies—would be required to directly link ice-core DFe to biological uptake in the ocean?
• Given that the ice-core record integrates long-range atmospheric transport, how representative are the inferred iron solubility patterns for North Pacific?
• Can the authors elaborate on the pathways by which similar iron species are deposited in both marine sediments and ice cores, and how post-depositional processes in snow, firn, and seawater may differentially modify iron speciation?

4. Lastly, one minor comment: although the study period extends slightly beyond the Holocene, it represents only a very limited interval of the late Pleistocene. As such, the term “Pleistocene–Holocene” in the title may be somewhat misleading with respect to the actual temporal scope of the study. I suggest revising the title to refer more specifically to the “last deglaciation” or to explicitly highlight the focus on the Younger Dryas interval.

       This manuscript reports high-resolution records of Fe_ICP and DFe from the EGRIP ice core in Greenland, covering the Pleistocene-Holocene transition (10.3–13.0 ka). The study demonstrates that while total iron concentrations increased 17-fold during the YD compared to the Early Holocene, the biologically active DFe fraction increased by only 29%. By integrating ice-core chemistry with marine productivity records from the North Pacific HNLC region, the authors effectively argue that aerosol alkalinity, rather than total dust flux, acted as a critical limit on iron fertilization during this climatic transition. The data are of high quality, utilizing CFA coupled with spICP-MS, and the findings provide significant new insights into the biogeochemical cycling of iron in the Northern Hemisphere. The manuscript is well-written and deserving of publication in Climate of the Past after addressing the following comments.

General comments

Comment 1: The manuscript effectively argues that aerosol alkalinity controls the iron solubility, however, the connection to marine primary productivity remains largely theoretical. The manuscript lacks a direct comparison with existing productivity records from the North Pacific HNLC region.

Comment 2: L12-15: The manuscript excludes the correlation between atmospheric Fe deposition and productivity due to alkalinity constraints, while implies DFe is the key driver. However, this does not confirm that other factors (stratification, sea-ice, etc.) were more relevant without direct evidence. Additionally, the manuscript lack of the comparison between DFe and productivity records.

Comment 3: The manuscript suggest that volcanic eruptions can drive DFe atmospheric iron solubility. However, the ice core record primarily reflects local deposition near the EGRIP site. It remains unclear to what extent these volcanic events influenced the productivity in the North Pacific, which need more sedimentary record in the North Pacific. I suggest add a figure comparing the DFe of 10 volcanic eruptions with a productivity record in the North Pacific.

Comment 4: There is a geographic distance between the study site and the North Pacific. Therefore, the manuscript should to discuss whether the characteristic (concentration and solubility) of DFe at the study site are truly representative of the aerosol deposition over the North Pacific, given the chemical evolution during long-range transport.

Specific comments

Comment 1: L213-215: I suggest adding panel labels (a, b and c) to the Figure 2 and captions.

Comment 2: Move Figure S1 to the main text to better introduce the location of these three ice cores in the text.

Comment 3: The relationship between the acidity and DFe is important evidence of the argument. I suggest (1) moving Figure S3 and S4 to the main text, (2) combining these two penal into one figure, which can better compare the correspondence across the different periods.

Comment 4: Add one figure superimposing the volcanic DFe and acidity data onto the records of the 8 selected periods, to provide further supporting evidence for the limiting effects of aerosol alkalinity on Fe solubility.

Burgay et al. present new high-resolution soluble iron data from the EGRIP ice core over the Younger Dryas-Holocene transition. The authors employ two continuous methods to measure soluble iron in the ice core. Consistent with previous studies, the authors show that soluble iron concentrations were higher in the dusty Younger Dryas period relative to the Holocene. They attribute this change to alkaline conditions. In addition, the authors suggest that volcanic eruptions caused episodic increases in the deposition of soluble iron to the Greenland ice sheet. As there is a complex interplay of sources and processes that drive the solubility of atmospheric iron, these new data from Greenland provide valuable insight into the relationship between soluble iron and mineral dust. Thank you for the valuable contribution to the literature. 

Outlined below are three areas that would benefit from further clarification and discussion.

Some aspects of this review are drawn on my earlier work in this area. References to this work provide an example of how the present-day understanding of the controls on iron solubility can be applied to paleorecords, thereby providing a framework for interpreting the soluble iron data presented in this manuscript. They are intended solely to support constructive suggestions for strengthening the manuscript.

Methods

1. Operational definition of dFe

Within the marine biogeochemistry community (aerosol and ocean chemistry), the standard operational definition of dissolved iron (dFe) is Fe <0.2 or 0.4 um. This is typically determined by filtering the sample through a 0.2 um or 0.4 um filter and the iron concentration in the leachate analysed. In addition, total dissolvable iron (TDFe) is the fraction of iron that is leached with weak acid in an unfiltered sample. Further information on these definitions can be found in the GEOTRACES programme https://www.geotraces.org/. These standard methods have been adopted in iron solubility studies of snow and ice, first by Edwards and Sedwick [2001] for TDFe and by Winton et al. [2016] for dFe, and subsequently by other studies e.g., Du et al. [2019]; Du et al. [2020]; Liu et al. [2019]; Liu et al. [2021]; Winton et al. [2022].

The ‘dFe’ method employed by Burgay et al. does not access the <0.2 or 0.4 um fraction of Fe and thus the term ‘dFe’ is misleading. Please rename with a different term such as ‘labile iron’ or other. Consistency of terms will avoid confusion especially for the marine biogeochemistry community who will be highly interested in this work. See Berger et al. [2008] for a background on labile iron methods.

There are various methods in the literature that assess a different pool of soluble iron in snow/ice/aerosols. These were developed to mimic certain processes of iron dissolution upon deposition to the ocean. Please clearly explain the rational for the two methods of soluble iron employed in this study and include what fraction of soluble iron each method assesses.

Assessment of bioavailable iron typically requires both the dFe <0.2 um and labile phases. I’m assuming this is why Burgay et al. have used two methods of soluble iron in this manuscript with ‘FeICP’ representing instantaneous soluble iron and ‘dFe’ representing a fraction of labile iron where iron continues to leach from particles over days after deposition to surface waters. Although this is currently unclear in the manuscript (to me at least). Have I understood this correctly? If I understand the intention of the methods, then please reframe the manuscript around  instantaneous soluble iron and labile iron.

It would be helpful to include a discussion on how these two continuous methods compare to dFe <0.2 um. Has a study been carried out to compare dFe <0.2 um or TDFe with the two continuous soluble iron methods used here? What consideration was made for an in-line filter in the CFA to assess the dFe <0.2 um fraction?

2. Lack of total or TDFe concentration and fractional iron solubility data

A limitation of this study is that total or total dissolvable iron (TDFe) data are not reported and thus there is no information on the fractional iron solubility. While soluble iron provides an idea of the concentration, fractional iron solubility identifies the efficiency of the conversion from total to soluble. This is important information as it helps understand what factors are controlling the solubility. Total iron with a low fractional iron solubility often comes from mineral dust, while high fractional iron solubility is derived from other sources, such as combustion, or indicates processing in the atmosphere that enhances the solubility. Thus, fractional iron solubility is an important metric for assessing changes in iron bioavailability across a significant climate and dust transition. In core cores studies, there is a trade-off between high resolution continuous data and discrete sampling - both having advantages and disadvantages. Please acknowledge this limitation in the manuscript.

3. Trace metal protocols and figures of merit

Measuring trace metals at low concentration levels in ice cores is not a trivial task. Please include additional information on the steps taken to minimise trace metal contamination.

To ensure the quality of the data, please report figures of merit for both types of soluble iron measurements including blank concentrations, accuracy and reproducibility.

Drivers of iron solubility

The current manuscript considers a single explanation (aerosol alkalinity) for the change in soluble iron concentrations between climate states. Yet iron solubility is driven by a complex interplay of sources and processes.

There is a well-established non-linear relationship between total/TDFe and soluble iron in the literature. The Sholkovitz et al. [2012] global compilation of aerosol iron data showed that fractional iron solubility is a function of total iron. The non-linear relationship is described by a simple two-component mixing model, whereby the fractional iron solubility of iron reflects the mixing of mineral dust (high total/TDFe Fe and low fractional iron solubility) and non-mineral combustion aerosols (low total/TDFe Fe and high fractional iron solubility).

This model was applied to Antarctic ice cores over the last glacial transition. See Winton et al. [2022] and Supplementary Figure 5. In the Holocene, dFe concentrations decreased and fractional iron solubility increased which is best explained by greater biomass burning and/or changes in the chemical processing of iron in the atmosphere that make it more soluble e.g., cloud processing.

Burgay et al. present a strong argument for changes in aerosol alkalinity driving the soluble iron concentration. However, there are a range of other factors (sources and processes) that are currently overlooked. The manuscript would benefit from a discussion of these.

Even better if the authors can measure TDFe in Younger Dryas and Holocene samples to investigate the relationship between TDFe and fractional iron solubility. This does not necessarily need to be a continuous record. Some constraint on the fractional iron solubility would be highly valuable and allows the authors to frame their discussion around the model described above. If there is no sample material available or additional analyses are out of scope, the authors could apply assumptions to estimate the total iron content of the dust which would help their interpretation of the soluble iron data over the Younger Dryas-Holocene transition. It would be a valuable exercise to compare Greenland vs Antarctica where the dust loading and atmospheric composition are different. Other datasets to support the discussion could include black carbon or other biomass burning proxies, mineral dust particle size, soluble iron fluxes.

The study assumes soluble iron in EGRIP comes from mineral dust. Please acknowledge this assumption. What about a changing dust source over the last deglaciation transition?

Volcanic sources of soluble iron

There is an emerging body of literature on this topic. I agree with the authors that volcanic eruptions are an important source of episodic soluble iron. There is an opportunity to strengthen the argument by:

1. Including a brief introduction to aerosol iron sources, including volcanic eruptions, as a source of soluble iron in the introduction.
2. Citing references relevant to enhanced aerosol iron solubility via volcanic eruptions and atmospheric processing in section 3.3. The emerging body of literature does support this interpretation, but they are currently not cited.
3. Supporting the argument by reporting values of soluble iron concentrations in volcanic vs background dust samples. A figure in the main manuscript could support this as well.

Specific comments

L10 ‘Total iron’ mentioned here but not reported.

L10 dissolved iron ‘concentration’.

L30-32 Large number of studies that support this statement. Please include additional references.

L46-70 This would benefit from restructuring to clearly show the difference between soluble vs total/TDFe fractions.

L58-59 Note that dFe method in snow/ice was employed by Winton et al. [2016] and uses HCl.

L134-137 Helpful to provide a bit more information on conductivity and particle measurements.

L154 Ca proxy may need a quick introduction.

L174-176 Helpful to explain data gaps in the caption.

L200-201 Could an in-line filter be added to the CFA?

L206 Support statement by showing seasonality data.

L317 Do you mean “increased dust deposition”? The measurements represent iron deposition to the ice sheet.

Figure 2 ECM and acidity y-axis is small compared to Fe.

References

Berger, C. J., S. M. Lippiatt, M. G. Lawrence, and K. W. Bruland (2008), Application of a chemical leach technique for estimating labile particulate aluminum, iron, and manganese in the Columbia River plume and coastal waters off Oregon and Washington, Journal of Geophysical Research: Oceans (1978–2012), 113(C2).

Du, Z., C. Xiao, M. J. Handley, P. A. Mayewski, C. Li, S. Liu, X. Ma, and J. Yang (2019), Fe variation characteristics and sources in snow samples along a traverse from Zhongshan Station to Dome A, East Antarctica, Science of the Total Environment, 675, 380-389.

Du, Z., C. Xiao, P. A. Mayewski, M. J. Handley, C. Li, M. Ding, J. Liu, J. Yang, and K. Liu (2020), The iron records and its sources during 1990-2017 from the Lambert Glacial Basin shallow ice core, East Antarctica, Chemosphere, 126399.

Edwards, R., and P. Sedwick (2001), Iron in East Antarctic snow: Implications for atmospheric iron deposition and algal production in Antarctic waters, Geophys. Res. Lett., 28(20), 3907-3910.

Liu, K., S. Hou, S. Wu, W. Zhang, X. Zou, H. Pang, J. Yu, X. Jiang, and Y. Wu (2019), Dissolved iron concentration in the recent snow of the Lambert Glacial Basin, Antarctica, Atmospheric environment, 196, 44-52.

Liu, K., S. Hou, S. Wu, H. Pang, W. Zhang, J. Song, J. Yu, X. Zou, and J. Wang (2021), The atmospheric iron variations during 1950–2016 recorded in snow at Dome Argus, East Antarctica, Atmospheric Research, 248, 105263.

Sholkovitz, E. R., P. N. Sedwick, T. M. Church, A. R. Baker, and C. F. Powell (2012), Fractional solubility of aerosol iron: Synthesis of a global-scale data set, Geochimica et cosmochimica acta, 89, 173-189.

Winton, V. H. L., A. R. Bowie, M. A. Curran, and A. D. Moy (2022), Enhanced Deposition of Atmospheric Soluble Iron by Intrusions of Marine Air Masses to East Antarctica, Journal of Geophysical Research: Atmospheres, 127(13), e2022JD036586.

Winton, V. H. L., R. Edwards, B. Delmonte, A. Ellis, P. S. Andersson, A. Bowie, N. A. N. Bertler, P. Neff, and A. Tuohy (2016), Multiple sources of soluble atmospheric iron to Antarctic waters, Global Biogeochemical Cycles, 30(3), 421-437.