Review of egusphere-2025-4525 “Iron isotope insights into equatorial Pacific biogeochemistry” by  Camin et al.

Summary

This study presents iron concentration and isotope data from the EUCFe cruise that extends along the equator from near Papua New Guinea to 140°W. Two earlier papers have resulted from this dataset, with this manuscript extending the dataset to some extra stations (Labatut et al., 2014; Radic et al., 2011). In their earlier papers, the authors reveal two distinct regimes: 1) a western margin zone dominated by lithogenic inputs from Papua New Guinea, including riverine, shelf and hydrothermal input, and 2) a central open ocean region where Fe is transported eastward via the Equatorial Undercurrent. Iron isotope signatures for particulate iron are close to the upper continental crust, and the offset between dissolved and particulate iron suggests equilibrium fractionation, perhaps via non-reductive dissolution. Measurements of iron isotopes in the deep chlorophyll maximum showed minimal isotopic fractionation with no clear preference for heavy or light isotopes.

Overall, the manuscript presents new iron isotope data; however, it appears that the main findings have been discussed previously in Labatut et al., 2014 and Radic et al., 2011. The new component of the manuscript presents a detailed examination of iron isotope fractionation in each water mass. I also wonder about the strength of the discussion relating to iron isotope fractionation by phytoplankton. The authors do not present any results relating to phytoplankton groups that are likely to take up iron and fractionate it. They just say that iron isotope fractionation “lies between -0.17 and +0.39 ‰ (+0.11 ± 0.28) at a 95% confidence level”. There is little consideration that various phytoplankton species might have different iron acquisition strategies e.g. Sutak et al., 2020, which likely influences iron isotope fractionation. If possible, I think this needs to be explored a bit more in the manuscript.

Below are my comments on the manuscript, and below that, I address the guideline questions for peer review and interactive public discussion.

Comments

Line 218. The presentation of the blank contributions is percentage values relative to dissolved and particulate matter iron data. Is it possible to present the amount or concentration values as well?

Line 331. “Fe concentrations in the western equatorial Pacific were approximately seven times ...”  It would be useful to explicitly state concentrations, perhaps in brackets, so the reader doesn’t have to search for the values.  Fe enrichment in the western equatorial Pacific, e.g., “Fe concentrations were approximately twice as high for DFe and seven times higher for PFe compared to central Pacific stations.”

Lines 411 to 413. Have you considered that heavier δ⁵⁶DFe relative to δ⁵⁶PFe could be preferential complexation of dissolved iron to natural organic ligands present in seawater? Under equilibrium control, ligands should selectively bind heavier isotopes relative to lighter isotopes. If the majority of DFe is bound to strong organic ligands (Fe-L), then Fe-L should be heavier than inorganic Fe (Fe’). Persumability, Fe’ is what exchanges with PFe. I guess this is what you are terming as non-reductive dissolution.

Lines 456 to 459 – Figure 8. The colour scheme used in this Figure and subsequent ones makes it very hard to determine the difference between isotope values.  I certainly found it difficult to see the subtle changes in blue between samples – this is highlighted in the lower panel with stations 13 and 14. Here are the values greater or less than 0‰? Perhaps change the colour palette away from Viridis to the ODV colour palette or Ferret_blue_orange.

Line 465. Any ideas on what phytoplankton species occupied the deep chlorophyll maximum (DCM)? This seems important when attributing isotope fractionation to biological production. Where nutrient and associated parameters were collected on the voyage to support the interpretation of how iron might be acquired by phytoplankton, new vs recycled iron etc? Cyanobacteria and diazotrophs vs eukaryotic species.

Line 531. It might be worth considering the work of John et al. (2024), who tried to determine iron isotope fractionation in phytoplankton cultures.

Lines 532 to 540. The discussion here is a little simplified and assumes that iron isotope fractionation by phytoplankton is likely to be similar across varying regions. At present, we have no real idea how cyanobacteria fractionate iron. The DCM is likely to be populated by Prochlorococcus and Synechococcus, as well as by unicellular diazotrophs, if nitrogen is limiting. Very little work has been done with these two bugs in fractionating iron under oxic conditions (Mulholland et al., 2015; Swanner et al., 2017). Perhaps this could be acknowledged. Again, it might also be worth referencing. John et al. (2024) here, who reported kinetic isotope effects during Fe(III) reduction in cultures. These findings could provide useful context for interpreting biological fractionation in this study.

Lined 701 to 702. A supporting reference for AAIW circulation in this region and the South Pacific is Bostock et al. (2013). It may be worth noting that this study region lies near the northern extent of AAIW influence, as discussed by Bostock et al. (2013), who reviewed AAIW circulation and mixing using geochemical tracers and Argo float data.

Review guidelines

1. Does the paper address relevant scientific questions within the scope of OS?

Yes

1. Does the paper present novel concepts, ideas, tools, or data?

Somewhat – as mentioned, the manuscript presents some new iron isotope data; however, it appears that the main findings have been discussed previously in (Labatut et al., 2014; Radic et al., 2011)

1. Are substantial conclusions reached?

More work is needed on how phytoplankton fractionate iron isotopes./

1. Are the scientific methods and assumptions valid and clearly outlined?

Yes the scientific method and measurements are sound.

1. Are the results sufficient to support the interpretations and conclusions?

Generally, see comments about iron isotope fractionation by phytoplankton

1. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?

This is fine

1. Do the authors give proper credit to related work and clearly indicate their own new/original contribution?

Yes they credit previous work

1. Does the title clearly reflect the contents of the paper?

I think a better title would be “Iron isotopes provide insights into the biogeochemical cycling of iron in the equatorial Pacific”

1. Does the abstract provide a concise and complete summary?

Yes

1. Is the overall presentation well structured and clear?

Generally, figure colours could be improved to allow the reader to distinguish between iron isotope values.

1. Is the language fluent and precise?

yes

1. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?

yes

1. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?

no

1. Are the number and quality of references appropriate?

yes

1. Is the amount and quality of supplementary material appropriate?

yes

references

Bostock, H.C., Sutton, P.J., Williams, M.J.M., Opdyke, B.N., 2013. Reviewing the circulation and mixing of Antarctic Intermediate Water in the South Pacific using evidence from geochemical tracers and Argo float trajectories. Deep-Sea Research Part I: Oceanographic Research Papers, 73: 84-98.

John, S.G. et al., 2024. Kinetic Isotope Effects During Reduction of Fe(III) to Fe(II): Large Normal and Inverse Isotope Effects for Abiotic Reduction and Smaller Fractionations by Phytoplankton in Culture. Geochemistry, Geophysics, Geosystems, 25(6): e2023GC010952.

Labatut, M. et al., 2014. Iron sources and dissolved-particulate interactions in the seawater of the Western Equatorial Pacific, iron isotope perspectives. Global Biogeochemical Cycles, 28(10): 1044-1065.

Mulholland, D.S. et al., 2015. Iron isotope fractionation during Fe(II) and Fe(III) adsorption on cyanobacteria. Chemical Geology, 400: 24-33.

Radic, A., Lacan, F., Murray, J.W., 2011. Iron isotopes in the seawater of the equatorial Pacific Ocean: New constraints for the oceanic iron cycle. Earth and Planetary Science Letters, 306(1-2): 1-10.

Sutak, R., Camadro, J.-M., Lesuisse, E., 2020. Iron Uptake Mechanisms in Marine Phytoplankton. Frontiers in Microbiology, 11(2831).

Swanner, E.D. et al., 2017. Iron Isotope Fractionation during Fe(II) Oxidation Mediated by the Oxygen-Producing Marine Cyanobacterium Synechococcus PCC 7002. Environmental Science & Technology, 51(9): 4897-4906.

General comments: “Iron isotope insights into equatorial Pacific biogeochemistry” by Camin et al presents an expanded dataset of dissolved and particulate iron concentrations and stable isotope measurements from the western and central equatorial Pacific. The manuscript builds upon work previously published from this project that focused on a subset of the stations along with aerosol iron measurements from the same cruise. The authors combine these datasets to construct a box model for iron transport in the region and also use the data to discuss iron transport in the various water masses of the region. Overall I think that the manuscript is a helpful addition to the literature on iron biogeochemistry in the Pacific but I think the authors could strengthen parts of the discussion – for example, discussing uncertainties and limitations associated with the box model approach and how consideration of separate authigenic and biogenic contributions to non-lithogenic particulate iron (highlighted by recent work in the North Atlantic) could influence their conclusions about isotope fractionation during biological uptake. There is also some geographical overlap between this work and a recently published paper by Sarthou et al (“Dissolved iron distribution and budget in the Solomon Sea”; accepted by Marine Chemistry shortly after this manuscript was submitted –full details below) with some of the same coauthors. There is sufficient overlap that some references to that paper could be incorporated into the discussion of this work during revision.

Specific comments:

I think the calculations for the box model need some more discussion, as outlined below:

• The calculation of Flux PFe_SW out appears to have been done by averaging all of the PFe concentrations from all depths from stations 21-30 to get a value of 3.6 nmol/kg, which was then multiplied by water transport. Is this the best approach? That dataset includes the one very high value of 29 nmol/kg from 39m at station 28. Excluding this one value would drop the average significantly from 3.6 to 2.9 nmol/kg. It also includes a handful of values calculated from different bottles at the same depth at certain stations (e.g. three samples at 218m depth at station 23), which potentially gives those depths more weight in the calculated average. And only 5 of the 40 values are from depths in the 400-1000m range, thus the average would be skewed towards <400m data. I think the uncertainty and the caveats associated with this approach should be acknowledged and explained more. For example, if the average of 3.6 nmol/kg is calculated as stated above, the associated standard deviation is 4.8 nmol/kg by my calculations.
• For the atmospheric deposition, the deposition velocity of 1000 m/d used is commonly applied where this cannot be measured more directly, but it is usually applied to calculate dry deposition only, rather than bulk (dry + wet) deposition, and wet deposition dominates atmospheric input over much of the ocean. An alternative approach to calculate total atmospheric input would be to apply the relationship between bulk deposition velocity and precipitation rate that has been derived from beryllium-7 data by Kadko et al (2020) and recently updated by He et al (2025). This would involve deriving an estimate of the precipitation rate over the area covered by the box model from satellite imagery.
• A related point is that the box model area to which atmospheric deposition is applied includes much of New Guinea. Any deposition over this area would technically be included in runoff (river input) to the study area rather than direct atmospheric input and so should not be included in the atmospheric input. I suspect that this detail would become more important when calculating bulk deposition due to enhanced rainfall over high elevations of the island.

I have a couple of questions about the calculation of phytoplankton PFe and d⁵⁶PFe in section 5.2.2.

• It is assumed that all non-lithogenic PFe consists entirely of organic (and specifically phytoplankton) PFe. However, recent work from the Atlantic Ocean stresses the importance of non-biogenic, non-lithogenic phases in controlling Fe dynamics in the upper ocean, with authigenic Fe often representing a greater fraction of PFe than biogenic PFe (Tagliabue et al, 2023; Sofen et al, 2023). The modeling output in Figure 4 from Tagliabue et al 2023 does suggest that biology may be the dominant control on the Fe cycle in the region of this study, but the authors should discuss how contributions from authigenic (as opposed to biogenic) PFe would affect their calculations.
• In calculating d⁵⁶PFe_Phyto, the authors use a lithogenic d⁵⁶PFe value equivalent to the crustal signature (+0.07 ‰). However, in a separate paper from the same study, the authors described atmospheric input to the region having a heavier than crustal signature of +0.31 ‰. Is the assumption that this atmospheric contribution is insignificant relative to lithogenic Fe advected from the western Pacific?

I was confused by the use of “tons” when describing the amount of Fe transported into and out of the study region throughout section 5.1. Presumably the amount referred to here is a metric ton(tonne), but ton can also refer to an imperial weight unit. I suggest expressing all values in grams instead (i.e. 45 tons would be 45 x 10⁶ g).

Line-specific comments:

Lines 108-109: How deep does the influence of these westward currents extend?

Lines 117-118: Some of the references cited in the figure caption are not in the reference list (Delcroix et al, 1992; Kashino et al, 1996; Kashino et al, 2007; Johnson et al, 2002).

Lines 201-212: Does “leachate” refer to the digested material? I suggest using “digest” instead, as leachate can often refer to the solution resulting from a treatment that gives a partial release of elements, rather than total solubilization.

Line 220: Does repeatability here refer to repeat analysis of a sample or analysis of two samples collected at the same depth and processed individually?

Lines 331-333: Does the comparison of concentrations between western and central equatorial stations (“twice as high for DFe…seven times higher for PFe”) refer to a specific depth range? Or averaged over all samples? I think this could be more specific.

Line 410: I’m confused as to how the exchange is both permanent and reversible. (also in the Conclusions at line 760 and the abstract at line 30).

Lines 696-704: Suggest not numbering statements as “1” and “2” twice in one paragraph. I don’t think they are necessary for the first usage – that sentence would work well enough without numbered points – “Its signatures fall within the range observed for those water masses in previous studies, but they fall in the heavy part of those ranges and again have a smaller variability…”

Lines 761-763: This sentence didn’t read clearly to me. It may help to use “occurs” instead of “occurring” and add parentheses for the “i.e.” parts of the sentence.

Figure 4: Suggest including something in the caption to the effect that error bars are not included for clarity but a scale showing relative size of typical error is included.

Figures 8-12: I found the choice of colour scheme or distribution of colours involved to be unhelpful in picking out some of the trends discussed in the text. For example, in section 5.2.3, it is noted that d56DFe at stations 1 and 3 differ significantly from the other samples (line 615), but the difference in the blue and green dots is quite subtle and difficult to pick out.

Technical corrections:

Line 34: Should be HNLC rather than HNCL. Also at Line 750.

Line 36: Should be “…that iron was originating…” rather than “…that was iron originating…”.

Line 39: Sentence needs some rearrangement. Maybe “…that any fractionation must be small, no larger than…”.

Line 52: “be” is not necessary here – “where Fe is believed to have a main source…”.

Line 54: Should be “…is an eastward-flowing…”.

Line 135: No need for semicolon after the reference.

Line 138: Degree symbol should be in front of “C”. Also at Lines 460, 549, 588, 639, 672.

Line 303: No need to type out “upper continental crust” again – it was defined as UCC on line 293. (also Line 370).

Line 479: No need for “the” before New Guinea.

Line 481: Insert “are” before “…equal to…”.

Line 584: Density units are given as g.m^-3 rather than kg.m^-3.

Line 596: No need for “the” before “Fe transport”.

Line 625: First comma of the line should be after “above” rather than “suggests”.

Line 645: “Figure” rather than “Figures”.

Line 727: No need for “coast” in “…all located coast within…”.

Line 755: Suggest using “averaging” rather than “average of”.

Line 777: Suggest changing to “Note that the fluorescence profiles were measured…”.

Line 781: Add “Fe” to “dissolved isotopic composition”.

Table 1: For origin description of NPEW, it is described as a “mixture between SPEW and NPWC”. I think the latter should be “NPCW”.

Figure 6: Superscript needed for two of the “day^-1” labels.

References cited in this review:

Sarthou et al (2025), Dissolved iron distribution and budget in the Solomon Sea. Marine Chemistry, https://doi.org/10.1016/j.marchem.2025.104567  

Kadko et al (2020), Quantifying atmospheric trace element deposition over the ocean on a global scale with satellite rainfall products. Geophysical Research Letters, https://doi.org/10.1029/2019GL086357

He et al (2025), Constraining aerosol deposition over the global ocean. Nature Geoscience, https://doi.org/10.1038/s41561-025-01785-2

Sofen et al (2023),Authigenic iron is a significant component of oceanic labile particulate iron inventories. Global Biogeochemical Cycles, https://doi.org/10.1029/2023GB007837

Tagliabue et al (2023), Authigenic mineral phases as a driver of the upper-ocean iron cycle. Nature, https://doi.org/10.1038/s41586-023-06210-5