| 06 Oct 2025
Iron isotope insights into equatorial Pacific biogeochemistry
Abstract. The EUCFe cruise (RV Kilo Moana, 2006) was designed to characterize sources of Fe to the western equatorial Pacific and its transport by the Equatorial Undercurrent (EUC), a narrow and fast eastward current flowing along the equator, to the eastern equatorial Pacific High Nutrient Low Chlorophyll (HNLC) region. This study presents seawater dissolved (DFe) and particulate (PFe) iron concentrations and isotopic compositions (δ56DFe and δ56PFe) from 15 stations in the equatorial band (2° N–2° S) between Papua New Guinea and 140° W, over more than 8,500 km along the equator and in the upper 1,000 m of the water column.
δ56DFe and δ56PFe ranged from -0.22 to +0.79 ± 0.07 ‰ and from -0.52 to +0.43 ± 0.07 ‰, respectively (relative to IRMM-14, 95 % confidence interval). Source signatures, biogeochemical processes and transport all contribute to these observations. Two distinct areas, one under continental influence (the western equatorial Pacific) and an open ocean region (the central equatorial Pacific), emerged from the data. In the area under continental influence, high PFe concentrations along with δ56DFe values systematically heavier than that of δ56PFe indicated a permanent and reversible dissolved-particulate exchange. This exchange occurs through non-reductive processes, as previously proposed from three of the eight stations of this area (Labatut et al., 2014). In the open ocean area, preservation of a DFe isotopic signature of ~+0.36 ‰ within the EUC, from Papua New Guinea to the central equatorial Pacific (7,800 km), confirmed the origin of the DFe carried within this current toward the HNCL region. At the same depth, bordering the EUC at 2° N and 2° S at 140° W, light isotopic signatures suggested that was iron originating from the eastern Pacific oxygen minimum zones. These light signatures were also observed in deeper central waters, between 200 and 500 m. Our data did not allow conclusions about fractionation during uptake by phytoplankton, but indicated that this fractionation must be if any, is small, no larger than a few tenths of a per mil.
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
Yes
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)
More work is needed on how phytoplankton fractionate iron isotopes./
Yes the scientific method and measurements are sound.
Generally, see comments about iron isotope fractionation by phytoplankton
This is fine
Yes they credit previous work
I think a better title would be “Iron isotopes provide insights into the biogeochemical cycling of iron in the equatorial Pacific”
Yes
Generally, figure colours could be improved to allow the reader to distinguish between iron isotope values.
yes
yes
no
yes
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.