the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Feb 2026
Frazil ice formation as a pathway for iron enrichment in Antarctic sea ice
Abstract. The Southern Ocean plays a vital role in regulating Earth’s climate by absorbing large quantities of carbon dioxide, but its productivity is strongly limited by the availability of iron needed for phytoplankton growth. Sea ice is a crucial seasonal reservoir of iron; however, the processes responsible for its enrichment remain poorly understood. Here we use a process-based model to show that frazil ice formation, a common feature of winter sea-ice growth in the Southern Ocean, can scavenge dissolved iron from seawater and concentrate it in newly formed ice. First-order estimates scaled to the Antarctic sea-ice zone suggest that frazil-origin ice could supply ~16–33% of the seasonal dissolved iron pool in the upper 10 m. Because this iron is released as a short-lived but intense pulse upon melt, frazil ice provides a pathway that can trigger phytoplankton blooms, linking polar sea-ice processes directly to global carbon cycling.
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Status: open (until 24 Mar 2026)
- RC1: 'Comment on egusphere-2025-6509', Anonymous Referee #1, 14 Feb 2026 reply
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- 1
Reviewer comments regarding the manuscript entitled “Frazil ice formation as a pathway for iron enrichment in Antarctic sea ice” submitted by Tedesco et al.
The manuscript, based on observational and modelling data, which describes the mechanisms involved in the transport and enrichment of the sea ice with both particulate and dissolved iron. The surplus of iron is hypothesized to sustain and fuel the pelagic primary production during ice melt, establishing a component in solving the low Chlorophyll – high nutrient Southern Ocean paradox. Based on the results the authors claim that sea ice could supply 16-33 % of the dissolved iron pool available for primary production. It’s new and very interesting results.
The manuscript is well written and generally well organized, but there are many statements with no references. The authors put a lot of emphasize on frazil ice formation and where and when the frazil ice forms? I’m fully aware that the spatial extension of frazil ice formation is needed to estimate the importance of this mechanism in percent. At present state it would be better to focus on the modelling and how this model actually is working, and what is actually modelled? I’m not a modelling person, but it appears that the authors have to make a lot of assumptions to run the model. Further, it’s not convincingly explained how frazil crystals select for dissolved iron and not any of the dissolved macro nutrients? Is the dissolved iron adsorbed to the particulate matter, and then what is the difference between dissolved and particulate matter apart from different size classes? This should be explained in more detail. There is a Fig. 1 where the processes are explained, but there is no explaining text? What is complexation and what is L and L’ in Fig. 1? The differentiation between dissolved and particulate iron is quite unclear and should be improved.
Specific comment:
L19: Frazil ice formation also in the Arctic
L22: should be upper 10 m of the water column
L29: Several? Please specify and give references
L31: What forms can be labile? Specify and give a reference.
L32: There are more up to date references that 1990?
L34: Resuspension? That’s only in coastal and very shallow shelf waters. The reference de Jong (2013) also adds wind-blow dust as a source of iron. Resuspension can only have limited spatial effect of the release of iron.
L47: The figure needs to be explained in much more detail. What are L and L’? Text at external inputs is difficult to read!
L51 is also enriched? Explain in more detail!
L49-54 No references here?
L69 barrier to particle incorporation. Any reference?
L71 Not only winds and waves also currents!
L78-80 I suggest to move this section up below L53 which deals with incorporation in the sea ice.
L86 what is sediment mobilization
L90 more? Compared to what?
L96 where is resuspension part of the model?
L116 Move figure 3 here where you present the data
L120 I’m not sure that a reference to a data set on Zenodo classifies for a reference, and here that Fe tends to follow Chl-a distribution in the ice core? It’s shown in the Lannuzel et al. 2016 paper.
L124 Why are macronutrients not shown? Could be in supplm.?
L129 Any reference on the statement?
L132 The figure is a conceptual model and not a result. Maybe combine Fig. 1 and Fig. 2? Further, is it snow coming from the clouds? A source of iron? A bit confusing.
L133 followed by adsorption. Yes, maybe but this processes is absolutely essential and need further and more explanation.
L140 rising? In the sense of increasing or upward vertical velocity?
L141 much like snowflakes? Please specify what that means?
L144 Any references?
L147 75% of the dissolved fraction and 25% of dFe’s is in the soluble form. A bit confusing as the dissolved fraction is soluble or what?
L147 on page 1 its’ < 0.2 ym?
L154, yes, but what model?
L156 Do the numbers refer to the bottom section of the ice? The bottom 10 cm was collected for analyses as mentioned some where. In that case I’m surprised that sea ice temperatures are that low?
L169-179 this could be shortened and condensed. It appears
L181 I checked some of the enrichment index numbers in table 2 and get 306 for Chl-a > 10 ym and not 366 in core 488, and only 5912 in core 496. Check again. I get same number of dFe enrichments. Check again.
L207 When crystal structure is important I suggest to enlarge and these images and replace them with the chl-a graph.
L232 ocean current velocities of 0.7 m s-1 is a very strong current, and this factor should have been discussed in more detail considering the turbulence levels and frazil ice formations.
L236 Yes, but how much below? Give a number.
L242. From where do you get the data points Fig. 4? Aggregation factor (?) on the x-axis. It does not look like alpha.
L258 larger crystals? How larger? Give a number.
L265-282 A very long and extended discussion about frazil ice formation. Consider to condense and shorten. Further I would start the discussion with the modelling results, which is what this paper is about.
L299 Where do I see that the model reached this values?
L305-306. Maybe I missed it, but why are there different concentrations of iron in the wasters between the years? It’s 3.2 Gg both years 2023 and 2002. Is there any difference?
L315 enrichment. Any reference here?
L317 multi-parameter. Yes, but specify the parameters! Targeted field campaigns. Yes, what samples should be collected and what should be measured to adequately run an improved model?
L320-333 this section is a bit out of focus and related to more general conditions.
L340 and particles. What particles? PFe?
L342. Remineralization. This needs to be clarified and discussed in more detail or at least give a reference.
L348-353. Very long. Try to condense.
L357-361. Also condense if possible. It’s quite general statements.
The manuscript needs some revisions and clarifications before accepted for publication. I acknowledge the authors courage to carry out this preliminary work based on a very limited field data-set with many assumptions to run the model.