| 16 Sep 2026
Constraining organic alkalinity in Antarctic sea ice brines containing high concentrations of dissolved organic matter: Current limitations and future directions
Abstract. Organic alkalinity (OrgAlk), the component of total alkalinity (AT) arising from non-inorganic species or uncharacterized bases, introduces error into calculated carbonate system parameters when AT is used as an input. OrgAlk has been identified in Arctic sea ice brines with elevated dissolved organic carbon (DOC) concentrations, suggesting organic contributions to AT may also be important in other sea ice environments. Antarctic sea ice brines are reported to contain higher DOC concentrations, yet estimating carbonate system components in Antarctic sea ice brines has relied on AT-based calculations although no direct OrgAlk measurements exist for Antarctic sea ice brines and the potential magnitude of organic contributions to Antarctic brine AT remains unknown. This study assesses potential OrgAlk contributions in Antarctic sea ice brines using published OrgAlk:DOC relationships as boundary conditions. Applying published OrgAlk:DOC relationships spanning 0.06–0.44 to Antarctic sea ice brine DOC concentrations produced a wide range of possible OrgAlk values, reinforcing the uncertainty of transferring DOC relationships across environments. These values are not interpreted as quantitative estimates of Antarctic sea ice brine OrgAlk, but rather as a sensitivity framework for evaluating plausible conditions. Sensitivity analyses indicate that unaccounted OrgAlk of this magnitude could affect calculated carbonate system parameters, including pCO2, when AT is used as an input parameter. These findings establish current uncertainty boundaries for OrgAlk in Antarctic sea ice brines, highlight a significant gap in Antarctic sea ice biogeochemistry and demonstrate the need for direct OrgAlk measurements to reduce uncertainty in Antarctic sea ice carbonate system calculations.
This paper examines the potential importance of organic alkalinity in antarctic sea ice brines. I can sum up its main outcome fairly concisely: a recent study (by other authors) showed the importance of organic alkalinity in Arctic sea ice, and reported a DOC:OrgAlk ratio; because Antarctic sea ice brine typically has higher DOC, it stands to reason that organic alkalinity might be even higher in Antarctic sea ice brine. That is essentially the premise, and the eventual conclusion of the paper.
I did not identify any major technical flaws in the study (although I have some comments about the final "sensitivity analysis"), and in fact it serves as a nice review of the issue of organic alkalinity, highlighting the specific case of sea ice. I think the challenge for the editor is simply to decide whether or not this simple, and fairly intuitive conclusion, is significant enough for publication in this particular journal.
Let me make the case for the strengths of this paper, and why it might warrant publication in The Cryosphere:
-The authors have put in the effort of compiling a dataset of DOC and TA measurements for Antarctic brine, and related seawater measurements. This does allow for us to understand the range of DOC in Antarctic brine, and the resulting OrgAlk range that may arise from applying different OrgAlk:DOC ratios. This certainly adds more value than a simple statement of "Antarctic ice has high DOC, therefore it probably has high OrgAlk" (see my summary above)
-The authors have also compiled a wide range of OrgAlk:DOC ratios from the literature, which allows them to do a very wide sensitivity analysis of what the OrgAlk contribution could be in Antarctic sea ice.
-The authors are very up front about limitations of their study, and thoroughly explain why the study is limited. Their review of carbonate system theory and its applications to sea ice is excellent, and is a strength of the paper.
-It does elevate the issue of OrgAlk in sea ice brine; it is important that the sea ice carbonate system community is aware of this issue, and in that way the paper could be impactful.
Here are the weaknesses of the paper, and why it may need work before publication:
-The core calculations that are performed with this carefully curated dataset are quite simplistic. They are really just multiplying their DOC data by a wide range of OrgAlk:DOC ratios, and reporting them in graph and table form.
-While overall the writing in this paper is strong, I found it at times to be redundant. Certain concepts are repeated at different points in the paper (sometimes separated by no more than a paragraph), and this affects the readability. Here are some examples of redundancy:
-Lines 115 - 120 provide significant detail on the range of OrgAlk:DOC ratios in literature, and the studies they arise from; Line 150 -154 repeat the same information.
-Lines 354 - 356 describe constraints on the Antarctic dataset, and mention that a sensitivity analysis is the only thing possible. Lines 361-362 repeat this.
-Section 4.2.1 contains many redundancies, both internally and with other sections of the paper
Basically, key concepts are introduced and then reiterated multiple times throughout the manuscript. Concepts that are prone to redundancy in this paper include: the published range of OrgAlk:DOC relationships (see above), the inability of existing Antarctic datasets to uniquely constrain OrgAlk, the uncertainty associated with transferring OrgAlk:DOC relationships between environments, and the statement that the present study provides sensitivity bounds rather than quantitative OrgAlk estimates
-I really thought the final sensitivity analysis (Section 4.3) could be improved. Here are my thoughts about that:
-Why not apply the full range of OrgAlk:DOC ratios to this analysis? One of the core purposes of the paper was to explain that the ratio was highly uncertain... Shouldn't the sensitivity analysis be applied to the full range of OrgAlk:DOC ratios that you think are feasible?
-I'm concerned that the effort to just proportionally scale the effect of OrgAlk on pCO2 calculations is unsupported. The authors just spent several sections (4.2.1, 4.2.2) explaining how complex the sea ice carbonate system becomes when you consider organic alkalinity and other factors (e.g. poorly constrained dissociation constants). I would not assume that the effect of organic alkalinity on pCO2 would just scale proportionally between Arctic and Antarctic sea ice. There could be many factors (many of which have been reported in this study!) that would preclude that... One would need to consider at the very least one other carbonate parameter (e.g. DIC or pH). Can the authors better justify this choice? Were additional carbonate parameters (pH, DIC) not available in their dataset? If they were available, why did they not actually do the sensitivity calculation in CO2SYS?
I think if the authors put some effort into reducing the redundancy of the paper, and were able to come up with a more robust final sensitivity analysis (impact of OrgAlk on pCO2 or other derived carbonate system variables), this paper could warrant publication in The Cryosphere.